186 Search Results

A first study is presented of the cross section for the scattering of same-sign W boson pairs via the detection of a τ lepton. The data from proton-proton collisions at the center-of-mass energy of 13 TeV were collected by the CMS detector at the LHC, and correspond to an integrated luminosity of 138 fb⁻¹. Events were selected that contain two jets with large pseudorapidity and large invariant mass, one τ lepton, one light lepton (e or μ), and significant missing transverse momentum. The measured cross section for electroweak same-sign WW scattering is $$1.44^{+0.63}_{-0.56}$$ times the standard model prediction. In addition,more » a search is presented for the indirect effects of processes beyond the standard model via the effective field theory framework, in terms of dimension-6 and dimension-8 operators.« 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

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 light-by-light scattering (LbL, γγ → γγ) and the Breit-Wheeler process (BW, γγ → e⁺e⁻) are reported in ultraperipheral PbPb collisions at a centre-of-mass energy per nucleon pair of 5.02 TeV. The data sample, corresponding to an integrated luminosity of 1.7 nb⁻¹, was collected by the CMS experiment at the CERN LHC in 2018. Events with an exclusively produced γγ or e⁺e⁻ pair with invariant masses m^γγ,ee > 5 GeV, along with other fiducial criteria, are selected. The measured BW fiducial production cross section, σ_fid(γγ → e⁺e⁻) = 263.5 ± 1.8(stat) ± 17.8(syst) μb, as well as the differentialmore » distributions for various kinematic observables, are in agreement with leading-order quantum electrodynamics predictions complemented with final-state photon radiation. The measured differential BW cross sections allow discrimination between different theoretical descriptions of the photon flux of the lead ion. In the LbL final state, 26 exclusive diphoton candidate events are observed compared with 12.0 ± 2.9 expected for the background. Combined with previous results, the observed significance of the LbL signal with respect to the background-only hypothesis is above five standard deviations. The measured fiducial LbL scattering cross section, σ_fid(γγ → γγ) = 107 ± 24(stat) ± 13(syst) nb, is in agreement with next- to-leading-order predictions. Limits on the production of axion-like particles coupled to photons are set over the mass range 5–100 GeV, including the most stringent limits to date in the range of 5–10 GeV.« less

A measurement of the Higgs boson mass and width via its decay to two 𝑍 bosons is presented. Proton-proton collision data collected by the CMS experiment, corresponding to an integrated luminosity of 138  fb⁻¹ at a center-of-mass energy of 13 TeV, is used. The invariant mass distribution of four leptons in the on-shell Higgs boson decay is used to measure its mass and constrain its width. This yields the most precise single measurement of the Higgs boson mass to date, 125.04 ± 0.12  GeV, and an upper limit on the width Γ_𝐻 < 330 MeV at 95% confidence level. A combination ofmore » the on- and off-shell Higgs boson production decaying to four leptons is used to determine the Higgs boson width, assuming that no new virtual particles affect the production, a premise that is tested by adding new heavy particles in the gluon fusion loop model. This result is combined with a previous CMS analysis of the off-shell Higgs boson production with decay to two leptons and two neutrinos, giving a measured Higgs boson width of 3.0$$^{+2.0}_{−1.5}$$ MeV, in agreement with the standard model prediction of 4.1 MeV. The strength of the off-shell Higgs boson production is also reported. The scenario of no off-shell Higgs boson production is excluded at a confidence level corresponding to 3.8 standard deviations.« 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

An analysis is presented based on models of the intrinsic transverse momentum (intrinsic 𝑘_T) of partons in nucleons by studying the dilepton transverse momentum in Drell-Yan events. Using parameter tuning in event generators and existing data from fixed-target experiments and from hadron colliders, our investigation spans 3 orders of magnitude in center-of-mass energy and 2 orders of magnitude in dilepton invariant mass. The results show an energy-scaling behavior of the intrinsic 𝑘_T parameters, independent of the dilepton invariant mass at a given center-of-mass energy.

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

Nuclear medium effects on 𝐵⁺ meson production are studied using the binary-collision scaled cross section ratio between events of different charged-particle multiplicities from proton-lead collisions. Data, collected by the CMS experiment in 2016 at a nucleon-nucleon center-of-mass energy of $$\sqrt{𝑠_{NN}}$$ = 8.16 TeV, corresponding to an integrated luminosity of 175 nb⁻¹, were used. The scaling factors in the ratio are determined using a novel approach based on the 𝑍 → 𝜇⁻⁢𝜇⁺ cross sections measured in the same events. The scaled ratio for 𝐵⁺ is consistent with unity for all event multiplicities, putting stringent constraints on nuclear modification for heavy flavor.

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