275 Search Results

A first search for beyond the standard model physics in jet multiplicity patterns of multilepton events is presented, using a data sample corresponding to an integrated luminosity of 138 fb⁻¹ of 13 TeV proton-proton collisions recorded by the CMS detector at the LHC. The search uses observed jet multiplicity distributions in one-, two-, and four-lepton events to explore possible enhancements in jet production rate in three-lepton events with and without bottom quarks. The data are found to be consistent with the standard model expectation. The results are interpreted in terms of supersymmetric production of electroweak chargino-neutralino superpartners with cascade decaysmore » terminating in prompt hadronic 𝑅-parity violating interactions.« less

Despite the f₀(980) hadron having been discovered half a century ago, the question about its quark content has not been settled: it might be an ordinary quark-antiquark ($$q\bar{q}$$) meson, a tetraquark ($$q\bar{q}$$$$q\bar{q}$$) exotic state, a kaon-antikaon ($$K\bar{K}$$) molecule, or a quark-antiquark-gluon ($$q\bar{q}$$g) hybrid. This paper reports strong evidence that the f₀(980) state is an ordinary $$q\bar{q}$$ meson, inferred from the scaling of elliptic anisotropies (v₂) with the number of constituent quarks (n_q), as empirically established using conventional hadrons in relativistic heavy ion collisions. The f₀(980) state is reconstructed via its dominant decay channel f₀(980) → π⁺π⁻, in proton-lead collisions recordedmore » by the CMS experiment at the LHC, and its v₂ is measured as a function of transverse momentum (p_T). It is found that the n_q = 2 ($$q\bar{q}$$ state) hypothesis is favored over n_q = 4 ($$q\bar{q}$$$$q\bar{q}$$ or $$K\bar{K}$$ states) by 7.7, 6.3, or 3.1 standard deviations in the p_T < 10, 8, or 6 GeV/c ranges, respectively, and over n_q = 3 ($$q\bar{q}$$g hybrid state) by 3.5 standard deviations in the p_T < 8 GeV/c range. This result represents the first determination of the quark content of the f₀(980) state, made possible by using a novel approach, and paves the way for similar studies of other exotic hadron candidates.« less

Salient aspects of the commissioning, calibration, and performance of the CMS silicon strip tracker are discussed, drawing on experience during operation with proton-proton collisions delivered by the CERN LHC. The data were obtained with a variety of luminosities. The operating temperature of the strip tracker was changed several times during this period and results are shown as a function of temperature in several cases. Details of the system performance are presented, including occupancy, signal-to-noise ratio, Lorentz angle, and single-hit spatial resolution. Saturation effects in the APV25 readout chip preamplifier observed during early Run 2 are presented, showing the effect onmore » various observables and the subsequent remedy. Studies of radiation effects on the strip tracker are presented both for the optical readout links and the silicon sensors. The observed effects are compared to simulation, where available, and they generally agree well with expectations.« less

A search for flavor-changing neutral current interactions of the top quark (𝑡) and the Higgs boson (𝐻) is presented. The search is based on proton-proton collision data collected in 2016–2018 at a center-of-mass energy of 13 TeV with the CMS detector at the LHC, and corresponding to an integrated luminosity of 138 fb⁻¹. Events containing a pair of leptons with the same-sign electric charge and at least one jet are considered. The results are used to constrain the branching fraction (ℬ) of the top quark decaying to a Higgs boson and an up (𝑢) or charm (𝑐) quark. No significantmore » excess above the estimated background was found. The observed (expected) upper limits at a 95% confidence level are found to be 0.072% (0.059%) for ℬ⁡(𝑡 → 𝐻⁡𝑢) and 0.043% (0.062%) for ℬ⁡(𝑡 → 𝐻⁡𝑐). These results are combined with two other searches performed by the CMS Collaboration for flavor-changing neutral current interactions of top quarks and Higgs bosons in final states where the Higgs boson decays to either a pair of photons or a pair of bottom quarks. The resulting observed (expected) upper limits at the 95% confidence level are 0.019% (0.027%) for ℬ⁡(𝑡 → 𝐻⁡𝑢) and 0.037% (0.035%) for ℬ⁡(𝑡 → 𝐻⁡𝑐).« less

Measurements of the production yields of charm baryons (Λ$$^+_c$$ ) and charm mesons (D⁰ ) in proton-lead collisions at a nucleon-nucleon center-of-mass energy of 8.16 TeV are presented. The data were collected in 2016 with the CMS experiment and correspond to an integrated luminosity of 186 nb^-1. The Λ$$^+_c$$ baryon is reconstructed from the decay channel Λ$$^+_c$$ → K$$^0_S$$p , while the D⁰ meson is reconstructed via D⁰ → K^-π⁺. The Λ$$^+_c$$ baryon and D⁰ meson yields are extracted in several charged-particle multiplicity classes. No strong multiplicity dependence of the Λ$$^+_c$$-to-D⁰ yield ratio is observed, unlike the observed strange baryonmore » to strange meson yield ratio of $$Λ/\bar{Λ}$$to K$$^0_S$$, which shows a strong multiplicity dependence. This observation indicates different mechanisms for the multiplicity evolution of hadronization processes for charm and strange quarks and provides new constraints to the understanding of heavy flavor production and collectivity in small collision systems.« less

Measurements of fiducial and total inclusive cross sections for W and Z boson production are presented in proton-proton collisions at $$ \sqrt{s} $$ = 5.02 and 13 TeV. Electron and muon decay modes (ℓ = e or μ) are studied in the data collected with the CMS detector in 2017, in dedicated runs with reduced instantaneous luminosity. The data sets correspond to integrated luminosities of 298 ± 6 pb$$^{−1}$$ at 5.02 TeV and 206 ± 5 pb$$^{−1}$$ at 13 TeV. Measured values of the products of the total inclusive cross sections and the branching fractions at 5.02 TeV are σ(ppmore » → W + X)$$ \mathcal{B} $$(W → ℓν) = 7300 ± 10 (stat) ± 60 (syst) ± 140 (lumi) pb, and σ(pp → Z+X)$$ \mathcal{B} $$(Z → ℓ$$^{+}$$ℓ$$^{−}$$) = 669 ± 2 (stat) ± 6 (syst) ± 13 (lumi) pb for the dilepton invariant mass in the range of 60–120 GeV. The corresponding results at 13 TeV are 20480 ± 10 (stat) ± 170 (syst) ± 470 (lumi) pb and 1952 ± 4 (stat) ± 18 (syst) ± 45 (lumi) pb. The measured values agree with cross section calculations at next-to-next-to-leading-order in perturbative quantum chromodynamics. Fiducial and total inclusive cross sections, ratios of cross sections of W$$^{+}$$ and W$$^{−}$$ production as well as inclusive W and Z boson production, and ratios of these measurements at 5.02 and 13 TeV are reported.« less

A search is presented for fractionally charged particles with charges below 1⁢𝑒, using their small energy loss in the tracking detector as a key variable to observe a signal. The analyzed dataset corresponds to an integrated luminosity of 138 fb⁻¹ of proton-proton collisions collected at $$\sqrt{𝑠}$$ = 13 TeV in 2016–2018 at the CERN LHC. This is the first search at the LHC for new particles with a charge between 𝑒/3 and 0.9⁢𝑒, including an extension of previous results at a charge of 2⁢𝑒/3. Masses up to 640 GeV and charges as low as 𝑒/3 are excluded at 95% confidencemore » level. These are the most stringent limits to date for the considered Drell-Yan-like production mode.« less

A measurement of the ratio of branching fractions 𝑅⁢(𝐽/𝜓) =ℬ⁡(𝐵$$^+_𝑐$$ →𝐽/𝜓⁢𝜏⁺⁢𝜈_𝜏)/ℬ⁡(𝐵$$^+_𝑐$$ →𝐽/𝜓⁢𝜇⁺⁢𝜈_𝜇) in the 𝐽/𝜓 →𝜇⁺⁢𝜇⁻, 𝜏⁺→𝜇⁺⁢𝜈_𝜇⁢$$\bar{𝜈}$$_𝜏 decay channel is presented. This measurement uses a sample of proton-proton collision data collected at a center-of-mass energy of 13 TeV by the CMS experiment in 2018, corresponding to an integrated luminosity of 59.7 fb⁻¹. The measured ratio, 𝑅⁢(𝐽/𝜓) = 0.1⁢7$$^{+0.18}_{−0.17}⁢$$(stat)$$^{+0.21}_{−0.22⁢}$$(syst)$$^{+0.19}_{−0.18}$$⁢(theo) =0.17 ± 0.33, agrees with the value of 0.2582 ± 0.0038 predicted by the standard model, which assumes lepton flavor universality. By testing lepton flavor universality, this measurement is a probe of new physics using 𝐵$$^+_𝑐$$ mesons, which are currently only producedmore » at the LHC.« less

A search is presented for an extended Higgs sector with two new particles, 𝑋 and 𝜙, in the process 𝑋 → 𝜙⁢𝜙 → (𝛾⁢𝛾)⁢(𝛾⁢𝛾). 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 thismore » channel.« less

A search is performed for charged-lepton flavor violating processes in top quark (𝑡) production and decay. The data were collected by the CMS experiment from proton-proton collisions at a center-of-mass energy of 13 TeV and correspond to an integrated luminosity of 138 fb⁻¹. The selected events are required to contain one opposite-sign electron-muon pair, a third charged lepton (electron or muon), and at least one jet of which no more than one is associated with a bottom quark. Boosted decision trees are used to distinguish signal from background, exploiting differences in the kinematics of the final states particles. The datamore » are consistent with the standard model expectation. Upper limits at 95% confidence level are placed in the context of effective field theory on the Wilson coefficients, which range between 0.024–0.424 TeV⁻² depending on the flavor of the associated light quark and the Lorentz structure of the interaction. These limits are converted to upper limits on branching fractions involving up (charm) quarks, 𝑡 → 𝑒⁢𝜇⁢𝑢 (𝑡 → 𝑒⁢𝜇⁢𝑐), of 0.032⁢(0.498) × 10⁻⁶, 0.022⁢(0.369) × 10⁻⁶, and 0.012⁢(0.216) × 10⁻⁶ for tensorlike, vectorlike, and scalarlike interactions, respectively.« less