Average Transverse Momentum Research Articles

Study of charged particle multiplicity, average transverse momentum and azimuthal anisotropy in Xe+Xe collisions at $\sqrt{s_{NN}} = 5.44$ TeV using AMPT model

We have studied the average charged particle density ($\langle \mathrm{d} \mathrm{N}_{ch}/\mathrm{d} \eta\rangle$), transverse momentum ($ p_{\mathrm{T}}$) spectra, average transverse momentum ($ \langle p_{\mathrm{T}}\rangle$) and azimuthal anisotropies of inclusive charged particles produced in Xe+Xe collisions at $\sqrt{s_{\mathrm{NN}}} = 5.44$ TeV using A Multiphase Transport Model (AMPT), which includes the deformation of 129Xe nucleus. Calculations have been performed with the string melting version of AMPT model and compared with the recent measurements from the ALICE experiment. The model result overestimates the measured $ \langle \mathrm{d} \mathrm{N}_{ch}/\mathrm{d} \eta\rangle$ for central collisions, agrees with the data for mid-central collisions and underestimates the measurements for the peripheral collisions. The centrality dependence of $ \langle p_{\mathrm{T}}\rangle$ of charged particles measured in ALICE is not reproduced by the model results. The calculated elliptic flow ($v_{2}$) from AMPT model is larger than the ALICE measurements in central collisions but is consistent with the data for 30-50% collisions centrality. We find that the model shows a mild centrality dependence of triangular flow and overestimates the ALICE measurements. Within the model framework, we have also studied various collision configurations of Xe nuclei such as body-body, tip-tip, side-side and random. We find a strong dependence of the above observable on the collision configurations.

Event-shape and multiplicity dependence of freeze-out radii in pp collisions at sqrt{s} = 7 TeV

Two-particle correlations in high-energy collision experiments enable the extraction of particle source radii by using the Bose-Einstein enhancement of pion production at low relative momentum q ∝ 1/R. It was previously observed that in pp collisions at sqrt{s} = 7TeV the average pair transverse momentum kT range of such analyses is limited due to large background correlations which were attributed to mini-jet phenomena. To investigate this further, an event-shape dependent analysis of Bose-Einstein correlations for pion pairs is performed in this work. By categorizing the events by their transverse sphericity ST into spherical (ST > 0:7) and jet-like (ST < 0:3) events a method was developed that allows for the determination of source radii for much larger values of kT for the first time. Spherical events demonstrate little or no background correlations while jet-like events are dominated by them. This observation agrees with the hypothesis of a mini-jet origin of the non-femtoscopic background correlations and gives new insight into the physics interpretation of the kT dependence of the radii. The emission source size in spherical events shows a substantially diminished kT dependence, while jet-like events show indications of a negative trend with respect to kT in the highest multiplicity events. Regarding the emission source shape, the correlation functions for both event sphericity classes show good agreement with an exponential shape, rather than a Gaussian one.

Influence of the Effects of Color Reconnection and the Formation of Hadronic Jets on the Distribution of Charged Particles over the Transverse Momentum in pp-Collisions at the Energies of the Large Hadron Collider

Dependences of the average transverse momentum of charged particles on rapidity $$y$$ and pseudorapidity $$\eta $$ are obtained, and the influence the effects of color reconnection and the formation of hadronic jets have on these dependences is studied. It is shown that the result depends largely on the interval of which physical quantity (rapidity or pseudorapidity) particle distributions over the transverse momentum are considered. It is important to compare theoretically calculated distributions to experimental data. It is shown in particular that allowing for the kinematic factor which arises in describing the distribution of particles over the transverse momentum in the specified pseudorapidity interval considerably influences the description of experimental data on pp-collisions at energies of $$\sqrt s = 7$$ TeV in the Large Hadron Collider.

Strongly Intensive Variables and Long-Range Correlations in the Model With a Lattice in the Transverse Plane

In the framework of the quark–gluon string fusion model on the transverse lattice, we study a strongly intensive variable characterizing correlations between the number of particles produced in hadronic interactions in two observation windows separated by a rapidity interval. We show that in the case of independent identical strings, this variable is indeed strongly intensive. It depends only on string characteristics and is independent of trivial so-called volume fluctuations in the string number resulting, in particular, from inevitable impact parameter fluctuations. With string fusion effects causing the production of string clusters with new properties taken into account, this variable turns out to be equal to the weighted average of its values for different string clusters. The weighting coefficients depend on the collision conditions, and the variable loses its strongly intensive character. In the framework of this model in a realistic case with a nonuniform string distribution in the transverse plane, we find explicit analytic formulas for the asymptotic coefficients of long-range correlations between different quantities including an intensive one, the average transverse momentum. We analyze the properties of the obtained correlation coefficients, the studied strongly intensive variable, and also the possibilities of its experimental observation.

Pion induced Drell-Yan: the transverse momentum structure of the pion

We present an analysis of unpolarized Drell-Yan pair production in pion-nucleus scattering with a particular focus into the pion dynamics. The study consists in analyzing the effect of the partonic longitudinal and, especially, transverse distributions of the pion in a Nambu–Jona-Lasinio (NJL) framework, with Pauli-Villars regularization. In order to consistently take into account the QCD evolution effects, we have estimated the hadronic scale corresponding to the NJL model’s degrees of freedom through a minimization procedure at NLO: The NLO evolved pion distributions have been compared to rapidity differential DrellYan cross sections data. That hadronic scale so determined represents the only free parameter in our approach.The NJL transverse momentum PDF, evolved up to next-to-leading logarithmic accuracy, is then tested against the transverse momentum spectrum of dilepton pairs up to a transverse momentum of 2 GeV. We found a fair agreement with available pion-nucleus data. We find sizable evolution effects on the shape of the distributions and on the generated average transverse momentum of the dilepton pair.

Production of light flavored charged hadron in pp collisions at 900 GeV with hadron production models

Differential yields of light flavored charged hadrons in [Formula: see text] collision at 0.9 TeV in the transverse momentum [Formula: see text] ranging from 0.1–2.5 GeV/c with pseudorapidity [Formula: see text] are presented here. The spectra obtained by Monte Carlo simulations with event generators — EPOS 1.99, EPOS-LHC, QGSJETII-04 and Sibyll2.3c — are compared with the ALICE measurements. For [Formula: see text] mesons, EPOS1.99, EPOS-LHC and QGSJETII-04 described the ALICE measurement very well almost in the entire [Formula: see text] range except QGSJETII-04 which underpredicts with [Formula: see text] below 0.4 GeV/c. The Sibyll2.3c model slightly overpredicts for [Formula: see text] GeV/c and underpredicts afterwards. For [Formula: see text] mesons, all models describe the experimental data qualitatively, except at high [Formula: see text], where the models slightly underpredict. The EPOS-LHC and QGSJETII-04 models, at low [Formula: see text], describe the distribution of the experimental measurements very well in both cases of protons and anti-protons, whereas the former underpredicts and the latter overpredicts at higher [Formula: see text]. The EPOS1.99 overpredicts at low [Formula: see text] below 1 GeV/c, whereas it reproduces the results well above it. Sibyll2.3c overpredicts the distribution over the entire range of [Formula: see text]. Models described the average transverse momentum [Formula: see text] of these hadron species as a function of their mass, showing small rise in the average [Formula: see text] with the mass of the particles. The same has been observed by the STAR collaboration with measurements in [Formula: see text] reactions at [Formula: see text] = 200 GeV and also in [Formula: see text] collisions at [Formula: see text] = 900 GeV.

Charged Particles pT Spectra and the Correlation between pT and all Charged Particles at √S = 900 GeV

The transverse momentum (pT) spectra, covering a region of 0.15–10 GeV/c, of charged particles as well as the correlation between the average transverse momentum and all charged particles (Nch) in pp collision at 900 GeV/c are investigated. Simulations by hadron production models are compared with the measurements of ALICE Collaboration. Qualitatively, all models’ predictions are in good agreement with experimental data for the pT distributions of invariant yield of all charged hadrons. For the as a function Nch presented in two pT regions, 0.15 < pT < 4 and 0.5 < pT < 4, the EPOS models, and HIJING produced good predictions of the experimental measurements as compared to QGSJETII-04 and Sibyll2.3c. Although the models’ prediction is compatible with the experimental data, none of them is enough comprehensive to provide complete description of all measurements.

Probing cold nuclear matter effects with the productions of isolated- and +jet in p+Pb collisions at 8.16 TeV * * Supported by the NSFC of China (11435004, 11322546, 11805167) and partly Supported by China University of Geosciences (Wuhan) (162301182691)

We investigate cold nuclear matter (CNM) effects on the productions of isolated prompt photons and jet in proton-lead collisions at TeV under next-to-leading order perturbative quantum chromodynamics calculations with four parameterizations for nuclear parton distribution functions (nPDFs), i.e., DSSZ, EPPS16, nCTEQ15, and nIMParton. Our theoretical calculations provide good descriptions of the pp baseline in the ATLAS collaboration and make predictions for future experimental results at + collisions. We calculate the dependence of the nuclear modification factor of isolated prompt photons on transverse momentum and pseudo-rapidity at very forward and backward rapidity regions, and we demonstrate that the forward-to-backward yield asymmetries as a function of with different nPDF parameterizations have diverse behaviors. Furthermore, the nuclear modification factor of isolated- jet as a function of jet's pseudo-rapidity at different average transverse momenta has been discussed. This can facilitate a tomographic study of CNM effects with precise locations in a rather wide kinematic region by varying the transverse momenta and rapidities of both isolated photons and jets in p+A collisions.

Recent developments in particle yield fluctuation measurements

In relativistic heavy-ion collisions, properties of the initial state and effects arising during evolution of the medium, such as a transition between the hadronic and partonic phases, should reflect themselves in event-by-event fluctuations of the number of produced particles. In this paper, recentmeasurements of several event-by-event observables, namely, dynamical fluctuations of relative particle yields and forwardbackward correlations of different types, are discussed. Also, new observables for forward-backward correlation studies are proposed: correlations between ratios of identified particle yields in two separated acceptance intervals and the correlation between the ratio in one interval and average transverse momentum in another.

Event shape variables measured using multijet final states in proton-proton collisions at sqrt{s}=13 TeV

The study of global event shape variables can provide sensitive tests of predictions for multijet production in proton-proton collisions. This paper presents a study of several event shape variables calculated using jet four momenta in proton-proton collisions at a centre-of-mass energy of 13 TeV and uses data recorded with the CMS detector at the LHC corresponding to an integrated luminosity of 2.2 fb−1. After correcting for detector effects, the resulting distributions are compared with several theoretical predictions. The agreement generally improves as the energy, represented by the average transverse momentum of the two leading jets, increases.

Role of transverse momentum dependence of unpolarized parton distribution and fragmentation functions in the analysis of azimuthal spin asymmetries

Information on the Sivers distribution and the Collins fragmentation functions and their transverse momentum dependence is mainly based on fitting single spin asymmetry data from semi-inclusive deep inelastic scattering (SIDIS). Independent information, respectively on the Sivers distribution and the Collins fragmentation, can be obtained from Drell-Yan and $e^+e^-$ annihilation processes. In the SIDIS case, the transverse momentum of the final observed hadron, which is the quantity measured, is generated both by the average transverse momentum in the distribution and in the fragmentation functions. As a consequence, these are strongly correlated and a separate extraction is made difficult. In this paper we investigate, in a simple kinematical Gaussian configuration, this correlation, its role on the transverse single spin asymmetries in SIDIS and the consequences for predictions of the Sivers asymmetry in Drell-Yan processes and for the Collins asymmetry in $e^+e^-$ annihilation. We find that, in some cases, these effects can be relevant and must be carefully taken into account.

Transverse momentum spectra and nuclear modification factors of charged particles in Xe–Xe collisions at [formula omitted

Transverse momentum (pT ) spectra of charged particles at mid-pseudorapidity in Xe–Xe collisions at sNN=5.44TeV measured with the ALICE apparatus at the Large Hadron Collider are reported. The kinematic range 0.15<pT<50GeV/c and |η|<0.8 is covered. Results are presented in nine classes of collision centrality in the 0–80% range. For comparison, a pp reference at the collision energy of s=5.44 TeV is obtained by interpolating between existing pp measurements at s=5.02 and 7 TeV. The nuclear modification factors in central Xe–Xe collisions and Pb–Pb collisions at a similar center-of-mass energy of sNN=5.02 TeV, and in addition at 2.76 TeV, at analogous ranges of charged particle multiplicity density 〈dNch/dη〉 show a remarkable similarity at pT>10GeV/c. The centrality dependence of the ratio of the average transverse momentum 〈pT〉 in Xe–Xe collisions over Pb–Pb collision at s=5.02 TeV is compared to hydrodynamical model calculations.

Constraining Gluon Distributions in Nuclei Using Dijets in Proton-Proton and Proton-Lead Collisions at sqrt[s_{NN}]=5.02 TeV.

The pseudorapidity distributions of dijets as functions of their average transverse momentum (p_{T}^{ave}) are measured in proton-lead (pPb) and proton-proton (pp) collisions. The data samples were collected by the CMS experiment at the CERN LHC, at a nucleon-nucleon center-of-mass energy of 5.02TeV. A significant modification of the pPb spectra with respect to the pp spectra is observed in all p_{T}^{ave} intervals investigated. The ratios of the pPb and pp distributions are compared to next-to-leading order perturbative quantum chromodynamics calculations with unbound nucleon and nuclear parton distribution functions (PDFs). These results give the first evidence that the gluon PDF at large Bjorken x in lead ions is strongly suppressed with respect to the PDF in unbound nucleons.

Energy and centrality dependent study of deconfinement phase transition in a color string percolation approach at RHIC energies

We take the experimental data for transverse momentum spectra of identified charged hadrons in different centrality classes for nucleus-nucleus (A+A) collisions at various Relativistic Heavy-Ion Collider (RHIC) energies measured by the STAR collaboration. We analyse these data in the framework of color string percolation model (CSPM) in order to extract various percolation parameters at different centralities at RHIC energies to study the effect of collision geometry and collision energy. We use these parameters to study the centrality dependent behaviour of initial temperature of the percolation cluster, energy density, average transverse momentum, shear viscosity to entropy density ratio ($\eta/s$) and trace anomaly for different energies at RHIC from $\sqrt{s_{\rm NN}}$ = 19.6 to 200 GeV. These observables are found to strongly depend on centrality at various collision energies. The critical percolation density, which is related to the deconfinement phase transition is achieved in the most central nucleus-nucleus collisions, while it fails in the peripheral collisions. A universal scaling is observed in color suppression factor and initial temperatures when studied as a function of charged particle pseudorapidity distribution scaled by nuclear overlap area at RHIC energies. The minimum of $\eta/s$ is observed in the most central collisions at $\sqrt{s_{\rm NN}}$ = 130 and 200 GeV.

Pion nucleus Drell\u2013Yan process and parton transverse momentum in the pion

We present a thorough analysis of unpolarized Drell–Yan (DY) pair production in pion–nucleus scattering. On the nucleus side, we use nuclear parton distributions along with parametrisations of the nucleon partonic transverse distribution available in the literature. Partonic longitudinal and transverse distributions of the pion are those obtained in a recent calculation in a Nambu–Jona Lasinio (NJL) framework, with Pauli–Villars regularization. The scale of the NJL model is determined with a minimisation procedure comparing NLO predictions based on NJL evolved pion distributions to rapidity differential DY cross sections data. The resulting distributions are then used to describe, up to next-to-leading logarithmic accuracy, the transverse momentum spectrum of dilepton pairs up to a transverse momentum of 2 GeV. With no additional parameters, fair agreement is found with available pion–nucleus data, confirming the virtues of the NJL description of pion parton structure. We find sizable evolution effects on the shape of the distributions and on the generated average transverse momentum of the dilepton pair. We furthermore discuss the possibility of gaining information about the behavior of the pion unpolarized transverse momentum dependent parton distribution from pion nucleus DY data.

Bose-Einstein correlations in pp, pPb , and PbPb collisions at sNN=0.9–7 TeV

Quantum statistical (Bose-Einstein) two-particle correlations are measured in pp collisions at $\sqrt{s}=$ 0.9, 2.76, and 7 TeV, as well as in pPb and peripheral PbPb collisions at nucleon-nucleon center-of-mass energies of 5.02 and 2.76 TeV, respectively, using the CMS detector at the LHC. Separate analyses are performed for same-sign unidentified charged particles as well as for same-sign pions and kaons identified via their energy loss in the silicon tracker. The characteristics of the one-, two-, and three-dimensional correlation functions are studied as functions of the pair average transverse momentum ($k_\mathrm{T}$) and the charged-particle multiplicity in the event. For all systems, the extracted correlation radii steadily increase with the event multiplicity, and decrease with increasing $k_\mathrm{T}$. The radii are in the range 1-5 fm, the largest values corresponding to very high multiplicity pPb interactions and to peripheral PbPb collisions with multiplicities similar to those seen in pPb data. It is also observed that the dependencies of the radii on multiplicity and $k_\mathrm{T}$ largely factorize. At the same multiplicity, the radii are relatively independent of the colliding system and center-of-mass energy.

Forward–backward Correlations Between Mean Transverse Momenta in Pb–Pb Collisions with ALICE

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Connections between collinear and transverse-momentum-dependent polarized observables within the Collins–Soper–Sterman formalism

We extend the improved Collins–Soper–Sterman (iCSS) W+Y construction recently presented in [1] to the case of polarized observables, where we focus in particular on the Sivers effect in semi-inclusive deep-inelastic scattering. We further show how one recovers the expected leading-order collinear twist-3 result from a (weighted) qT-integral of the differential cross section. We are also able to demonstrate the validity of the well-known relation between the (TMD) Sivers function and the (collinear twist-3) Qiu–Sterman function within the iCSS framework. This relation allows for their interpretation as functions yielding the average transverse momentum of unpolarized quarks in a transversely polarized spin-12 target. We further outline how this study can be generalized to other polarized quantities.

Transverse-momentum-dependent multiplicities of charged hadrons in muon-deuteron deep inelastic scattering

A semi-inclusive measurement of charged hadron multiplicities in deep inelastic muon scattering off an isoscalar target was performed using data collected by the COMPASS Collaboration at CERN. The following kinematic domain is covered by the data: photon virtuality $Q^{2}>1$ (GeV/$c$)$^2$, invariant mass of the hadronic system $W > 5$ GeV/$c^2$, Bjorken scaling variable in the range $0.003 < x < 0.4$, fraction of the virtual photon energy carried by the hadron in the range $0.2 < z < 0.8$, square of the hadron transverse momentum with respect to the virtual photon direction in the range 0.02 (GeV/$c)^2 < P_{\rm{hT}}^{2} < 3$ (GeV/$c$)$^2$. The multiplicities are presented as a function of $P_{\rm{hT}}^{2}$ in three-dimensional bins of $x$, $Q^2$, $z$ and compared to previous semi-inclusive measurements. We explore the small-$P_{\rm{hT}}^{2}$ region, i.e. $P_{\rm{hT}}^{2} < 1$ (GeV/$c$)$^2$, where hadron transverse momenta are expected to arise from non-perturbative effects, and also the domain of larger $P_{\rm{hT}}^{2}$, where contributions from higher-order perturbative QCD are expected to dominate. The multiplicities are fitted using a single-exponential function at small $P_{\rm{hT}}^{2}$ to study the dependence of the average transverse momentum $\langle P_{\rm{hT}}^{2}\rangle$ on $x$, $Q^2$ and $z$. The power-law behaviour of the multiplicities at large $P_{\rm{hT}}^{2}$ is investigated using various functional forms. The fits describe the data reasonably well over the full measured range.

Prompt photon-jet angular correlations at central rapidities in p+A collisions

Photon-jet azimuthal correlations in proton-nucleus collisions are a promising tool for gaining information on the gluon distribution of the nucleus in the regime of non-linear color fields. We compute such correlations from the process $g+A\to q \bar{q} \gamma$ in the rapidity regime where both the projectile and target light-cone momentum fractions are small. By integrating over the phase space of the quark which emits the photon, subject to the restriction that the photon picks up most of the transverse momentum (to pass an isolation cut), we effectively obtain a $g+A\to q \gamma$ process. For nearly back-to-back photon-jet configurations we find that it dominates over the leading order process $q+A\to q \gamma$ by two less powers of $Q_\perp / Q_S$, where $\boldsymbol{Q}_\perp$ and $Q_S$ denote the net photon-jet pair momentum and the saturation scale of the nucleus, respectively. We determine the transverse momentum dependent gluon distributions involved in $g+A\to q \gamma$ and the scale where they are evaluated. Finally, we provide analytic expressions for $\langle\cos n\phi\rangle$ moments, where $\phi$ is the angle between $\boldsymbol{Q}_\perp$ and the average photon-jet transverse momentum $\tilde{\boldsymbol{P}}_\perp$, and first qualitative estimates of their transverse momentum dependence.