Showing papers by "Serguei Afanasiev published in 2017"

Particle-flow reconstruction and global event description with the CMS detector

Abstract: The CMS apparatus was identified, a few years before the start of the LHC operation at CERN, to feature properties well suited to particle-flow (PF) reconstruction: a highly-segmented tracker, a fine-grained electromagnetic calorimeter, a hermetic hadron calorimeter, a strong magnetic field, and an excellent muon spectrometer. A fully-fledged PF reconstruction algorithm tuned to the CMS detector was therefore developed and has been consistently used in physics analyses for the first time at a hadron collider. For each collision, the comprehensive list of final-state particles identified and reconstructed by the algorithm provides a global event description that leads to unprecedented CMS performance for jet and hadronic τ decay reconstruction, missing transverse momentum determination, and electron and muon identification. This approach also allows particles from pileup interactions to be identified and enables efficient pileup mitigation methods. The data collected by CMS at a centre-of-mass energy of 8\TeV show excellent agreement with the simulation and confirm the superior PF performance at least up to an average of 20 pileup interactions.

Jet energy scale and resolution in the CMS experiment in pp collisions at 8 TeV

Abstract: Improved jet energy scale corrections, based on a data sample corresponding to an integrated luminosity of 19.7 fb^(-1) collected by the CMS experiment in proton-proton collisions at a center-of-mass energy of 8 TeV, are presented. The corrections as a function of pseudorapidity η and transverse momentum p_T are extracted from data and simulated events combining several channels and methods. They account successively for the effects of pileup, uniformity of the detector response, and residual data-simulation jet energy scale differences. Further corrections, depending on the jet flavor and distance parameter (jet size) R, are also presented. The jet energy resolution is measured in data and simulated events and is studied as a function of pileup, jet size, and jet flavor. Typical jet energy resolutions at the central rapidities are 15–20% at 30 GeV, about 10% at 100 GeV, and 5% at 1 TeV. The studies exploit events with dijet topology, as well as photon+jet, Z+jet and multijet events. Several new techniques are used to account for the various sources of jet energy scale corrections, and a full set of uncertainties, and their correlations, are provided. The final uncertainties on the jet energy scale are below 3% across the phase space considered by most analyses (p_T > 30 GeV and 0|η| 30 GeV is reached, when excluding the jet flavor uncertainties, which are provided separately for different jet flavors. A new benchmark for jet energy scale determination at hadron colliders is achieved with 0.32% uncertainty for jets with p_T of the order of 165–330 GeV, and |η| < 0.8.

Springer : Search for dark matter and unparticles in events with a Z boson and missing transverse momentum in proton-proton collisions at $ \sqrt{s}=13 $ TeV

Abstract: In the original paper, figure 10 was incorrect. The correct figure is shown below. Additionally, the unparticle entries in table 3, as well as figure 4 and 5 were labelled with incorrect values of ΛU.

Observation of Charge-Dependent Azimuthal Correlations in p−Pb Collisions and Its Implication for the Search for the Chiral Magnetic Effect

Abstract: Charge-dependent azimuthal particle correlations with respect to the second-order event plane in pPb and PbPb collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV have been studied with the CMS experiment at the LHC. The measurement is performed with a three-particle correlation technique, using two particles with the same or opposite charge within the pseudorapidity range abs(eta)<2.4, and a third particle measured in the hadron forward calorimeters (4.4< abs(eta)<5). The observed differences between the same and opposite sign correlations, as functions of multiplicity and eta gap between the two charged particles, are of similar magnitude in pPb and PbPb collisions at the same multiplicities. These results pose a challenge for the interpretation of charge-dependent azimuthal correlations in heavy ion collisions in terms of the chiral magnetic effect.

Measurement of charged pion, kaon, and proton production in proton-proton collisions at √s = 13 TeV

Abstract: Transverse momentum spectra of charged pions, kaons, and protons are measured in proton-proton collisions at root s = 13 TeV with the CMS detector at the LHC. The particles, identified via their energy loss in the silicon tracker, are measured in the transverse momentum range of p(T) approximate to 0.1-1.7 GeV/c and rapidities vertical bar y vertical bar < 1. The pT spectra and integrated yields are compared to previous results at smaller ffiffiffi root s and to predictions of Monte Carlo event generators. The average pT increases with particle mass and charged particle multiplicity of the event. Comparisons with previous CMS results at root s = 0.9, 2.76, and 7 TeV show that the average pT and the ratios of hadron yields feature very similar dependences on the particle multiplicity in the event, independently of the center-of-mass energy of the pp collision.

Relative modification of prompt ψ (2S) and J /ψ yields from pp to PbPb collisions at sNN =5.02 TeV

Abstract: The relative modification of the prompt ψ(2S) and J/ψ yields from pp to PbPb collisions, at the center-of-mass energy of 5.02 TeV per nucleon pair, is presented. The analysis is based on pp and PbPb data samples collected by the CMS experiment at the LHC in 2015, corresponding to integrated luminosities of 28.0 pb^(−1) and 464 μb^(−1), respectively. The double ratio of measured yields of prompt charmonia reconstructed through their decays into muon pairs, (N_ψ(2S)/N_J/ψ)_PbPb/(N_ψ(2S)/N_J/ψ)_pp, is determined as a function of PbPb collision centrality and charmonium transverse momentum p_T, in two kinematic intervals: |y| < 1.6 covering 6.5 < p_T < 30 GeV/c and 1.6 < |y| < 2.4 covering 3 < p_T < 30 GeV/c. The centrality-integrated double ratios are 0.36 ± 0.08 (stat) ± 0.05 (syst) in the first interval and 0.24 ± 0.22 (stat) ± 0.09 (syst) in the second. The double ratio is lower than unity in all the measured bins, suggesting that the ψ(2S) yield is more suppressed than the J/ψ yield in the explored phase space.

Search for associated production of a Z boson with a single top quark and for tZ flavour-changing interactions in pp collisions at s√=8 TeV

Abstract: A search for the production of a single top quark in association with a Z boson is presented, both to identify the expected standard model process and to search for flavour-changing neutral current interactions. The data sample corresponds to an integrated luminosity of 19.7 fb^(−1) recorded by the CMS experiment at the LHC in proton-proton collisions at √s = 8 TeV. Final states with three leptons (electrons or muons) and at least one jet are investigated. An events yield compatible with tZq standard model production is observed, and the corresponding cross section is measured to be σ(pp → tZq → lνbl^+l^−q) = 10_(−7)^(+8) fb with a significance of 2.4 standard deviations. No presence of flavour-changing neutral current production of tZq is observed. Exclusion limits at 95% confidence level on the branching fractions of a top quark decaying to a Z boson and an up or a charm quark are found to be ℬ(t → Zu) < 0.022% and ℬ(t → Zc) < 0.049%.

Measurement of the t t ¯ production cross section using events with one lepton and at least one jet in pp collisions at √s=13 TeV

Abstract: A measurement of the tt production cross section at √s = 13 TeV is presented using proton-proton collisions, corresponding to an integrated luminosity of 2.2 fb^(−1), collected with the CMS detector at the LHC. Final states with one isolated charged lepton (electron or muon) and at least one jet are selected and categorized according to the accompanying jet multiplicity. From a likelihood fit to the invariant mass distribution of the isolated lepton and a jet identified as coming from the hadronization of a bottom quark, the cross section is measured to be σ_(tt) = 888 ± 2,(stat)^(+26)_(−28)(syst)±20(lumi)pb, in agreement with the standard model prediction. Using the expected dependence of the cross section on the pole mass of the top quark (m_t), the value of m_t is found to be 170.6 ± 2.7 GeV.

Search for single production of vector-like quarks decaying to a Z boson and a top or a bottom quark in proton-proton collisions at √s=13 TeV

Abstract: A search for single production of vector-like quarks, T and B, decaying into a Z boson and a top or a bottom quark, respectively, is presented. The search is performed using data collected by the CMS experiment at the LHC in proton-proton collisions at sqrt(s) = 13 TeV, corresponding to an integrated luminosity of 2.3 inverse femtobarns. An exotic T quark production mode through the decay of a heavy Z' resonance is also considered. The search is performed in events with a Z boson decaying leptonically, accompanied by a bottom or a top quark decaying hadronically. No excess of events is observed over the standard model background expectation. Products of production cross section and branching fraction for T and B quarks from 1.26 and 0.13 pb are excluded at 95% confidence level for the range of resonance mass considered, which is between 0.7 and 1.7 TeV. Limits on the product of the Z' boson production cross section and branching fraction, with the Z' boson decaying to the Tt final state, are set between 0.31 and 0.13 pb, for Z' boson masses in the range from 1.5 to 2.5 TeV. This is the first search at 13 TeV for single production of vector-like quarks in events with a Z boson decaying leptonically accompanied by boosted jets.

Search for electroweak production of charginos and neutralinos in WH events in proton-proton collisions at √s=13 TeV

Abstract: Results are reported from a search for physics beyond the standard model in proton-proton collision events with a charged lepton (electron or muon), two jets identified as originating from a bottom quark decay, and significant imbalance in the transverse momentum. The search was performed using a data sample corresponding to 35.9 fb$^{−1}$, collected by the CMS experiment in 2016 at $ \sqrt{s}=13 $ TeV. Events with this signature can arise, for example, from the electroweak production of gauginos, which are predicted in models based on supersymmetry. The event yields observed in data are consistent with the estimated standard model backgrounds. Limits are obtained on the cross sections for chargino-neutralino $ \left({\tilde{\chi}}_1^{\pm }{\tilde{\chi}}_2^0\right) $ production in a simplified model of supersymmetry with the decays $ {\chi}_1^{\pm}\to {\mathrm{W}}^{\pm }{\tilde{\chi}}_1^0\kern0.5em \mathrm{and}\kern0.5em {\tilde{\chi}}_2^0\to \mathrm{H}{\chi}_1^0 $ . Values of $ {m}_{{\tilde{\chi}}_1^{\pm }} $ between 220 and 490 GeV are excluded at 95% confidence level by this search when the $ {\tilde{\chi}}_1^0 $ is massless, and values of $ {m}_{{\tilde{\chi}}_1^0} $ are 1 excluded up to 110 GeV for $ {m}_{{\tilde{\chi}}_1^{\pm }}\approx 450 $ GeV.

Measurements of jet charge with dijet events in pp collisions at √s = 8 TeV

Abstract: Jet charge is an estimator of the electric charge of a quark, antiquark, or gluon initiating a jet. It is based on the momentum-weighted sum of the electric charges of the jet constituents. Measurements of three charge observables of the leading jet in transverse momentum pT are performed with dijet events. The analysis is carried out with data collected by the CMS experiment at the CERN LHC in proton-proton collisions at root s = 8 TeV corresponding to an integrated luminosity of 19.7 fb(-1). The results are presented as a function of the pT of the leading jet and compared to predictions from leading and next-to-leading-order event generators combined with parton showers. Measured jet charge distributions, unfolded for detector effects, are reported, which expand on previous measurements of the jet charge average and standard deviation in pp collisions.

Measurement of the transverse momentum spectrum of the Higgs boson produced in pp collisions at s=8 TeV using H → WW decays

Abstract: The cross section for Higgs boson production in pp collisions is studied using the H → W+W− decay mode, followed by leptonic decays of the W bosons to an oppositely charged electron-muon pair in the final state. The measurements are performed using data collected by the CMS experiment at the LHC at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 19.4 fb−1. The Higgs boson transverse momentum (pT) is reconstructed using the lepton pair pT and missing pT. The differential cross section times branching fraction is measured as a function of the Higgs boson pT in a fiducial phase space defined to match the experimental acceptance in terms of the lepton kinematics and event topology. The production cross section times branching fraction in the fiducial phase space is measured to be 39 ± 8 (stat) ± 9 (syst) fb. The measurements are found to agree, within experimental uncertainties, with theoretical calculations based on the standard model.

Principal-component analysis of two-particle azimuthal correlations in PbPb and pPb collisions at CMS

Abstract: For the first time a principle-component analysis is used to separate out different orthogonal modes of the two-particle correlation matrix from heavy ion collisions. The analysis uses data from sNN=2.76TeV PbPb and sNN=5.02TeV pPb collisions collected by the CMS experiment at the CERN Large Hadron Collider. Two-particle azimuthal correlations have been extensively used to study hydrodynamic flow in heavy ion collisions. Recently it was shown that the expected factorization of two-particle results into a product of the constituent single-particle anisotropies is broken. The new information provided by these modes may shed light on the breakdown of flow factorization in heavy ion collisions. The first two modes (“leading” and “subleading”) of two-particle correlations are presented for elliptical and triangular anisotropies in PbPb and pPb collisions as a function of pT over a wide range of event activity. The leading mode is found to be essentially equivalent to the anisotropy harmonic previously extracted from two-particle correlation methods. The subleading mode represents a new experimental observable and is shown to account for a large fraction of the factorization breaking recently observed at high transverse momentum. The principle-component analysis technique was also applied to multiplicity fluctuations. These also show a subleading mode. The connection of these new results to previous studies of factorization is discussed.

Search for a light pseudoscalar Higgs boson produced in association with bottom quarks in pp collisions at sqrt(s)=8 TeV

Abstract: A search for a light pseudoscalar Higgs boson (A) produced in association with bottom quarks and decaying into a muon pair is reported. The search uses 19.7 inverse femtobarns of proton-proton collisions at a center-of-mass energy of 8 TeV, collected by the CMS experiment. No signal is observed in the dimuon mass range from 25 to 60 GeV. Upper limits on the cross section times branching fraction, sigma(pp to b bbar A) B(A to mu mu), are set.

Measurement of the top quark mass using single top quark events in proton-proton collisions at √s = 8 TeV

Abstract: A measurement of the top quark mass is reported in events containing a single top quark produced via the electroweak t channel. The analysis is performed using data from proton-proton collisions collected with the CMS detector at the LHC at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 19.7 fb$^{-1}$ . Top quark candidates are reconstructed from their decay to a $\mathrm {W}$ boson and a b quark, with the $\mathrm {W}$ boson decaying leptonically to a muon and a neutrino. The final state signature and kinematic properties of single top quark events in the t channel are used to enhance the purity of the sample, suppressing the contribution from top quark pair production. A fit to the invariant mass distribution of reconstructed top quark candidates yields a value of the top quark mass of $172.95 \pm 0.77\,\text {(stat)} ^{+0.97-0.93}\,\text {(syst)} \,\text {GeV} $ . This result is in agreement with the current world average, and represents the first measurement of the top quark mass in event topologies not dominated by top quark pair production, therefore contributing to future averages with partially uncorrelated systematic uncertainties and a largely uncorrelated statistical uncertainty.

Measurement of the top quark mass in the dileptonic t(t)over-bar decay channel using the mass observables M-bl, M-T2, and M-blv in pp collisions at root=8 TeV

Abstract: SEIDI, CPAN, PCTI and FEDER (Spain). Individuals have received support from the Marie-Curie program and the European Research Council and EPLANET (European Union); the Programa Clarin-COFUND del Principado de Asturias (...)

Measurement of the inclusive energy spectrum in the very forward direction in proton-proton collisions at √s=13 TeV

Abstract: The differential cross section for inclusive particle production as a function of energy in proton-proton collisions at a center-of-mass energy of 13 TeV is measured in the very forward region of the CMS detector. The measurement is based on data collected with the CMS apparatus at the LHC, and corresponds to an integrated luminosity of 0.34 mu b(-1). The energy is measured in the CASTOR calorimeter, which covers the pseudorapidity region -6.6 < eta < -5.2. The results are given as a function of the total energy deposited in CASTOR, as well as of its electromagnetic and hadronic components. The spectra are sensitive to the modeling of multiparton interactions in pp collisions, and provide new constraints for hadronic interaction models used in collider and in high energy cosmic ray physics.

arXiv : Measurement of b hadron lifetimes in pp collisions at $\sqrt{s} =$ 8 TeV

Abstract: Measurements are presented of the lifetimes of the $${\\mathrm {B}^0}$$B0, $$\\mathrm {B}^0_\\mathrm {s}$$Bs0, $$\\mathrm {\\Lambda }_\\mathrm {b} ^0$$Λb0, and $$\\mathrm {B}_\\mathrm {c}^+$$Bc+ hadrons using the decay channels $${\\mathrm {B}^0}\\!\\rightarrow \\! \\mathrm {J}/\\psi \\mathrm {K^{*}(892)}{}^{0}$$B0→J/ψK∗(892)0, $${\\mathrm {B}^0}\\!\\rightarrow \\! \\mathrm {J}/\\psi \\mathrm {K^0_S}$$B0→J/ψKS0, $$\\mathrm {B}^0_\\mathrm {s} \\!\\rightarrow \\! \\mathrm {J}/\\psi \\mathrm {\\pi ^+}\\mathrm {\\pi ^-}$$Bs0→J/ψπ+π-, $$\\mathrm {B}^0_\\mathrm {s} \\!\\rightarrow \\! \\mathrm {J}/\\psi \\mathrm {\\phi (1020)}$$Bs0→J/ψϕ(1020), $$\\varLambda _\\mathrm {b}^0\\!\\rightarrow \\!\\mathrm {J}/\\psi \\mathrm {\\Lambda }^{0}$$Λb0→J/ψΛ0, and $$\\mathrm {B}_\\mathrm {c}^+ \\!\\rightarrow \\!\\mathrm {J}/\\psi \\mathrm {\\pi ^+}$$Bc+→J/ψπ+. The data sample, corresponding to an integrated luminosity of 19.7$$\\,\\text {fb}^\\text {-1}$$fb-1, was collected by the CMS detector at the LHC in proton–proton collisions at $$\\sqrt{s}=8\\,\\text {Te}\\text {V} $$s=8TeV. The $${\\mathrm {B}^0}$$B0 lifetime is measured to be $$453.0 \\pm 1.6\\,\\text {(stat)} \\pm 1.8\\,\\text {(syst)} \\,\\upmu \\text {m} $$453.0±1.6(stat)±1.8(syst)μm in $$\\mathrm {J}/\\psi \\mathrm {K^{*}(892)}{}^{0}$$J/ψK∗(892)0and $$457.8 \\pm 2.7\\,\\text {(stat)} \\pm 2.8\\,\\text {(syst)} \\,\\upmu \\text {m} $$457.8±2.7(stat)±2.8(syst)μm in $$ \\mathrm {J}/\\psi \\mathrm {K^0_S}$$J/ψKS0, which results in a combined measurement of $$c\\tau _{{\\mathrm {B}^0}} = 454.1 \\pm 1.4\\,\\text {(stat)} \\pm 1.7\\,\\text {(syst)} \\,\\upmu \\text {m} $$cτB0=454.1±1.4(stat)±1.7(syst)μm. The effective lifetime of the $$\\mathrm {B}^0_\\mathrm {s}$$Bs0 meson is measured in two decay modes, with contributions from different amounts of the heavy and light eigenstates. This results in two different measured lifetimes: $$c\\tau _{\\mathrm {B}^0_\\mathrm {s} \\rightarrow \\mathrm {J}/\\psi \\mathrm {\\pi ^+}\\mathrm {\\pi ^-}} = 502.7 \\pm 10.2\\,\\text {(stat)} \\pm 3.4\\,\\text {(syst)} \\,\\upmu \\text {m} $$cτBs0→J/ψπ+π-=502.7±10.2(stat)±3.4(syst)μm and $$c\\tau _{\\mathrm {B}^0_\\mathrm {s} \\rightarrow \\mathrm {J}/\\psi \\mathrm {\\phi (1020)}} = 443.9 \\pm 2.0\\,\\text {(stat)} \\pm 1.5\\,\\text {(syst)} \\,\\upmu \\text {m} $$cτBs0→J/ψϕ(1020)=443.9±2.0(stat)±1.5(syst)μm. The $$\\mathrm {\\Lambda }_\\mathrm {b} ^0$$Λb0 lifetime is found to be $$442.9 \\pm 8.2\\,\\text {(stat)} \\pm 2.8\\,\\text {(syst)} \\,\\upmu \\text {m} $$442.9±8.2(stat)±2.8(syst)μm. The precision from each of these channels is as good as or better than previous measurements. The $$\\mathrm {B}_\\mathrm {c}^+$$Bc+ lifetime, measured with respect to the $${\\mathrm {B}^{+}}$$B+ to reduce the systematic uncertainty, is $$162.3 \\pm 7.8\\,\\text {(stat)} \\pm 4.2\\,\\text {(syst)} \\pm 0.1\\,(\\tau _{{\\mathrm {B}^{+}}})\\,\\upmu \\text {m} $$162.3±7.8(stat)±4.2(syst)±0.1(τB+)μm. All results are in agreement with current world-average values.

Challenges to the chiral magnetic wave using charge-dependent azimuthal anisotropies in pPb and PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV : arXiv

Abstract: Charge-dependent anisotropy Fourier coefficients ($v_n$) of particle azimuthal distributions are measured in pPb and PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV with the CMS detector at the LHC. The normalized difference in the second-order anisotropy coefficients ($v_2$) between positively and negatively charged particles is found to depend linearly on the observed event charge asymmetry with comparable slopes for both pPb and PbPb collisions over a wide range of charged particle multiplicity. In PbPb, the third-order anisotropy coefficient, $v_3$, shows a similar linear dependence with the same slope as seen for $v_2$. The observed similarities between the $v_2$ slopes for pPb and PbPb, as well as the similar slopes for $v_2$ and $v_3$ in PbPb, are compatible with expectations based on local charge conservation in the decay of clusters or resonances, and constitute a challenge to the hypothesis that the observed charge asymmetry dependence of $v_2$ in heavy ion collisions arises from a chiral magnetic wave.

Exclusive and semi-exclusive pi+pi- production in proton-proton collisions at sqrt(s) = 7 TeV

Abstract: A measurement is presented of the exclusive and semi-exclusive production of charged pion pairs in proton-proton collisions, pp to p(p*) + pi+pi- + p(p*) here the pi+pi- pair is emitted at central rapidities, and the scattered protons stay intact (p) or diffractively dissociate (p*) without detection. The measurement is performed with the CMS detector at the LHC, using a data sample corresponding to an integrated luminosity of 450 inverse microbarns collected at a center-of-mass energy of 7 TeV. The dipion cross section, measured for single-pion transverse momentum pt >0.2 GeV and rapidity abs(y)<2, is 26.5 +/- 0.3 (stat) +/- 5.0 (syst) +/- 1.1 (lumi) microbarns. The differential cross sections measured as a function of the invariant mass, pt, and y of the pion pair, and as a function of single-pion pt, are compared to phenomenological predictions.

Search for Supersymmetry in pp Collisions at root s=13 TeV in the Single-Lepton Final State Using the Sum of Masses of Large-Radius Jets

Abstract: Results are reported from a search for supersymmetric particles in proton-proton collisions in the final state with a single lepton, multiple jets, including at least one b-tagged jet, and large missing transverse momentum. The search uses a sample of proton-proton collision data at root s = 13 TeV recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of 35.9 fb(-1). The observed event yields in the signal regions are consistent with those expected from standard model backgrounds. The results are interpreted in the context of simplified models of supersymmetry involving gluino pair production, with gluino decay into either on- or off-mass-shell top squarks. Assuming that the top squarks decay into a top quark plus a stable, weakly interacting neutralino, scenarios with gluino masses up to about 1.9 TeV are excluded at 95% confidence level for neutralino masses up to about 1 TeV.