Measurements of the Electroweak Diboson Production Cross Sections in Proton-Proton Collisions at root s=5.02 TeV Using Leptonic Decays

The first measurements of diboson production cross sections in proton-proton interactions at a center-of-mass energy of 5.02 TeV are reported. They are based on data collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 302 pb(-1). Events with two, three, or four charged light leptons (electrons or muons) in the final state are analyzed. The WW, WZ, and ZZ total cross sections are measured as sigma(WW) = 37:0(-5.2)(+5.5) (stat)(-2.6)(+2.7) (syst) pb, sigma(WZ) = 6.4(-2.1)(+2.5) (stat)(-0.3)(+0.5)(syst) pb, and sigma(ZZ) = 5.3(-2.1)(+2.5)(stat)(-0.4)(+0.5) (syst) pb. All measurements are in good agreement with theoretical calculations at combined next-to-next-to-leading order quantum chromodynamics and next-to-leading order electroweak accuracy

Search for lepton-flavor violating decays of the Higgs boson in the mu tau and e tau final states in proton-proton collisions at root s=13 TeV

A search is presented for lepton-flavor violating decays of the Higgs boson to mu t and et. The dataset corresponds to an integrated luminosity of 137 fb(-1) collected at the LHC in proton-proton collisions at a center-of-mass energy of 13 TeV. No significant excess has been found, and the results are interpreted in terms of upper limits on lepton-flavor violating branching fractions of the Higgs boson. The observed (expected) upper limits on the branching fractions are, respectively, B(H -> mu t) e tau) < 0.22(0.16)% at 95% confidence level.Peer reviewe

Search for new particles in events with energetic jets and large missing transverse momentum in proton-proton collisions at root s=13 TeV

A search is presented for new particles produced at the LHC in proton-proton collisions at root s = 13 TeV, using events with energetic jets and large missing transverse momentum. The analysis is based on a data sample corresponding to an integrated luminosity of 101 fb(-1), collected in 2017-2018 with the CMS detector. Machine learning techniques are used to define separate categories for events with narrow jets from initial-state radiation and events with large-radius jets consistent with a hadronic decay of a W or Z boson. A statistical combination is made with an earlier search based on a data sample of 36 fb(-1), collected in 2016. No significant excess of events is observed with respect to the standard model background expectation determined from control samples in data. The results are interpreted in terms of limits on the branching fraction of an invisible decay of the Higgs boson, as well as constraints on simplified models of dark matter, on first-generation scalar leptoquarks decaying to quarks and neutrinos, and on models with large extra dimensions. Several of the new limits, specifically for spin-1 dark matter mediators, pseudoscalar mediators, colored mediators, and leptoquarks, are the most restrictive to date.Peer reviewe

Combined searches for the production of supersymmetric top quark partners in proton-proton collisions at root s=13 TeV

A combination of searches for top squark pair production using proton-proton collision data at a center-of-mass energy of 13 TeV at the CERN LHC, corresponding to an integrated luminosity of 137 fb(-1) collected by the CMS experiment, is presented. Signatures with at least 2 jets and large missing transverse momentum are categorized into events with 0, 1, or 2 leptons. New results for regions of parameter space where the kinematical properties of top squark pair production and top quark pair production are very similar are presented. Depending on themodel, the combined result excludes a top squarkmass up to 1325 GeV for amassless neutralino, and a neutralinomass up to 700 GeV for a top squarkmass of 1150 GeV. Top squarks with masses from 145 to 295 GeV, for neutralino masses from 0 to 100 GeV, with a mass difference between the top squark and the neutralino in a window of 30 GeV around the mass of the top quark, are excluded for the first time with CMS data. The results of theses searches are also interpreted in an alternative signal model of dark matter production via a spin-0 mediator in association with a top quark pair. Upper limits are set on the cross section for mediator particle masses of up to 420 GeV

Probing effective field theory operators in the associated production of top quarks with a Z boson in multilepton final states at root s=13 TeV

Peer reviewe

Observation of γγ → ττ in proton-proton collisions and limits on the anomalous electromagnetic moments of the τ lepton

The production of a pair of τ leptons via photon–photon fusion, γγ → ττ, is observed for the f irst time in proton–proton collisions, with a significance of 5.3 standard deviations. This observation is based on a data set recorded 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. Events with a pair of τ leptons produced via photon–photon fusion are selected by requiring them to be back-to-back in the azimuthal direction and to have a minimum number of charged hadrons associated with their production vertex. The τ leptons are reconstructed in their leptonic and hadronic decay modes. The measured fiducial cross section of γγ → ττ is σfid obs = 12.4+3.8 −3.1 fb. Constraints are set on the contributions to the anomalous magnetic moment (aτ) and electric dipole moments (dτ) of the τ lepton originating from potential effects of new physics on the γττ vertex: aτ = 0.0009+0.0032 −0.0031 and |dτ| &lt; 2.9×10−17ecm (95% confidence level), consistent with the standard model

Measurement of the top quark mass using events with a single reconstructed top quark in pp collisions at root s=13 TeV

Abstract:A measurement of the top quark mass is performed using a data sample en-riched with single top quark events produced in thetchannel. The study is based on proton-proton collision data, corresponding to an integrated luminosity of 35.9 fb−1, recorded at√s= 13TeV by the CMS experiment at the LHC in 2016. Candidate events are selectedby requiring an isolated high-momentum lepton (muon or electron) and exactly two jets,of which one is identified as originating from a bottom quark. Multivariate discriminantsare designed to separate the signal from the background. Optimized thresholds are placedon the discriminant outputs to obtain an event sample with high signal purity. The topquark mass is found to be172.13+0.76−0.77GeV, where the uncertainty includes both the sta-tistical and systematic components, reaching sub-GeV precision for the first time in thisevent topology. The masses of the top quark and antiquark are also determined separatelyusing the lepton charge in the final state, from which the mass ratio and difference aredetermined to be0.9952+0.0079−0.0104and0.83+1.79−1.35GeV, respectively. The results are consistentwithCPTinvariance

Measurements of Higgs boson production cross sections and couplings in the diphoton decay channel at root s=13 TeV

Measurements of Higgs boson production cross sections and couplings in events where the Higgs boson decays into a pair of photons are reported. Events are selected from a sample of proton-proton collisions at s√ = 13 TeV collected by the CMS detector at the LHC from 2016 to 2018, corresponding to an integrated luminosity of 137 fb−1. Analysis categories enriched in Higgs boson events produced via gluon fusion, vector boson fusion, vector boson associated production, and production associated with top quarks are constructed. The total Higgs boson signal strength, relative to the standard model (SM) prediction, is measured to be 1.12±0.09. Other properties of the Higgs boson are measured, including SM signal strength modifiers, production cross sections, and its couplings to other particles. These include the most precise measurements of gluon fusion and vector boson fusion Higgs boson production in several different kinematic regions, the first measurement of Higgs boson production in association with a top quark pair in five regions of the Higgs boson transverse momentum, and an upper limit on the rate of Higgs boson production in association with a single top quark. All results are found to be in agreement with the SM expectations

The BEST Thesis: The Boosted Event Shape Tagger, A Search for Vector-like Quarks, and A Real GEM in CMS

This thesis is a collection of three topics that take place at the Compact Muon Solenoid experiment---a particle detector which observes proton-proton collisions at CERN's Large Hadron Collider. These topics are the installation of GEMs into the CMS experiment, the development of the Boosted Event Shape Tagger, and a search for vector-like quarks. The LHC is undergoing upgrades which will increase the instantaneous luminosity to $5\times10^{34}$cm$^{-2}$s$^{-1}$, a factor of 2.5 higher than the current maximum value. Therefore, the experiments are implementing upgrades to cope with the augmented particle rates. In the muon system of the Compact Muon Solenoid (CMS) experiment, Gas Electron Multipliers (GEMs) are being installed to complement the existing Cathode Strip Chambers (CSCs). This will provide a more precise measurement of the muon bending angle and thus improve the muon trigger capabilities. GEMs are micro-pattern gaseous detectors with high rate capabilities–ideal for the forward regions of the CMS muon system. In preparation for the LHC Run 3, 144 GEM chambers were installed in the first muon station and are now operational in Run 3. This thesis introduces the GEM technology and discusses the production, installation, commissioning, and operation of the new GEM muon detectors at CMS. The first GEMs in CMS will improve the identification of muons, but proper identification of hadronic decays requires the development of new analysis tools. Jets from heavy particles (top, Higgs, $W$, $Z$) have characteristic patterns that can be identified by Lorentz boosting the jet to various hypothetical rest frames. The Boosted Event Shape Tagger (BEST) is a deep neural network that utilizes this technique to classify heavy particles from QCD background. A version of BEST was previously used for 2016 collision data. This version was improved on for the full Run 2 dataset. In the effort to improve BEST resulted in a method for creating images of jets in rest frames. These images were passed to a convolutional neural network for classification. This thesis discusses this method and the other improvements to BEST in detail. The improved version of BEST was used to search for a pair of vector-like quarks in an all hadronic final state in LHC Run 2 data. Vector-like quarks arise in extensions to the Standard Model which aim to solve the gauge hierarchy problem. This search uses BEST to classify collision events into 126 orthogonal regions. The $H_{T}$ distributions are tested in each region for the presence of signal and exclusion limits are set for $T'$ and $B'$ masses. This search is currently being approved by the CMS experiment, so only expected limits are presented---the expected sensitivity of the search if no signal is present. The process for setting expected limits is completed using Monte Carlo simulated data and data driven estimates, so no collision data from the signal region are included

The experimental setup of SHADOWS proposed for ECN3

The SHADOWS (Search for Hidden And Dark Objects With the SPS) experiment is one of three proposed detectors for installation in the ECN3 SPS beam dump in the North Area at CERN. SHADOWS would search for a large variety of feebly-interacting particles (FIPs) created in the dump of the 400 GeV SPS proton beam. SHADOWS is placed off axis in order to reduce background from the beam, which also allows it to co-exist with the proposed HIKE (High Intensity Kaon Experiment) experiment. The proposed SHADOWS detector consists of: an upstream veto, a Magnetized Iron Block lateral veto, a 20 m decay volume, a tracking system utilizing the magnetic field produced by a dipole magnet, a timing system, an electromagnetic calorimeter, and a muon system. SHADOWS will allow the exploration of a large parameter space for many FIPs—such as light dark scalars, axion-like particles, and heavy neutral leptons with, masses ranging between 0.1 and 5 GeV. It might also be complemented by a dedicated neutrino experiment (NaNu@SHADOWS) which would extend the physics reach by studying tau-neutrino events. This talk will provide an overview of SHADOWS, including a brief status of FIP searches, and then focus on the detector technologies and challenges involved.Coffee will be served at 10:30.</p