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CMS Collaboration

Publications and source records attributed to CMS Collaboration.

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arXiv2020

Search for top squark pair production in a final state with two tau leptons in proton-proton collisions at $\sqrt{s} =$ 13 TeV

A search for pair production of the supersymmetric partner of the top quark, the top squark, in proton-proton collision events at $\sqrt{s} =$ 13 TeV is presented in a final state containing hadronically decaying tau leptons and large missing transverse momentum. This final state is highly sensitive to high-$\tanβ$ or higgsino-like scenarios in which decays of electroweak gauginos to tau leptons are dominant. The search uses a data set corresponding to an integrated luminosity of 77.2 fb$^{-1}$, which was recorded with the CMS detector during 2016 and 2017. No significant excess is observed with respect to the background prediction. Exclusion limits at 95% confidence level are presented in the top squark and lightest neutralino mass plane within the framework of simplified models, in which top squark masses up to 1100 GeV are excluded for a nearly massless neutralino.

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arXiv2020

Search for supersymmetry with a compressed mass spectrum in events with a soft $τ$ lepton, a highly energetic jet, and large missing transverse momentum in proton-proton collisions at $\sqrt{s} =$ 13 TeV

The first search for supersymmetry in events with an experimental signature of one soft, hadronically decaying $τ$ 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 $τ$ leptons ($\tildeτ$) in supersymmetric models that exhibit coannihilation between the $\tildeτ$ and the lightest neutralino ($\tildeχ^0_1$), 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 $\sqrt{s} =$ 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 ($Δm$) between the chargino ($\tildeχ^\pm_1$) or next-to-lightest neutralino ($\tildeχ^0_2$), and the $\tildeχ^0_1$. The mass of the $\tildeτ$ is assumed to be the average of the $\tildeχ^\pm_1$ and $\tildeχ^0_1$ masses. The data are consistent with standard model background predictions. Upper limits at 95% confidence level are set on the sum of the $\tildeχ^\pm_1$, $\tildeχ^0_2$, and $\tildeτ$ production cross sections for $Δm(\tildeχ^\pm_1, \tildeχ^0_1) =$ 50 GeV, resulting in a lower limit of 290 GeV on the mass of the $\tildeχ^\pm_1$, which is the most stringent to date and surpasses the bounds from the LEP experiments.

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arXiv2020

Search for production of four top quarks in final states with same-sign or multiple leptons in proton-proton collisions at $\sqrt{s} =$ 13 TeV

The standard model (SM) production of four top quarks ($\mathrm{t\bar{t}t\bar{t}}$) in proton-proton collision 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 fb$^{-1}$ at a center-of-mass energy of 13 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 $\mathrm{t\bar{t}t\bar{t}}$ signal are respectively 2.6 and 2.7 standard deviations, and the $\mathrm{t\bar{t}t\bar{t}}$ cross section is measured to be 12.6 $^{+5.8}_{-5.2}$ fb. The results are used to constrain the Yukawa coupling of the top quark to the Higgs boson, $y_{\mathrm{t}}$, yielding a limit of $|y_{\mathrm{t}}$ $/$ $y_{\mathrm{t}}^{\mathrm{SM}}|$ $<$ 1.7 at 95% confidence level, where $y_{\mathrm{t}}^{\mathrm{SM}}$ is the SM value of $y_{\mathrm{t}}$. 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 GeV and 350-550 GeV for scalar and pseudoscalar bosons, respectively. Upper bounds are also set on couplings of the top quark to new light particles.

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arXiv2020

Search for a charged Higgs boson decaying into top and bottom quarks in events with electrons or muons in proton-proton collisions at $\sqrt{s} =$ 13 TeV

A search is presented for a charged Higgs boson heavier than the top quark, produced in association with a top quark, or with a top and a bottom quark, and decaying into a top-bottom quark-antiquark pair. The search is performed using proton-proton collision data collected by the CMS experiment at the LHC at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. Events are selected by the presence of a single isolated charged lepton (electron or muon) or an opposite-sign dilepton (electron or muon) pair, categorized according to the jet multiplicity and the number of jets identified as originating from b quarks. Multivariate analysis techniques are used to enhance the discrimination between signal and background in each category. The data are compatible with the standard model, and 95% confidence level upper limits of 9.6-0.01 pb are set on the charged Higgs boson production cross section times branching fraction to a top-bottom quark-antiquark pair, for charged Higgs boson mass hypotheses ranging from 200 GeV to 3 TeV. The upper limits are interpreted in different minimal supersymmetric extensions of the standard model.

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arXiv2020

Observation of the $Λ_\mathrm{b}^0$ $\to$ J/$ψΛϕ$ decay in proton-proton collisions at $\sqrt{s} =$ 13 TeV

The observation of the $Λ_\mathrm{b}^0$ $\to$ J/$ψΛϕ$ decay is reported using proton-proton collision data collected at $\sqrt{s} =$ 13 TeV by the CMS experiment at the LHC in 2018, corresponding to an integrated luminosity of 60 fb$^{-1}$. The ratio of the branching fractions $\mathcal{B}(Λ_\mathrm{b}^0$ $\to$ J/$ψΛϕ)$ $/$ $\mathcal{B}(Λ_\mathrm{b}^0$ $\to$ $ψΛ)$ is measured to be (8.26 $\pm$ 0.90 (stat) $\pm$ 0.68 (syst) $\pm$ 0.11 $(\mathcal{B}))$ $\times$ 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.

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arXiv2020

Multiparticle correlation studies in pPb collisions at $\sqrt{s_\mathrm{NN}} =$ 8.16 TeV

The second- and third-order azimuthal anisotropy Fourier harmonics of charged particles produced in pPb collisions, at $\sqrt{s_\mathrm{NN}} =$ 8.16 TeV, are studied over a wide range of event multiplicities. Multiparticle correlations are used to isolate global properties stemming from the collision overlap geometry. The second-order "elliptic" harmonic moment is obtained with high precision through four-, six-, and eight-particle correlations and, for the first time, the third-order "triangular" harmonic moment is studied using four-particle correlations. A sample of peripheral PbPb collisions at $\sqrt{s_\mathrm{NN}} =$ 5.02 TeV that covers a similar range of event multiplicities as the pPb results is also analyzed. Model calculations of initial-state fluctuations in pPb and PbPb collisions can be directly compared to the high precision experimental results. This work provides new insight into the fluctuation-driven origin of the $v_3$ coefficients in pPb and PbPb collisions, and into the dominating overall collision geometry in PbPb collisions at the earliest stages of heavy ion interactions.

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arXiv2020

Measurement of electroweak production of a W boson in association with two jets in proton-proton collisions at $\sqrt{s} =$ 13 TeV

A measurement is presented of electroweak (EW) production of a W boson in association with two jets in proton-proton collisions at $\sqrt{s} =$ 13 TeV. The data sample was recorded by the CMS Collaboration at the LHC and corresponds to an integrated luminosity of 35.9 fb$^{-1}$. The measurement is performed for the $\ellν$jj final state (with $\ellν$ indicating a lepton-neutrino pair, and j representing the quarks produced in the hard interaction) in a kinematic region defined by invariant mass $m_\mathrm{jj}$ $>$ 120 GeV and transverse momenta $p_\mathrm{T j}$ $>$ 25 GeV. The cross section of the process is measured in the electron and muon channels yielding $σ_\mathrm{EW}$(Wjj) = 6.23 $\pm$ 0.12 (stat) $\pm$ 0.61 (syst) pb per channel, in agreement with leading-order standard model predictions. The additional hadronic activity of events in a signal-enriched region is studied, and the measurements are compared with predictions. The final state is also used to perform a search for anomalous trilinear gauge couplings. Limits on anomalous trilinear gauge couplings associated with dimension-six operators are given in the framework of an effective field theory. The corresponding 95% confidence level intervals are $-$2.3 $<$ $c_{\mathrm{WWW}}/Λ^2$ $<$ 2.5 TeV$^{-2}$, $-$8.8 $<$ $c_{\mathrm{W}}/Λ^2$ $<$ 16 TeV$^{-2}$, and $-$45 $<$ $c_{\mathrm{B}}/Λ^2$ $<$ 46 TeV$^{-2}$. These results are combined with the CMS EW Zjj analysis, yielding the constraint on the $c_{\mathrm{WWW}}$ coupling: $-$1.8 $<$ $c_{\mathrm{WWW}}/Λ^2$ $<$ 2.0 TeV$^{-2}$.

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arXiv2020

Evidence for WW production from double-parton interactions in proton-proton collisions at $\sqrt{s} =$ 13 TeV

A search for WW production from double-parton scattering processes using same-charge electron-muon and dimuon events is reported, based on proton-proton collision data collected at a center-of-mass energy of 13 TeV. The analyzed data set corresponds to an integrated luminosity of 77.4 fb$^{-1}$, collected using the CMS detector at the LHC in 2016 and 2017. Multivariate classifiers are used to discriminate between the signal and the dominant background processes. A maximum likelihood fit is performed to extract the signal cross section. This leads to the first evidence for WW production via double-parton scattering, with a significance of 3.9 standard deviations. The measured inclusive cross section is 1.41 $\pm$ 0.28 (stat) $\pm$ 0.28 (syst) pb.

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arXiv2020

Search for electroweak production of a vector-like T quark using fully hadronic final states

A search is performed for electroweak production of a vector-like top quark partner T of charge 2/3 in association with a top or bottom quark, using proton-proton collision data at $\sqrt{s} =$ 13 TeV collected by the CMS experiment at the LHC in 2016. The data sample corresponds to an integrated luminosity of 35.9 fb$^{-1}$. The search targets T quarks over a wide range of masses and fractional widths, decaying to a top quark and either a Higgs boson or a Z boson in fully hadronic final states. The search is performed using two experimentally distinct signatures that depend on whether or not each quark from the decays of the top quark, Higgs boson, or Z boson produces an individual resolved jet. Jet substructure, b tagging, and kinematic variables are used to identify the top quark and boson jets, and also to suppress the standard model backgrounds. The data are found to be consistent with the expected backgrounds. Upper limits at 95% confidence level are set on the cross sections for T quark-mediated production of tHQq, tZQq, and their sum, where Q is the associated top or bottom heavy quark and q is another associated quark. The limits are given for each search signature for various T quark widths up to 30% of the T quark mass, and are between 2 pb and 20 fb for T quark masses in the range 0.6-2.6 TeV. These results are significantly more sensitive than prior searches for electroweak single production of T $\to$ tH and represent the first constraints on T $\to$ tZ using hadronic decays of the Z boson with this production mode.

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arXiv2020

Combined search for supersymmetry with photons in proton-proton collisions at $\sqrt{s} =$ 13 TeV

A combination of four searches for new physics involving signatures with at least one photon and large missing transverse momentum, motivated by generalized models of gauge-mediated supersymmetry (SUSY) breaking, is presented. All searches make use of proton-proton collision data at $\sqrt{s} =$ 13 TeV, which were recorded with the CMS detector at the LHC in 2016, and correspond to an integrated luminosity of 35.9 fb$^{-1}$. Signatures with at least one photon and large missing transverse momentum are categorized into events with two isolated photons, events with a lepton and a photon, events with additional jets, and events with at least one high-energy photon. No excess of events is observed beyond expectations from standard model processes, and limits are set in the context of gauge-mediated SUSY. Compared to the individual searches, the combination extends the sensitivity to gauge-mediated SUSY in both electroweak and strong production scenarios by up to 100 GeV in neutralino and chargino masses, and yields the first CMS result combining various SUSY searches in events with photons at $\sqrt{s} =$ 13 TeV.

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arXiv2020

Search for low mass vector resonances decaying into quark-antiquark pairs in proton-proton collisions at $\sqrt{s} =$ 13 TeV

A search for low mass narrow vector resonances decaying into quark-antiquark pairs is presented. The analysis is based on data collected in 2017 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 41.1 fb$^{-1}$. The results of this analysis are combined with those of an earlier analysis based on data collected at the same collision energy in 2016, corresponding to 35.9 fb$^{-1}$. Signal candidates will be recoiling against initial state radiation and are identified as energetic, large-radius jets with two pronged substructure. The invariant jet mass spectrum is probed for a potential narrow peaking signal over a smoothly falling background. No evidence for such resonances is observed within the mass range of 50-450 GeV. Upper limits at the 95% confidence level are set on the coupling of narrow resonances to quarks, as a function of the resonance mass. For masses between 50 and 300 GeV these are the most sensitive limits to date. This analysis extends the earlier search to a mass range of 300-450 GeV, which is probed for the first time with jet substructure techniques.

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arXiv2020

Searches for physics beyond the standard model with the $M_\mathrm{T2}$ variable in hadronic final states with and without disappearing tracks in proton-proton collisions at $\sqrt{s} =$ 13 TeV

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 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.

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arXiv2020

Extraction and validation of a new set of CMS PYTHIA8 tunes from underlying-event measurements

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.

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arXiv2019

Search for physics beyond the standard model in events with overlapping photons and jets

Results are reported from a search for new particles that decay into a photon and two gluons, in events with jets. Novel jet substructure techniques are developed that allow photons to be identified in an environment densely populated with hadrons. The analyzed proton-proton collision data were collected by the CMS experiment at the LHC, in 2016 at $\sqrt{s} =$ 13 TeV, and correspond to an integrated luminosity of 35.9 fb$^{-1}$. The spectra of total transverse hadronic energy of candidate events are examined for deviations from the standard model predictions. No statistically significant excess is observed over the expected background. The first cross section limits on new physics processes resulting in such events are set. The results are interpreted as upper limits on the rate of gluino pair production, utilizing a simplified stealth supersymmetry model. The excluded gluino masses extend up to 1.7 TeV, for a neutralino mass of 200 GeV and exceed previous mass constraints set by analyses targeting events with isolated photons.

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arXiv2019

Observation of nuclear modifications in W$^\pm$ boson production in pPb collisions at $\sqrt{s_\mathrm{NN}} =$ 8.16 TeV

The production of W$^\pm$ bosons is studied in proton-lead (pPb) collisions at a nucleon-nucleon centre-of-mass energy of $\sqrt{s_\mathrm{NN}} =$ 8.16 TeV. Measurements are performed in the W$^\pm$ $\to$ $μ^\pmν_μ$ channel using a data sample corresponding to an integrated luminosity of 173.4 $\pm$ 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 $|η^μ_\mathrm{lab}|$ $<$ 2.4 and transverse momentum $p_\mathrm{T}^μ$ $>$ 25 GeV/$c$. The W$^\pm$ boson differential cross sections, muon charge asymmetry, and the ratios of W$^\pm$ 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.

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arXiv2019

Study of the B$^+$ $\to$ J$/ψ\overlineΛ$p decay in proton-proton collisions at $\sqrt{s} =$ 8 TeV

A study of the B$^+$ $\to$ J$/ψ\overlineΛ$p decay using proton-proton collision data collected at $\sqrt{s} =$ 8 TeV by the CMS experiment at the LHC, corresponding to an integrated luminosity of 19.6 fb$^{-1}$, is presented. The ratio of branching fractions $\mathcal{B}$(B$^+$ $\to$ J$/ψ\overlineΛ$p ) / $\mathcal{B}$(B$^+$ $\to$ J$/ψ$K$^*$(892)$^+$) is measured to be (1.054 $\pm$ 0.057 (stat) $\pm$ 0.035 (syst) $\pm$ 0.011 ($\mathcal{B}$))%, where the last uncertainty reflects the uncertainties in the world-average branching fractions of $\overlineΛ$ and K$^*$(892)$^+$ decays to reconstructed final states. The invariant mass distributions of the J$/ψ\overlineΛ$, J$/ψ$p, and $\overlineΛ$p systems produced in the B$^+$ $\to$ J$/ψ\overlineΛ$p decay are investigated and found to be inconsistent with the pure phase space hypothesis. The analysis is extended by using a model-independent angular amplitude analysis, which shows that the observed invariant mass distributions are consistent with the contributions from excited kaons decaying to the $\overlineΛ$p system.

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arXiv2019

Probing the chiral magnetic wave in pPb and PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV using charge-dependent azimuthal anisotropies

Charge-dependent anisotropy Fourier coefficients ($v_n$) of particle azimuthal distributions are measured in pPb and PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV with the CMS detector at the LHC. The normalized difference in the second-order anisotropy coefficients ($v_2$) between positively and negatively charged particles is found to depend linearly on the observed event charge asymmetry with comparable slopes for both pPb and PbPb collisions over a wide range of charged particle multiplicity. In PbPb, the third-order anisotropy coefficient, $v_3$, shows a similar linear dependence with the same slope as seen for $v_2$. The observed similarities between the $v_2$ slopes for pPb and PbPb, as well as the similar slopes for $v_2$ and $v_3$ in PbPb, are compatible with expectations based on local charge conservation in the decay of clusters or resonances, and constitute a challenge to the hypothesis that, at LHC energies, the observed charge asymmetry dependence of $v_2$ in heavy ion collisions arises from a chiral magnetic wave.

nucl-ex↗

arXiv2019

Measurement of the differential Drell-Yan cross section in proton-proton collisions at $\sqrt{s} =$ 13 TeV

Measurements of the differential cross section for the Drell-Yan process, based on proton-proton collision data at a centre-of-mass energy of 13 TeV, collected by the CMS experiment, are presented. The data correspond to an integrated luminosity of 2.8 (2.3) fb$^{-1}$ in the dimuon (dielectron) channel. The total and fiducial cross section measurements are presented as a function of dilepton invariant mass in the range 15 to 3000 GeV, and compared with the perturbative predictions of the standard model. The measured differential cross sections are in good agreement with the theoretical calculations.

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