Charged Particle Transverse Momentum Research Articles

The Parameters of The Tsallis Distribution at the LHC

This talk focuses on fits, using the Tsallis distribution, extending to very large values of the transverse momentum of charged particles in p-p collisions and on the energy dependence of the parameters. A thermodynamically consistent form of the distribution is used for fitting the transverse momentum spectra at mid-rapidity. The fits based on the proposed distribution provide a very good description over 14 orders of magnitude. The parameters obtained show a smooth increase in the value of q and a corresponding smooth decrease in the value of T with increasing beam energy.

Transverse momentum, rapidity, and centrality dependence of inclusive charged-particle production in sNN=5.02 TeV p+ Pb collisions measured by the ATLAS experiment

Measurements of the per-event charged-particle yield as a function of the charged-particle transverse momentum and rapidity are performed using $p+$Pb collision data collected by the ATLAS experiment at the LHC at a centre-of-mass energy of $\sqrt{s_{NN}}=5.02$ TeV. Charged particles are reconstructed over pseudorapidity $|\eta|<2.3$ and transverse momentum between $0.1$ GeV and $22$ GeV in a dataset corresponding to an integrated luminosity of $1$ $\mu b^{-1}$. The results are presented in the form of charged-particle nuclear modification factors, where the $p+$Pb charged-particle multiplicities are compared between central and peripheral $p+$Pb collisions as well as to charged-particle cross sections measured in pp collisions. The $p+$Pb collision centrality is characterized by the total transverse energy measured in $-4.9<\eta<-3.1$, which is in the direction of the outgoing lead beam. Three different estimations of the number of nucleons participating in the $p+$Pb collision are carried out using the Glauber model and two Glauber-Gribov colour-fluctuation extensions to the Glauber model. The values of the nuclear modification factors are found to vary significantly as a function of rapidity and transverse momentum. A broad peak is observed for all centralities and rapidities in the nuclear modification factors for charged-particle transverse momentum values around $3$ GeV. The magnitude of the peak increases for more central collisions as well as rapidity ranges closer to the direction of the outgoing lead nucleus.

Measurement of Long-Range Near-Side Two-Particle Angular Correlations in pp Collisions at sqrt[s]=13 TeV.

Results on two-particle angular correlations for charged particles produced in pp collisions at a center-of-mass energy of 13TeV are presented. The data were taken with the CMS detector at the LHC and correspond to an integrated luminosity of about 270 nb^{-1}. The correlations are studied over a broad range of pseudorapidity (|η|<2.4) and over the full azimuth (ϕ) as a function of charged particle multiplicity and transverse momentum (p_{T}). In high-multiplicity events, a long-range (|Δη|>2.0), near-side (Δϕ≈0) structure emerges in the two-particle Δη-Δϕ correlation functions. The magnitude of the correlation exhibits a pronounced maximum in the range 1.0<p_{T}<2.0 GeV/c and an approximately linear increase with the charged particle multiplicity, with an overall correlation strength similar to that found in earlier pp data at sqrt[s]=7 TeV. The present measurement extends the study of near-side long-range correlations up to charged particle multiplicities N_{ch}∼180, a region so far unexplored in pp collisions. The observed long-range correlations are compared to those seen in pp, pPb, and PbPb collisions at lower collision energies.

Large Transverse Momenta and Tsallis Thermodynamics

The charged particle transverse momentum (pT) spectra measured by the ATLAS and CMS collaborations in proton - proton collisions at √s = 0.9 and 7 TeV have been studied using Tsallis thermodynamics. A thermodynamically consistent form of the Tsallis distribution is used for fitting the transverse momentum spectra at mid-rapidity. It is found that the fits based on the proposed distribution provide an excellent description over 14 orders of magnitude with pT values up to 200 GeV/c.

Two-particle Correlations in $pp$ Collisions at 13 TeV Measured with CMS

Results on two-particle angular correlations for charged particles emitted in pp collisions at a centerof-mass energy of 13 TeV are presented as a function of charged-particle multiplicity and transverse momentum (pT ). In high−multiplicity events, a long-range (| η |> 2.0), near-side (∆φ = 0) structure emerges in the two-particle ∆η − ∆φ correlation functions. The overall correlation strength is similar to that found in earlier pp data at 7 TeV, but is measured up to much higher multiplicity values. A detailed study in pp collisions at 7 TeV of the second-order (v2) azimuthal anisotropy harmonics of charged particles, K S and Λ/Λ particles are extracted from long-range two-particle correlations as a function of particle multiplicity and transverse momentum and are also compared with values obtained in pPb and PbPb collisions at similar multiplicities. Presented at WPCF-2015 XI Workshop on Particle Correlations and Femtoscopy Two-Particle Correlations in pp Collisions at 13 TeV Measured with CMS ∗

Nucleon-gold collisions at200AGeV using taggedd + Auinteractions in the PHOBOS detector

Forward calorimetry in the PHOBOS detector has been used to study charged hadron production in d+Au, p+Au and n+Au collisions at sqrt(s_nn) = 200 GeV. The forward proton calorimeter detectors are described and a procedure for determining collision centrality with these detectors is detailed. The deposition of energy by deuteron spectator nucleons in the forward calorimeters is used to identify p+Au and n+Au collisions in the data. A weighted combination of the yield of p+Au and n+Au is constructed to build a reference for Au+Au collisions that better matches the isospin composition of the gold nucleus. The p_T and centrality dependence of the yield of this improved reference system is found to match that of d+Au. The shape of the charged particle transverse momentum distribution is observed to extrapolate smoothly from pbar+p to central d+Au as a function of the charged particle pseudorapidity density. The asymmetry of positively- and negatively-charged hadron production in p+Au is compared to that of n+Au. No significant asymmetry is observed at mid-rapidity. These studies augment recent results from experiments at the LHC and RHIC facilities to give a more complete description of particle production in p+A and d+A collisions, essential for the understanding the medium produced in high energy nucleus-nucleus collisions.

Centrality dependence of particle production inp−Pbcollisions atsNN=5.02TeV

We report measurements of the primary charged particle pseudorapidity density and transverse momentum distributions in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV, and investigate their correlation with experimental observables sensitive to the centrality of the collision. Centrality classes are defined using different event activity estimators, i.e. charged particle multiplicities measured in three disjunct pseudorapidity regions as well as the energy measured at beam rapidity (zero-degree). The procedures to determine the centrality, quantified by the number of participants ($N_{\rm part}$), or the number of nucleon-nucleon binary collisions ($N_{\rm coll}$), are described. We show that, in contrast to Pb-Pb collisions, in p-Pb collisions large multiplicity fluctuations together with the small range of participants available, generate a dynamical bias in centrality classes based on particle multiplicity. We propose to use the zero-degree energy, which we expect not to introduce a dynamical bias, as an alternative event-centrality estimator. Based on zero-degree energy centrality classes, the $N_{\rm part}$ dependence of particle production is studied. Under the assumption that the multiplicity measured in the Pb-going rapidity region scales with the number of Pb-participants, an approximate independence of the multiplicity per participating nucleon measured at mid-rapitity of the number of participating nucleons is observed. Furthermore, at high-$p_{\rm T}$ the p-Pb spectra are found to be consistent with the pp spectra scaled by $N_{\rm coll}$ for all centrality classes. Our results represent valuable input for the study of the event activity dependence of hard probes in p-Pb collision and, hence, help to establish baselines for the interpretation of the Pb-Pb data.

Measurement of distributions sensitive to the underlying event in inclusive Z-boson production in [Formula: see text] collisions at [Formula: see text] TeV with the ATLAS detector.

A measurement of charged-particle distributions sensitive to the properties of the underlying event is presented for an inclusive sample of events containing a Z-boson, decaying to an electron or muon pair. The measurement is based on data collected using the ATLAS detector at the LHC in proton–proton collisions at a centre-of-mass energy of 7 TeV with an integrated luminosity of 4.6 fb^{-1}. Distributions of the charged particle multiplicity and of the charged particle transverse momentum are measured in regions of azimuthal angle defined with respect to the Z-boson direction. The measured distributions are compared to similar distributions measured in jet events, and to the predictions of various Monte Carlo generators implementing different underlying event models.

Measurement of inclusive jet charged-particle fragmentation functions in Pb+Pb collisions at [formula omitted] with the ATLAS detector

Measurements of charged-particle fragmentation functions of jets produced in ultra-relativistic nuclear collisions can provide insight into the modification of parton showers in the hot, dense medium created in the collisions. ATLAS has measured jets in sNN=2.76 TeV Pb+Pb collisions at the LHC using a data set recorded in 2011 with an integrated luminosity of 0.14 nb−1. Jets were reconstructed using the anti-kt algorithm with distance parameter values R=0.2,0.3,and 0.4. Distributions of charged-particle transverse momentum and longitudinal momentum fraction are reported for seven bins in collision centrality for R=0.4 jets with pTjet>100 GeV. Commensurate minimum pT values are used for the other radii. Ratios of fragment distributions in each centrality bin to those measured in the most peripheral bin are presented. These ratios show a reduction of fragment yield in central collisions relative to peripheral collisions at intermediate z values, 0.04≲z≲0.2, and an enhancement in fragment yield for z≲0.04. A smaller, less significant enhancement is observed at large z and large pT in central collisions.

Event-by-event mean $$\varvec{p}_{\mathbf {T}}$$ p T fluctuations in pp and Pb–Pb collisions at the LHC

Event-by-event fluctuations of the mean transverse momentum of charged particles produced in pp collisions at $\sqrt{s}$ = 0.9, 2.76 and 7 TeV, and Pb-Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV are studied as a function of the charged-particle multiplicity using the ALICE detector at the LHC. Dynamical fluctuations indicative of correlated particle emission are observed in all systems. The results in pp collisions show little dependence on collision energy. The Monte Carlo event generators PYTHIA and PHOJET are in qualitative agreement with the data. Peripheral Pb-Pb data exhibit a similar multiplicity dependence as that observed in pp. In central Pb-Pb, the results deviate from this trend, featuring a significant reduction of the fluctuation strength. The results in Pb--Pb are in qualitative agreement with previous measurements in Au-Au at lower collision energies and with expectations from models that incorporate collective phenomena.

Charged particle production in p+Pb collisions with the ATLAS detector

Abstract We report on the measurements of centrality dependence of the charged particle pseudorapidity distribution and transverse momentum distributions in proton–lead collisions with the ATLAS detector at LHC. The data were collected in 2012 run providing proton–nucleus collisions at s NN = 5.02 TeV . These measurements are expected to contribute to understanding the initial state of the nuclear wave function. The presented results also point to the importance of understanding the fluctuating nature of the nucleon–nucleon collisions.

Simultaneously study for particle transverse sphericity and ellipticity in pp collisions at LHC energies

Abstract Since the PYTHIA6 and PYTHIA8 results are inconsistent with ALICE data of charged particle transverse momentum sphericity, we introduce a new method in which the p x and p y components of produced particles are randomly rearranged on the circumference of an ellipse with a half major and minor axes being p T ( 1 + δ p ) and p T ( 1 − δ p ) . Based on the particles generated in the PYTHIA6.4 simulations this rearrangement method fairly well reproduces the ALICE data of the transverse sphericity as a function of charged multiplicity in the minimum bias pp collisions at s = 0.9 and 7 TeV . Meanwhile, this work predicts a measurable value of v 2 ∼ 0.2 – 0.3 for the charged particles in the above pp collisions and suggests a simultaneous study of the particle transverse momentum sphericity and elliptic flow parameter as well as other harmonic coefficients.

Comparison of the Color Glass Condensate to multiplicity dependence of the average transverse momentum in p+p, p+Pb and Pb+Pb collisions at the LHC

The first moment 〈pT〉 of the charged-particle transverse momentum spectrum and its correlation with the charged-particle multiplicity Nch provide vital information about the underlying particle production mechanism. The ALICE Collaboration recently reported that 〈pT〉 versus Nch in Pb+Pb collisions is smaller than in p+p and p+Pb collisions. Other interesting features of data is rather flatness of 〈pT〉 at high Nch in Pb+Pb and p+Pb collisions in seemingly striking contrast to the case of p+p collisions. With a detailed calculation, we show all these peculiar features in a wide range of energies and system sizes can be well described by the idea of gluon saturation within the Color Glass Condensate framework using the kT-factorization. This establishes an important fact that the bulk of the produced particles in heavy-ion collisions at the LHC carries signature of the initial stage of collisions. We also show that the recent scaling property seen by the CMS Collaboration between the number of tracks in p+p and p+Pb collisions may provide a strong evidence in favor of geometric-scaling phenomenon and gluon saturation, indicating that the underlying dynamics of high multiplicity events in p+p and p+Pb collisions should be similar.

Observation of near-side angular correlations in pPb collisions at 5.02TeV with CMS

The first measurement of two-particle correlations in pPb collisions at a center-of-mass energy of 5.02 TeV is presented. The correlations are studied in azimuth, ϕ, and pseudorapidity, η, as a function of charged-particle transverse momentum, pT, and multiplicity. A total of two million collisions are used in this analysis. A long range (2 < |Δη| < 4), near-side (Δϕ ≈ 0) ridge-like structure emerges in the two-particle Δη−Δϕ correlation functions of high-multiplicity events. This is the first time such a correlation has been seen in proton-nucleus collisions, which which resembles the correlations seen in a broad energy range of nucleus-nucleus collisions and in high-multiplicity proton-proton collisions at = 7 TeV. The correlation strength rises approximately linearly with multiplicity and has a maximum in the range of pT = 1 – 1.5 GeV/c.

Measurement of charged-particle event shape variables in inclusive(s)=7 TeVproton-proton interactions with the ATLAS detector

The measurement of charged-particle event shape variables is presented in inclusive inelastic pp collisions at a center-of-mass energy of 7 TeV using the ATLAS detector at the LHC. The observables studied are the transverse thrust, thrust minor and transverse sphericity, each defined using the final-state charged particles' momentum components perpendicular to the beam direction. Events with at least six charged particles are selected by a minimum-bias trigger. In addition to the differential distributions, the evolution of each event shape variable as a function of the leading charged particle transverse momentum, charged particle multiplicity and summed transverse momentum is presented. Predictions from several Monte Carlo models show significant deviations from data.

Measurement of charged particle RAA at high pT in PbPb collisions at sNN=2.76TeV with CMS

Charged particle transverse momentum (pT) spectra have been measured by CMS for pp and PbPb collisions at the same sqrt(sNN)=2.76 TeV collision energy per nucleon pairs. Calorimeter-based jet triggers are employed to enhance the statistical reach of the high-pT measurements. The nuclear modification factor (R_AA) is obtained in bins of collision centrality for the PbPb data sample dividing by the measured pp reference spectrum. In the range pT = 5-10 GeV/c, the charged particle yield in the most central PbPb collisions is suppressed by up to a factor of 7. At higher pT, this suppression is significantly reduced, approaching a factor of 2 for particles with pT = 40-100 GeV/c.

Charged particle production in Pb–Pb collisions at the LHC with the ALICE detector

The ALICE collaboration measured charged particle production in $\sqrt{s_{NN}}=2.76$ TeV Pb--Pb collisions at the LHC. We report on results on charged particle multiplicity and transverse momentum spectra. All the results are presented as a function of the centrality of the collision, estimated with a Glauber Monte Carlo fit to multiplicity distributions reconstructed in various detectors. The applicability of the Glauber model at LHC energies, the precision of the centrality determination and the related systematic uncertainties are discussed in detail. Particles are tracked in the pseudorapidity window $|\eta| \lesssim 0.9$\ with the silicon Inner Tracking System (ITS) and the Time Projection Chamber (TPC), over the range $0.15 < \pt \lesssim 50$ GeV/$c$. The low-$p_t$ cut-off is further reduced in the multiplicity measurement using "tracklets", reconstructed in the 2 innermost layers of the ITS. The charged particle multiplicity is measured in $|\eta| < 0.5$ to be $\mathrm{d}N_{ch}/\mathrm{d}\eta = 1601 \pm 60$ in 5% most central Pb--Pb collisions, indicating an energy density a factor $\sim 3$ higher than at RHIC. Its evolution with centrality shows a pattern strikingly similar to the one measured at RHIC. Intermediate ($5 \lesssim \pt \lesssim 15$ GeV/$c$) transverse momentum particles are found to be most strongly suppressed with respect to pp collisions, consistent with a large energy loss of hard-scattered partons in the hot and dense medium. The results are presented in terms of the nuclear modification factor $R_{\mathrm{AA}}$ and compared to theoretical expectations.

Mid-rapidity antibaryon-to-baryon ratios in pp and Pb–Pb collisions measured by the ALICE experiment

Abstract Measurement of the antibaryon-baryon ratios ( B ¯ / B ) at mid rapidity probes the baryon transport and the degree of baryon stopping in high-energy collisions, providing insight into the collision dynamics. In this paper, we discuss the measurement of different B ¯ / B ratios ( p ¯ / p , Λ ¯ / Λ , Ξ ¯ + / Ξ − ) in pp collisions at s = 0.9 , 2.76 , and 7 TeV and in Pb–Pb collisions at s NN = 2.76 TeV , as a function of charged particle multiplicity, rapidity and transverse momentum. Results from pp and Pb–Pb collisions are presented and compared to models.

PACIAE 2.1: An updated issue of the parton and hadron cascade model PACIAE 2.0

We have updated the parton and hadron cascade model PACIAE 2.0 for relativistic nuclear collisions (cf. Comput. Phys. Comm. 183(2012)333) to the new issue of PACIAE 2.1. The PACIAE model is based on PYTHIA. In the PYTHIA model, once the generated particle/parton transverse momentum $p_T$ is randomly selected, its component of $[p_x,p_y]$ was originally put on the circle with radius $p_T$ randomly. It is now replaced by put on the circumference of ellipse with half major and minor axes of $p_T(1+\delta_p)$ and $p_T(1-\delta_p)$, respectively, in order to investigate better the transverse momentum anisotropy in final hadronic state.

HYDRO-PACIAE, a hydrodynamic and transport hybrid model for ultra-relativistic heavy ion collisions

Combining the particle in cell hydrodynamic model and the PACIAE transport model, we present the HYDRO-PACIAE hybrid model and then use it to investigate, as an example, the charged particle production in the Au + Au collisions at = 130 GeV and the Pb + Pb collisions at = 2.76 TeV. The calculated charged particle pseudo-rapidity distribution and transverse momentum distribution reproduce the corresponding experimental data. The experimental data of the elliptic flow parameter may also be explained. The HYDRO-PACIAE model is able to describe most features of the ultra-relativistic heavy ion collisions at RHIC and LHC energies, while it describes rapid non-equilibrium hadronization (freeze out) and allows for supercooled quark–gluon plasma. This is a feature not used in other hybrid models, which assume a phase transition via a mixed phase, phase equilibrium and/or ‘crossover’.