88 Search Results

At the start of Run 2 in 2015, the LHC delivered proton-proton collisions at a center-of-mass energy of 13\TeV. During Run 2 (years 2015–2018) the LHC eventually reached a luminosity of 2.1× 1034 cm-2s-1, almost three times that reached during Run 1 (2009–2013) and a factor of two larger than the LHC design value, leading to events with up to a mean of about 50 simultaneous inelastic proton-proton collisions per bunch crossing (pileup). The CMS Level-1 trigger was upgraded prior to 2016 to improve the selection of physics events in the challenging conditions posed by the second run of the LHC.more » This paper describes the performance of the CMS Level-1 trigger upgrade during the data taking period of 2016–2018. The upgraded trigger implements pattern recognition and boosted decision tree regression techniques for muon reconstruction, includes pileup subtraction for jets and energy sums, and incorporates pileup-dependent isolation requirements for electrons and tau leptons. In addition, the new trigger calculates high-level quantities such as the invariant mass of pairs of reconstructed particles. The upgrade reduces the trigger rate from background processes and improves the trigger efficiency for a wide variety of physics signals.« less

With increasing instantaneous luminosity at the LHC come additional reconstruction challenges. At high luminosity, many collisions occur simultaneously within one proton-proton bunch crossing. The isolation of an interesting collision from the additional “pileup” collisions is needed for effective physics performance. In the CMS Collaboration, several techniques capable of mitigating the impact of these pileup collisions have been developed. Such methods include charged-hadron subtraction, pileup jet identification, isospin-based neutral particle “δβ” correction, and, most recently, pileup per particle identification. This paper surveys the performance of these techniques for jet and missing transverse momentum reconstruction, as well as muon isolation. The analysismore » makes use of data corresponding to 35.9 fb−1 collected with the CMS experiment in 2016 at a center-of-mass energy of 13 TeV. The performance of each algorithm is discussed for up to 70 simultaneous collisions per bunch crossing. Significant improvements are found in the identification of pileup jets, the jet energy, mass, and angular resolution, missing transverse momentum resolution, and muon isolation when using pileup per particle identification.« less

The fiducial cross section for Y(1S) pair production in proton-proton collisions at a center-of-mass energy of 13 TeV in the region where both Y(1S) mesons have an absolute rapidity below 2.0 is measured to be 79±11(stat)±6(syst)±3(B) pb assuming the mesons are produced unpolarized. The last uncertainty corresponds to the uncertainty in the Y(1S) meson dimuon branching fraction. The measurement is performed in the final state with four muons using proton-proton collision data collected in 2016 by the CMS experiment at the LHC, corresponding to an integrated luminosity of 35.9 fb−1 . This process serves as a standard model reference inmore » a search for narrow resonances decaying to Y(1S)μ+μ− in the same final state. Such a resonance could indicate the existence of a tetraquark that is a bound state of two b quarks and two b¯ antiquarks. The tetraquark search is performed for masses in the vicinity of four times the bottom quark mass, between 17.5 and 19.5 GeV, while a generic search for other resonances is performed for masses between 16.5 and 27 GeV. No significant excess of events compatible with a narrow resonance is observed in the data. Limits on the production cross section times branching fraction to four muons via an intermediate Y(1S) resonance are set as a function of the resonance mass.« less

Ratios of cross sections, σ(Z+c  jets)/σ(Z+jets), σ(Z+b  jets)/σ(Z+jets), and σ(Z+c  jets)/σ(Z+b  jets) in the associated production of a Z boson with at least one charm or bottom quark jet are measured in proton-proton collisions at s=13  TeV. The data sample, collected by the CMS experiment at the CERN LHC, corresponds to an integrated luminosity of 35.9  fb-1, with a fiducial volume of pT>30  GeV and |η|<2.4 for the jets, where pT and η represent transverse momentum and pseudorapidity, respectively. The Z boson candidates come from leptonic decays into electrons or muons with pT>25  GeV and |η|<2.4, and the dilepton mass satisfies 71

A data sample of events from proton-proton collisions with at least two jets, and two isolated same-sign or three or more charged leptons, is studied in a search for signatures of new physics phenomena. The data correspond to an integrated luminosity of $$137{\,{\text {fb}}^{-1}} $$ at a center-of-mass energy of $$13\,{\text {TeV}} $$, collected in 2016–2018 by the CMS experiment at the LHC. The search is performed using a total of 168 signal regions defined using several kinematic variables. The properties of the events are found to be consistent with the expectations from standard model processes. Exclusion limits at 95%more » confidence level are set on cross sections for the pair production of gluinos or squarks for various decay scenarios in the context of supersymmetric models conserving or violating R parity. The observed lower mass limits are as large as $$2.1\,{\text {TeV}} $$ for gluinos and $$0.9\,{\text {TeV}} $$ for top and bottom squarks. To facilitate reinterpretations, model-independent limits are provided in a set of simplified signal regions.« less

Central exclusive and semiexclusive production of pairs is measured with the CMS detector in proton-proton collisions at the LHC at center-of-mass energies of 5.02 and 13TeV. The theoretical description of these nonperturbative processes, which have not yet been measured in detail at the LHC, poses a significant challenge to models. The two pions are measured and identified in the CMS silicon tracker based on specific energy loss, whereas the absence of other particles is ensured by calorimeter information. The total and differential cross sections of exclusive and semiexclusive central production are measured as functions of invariant mass, transverse momentum, andmore » rapidity of the system in the fiducial region defined as transverse momentum and pseudorapidity . The production cross sections for the four resonant channels [inline-graphic not available: see fulltext], , [inline-graphic not available: see fulltext], and [inline-graphic not available: see fulltext]are extracted using a simple model. These results represent the first measurement of this process at the LHC collision energies of 5.02 and 13TeV.« less

The combination of measurements of the W boson polarization in top quark decays performed by the ATLAS and CMS collaborations is presented. The measurements are based on proton-proton collision data produced at the LHC at a centre-of-mass energy of 8 TeV, and corresponding to an integrated luminosity of about 20 fb$$^{−1}$$ for each experiment. The measurements used events containing one lepton and having different jet multiplicities in the final state. The results are quoted as fractions of W bosons with longitudinal (F$$_{0}$$), left-handed (F$$_{L}$$), or right-handed (F$$_{R}$$) polarizations. The resulting combined measurements of the polarization fractions are F$$_{0}$$ = 0.693more » ± 0.014 and F$$_{L}$$ = 0.315 ± 0.011. The fraction F$$_{R}$$ is calculated from the unitarity constraint to be F$$_{R}$$ = −0.008 ± 0.007. These results are in agreement with the standard model predictions at next-to-next-to-leading order in perturbative quantum chromodynamics and represent an improvement in precision of 25 (29)% for F$$_{0}$$ (F$$_{L}$$) with respect to the most precise single measurement. A limit on anomalous right-handed vector (V$$_{R}$$), and left- and right-handed tensor (g$$_{L}$$, g$$_{R}$$) tWb couplings is set while fixing all others to their standard model values. The allowed regions are [−0.11, 0.16] for V$$_{R}$$, [−0.08, 0.05] for g$$_{L}$$, and [−0.04, 0.02] for g$$_{R}$$, at 95% confidence level. Limits on the corresponding Wilson coefficients are also derived.[graphic not available: see fulltext]« less

A search for charged Higgs bosons (H$$^{±}$$) decaying into a top and a bottom quark in the all-jet final state is presented. The analysis uses LHC proton-proton collision data recorded with the CMS detector in 2016 at $$ \sqrt{s} $$ = 13 TeV, corresponding to an integrated luminosity of 35.9 fb$$^{−1}$$. No significant excess is observed above the expected background. Model-independent upper limits at 95% confidence level are set on the product of the H$$^{±}$$ production cross section and branching fraction in two scenarios. For production in association with a top quark, limits of 21.3 to 0.007 pb are obtainedmore » for H$$^{±}$$ masses in the range of 0.2 to 3 TeV. Combining this with a search in leptonic final states results in improved limits of 9.25 to 0.005 pb. The complementary s-channel production of an H$$^{±}$$ is investigated in the mass range of 0.8 to 3 TeV and the corresponding upper limits are 4.5 to 0.023 pb. These results are interpreted using different minimal supersymmetric extensions of the standard model.[graphic not available: see fulltext]« less

Measurements of the cross section for the production of top quark pairs in association with a pair of jets from bottom quarks $$ \left({\sigma}_{\mathrm{t}\overline{\mathrm{t}}\mathrm{b}\overline{\mathrm{b}}}\right) $$ and in association with a pair of jets from quarks of any flavor or gluons $$ \left({\sigma}_{\mathrm{t}\overline{\mathrm{t}}\mathrm{jj}}\right) $$ and their ratio are presented. The data were collected in proton-proton collisions at a center-of-mass energy of 13 TeV by the CMS experiment at the LHC in 2016 and correspond to an integrated luminosity of 35.9 fb$$^{−1}$$. The measurements are performed in a fiducial phase space and extrapolated to the full phase space, separately for the dileptonmore » and lepton+jets channels, where lepton corresponds to either an electron or a muon. The results of the measurements in the fiducial phase space for the dilepton and lepton+jets channels, respectively, are $$ {\sigma}_{\mathrm{t}\overline{\mathrm{t}}\mathrm{jj}} $$ = 2.36±0.02 (stat)±0.20 (syst) pb and 31.0±0.2 (stat)±2.9 (syst) pb, and for the cross section ratio 0.017 ± 0.001 (stat) ± 0.001 (syst) and 0.020 ± 0.001 (stat) ± 0.001 (syst). The values of $$ {\sigma}_{\mathrm{t}\overline{\mathrm{t}}\mathrm{b}\overline{\mathrm{b}}} $$ are determined from the product of the $$ {\sigma}_{\mathrm{t}\overline{\mathrm{t}}\mathrm{jj}} $$ and the cross section ratio, obtaining, respectively, 0.040±0.002 (stat)±0.005 (syst) pb and 0.62±0.03 (stat)±0.07 (syst) pb. These measurements are the most precise to date and are consistent, within the uncertainties, with the standard model expectations obtained using a matrix element calculation at next-to-leading order in quantum chromodynamics matched to a parton shower.[graphic not available: see fulltext]« less

The transverse energy ($$ {E}_{\mathrm{T}}^{\upgamma} $$) spectra of photons isolated from other particles are measured using proton-proton (pp) and lead-lead (PbPb) collisions at the LHC at $$ \sqrt{s_{\mathrm{NN}}} $$ = 5.02 TeV with integrated luminosities of 27.4 pb$$^{−1}$$ and 404 μb$$^{−1}$$ for pp and PbPb data, respectively. The results are presented for photons with 25 <$$ {E}_{\mathrm{T}}^{\upgamma} $$< 200 GeV in the pseudorapidity range |η| < 1.44, and for different centrality intervals for PbPb collisions. Photon production in PbPb collisions is consistent with that in pp collisions scaled by the number of binary nucleon-nucleon collisions, demonstrating that photons do notmore » interact with the quark-gluon plasma. Therefore, isolated photons can provide information about the initial energy of the associated parton in photon+jet measurements. The results are compared with predictions from the next-to-leading-order jetphox generator for different parton distribution functions (PDFs) and nuclear PDFs (nPDFs). The comparisons can help to constrain the nPDFs global fits.[graphic not available: see fulltext]« less