Giacomo Bruno

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 and 2016 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.

Giacomo Bruno

The detection of TeV muons is a fundamental ingredient of a number of key analyses performed by the CMS experiment at the LHC collider, like the search for new high-mass resonances decaying into di-muons or one muon and one neutrino. Muons with an energy of a few hundred GeV or more experience catastrophic energy losses in the material they traverse. These energy losses have a very significant negative imact on the most important parameters of the muon energy measurement distribution: central value, resolution, and tails.

In order to mitigate these effects, a new muon reconstruction algorithm, called DYnamic Truncation (DYT), has been developed. The DYT identifies the muon position measurements that are produced after a catastrophic energy loss. The inclusion of these measurements in the muon track fit is responsible for the degradation of the muon energy measurement. The identification of such measuremnts is based on the level of incompatibility between the position measurement itself and the expected position obtained using the previous measurements.

Giacomo Bruno

A search for a yet-unobserved baryon number violating top quark decay has been performed using data collected in 2011 and 2012 by the CMS experiment at the LHC. This search was motivated by a theoretical work from the UCL-CP3 phenomenology group, who have noticed that the existence of physics beyond the standard model would imply, under certain conditions, baryon number violation both in the production of top quarks and in their decay process. In the latter case top quarks would decay with a certain branching fraction into a lepton and two jets. The CP3 Louvain experimental group has searched for such decays in a final state containing a pair of top quarks, where the second top quark experiences a SM hadronic decay. No evidence of such an exotic decay has been found and limits have been set at the level of per mille on the branching fraction of the top quark.
More recently, the CP3 Louvain group has been preparing a new search for boosted same sign top quark pairs, possibly accompanied by additional ligh-flavor jets. This is also a signature of baryon number violation. Notable models where such topologies can be realized are supersymmetric ones with R-parity violation.