High Transverse Momentum Research Articles

AdS/CFT predictions for azimuthal and momentum correlations of [formula omitted] pairs in heavy ion collisions

We use an energy loss model sensitive to thermal fluctuations [W. A. Horowitz, Phys. Rev. D 91, no. 8, 085019 (2015)] to compute the azimuthal and momentum correlations of bb¯ pairs traversing a strongly coupled plasma from Pb+Pb collisions at LHC (s=2.76 TeV). The azimuthal correlations are compared with those from perturbative QCD based simulations [M. Nahrgang, J. Aichelin, P. B. Gossiaux and K. Werner, Phys. Rev. C 90, no. 2, 024907 (2014)]. When restricted to leading order production processes, we find that the strongly coupled correlations of high transverse momentum pairs (>4 GeV) are broadened less efficiently than the corresponding weak coupling based correlations, while low transverse momentum pairs (1–4GeV) are broaded with similar efficiency, but with an order of magnitude more particles ending up in this momentum class.The strong coupling momentum correlations we compute account for initial correlations and reveal that the particle pairs suppressed from initially high momenta to the low momentum domain do not suffice to explain the stark difference to the weak coupling results in momentum correlations for 1–4GeV. From this, we conclude that bb¯ pairs are more likely to stay correlated in momentum when propagating through a strongly coupled plasma than a weakly coupled one.

Measurement of the nuclear modification factor for high-pT charged hadrons in p+Pb collisions with the ATLAS detector

The charged hadron spectra in p+Pb and pp collisions at sNN=s=5.02 TeV are measured with the ATLAS experiment at the LHC. The measurements are performed with p+Pb data recorded in 2013 with an integrated luminosity of 25 nb−1 and pp data recorded in 2015 with an integrated luminosity of 28 pb−1. The p+Pb results are compared to pp spectra, presented as a ratio of transverse momentum distributions in the two systems scaled by the number of binary nucleon–nucleon collisions, the nuclear modification factor RpPb. The study of RpPb allows a detailed comparison of the collision systems in different centrality intervals and in a wide range of transverse momentum. It is shown that the nuclear modification factor does not have any significant deviation from unity in the high transverse momentum region.

Hard Probe Measurements in Cu+Au Collisions at PHENIX: Jets and Leading Particles

Collision of asymmetric nuclei like Cu+Au provides the means to study hard probes in different collision geometry compared to collisions of symmetric nuclei like Cu+Cu and Au+Au. A systematic study of the production of high transverse momentum particles and jets in systems with different geometry may provide information about the path-length dependence of energy loss in the medium. The PHENIX experiment has measured the transverse momentum distributions and the nuclear modification factors, RAA of jets and leading particles, such as π0 mesons, in Cu+Au collisions at sNN=200 GeV. The jets are reconstructed with the anti-kT clustering algorithm with a distance parameter of R=0.2, chosen to minimize the contribution of the underlying event. The RAA measurements in Cu+Au collisions are presented as a function of pT and centrality and compared to theoretical calculations.

First measurement of the Sivers asymmetry for gluons using SIDIS data

The Sivers function describes the correlation between the transverse spin of a nucleon and the transverse motion of its partons. For quarks, it was studied in previous measurements of the azimuthal asymmetry of hadrons produced in semi-inclusive deep inelastic scattering of leptons off transversely polarised nucleon targets, and it was found to be non-zero. In this letter the evaluation of the Sivers asymmetry for gluons is presented. The contribution of the photon–gluon fusion subprocess is enhanced by requiring two high transverse-momentum hadrons. The analysis method is based on a Monte Carlo simulation that includes three hard processes: photon–gluon fusion, QCD Compton scattering and the leading-order virtual-photon absorption process. The Sivers asymmetries of the three processes are simultaneously extracted using the LEPTO event generator and a neural network approach. The method is applied to samples of events containing at least two hadrons with large transverse momentum from the COMPASS data taken with a 160 GeV/c muon beam scattered off transversely polarised deuterons and protons. With a significance of about two standard deviations, a negative value is obtained for the gluon Sivers asymmetry. The result of a similar analysis for a Collins-like asymmetry for gluons is consistent with zero.

NRQCD Confronts LHCb Data on Quarkonium Production within Jets.

We analyze the recent LHCb measurement of the distribution of the fraction of the transverse momentum, z(J/ψ), carried by the J/ψ within a jet. LHCb data are compared with analytic calculations using the fragmenting jet function (FJF) formalism for studying J/ψ in jets. Logarithms in the FJFs are resummed using Dokshitzer-Gribov-Lipatov-Altarelli-Parisi evolution. We also convolve hard QCD partonic cross sections, showered with pythia, with leading order nonrelativistic quantum chromodynamics (NRQCD) fragmentation functions and obtain consistent results. Both approaches use madgraph to calculate the hard process that creates the jet initiating parton. These calculations give reasonable agreement with the z(J/ψ) distribution that was shown to be poorly described by default pythia simulations in the LHCb paper. We compare our predictions for the J/ψ distribution using various extractions of nonperturbative NRQCD long-distance matrix elements (LDMEs) in the literature. NRQCD calculations agree with LHCb data better than default pythia regardless of which fit to the LDMEs is used. LDMEs from fits that focus exclusively on high transverse momentum data from colliders are in good agreement with the LHCb measurement.

Measurement of electrons from beauty-hadron decays in p-Pb collisions at sqrt{s_{mathrm{NN}}}=5.02 TeV and Pb-Pb collisions at sqrt{s_{mathrm{NN}}}=2.76 TeV

The production of beauty hadrons was measured via semi-leptonic decays at mid-rapidity with the ALICE detector at the LHC in the transverse momentum interval 1<pT< 8 GeV/c in minimum-bias p-Pb collisions at sqrt{s_{mathrm{NN}}}=5.02 TeV and in 1.3 < pT< 8 GeV/c in the 20% most central Pb-Pb collisions at sqrt{s_{mathrm{NN}}}=2.76 TeV. The pp reference spectra at sqrt{s_{mathrm{NN}}}=5.02 TeV and sqrt{s}=2.76 TeV, needed for the calculation of the nuclear modification factors RpPb and RPbPb, were obtained by a pQCD-driven scaling of the cross section of electrons from beauty-hadron decays measured at sqrt{s}=7 TeV. In the pT interval 3 < pT< 8 GeV/c, a suppression of the yield of electrons from beauty-hadron decays is observed in Pb-Pb compared to pp collisions. Towards lower pT, the RPbPb values increase with large systematic uncertainties. The RpPb is consistent with unity within systematic uncertainties and is well described by theoretical calculations that include cold nuclear matter effects in p-Pb collisions. The measured RpPb and these calculations indicate that cold nuclear matter effects are small at high transverse momentum also in Pb-Pb collisions. Therefore, the observed reduction of RPbPb below unity at high pT may be ascribed to an effect of the hot and dense medium formed in Pb-Pb collisions.

Collisional broadening of angular correlations in a multiphase transport model

Systematic comparisons of jetlike correlation data to radiative and collisional energy loss model calculations are essential to extract transport properties of the quark-gluon medium created in relativistic heavy ion collisions. This paper presents a transport study of collisional broadening of jetlike correlations, by following parton–parton collision history in a multiphase transport (AMPT) model. The correlation shape is studied as functions of the number of parton–parton collisions suffered by a high transverse momentum probe parton (Ncoll) and the azimuth of the probe relative to the reaction plane (ϕfin.probe). Correlation is found to broaden with increasing Ncoll and ϕfin.probe from in- to out-of-plane direction. This study provides a transport model reference for future jet-medium interaction studies.

Implementation of NLO high energy factorization in single inclusive forward hadron production

Single inclusive particle production cross sections in high energy hadron collisions at forward rapidity are an important benchmark process for the CGC picture of small x QCD. Recent calculations of this process have not led to a stable perturbative expansion for this quantity at high transverse momenta. We consider the quark channel production cross section using the new rapidity factorization procedure proposed by Iancu et al. We show that for fixed coupling one does indeed obtain a physically meaningful cross section which is positive and reduces in a controlled way to previous leading order calculations. We also consider a running coupling that depends on the transverse momentum of the produced particle. This gives a stable result which, however, is not fully consistent with previous leading order calculations that use a coordinate space running coupling.

Extraction of partonic transverse momentum distributions from semi-inclusive deep-inelastic scattering, Drell-Yan and Z-boson production

We present an extraction of unpolarized partonic transverse momentum distributions (TMDs) from a simultaneous fit of available data measured in semi-inclusive deep-inelastic scattering, Drell-Yan and Z boson production. To connect data at different scales, we use TMD evolution at next-to-leading logarithmic accuracy. The analysis is restricted to the low-transverse-momentum region, with no matching to fixed-order calculations at high transverse momentum. We introduce specific choices to deal with TMD evolution at low scales, of the order of 1 GeV2. This could be considered as a first attempt at a global fit of TMDs.

Measurement of jet fragmentation in Pb+Pb and pp collisions at sqrt{{s_mathrm{NN}}} = 2.76 TeV with the ATLAS detector at the LHC

The distributions of transverse momentum and longitudinal momentum fraction of charged particles in jets are measured in Pb+Pb and pp collisions with the ATLAS detector at the LHC. The distributions are measured as a function of jet transverse momentum and rapidity. The analysis utilises an integrated luminosity of 0.14 nb^{-1} of Pb+Pb data and 4.0 pb^{-1} of pp data collected in 2011 and 2013, respectively, at the same centre-of-mass energy of 2.76 TeV per colliding nucleon pair. The distributions measured in pp collisions are used as a reference for those measured in Pb+Pb collisions in order to evaluate the impact on the internal structure of jets from the jet energy loss of fast partons propagating through the hot, dense medium created in heavy-ion collisions. Modest but significant centrality-dependent modifications of fragmentation functions in Pb+Pb collisions with respect to those in pp collisions are seen. No significant dependence of modifications on jet p_mathrm{T} and rapidity selections is observed except for the fragments with the highest transverse momenta for which some reduction of yields is observed for more forward jets.

Elliptic flow of light nuclei

Using the coalescence model based on nucleons from a blast-wave model with its parameters fitted to the measured proton transverse momentum spectrum and elliptic flow in heavy ion collisions at the Relativistic Heavy Ion Collider, we study the elliptic flows of light nuclei in these collisions. We find that to describe the measured elliptic flows of deuterons (anti-deuterons) and tritons (helium-3) requires that the emission source for nucleons of high transverse momentum is more elongated along the reaction plane than in the perpendicular direction. Our results thus suggest that the elliptic flows of light nuclei can be used to study the nucleon emission source in relativistic heavy ion collisions.

Effect of an electromagnetic field on the spectra and elliptic flow of particles

In 2+1 dimensions, the evolution of flow under the influence of an external electromagnetic field is simulated. The external electromagnetic field is exponentially decaying with time. Under the same initial conditions, flow evolution with and without the external electromagnetic field is compared. It was found that the production of particles was enhanced when the external electromagnetic field was present. As the strength of the electromagnetic field increased, more particles were produced. Enhancement is greater at the high transverse momentum than at the low transverse momentum. In addition, the electromagnetic field also modifies the elliptic flow and the modification is related to the mass and charge of the particles.

Measurement of the inclusive cross-sections of single top-quark and top-antiquark t-channel production in pp collisions at sqrt{s}=13 TeV with the ATLAS detector

A measurement of the t-channel single-top-quark and single-top-antiquark production cross-sections in the lepton+jets channel is presented, using 3.2 fb−1 of proton-proton collision data at a centre-of-mass energy of 13 TeV, recorded with the ATLAS detector at the LHC in 2015. Events are selected by requiring one charged lepton (electron or muon), missing transverse momentum, and two jets with high transverse momentum, exactly one of which is required to be b-tagged. Using a binned maximum-likelihood fit to the discriminant distribution of a neural network, the cross-sections are determined to be σ(tq) = 156 ± 5 (stat.) ± 27 (syst.) ± 3 (lumi.) pb for single top-quark production and sigma left(overline{t}qright)=91pm 4 (stat.) ± 18 (syst.) ± 2 (lumi.) pb for single top-antiquark production, assuming a top-quark mass of 172.5 GeV. The cross-section ratio is measured to be {R}_t=sigma (tq)/sigma left(overline{t}qright)=1.72pm 0.09 (stat.) ± 0.18 (syst.). All results are in agreement with Standard Model predictions.

Transverse momentum distribution of primary charged particles in p–Pb interactions at forward pseudorapidity at LHC energies

Transverse momentum distributions of primary charged particles have been studied using simulated data from the HIJING 1.0 event generator in the minimum bias p–Pb collisions at [Formula: see text] = 0.9, 1.8, 2.76 and 5.02[Formula: see text], in the two forward pseudorapidity ([Formula: see text]) regions: [Formula: see text] and [Formula: see text] and in the transverse momentum range of [Formula: see text]. The simulated data in the pseudorapidity region of [Formula: see text] at 5.02[Formula: see text] depicts some differences in the region of [Formula: see text] [Formula: see text] 2[Formula: see text] when compared with CMS data. Model shows systematically higher values than the experimental measurements pointing out absorption effect for the experimental data. It is also observed that with increasing rapidity interval from [Formula: see text] to [Formula: see text] observed differences for the behavior of the transverse momentum distributions are shifted to high transverse momentum region. The nuclear modification factor as a function of transverse momentum is constructed using the HIJING 1.0 code. With incident energy, the values of nuclear modification factor increase, for 0.9 and 1.8 [Formula: see text], the distributions seem to increase, but for 2.76 and 5.02 [Formula: see text], the distributions look flat. Numerically, the value of nuclear modification factor increases with the increase in the number of jets. This result shows that for the considered more forward pseudorapidiry area, the influence of the incident energy dominates and this is the reason that main results in the areas are connected with the leading particles.

Performance of algorithms that reconstruct missing transverse momentum in sqrt{s}= 8 TeV proton\u2013proton collisions in the ATLAS detector

The reconstruction and calibration algorithms used to calculate missing transverse momentum (E_{text {T}}^{text {miss}} ) with the ATLAS detector exploit energy deposits in the calorimeter and tracks reconstructed in the inner detector as well as the muon spectrometer. Various strategies are used to suppress effects arising from additional proton–proton interactions, called pileup, concurrent with the hard-scatter processes. Tracking information is used to distinguish contributions from the pileup interactions using their vertex separation along the beam axis. The performance of the E_{text {T}}^{text {miss}} reconstruction algorithms, especially with respect to the amount of pileup, is evaluated using data collected in proton–proton collisions at a centre-of-mass energy of 8 text {TeV} during 2012, and results are shown for a data sample corresponding to an integrated luminosity of 20.3, mathrm{fb}^{-1}. The simulation and modelling of E_{text {T}}^{text {miss}} in events containing a Z boson decaying to two charged leptons (electrons or muons) or a W boson decaying to a charged lepton and a neutrino are compared to data. The acceptance for different event topologies, with and without high transverse momentum neutrinos, is shown for a range of threshold criteria for E_{text {T}}^{text {miss}} , and estimates of the systematic uncertainties in the E_{text {T}}^{text {miss}} measurements are presented.

James Watson Cronin

James Watson Cronin

Search for heavy neutrinos or third-generation leptoquarks in final states with two hadronically decaying \u03c4 leptons and two jets in proton-proton collisions at sqrt{s}=13 TeV

A search for new particles has been conducted using events with two high transverse momentum (pT) τ leptons that decay hadronically, at least two high-pT jets, and missing transverse energy from the τ lepton decays. The analysis is performed using data from proton-proton collisions, collected by the CMS experiment in 2015 at sqrt{s}=13 TeV, corresponding to an integrated luminosity of 2.1 fb−1. The results are interpreted in two physics models. The first model involves heavy right-handed neutrinos, Nℓ (ℓ = e, μ, τ), and right-handed charged bosons, WR, arising in a left-right symmetric extension of the standard model. Masses of the WR boson below 2.35 (1.63) TeV are excluded at 95% confidence level, assuming the Nτ mass is 0.8 (0.2) times the mass of the WR boson and that only the Nτ flavor contributes to the WR decay width. In the second model, pair production of third-generation scalar leptoquarks that decay into ττbb is considered. Third-generation scalar leptoquarks with masses below 740 GeV are excluded, assuming a 100% branching fraction for the leptoquark decay to a τ lepton and a bottom quark. This is the first search at hadron colliders for the third-generation Majorana neutrino, as well as the first search for third-generation leptoquarks in the final state with a pair of hadronically decaying τ leptons and jets.

Transverse momentum spectra and the nuclear modification factor of charged particles with ALICE

The observed suppression of particle production at high transverse momentum in nucleus-nucleus collisions at high energies is an effect of the energy loss of partons as they propagate through the hot and dense deconfined QCD medium. The measurement of transverse momentum spectra of primary charged particles in Pb–Pb collisions and the comparison to the measurement in pp collisions are important to study the properties of the deconfined medium. In 2015, pp and Pb–Pb collisions have been recorded at the LHC at the energy of = 5.02 TeV. The inclusive charged particle production is measured with the ALICE detector in the pseudo-rapidity range |η| < 0.8 and in the transverse momentum range 0.15 < pT < 40 GeV/c. The spectra in Pb–Pb, determined for several centrality intervals, are compared to the reference spectrum in pp collisions by calculating the nuclear modification factor RAA. The data are compared to measurements at the lower collision energy of 2.76 TeV and to model predictions.

Measurements of Open Heavy Flavor Production in Semi-leptonic Decay Channels at the STAR experiment

In these proceedings, we will present the new results of the Non-Photonic Electron (NPE) production from semi-leptonic decays of open heavy flavor hadrons at the STAR experiment. Firstly we will report the updated results on NPE production in p+p collisions at with much improved precision and wider kinematic coverage than previous measurements. Secondly we will report measurements of the nuclear modification factor, RAA, for NPE production in Au+Au collisions at with this new p+p reference. The NPE RAA shows large suppression at high transverse momenta (pT) in central collisions, which reduces gradually towards more peripheral collisions, and an enhancement at low pT with large systematic uncertainties from the p+p reference. We will compare NPE RAA to the RAA of charm decayed electrons, D0 mesons and light hadrons in Au+Au collisions as well as to NPE and D0 mesons in central U+U collisions and show that they are all consistent within the uncertainties. Finally we will report the results on measurements of NPE from open bottom hadron decays and discuss the prospect of measuring NPE from open bottom and charm hadron decays separately utilizing the Heavy Flavor Tracker.

Measurement of charged-particle distributions sensitive to the underlying event in sqrt{s}=13 TeV proton-proton collisions with the ATLAS detector at the LHC

We present charged-particle distributions sensitive to the underlying event, measured by the ATLAS detector in proton-proton collisions at a centre-of-mass energy of 13 TeV, in low-luminosity Large Hadron Collider fills corresponding to an integrated luminosity of 1.6 nb−1. The distributions were constructed using charged particles with absolute pseudorapidity less than 2.5 and with transverse momentum greater than 500 MeV, in events with at least one such charged particle with transverse momentum above 1 GeV. These distributions characterise the angular distribution of energy and particle flows with respect to the charged particle with highest transverse momentum, as a function of both that momentum and of charged-particle multiplicity. The results have been corrected for detector effects and are compared to the predictions of various Monte Carlo event generators, experimentally establishing the level of underlying-event activity at LHC Run 2 energies and providing inputs for the development of event generator modelling. The current models in use for UE modelling typically describe this data to 5% accuracy, compared with data uncertainties of less than 1%.