CMS Tracker commissioning and performance assessment
The CMS silicon strip tracker is the largest device of its type ever built. There are 24244 single-sided micro-strip sensors covering an active area of 198m2.
Physics performance of the detector are being constantly assessed and optimized as new data comes.
Members of UCL are playing a major role in the understanding of the silicon strip tracker and in the maintenance and development of the local reconstruction code.
External collaborators: CMS tracker collaboration.
Higgs bosons to ZZ -> 2l 2nu and measurement of the Higgs natural width
The final state containing two Z bosons decaying into a pair of leptons and a pair of neutrinos has been exploited by the CMS experiment at the LHC to produce a number of results related to the Higgs boson, including measurements of related standard model cross sections.
Constraints have been set on the total width of the 125 GeV Higgs boson, using its relative on-shell and off-shell production and decay rates to a pair of Z bosons, where one Z boson decays to an electron or muon pair, and the other to an electron, muon, or neutrino pair. The analysis is based on the data collected by the CMS experiment at the LHC in 2011 and 2012. A simultaneous maximum likelihood fit to the measured kinematic distributions near the resonance peak and above the Z-boson pair production threshold leads to an upper limit on the Higgs boson width of < 22 MeV at a 95% confidence level, which is 5.4 times the expected value in the standard model at the measured mass of 125.6 GeV.
A search for heavy Higgs bosons in the H → ZZ → 2l2ν decay channel, where l = e or µ, has also been performed using data collected in 2015 at the center of mass energy of 13 TeV. No significant excess is observed above the background expectation. The results are interpreted to set exclusion limits on a number of extensions of the standard model scalar sectors: models with an additional electroweak singlet, as well as Type-I and Type-II two-Higgs doublets models.
External collaborators: CMS collaboration.
Particle Identification with ionization energy loss in the CMS experiment at the LHC
The CMS detector at the LHC can be used to identify particles via the measurement of their ionization energy loss. The sub-detectors that have provided so far useful information for this experimental technique are the silicon strip tracker and the pixel detectors. Identification of low momentum hadrons and detection of new exotic massive long-lived charged particles have all benefited from this experimental method. Members of UCL pioneered this technique in the early LHC times and have been developing the tools for its use and calibration. Since 2010 particle identification with ionization energy loss has been the basis of the CMS inclusive search for new massive long-lived charged particles, which has been providing the most stringent and model-independent limits existing to date on any model of new physics predicting such particles.
External collaborators: CMS collaboration.
Search for massive long-lived charged particles with the CMS detector at the LHC
The CMS detector at the LHC is used to search for yet unobserved heavy (mass >100 GeV/c$^2$), long-lived (lifetime > 1 ns), electrically charged particles, called generically HSCPs.
HSCPs can be distinguished from Standard Model particles by exploiting their unique signature: very high momentum and low velocity. These features are a consequence of their high mass and the relatively limited LHC collision energy. Two experimental techniques are used to identify such hypothetical heavy and low-velocity particles: the measurement of the ionization energy loss rate using the all-silicon tracker detector and the time-of-flight measurement with the muon detectors.
UCL members have developed the ionization energy loss identification technique and have lead the CMS HSCP search since 2010, when the first HSCP paper became one of the first published LHC search papers. Updated results, using the 2011 dataset, were then published followed by a comprehensive paper including also searches for fractional and multiply-charged particles published using the full CMS Run-1 dataset. The results obtained by analysing the 2015 Run 2 data at 13 TeV have also been published.
The analysis, which is very inclusive, doesn't find evidence of HSCP. It currently excludes, among various models, the existence of quasi-stable gluinos, predicted by certain realizations of supersymmetry, and Drell-Yan-produced staus with masses lower than about 1.3 TeV and 350 GeV, respectively. These and the other limits set by the analysis are the most stringent to date. The CMS HSCP papers total to date more than 300 citations.