CERN Accelerating science

The Large Hadron Collid

Since the LHC collider started operating, more than a decade ago, several trillions of proton-proton collisions have taken place.

Using events containing multiple leptons, the CMS Collaboration looked for production of a pair of Higgs bosons, but could not find any evidence. However there is some extra events in data with high mass values for the Higgs boson pair when compared with total expectation of Higgs boson pair signal and background. Only with more data the situation can be clarified.

Vector boson (W, Z) scattering process is a purely weak process which have been studied here to investigate interesting characteristics of the standard model and to test new physics scenarios that have a sizable indirect impact on the measurable quantities. This analysis looks for production of two opposite sign W bosons accompanied by a pair of forward-backward jets with large mass and wide separation. In addition there is missing energy in the event due to two neutrinos produced with the muons. Since the rate of this process in standard model is extremely low, using machine learning techniques signal-to-background ratio has been increased considerably allowing observation of the process with good confidence. Nothing usual has been found.

A search for long-lived particles decaying into two muons has been performed by the CMS collaboration. Many scenarios of beyond standard model physics envisage such a possibility, whereby the exotic particles, travel before decaying, a considerable distance which can range from few microns to several kilometres depending on detailed properties. The striking experimental signature of such a phenomena is two muon tracks originating away from the collision region.

CMS looked for a relatively rare decay mode of the Higgs boson, viz., to a Z boson and a photon (H → Z𝛾 ). The Z was identified via its decays to electron and muon pairs (e+e- or 𝝁+ 𝝁-). The spectrum for the combined mass of the three particles (e+, e- and 𝛾 and 𝝁+ , 𝝁- and 𝛾 ) in data showed a small bulge near the measured mass of the H, indicating possible presence of excess events due to the decay H → Z𝛾. To be more confident about it, more data is required. Stay tuned!

Experiments have established that the neutrinos do have non-zero mass but extremely tiny, though in standard model they are treated as mass-less. This can be explained using theoretical ideas of Majorana and Weinberg. The testable prediction at the LHC is the production of 2 same-sign muons unaccompanied by neutrinos. Data collected by the CMS experiment were found to be compatible with the backgrounds only for such a process, unfortunately.

Ask someone what experiments at the CERN-LHC do, they will tell you: ‘You accelerate two protons to very high energy, smash them together and look in great detail at all particles created from the disintegration of the protons.’ etcetera. Well… yes… mostly.

The CMS collaboration has substantially improved the precision with which the top quark mass is measured. The latest result, a measurement of the top quark mass of 171.77 ± 0.38 GeV, was presented at CERN last week and estimates the value of the top quark mass with a precision of about 0.22%. The gain comes from new analysis methods and improved procedures to consistently treat uncertainties in the measurement simultaneously.

In heavy ion collisions, for ultra-peripheral processes, simultaneous emission of photons by oppositely moving 2 lead ions can occasionally produce a pair of tau leptons. The CMS Collaboration has observed such a phenomena, for the first time in the LHC data. The results were presented in the recently concluded Moriond and Quark Matter conferences.