CMS Collaboration’s research while affiliated with Purdue University West Lafayette and other places

Triple-GEM detectors for the GE2/1 and ME0 stations of the endcap muon system for the Phase-2 upgrade of the CMS Experiment have been operated in a test beam to measure their efficiency and spatial resolution, together with a high spatial resolution triple-GEM tracker. A production module of GE2/1 detectors and a prototype ME0 detector show excellent local efficiency. A prototype detector with GEM foils employing random hole sectorization showed significant reduction of dead areas. The spatial resolution of the tracker has been measured and found close to the expected value of 75 µm.

Inclusive and differential cross sections of single top quark production in association with a Z boson are measured in proton-proton collisions at a center-of-mass energy of 13 TeV with a data sample corresponding to an integrated luminosity of 138 fb−1 recorded by the CMS experiment. Events are selected based on the presence of three leptons, electrons or muons, associated with leptonic Z boson and top quark decays. The measurement yields an inclusive cross section of 87.9+7.5−7.3(stat)+7.3−6.0(syst) fb for a dilepton invariant mass greater than 30 GeV, in agreement with standard model (SM) calculations and represents the most precise determination to date. The ratio between the cross sections for the top quark and the top antiquark production in association with a Z boson is measured as 2.37+0.56−0.42(stat)+0.27−0.13(syst). Differential measurements at parton and particle levels are performed for the first time. Several kinematic observables are considered to study the modeling of the process. Results are compared to theoretical predictions with different assumptions on the source of the initial-state b quark and found to be in agreement, within the uncertainties. Additionally, the spin asymmetry, which is sensitive to the top quark polarization, is determined from the differential distribution of the polarization angle at parton level to be 0.54 ± 0.16 (stat) ± 0.06 (syst), in agreement with SM predictions.

Production cross sections of Image 1, Image 2, and Image 3 states decaying into Image 4 in proton-lead () collisions are reported using data collected by the CMS experiment at . A comparison is made with corresponding cross sections obtained with data measured at the same collision energy and scaled by the Pb nucleus mass number. The nuclear modification factor for Image 1 is found to be Image 5. Similar results for the excited states indicate a sequential suppression pattern, such that Image 6. The suppression of all states is much less pronounced in than in PbPb collisions, and independent of transverse momentum Image 7 and center-of-mass rapidity Image 8 of the individual Image 9 state in the studied range Image 10 and Image 11. Models that incorporate final-state effects of bottomonia in pPb collisions are in better agreement with the data than those which only assume initial-state modifications.

The leptonic and inclusive hadronic decay branching fractions of the W boson are measured using proton-proton collision data collected at √s=13 TeV by the CMS experiment at the CERN LHC, corresponding to an integrated luminosity of 35.9 fb−1. Events characterized by the production of one or two W bosons are selected and categorized based on the multiplicity and flavor of reconstructed leptons, the number of jets, and the number of jets identified as originating from the hadronization of b quarks. A binned maximum likelihood estimate of the W boson branching fractions is performed simultaneously in each event category. The measured branching fractions of the W boson decaying into electron, muon, and tau lepton final states are (10.83±0.10)%, (10.94±0.08)%, and (10.77±0.21)%, respectively, consistent with lepton flavor universality for the weak interaction. The average leptonic and inclusive hadronic decay branching fractions are estimated to be (10.89±0.08)% and (67.32±0.23)%, respectively. Based on the hadronic branching fraction, three standard model quantities are subsequently derived: the sum of squared elements in the first two rows of the Cabibbo–Kobayashi–Maskawa (CKM) matrix ∑ij|Vij|2=1.984±0.021, the CKM element |Vcs|=0.967±0.011, and the strong coupling constant at the W boson mass scale, αS(m2W)=0.095±0.033.

The B and B+ production yields are measured in PbPb collisions at a center-of-mass energy per nucleon pair of 5.02 TeV. The data sample, collected with the CMS detector at the LHC, corresponds to an integrated luminosity of 1.7 nb−1. The mesons are reconstructed in the exclusive decay channels B and B, in the transverse momentum range 7–50 GeV/c and absolute rapidity 0–2.4. The B meson is observed with a statistical significance in excess of five standard deviations for the first time in nucleus-nucleus collisions. The measurements are performed as functions of the transverse momentum of the B mesons and of the PbPb collision centrality. The ratio of production yields of B and B+ is measured and compared to theoretical models that include quark recombination effects.

The High Luminosity upgrade of the LHC (HL-LHC) at CERN will provide unprecedented instantaneous luminosity of 5 × 1034 cm−2 s−1, leading to an average of 150–200 simultaneous collisions. This high instantaneous luminosity scenario presents a significant challenge for the detectors. The barrel region of the CMS electromagnetic calorimeter (ECAL) will be preserved but will be operated at a lower temperature and with a completely new readout and trigger electronics. A dual gain transimpedance amplifier and an ASIC providing two 160 MHz ADC channels, gain selection, and data compression will be used in the new readout electronics. The trigger decision will be moved off-detector and performed by powerful and flexible FPGA processors, allowing for more sophisticated trigger algorithms to be applied. The upgraded ECAL will be capable of high-precision energy measurements throughout HL-LHC and will greatly improve the time resolution for photons and electrons above 10 GeV. The performance obtained with the new electronics has been tested at CERN under an electron beam with a matrix of ECAL crystals equipped with APDs.

Measurements of the inclusive and differential fiducial cross sections of the Higgs boson are presented, using the τ lepton decay channel. The differential cross sections are measured as functions of the Higgs boson transverse momentum, jet multiplicity, and transverse momentum of the leading jet in the event, if any. The analysis is performed using proton-proton collision data collected with the CMS detector at the LHC at a center-of-mass energy of 13 TeV and corresponding to an integrated luminosity of 138 fb−1. These are the first differential measurements of the Higgs boson cross section in the final state of two τ leptons. In final states with a large jet multiplicity or with a Lorentz-boosted Higgs boson, these measurements constitute a significant improvement over measurements performed in other final states.