617 Search Results

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 dependence of inclusive jet production in proton-proton collisions with a center-of-mass energy of 13 TeV on the distance parameter R of the anti-k_T algorithm is studied using data corresponding to integrated luminosities up to 35.9 fb^–1 collected by the CMS experiment in 2016. The ratios of the inclusive cross sections as functions of transverse momentum p_T and rapidity y, for R in the range 0.1 to 1.2 to those using R = 0.4 are presented in the region 84 < p_T < 1588 GeV and |y| < 2.0. The results are compared to calculations at leading and next-to-leading ordermore » in the strong coupling constant using different parton shower models. The variation of the ratio of cross sections with R is well described by calculations including a parton shower model, but not by a leading-order quantum chromodynamics calculation including nonperturbative effects. The agreement between the data and the theoretical predictions for the ratios of cross sections is significantly improved when next-to-leading order calculations with nonperturbative effects are used.« less

We describe a method to obtain point and dispersion estimates for the energies of jets arising from b quarks produced in proton–proton collisions at an energy of √s = 13TeV at the CERN LHC. The algorithm is trained on a large sample of simulated b jets and validated on data recorded by the CMS detector in 2017 corresponding to an integrated luminosity of 41 fb^-1. A multivariate regression algorithm based on a deep feed-forward neural network employs jet composition and shape information, and the properties of reconstructed secondary vertices associated with the jet. The results of the algorithm are usedmore » to improve the sensitivity of analyses that make use of b jets in the final state, such as the observation of Higgs boson decay to $$b\bar{b}$$.« less

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 analysismore » 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.« less

Ratios of cross sections, $$\sigma$$(Z+c jets)/$$\sigma$$(Z+jets), $$\sigma$$(Z+b jets)/$$\sigma$$(Z+jets), and $$\sigma$$(Z+c jets)/$$\sigma$$(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 $$|\eta|<$$ 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 GeV and $$|\eta|<$$more » 2.4, and the dilepton mass satisfies 71 $$\lt m_\mathrm{Z}\lt$$ 111 GeV. The measured values are $$\sigma$$(Z+c jets)/$$\sigma$$(Z+jets) = 0.102 $$\pm$$ 0.002 $$\pm$$ 0.009, $$\sigma$$(Z+b jets)/$$\sigma$$(Z+jets) = 0.0633 $$\pm$$ 0.0004 $$\pm$$ 0.0015, and $$\sigma$$(Z+c jets)/$$\sigma$$(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, and the measurement precision is better than that of the current theoretical predictions.« less

Central exclusive production of $$\pi^+\pi^-$$ 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 central $$\pi^+\pi^-$$ production are measured as functions of invariant mass, transverse momentum, and rapiditymore » of the $$\pi^+\pi^-$$ system in the fiducial region defined as transverse momentum $$p_\mathrm{T}(\pi) > $$ 0.2 GeV and pseudorapidity $$|\eta(\pi)| <$$ 2.4. The production cross sections for the four resonant channels f$$_0(500)$$, $$\rho^0(770)$$, 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.« less

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

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$$^\pm$$more » masses in the range of 0.2 to 3 TeV. Combining this with data from 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.« 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

Measurements are presented of the reduction of signal output due to radiation dam-age for two types of plastic scintillator tiles used in the hadron endcap (HE) calorimeter of the CMS detector. The tiles were exposed to particles produced in proton-proton(pp) collisions at the CERN LHC with a center-of-mass energy of 13 TeV, corresponding to a delivered luminosity of 50 fb^–1. The measurements are based on readout channels of the HE that were instrumented with silicon photomultipliers, and are derived using data from several sources: a laser calibration system, a movable radioactive source, as well as hadrons and muons produced inmore » pp collisions. Results from several irradiation campaigns using ⁶⁰Co sources are also discussed. The damage is presented as a function of dose rate. Within the range of these measurements,for a fixed dose the damage increases with decreasing dose rate.« less