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, comparisons 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.
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 Te , 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 T 2 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.
A measurement is presented of the Z / γ ∗ → τ τ cross section in pp collisions at s = 13 TeV , using data recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of 2.3 fb - 1 . The product of the inclusive cross section and branching fraction is measured to be σ ( pp → Z / γ ∗ +X ) B ( Z / γ ∗ → τ τ ) = 1848 ± 12 ( stat ) ± 67 (syst \,+\,lumi) pb , in agreement with the standard model expectation, computed at next-to-next-to-leading order accuracy in perturbative quantum chromodynamics. The measurement is used to validate new analysis techniques relevant for future measurements of τ lepton production. The measurement also provides the reconstruction efficiency and energy scale for τ decays to hadrons + ν τ final states, determined with respective relative uncertainties of 2.2 and 0.9%.
The nuclear modification factors of J / ψ and ψ (2S) mesons are measured in PbPb collisions at a centre-of-mass energy per nucleon pair of s NN = 5.02 TeV . The analysis is based on PbPb and p p data samples collected by CMS at the LHC in 2015, corresponding to integrated luminosities of 464 μ b - 1 and 28 pb -1 , respectively. The measurements are performed in the dimuon rapidity range of | y | < 2.4 as a function of centrality, rapidity, and transverse momentum ( p T ) from p T = 3 GeV / c in the most forward region and up to 50 GeV / c . Both prompt and nonprompt (coming from b hadron decays) J / ψ mesons are observed to be increasingly suppressed with centrality, with a magnitude similar to the one observed at s NN = 2.76 TeV for the two J / ψ meson components. No dependence on rapidity is observed for either prompt or nonprompt J / ψ mesons. An indication of a lower prompt J / ψ meson suppression at p T > 25 GeV / c is seen with respect to that observed at intermediate p T . The prompt ψ (2S) meson yield is found to be more suppressed than that of the prompt J / ψ mesons in the entire p T range.
Four-lepton production in proton-proton collisions, p p → ( Z / γ ∗ ) ( Z / γ ∗ ) → 4 ℓ , where ℓ = e or μ , is studied at a center-of-mass energy of 13 TeV with the CMS detector at the LHC. The data sample corresponds to an integrated luminosity of 35.9 fb - 1 . The ZZ production cross section, σ ( p p → Z Z ) = 17.2 ± 0.5 (stat) ± 0.7 (syst) ± 0.4 (theo) ± 0.4 (lumi) pb , measured using events with two opposite-sign, same-flavor lepton pairs produced in the mass region 60 < m ℓ + ℓ - < 120 GeV , is consistent with standard model predictions. Differential cross sections are measured and are well described by the theoretical predictions. The Z boson branching fraction to four leptons is measured to be B ( Z → 4 ℓ ) = 4 . 83 - 0.22 + 0.23 ( s t a t ) - 0.29 + 0.32 ( s y s t ) ± 0.08 ( t h e o ) ± 0.12 ( l u m i ) × 10 - 6 for events with a four-lepton invariant mass in the range 80 < m 4 ℓ < 100 GeV and a dilepton mass m ℓ ℓ > 4 GeV for all opposite-sign, same-flavor lepton pairs. The results agree with standard model predictions. The invariant mass distribution of the four-lepton system is used to set limits on anomalous ZZZ and ZZ γ couplings at 95% confidence level: - 0.0012 < f 4 Z < 0.0010 , - 0.0010 < f 5 Z < 0.0013 , - 0.0012 < f 4 γ < 0.0013 , - 0.0012 < f 5 γ < 0.0013 .
Measurements are presented of the lifetimes of the B 0 , B s 0 , Λ b 0 , and B c + hadrons using the decay channels B 0 → J / ψ K ∗ ( 892 ) 0 , B 0 → J / ψ K S 0 , B s 0 → J / ψ π + π - , B s 0 → J / ψ ϕ ( 1020 ) , Λ b 0 → J / ψ Λ 0 , and B c + → J / ψ π + . The data sample, corresponding to an integrated luminosity of 19.7 fb -1 , was collected by the CMS detector at the LHC in proton-proton collisions at s = 8 TeV . The B 0 lifetime is measured to be 453.0 ± 1.6 (stat) ± 1.8 (syst) μ m in J / ψ K ∗ ( 892 ) 0 and 457.8 ± 2.7 (stat) ± 2.8 (syst) μ m in J / ψ K S 0 , which results in a combined measurement of c τ B 0 = 454.1 ± 1.4 (stat) ± 1.7 (syst) μ m . The effective lifetime of the B s 0 meson is measured in two decay modes, with contributions from different amounts of the heavy and light eigenstates. This results in two different measured lifetimes: c τ B s 0 → J / ψ π + π - = 502.7 ± 10.2 (stat) ± 3.4 (syst) μ m and c τ B s 0 → J / ψ ϕ ( 1020 ) = 443.9 ± 2.0 (stat) ± 1.5 (syst) μ m . The Λ b 0 lifetime is found to be 442.9 ± 8.2 (stat) ± 2.8 (syst) μ m . The precision from each of these channels is as good as or better than previous measurements. The B c + lifetime, measured with respect to the B + to reduce the systematic uncertainty, is 162.3 ± 7.8 (stat) ± 4.2 (syst) ± 0.1 ( τ B + ) μ m . All results are in agreement with current world-average values.
A search for new phenomena is performed using events with jets and significant transverse momentum imbalance, as inferred through the M T 2 variable. The results are based on a sample of proton-proton collisions collected in 2016 at a center-of-mass energy of 13 TeV with the CMS detector and corresponding to an integrated luminosity of 35.9 fb -1 . No excess event yield is observed above the predicted standard model background, and the results are interpreted as exclusion limits at 95% confidence level on the masses of predicted particles in a variety of simplified models of R-parity conserving supersymmetry. Depending on the details of the model, 95% confidence level lower limits on the gluino (light-flavor squark) masses are placed up to 2025 (1550) GeV . Mass limits as high as 1070 (1175) GeV are set on the masses of top (bottom) squarks. Information is provided to enable re-interpretation of these results, including model-independent limits on the number of non-standard model events for a set of simplified, inclusive search regions.
A search is presented for narrow heavy resonances X decaying into pairs of Higgs bosons ([Formula: see text]) in proton-proton collisions collected by the CMS experiment at the LHC at [Formula: see text]. The data correspond to an integrated luminosity of 19.7[Formula: see text]. The search considers [Formula: see text] resonances with masses between 1 and 3[Formula: see text], having final states of two b quark pairs. Each Higgs boson is produced with large momentum, and the hadronization products of the pair of b quarks can usually be reconstructed as single large jets. The background from multijet and [Formula: see text] events is significantly reduced by applying requirements related to the flavor of the jet, its mass, and its substructure. The signal would be identified as a peak on top of the dijet invariant mass spectrum of the remaining background events. No evidence is observed for such a signal. Upper limits obtained at 95 % confidence level for the product of the production cross section and branching fraction [Formula: see text] range from 10 to 1.5[Formula: see text] for the mass of X from 1.15 to 2.0[Formula: see text], significantly extending previous searches. For a warped extra dimension theory with a mass scale [Formula: see text] [Formula: see text], the data exclude radion scalar masses between 1.15 and 1.55[Formula: see text].
Inclusive jet production in pPb collisions at a nucleon-nucleon (NN) center-of-mass energy of [Formula: see text] is studied with the CMS detector at the LHC. A data sample corresponding to an integrated luminosity of 30.1 nb[Formula: see text] is analyzed. The jet transverse momentum spectra are studied in seven pseudorapidity intervals covering the range [Formula: see text] in the NN center-of-mass frame. The jet production yields at forward and backward pseudorapidity are compared and no significant asymmetry about [Formula: see text] is observed in the measured kinematic range. The measurements in the pPb system are compared to reference jet spectra obtained by extrapolation from previous measurements in pp collisions at [Formula: see text]. In all pseudorapidity ranges, nuclear modifications in inclusive jet production are found to be small, as predicted by next-to-leading order perturbative QCD calculations that incorporate nuclear effects in the parton distribution functions.
A measurement of the top quark-antiquark pair production cross section σ t t ¯ in proton-proton collisions at a centre-of-mass energy of 13 Te is presented. The data correspond to an integrated luminosity of 35.9 fb - 1 , recorded by the CMS experiment at the CERN LHC in 2016. Dilepton events ( e ± μ ∓ , μ + μ - , e + e - ) are selected and the cross section is measured from a likelihood fit. For a top quark mass parameter in the simulation of m t MC = 172.5 Ge the fit yields a measured cross section σ t t ¯ = 803 ± 2 (stat) ± 25 (syst) ± 20 (lumi) pb , in agreement with the expectation from the standard model calculation at next-to-next-to-leading order. A simultaneous fit of the cross section and the top quark mass parameter in the powheg simulation is performed. The measured value of m t MC = 172.33 ± 0.14 (stat) - 0.72 + 0.66 (syst) Ge is in good agreement with previous measurements. The resulting cross section is used, together with the theoretical prediction, to determine the top quark mass and to extract a value of the strong coupling constant with different sets of parton distribution functions.
The observation of WWγ production in proton-proton collisions at a center-of-mass energy of 13 TeV with an integrated luminosity of 138 fb^{-1} is presented. The observed (expected) significance is 5.6 (5.1) standard deviations. Events are selected by requiring exactly two leptons (one electron and one muon) of opposite charge, moderate missing transverse momentum, and a photon. The measured fiducial cross section for WWγ is 5.9±0.8(stat)±0.8(syst)±0.7(modeling) fb, in agreement with the next-to-leading order quantum chromodynamics prediction. The analysis is extended with a search for the associated production of the Higgs boson and a photon, which is generated by a coupling of the Higgs boson to light quarks. The result is used to constrain the Higgs boson couplings to light quarks.
A search is reported for near-threshold structures in the J/ψJ/ψ invariant mass spectrum produced in proton-proton collisions at sqrt[s]=13 TeV from data collected by the CMS experiment, corresponding to an integrated luminosity of 135 fb^{-1}. Three structures are found, and a model with quantum interference among these structures provides a good description of the data. A new structure is observed with a local significance above 5 standard deviations at a mass of 6638_{-38}^{+43}(stat)_{-31}^{+16}(syst) MeV. Another structure with even higher significance is found at a mass of 6847_{-28}^{+44}(stat)_{-20}^{+48}(syst) MeV, which is consistent with the X(6900) resonance reported by the LHCb experiment and confirmed by the ATLAS experiment. Evidence for another new structure, with a local significance of 4.7 standard deviations, is found at a mass of 7134_{-25}^{+48}(stat)_{-15}^{+41}(syst) MeV. Results are also reported for a model without interference, which does not fit the data as well and shows mass shifts up to 150 MeV relative to the model with interference.
The first search for singly produced narrow resonances decaying to three well-separated hadronic jets is presented. The search uses proton-proton collision data corresponding to an integrated luminosity of 138 fb^{-1} at sqrt[s]=13 TeV, collected at the CERN LHC. No significant deviations from the background predictions are observed between 1.75 and 9.00 TeV. The results provide the first mass limits on a right-handed boson Z_{R} decaying to three gluons and on an excited quark decaying via a vector boson to three quarks, as well as updated limits on a Kaluza-Klein gluon decaying via a radion to three gluons.
The first search for scalar leptoquarks produced in τ-lepton-quark collisions is presented. It is based on a set of proton-proton collision data recorded with the CMS detector at the LHC at a center-of-mass energy of 13 TeV corresponding to an integrated luminosity of 138 fb^{-1}. The reconstructed final state consists of a jet, significant missing transverse momentum, and a τ lepton reconstructed through its hadronic or leptonic decays. Limits are set on the product of the leptoquark production cross section and branching fraction and interpreted as exclusions in the plane of the leptoquark mass and the leptoquark-τ-quark coupling strength.
A search is presented for baryon number violating interactions in top quark production and decay. The analysis uses data from proton-proton collisions at a center-of-mass energy of 13 TeV, collected with the CMS detector at the LHC with an integrated luminosity of 138 fb^{-1}. Candidate events are selected by requiring two oppositely charged leptons (electrons or muons) and exactly one jet identified as originating from a bottom quark. Multivariate discriminants are used to separate the signal from the background. No significant deviation from the standard model prediction is observed. Upper limits are placed on the strength of baryon number violating couplings. For the first time the production of single top quarks via baryon number violating interactions is studied. This allows the search to set the most stringent constraints to date on the branching fraction of the top quark decay to a lepton, an up-type quark (u or c), and a down-type quark (d, s, or b). The results improve the previous bounds by 3 to 6 orders of magnitude based on the fermion flavor combination of the baryon number violating interactions.
The production of ϒ(2S) and ϒ(3S) mesons in lead-lead (Pb-Pb) and proton-proton (pp) collisions is studied in their dimuon decay channel using the CMS detector at the LHC. The ϒ(3S) meson is observed for the first time in Pb-Pb collisions, with a significance above 5 standard deviations. The ratios of yields measured in Pb-Pb and pp collisions are reported for both the ϒ(2S) and ϒ(3S) mesons, as functions of transverse momentum and Pb-Pb collision centrality. These ratios, when appropriately scaled, are significantly less than unity, indicating a suppression of ϒ yields in Pb-Pb collisions. This suppression increases from peripheral to central Pb-Pb collisions. Furthermore, the suppression is stronger for ϒ(3S) mesons compared to ϒ(2S) mesons, extending the pattern of sequential suppression of quarkonium states in nuclear collisions previously seen for the J/ψ, ψ(2S), ϒ(1S), and ϒ(2S) mesons.
Energy correlators that describe energy-weighted distances between two or three particles in a hadronic jet are measured using an event sample of sqrt[s]=13 TeV proton-proton collisions collected by the CMS experiment and corresponding to an integrated luminosity of 36.3 fb^{-1}. The measured distributions are consistent with the trends in the simulation that reveal two key features of the strong interaction: confinement and asymptotic freedom. By comparing the ratio of the measured three- and two-particle energy correlator distributions with theoretical calculations that resum collinear emissions at approximate next-to-next-to-leading-logarithmic accuracy matched to a next-to-leading-order calculation, the strong coupling is determined at the Z boson mass: α_{S}(m_{Z})=0.1229_{-0.0050}^{+0.0040}, the most precise α_{S}(m_{Z}) value obtained using jet substructure observables.
Searches for pair-produced multijet signatures using data corresponding to an integrated luminosity of 128 fb^{-1} of proton-proton collisions at sqrt[s]=13 TeV are presented. A data scouting technique is employed to record events with low jet scalar transverse momentum sum values. The electroweak production of particles predicted in R-parity violating supersymmetric models is probed for the first time with fully hadronic final states. This is the first search for prompt hadronically decaying mass-degenerate higgsinos, and extends current exclusions on R-parity violating top squarks and gluinos.
A search is presented for an extended Higgs sector with two new particles, X and ϕ, in the process X→ϕϕ→(γγ)(γγ). Novel neural networks classify events with diphotons that are merged and determine the diphoton masses. The search uses LHC proton-proton collision data at sqrt[s]=13 TeV collected with the CMS detector, corresponding to an integrated luminosity of 138 fb^{-1}. No evidence of such resonances is seen. Upper limits are set on the production cross section for m_{X} between 300 and 3000 GeV and m_{ϕ}/m_{X} between 0.5% and 2.5%, representing the most sensitive search in this channel.
The first search for the Z boson decay to ττμμ at the CERN LHC is presented, based on data collected by the CMS experiment at the LHC in proton-proton collisions at a center-of-mass energy of 13 TeV and corresponding to an integrated luminosity of 138 fb^{-1}. The data are compatible with the predicted background. For the first time, an upper limit at the 95% confidence level of 6.9 times the standard model expectation is placed on the ratio of the Z→ττμμ to Z→4μ branching fractions. Limits are also placed on the six flavor-conserving four-lepton effective-field-theory operators involving two muons and two tau leptons, for the first time testing all such operators.