37 Search Results

Machine-learning (ML) techniques are explored to identify and classify hadronic decays of highly Lorentz-boosted W/Z/Higgs bosons and top quarks. Techniques without ML have also been evaluated and are included for comparison. The identification performances of a variety of algorithms are characterized in simulated events and directly compared with data. The algorithms are validated using proton-proton collision data at $$\sqrt{s}$$ = 13TeV, corresponding to an integrated luminosity of 35.9 fb^-1. Systematic uncertainties are assessed by comparing the results obtained using simulation and collision data. The new techniques studied in this paper provide significant performance improvements over non-ML techniques, reducing the background rate bymore » up to an order of magnitude at the same signal efficiency.« less

Methods are presented for calibrating the hadron calorimeter system of the CMS detector at the LHC. The hadron calorimeters of the CMS experiment are sampling calorimeters of brass and scintillator, and are in the form of one central detector and two endcaps. These calorimeters cover pseudorapidities |η| < 3 and are positioned inside the solenoidal magnet. An outer calorimeter, outside the magnet coil, covers |η| < 1.26, and a steel and quartz-fiber Cherenkov forward calorimeter extends the coverage to |η| < 5.19. The initial calibration of the calorimeters was based on results from test beams, augmented with the use ofmore » radioactive sources and lasers. The calibration was improved substantially using proton-proton collision data collected at √s = 7, 8, and 13 TeV, as well as cosmic ray muon data collected during the periods when the LHC beams were not present. The present calibration is performed using the 13 TeV data collected during 2016 corresponding to an integrated luminosity of 35.9 fb^-1. The intercalibration of channels exploits the approximate uniformity of energy collection over the azimuthal angle. The absolute energy scale of the central and endcap calorimeters is set using isolated charged hadrons. The energy scale for the electromagnetic portion of the forward calorimeters is set using Z→ ee data. The energy scale of the outer calorimeters has been determined with test beam data and is confirmed through data with high transverse momentum jets. In this paper, we present the details of the calibration methods and accuracy.« less

A measurement is presented of differential cross sections for t-channel single top quark and antiquark production in proton–proton collisions at a centre-of-mass energy of 13$$\,\text {Te}\text {V}$$ by the CMS experiment at the LHC. From a data set corresponding to an integrated luminosity of 35.9$$\,\text {fb}^{-1}$$, events containing one muon or electron and two or three jets are analysed. The cross section is measured as a function of the top quark transverse momentum ($$p_{\mathrm{T}} $$), rapidity, and polarisation angle, the charged lepton $$p_{\mathrm{T}} $$ and rapidity, and the $$p_{\mathrm{T}} $$ of the $$\text {W}{}{}$$  boson from the top quark decay.more » In addition, the charge ratio is measured differentially as a function of the top quark, charged lepton, and $$\text {W}{}{}$$  boson kinematic observables. The results are found to be in agreement with standard model predictions using various next-to-leading-order event generators and sets of parton distribution functions. Additionally, the spin asymmetry, sensitive to the top quark polarisation, is determined from the differential distribution of the polarisation angle at parton level to be $$0.440 \pm 0.070$$, in agreement with the standard model prediction.« less

A search in an all-jet final state for new massive resonances decaying to $$\text{ W }{}{}\text{ W }{}{}$$, $$\text{ W }{}{}\text{ Z }{}{}$$, or $$\text{ Z }{}{}\text{ Z }{}{}$$ boson pairs using a novel analysis method is presented. The analysis is performed on data corresponding to an integrated luminosity of 77.3 $$\,\text {fb}^{-1}$$ recorded with the CMS experiment at the LHC at a centre-of-mass energy of 13 $$\text {Te}\text {V}$$. The search is focussed on potential narrow-width resonances with masses above 1.2 $$\text {Te}\text {V}$$, where the decay products of each $$\text{ W }{}{}$$ or $$\text{ Z }{}{}$$ boson are expected tomore » 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 $${\text {Z}}^{\prime }$$ and $${\text {W}}^{\prime }$$ resonances with masses below 3.5 and 3.8  $$\text {Te}\text {V}$$, respectively, are excluded at 95% confidence level. In a bulk graviton model, upper limits on cross sections are set between 27 and 0.2 $$\,\text {fb}$$ for resonance masses between 1.2 and 5.2  $$\text {Te}\text {V}$$, respectively. The limits presented in this paper are the best to date in the dijet final state.« less

A search is presented for $$\tau$$ slepton pairs produced in proton-proton collisions at a center-of-mass energy of 13 TeV. The search is carried out in events containing two $$\tau$$ leptons in the final state, on the assumption that each $$\tau$$ slepton decays primarily to a $$\tau$$ lepton and a neutralino. Events are considered in which each $$\tau$$ lepton decays to one or more hadrons and a neutrino, or in which one of the $$\tau$$ leptons decays instead to an electron or a muon and two neutrinos. The data, collected with the CMS detector in 2016 and 2017, correspond to anmore » integrated luminosity of 77.2 fb$$^{-1}$$. The observed data are consistent with the standard model background expectation. The results are used to set 95% confidence level upper limits on the cross section for $$\tau$$ slepton pair production in various models for $$\tau$$ slepton masses between 90 and 200 GeV and neutralino masses of 1, 10, and 20 GeV. In the case of purely left-handed $$\tau$$ slepton production and decay to a $$\tau$$ lepton and a neutralino with a mass of 1 GeV, the strongest limit is obtained for a $$\tau$$ slepton mass of 125 GeV at a factor of 1.14 larger than the theoretical cross section.« less

The standard model (SM) production of four top quarks ($$\text {t} {}{\overline{\text {t}}} \text {t} {}{\overline{\text {t}}} $$) in proton–proton collisions is studied by the CMS Collaboration. The data sample, collected during the 2016–2018 data taking of the LHC, corresponds to an integrated luminosity of 137$$\,\text {fb}^{-1}$$ at a center-of-mass energy of 13$$\,\text {TeV}$$. The events are required to contain two same-sign charged leptons (electrons or muons) or at least three leptons, and jets. The observed and expected significances for the $$\text {t} {}{\overline{\text {t}}} \text {t} {}{\overline{\text {t}}} $$ signal are respectively 2.6 and 2.7 standard deviations, and themore » $$\text {t} {}{\overline{\text {t}}} \text {t} {}{\overline{\text {t}}} $$ cross section is measured to be $$12.6^{+5.8}_{-5.2}\,\text {fb} $$. The results are used to constrain the Yukawa coupling of the top quark to the Higgs boson, $$y_{\text {t}}$$, yielding a limit of $$|y_{\text {t}}/y_{\text {t}}^{\mathrm {SM}} | < 1.7$$ at $$95\%$$ confidence level, where $$y_{\text {t}}^{\mathrm {SM}}$$ is the SM value of $$y_{\text {t}}$$. They are also used to constrain the oblique parameter of the Higgs boson in an effective field theory framework, $$\hat{H}<0.12$$. Limits are set on the production of a heavy scalar or pseudoscalar boson in Type-II two-Higgs-doublet and simplified dark matter models, with exclusion limits reaching 350–470$$\,\text {GeV}$$ and 350–550$$\,\text {GeV}$$ for scalar and pseudoscalar bosons, respectively. Upper bounds are also set on couplings of the top quark to new light particles.« less

New sets of CMS underlying-event parameters ("tunes") are presented for the PYTHIA8 event generator. These tunes use the NNPDF3.1 parton distribution functions (PDFs) at leading (LO), next-to-leading (NLO), or next-to-next-to-leading (NNLO) orders in perturbative quantum chromodynamics, and the strong coupling evolution at LO or NLO. Measurements of charged-particle multiplicity and transverse momentum densities at various hadron collision energies are fit simultaneously to determine the parameters of the tunes. Comparisons of the predictions of the new tunes are provided for observables sensitive to the event shapes at LEP, global underlying event, soft multiparton interactions, and double-parton scattering contributions. In addition, comparisonsmore » are made for observables measured in various specific processes, such as multijet, Drell-Yan, and top quark-antiquark pair production including jet substructure observables. The simulation of the underlying event provided by the new tunes is interfaced to a higher-order matrix-element calculation. For the first time, predictions from PYTHIA8 obtained with tunes based on NLO or NNLO PDFs are shown to reliably describe minimum-bias and underlying-event data with a similar level of agreement to predictions from tunes using LO PDF sets.« less

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 13 TeV, collected by the CMS experiment at the LHC in 2016-2018 and corresponding to an integrated luminosity of 137 fb$$^{-1}$$. 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 $$M_\mathrm{T2}$$ for events with atmore » 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.« less

Measurements are presented of the triple-differential cross section for inclusive isolated-photon+jet events in pp collisions at $$\sqrt{s} =$$ 8 TeV as a function of photon transverse momentum ($$p_\mathrm{T}^\gamma$$), photon pseudorapidity ($$\eta^\gamma$$), and jet pseudorapidity ($$\eta^\text{jet}$$). The data correspond to an integrated luminosity of 19.7 fb$$^{-1}$$ that probe a broad range of the available phase space, for $$|\eta^\gamma|<1.44$$ and 1.57$$<|\eta^\gamma|<$$2.50, $$|\eta^\text{jet}|<$$2.5, 40$$< p_\mathrm{T}^\gamma<$$ 1000 GeV, and jet transverse momentum, $$p_\mathrm{T}^\text{jet}$$, $$>$$25 GeV. The measurements are compared to next-to-leading order perturbative quantum chromodynamics calculations, which reproduce the data within uncertainties.

The average total energy as well as its hadronic and electromagnetic components are measured with the CMS detector at pseudorapidities $$-6.6<\eta <-5.2$$ in proton-proton collisions at a centre-of-mass energy $$\sqrt{s}=13\,\text {TeV} $$ . The results are presented as a function of the charged particle multiplicity in the region $$|\eta |<2$$ . This measurement is sensitive to correlations induced by the underlying event structure over a very wide pseudorapidity region. The predictions of Monte Carlo event generators commonly used in collider experiments and ultra-high energy cosmic ray physics are compared to the data. All generators considered overestimate the fraction of energymore » going into hadrons.« less