Fluctuations Of Mean Transverse Momentum Research Articles

Skewness and kurtosis of mean transverse momentum fluctuations at the LHC energies

The first measurements of skewness and kurtosis of mean transverse momentum (〈pT〉) fluctuations are reported in Pb–Pb collisions at sNN = 5.02 TeV, Xe–Xe collisions at sNN = 5.44 TeV and pp collisions at s=5.02 TeV using the ALICE detector. The measurements are carried out as a function of system size 〈dNch/dη〉|η|<0.51/3, using charged particles with transverse momentum (pT) and pseudorapidity (η), in the range 0.2<pT<3.0 GeV/c and |η|<0.8, respectively. In Pb–Pb and Xe–Xe collisions, positive skewness is observed in the fluctuations of 〈pT〉 for all centralities, which is significantly larger than what would be expected in the scenario of independent particle emission. This positive skewness is considered a crucial consequence of the hydrodynamic evolution of the hot and dense nuclear matter created in heavy-ion collisions. Furthermore, similar observations of positive skewness for minimum bias pp collisions are also reported here. Kurtosis of 〈pT〉 fluctuations is found to be in good agreement with the kurtosis of Gaussian distribution, for most central Pb–Pb collisions. Hydrodynamic model calculations with MUSIC using Monte Carlo Glauber initial conditions are able to explain the measurements of both skewness and kurtosis qualitatively from semicentral to central collisions in Pb–Pb system. Color reconnection mechanism in PYTHIA8 model seems to play a pivotal role in capturing the qualitative behavior of the same measurements in pp collisions.

Event-by-event fluctuations of mean transverse momentum in Pb–Pb and Xe–Xe collisions with ALICE

Event-by-event fluctuations of the mean transverse momentum of charged particles produced in Pb–Pb and Xe–Xe collisions at TeV and TeV, respectively, are studied as a function of the charged-particle multiplicity using the ALICE detector at the LHC. Dynamical fluctuations are observed in both collision systems which indicate correlated particle emission. The central Pb–Pb and Xe–Xe collisions show a significant reduction of the fluctuation in comparison to peripheral collisions and are in qualitative agreement with previous measurements in Pb–Pb collisions at TeV. The results are compared with the HIJING model. A clear deviation from simple superposition of independent nucleon-nucleon collisions scenario, where the final state particles are produced from superposition of particle emitting sources, is observed.

Higher-order transverse momentum fluctuations in heavy-ion collisions

In relativistic heavy-ion collisions, the event-by-event mean transverse momentum fluctuations are sensitive to overlap area and energy density fluctuations in initial state. We present a framework to calculate $p_{\mathrm{T}}$ fluctuations up to $4^{\mathrm{th}}$-order using standard and subevent methods, which is validated using the HIJING model. We observe a power-law dependence for cumulants of all orders as a function of charged particle multiplicity $N_{\mathrm{ch}}$, consistent with a simple independent source picture. The fluctuation in $pp$ collisions is observed to be larger than for $p+$Pb, Pb+Pb and Xe+Xe collisions at the same $N_{\mathrm{ch}}$ due to bias in number of contributing sources. The short-range correlations are greatly suppressed in subevent method in comparison to calculations based on the standard method. This study provides a baseline for transverse momentum fluctuations without the presence of final state effects.

Skewness of mean transverse momentum fluctuations in heavy-ion collisions

We propose the skewness of mean transverse momentum, $\langle p_t \rangle$, fluctuations as a fine probe of hydrodynamic behavior in relativistic nuclear collisions. We describe how the skewness of the $\langle p_t \rangle$ distribution can be analyzed experimentally, and we use hydrodynamic simulations to predict its value. We predict in particular that $\langle p_t \rangle$ fluctuations have positive skew, which is significantly larger than if particles were emitted independently. We elucidate the origin of this result by deriving generic formulas relating the fluctuations of $\langle p_t \rangle$ to the fluctuations of the early-time thermodynamic quantities. We postulate that the large positive skewness of $\langle p_t \rangle$ fluctuations is a generic prediction of hydrodynamic models.

Probing the QCD phase structure using event-by-event fluctuations

Heavy-ion collisions at relativistic energies probe matter at extreme conditions of temperatures and energy densities. The study of event-by-event fluctuations of experimental observables is crucial to probe the QCD phase transition, locate the critical point, and learn about the associated critical phenomena. At the critical point, all thermodynamic quantities behave anomalously. Fluctuation measurements provide access to thermodynamic response functions. We discuss the methods for obtaining the isothermal compressibility using particle multiplicity fluctuation, and specific heat using fluctuations in mean transverse momentum, temperature, and energy. Lattice QCD calculations have predicted non-monotonic behavior in the higher-order cumulants of conserved quantities at the critical point. Fluctuations in the multiplicity of charged to neutral particles have been measured to understand the formation of domains of disoriented chiral condensates. We review the recent fluctuation results as a function of collision centrality and energy from experiments at SPS, RHIC, and LHC. In addition, we propose to map the temperature fluctuations in η-φ plane to probe local fluctuations of temperature and energy density.

Event-by-event mean fluctuations and transverse size of color flux tube generated in p–p collisions at = 0.90 TeV

We propose a novel phenomenological model of mean transverse momentum fluctuations based on the geometrical scaling hypothesis. Bose–Einstein correlations between two gluons generated from an identical color flux tube are taken into account as a source of the fluctuation. We calculate an event-by-event fluctuation measure and show that ALICE data observed at TeV for p+p collisions are reproduced. By fitting our model to the experimental data, we evaluate the transverse size of the color flux tube as a function of the multiplicity.

Recent results on event-by-event fluctuations from the RHIC Beam Energy Scan program in the STAR experiment

Event-by-event fluctuations of global observables in relativistic heavy-ion collisions are studied as probes for the QCD phase transition and as tools to search for critical phenomena near the phase boundary. Dynamical fluctuations in mean transverse momentum, identified particle ratios and conserved quantities (such as net-charge, net-baryon) are expected to provide signatures of a de-confined state of matter. Non-monotonic behavior in the higher-moments of conserved quantities as a function of beam energy and collision centrality are proposed as signatures of the QCD critical point. To study the QCD phase transition and locate the critical point, the STAR experiment at RHIC has collected a large amount of data for Au+Au collisions from = 7.7 - 200 GeV in the RHIC Beam Energy Scan (BES) program. We present the recent beam energy scan results on dynamical fluctuations of particle ratios and two-particle transverse momentum correlations at mid-rapidity. Higher-moments of the net-charge and net- proton multiplicity distributions as a function of beam energy will be presented. We give a summary of what has been learnt so far and future prospectives for the BES-II program.

Correlations and fluctuations studied with ALICE

The measurement of particle correlations and event-by-event fluctuations of physical observables allows to study a large variety of properties of the matter produced in ultra relativistic heavy-ion collisions. We will present results for two-particle correlations, mean transverse momentum fluctuations, and net charge fluctuations in Pb-Pb collisions at = 2.76 TeV.

First Event-by-Event Fluctuation Studies in Pb-Pb Collisions at LHC Energy by the ALICE Experiment

The presence of the phase transition can manifest itself by the characteristic behavior of several observables which may vary dramatically from one event to the other. Thus, the study of various conserved quantities on an event-by-event basis offers the possibility to study the phase transition and the nature of high density matter. The ALICE experiment is well suited for precise event-by-event measurements of various quantities. In this article, the event-by-event fluctuations of mean transverse momentum and net-charge distributions as measured by the ALICE experiment are presented.

Event-by-event fluctuations of transverse momentum and multiparticle clusters in relativistic heavy-ion collisions

We analyze the event-by-event fluctuations of mean transverse momentum measured recently by the PHENIX and STAR Collaborations at RHIC. We argue that the observed scaling of strength of dynamical fluctuations with the inverse number of particles can be naturally explained by formation of multiparticle clusters.

Comparison of two types of event-by-event transverse momentum fluctuation measurements

Physically more transparent formulas for measuring event-by-event dynamical mean transverse momentum fluctuations ${\ensuremath{\sigma}}^{2}({M}_{{p}_{t},\mathrm{dyn}})$ are derived. It turns out that ${\ensuremath{\sigma}}^{2}({M}_{{p}_{t},\mathrm{dyn}})$ equals the dynamical fluctuations of the event total transverse momentum divided by the square of the average sample multiplicities. It is equivalent to the dynamical fluctuations of the event average transverse momentum ${\ensuremath{\sigma}}^{2}({M}_{{p}_{t}}){}_{\mathrm{dyn}}$ when multiplicity n is fixed and different from ${\ensuremath{\sigma}}^{2}({M}_{{p}_{t}}){}_{\mathrm{dyn}}$ when n varies from event to event. HIJING-1.382 simulations confirm the analytical results.

Event-wise〈pt〉fluctuations in Au-Au collisions atsNN=130GeV

We present the first large-acceptance measurement of event-wise mean transverse momentum fluctuations for Au-Au collisions at nucleon-nucleon center-of-momentum collision energy root s(NN) = 130 GeV. The observed nonstatistical fluctuations substantially exceed in magnitude fluctuations expected from the finite number of particles produced in a typical collision. The r.m.s. fractional width excess of the event-wise distribution is 13.7 +/- 0.1(stat) +/- 1.3(syst)% relative to a statistical reference, for the 15% most-central collisions and for charged hadrons within pseudorapidity range vertical bar eta vertical bar fluctuation excess is qualitatively larger than those observed at lower energies and differs markedly from theoretical expectations. Contributions to fluctuations from semihard parton scattering in the initial state and dissipation in the bulk colored medium are discussed.

Global variables and identified hadrons in the PHENIX experiment

PHENIX measurements related to global variables and identified hadrons are discussed. These include two-pion correlations, elliptic flow, and dN/dη. Measurements of event-by-event fluctuations in mean transverse momentum, mean transverse energy, and net charge are presented for particles within the PHENIX acceptance. The centrality dependence of these measurements is also discussed.

Event-by-event fluctuations of mean transverse momentum in AuAu collisions in STAR experiment at relativistic heavy-ion collider

We report results on event-by-event fluctuations in mean transverse momentum in AuAu collisions at\(\sqrt {^s NN} = 130\) GeV measured by the STAR experiment at RHIC. The dynamical fluctuations,\(\sigma _{\langle P_t \rangle ,dynam} \) is found to be about 1.2 ±02% of the mean transverse momentum for particles in pseudo-rapidity range of −0.5 < η < 0.5 and for the top 6% central collisions.

Correlations, fluctuations, and flow measurements from the STAR experiment

New measurements of short-range and long-range two-particle correlations, azimuthal anisotropy, and event-by-event fluctuations from the STAR experiment for S N N = 130 and 200 GeV Au+Au collisions are summarized. Striking evidence is presented for large, non-statistical fluctuations in mean transverse momentum. Descriptions of the data in terms of phenomenological source function models are also presented.

Event-by-event fluctuations in meanpTand meaneTinsNN=130 GeVAu+Au collisions

Distributions of event-by-event fluctuations of the mean transverse momentum and mean transverse energy near mid-rapidity have been measured in Au+Au collisions at roots(NN)=130 GeV at the Relativistic Heavy-Ion Collider. By comparing the distributions to what is expected for statistically independent particle emission, the magnitude of nonstatistical fluctuations in mean transverse momentum is determined to be consistent with zero. Also, no significant nonrandom fluctuations in mean transverse energy are observed. By constructing a fluctuation model with two event classes that preserve the mean and variance of the semi-inclusive p(T) or e(T) spectra, we exclude a region of fluctuations in roots(NN)=130 GeV Au+Au collisions.