Perhaps the most anticipated result of the LHC involves the search for the Higgs boson, the only particle predicted by the Standard Model (SM) that has not yet been seen by experiments. The Higgs boson helps explain how elementary particles acquire mass. If the SM Higgs boson exists it will be produced at the LHC and swiftly decay into various known and well-studied particles, with the dominant decay products depending on the actual Higgs mass. ATLAS and CMS search for the SM Higgs boson using a range of decay products: two photons; two tau leptons; two b quarks; two W bosons; and two Z bosons. Analysing all these channels ensures that the search is sensitive to observing the Higgs irrespective of its mass. First results from ATLAS and CMS on searches for the SM Higgs boson with about 1 inverse femtobarn^[1] (fb⁻¹) of data were reported already in July at the EPS2011 meeting in Grenoble, and updates were shown one month later at the Lepton Photon 2011 conference in Mumbai with data samples of up to 2.3 fb⁻¹. By combining the individual results of the two experiments an increased sensitivity in the search can be obtained. Hence, as a next logical step, the experiments have now combined the search data released this summer. A team of physicists from ATLAS and CMS was put in place early this year to prepare for such a combination. Correlations between the individual results have been carefully taken into account. Various statistical combination techniques have been explored to perform the combination, and a huge number of cross checks were made in the last few months. Finally, the combined search result is now ready. The results of the combination are shown in the figure, which depicts in a very clear way the enormous impact of the 2011 LHC data on the search for the SM Higgs boson. The CMS and ATLAS combined search excludes at >95% confidence level^[2] the presence of the Standard Model Higgs in the mass range^[3] 141-476 GeV. Indeed, the region from 146 to 443 GeV is excluded at 99% confidence level with the exception of three small regions between 220 and 320 GeV. Nonetheless, exploration continues of the region at low mass that is most favoured by indirect measurements, as the experiments collect and analyse more data to increase their sensitivity. Clearly this region will be the main focus of the searches with the complete 2011 data sample, which amounts to more than 5 fb⁻¹ recorded by each of the experiments, and in 2012. At the same time, the search in the full range will continue to look for production of non-Standard Model Higgs bosons, with a possible lower production rate in the channels studied so far, and by exploring new decay channels. The next few months are definitely going to be very exciting. More information can be found in CMS-PAS-HIG-11-023, ATLAS-CONF-2011-157.

Video

Follow the combination process from the CMS perspective (includes exclusive footage from internal CMS meetings):

Additional information

• All CMS Higgs plots and results
• All CMS Papers
• All CMS Physics Analysis Summaries
• All CMS results
• Images of real collisions in CMS
• Animations of real collisions in CMS

About CMS

CMS is one of two general-purpose experiments at the LHC that have been built to search for new physics. It is designed to detect a wide range of particles and phenomena produced in the LHC's high-energy proton-proton and heavy-ion collisions and will help to answer questions such as: "What is the Universe really made of and what forces act within it?" and "What gives everything substance?" It will also measure the properties of well-known particles with unprecedented precision and be on the lookout for completely new, unpredicted phenomena. Such research not only increases our understanding of the way the Universe works, but may eventually spark new technologies that change the world in which we live as has often been true in the past. The conceptual design of the CMS experiment dates back to 1992. The construction of the gigantic detector (15 m diameter by nearly 29 m long with a weight of 14000 tonnes) took 16 years of effort from one of the largest international scientific collaborations ever assembled: more than 3100 scientists and engineers from 169 institutions and research laboratories distributed in 39 countries all over the world. For further information, contact: cms.outreach@cern.ch.

• [1] http://news.stanford.edu/news/2004/july21/femtobarn-721.html
  http://www.quantumdiaries.org/2011/03/02/why-don%E2%80%99t-we-just-say-collision-rate/
• [2] Confidence level is a statistical measure of the number of times out of 100 that test results can be expected to be within a specified range. For example, a confidence level of 95% means that the result of an action will probably meet expectations 95% of the time. (Source: NADbank)
• [3] By mass-energy equivalence, the electron volt is also a unit of mass. It is common in particle physics, where mass and energy are often interchanged, to use eV/c², where c is the speed of light in a vacuum (from E = mc²). Even more common is to use a system of natural units with c set to 1 (hence, E = m), and simply use eV as a unit of mass. (Source: Wikipedia)

CERN, 13th December 2011

The statement below is also available as PDF files in: Chinese (traditional) | Chinese (simplified) | Croatian | Dutch | English | Finnish | French | German | Greek | Hungarian | Italian | Persian | Polish | Spanish | Portuguese | Russian | Serbian | Turkish

The Higgs boson is the only particle predicted by the Standard Model (SM) of particle physics that has not yet been experimentally observed. Its observation would be a major step forward in our understanding of how particles acquire mass. Conversely, not finding the SM Higgs boson at the LHC would be very significant and would lead to a greater focus on alternative theories that extend beyond the Standard Model, with associated Higgs-like particles. Today the CMS Collaboration presented their latest results in the search for the Standard Model Higgs boson, using the entire data sample of proton-proton collisions collected up to the end of 2011. These data amount to 4.7 fb^-1 of integrated luminosity[1], meaning that CMS can study Higgs production in almost the entire mass range above the limit from CERN’s Large Electron Positron (LEP) collider of 114 GeV/c² (or 114 GeV in natural units [2]) and up to 600 GeV. Our results were achieved by combining searches in a number of predicted Higgs “decays channels” including: pairs of W or Z bosons, which decay to four leptons; pairs of heavy quarks; pairs of tau leptons; and pairs of photons (Figure 1). Our preliminary results, for several statistical confidence levels [3], exclude the existence of the SM Higgs boson in a wide range of possible Higgs boson masses:

• 127 – 600 GeV at 95% confidence level, as shown in Figure 2a; and
• 128 – 525 GeV at 99% confidence level.

A mass is said to be “excluded at 95% confidence level” if the Standard Model Higgs boson with that mass would yield more evidence than that observed in our data at least 95% of the time in a set of repeated experiments. We do not exclude a SM Higgs boson with a mass between 115 GeV and 127 GeV at 95% confidence level. Compared to the SM prediction there is an excess of events in this mass region (see Figure 2b), that appears, quite consistently, in five independent channels. With the amount of data collected so far, it is inherently difficult to distinguish between the two hypotheses of existence vs non-existence of a Higgs signal in this low mass region. The observed excess of events could be a statistical fluctuation of the known background processes, either with or without the existence of the SM Higgs boson in this mass range. The larger data samples to be collected in 2012 will reduce the statistical uncertainties, enabling us to make a clear statement on the possible existence, or not, of the SM Higgs boson in this mass region. The excess is most compatible with an SM Higgs hypothesis in the vicinity of 124 GeV and below, but with a statistical significance of less than 2 standard deviations (2σ) from the known backgrounds, once the so-called Look-Elsewhere Effect [4] has been taken into account. This is well below the significance level that traditionally has been associated with excesses that stand the test of time. If we explore the hypothesis that our observed excess could be the first hint of the presence of the SM Higgs boson, we find that the production rate (“cross section” relative to the SM, σ/σ_SM) for each decay channel is consistent with expectations, albeit with large uncertainties. However, the low statistical significance means that this excess can reasonably be interpreted as fluctuations of the background. More data, to be collected in 2012, will help ascertain the origin of the excess. UPDATE: CMS publishes Higgs boson search results using 2010–2011 data

• This statement is available online at:
  http://cern.ch/cms/news/cms-search-standard-model-higgs-boson-lhc-data-2010-and-2011
• For more details see the CMS public website: http://cern.ch/cms

Event images and animations of real CMS data

• Collision event display images from the Higgs analysis of 2010 and 2011 data
• Collision event animation (4 electrons) from the Higgs analysis of 2010 and 2011 data
• Collision event animation (2 photon) from the Higgs analysis of 2010 and 2011 data
• Collision event animation (4 muons) from the Higgs analysis of 2010 and 2011 data

Short movies about the Higgs

• Animation of the Higgs mechanism [ with subtitles] See also [ without subtitles]
• What is the Higgs? by Don Lincoln
• Higgs Boson: How do we search for it? by Don Lincoln

Seminar at CERN, 13th December 2011

• CMS seminar slides (Prof. G. Tonelli)
• CERN Press Release, 13th December 2011 [ Also available in French ]
• ATLAS Experiment Higgs results

More about the Higgs at CMS

• CMS Physics Analysis summary papers about the Higgs
• Simple explanations of Higgs search terminology
• Search for the Standard Model Higgs Boson in CMS
• All CMS Higgs plots and results

About CMS Physics

• All CMS Papers
• All CMS Physics Analysis Summaries
• All CMS results
• Images of real collisions in CMS
• Animations of real collisions in CMS

About CMS

More information may be found on the CMS web site: http://cern.ch/cms. CMS is one of two general-purpose experiments at the LHC that have been built to search for new physics. It is designed to detect a wide range of particles and phenomena produced in the LHC's high-energy proton-proton and heavy-ion collisions and will help to answer questions such as: "What is the Universe really made of and what forces act within it?" and "What gives everything substance?" It will also measure the properties of well-known particles with unprecedented precision and be on the lookout for completely new, unpredicted phenomena. Such research not only increases our understanding of the way the Universe works, but may eventually spark new technologies that change the world in which we live as has often been true in the past. The conceptual design of the CMS experiment dates back to 1992. The construction of the gigantic detector (15 m diameter by nearly 29 m long with a weight of 14000 tonnes) took 16 years of effort from one of the largest international scientific collaborations ever assembled: more than 3100 scientists and engineers from 169 institutions and research laboratories distributed in 39 countries all over the world. For further information, contact: cms.outreach@cern.ch.

Footnotes

• [1] http://news.stanford.edu/news/2004/july21/femtobarn-721.html
• [2] The electron volt is a unit of energy. In particle physics, where mass and energy are often interchanged, it is common to use eV/c² as a unit of mass (from E = mc², where c is the speed of light in vacuum). Even more common is to use a system of natural units with c set to 1 (hence, E = m), and simply use eV as a unit of mass.
• [3] Confidence level is a statistical measure of the percentage of test results that can be expected to be within a specified range. For example, a confidence level of 95% means that the result of an action will probably meet expectations 95% of the time.
• [4] http://cmsexperiment.web.cern.ch/news/should-you-get-excited-your-data-…

The Higgs boson is the only particle predicted by the Standard Model that has not yet been seen by experiments. It helps explain how elementary particles acquire mass. If the Higgs boson exists it will be produced in proton-proton collisions at the LHC. It then swiftly decays into various known and well-studied particles (depending on the actual Higgs mass). The CMS search for the Higgs boson is being carried out using a range of decay products: two photons; two tau leptons; two b quarks; two W bosons; and two Z bosons. Analysing all these channels ensures that the search is sensitive to observing the Higgs irrespective of its own mass. An example of an event passing the selection criteria is shown in Figure 1, in which two Z bosons are identified through their decays to a pair of electrons and a pair of muons respectively. These results are based on data recorded during LHC running at an energy of 7 TeV in 2010 and 2011. The analysis uses 1.1 – 1.7 inverse femtobarns^[1] (integrated luminosity) of data, depending on the channel — an inverse femtobarn corresponds to about 100 trillion proton-proton collisions. CMS observes no convincing excess of events in the explored mass^[2] range of 110-600 GeV (for full details see [HIG-11-022]). The analysis excludes, with a confidence level (C.L.) of 95%^[3], the existence of a Standard Model Higgs boson in three Higgs mass ranges: 145-216 GeV, 226-288 GeV and 310-400 GeV (Figure 2). For the quantity of data we have collected, on average we would expect to exclude the range 130-440 GeV in the absence of a signal. We believe that the differences between the expected and observed exclusion mass ranges are consistent with statistical fluctuations. At 90% C.L., we exclude the SM Higgs boson in the mass range from 144-440 GeV. All exclusion regions were obtained using the modified frequentist construction confidence levels. It should be noted that a modest excess of events is observed for Higgs boson masses below 145 GeV. With the data we will collect in the coming months we will be able to distinguish between the possible interpretations: the production of a SM Higgs boson or a statistical fluctuation of the backgrounds. During the ongoing LHC proton-proton data-taking period, expected to terminate at the end of 2012, CMS will record substantially more data that should be sensitive to observing a SM Higgs boson, if it exists, over the full range of possible masses.

More information

• All CMS Papers (see also: timeline graphical index of all papers)
• All CMS Physics Analysis Summaries
• All CMS results

About CMS

More information, including images and animations of CMS collision events, may be found on the CMS web site: http://cms.cern.ch. CMS is one of two general-purpose experiments at the LHC that have been built to search for new physics. It is designed to detect a wide range of particles and phenomena produced in the LHC's high-energy proton-proton and heavy-ion collisions and will help to answer questions such as: "What is the Universe really made of and what forces act within it?" and "What gives everything substance?" It will also measure the properties of well-known particles with unprecedented precision and be on the lookout for completely new, unpredicted phenomena. Such research not only increases our understanding of the way the Universe works, but may eventually spark new technologies that change the world in which we live as has often been true in the past. The conceptual design of the CMS experiment dates back to 1992. The construction of the gigantic detector (15 m diameter by nearly 29 m long with a weight of 14000 tonnes) took 16 years of effort from one of the largest international scientific collaborations ever assembled: more than 3100 scientists and engineers from 169 institutions and research laboratories distributed in 39 countries all over the world. For further information, contact: cms.outreach@cern.ch.

• [1] http://news.stanford.edu/news/2004/july21/femtobarn-721.html
  http://www.quantumdiaries.org/2011/03/02/why-don%E2%80%99t-we-just-say-collision-rate/
• [2] By mass-energy equivalence, the electron volt is also a unit of mass. It is common in particle physics, where mass and energy are often interchanged, to use eV/c², where c is the speed of light in a vacuum (from E = mc²). Even more common is to use a system of natural units with c set to 1 (hence, E = m), and simply use eV as a unit of mass. (Source: Wikipedia)
• [3] Confidence level is a statistical measure of the number of times out of 100 that test results can be expected to be within a specified range. For example, a confidence level of 95% means that the result of an action will probably meet expectations 95% of the time. (Source: NADbank)

The CMS collaboration is presenting its latest results this week at the 2011 Europhysics Conference on High-Energy Physics, held in Grenoble, France. These results are based on about 1 inverse femtobarn^[1] of data (100 trillion proton-proton collisions) from LHC running at an energy of 7 TeV, which were collected in 2010 and 2011. They include a wide range of searches for new physics and precise measurements of Standard Model processes.

Higgs Search

The Higgs boson is the only particle predicted by the Standard Model that has not yet been seen by experiments. It helps explain how elementary particles acquire mass. If the Higgs boson exists it will be produced in proton-proton collisions at the LHC and then swiftly decay into several known and well-studied particles. The CMS search for the Higgs boson is being carried out using a range of decay products: two photons; two tau leptons; two W bosons; and two Z bosons. The W is observed through its decay to an electron plus a neutrino, or a muon plus a neutrino. The Z is observed through its decay to a pair of electrons, or a pair of muons, (see Figure 1) or a pair of jets of hadronic particles. Analysing all these channels ensures that the search is sensitive to observing the Higgs irrespective of its own mass. CMS observes no convincing excesses of events in the explored mass^[2] range of 120-600 GeV (for full details see [HIG-11-011]). The analysis excludes, with a confidence level of 95%^[3], the existence of a Higgs boson in two broad Higgs mass ranges, 149-206 GeV and 300-440 GeV, as well as several narrower intervals in between (Figure 2). At a lower confidence level of 90%, the existence of a Higgs boson is excluded for the range 145-480 GeV. Re-interpreting the results in the context of the Standard Model with a fourth generation of fermions in addition to the known three generations (SM4), allows us to exclude the SM4 Higgs boson with a mass in the range 120-600 GeV with a confidence level of 95%. It should be noted that a modest excess of events is observed for Higgs boson masses below 145 GeV. With the data we will collect in the next few months we will be able to distinguish between the possible interpretations: the production of a Higgs boson or a statistical fluctuation of the backgrounds. During the ongoing LHC proton-proton run CMS will record substantially more data that should be sensitive to observing a Higgs boson, if it exists, over the full range of possible masses.

Search for rare decays of B_s→μμ

B_s mesons, comprising a 'bottom' and a 'strange' quark, are produced copiously at the LHC. The fraction that subsequently decays (known as the "branching fraction") to a pair of easily-detected muons is highly suppressed in the Standard Model — only about three such decays are expected per billion B_s particles produced. Several extensions of the SM, for instance Supersymmetric models, predict significant enhancements in the number of decays to pairs of muons, thanks to new particles that would contribute to the decay through "virtual loops". Therefore, any enhancement of this B_s decay rate could indicate the existence of new physics. CMS has searched for the decays of B_s (and B⁰ particles, comprising a 'bottom' and a 'down' quark) to muon pairs using proton-proton collision data collected up to June 2011 (Figure 3). A challenging aspect of this search is reducing the large backgrounds from other B-hadron decays or particles misidentified as muons. The number of candidate decays observed in the available data sample is consistent with the Standard Model expectations for signal and background (see Figure 4). Given the absence of a significant excess, CMS has excluded (at 95% confidence level) branching fractions larger than 1.9x10^-8 and 4.6x10^-9 for the decay of B_s and B⁰ particles, respectively. This result constitutes one of the most stringent exclusion limits achieved until now. The data CMS will collect in the remainder of 2011 and in 2012 will be sensitive to smaller branching fractions, at the level of SM expectations, and may lead to the observation of an enhanced decay rate which could be indicative of a non-Standard-Model physics process.

Other Results

CMS has searched for many other possible signatures of exotic new physics. No significant signals for new physics have yet been seen and therefore new constraints, at 95% confidence level, have been placed on many extensions to the Standard Model, including the following:

• The Supersymmetry (SUSY) theory provides a symmetry between matter and forces, and predicts that for each known particle there is a 'supersymmetric' partner. SUSY quarks, or squarks, are ruled out by CMS up to 1.2 TeV for a large range of SUSY parameters in the models tested [SUS-11-003].
• High-mass W′ and Z′ particles with Standard Model couplings are ruled out up to masses of 2.27 TeV [EXO-11-024] and 1.94 TeV [EXO-11-019] respectively. Constraints are placed on other models producing high-mass di-lepton resonances. Analysis of top-anti-top final states rules out the "Kaluza Klein gluon" (in Randal Sundrum models) with mass below 1.5 TeV [EXO-11-006].
• A study of di-jet resonances excludes excited quarks in the mass range of 1.0 TeV to 2.5 TeV and similarly constrains models involving E6 di-quarks, string resonances, the W′, axigluons and colorons [EXO-11-015].
• A search for the pair production of a fourth-generation t′ quark and its antiparticle ruled them out up to a t′ mass of 450GeV [EXO-11-051].
• Microscopic black-hole production is ruled out for black-hole masses up to between 4 and 5 TeV, depending on the model in question [EXO-11-071].

CMS also has a rich programme that confronts the predictions of the Standard Model with precise experimental measurements at the high-energy frontier of the LHC. These include top-quark production that has been studied in detail, with recent measurements made of the cross-section for top quark-top antiquark pairs decaying fully hadronically [TOP-11-007], the cross-section measurement using events with a tau lepton in the final state [TOP-11-006] and the top charge asymmetry [TOP-11-014]. Many aspects of the Electroweak and Strong forces as well as b-quarks are being studied. These analyses have already resulted in more than thirty publications in refereed journals, with many more results in the pipeline. A good understanding of Standard Model processes is also important as they contribute to the backgrounds to new physics processes.

Outlook

The LHC machine is running extremely well and has already delivered more proton-collision data than was expected in the entire 2011 run. The high-precision CMS experiment is recording good quality data with high efficiency. By the end of 2012 we expect to increase the size of our data sample by a factor of about ten. We are just starting to explore a vast territory where new physics could manifest itself and the CMS experiment is ready for whatever Nature has in store for us.

More information

• All CMS Papers (see also: timeline graphical index of all papers)
• All CMS Physics Analysis Summaries
• All CMS results

About CMS

More information, including images and animations of CMS collision events, may be found on the CMS web site: http://cms.cern.ch. CMS is one of two general-purpose experiments at the LHC that have been built to search for new physics. It is designed to detect a wide range of particles and phenomena produced in the LHC's high-energy proton-proton and heavy-ion collisions and will help to answer questions such as: "What is the Universe really made of and what forces act within it?" and "What gives everything substance?" It will also measure the properties of well-known particles with unprecedented precision and be on the lookout for completely new, unpredicted phenomena. Such research not only increases our understanding of the way the Universe works, but may eventually spark new technologies that change the world in which we live as has often been true in the past. The conceptual design of the CMS experiment dates back to 1992. The construction of the gigantic detector (15 m diameter by nearly 29 m long with a weight of 14000 tonnes) took 16 years of effort from one of the largest international scientific collaborations ever assembled: more than 3100 scientists and engineers from 169 institutions and research laboratories distributed in 39 countries all over the world. For further information, contact: cms.outreach@cern.ch.

• [1] http://news.stanford.edu/news/2004/july21/femtobarn-721.html
  http://www.quantumdiaries.org/2011/03/02/why-don%E2%80%99t-we-just-say-collision-rate/
• [2] By mass-energy equivalence, the electron volt is also a unit of mass. It is common in particle physics, where mass and energy are often interchanged, to use eV/c², where c is the speed of light in a vacuum (from E = mc²). Even more common is to use a system of natural units with c set to 1 (hence, E = m), and simply use eV as a unit of mass. (Source: Wikipedia)
• [3] Confidence level is a statistical measure of the number of times out of 100 that test results can be expected to be within a specified range. For example, a confidence level of 95% means that the result of an action will probably meet expectations 95% of the time. (Source: NADbank)

CMS performs searches for new particles by looking for signals amidst a background of known physics. If the data begin to indicate something more interesting than merely background - for instance, more decay events than expected in a certain region - it is important to make sure that the observation is statistically significant by collecting and analysing more data. However, we don't want to bias our analyses by optimising the analysis based on what was already seen. To avoid such bias while analysing new data, physicists draw "blinds” over the region where an excess of decay events is expected; this region is only "unblinded" when they are satisfied with their procedures. This ensures objectivity when it comes to looking for much-sought-after signs of new physics, and gives confidence in the ultimate result.

The procedure is similar to that used by medical researchers when testing a new treatment. Even though the Higgs boson was discovered a decade ago, we still employ this procedure for measuring the Higgs boson's properties - we still do not want to bias these measurements by optimising towards deviations from the standard model expectations! All of the available data, including those in the blinded region, are used in the analysis; you just don’t peek at this region to see how it changes as you tweak various selection criteria. In many of the on-going searches and measurements, data corresponding to a possible signal look very similar to data corresponding to background.

Therefore, once the analysis is blinded, physicists must optimise two things: 1) simulations, to help determine the criteria for selecting signals and 2) the methods for rejecting or quantifying background events using data from the region where there is no excess. To cross-check all the details, the various steps involved in the analysis procedures are usually carried out by at least two independent teams. If both teams see similar results in the background region, the analysis is signed off by the wider collaboration. It is only after the sign-off that the signal region is unblinded. The results of the unblinding are put through further scrutiny by the collaboration before being made public.

A dangerous beast is hiding in today’s searches for the rare signs of new physics — or even “old” physics, such as the Higgs boson — at the Large Hadron Collider. It is called “Look-Elsewhere Effect”, LEE for insiders. What is it, and why should you care? Imagine you look for a heavy particle decaying into a pair of hadronic jets: a commonplace test case in high-energy physics. You have your background model, which predicts the observable shape of the di-jet mass distribution, and you know what kind of a bump a new particle signal would produce in that shape. So you search for such a bump in the data, but — not knowing where it might appear — you search everywhere. You have worked all day, and the night is nearing; you prepare yourself a martini and spin your analysis program. To your amazement, the program finds a significant bump at some particular mass value: is it a real signal? To claim it is a new signal, the effect must reach or exceed the “five-sigma” significance level, five standard deviations away from the expectation: that's a rather silly but well-established rule. But if yours gives only 3.5 or four sigma, are you allowed to get excited and wake up your boss, or should you sit back and sip your martini, with an “I know better” grin on your face? I claim the latter is a better option. You have fallen prey to the LEE: you looked in many places for a possible signal, and found a significant effect somewhere; the likelihood of finding something in the entire region you probed is greater than it would be if you had stated beforehand where the signal would be, because of the “probability boost” of looking in many places. A good rule of thumb is the following: if your signal has a width W, and if you examined a spectrum spanning a mass range from M1 to M2, then the “boost factor” due to the LEE is (M2-M1)/W. This may easily amount to a factor of 10 or 100, depending on the details of your search. An effect occurring by chance once in ten thousand cases in a given place on your spectrum may actually be just an unexciting one-in-a-hundred fluctuation! In fact, the “five-sigma” rule I mentioned above was conceived with exactly this particular effect in mind. Five sigma is a really, really rare occurrence (three in ten million), so even including the LEE, it is still something to take quite seriously. Now-a-days, we publish three-sigma results that may or may not go away with more data, and our scientific integrity requires us to account for the LEE. This is actually less easy to do than by just multiplying a probability by (M2-M1)/W as in the example above: in complex searches such as that for the Higgs boson, which combine bump hunts in many channels, this is not a trivial task. A recent paper by E. Gross and O. Vitells (Eur. Phys. J. C70:525-530,2010) has clarified some of the technical issues. The searches for the Higgs boson by CMS and ATLAS are sizing up the LEE by studying the probability of the background-only hypothesis as a function of the Higgs mass: the more the observed p-value distribution varies up and down as the signal mass hypothesis change, the stronger is the Look-Elsewhere-Effect correction that is required. Stuff for experts, for sure. But outsiders have better be aware that a three-sigma effect should not be blindly dubbed “evidence” for something new in the data. As travellers to a foreign country whose tax habits are unknown, you better ask before you buy, “LEE included or not”? — Submitted by Tommaso Dorigo

Spot the difference

So how did the universe evolve into this very asymmetric state, dominated by matter, if the underlying forces can barely tell the difference between the two? One possible explanation is that there exists yet another, undiscovered, force in nature that is not matter-antimatter symmetric, i.e. has different effects on matter and antimatter. Another, more popular explanation is that the weak interaction, through which such particles decay, can actually distinguish between matter and antimatter particles.

A clue to the answer into this question may be provided by the phenomenon of Charge-Parity (CP) violation, discovered over four decades ago. CP violation implies that there is a small difference in the rates at which certain particles decay and the corresponding rates at which their antiparticles decay.

One such particle is the B meson, a relative of the proton but made up of quarks known as “beauty”, “bottom” or more often just “b” quarks. Huge numbers of these will be generated in the LHC, and at CMS we can study their decay. Results from CMS will complement those generated in LHCb, a fellow LHC experiment that has been designed specifically for this task. Observing a large asymmetry in the decay rates of b quarks versus anti-b quarks might tell us more about why nature prefers matter over antimatter

The B meson and its antiparticle can both decay into two muons and a different meson made of “charm” quarks, which in turn decays further to form two pions (yet another of the meson family). This decay, shown in the diagram, presents a fairly simple signature for the experiment to detect. We can tell whether the decayed particle was the B meson or its antiparticle by looking at the type of muon produced by the decay of the opposite b quark in the event, and we will be able to compare the rates of the two.

Supersymmetry: Uniting the forces

Towards a superforce

Our understanding of the workings of the Universe often progress when unexpected connections are found between what appeared at first to be separate entities. A major breakthrough occurred in the 1860s when James Clerk Maxwell recognized the similarities between electricity and magnetism and developed his theory of a single electromagnetic force. A similar breakthrough came a century later, when theorists began to develop links between electromagnetism, with its obvious effects in everyday life, and the weak force, which normally hides within the atomic nucleus. Vital support for these ideas came first from the Gargamelle experiment at CERN, and then with the Nobel prize winning discovery of the W and Z particles, which carry the electroweak force. But take note – it is only at the higher energies explored in particle collisions at CERN and other laboratories that the electromagnetic and weak forces begin to act on equal terms.

So will other forces join the club at even higher energies? Experiments already show that the effect of the strong force becomes weaker as energies increase. This is a good indication that at incredibly high energies, the strengths of the electromagnetic, weak and strong forces may be the same. The energies involved are at least a thousand million times greater than particle accelerators can reach, but such conditions would have existed in the very early Universe, almost immediately (10^-34 s) after the Big Bang. Pushing the concept a step further, theorists even contemplate the possibility of including gravity at still higher energies, thereby unifying all the fundamental forces into a single force. This would have ruled the first instants of the Universe, before its different components separated out as the Universe cooled.

Enter superparticles

Although at present we cannot recreate conditions with energy high enough to test these ideas directly, we can look for the consequences of ‘grand unification’ at lower energies, for instance at the Large Hadron Collider. A very popular idea that allows the stong and electroweak forces to unite and become a single interaction at a common energy is called supersymmetry, or SUSY for short. SUSY provides a symmetry between matter and forces, and predicts that for each known particle there is a 'supersymmetric' partner. If this is correct, then supersymmetric particles should appear in collisions at the LHC.

Read more about Supersymmetry in CMS

Evidence from the depths of the Universe has ruled out a number of models for what the mysterious dark matter might be, but one candidate that fits so far is the lightest supersymmetric particle (LSP) otherwise known as the “neutralino”, the lightest of a whole range of new particles suggested by a theory called supersymmetry. If the neutralino exists it will likely be stable, heavy, neutral, and will not interact electromagnetically. This makes it a perfect candidate for a substance that pervades the universe without being spotted.

If supersymmetric particles exist, they are very likely to be produced in collisions in the LHC. The heavy particles will decay into combinations of leptons (like electrons and muons) and quarks (which will cause sprays of particles called jets) as well as into neutralinos that will  not decay any further. Therefore many neutralinos will pass through the CMS detector, without depositing any energy or leaving a trail.

So how do you detect an “invisible” particle? CMS will be able to find the neutralino indirectly – by identifying when the energy used to make it goes missing.

Momentum in equals momentum out…

One of the most fundamental laws of physics is that ‘momentum is conserved’. In other words, the total momentum before a collision is equal to the total momentum after. If the total momentum of the observed particles that emerge from a proton-proton collision does not equal the momentum of the two protons, we can deduce there must be an invisible particle somewhere that carried away that missing momentum.

As we collect all the particles we can also add up their momenta and energies (in the “transverse” direction, i.e. at right angles to the beam line) and reconstruct the entire collision; like building a giant jigsaw puzzle. When a neutralino is formed and we can’t detect it we see an imbalance in the collision, with particles flying out one side but not the other and the energy not adding up. This shows up as a hole in the jigsaw puzzle: a missing particle seen through its missing energy or momentum.

The CMS is “hermetic” when it comes to finding missing particles. This means that, to the extent possible, it catches every detectable particle emerging from a collision. Large detectors have “channels for escape”, regions where particles cannot be detected because of cables or other mechanical support. These regions must be minimised to ensure that standard particles can't slip by undetected. This way, if the energy or momentum is "missing", it really is due to an invisible particle.

To search for this missing energy, it is important that the CMS has a good hadron calorimeter as well as detectors at every angle around the beam line. To ensure that particles flying from all directions will be detected, this includes the very shallow angles known as the “forward region”.

Superstring theory is elegant but speculative and throws up a number of possible experimental outcomes, some more likely than others.

One option would be to find evidence of another host of particles that can only exist if there are more dimensions. Theories that postulate these extra dimensions predict that, like an atom having a low energy ground state and then more energetic states, there must be heavier versions of standard particles recurring at higher and higher energies as they navigate smaller dimensions. These have been called Kaluza-Klein recurrences and would have exactly the same properties as standard particles (and so be visible to our detector) but at a greater mass. If CMS were to find a Z-like particle (the Z boson being one of the carriers of the electroweak force) at 2 TeV for instance, this might suggest the presence of extra dimensions. Such heavy particles wouldn't have been seen at lower energies.

As gravity is thought to be a force able to probe these extra dimensions, another way in which we may be find evidence for string theory is through the disappearance of gravitons, the hypothesised carrier of gravity, into these other dimensions. The particle might be carried away without a trace, but it would leave behind an imbalance in momentum and energy.

Two of the most fundamental laws of physics are that momentum and energy “are conserved”. In other words, the total momentum and energy before a collision is equal to the total momentum and energy after. So if we add these up in all the observed particles emerging from a collision (in the case of CMS we do this is the sideways “transverse” direction) and it does not equal the amount in the original particles, there must be a missing particle that carried away the momentum and energy: a lost piece leaving a hole in the jigsaw.

This is why the detector must be as “hermetic” as possible, that is, it must be able to catch, to the extent possible, every particle emerging from the collisions, so we can deduce that particles have genuinely disappeared, and have not just been missed by the detector.

This method of searching is similar to the way in which we will detect supersymmetric particles , so careful analysis would be needed to decide if a graviton has travelled to another dimension or we have instead produced a SUSY particle, whose decay products would also be invisible to our detector.

Finally, another spectacular yet speculative way of revealing extra dimensions would be through the production of “Micro Quantum Black Holes”, which, if there are extra dimensions, might be produced at the LHC. What exactly we would see would depend on the number of extra dimensions, the mass of the black hole, the size of the dimensions and the energy at which the black hole occurs. If micro black holes do appear they would disintegrate extremely rapidly, in around 10^-27 seconds, as they decay into Standard Model or SUSY particles, producing many jets and leptons.

To find these would be an incredible feat that would tell us about how quantum mechanics, thermodynamics and gravity all work together on miniscule scale, allowing us to study quantum gravity in the laboratory. We could use what we see in our three dimensions - for instance, the type of particles emerging, their energy distributions and masses - to decipher the geometry of the extra dimensions and this would be a challenge for physicists to unravel for the coming century.

Read more about the safety of micro quantum black holes and to download the most recent expert reports on the subject.

String theory and extra dimensions

Loose ends

Will the string tie the Standard package? Hot on the heels of the Standard Model, some physicists are working to support an idea called string theory. This attempts to tie up the loose ends in the Standard Model by explaining all the fundamental particles and forces (including gravity) in a unified framework.

Underlying string theory is the radical idea that fundamental particles are not really like points or dots, but rather small loops of vibrating strings. All the different particles and forces are just different oscillation modes of a unique type of string. Bizarrely, the theory also implies that besides the familiar three–dimensional world and the fourth dimension of time, there are six additional spatial dimensions! These extra dimensions are apparently 'curled up' so small that we do not see them.
 

Which string?

String theory is conceptually complex, with a fascinating but very difficult mathematical structure. This has so far prevented researchers from deriving concrete predictions from the theory for comparison with experimental results. Not only does string theory involve the complex study of the geometry of extra dimensions, but the way the structure of the dimensions are chosen appears arbitrary and can lead to different outcomes.

For instance, there seem to be many possible ways to curl up the extra dimensions, by choosing different shapes and sizes. This leads to many alternative versions of the theory. In certain cases, the sizes of the extra dimensions are very small and it will be difficult to obtain direct evidence for them. In others, the sizes are far larger and could be observed at new accelerators such as CERN’s Large Hadron Collider.
 

Secret dimensions

In everyday life, we inhabit a space of three dimensions – a vast ‘cupboard’ with height, width and depth, well known for centuries. Less obviously, we can consider time as an additional, fourth dimension, as Einstein famously revealed. But just as we are becoming more used to the idea of four dimensions, some theorists have made predictions wilder than even Einstein had imagined.

String theory intriguingly suggests that six more dimensions exist, but are somehow hidden from our senses. They could be all around us, but curled up to be so tiny that we have never realized their existence.
 

What is a dimension?

Dimensions are really just the number of co-ordinates we need to describe things. We can compare this to a tightrope walker travelling along a rope. For the acrobat there is only one dimension – forwards or backwards, and we can state her or his position with just one number. But if we look on a smaller level, for an ant crawling about on the rope there would be two dimensions of travel: we’d need to know how far around the rope it is, as well as how far along. If we zoom in even further, for atoms inside the rope, the world would be in three dimensions, the x, y and z of everyday coordinates. Who is to say that as we go smaller and smaller the number of directions to travel in, the number of dimensions, does not increase even further?
 

Beyond the third dimension

Some string theorists have taken this idea further to explain a mystery of gravity that has perplexed physicists for some time – why is gravity so much weaker than the other fundamental forces? Why do we need objects the size of planets in order to feel its force when we can experience the electromagnetic force with just a small magnet? And if gravity’s quantum carrier, the graviton, exists, how can we find it? The idea is that we do not feel gravity’s full effect in the everyday world. Gravity may appear weak only because its force is being shared with other spatial dimensions.

To find out whether these ideas are just products of wild imaginations or an incredible leap in understanding will require experimental evidence. But how?

High-energy experiments could prise open the inconspicuous dimensions just enough to allow particles to move between the normal 3D world and other dimensions, manifesting itself in the sudden appearance or disappearance of a particle. Or we might detect some of the new phenomena that a world with extra dimensions predicts. Who knows where such a discovery could lead!

Read more about Extra dimensions in CMS via the link below

Probing the Plasma

Lead nuclei consist of large numbers of protons and neutrons, both made up of quarks. When the nuclei collide, a range of particles will be produced, some of which are expected to behave differently if a QGP is produced and such behaviour will tell us something about the plasma.

The QGP will for instance affect the frequency with which we see special kinds of mesons made of a pair of “heavy” quarks. When no QGP is formed, quarks remain locked within their particles and a host of these particles fly into the detector, usually seen through their decays to muon pairs. However, when a QGP has been produced, the more loosely-bound of these mesons are no longer seen because they dissolve into the scalding quark soup.

When the plasma gets denser and hotter, the more strongly-bound of these particles also “melt”. By studying the number and types of the surviving particles we can then deduce something about the state of the matter and estimate the temperature of the QGP.

By looking at the energies of the detected jets (narrow sprays of particles produced by quarks or gluons) we can also deduce how dense the plasma is.

Jets are produced in pairs in collisions and fire out in opposite directions. Jets produced in the centre of the dense plasma are “quenched”, losing energy like bullets in water, and most never emerge. Those produced at the edge however might escape, but only in one direction, because the opposite jet will be lost to the plasma mass. If the plasma is dense enough we therefore expect to see single jets rather than jet pairs.

The barely emerging jet will have lost much of its energy. Knowing how much energy loss the plasma caused tells us how dense it is, but to calculate this loss we need to know the jet’s original energy. To do this we look at jets where on the opposite side of the collision a photon or Z boson was produced instead of the second jet. Because these particles do not interact via the strong force they can sail through the QGP unaffected, and tell us the original energy of the interaction.

Z bosons decay into two muons; therefore the final particles that CMS must detect are photons, muon pairs and jets. With very precise hadron and electromagnetic calorimeters and a high performance muon detector system, CMS is ideally equipped to probe the existence of the quark-gluon plasma.

Colliding heavy nuclei

Clues to the early Universe

The Universe has changed a great deal in the 13.7 billion years since the Big Bang, but the basic building blocks of everything from microbes to galaxies were signed, sealed and delivered in the first few millionths of a second. This is when the fundamental quarks became locked up within the protons and neutrons that form atomic nuclei. And there they remain, stuck together with gluons, the carrier particles of the strong force. This force is so strong that experiments are not able to prise individual quarks or gluons out of their particles for long before they recombine quickly to produce new particles.
 

Primordial soup

Suppose, however, you could reverse the process. The current theory of the strong interaction predicts that at very high temperatures and very high densities, quarks and gluons should no longer be confined inside composite particles. Instead they should exist in a new state of matter known as ‘quark-gluon plasma’ (QGP).

Such a transition should occur when the temperature goes above a value around 2000 billion degrees - about 100 000 times hotter than the core of the Sun! For a few millionths of a second after the Big Bang the temperature of the Universe was indeed above this value; as it grew from being the size of a grapefruit right up to the size of our solar system, the entire Universe would have been in a state of quark-gluon plasma – a hot, dense ‘soup’ of quarks and gluons. Then as the Universe cooled below the critical value, the soup condensed into composite particles, including protons and neutrons.

Experiments at CERN’s Super Proton Synchrotron reported tantalising evidence for quark-gluon plasma in 2000 and the Relativistic heavy Ion Collider (RHIC) at Brookhaven National Laboratory has since pursued a broad and successful programme in which some fascinating and unexpected properties of the quark-gluon system where observed, giving us new insights into what to look for. The next big step will be with the Large Hadron Collider (LHC) .

For a period of time each year the LHC will, in place of colliding protons, collide heavy lead nuclei at close to the speed of light, recreating conditions similar to those just after the “Big Bang”, only on a much smaller scale. The quarks and gluons previously confined in each proton or neutron will then form part of a QGP. This will very quickly expand and cool and at a low enough temperature reassemble as ordinary matter. The reassembled particles fly out into the detector and studying them will help us understand how quarks and gluons behave in their plasma state. This in turn will help us to understand why and how these basic constituents of matter ever formed into protons and neutrons at all, and what keeps them in that state.

Read more about Heavy ions in CMS via the link below

Will SUSY be found?

If supersymmetry exists it will likely show itself through the presence of missing energy and through some particle signatures being produced more frequently than the normal “background” rate. A thorough understanding of the detector and all possible read-outs is therefore necessary to establish that we can’t explain these results by any known phenomena or experimental error before looking to a new explanation.

CMS is well equipped to discover such elusive particles and if SUSY is a true symmetry of Nature at the energy scales of the LHC, it will almost certainly be discovered in CMS.

This means a doubling of every particle we know to make SUSY partners or “sparticles”: matter partners are given an “s”, so the partners of electrons and quarks are called “selectrons” and “squarks”, and force partners become an “ino”, for example a “gluino” for the gluon and a “zino” for the Z boson.

Source: DESY

Given that in the 1930s physicist Paul Dirac already doubled the number of known particles by suggesting that each had an associated “antiparticle”, this idea isn’t so strange. But why haven't we ever seen these SUSY particles?

Physicists think that we’ve yet to see the superpartners because SUSY is a broken symmetry, where sparticles have much heavier masses and are less stable than their counterparts. So though SUSY particles would have featured in the same numbers as standard particles in the early Universe, as it expanded and cooled they would have decayed to now only exist in a lightest “fossil” form that is hard to find and detect. But if SUSY particles exist at energy scales compatible with the LHC, up to 2-3 TeV, we will be able to produce and study them in large quantities right here.
 

How to detect a superparticle

SUSY particles produced in the LHC are likely to be pairs of gluinos and/or squarks, heavy particles that will only live for 10^-23 seconds before they decay (turn into other more stable and familiar particles) in multiple stages, forming long “decay chains”. The final products will consist of leptons like electrons and muons (producing clean tracks in the experiment), quarks (producing sprays of particles known as jets), and always the lightest supersymmetric particle (LSP).

An event display showing SUSY decay to three leptons, two jets and two LSPs

The LSP is thought to be the lightest of the four "neutralinos", formed as combinations of a zino, a photino and two higgsinos: the SUSY partners of the Z boson, photon and Higgs boson respectively. The LSP is thought to be stable and so marks the end of the decay chain. But because it is weakly interacting and neutral, like the neutrino, it will pass straight through material and not be detected in CMS.

So how do you detect a particle you can’t see and that doesn’t interact with your detector? CMS will be able to detect LSPs, and so SUSY particles, by instead spotting when energy or momentum from the initial collision goes missing.
 

Momentum in equals momentum out…

One of the most fundamental laws of physics is that ‘momentum is conserved’. In other words, the total momentum before a collision is equal to the total momentum after; in this instance zero, because the protons travel with equal speeds in opposite directions so their momenta cancel out. If the total momentum of the observed particles that emerge from a proton-proton collision does not equal zero, we can deduce there must be an invisible particle somewhere that carried away that missing momentum.

CMS does exactly this: collecting and adding up the momenta and energies of all the emerging particles from a collision (in the “transverse” direction, i.e. at right angles to the beam line). CMS reconstructs the collision like a giant jigaw, and if the emerging particles seem imbalanced, flying out one side but not the other because an undetected LSP has been produced, we see the missing particle as a hole in the puzzle, identifiable due to its missing momentum or energy.

To spot the missing particles CMS is “hermetic” meaning that, to the extent possible, it catches every detectable particle emerging from a collision. Large detectors have cables and support regions that don't detect any kinds of particle and these channels for escape must be minimised to ensure that standard particles can't slip by undetected. This way if the energy or momentum is ‘missing’ it really is due to an invisible particle.

Finding missing energy is why CMS needs a good hadron calorimeter and why it has detectors in the “forward region”, at very shallow angles to the beam line (in addition to the detectors that surround the collision at larger angles) so that particles flying in all directions will be picked up.

Another important consideration in detecting SUSY particles is the ability to detect the “beauty” or “bottom” quarks. The b quark features in a number of decay chains, its sparticle expected to be one of the lighter squarks, but only lives for a very short amount of time before decaying (1 picosecond, one millionth of a millionth of a second). To spot these detectors must have very fine resolution and be placed very close to the beam line.
 

SUSY signatures

Pairs of SUSY particles decay in multiple stages producing a cascade of particles, often including further SUSYs, but resulting in three main “signatures” that CMS can look for.

An example of a pair of oppositely charged muons (µ+ and µ-) produced as a result of the decay of a squark (~q)

The first uses the fact that the final decay products will often consist of pairs of leptons of opposite charge (e.g. a muon and an anti-muon), along with the ever present LSPs. Looking for unusually high levels of pairs of these particles should be a clear signal of SUSY production and is why we need a high performance detector system for muons , and a good ECAL to detect electrons.

The second common SUSY decay is for gluino or squark pairs to decay into lots of quarks and anti-quarks, along with the two LSPs; in the experiment seen as many narrow sprays of particles called jets and missing energy.

The third way involves spotting an otherwise rare occurrence, the production of pairs of leptons with the same charge. Usually pairs of leptons are produced from decays of a pair of oppositely charged particles, or from the decay of a single neutral particle; either way, conservation of charge means that the two leptons will have opposite charges. However with SUSY we expect to produce pairs of gluinos, which are neutral particles. Furthermore the decay of each gluino is just as likely to produce a positively charged lepton as a negatively charged one (balanced by negatively or positively charged quarks), so a pair of gluinos will often give a pair of leptons with the same charges. Finding unexpectedly large number of same sign leptons produced in events (e.g. two muons rather than a muon and an anti-muon) would also strongly point towards SUSY production.
 

SUSY Higgs

Supersymmetry doesn’t only increase the number of familiar particles: within supersymmetric models, there are also at least five Higgs bosons – the light scalar (h^o), the heavy scalar (H^o), the pseudo scalar (A^o) and the negative and positive charged Higgs particles (H^±). These can decay to photons, leptons (including the heaviest known lepton, the tau) and mesons (particles made of quarks), seen as jets in the detector.

Shown in the decay diagram and event display is how a SUSY Higgs might decay to two taus, giving an end product of an electron, jets (caused by the pions) and neutrinos, which will be detected through their missing energy. If supersymmetric Higgs particles exist, one way they could therefore be seen is as an increased frequency of pairs of taus produced as specific energies, as shown in the graph.

An image of a SUSY collision showing an electron plus jets.
 

Antimatter detectives

The antimatter is missing – not from CERN, but from the Universe! At least that is what we can deduce so far from careful examination of the evidence. For each basic particle of matter, there exists an antiparticle with the same mass, but the opposite electric charge. The negatively charged electron, for example, has a positively charged antiparticle called the positron. When a particle and its antiparticle come together, they both disappear, quite literally in a flash, as the annihilation process transforms their mass into energy.
 

The evidence spoke for itself

The ‘case file’ of antimatter was opened in 1928 by physicist Paul Dirac. He developed a theory that combined quantum mechanics and Einstein’s special relativity to provide a more complete description of electron interactions. The basic equation he derived turned out to have two solutions, one for the electron and one that seemed to describe something with positive charge (in fact, it was the positron). Then in 1932 the evidence was found to prove these ideas correct, when the positron was discovered occurring naturally in cosmic rays.

For the past 50 years and more, laboratories like CERN have routinely produced antiparticles, and in 1995 CERN became the first laboratory to create anti-atoms artificially. But no one has ever produced antimatter without also obtaining the corresponding matter particles. The scenario should have been the same during the birth of the Universe, when equal amounts of matter and antimatter would have been produced in the Big Bang.
 

“Just one more thing…”

So if matter and antimatter annihilate, and we and everything else are made of matter, why do we still exist? This mystery arises because we find ourselves living in a Universe made exclusively of matter. Didn't matter and antimatter completely annihilate at the time of the Big Bang? Perhaps this antimatter still exists somewhere else? Otherwise where did it go and what happened to it in the first place?

Such questions have led to speculative theories, from a break in the rules to the existence of an entire anti-Universe somewhere else! The way to solve the baffling disappearance of antimatter, and to learn more about this substance in general, is by studying both particles and antiparticles to find and decipher the subtle clues.

Click the link below to read more about Antimatter in CMS

The standard package

The theories and discoveries of thousands of physicists over the past century have resulted in a remarkable insight into the fundamental structure of matter: everything that has been directly observed in the Universe until now has been found to be made from twelve basic building blocks called fundamental particles, governed by four fundamental forces. Our best understanding of how these twelve particles and three of the forces are related to each other is encapsulated in the Standard Model of particles and forces. Developed in the early 1970s, it has successfully explained a host of experimental results and precisely predicted a wide variety of phenomena. Over time and through many experiments by many physicists, the Standard Model has become established as a well-tested physics theory.
 

Matter particles

Everything around us is made of matter particles.These occur in two basic types called quarks and leptons.

Each group consists of six particles, which are related in pairs, or ‘generations’. The six quarks are paired in the three generations – the 'up quark' and the 'down quark' form the first generation, followed by the 'charm quark' and 'strange quark', then the 'top quark' and 'bottom quark'. The six leptons are similarly arranged in three generations – the 'electron' and the 'electron-neutrino', the 'muon' and the 'muon-neutrino', and the 'tau' and the 'tau-neutrino'. The electron, the muon and the tau all have an electric charge and mass, whereas the neutrinos are electrically neutral with very little mass.

The lightest and most stable charged particles are in the first generation, whereas the heavier and less stable particles belong to the second and third generations. All stable matter observed in the Universe is made from particles that belong to the first generation; any heavier particles quickly decay to the next most stable level.
 

Forces and carrier particles

There are four fundamental forces at work in the Universe: the strong force, the weak force, the electromagnetic force, and the gravitational force. They work over different ranges and have different strengths. Gravity is the weakest but it has an infinite range. The electromagnetic force also has infinite range but it is many times stronger than gravity. The weak and strong forces are effective only over a very short range (the size of a proton) and dominate only at the level of subatomic particles. Despite its name, the weak force is much stronger than gravity but it is indeed the weakest of the other three. The strong force is, as the name implies, the strongest among all the four fundamental interactions.

We know that three of the fundamental forces result from the exchange of force carrier particles, which belong to a broader group called ‘bosons’. Matter particles transfer discrete amounts of energy by exchanging bosons with each other. Each fundamental force has its own corresponding boson particle – the strong force is carried by the ‘gluon’, the electromagnetic force is carried by the ‘photon’, and the ‘W and Z bosons’ are responsible for the weak force. Although not yet found, the ‘graviton’ should be the corresponding force-carrying particle of gravity.

The Standard Model includes the electromagnetic, strong and weak forces and all their carrier particles, and explains extremely well how these forces act on all the matter particles. However, the most familiar force in our everyday lives, gravity, is not part of the Standard Model. In fact, fitting gravity comfortably into the framework has proved to be a difficult challenge. The quantum theory used to describe the micro world, and the general theory of relativity used to describe the macro world, are like two children who refuse to play nicely together. No one has managed to make the two mathematically compatible in the context of the Standard Model. But luckily for particle physics, the effects of gravity on their experiments have been so weak as to be negligible. Only when we have matter in bulk, such as in ourselves or in planets, is gravity observed to dominate. So the Standard Model still works well despite its reluctant exclusion of one of the fundamental forces.
 

So far so good, but...

...it is not time for physicists to call it a day just yet. Even though the Standard Model is currently the best description we have of the subatomic world, it does not explain the complete picture. The theory incorporates only three out of the four fundamental forces, omitting gravity. Alas, Newton would be turning in his grave! Nor does it explain why the many well-established basic parameters such as particles' masses have the values they do. There are also important questions it cannot answer, such as what is dark matter, what happened to the missing antimatter, and more.

Last but not least, an essential ingredient of the Standard Model, a particle called the Higgs boson, has yet to be found in an experiment. The race is on to hunt for the Higgs – the key to the origin of particle mass. Finding it would be a big step for particle physics, although its discovery would not write the final ending to the story.

So despite the Standard Model's effectiveness at describing the phenomena within its domain, it is nevertheless incomplete. Perhaps it is only a part of a bigger picture that includes new physics that has so far been hidden deep in the subatomic world or in the dark recesses of the Universe. New information from experiments at the Large Hadron Collider are sure to help us find more of these missing pieces.

Dark secrets of the Universe

It’s perhaps natural that we don’t know much about how the Universe was created – after all, we were never there ourselves. But it’s surprising to realise that when it comes to the Universe today, we don’t necessarily have a much better knowledge of what is out there. In fact, astronomers and physicists have found that all we see in the Universe – planets, stars, galaxies – accounts for only a tiny 4% of it! In a way, it is not so much the visible things that define the Universe, but rather the void around them.

Cosmological and astrophysical observations indicate that most of the Universe is made up of invisible substances that do not emit electromagnetic radiation – that is, we cannot detect them directly through telescopes or similar instruments. We detect them only through their gravitational effects, which makes them very difficult to study. These mysterious substances are known as ‘dark matter’ and ‘dark energy’. What they are and what role they played in the evolution of the Universe are a mystery, but within this darkness lie intriguing possibilities of hitherto undiscovered physics beyond the established Standard Model.
 

Dark matter

Dark matter makes up about 26% of the Universe. The first hint of its existence came in 1933, when astronomical observations and calculations of gravitational effects revealed that there must be more 'stuff' present in the Universe than telescopes could see.

Researchers now believe that the gravitational effect of dark matter makes galaxies spin faster than expected, and that its gravitational field bends the light coming from objects behind it. Measurements of these effects show that dark matter exists, and they can be used to estimate the density of dark matter even though we cannot directly observe it.

But what is dark matter? One idea is that it could contain ‘supersymmetric particles’ - hypothesized particles that are partners to those already known in the Standard Model. Experiments at the Large Hadron Collider may be able to find them.
 

Dark energy

Dark energy makes up approximately 70% of the Universe and appears to be associated with the vacuum in space. The LHC will not directly detect dark energy.