Average Transverse Momentum Research Articles

Femtoscopy with identified charged pions in proton-lead collisions at sNN=5.02 TeV with ATLAS

Bose-Einstein correlations between identified charged pions are measured for $p$+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV using data recorded by the ATLAS detector at the LHC corresponding to a total integrated luminosity of $28$ $\mathrm{nb}^{-1}$. Pions are identified using ionization energy loss measured in the pixel detector. Two-particle correlation functions and the extracted source radii are presented as a function of collision centrality as well as the average transverse momentum ($k_{\mathrm{T}}$) and rapidity ($y^{\star}_{\pi\pi}$) of the pair. Pairs are selected with a rapidity $-2 < y^{\star}_{\pi\pi} < 1$ and with an average transverse momentum $0.1 < k_{\mathrm{T}} < 0.8$ GeV. The effect of jet fragmentation on the two-particle correlation function is studied, and a method using opposite-charge pair data to constrain its contributions to the measured correlations is described. The measured source sizes are substantially larger in more central collisions and are observed to decrease with increasing pair $k_{\mathrm{T}}$. A correlation of the radii with the local charged-particle density is demonstrated. The scaling of the extracted radii with the mean number of participating nucleons is also used to compare a selection of initial-geometry models. The cross-term $R_\mathrm{ol}$ is measured as a function of rapidity, and a nonzero value is observed with $5.1\sigma$ combined significance for $-1 < y^{\star}_{\pi\pi} < 1$ in the most central events.

New insights into hadron production mechanism from pT spectra in pp collisions at s=7 TeV

We show that the experimental data of mid-rapidity $p_{T}$ spectra for proton, $\Lambda$, $\Xi$ , $\Omega^{-}$, $\text{K(892)}^{*0}$ and $\Xi(1530)^{*0}$ in minimum-bias $pp$ collisions at $\sqrt{s}=7$ TeV can be systemically explained by the quark combination mechanism of hadronization. The averaged transverse momentum $\langle p_{T}\rangle$ and spectra ratios such as $\Xi/\Lambda$ and $\Omega/\phi$ calculated from quark combination reproduce the data much better than those from traditional string and/or cluster fragmentation. The available data of hadronic $p_{T}$ spectra released by ALICE collaboration in the first three high-multiplicity classes of $pp$ collisions at $\sqrt{s}=7$ TeV are also well explained. We make predictions for other hadrons, and propose two scaling behaviors among decuplet baryons and vector mesons as the effective probe of hadron production mechanism at such high collision energy.

Modeling of Transverse Momentum Spectra for Charged Particles in Proton–Proton Collisions Based on Soft Computing Approaches

ABSTRACTThe transverse momentum (pT) distributions (or spectra) of charged particles produced in high and ultra-high energy proton–proton (pp) collisions are studied by the neuro-fuzzy model. The objective of the present work is developing an Adaptive Neuro-Fuzzy Inference System (ANFIS) for calculating and predicting the transverse momentum spectra of charged particles as a function of pT and the center-of-mass energy (), as well as the modeling of the average transverse momentum ⟨pT⟩ as a function of , i.e., we have proposed and developed two models. The ANFIS models were designed based on available experimental data for = 53 GeV, 200 GeV, 546 GeV, 900 GeV, 1800 GeV, 2360 GeV and 7 TeV. The empirical results from the developed ANFIS models for pT distributions, as well as ⟨pT⟩ for pp collisions are compared with the theoretical ones which are obtained from other models. The comparison results show a great deal of agreement between the available experimental data (up to 7 TeV) and the theoretical ones. At full large hadron collider (LHC) energy ( = 14 TeV), we have predicted the pT spectra and ⟨pT⟩ which also, show a good agreement with different models.

J/ψ production as a function of charged-particle pseudorapidity density in p–Pb collisions at [formula omitted

We report measurements of the inclusive J/ψ yield and average transverse momentum as a function of charged-particle pseudorapidity density dNch/dη in p–Pb collisions at sNN=5.02TeV with ALICE at the LHC. The observables are normalised to their corresponding averages in non-single diffractive events. An increase of the normalised J/ψ yield with normalised dNch/dη, measured at mid-rapidity, is observed at mid-rapidity and backward rapidity. At forward rapidity, a saturation of the relative yield is observed for high charged-particle multiplicities. The normalised average transverse momentum at forward and backward rapidities increases with multiplicity at low multiplicities and saturates beyond moderate multiplicities. In addition, the forward-to-backward nuclear modification factor ratio is also reported, showing an increasing suppression of J/ψ production at forward rapidity with respect to backward rapidity for increasing charged-particle multiplicity.

Measurement of the triple-differential dijet cross section in proton-proton collisions at sqrt{s}=8,text {TeV} and constraints on parton distribution functions

A measurement is presented of the triple-differential dijet cross section at a centre-of-mass energy of 8,text {TeV} using 19.7,text {fb}^text {-1} of data collected with the CMS detector in proton-proton collisions at the LHC. The cross section is measured as a function of the average transverse momentum, half the rapidity separation, and the boost of the two leading jets in the event. The cross section is corrected for detector effects and compared to calculations in perturbative quantum chromodynamics at next-to-leading order accuracy, complemented with electroweak and nonperturbative corrections. New constraints on parton distribution functions are obtained and the inferred value of the strong coupling constant is alpha _S(M_text {Z}) = 0.1199,pm {0.0015},(mathrm {exp}), _{-0.0020}^{+0.0031},(mathrm {theo}), where M_text {Z} is the mass of the Z boson.

Anisotropic hydrodynamic modeling of 2.76 TeV Pb-Pb collisions

We compare phenomenological results from 3+1d quasiparticle anisotropic hydrodynamics (aHydroQP) with experimental data collected in LHC 2.76 TeV Pb-Pb collisions. In particular, we present comparisons of particle spectra, average transverse momentum, elliptic flow, and HBT radii. The aHydroQP model relies on the introduction of a single temperature-dependent quasiparticle mass which is fit to lattice QCD data. By taking moments of the resulting Boltzmann equation, we obtain the dynamical equations used in the hydrodynamic stage which include the effects of both shear and bulk viscosities. At freeze-out, we use anisotropic Cooper-Frye freeze-out performed on a fixed-energy-density hypersurface to convert to hadrons. To model the production and decays of the hadrons we use THERMINATOR 2 which is customized to sample from ellipsoidal momentum-space distribution functions. Using smooth Glauber initial conditions, we find very good agreement with many heavy-ion collision observables.

Bulk properties of the medium produced in relativistic heavy-ion collisions from the beam energy scan program

We present measurements of bulk properties of the matter produced in Au+Au collisions at sNN=7.7,11.5,19.6,27, and 39 GeV using identified hadrons (π±, K±, p, and p¯) from the STAR experiment in the Beam Energy Scan (BES) Program at the Relativistic Heavy Ion Collider (RHIC). Midrapidity (|y|<0.1) results for multiplicity densities dN/dy, average transverse momenta 〈pT〉, and particle ratios are presented. The chemical and kinetic freeze-out dynamics at these energies are discussed and presented as a function of collision centrality and energy. These results constitute the systematic measurements of bulk properties of matter formed in heavy-ion collisions over a broad range of energy (or baryon chemical potential) at RHIC.31 MoreReceived 25 January 2017DOI:https://doi.org/10.1103/PhysRevC.96.044904©2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasParticle & resonance productionQCD phase transitionsQuark-gluon plasmaRelativistic heavy-ion collisionsThermal & statistical modelsNuclear Physics

Femtoscopy in [formula omitted] p+Pb collisions with ATLAS

Bose-Einstein correlations between identified charged pions are measured for p+Pb collisions at sNN=5.02 TeV with the ATLAS detector using a total integrated luminosity of 28 nb−1. Pions are identified using ionisation energy loss measured in the pixel silicon detector. Two-particle correlation functions and the extracted source radii are presented as a function of collision centrality as well as average transverse pair momentum (kT), rapidity (yππ⋆), and azimuthal angle with respect to the second-order event plane. Pairs are selected with a rapidity −2<yππ⋆<1 and with an average transverse momentum 0.1<kT<0.8 GeV. The effect on the two-particle correlation function from jet fragmentation is studied, and a new method for constraining its contributions to the measured correlations is described. The measured source sizes are substantially larger in more central collisions and are observed to decrease with increasing pair kT. A correlation with the local single-particle multiplicity dNch/dy⋆ is demonstrated. The cross term Rol, which couples radial and longitudinal expansion, is measured as a function of rapidity, and a departure from zero is observed with 5.1σ combined significance for yππ⋆>−1 in the most central events. The azimuthal modulation of the radii in central events is observed to be consistent with that predicted by hydrodynamics and observed in A+A collisions.

Transport coefficients from energy loss studies in an expanding QGP

We use linear viscous hydrodynamics to describe the energy and momentum deposited by a fast moving parton in a quark gluon plasma. This energy-momentum is used to compute the probability density for the production of soft partons by means of the Cooper-Frye formula. Using this probability density, we render manifest a relation between the average transverse momentum given to the fast moving parton from the medium, q̂, the shear viscosity to entropy density ratio, η/s, and the energy lost by the fast moving parton, ΔE, in an expanding medium under similar conditions to those generated in nucleus-nucleus collisions at the LHC and RHIC energies. We find that q̂ increases with ΔE. On the other hand, η/s is more stable with ΔE. The behavior of q̂, with ΔE is understood as arising from the length of medium the parton traverses from the point where it is produced. However, since η/s is proportional to the ratio of the length of medium traversed by the fast parton and the average number of scatterings it experiences, it has a milder dependence on the energy it loses. This study represents a tool to obtain a direct connection between transport coefficients and the description of in-medium energy loss within a linear viscous hydrodynamical evolution of the bulk.

(3+1)D Quasiparticle Anisotropic Hydrodynamics for Ultrarelativistic Heavy-Ion Collisions.

We present the first comparisons of experimental data with phenomenological results from (3+1)D quasiparticle anisotropic hydrodynamics (aHydroQP). We compare particle spectra, average transverse momentum, and elliptic flow. The dynamical equations used for the hydrodynamic stage utilize aHydroQP, which naturally includes both shear and bulk viscous effects. The (3+1)D aHydroQP evolution obtained is self-consistently converted to hadrons using anisotropic Cooper-Frye freeze-out. Hadron production and decays are modeled using a customized version of therminator 2. In this first study, we utilized smooth Glauber-type initial conditions and a single effective freeze-out temperature T_{FO}=130 MeV with all hadronic species in full chemical equilibrium. With this rather simple setup, we find a very good description of many heavy-ion observables.

Pseudorapidity profile of transverse momentum fluctuations in heavy ion collisions

We investigate pseudorapidity correlations of the average transverse flow of particles emitted in relativistic heavy-ion collisions. We employ 3+1 dimensional viscous relativistic hydrodynamics with initial conditions from the quark Glauber Monte Carlo model to confront the recent measurements on the pseudorapidity correlations of the transverse momentum fluctuations in Pb+Pb collisions at $\sqrt{s_{NN}}=2760$GeV. We find good agreement between the model predictions and data. Further, we study two other observables build with the covariance of the average transverse momentum in different rapidity bins. These observables have better stability under various systematics, thus allowing for a robust comparison between data and model. The transverse flow-transverse flow correlation coefficient is directly related to correlations of the underlying collective flow at different pseudorapidities. The 3-bin measure of $p_T$ factorization breaking in pseudorapidity gives an estimate of possible decorrelation of the average transverse flow in the longitudinal direction.

Transverse momentum fluctuations in ultrarelativistic Pb + Pb and p + Pb collisions with “wounded” quarks

We analyze the phenomenon of size-flow transmutation in ultrarelativistic nuclear collisions in a model where the initial size fluctuations are driven by the wounded quarks and the collectivity is provided by viscous hydrodynamics. It is found that the model properly reproduces the data for the transverse momentum fluctuations measured for Pb+Pb collisions at 2.76TeV by the ALICE Collaboration. The agreement holds for a remarkably wide range of centralities, from 0-5% up to 70-80%, and displays a departure from a simple scaling with $(dN_{\rm ch}/d\eta)^{1/2}$ in the form seen in the data. The overall agreement in the model with wounded quarks is significantly better than with nucleon participants. This feature joins the previously-found wounded quark multiplicity scaling in the argumentation in favor of subnucleonic degrees of freedom in the early dynamics. We also examine in detail the correlations between measures of the initial size and final average transverse momentum of hadrons. Predictions are made for the transverse momentum fluctuations in p+Pb collisions at 5.02TeV.

Coherent fragmentation of 12C nuclei of momentum 4.5 GeV/c per nucleon through the 8Beg.s.+4He channel in a nuclear photoemulsion containing lead nuclei

A two-particle channel in which an unbound nucleus of 8Be in the ground state (8Beg.s.) was one of the fragments was selected among events where 12C nuclei of momentum 4.5 GeV/c per nucleon undergo coherent dissociation into three alpha particles. The events in question were detected in a track nuclear photoemulsion containing lead nuclei, which was irradiated at the synchrophasotron of the Laboratory of High Energies at the Joint Institute for Nuclear Research (JINR, Dubna). The average transverse momentum of alpha particles produced upon the decay of 8Beg.s. nuclei was 87±6 MeV/c, while that for “single” alpha (αs) particles was 123±15 MeV/c. The average value of the transverse-momentum transfer in the reaction being considered, Pt(12C), was 223 ± 20 MeV/c. The average value of the cross section for this channel involving Ag and Br target nuclei was 13 ± 4 mb, while the cross section for the reaction on the Pb nucleus was 40 ± 15 mb. The Coulomb dissociation contribution evaluated on the basis of the number of events where the momentum P t(12C) did not exceed 0.1 GeV/c saturated about 20%. In nine events, the measured total transverse energy of the fragments in the reference frame comoving with the decaying carbon nucleus did not exceed 0.45 MeV, which did not contradict the excitation of the participant 12C nucleus to the level at 7.65 MeV. The average value of the transverse momentum in those events was 234 ± 25 MeV/c.

Rapidity dependence of the average transverse momentum in p+p and p+Pb collisions revisited

We revisit the calculation of rapidity dependence of the average transverse momentum $⟨{p}_{T}⟩$ of single inclusive jet production or particle in a $p+p$ and $p\ensuremath{-}\mathrm{Pb}$ collision, using the high energy factorization. We update previous predictions for the $⟨{p}_{T}⟩$ both in central and forward rapidity region using parton densities following from extended Balitsky-Kovchegov and Balitsky-Fadin-Kuraev-Lipatov evolution equations to account for corrections of higher orders as well as one obtained by Sudakov resummation. Furthermore, we demonstrate that in the midrapidity region the saturation based formalisms predict increase of the transversal momentum while in the forward rapidity region the $⟨{p}_{T}(y)⟩$ decreases.

Estimating nonlinear effects in forward dijet production in ultra-peripheral heavy ion collisions at the LHC

Using the framework that interpolates between the leading power limit of the color glass condensate and the high energy (or k_{T}) factorization we calculate the direct component of the forward dijet production in ultra-peripheral mathrm {Pb}–mathrm {Pb} collisions at CM energy 5.1,mathrm {TeV} per nucleon pair. The formalism is applicable when the average transverse momentum of the dijet system P_{T} is much bigger than the saturation scale Q_{s}, P_{T}gg Q_{s}, while the imbalance of the dijet system can be arbitrary. The cross section is uniquely sensitive to the Weizsäcker–Williams (WW) unintegrated gluon distribution, which is far less known from experimental data than the most common dipole gluon distribution appearing in inclusive small-x processes. We have calculated cross sections and nuclear modification ratios using WW gluon distribution obtained from the dipole gluon density through the Gaussian approximation. The dipole gluon distribution used to get WW was fitted to the inclusive HERA data with the nonlinear extension of unified BFKL + DGLAP evolution equation. The saturation effects are visible but rather weak for realistic p_{T} cut on the dijet system, reaching about 20% with the cut as low as 6,mathrm {GeV}. We find that the LO collinear factorization with nuclear leading-twist shadowing predicts quite similar effects.

Quasiparticle anisotropic hydrodynamics for central collisions

We use quasiparticle anisotropic hydrodynamics to study an azimuthally-symmetric boost-invariant quark-gluon plasma including the effects of both shear and bulk viscosities. In quasiparticle anisotropic hydrodynamics, a single finite-temperature quasiparticle mass is introduced and fit to the lattice data in order to implement a realistic equation of state. We compare results obtained using the quasiparticle method with the standard method of imposing the equation of state in anisotropic hydrodynamics and viscous hydrodynamics. Using these three methods, we extract the primordial particle spectra, total number of charged particles, and average transverse momentum for various values of the shear viscosity to entropy density ratio eta/s. We find that the three methods agree well for small shear viscosity to entropy density ratio, eta/s, but differ at large eta/s. We find, in particular, that when using standard viscous hydrodynamics, the bulk-viscous correction can drive the primordial particle spectra negative at large p_T which is clearly unphysical. Such a behavior is not seen in either anisotropic hydrodynamics approach, irrespective of the value of eta/s.

Femtoscopy in = 5.02 TeV p-Pb collisions with ATLAS

Bose-Einstein correlations between identified charged pions are measured for p+Pb collisions at = 5.02 TeV with the ATLAS detector using a total integrated luminosity of 28 nb−1. Pions are identified using ionisation energy loss measured in the pixel detector. Two-particle correlation functions and the extracted source radii are presented as a function of average transverse pair momentum (kT) and rapidity () as well as collision centrality. Pairs are selected with a rapidity −2 < < 1 and with an average transverse momentum 0.1 < kT < 0.8 GeV. The effect on the two-particle correlation function from jet fragmentation is studied, and a new method for constraining its contributions to the measured correlations is described. The measured source sizes are substantially larger in more central collisions and are observed to decrease with increasing pair kT. A correlation with the local single-particle multiplicity dNch/dy* is demonstrated. The scaling of the extracted radii with the mean number of participants is also used to compare a selection of initial-geometry models. The cross term Rol, which couples radial and longitudinal expansion, is measured as a function of rapidity, and a departure from zero is observed with 4.8 σ combined significance for > −1 in the most central events.

Asymptotic behavior of the correlation coefficients of transverse momenta in the model with string fusion

In the framework of the model with fusion of quark–gluon strings on the transverse lattice, we find the asymptotic behavior of the correlation coefficients between observables in separated rapidity intervals with a high string density in a realistic case with an inhomogeneous distribution of strings in the impact parameter plane. We calculate the asymptotic forms for three types of correlations: between the average transverse momenta of particles with rapidity in these intervals, between the average transverse momentum of particles in one rapidity interval and the multiplicity of particles in another, and also between the multiplicities of charged particles in these intervals. We show that the previously found independence of the asymptotic form of the correlation coefficient between the average transverse momenta from the variance in the number of particles produced in string fragmentation holds only in the case of a uniform distribution of strings in the transverse plane. We also show that the found general expressions for the long-range correlation coefficients in the particular case with a uniform distribution of strings in the transverse plane become the formulas previously obtained by another method applicable only in this simple case.

Femtoscopy in √SNN= 5.02 TeVp-Pb collisions withATLAS

Bose-Einstein correlations between identified charged pions are measured for p +Pb collisions at √S NN = 5.02 TeV with the ATLAS detector using a total integrated luminosity of 28 nb−1 . Pions are identified using ionisation energy loss measured in the pixel detector. Two-particle correlation functions and the extracted source radii are presented as a function of average transverse pair momentum (k T ) and rapidity (y ππ *) as well as collision centrality. Pairs are selected with a rapidity −2 ππ * T T . A correlation with the local single-particle multiplicity dN ch /dy * is demonstrated. The scaling of the extracted radii with the mean number of participants is also used to compare a selection of initial-geometry models. The cross term R o1 , which couples radial and longitudinal expansion, is measured as a function of rapidity, and a departure from zero is observed with 4.8 σ combined significance for y ππ * > −1 in the most central events.

Thermodynamical string fragmentation

The observation of heavy-ion-like behaviour in pp collisions at the LHC suggests that more physics mechanisms are at play than traditionally assumed. The introduction e.g. of quark-gluon plasma or colour rope formation can describe several of the observations, but as of yet there is no established paradigm. In this article we study a few possible modifications to the Pythia event generator, which describes a wealth of data but fails for a number of recent observations. Firstly, we present a new model for generating the transverse momentum of hadrons during the string fragmentation process, inspired by thermodynamics, where heavier hadrons naturally are suppressed in rate but obtain a higher average transverse momentum. Secondly, close-packing of strings is taken into account by making the temperature or string tension environment-dependent. Thirdly, a simple model for hadron rescattering is added. The effect of these modifications is studied, individually and taken together, and compared with data mainly from the LHC. While some improvements can be noted, it turns out to be nontrivial to obtain effects as big as required, and further work is called for.