Search

The first study of charm quark diffusion with respect to the jet axis in heavy ion collisions is presented. The measurement is performed using jets with p(T)(jet) > 60 GeV/c and D-0 mesons with p(T)(D) > 4 GeV/c in lead-lead (Pb-Pb) and proton-proton (pp) collisions at a nucleon-nucleon center-of-mass energy of root s(NN) = 5.02 TeV, recorded by the CMS detector at the LHC. The radial distribution of D-0 mesons with respect to the jet axis is sensitive to the production mechanisms of the meson, as well as to the energy loss and diffusion processes undergone by its parent parton inside the strongly interacting medium produced in Pb-Pb collisions. When compared to Monte Carlo event generators, the radial distribution in pp collisions is found to be well described by PYTHIA, while the slope of the distribution predicted by SHERPA is steeper than that of the data. In Pb-Pb collisions, compared to the pp results, the D-0 meson distribution for 4 < p(T)(D) < 20 GeV/c hints at a larger distance on average with respect to the jet axis, reflecting a diffusion of charm quarks in the medium created in heavy ion collisions. At higher p(T)(D), the Pb-Pb and pp radial distributions are found to be similar.

A study of the production of prompt J/ψ mesons contained in jets in proton-proton collisions at s=8TeV is presented. The analysis is based on data corresponding to an integrated luminosity of 19.1 fb−1 collected with the CMS detector at the LHC. For events with at least one observed jet, the angular separation between the J/ψ meson and the jet is used to test whether the J/ψ meson is part of the jet. The analysis shows that most prompt J/ψ mesons having energy above 15 GeV and rapidity |y

This paper reports on a search for an extended scalar sector of the standard model, where a new CP-even (odd) boson decays to a Z boson and a lighter CP-odd (even) boson, and the latter further decays to a b quark pair. The Z boson is reconstructed via its decays to electron or muon pairs. The analysed data were recorded in proton- proton collisions at a center-of-mass energy √s = 13 TeV, collected by the CMS experiment at the LHC during 2016, corresponding to an integrated luminosity of 35.9 fb−1. Data and predictions from the standard model are in agreement within the uncertainties. Upper limits at 95% confidence level are set on the production cross section times branching fraction, with masses of the new bosons up to 1000 GeV. The results are interpreted in the context of the two-Higgs- doublet model.

The first search for supersymmetry in events with an experimental signature of one soft, hadronically decaying tau lepton, one energetic jet from initial-state radiation, and large transverse momentum imbalance is presented. These event signatures are consistent with direct or indirect production of scalar tau leptons ((tau) over tilde) in supersymmetric models that exhibit coannihilation between the (tau) over tilde and the lightest neutralino ((chi) over tilde (0)(1)), and that could generate the observed relic density of dark matter. The data correspond to an integrated luminosity of 77.2 fb(-1) of proton-proton collisions at root s = 13 TeV collected with the CMS detector at the LHC in 2016 and 2017. The results are interpreted in a supersymmetric scenario with a small mass difference (Delta m) between the chargino ((chi) over tilde (+/-) (1)) or next-to-lightest neutralino ((chi) over tilde (0)(2)), and the (chi) over tilde (0)(1). The mass of the (tau) over tilde is assumed to be the average of the (chi) over tilde (1)(+/-) and (chi) over tilde (0)(1) masses. The data are consistent with standard model background predictions. Upper limits at 95% confidence level are set on the sum of the (chi) over tilde (+/-)(1). (chi) over tilde (0)(2), and (tau) over tilde production cross sections for Delta m ((chi) over tilde (+/-)(1), (chi) over tilde (0)(1)) = 50 GeV, resulting in a lower limit of 290 GeVon the mass of the (chi) over tilde (+/-)(1), which is the most stringent to date and surpasses the bounds from the LEP experiments.

Two related searches for phenomena beyond the standard model (BSM) are performed using events with hadronic jets and significant transverse momentum imbalance. The results are based on a sample of proton-proton collisions at a center- of-mass energy of 13Te, collected by the CMS experiment at the LHC in 2016-2018 and corresponding to an integrated luminosity of 137fb-1. The first search is inclusive, based on signal regions defined by the hadronic energy in the event, the jet multiplicity, the number of jets identified as originating from bottom quarks, and the value of the kinematic variable MT2 for events with at least two jets. For events with exactly one jet, the transverse momentum of the jet is used instead. The second search looks in addition for disappearing tracks produced by BSM long-lived charged particles that decay within the volume of the tracking detector. No excess event yield is observed above the predicted standard model background. This is used to constrain a range of BSM models that predict the following: the pair production of gluinos and squarks in the context of supersymmetry models conserving R- parity, with or without intermediate long-lived charginos produced in the decay chain ; the resonant production of a colored scalar state decaying to a massive Dirac fermion and a quark ; or the pair production of scalar and vector leptoquarks each decaying to a neutrino and a top, bottom, or light-flavor quark. In most of the cases, the results obtained are the most stringent constraints to date.

Anisotropies in the initial energy density distribution of the quark-gluon plasma created in high energy heavy ion collisions lead to anisotropies in the azimuthal distributions of the final-state particles known as collective anisotropic flow. Fourier harmonic decomposition is used to quantify these anisotropies. The higher-order harmonics can be induced by the same order anisotropies (linear response) or by the combined influence of several lower order anisotropies (nonlinear response) in the initial state. The mixed highe-rorder anisotropic flow and nonlinear response coefficients of charged particles are measured as functions of transverse momentum and centrality in PbPb collisions at nucleon-nucleon center-of-mass energies root s(NN) = 2.76 and 5.02 TeV with the CMS detector. The results are compared with viscous hydrodynamic calculations using several different initial conditions, aswell asmicroscopic transport model calculations. None of the models provides a simultaneous description of the mixed higher-order flow harmonics and nonlinear response coefficients.

The transverse momentum (P-T) spectra of inclusively produced A c P baryons are measured via the exclusive decay channel Lambda(+)(c) -> pK(-)pi(+) using the CMS detector at the LHC. Spectra are measured as a function of transverse momentum in proton-proton (pp) and lead-lead (PbPb) collisions at a nucleonnucleon center-of-mass energy of 5.02TeV. The measurement is performed within the Lambda(+)(c) rapidity interval vertical bar Y vertical bar < 1 in the p(T) range of 5-20GeV/c in pp and 10-20GeV/c in PbPb collisions. The observed yields of Lambda(+)(c) for p(T) of 10-20 GeV/c suggest a suppression in central PbPb collisions compared to pp collisions scaled by the number of nucleon-nucleon (NN) interactions. The Lambda(+)(c)/D-0 production ratio in pp collisions is compared to theoretical models. In PbPb collisions, this ratio is consistent with the result from pp collisions in their common p-r range.

The production of W± bosons is studied in proton-lead (pPb) collisions at a nucleon- nucleon centre-of-mass energy of √sNN = 8.16 TeV. Measurements are performed in the W± → μ±νμ channel using a data sample corresponding to an integrated luminosity of 173.4 ± 8.7 nb−1, collected by the CMS Collaboration at the LHC. The number of positively and negatively charged W bosons is determined separately in the muon pseudorapidity region in the laboratory frame |ημlab| < 2.4 and transverse momentum pμT > 25 GeV/c. The W± boson differential cross sections, muon charge asymmetry, and the ratios of W± boson yields for the proton-going over the Pb-going beam directions are reported as a function of the muon pseudorapidity in the nucleon-nucleon centre-of-mass frame. The measurements are compared to the predictions from theoretical calculations based on parton distribution functions (PDFs) at next-to-leading-order. The results favour PDF calculations that include nuclear modifications and provide constraints on the nuclear PDF global fits.

A search in an all-jet final state for new massive resonances decaying to WW, WZ, or ZZ boson pairs using a novel analysis method is presented. The analysis is performed on data corresponding to an integrated luminosity of 77.3 fb$^{; ; ; -1}; ; ; $ recorded with the CMS experiment at the LHC at a centre-of-mass energy of 13 TeV. The search is focussed on potential resonances with masses above 1.2 TeV, where the decay products of each W or Z boson are expected to be collimated into a single, large-radius jet. The signal is extracted using a three-dimensional maximum likelihood fit of the two jet masses and the dijet invariant mass, yielding an improvement in sensitivity of up to 30% relative to previous search methods. No excess is observed above the estimated standard model background. In a heavy vector triplet model, spin-1 Z' and W' resonances with masses below 3.5 and 3.8 TeV, respectively, are excluded at 95% confidence level. In a narrow-width bulk graviton model, upper limits on cross sections are set between 27 and 0.2 fb for resonance masses between 1.2 and 5.2 TeV, respectively. The limits presented in this paper are the best to date in the dijet final state.

A search is presented for pairs of light pseudoscalar bosons, in the mass range from 4 to 15 GeV, produced from decays of the 125 GeV Higgs boson. The decay modes considered are final states that arise when one of the pseudoscalars decays to a pair of tau leptons, and the other one either into a pair of tau leptons or muons. The search is based on proton-proton collisions collected by the CMS experiment in 2016 at a center-of-mass energy of 13 TeV that correspond to an integrated luminosity of 35.9 fb−1. The 2μ2τ and 4τ channels are used in combination to constrain the product of the Higgs boson production cross section and the branching fraction into 4τ final state, σB, exploiting the linear dependence of the fermionic coupling strength of pseudoscalar bosons on the fermion mass. No significant excess is observed beyond the expectation from the standard model. The observed and expected upper limits at 95% confidence level on σB, relative to the standard model Higgs boson production cross section, are set respectively between 0.022 and 0.23 and between 0.027 and 0.19 in the mass range probed by the analysis.

The observation of the Λ⁰_b→J/ψΛϕ decay is reported using proton-proton collision data collected at √s = 13 TeV by the CMS experiment at the LHC in 2018, corresponding to an integrated luminosity of 60fb⁻¹. The ratio of the branching fractions B(Λ⁰_b →J/ψΛϕ)/B(Λ⁰_b →ψ(2S)Λ) is measured to be (8.26±0.90(stat) ±0.68(syst)±0.11(B))×10⁻², where the first uncertainty is statistical, the second is systematic, and the last uncertainty reflects the uncertainties in the world-average branching fractions of ϕ and ψ(2S) decays to the reconstructed final states.

The polarizations of promptly produced χc1 and χc2 mesons are studied using data collected by the CMS experiment at the LHC, in proton-proton collisions at √s = 8 TeV. The χc states are reconstructed via their radiative decays χc→J/ ψγ, with the photons being measured through conversions to e+e−, which allows the two states to be well resolved. The polarizations are measured in the helicity frame, through the analysis of the χc2 to χc1 yield ratio as a function of the polar or azimuthal angle of the positive muon emitted in the J/ψ→μ+μ− decay, in three bins of J/ψ transverse momentum. While no differences are seen between the two states in terms of azimuthal decay angle distributions, they are observed to have significantly different polar anisotropies. The measurement favors a scenario where at least one of the two states is strongly polarized along the helicity quantization axis, in agreement with nonrelativistic quantum chromodynamics predictions. This is the first measurement of significantly polarized quarkonia produced at high transverse momentum.

Normalised multi-differential cross sections for top quark pair (t (t) over bar) production are measured in proton-proton collisions at a centre-of-mass energy of 13 TeV using events containing two oppositely charged leptons. The analysed data were recorded with the CMS detector in 2016 and correspond to an integrated luminosity of 35.9 fb(-1). The doubledifferential t (t) over bar cross section is measured as a function of the kinematic properties of the top quark and of the t (t) over bar system at parton level in the full phase space. A triple-differential measurement is performed as a function of the invariant mass and rapidity of the t (t) over bar system and the multiplicity of additional jets at particle level. The data are compared to predictions of Monte Carlo event generators that complement next-to-leading-order (NLO) quantum chromodynamics (QCD) calculations with parton showers. Together with a fixed-order NLO QCD calculation, the triple-differential measurement is used to extract values of the strong coupling strength alpha(S) and the top quark pole mass (m(t)(pole)) using several sets of parton distribution functions (PDFs). The measurement of m(t)(pole) exploits the sensitivity of the t (t) over bar invariant mass distribution to m(t)(pole) near the production threshold. Further-more, a simultaneous fit of the PDFs, aS, and m(t) (pole) is performed at NLO, demonstrating that the new data have significant impact on the gluon PDF, and at the same time allow an accurate determination of alpha(S) and m(t) (pole). The values alpha S(mZ) = 0.1135+0.0021- 0.0017 and m(t)(pole) = 170.5 +/- 0.8GeV are extracted, which account for experimental and theoretical uncertainties, the latter being estimated from NLO scale variations. Possible effects from Coulomb and soft-gluon resummation near the t (t) over bar production threshold are neglected in these parameter extractions. A rough estimate of these effects indicates an expected correction of m(t)(pole) of the order of +1 GeV, which can be regarded as additional theoretical uncertainty in the current m(t) (pole) extraction.

A study of excited Λ0b baryons is reported, based on a data sample collected in 2016-2018 with the CMS detector at the LHC in proton- proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of up to 140 fb−1. The existence of four excited Λ0b states: Λb(5912)0, Λb(5920)0, Λb(6146)0, and Λb(6152)0 in the Λ0bπ+π− mass spectrum is confirmed, and their masses are measured. The Λ0bπ+π− mass distribution exhibits a broad excess of events in the region of 6040-6100 MeV, whose origin cannot be discerned with the present data.

The first observation of the (tt) over barH process in a single Higgs boson decay channel with the full reconstruction of the final state (H -> gamma gamma) is presented, with a significance of 6.6 standard deviations (sigma). The CP structure of Higgs boson couplings to fermions is measured, resulting in an exclusion of the pure CP-odd structure of the top Yukawa coupling at 3.2 sigma. The measurements are based on a sample of protonproton collisions at a center-of-mass energy root s = 13 TeV collected by the CMS detector at the LHC, corresponding to an integrated luminosity of 137 fb(-1). The cross section times branching fraction of the (tt) over barH process is measured to be sigma B- (tt) over barH(gamma gamma) = 1.56(-0.32) (+0.34) th, which is compatible with the standard model prediction of 1.13(-0.11)(+0.08) fb. The fractional contribution of the CP-odd component is measured to be f(CP)(Hu) = 0.00 +/- 0.33.

The search for the Flavor Changing Neutral Current (FCNC) processes, has been one of the leading tools to test the Standard Model (SM), in an attempt of either discovering or putting stringent limits on the new physics scenarios. Within the SM, there are no FCNC transitions at tree level, mediated by the Higgs Boson. These processes are severely suppressed by the unitarity constraints of the Cabibo-Kobayashi-Maskawa matrix. The top quark being the heaviest of all the quarks has the largest Yukawa coupling to the Higgs boson. The large production rate of the top quarks in the LHC, allows one to look for transition of the top quark to a quark of different flavor but same charge. This suggests looking for rare neutral flavor changing transitions, t → cH, t → uH, as no symmetry prohibits this decay. The SM branching ratio of this process is extremely small, of the order BR(t → cH) SM ≈ 10 ^(−15). This is many orders of magnitude too small to be measured in the 14 TeV LHC. Therefore an affirmative observation of the process t → qH, well above the SM rate, will be a conclusive indication of new physics beyond the SM. We explore the FCNC top-Higgs decays in detail at polarized linear colliders and compare results with the existing LHC searches to constrain the FCNC couplings. Moreover, we emphasis advantages of linear collider searches over the hadronic ones, in particular for discriminating between chiralites of FCNC couplings.

A search for baryon number violation (BNV) in top-quark decays is performed using pp collisions produced by the LHC at √s = 8 TeV. The top-quark decay considered in this search results in one light lepton (muon or electron), two jets, but no neutrino in the final state. Data used for the analysis were collected by the CMS detector and correspond to an integrated luminosity of 19.5 fb−1. The event selection is optimized for top quarks produced in pairs, with one undergoing the BNV decay and the other the standard model hadronic decay to three jets. No significant excess of events over the expected yield from standard model processes is observed. The upper limits at 95% confidence level on the branching fraction of the BNV top-quark decay are calculated to be 0.0016 and 0.0017 for the muon and the electron channels, respectively. Assuming lepton universality, an upper limit of 0.0015 results from the combination of the two channels. These limits are the first that have been obtained on a BNV process involving the top quark.