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CMS Collaboration

Publications and source records attributed to CMS Collaboration.

At least 595 records · Page 33Linked to original sources

arXiv2020

Pileup mitigation at CMS in 13 TeV data

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 analysis 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.

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arXiv2020

Study of central exclusive $π^+π^-$ production in proton-proton collisions at $\sqrt{s} =$ 5.02 and 13 TeV

Central exclusive and semiexclusive production of $π^+π^-$ pairs is measured with the CMS detector in proton-proton collisions at the LHC at center-of-mass energies of 5.02 and 13 TeV. 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 $π^+π^-$ production are measured as functions of invariant mass, transverse momentum, and rapidity of the $π^+π^-$ system in the fiducial region defined as transverse momentum $p_\mathrm{T}(π)$ $>$ 0.2 GeV and pseudorapidity $|η(π)|$ $<$ 2.4. The production cross sections for the four resonant channels f$_0(500)$, $ρ^0(770)$, f$_0(980)$, and f$_2(1270)$ are extracted using a simple model. These results represent the first measurement of this process at the LHC collision energies of 5.02 and 13 TeV.

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arXiv2020

Studies of charm quark diffusion inside jets using PbPb and pp collisions at $\sqrt{s_\mathrm{NN}} =$ 5.02 TeV

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 $p_\mathrm{T}^\mathrm{jet}$ $>$ 60 GeV and D$^0$ mesons with $p_\mathrm{T}^\mathrm{D}$ $>$ 4 GeV in lead-lead (PbPb) and proton-proton (pp) collisions at a nucleon-nucleon center-of-mass energy of $\sqrt{s_\mathrm{NN}} =$ 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 PbPb 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 PbPb collisions, compared to the pp results, the D$^0$ meson distribution for 4 $<$ $p_\mathrm{T}^\mathrm{D}$ $<$ 20 GeV 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 $p_\mathrm{T}^\mathrm{D}$, the PbPb and pp radial distributions are found to be similar.

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arXiv2020

Search for a light pseudoscalar Higgs boson in the boosted $μμττ$ final state in proton-proton collisions at $\sqrt{s} =$ 13 TeV

A search for a light pseudoscalar Higgs boson (a) decaying from the 125 GeV (or a heavier) scalar Higgs boson (H) is performed using the 2016 LHC proton-proton collision data at $\sqrt{s} =$ 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$, collected by the CMS experiment. The analysis considers gluon fusion and vector boson fusion production of the H, followed by the decay H $\to$ aa $\to$ $μμττ$, and considers pseudoscalar masses in the range 3.6 $\lt$ $m_\mathrm{a}$ $\lt$ 21 GeV. Because of the large mass difference between the H and the a bosons and the small masses of the a boson decay products, both the $μμ$ and the $ττ$ pairs have high Lorentz boost and are collimated. The $ττ$ reconstruction efficiency is increased by modifying the standard technique for hadronic $τ$ lepton decay reconstruction to account for a nearby muon. No significant signal is observed. Model-independent limits are set at 95% confidence level, as a function of $m_\mathrm{a}$, on the branching fraction ($\mathcal{B}$) for H $\to$ aa $\to$ $μμττ$, down to 1.5 (2.0) $\times$ 10$^{-4}$ for $m_\mathrm{H} =$ 125 (300) GeV. Model-dependent limits on $\mathcal{B}$(H $\to$ aa) are set within the context of two Higgs doublets plus singlet models, with the most stringent results obtained for Type-III models. These results extend current LHC searches for heavier a bosons that decay to resolved lepton pairs and provide the first such bounds for an H boson with a mass above 125 GeV.

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arXiv2020

Measurements of production cross sections of WZ and same-sign WW boson pairs in association with two jets in proton-proton collisions at $\sqrt{s} =$ 13 TeV

Measurements of production cross sections of WZ and same-sign WW boson pairs in association with two jets in proton-proton collisions at $\sqrt{s} =$ 13 TeV at the LHC are reported. The data sample corresponds to an integrated luminosity of 137 fb$^{-1}$, collected with the CMS detector during 2016-2018. The measurements are performed in the leptonic decay modes W$^\pm$Z $\to$ $\ell^\pmν\ell'^\pm\ell'^\mp$ and WW $\to$ $\ell^\pmν\ell'^\pmν$, where $\ell, \ell' = $ e, $μ$. Differential fiducial cross sections as functions of the invariant masses of the jet and charged lepton pairs, as well as of the leading-lepton transverse momentum, are measured for WW production and are consistent with the standard model predictions. The dependence of differential cross sections on the invariant mass of the jet pair is also measured for WZ production. An observation of electroweak production of WZ boson pairs is reported with an observed (expected) significance of 6.8 (5.3) standard deviations. Constraints are obtained on the structure of quartic vector boson interactions in the framework of effective field theory.

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arXiv2020

Measurement of the associated production of a Z boson with charm or bottom quark jets in proton-proton collisions at $\sqrt{s} =$ 13 TeV

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 $\sqrt{s} =$ 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 $p_\mathrm{T}$ $>$ 30 GeV and $|η|$ $<$ 2.4 for the jets, where $p_\mathrm{T}$ and $η$ represent transverse momentum and pseudorapidity, respectively. The Z boson candidates come from leptonic decays into electrons or muons with $p_\mathrm{T}$ $>$ 25 GeV and $|η|$ $<$ 2.4, and the dilepton mass satisfies 71 $\lt$ $m_\mathrm{Z}$ $\lt$ 111 GeV. The measured values are $σ$(Z+c jets) / $σ$(Z+jets) = 0.102 $\pm$ 0.002 $\pm$ 0.009, $σ$(Z+b jets) / $σ$(Z+jets) = 0.0633 $\pm$ 0.0004 $\pm$ 0.0015, and $σ$(Z+c jets) / $σ$(Z+b jets) = 1.62 $\pm$ 0.03 $\pm$ 0.15. Results on the inclusive and differential cross section ratios as functions of jet and Z boson transverse momentum are compared with predictions from leading and next-to-leading order perturbative quantum chromodynamics calculations. These are the first measurements of the cross section ratios at 13 TeV.

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arXiv2020

Search for physics beyond the standard model in events with jets and two same-sign or at least three charged leptons in proton-proton collisions at $\sqrt{s} =$ 13 TeV

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 fb$^{-1}$ at a center-of-mass energy of 13 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% 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 TeV for gluinos and 0.9 TeV for top and bottom squarks. To facilitate reinterpretations, model-independent limits are provided in a set of simplified signal regions.

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arXiv2020

Search for resonant pair production of Higgs bosons in the bbZZ channel in proton-proton collisions at $\sqrt{s} =$ 13 TeV

A search for the production of a narrow-width resonance decaying into a pair of Higgs bosons decaying into the bbZZ channel is presented. The analysis is based on data collected with the CMS detector during 2016, in proton-proton collisions at the LHC, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The final states considered are the ones where one of the Z bosons decays into a pair of muons or electrons, and the other Z boson decays either to a pair of quarks or a pair of neutrinos. Upper limits at 95% confidence level are placed on the production of narrow-width spin-0 or spin-2 particles decaying to a pair of Higgs bosons, in models with and without an extended Higgs sector. For a resonance mass range between 260 and 1000 GeV, limits on the production cross section times branching fraction of a spin-0 and spin-2 resonance range from 0.1 to 5.0 pb and 0.1 to 3.6 pb, respectively. These results set limits in parameter space in bulk Randall-Sundrum radion, Kaluza-Klein excitation of the graviton, and N2HDM models. For specific choices of parameters the N2HDM can be excluded in a mass range between 360 and 620 GeV for a resonance decaying to two Higgs bosons. This is the first search for Higgs boson resonant pair production in the bbZZ channel.

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arXiv2020

Measurements of $\mathrm{t\bar{t}}$H production and the CP structure of the Yukawa interaction between the Higgs boson and the top quark in the diphoton decay channel

The first observation of the $\mathrm{t\bar{t}}$H process in a single Higgs boson decay channel with the full reconstruction of the final state (H$\toγγ$) is presented, with a significance of 6.6 standard deviations ($σ$). 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$σ$. The measurements are based on a sample of proton-proton collisions at a center-of-mass energy $\sqrt{s} =$ 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 $\mathrm{t\bar{t}}$H process is measured to be $σ_{\mathrm{t\bar{t}H}}\mathcal{B}_{γγ} =$ 1.56 $^{+0.34}_{-0.32}$ fb, which is compatible with the standard model prediction of 1.13 $^{+0.08}_{-0.11}$ fb. The fractional contribution of the CP-odd component is measured to be $f^{\mathrm{Htt}}_\mathrm{CP} =$ 0.00 $\pm$ 0.33.

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arXiv2020

Search for charged Higgs bosons decaying into a top and a bottom quark in the all-jet final state of pp collisions at $\sqrt{s} =$ 13 TeV

A search for charged Higgs bosons (H$^\pm$) decaying into a top and a bottom quark in the all-jet final states 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$^\pm$ 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 obtained for H$^\pm$ 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$^\pm$ 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.

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arXiv2020

Measurement of the cross section for $\text{t}\bar{\text{t}}$ production with additional jets and b jets in pp collisions at $\sqrt{s} =$ 13 TeV

Measurements of the cross section for the production of top quark pairs in association with a pair of jets from bottom quarks ($σ_\mathrm{t\bar{t}b\bar{b}}$) and in association with a pair of jets from quarks of any flavor or gluons ($σ_\mathrm{t\bar{t}jj}$) 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 in the dilepton and lepton+jets channels, where lepton corresponds to either an electron or a muon. The results for the dilepton and lepton+jets channels, respectively, are $σ_\mathrm{t\bar{t}jj}$ = 2.36 $\pm$ 0.02 (stat) $\pm$ 0.20 (syst) pb and 31.0 $\pm$ 0.2 (stat) $\pm$ 2.9 (syst) pb, and for the cross section ratio 0.017 $\pm$ 0.001 (stat) $\pm$ 0.001 (syst) and 0.020 $\pm$ 0.001 (stat) $\pm$ 0.001 (syst). The values of $σ_\mathrm{t\bar{t}b\bar{b}}$ are determined from the product of the $σ_\mathrm{t\bar{t}jj}$ and the cross section ratio, obtaining, respectively, 0.040 $\pm$ 0.002 (stat) $\pm$ 0.005 (syst) pb and 0.62 $\pm$ 0.03 (stat) $\pm$ 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.

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arXiv2020

Measurement of CKM matrix elements in single top quark $t$-channel production in proton-proton collisions at $\sqrt{s} = $ 13 TeV

The first direct, model-independent measurement is presented of the modulus of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements $|V_\mathrm{tb}|$, $|V_\mathrm{td}|$, and $|V_\mathrm{ts}|$, in final states enriched in single top quark $t$-channel events. The analysis uses proton-proton collision data from the LHC, collected during 2016 by the CMS experiment, at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. Processes directly sensitive to these matrix elements are considered at both the production and decay vertices of the top quark. In the standard model hypothesis of CKM unitarity, a lower limit of $|V_\mathrm{tb}|$ $>$ 0.970 is measured at the 95% confidence level. Several theories beyond the standard model are considered, and by releasing all constraints among the involved parameters, the values $|V_\mathrm{tb}| =$ 0.988 $\pm$ 0.024, and $|V_\mathrm{td}|^2 + |V_\mathrm{ts}|^2 =$ 0.06 $\pm$ 0.06, where the uncertainties include both statistical and systematic components, are measured.

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arXiv2020

Measurement of quark- and gluon-like jet fractions using jet charge in PbPb and pp collisions at 5.02 TeV

The momentum-weighted sum of the electric charges of particles inside a jet, known as jet charge, is sensitive to the electric charge of the particle initiating the parton shower. This paper presents jet charge distributions in $\sqrt{s_\mathrm{NN}} =$ 5.02 TeV lead-lead (PbPb) and proton-proton (pp) collisions recorded with the CMS detector at the LHC. These data correspond to integrated luminosities of 404 $μ$b$^{-1}$ and 27.4 pb$^{-1}$ for PbPb and pp collisions, respectively. Leveraging the sensitivity of the jet charge to fundamental differences in the electric charges of quarks and gluons, the jet charge distributions from simulated events are used as templates to extract the quark- and gluon-like jet fractions from data. The modification of these jet fractions is examined by comparing pp and PbPb data as a function of the overlap of the colliding Pb nuclei (centrality). This measurement tests the color charge dependence of jet energy loss due to interactions with the quark-gluon plasma. No significant modification between different centrality classes and with respect to pp results is observed in the extracted fractions of quark- and gluon-like jet fractions.

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arXiv2020

The production of isolated photons in PbPb and pp collisions at $\sqrt{s_\mathrm{NN}} =$ 5.02 TeV

The transverse energy ($E_\mathrm{T}^γ$) 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 $μ$b$^{-1}$ for pp and PbPb data, respectively. The results are presented for photons with 25 $<$ $E_\mathrm{T}^γ$ $<$ 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 not 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.

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arXiv2020

Measurement of the $Υ$(1S) pair production cross section and search for resonances decaying to $Υ$(1S)$μ^+μ^-$ in proton-proton collisions at $\sqrt{s} =$ 13 TeV

The fiducial cross section for $Υ$(1S) pair production in proton-proton collisions at a center-of-mass energy of 13 TeV in the region where both $Υ$(1S) mesons have an absolute rapidity below 2.0 is measured to be 79 $\pm$ 11 (stat) $\pm$ 6 (syst) $\pm$ 3 ($\mathcal{B}$) pb assuming the mesons are produced unpolarized. The last uncertainty corresponds to the uncertainty in the $Υ$(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 in a search for narrow resonances decaying to $Υ$(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 $\bar{\mathrm{b}}$ 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 $Υ$(1S) resonance are set as a function of the resonance mass.

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arXiv2020

Strange hadron production in pp and pPb collisions at $\sqrt{s_\mathrm{NN}} =$ 5.02 TeV

The transverse momentum ($p_\mathrm{T}$) distributions of $Λ$, $Ξ^-$, and $Ω^-$ baryons, their antiparticles, and K$^0_\mathrm{S}$ mesons are measured in proton-proton (pp) and proton-lead (pPb) collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV over a broad rapidity range. The data, corresponding to integrated luminosities of 40.2 nb$^{-1}$ and 15.6 $μ$b$^{-1}$ for pp and pPb collisions, respectively, were collected by the CMS experiment. The nuclear modification factor $R_\mathrm{pPb}$, defined as the ratio of the particle yield in pPb collisions and a scaled pp reference, is measured for each particle. A strong dependence on particle species is observed in the $p_\mathrm{T}$ range from 2 to 7 GeV, where $R_\mathrm{pPb}$ for K$^0_\mathrm{S}$ is consistent with unity, while an enhancement ordered by strangeness content and/or particle mass is observed for the three baryons. In pPb collisions, the strange hadron production is asymmetric about the nucleon-nucleon center-of-mass rapidity. Enhancements, which depend on the particle type, are observed in the direction of the Pb beam. The results are compared to predictions from EPOS LHC, which includes parametrized radial flow. The model is in qualitative agreement with the $R_\mathrm{pPb}$ data, but fails to describe the dependence on particle species in the yield asymmetries measured away from mid-rapidity in pPb collisions.

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arXiv2020

Mixed higher-order anisotropic flow and nonlinear response coefficients of charged particles in PbPb collisions at $\sqrt{s_\mathrm{NN}} =$ 2.76 and 5.02 TeV

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 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 higher-order 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 $\sqrt{s_\mathrm{NN}} =$ 2.76 and 5.02 TeV with the CMS detector. The results are compared with viscous hydrodynamic calculations using several different initial conditions, as well as microscopic transport model calculations. None of the models provides a simultaneous description of the mixed higher-order flow harmonics and nonlinear response coefficients.

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arXiv2020

Measurement of the top quark forward-backward production asymmetry and the anomalous chromoelectric and chromomagnetic moments in pp collisions at $\sqrt{s} =$ 13 TeV

The parton-level top quark (t) forward-backward asymmetry and the anomalous chromoelectric ($\hat{d}_\mathrm{t}$) and chromomagnetic ($\hatμ_\mathrm{t}$) moments have been measured using LHC pp collisions at a center-of-mass energy of 13 TeV, collected in the CMS detector in a data sample corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The linearized variable $A_\mathrm{FB}^{(1)}$ is used to approximate the asymmetry. Candidate $\mathrm{t\bar{t}}$ events decaying to a muon or electron and jets in final states with low and high Lorentz boosts are selected and reconstructed using a fit of the kinematic distributions of the decay products to those expected for $\mathrm{t\bar{t}}$ final states. The values found for the parameters are $A_\mathrm{FB}^{(1)} =$ 0.048 $^{+0.095}_{-0.087}$ (stat) $^{+0.020}_{-0.029}$ (syst), $\hatμ_\mathrm{t} =-$ 0.024 $^{+0.013}_{-0.009}$ (stat) $^{+0.016}_{-0.011}$ (syst), and a limit is placed on the magnitude of $|\hat{d}_\mathrm{t}|$ $<$ 0.03 at 95% confidence level.

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