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)