Search

Three of the most significant measured deviations from standard model predictions, the enhanced decay rate for $B\to D^{(*)}\tau \nu$, hints of lepton universality violation in $B\to K^{(*)}\ell \ell$ decays, and the anomalous magnetic moment of the muon, can be explained by the existence of leptoquarks (LQs) with large couplings to third-generation quarks and masses at the TeV scale. The existence of these states can be probed at the LHC in high energy proton-proton collisions. A novel search is presented for pair production of LQs coupled to a top quark and a muon using data at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of $35.9{\mathrm{fb}}^{-1}$, recorded by the CMS experiment. No deviation from the standard model prediction has been observed and scalar LQs decaying exclusively into $t\mu$ are excluded up to masses of 1420 GeV. The results of this search are combined with those from previous searches for LQ decays into $t\tau$ and $b\nu$, which excluded scalar LQs below masses of 900 and 1080 GeV. Vector LQs are excluded up to masses of 1190 GeV for all possible combinations of branching fractions to $t\mu$, $t\tau$ and $b\nu$. With this analysis, all relevant couplings of LQs with an electric charge of $-1/3$ to third-generation quarks are probed for the first time.

A search for a light charged Higgs boson ($H^{+}$) decaying to a $W$ boson and a $CP$-odd Higgs boson ($A$) in final states with $e\mu \mu$ or $\mu \mu \mu$ is performed using data from $pp$ collisions at $\sqrt{s}=13\mathrm{TeV}$, recorded by the CMS detector at the LHC and corresponding to an integrated luminosity of $35.9{\mathrm{fb}}^{-1}$. In this search, it is assumed that the $H^{+}$ boson is produced in decays of top quarks, and the $A$ boson decays to two oppositely charged muons. The presence of signals for $H^{+}$ boson masses between 100 and 160 GeV and $A$ boson masses between 15 and 75 GeV is investigated. No evidence for the production of the $H^{+}$ boson is found. Upper limits at 95% confidence level are obtained on the combined branching fraction for the decay chain, $t\to b{H}^{+}\to b{W}^{+}A\to b{W}^{+}{\mu }^{+}{\mu }^{-}$, of $1.9\times {10}^{-6}$ to $8.6\times {10}^{-6}$, depending on the masses of the $H^{+}$ and $A$ bosons. These are the first limits for these decay modes of the $H^{+}$ and $A$ bosons.

A search has been performed for pair-produced resonances decaying into three jets. The proton-proton collision data used for this analysis were collected with the CMS detector in 2016 at a center-of-mass energy of 13 TeV and correspond to an integrated luminosity of $35.9{\mathrm{fb}}^{-1}$. The mass range from 200 to 2000 GeV is explored in four separate mass regions. The observations show agreement with standard model expectations. The results are interpreted within the framework of $R$-parity violating SUSY, where pair-produced gluinos decay to a six quark final state. Gluino masses below 1500 GeV are excluded at 95% confidence level. An analysis based on data with multijet events reconstructed at the trigger level extends the reach to masses as low as 200 GeV. Improved analysis techniques have led to enhanced sensitivity, allowing the most stringent limits to date to be set on gluino pair production.

A search is presented for the production of a Higgs boson in association with a single top quark, based on data collected in 2016 by the CMS experiment at the LHC at a center-of-mass energy of 13 TeV, which corresponds to an integrated luminosity of $35.9{\mathrm{fb}}^{-1}$. The production cross section for this process is highly sensitive to the absolute values of the top quark Yukawa coupling, $y_t$; the Higgs boson coupling to vector bosons, $g_{H\mathrm{VV}}$; and, uniquely, their relative sign. Analyses using multilepton signatures, targeting $H\to WW$, $H\to \tau \tau$, and $H\to ZZ$ decay modes, and signatures with a single lepton and a $b\overline{b}$ pair, targeting the $H\to b\overline{b}$ decay, are combined with a reinterpretation of a measurement in the $H\to \gamma \gamma$ channel to constrain $y_t$. For a standard model–like value of $g_{H\mathrm{VV}}$, the data favor positive values of $y_t$ and exclude values of $y_t$ below about $-0.9{y_t}^{\mathrm{SM}}$.

The observation of single top quark production in association with a $Z$ boson and a quark ($tZq$) is reported. Events from proton-proton collisions at a center-of-mass energy of 13 TeV containing three charged leptons (either electrons or muons) and at least two jets are analyzed. The data were collected with the CMS detector in 2016 and 2017 and correspond to an integrated luminosity of $77.4{\mathrm{fb}}^{-1}$. The increased integrated luminosity, a multivariate lepton identification, and a redesigned analysis strategy improve significantly the sensitivity of the analysis compared to previous searches for $tZq$ production. The $tZq$ signal is observed with a significance well over 5 standard deviations. The measured $tZq$ production cross section is $\sigma (pp\to tZq\to t{\ell }^{+}{\ell }^{-}q)=111\pm 13{(\text{stat})}_{-9}^{+11}(\text{syst})\mathrm{fb}$, for dilepton invariant masses above 30 GeV, in agreement with the standard model expectation.

Ratios of cross sections, $\sigma (Z+c\text{jets})/\sigma (Z+\text{jets})$, $\sigma (Z+b\text{jets})/\sigma (Z+\text{jets})$, and $\sigma (Z+c\text{jets})/\sigma (Z+b\text{jets})$ in the associated production of a $Z$ boson with at least one charm or bottom quark jet are measured in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$. The data sample, collected by the CMS experiment at the CERN LHC, corresponds to an integrated luminosity of $35.9{\mathrm{fb}}^{-1}$, with a fiducial volume of $p_{\mathrm{T}}>30\mathrm{GeV}$ and $|\eta |<2.4$ for the jets, where $p_{\mathrm{T}}$ and $\eta$ represent transverse momentum and pseudorapidity, respectively. The $Z$ boson candidates come from leptonic decays into electrons or muons with $p_{\mathrm{T}}>25\mathrm{GeV}$ and $|\eta |<2.4$, and the dilepton mass satisfies $71<m_Z<111\mathrm{GeV}$. The measured values are $\sigma (Z+c\text{jets})/\sigma (Z+\text{jets})=0.102\pm 0.002\pm 0.009$, $\sigma (Z+b\text{jets})/\sigma (Z+\text{jets})=0.0633\pm 0.0004\pm 0.0015$, and $\sigma (Z+c\text{jets})/\sigma (Z+b\text{jets})=1.62\pm 0.03\pm 0.15$. 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.

The results of two searches for pair production of vectorlike $T$ or $B$ quarks in fully hadronic final states are presented, using data from the CMS experiment at a center-of-mass energy of 13 TeV. The data were collected at the LHC during 2016 and correspond to an integrated luminosity of $35.9{\mathrm{fb}}^{-1}$. A cut-based analysis specifically targets the $bW$ decay mode of the $T$ quark and allows for the reconstruction of the $T$ quark candidates. In a second analysis, a multiclassification algorithm, the “boosted event shape tagger,” is deployed to label candidate jets as originating from top quarks, and $W$, $Z$, and $H$. Candidate events are categorized according to the multiplicities of identified jets, and the scalar sum of all observed jet momenta is used to discriminate signal events from the quantum chromodynamics multijet background. Both analyses probe all possible branching fraction combinations of the $T$ and $B$ quarks and set limits at 95% confidence level on their masses, ranging from 740 to 1370 GeV. These results represent a significant improvement relative to existing searches in the fully hadronic final state.

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 $41.1{\mathrm{fb}}^{-1}$. 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 $35.9{\mathrm{fb}}^{-1}$. 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.

Results are reported from a search for new physics beyond the standard model in proton-proton collisions in final states with a single lepton; multiple jets, including at least one jet tagged as originating from the hadronization of a bottom quark; and large missing transverse momentum. The search uses a sample of proton-proton collision data at $\sqrt{s}=13\mathrm{TeV}$, corresponding to $137{\mathrm{fb}}^{-1}$, recorded by the CMS experiment at the LHC. The signal region is divided into categories characterized by the total number of jets, the number of bottom quark jets, the missing transverse momentum, and the sum of masses of large-radius jets. The observed event yields in the signal regions are consistent with estimates of standard model backgrounds based on event yields in the control regions. The results are interpreted in the context of simplified models of supersymmetry involving gluino pair production in which each gluino decays into a top quark-antiquark pair and a stable, unobserved neutralino, which generates missing transverse momentum in the event. Scenarios with gluino masses up to about 2150 GeV are excluded at 95% confidence level (or more) for neutralino masses up to 700 GeV. The highest excluded neutralino mass is about 1250 GeV, which holds for gluino masses around 1850 GeV.

A flavor-tagged time-dependent analysis of $B^0\to K_{\mathrm{S}}^0{\mu }^{+}{\mu }^{-}$ decays is performed across the full dimuon mass range excluding the $J/\psi$ and $\psi (2S)$ resonance regions. The analysis uses proton-proton collision data collected by the LHCb experiment in 2011–2018 at center-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$. The $CP$ violation parameters are determined to be $C=-0.13\pm 0.32\pm 0.04$, $S=+0.82\pm 0.29\pm 0.05$, where the first uncertainties are statistical and the second are systematic. The results are consistent with the standard model prediction. This is the first experimental study of time-dependent $CP$ violation in $b\to s{\ell }^{+}{\ell }^{-}$ processes.

The collinear and transverse-momentum-dependent jet fragmentation function and the radial profile for $B^{\pm }$ mesons in jets are measured. The $B^{\pm }$ mesons are reconstructed through the $J/\psi (\to {\mu }^{+}{\mu }^{-})K^{\pm }$ decay channel using proton-proton collision data collected during 2016–2018 with the LHCb detector at a center-of-mass (c.m.) energy of $\sqrt{s}=13\mathrm{TeV}$, corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$. The results complement recent measurements of jet fragmentation functions for heavy-flavor hadrons and suggest a growing contribution of gluon fragmentation to $B^{\pm }$ mesons as the jet transverse momentum increases.

A sample of $pp$ collision data, corresponding to an integrated luminosity of $5.5{\mathrm{fb}}^{-1}$ and collected by the LHCb experiment during LHC Run 2, is used to measure the ratio of the lifetime of the ${\mathrm{\Xi }}_b^{-}$ baryon to that of the ${\mathrm{\Lambda }}_b^0$ baryon, $r_{\tau }\equiv {\tau }_{{\mathrm{\Xi }}_b^{-}}/{\tau }_{{\mathrm{\Lambda }}_b^0}$. The value $r_{\tau }=1.076\pm 0.013\pm 0.006$ is obtained, where the first uncertainty is statistical and the second systematic. This value is averaged with the corresponding value from Run 1 to obtain $r_{\tau }^{\mathrm{Run}1,2}=1.078\pm 0.012\pm 0.007$. Multiplying by the world-average value of the ${\mathrm{\Lambda }}_b^0$ lifetime yields ${\tau }_{{\mathrm{\Xi }}_b^{-}}^{\mathrm{Run}1,2}=1.578\pm 0.018\pm 0.010\pm 0.011\mathrm{ps}$, where the uncertainties are statistical, systematic, and due to the limited knowledge of the ${\mathrm{\Lambda }}_b^0$ lifetime. This measurement improves the precision of the current world average of the ${\mathrm{\Xi }}_b^{-}$ lifetime by about a factor of 2, and is in good agreement with the most recent theoretical predictions.

The central exclusive production of charged-hadron pairs in $pp$ collisions at a center-of-mass energy of 13 TeV is examined, based on data collected in a special high-${\beta }^{*}$ run of the LHC. The nonresonant continuum processes are studied with the invariant mass of the centrally produced two-pion system in the resonance-free region, $m_{{\pi }^{+}{\pi }^{-}}<0.7$ or $m_{{\pi }^{+}{\pi }^{-}}>1.8\mathrm{GeV}$. Differential cross sections as functions of the azimuthal angle between the surviving protons, squared exchanged four-momenta, and $m_{{\pi }^{+}{\pi }^{-}}$ are measured in a wide region of scattered proton transverse momenta, between 0.2 and 0.8 GeV, and for pion rapidities $|y|<2$. A rich structure of interactions related to double-pomeron exchange is observed. A parabolic minimum in the distribution of the two-proton azimuthal angle is observed for the first time. It can be interpreted as an effect of additional pomeron exchanges between the protons from the interference between the bare and the rescattered amplitudes. After model tuning, various physical quantities are determined that are related to the pomeron cross section, proton-pomeron and meson-pomeron form factors, pomeron trajectory and intercept, and coefficients of diffractive eigenstates of the proton.

A search for the lepton-flavor violating decays $B_s^0\to \varphi {\mu }^{\pm }{\tau }^{\mp }$ is presented, using a sample of proton-proton collisions at center-of-mass energies of 7, 8, and 13 TeV, collected with the LHCb detector and corresponding to a total integrated luminosity of $9{\mathrm{fb}}^{-1}$. The $\tau$ leptons are selected using decays with three charged pions. No significant excess is observed, and an upper limit on the branching fraction is determined to be $\mathcal{B}(B_s^0\to \varphi {\mu }^{\pm }{\tau }^{\mp })<1.0\times {10}^{-5}$ at 90% confidence level.

A search for pair production of scalar and vector leptoquarks (LQs) each decaying to a muon and a bottom quark is performed using proton-proton collision data collected at $\sqrt{s}=13\mathrm{TeV}$ with the CMS detector at the CERN LHC, corresponding to an integrated luminosity of $138{\mathrm{fb}}^{-1}$. No excess above standard model expectation is observed. Scalar (vector) LQs with masses less than 1810 (2120) GeV are excluded at 95% confidence level, assuming a 100% branching fraction of the LQ decaying to a muon and a bottom quark. These limits represent the most stringent to date.

Flow harmonic coefficients, $v_n$, which are the key to studying the hydrodynamics of the quark-gluon plasma (QGP) created in heavy-ion collisions, have been measured in various collision systems and kinematic regions and using various particle species. The study of flow harmonics in a wide pseudorapidity range is particularly valuable to understand the temperature dependence of the shear viscosity to entropy density ratio of the QGP. This paper presents the first LHCb results of the second- and the third-order flow harmonic coefficients of charged hadrons as a function of transverse momentum in the forward region, corresponding to pseudorapidities between 2.0 and 4.9, using the data collected from PbPb collisions in 2018 at a center-of-mass energy of 5.02 $\text{TeV}$. The coefficients measured using the two-particle angular correlation analysis method are smaller than the central-pseudorapidity measurements at ALICE and ATLAS from the same collision system but share similar features.

A search for long-lived particles (LLPs) decaying in the CMS muon detectors is presented. A data sample of proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$ corresponding to an integrated luminosity of $138{\mathrm{fb}}^{-1}$, recorded at the LHC in 2016–2018, is used. The decays of LLPs are reconstructed as high multiplicity clusters of hits in the muon detectors. In the context of twin Higgs models, the search is sensitive to LLP masses from 0.4 to 55 GeV and a broad range of LLP decay modes, including decays to hadrons, $\tau$ leptons, electrons, or photons. No excess of events above the standard model background is observed. The most stringent limits to date from LHC data are set on the branching fraction of the Higgs boson decay to a pair of LLPs with masses below 10 GeV. This search also provides the best limits for various intervals of LLP proper decay length and mass. Finally, this search sets the first limits at the LHC on a dark quantum chromodynamic sector whose particles couple to the Higgs boson through gluon, Higgs boson, photon, vector, and dark-photon portals, and is sensitive to branching fractions of the Higgs boson to dark quarks as low as $2\times {10}^{-3}$.

A search is described for the production of a pair of bottom-type vectorlike quarks ($B$ VLQs) with mass greater than 1000 GeV. Each $B$ VLQ decays into a $b$ quark and a Higgs boson, a $b$ quark and a $Z$ boson, or a $t$ quark and a $W$ boson. This analysis considers both fully hadronic final states and those containing a charged lepton pair from a $Z$ boson decay. The products of the $H\to bb$ boson decay and of the hadronic $Z$ or $W$ boson decays can be resolved as two distinct jets or merged into a single jet, so the final states are classified by the number of reconstructed jets. The analysis uses data corresponding to an integrated luminosity of $138{\mathrm{fb}}^{-1}$ collected in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$ with the CMS detector at the LHC from 2016 to 2018. No excess over the expected background is observed. Lower limits are set on the $B$ VLQ mass at the 95% confidence level. These depend on the $B$ VLQ branching fractions and are 1570 and 1540 GeV for 100% $B\to bH$ and 100% $B\to bZ$, respectively. In most cases, the mass limits obtained exceed previous limits by at least 100 GeV.

A search is described for the production of a pair of bottom-type vectorlike quarks (VLQs), each decaying into a $b$ or $\overline{b}$ quark and either a Higgs or a $Z$ boson, with a mass greater than 1000 GeV. The analysis is based on data from proton-proton collisions at a 13 TeV center-of-mass energy recorded at the CERN LHC, corresponding to a total integrated luminosity of $137{\mathrm{fb}}^{-1}$. As the predominant decay modes of the Higgs and $Z$ bosons are to a pair of quarks, the analysis focuses on final states consisting of jets resulting from the six quarks produced in the events. Since the two jets produced in the decay of a highly Lorentz-boosted Higgs or $Z$ boson can merge to form a single jet, nine independent analyses are performed, categorized by the number of observed jets and the reconstructed event mode. No signal in excess of the expected background is observed. Lower limits are set on the VLQ mass at 95% confidence level equal to 1570 GeV in the case where the VLQ decays exclusively to a $b$ quark and a Higgs boson, 1390 GeV for when it decays exclusively to a $b$ quark and a $Z$ boson, and 1450 GeV for when it decays equally in these two modes. These limits represent significant improvements over the previously published VLQ limits.

A measurement of the Higgs boson Yukawa coupling to the top quark is presented using proton-proton collision data at $\sqrt{s}=13\mathrm{TeV}$, corresponding to an integrated luminosity of $137{\mathrm{fb}}^{-1}$, recorded with the CMS detector. The coupling strength with respect to the standard model value, $Y_t$, is determined from kinematic distributions in $t\overline{t}$ final states containing $ee$, $\mu \mu$, or $e\mu$ pairs. Variations of the Yukawa coupling strength lead to modified distributions for $t\overline{t}$ production. In particular, the distributions of the mass of the $t\overline{t}$ system and the rapidity difference of the top quark and antiquark are sensitive to the value of $Y_t$. The measurement yields a best fit value of ${Y_t=1.16}_{-0.35}^{+0.24}$, bounding $Y_t<1.54$ at a 95% confidence level.