283 Search Results

The second-order Fourier coefficients ( $v_2$) characterizing the azimuthal distributions of Υ(1S) and Υ(2S) mesons produced in PbPb collisions at $\sqrt{s_{{}_{\mathrm{NN}}}}=5.02\text{TeV}$ are studied. The Υ mesons are reconstructed in their dimuon decay channel, as measured by the CMS detector. The collected data set corresponds to an integrated luminosity of $1.7{\text{nb}}^{-1}$. The scalar product method is used to extract the ${more »}_{}$ v 2 coefficients of the azimuthal distributions. Results are reported for the rapidity range $\left|y\right|<2.4$, in the transverse momentum interval $0<p_{\mathrm{T}}<50\text{GeV}/c$, and in three centrality ranges of 10–30%, 30–50% and 50–90%. In contrast to the $\mathrm{J}/\mathit{\psi }$ mesons, the measured $v_2$ values for the Image 5 mesons are found to be consistent with zero.« less

The strong Coulomb field created in ultrarelativistic heavy ion collisions is expected to produce a rapidity-dependent difference ( Δv₂ ) in the second Fourier coefficient of the azimuthal distribution (elliptic flow, v₂ ) between D⁰ ( $$\bar{u}c$$ ) and $${\mathrm{\overline{D}}{}^0}$$ ( $$u\bar{c}$$ ) mesons. Motivated by the search for evidence of this field, the CMS detector at the LHC is used to perform the first measurement of Δv₂. The rapidity-averaged value is found to be ⟨Δv2⟩=0.001±0.001(stat)±0.003(syst) in PbPb collisions at $$\sqrt{s_{NN}}$$=5.02 TeV . In addition, the influence of the collision geometry is explored by measuring the D⁰ and $${\mathrm{\overline{D}}{}^0}$$ mesonsmore » v₂ and triangular flow coefficient ( v₃ ) as functions of rapidity, transverse momentum ( p_T ), and event centrality (a measure of the overlap of the two Pb nuclei). A clear centrality dependence of prompt D⁰ meson v₂ values is observed, while the v₃ is largely independent of centrality. These trends are consistent with expectations of flow driven by the initial-state geometry.« less

Event-by-event long-range correlations of azimuthal anisotropy Fourier coefficients ($$v_n$$) in 8.16 TeV pPb data, collected by the CMS experiment at the CERN Large Hadron Collider, are extracted using a subevent four-particle cumulant technique applied to very low multiplicity events. Each combination of four charged particles is selected from either two, three, or four distinct subevent regions of a pseudorapidity range from -2.4 to 2.4 of the CMS tracker, and with transverse momentum between 0.3 and 3.0 GeV. Using the subevent cumulant technique, correlations between $$v_n$$ of different orders are measured as functions of particle multiplicity and compared to the standardmore » cumulant method without subevents over a wide event multiplicity range. At high multiplicities, the $$v_2$$ and $$v_3$$ coefficients exhibit an anticorrelation; this behavior is observed consistently using various methods. The $$v_2$$ and $$v_4$$ correlation strength is found to depend on the number of subevents used in the calculation. As the event multiplicity decreases, the results from different subevent methods diverge because of different contributions of noncollective or few-particle correlations. Correlations extracted with the four-subevent method exhibit a tendency to diminish monotonically toward the lowest multiplicity region (about 20 charged tracks) investigated. These findings extend previous studies to a significantly lower event multiplicity range and establish the evidence for the onset of long-range collective multiparticle correlations in small system collisions.« less

The first observation of the $t\overline{t}H$ process in a single Higgs boson decay channel with the full reconstruction of the final state ( $H\to \gamma \gamma$) is presented, with a significance of 6.6 standard deviations ( $\sigma$). 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 $3.2\sigma$. The measurements are based on a sample of proton-proton collisions at a center-of-mass energy $\sqrt{s}=13\mathrm{TeV}$ collected bymore » the CMS detector at the LHC, corresponding to an integrated luminosity of $137{\mathrm{fb}}^{–1}$. The cross section times branching fraction of the $t\overline{t}H$ process is measured to be ${\sigma }_{t\overline{t}H}{\mathcal{B}}_{\gamma \gamma }={1.56}_{–0.32}^{+0.34}\mathrm{fb}$, which is compatible with the standard model prediction of ${1.13}_{–0.11}^{+0.08}\mathrm{fb}$. The fractional contribution of the $CP$-odd component is measured to be $f_{CP}^{Htt}=0.00\pm 0.33$.« less

The transverse momentum $\left(p_{\mathrm{T}}\right)$ distributions of $\mathrm{\Lambda }$, ${\mathrm{\Xi }}^{-}$, and ${\mathrm{\Omega }}^{-}$ baryons, their antiparticles, and $K_S^0$ mesons are measured in proton-proton $\left(pp\right)$ and proton-lead $\left(p\mathrm{Pb}\right)$ 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 ${\mathrm{nb}}^{-1}$ and 15.6 ${\mu \mathrm{b}}^{-more »}$ 1 for $pp$ and $p\mathrm{Pb}$ collisions, respectively, were collected by the CMS experiment. The nuclear modification factor $R_{p\text{Pb}}$, which is defined as the ratio of the particle yield in $p\mathrm{Pb}$ 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 $R_{p\text{Pb}}$ for $K_S^0$ is consistent with unity, while an enhancement ordered by strangeness content and/or particle mass is observed for the three baryons. In $p\mathrm{Pb}$ 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 with predictions from epos lhc, which includes parametrized radial flow. The model is in qualitative agreement with the $R_{p\text{Pb}}$ data, but fails to describe the dependence on particle species in the yield asymmetries measured away from midrapidity in $p\mathrm{Pb}$ collisions.« less

Machine-learning (ML) techniques are explored to identify and classify hadronic decays of highly Lorentz-boosted W/Z/Higgs bosons and top quarks. Techniques without ML have also been evaluated and are included for comparison. The identification performances of a variety of algorithms are characterized in simulated events and directly compared with data. The algorithms are validated using proton-proton collision data at $$\sqrt{s}$$ = 13TeV, corresponding to an integrated luminosity of 35.9 fb^-1. Systematic uncertainties are assessed by comparing the results obtained using simulation and collision data. The new techniques studied in this paper provide significant performance improvements over non-ML techniques, reducing the background rate bymore » up to an order of magnitude at the same signal efficiency.« less