Forward-backward Multiplicity Correlations Research Articles

Removing the influence of centrality fluctuations in the analysis of forward-backward multiplicity correlations in heavy-ion collisions

In high-energy heavy-ion collisions, forward-backward multiplicity (FB) correlation strengths are affected greatly by centrality fluctuation and centrality window width. A method called ${\mathrm{FB}}_{\mathrm{relative}}$ is raised to reduce or remove the influence. This method is tested by a Monte Carlo simulation and is compared with the ${\mathrm{FB}}_{\mathrm{average}}$ and ${\mathrm{FB}}_{\mathrm{profile}}$ methods. The ${\mathrm{FB}}_{\mathrm{relative}}$ method is also used for the hijing event generator of Au+Au collisions, and comparisons between the correlation strengths of different collision energies $\sqrt{{s}_{NN}}$ from 7.7 to 200 GeV are shown. As a result, the correlation strengths are all very weak, and the similarities of correlation strengths $b$ of different centrality windows within 0%--50% are obvious, and it is observed that the correlation strengths decrease with the increasing pseudorapidity gap at $\left|\ensuremath{\eta}\right|l2$.

Forward-backward multiplicity correlations caused by centrality fluctuations in high-energy heavy-ion collisions

We consider that most of the long-range forward-backward multiplicity (FB) correlations in high energy $A\text{\ensuremath{-}}A$ collisions are caused by the centrality fluctuation, and this phenomenon interferes with the measurement of the dynamical correlations greatly. We investigate the relationship between FB correlation strength and centrality by a Monte Carlo simulation and a derivation which are tested by A MultiPhases Transport (AMPT) model in Au+Au collisions at $\sqrt{{s}_{\mathrm{NN}}}=200$ GeV. We compare the FB correlation strengths of AMPT model with the results of the derivation at $\sqrt{{s}_{\mathrm{NN}}}$ $=$ $7.7$ to 200 GeV. A comparison between the default AMPT model and string melting AMPT model with different partonic scattering sections is made. As a result, we consider that the FB correlation strengths may be dominated by the mixing of different centrality events, and the short-range correlation may be overwhelmed for the most central collisions.

Forward-Backward Multiplicity Correlations in Relativistic Heavy-Ion Collisions

Forward-backward multiplicitycorrelations are investigated by analyzing the experimental data on 6O-AgBr collisions at 14.5A, 60A and 200A GeV/c and the findings are compared with the predictions of Monte Carlo model, hIJING. the findings indicate that the observed correlations are mainly of short-range in nature which arise due to the decay of clusters and (or) resonances produced in the central rapidity region. the result reveal that the range of F-B correlations extend to rather longer range with increasing beam energy which might be due to overall multiplicity fluctuations arising because of nuclear geometry. however, there is no evidence for the presence of long-range correlations even at the highest beam energy considered.

Forward-backward multiplicity correlations in pp collisions at s $$ \sqrt{s} $$ = 0.9, 2.76 and 7 TeV

The strength of forward-backward (FB) multiplicity correlations is measured by the ALICE detector in proton-proton (pp) collisions at $\sqrt{s}=0.9$, 2.76 and 7 TeV. The measurement is performed in the central pseudorapidity region ($|\eta| < 0.8$) for the transverse momentum $p_{\rm T}>0.3$ GeV/$c$. Two separate pseudorapidity windows of width ($\delta \eta$) ranging from 0.2 to 0.8 are chosen symmetrically around $\eta=0$. The multiplicity correlation strength ($b_{\rm cor}$) is studied as a function of the pseudorapidity gap ($\eta_{\rm gap}$) between the two windows as well as the width of these windows. The correlation strength is found to decrease with increasing $\eta_{\rm gap}$ and shows a non-linear increase with $\delta\eta$. A sizable increase of the correlation strength with the collision energy, which cannot be explained exclusively by the increase of the mean multiplicity inside the windows, is observed. The correlation coefficient is also measured for multiplicities in different configurations of two azimuthal sectors selected within the symmetric FB $\eta$-windows. Two different contributions, the short-range (SR) and the long-range (LR), are observed. The energy dependence of $b_{\rm cor}$ is found to be weak for the SR component while it is strong for the LR component. Moreover, the correlation coefficient is studied for particles belonging to various transverse momentum intervals chosen to have the same mean multiplicity. Both SR and LR contributions to $b_{\rm cor}$ are found to increase with $p_{\rm T}$ in this case. Results are compared to PYTHIA and PHOJET event generators and to a string-based phenomenological model. The observed dependencies of $b_{\rm cor}$ add new constraints on phenomenological models.

Search for Long-Range Correlations in Relativistic Heavy-Ion Collisions at SPS Energies

Long-range correlations are searched for by analysing the experimental data on16O-AgBr and32S-AgBr collisions at 200 A GeV/c and the results are compared with the predictions of a multi phase transport (AMPT) model. The findings reveal that the observed forward-backward (F-B) multiplicity correlations are mainly of short range in nature. The range of F-B correlations are observed to extend with increasing projectile mass. The observed extended range of F-B correlations might be due to overall multiplicity fluctuations arising because of nuclear geometry. The findings are not sufficient for making any definite conclusions regarding the presence of long-range correlations.

Method for High Accuracy Multiplicity Correlation Measurements

A novel method for measuring particle multiplicity correlations is presented. The method takes reduced acceptance and detection efficiency into account, in a purely statistical manner. The method does therefore not require any information from simulations - only knowledge of the detectors capability. The method provides the ability to measure forward-backward particle multiplicity correlations with high accuracy and negligible bias. The validity of the method is provided through numerous simulations. The correlation values calculated from carefully selected detection acceptances and efficiencies are compared to the con-elation values at the event-generator level. Further validity to the method is given by incorporating different event generators into the simulations. Although the emphasis is on fwd-bwd correlations, the method can be extended to other multiplicity correlation measurements. (Less)

Forward-backward multiplicity correlations of target fragments in nucleus-emulsion collisions at a few hundred MeV/u

The forward-backward multiplicity and correlations of a target evaporated fragment (black track particle) and target recoiled proton (grey track particle) emitted from 150 A MeV 4He, 290 A MeV 12C, 400 A MeV 12C, 400 A MeV 20Ne and 500 A MeV 56Fe induced different types of nuclear emulsion target interactions are investigated. It is found that the forward and backward averaged multiplicity of a grey, black and heavily ionized track particle increases with the increase of the target size. The averaged multiplicity of a forward black track particle, backward black track particle, and backward grey track particle do not depend on the projectile size and energy, but the averaged multiplicity of a forward grey track particle increases with an increase of projectile size and energy. The backward grey track particle multiplicity distribution follows an exponential decay law and the decay constant decreases with an increase of target size. The backward-forward multiplicity correlations follow linear law which is independent of the projectile size and energy, and the saturation effect is observed in some heavy target data sets.

Forward--Backward Multiplicity Correlations in Relativistic Heavy-ion Collisions

Forward-backward multiplicitycorrelations are investigated by analyzing the experimental data on O-AgBr collisions at 14.5A, 60A and 200A GeV/c and the findings are compared with the predictions of Monte Carlo model, hIJING. the findings indicate that the observed correlations are mainly of short-range in nature which arise due to the decay of clusters and (or) resonances produced in the central rapidity region. the result reveal that the range of F-B correlations extend to rather longer range with increasing beam energy which might be due to overall multiplicity fluctuations arising because of nuclear geometry. however, there is no evidence for the presence of long-range correlations even at the highest beam energy considered.

Forward–backward multiplicity correlation in high-energy nucleus–nucleus interactions at a few AGeV/c

We have presented a systematic study of two-particle rapidity correlations in terms of investigating the dynamical fluctuation observable in the forward–backward pseudo-rapidity windows by analyzing the experimental data of interactions at 4.5 AGeV/c, interactions at 4.1 AGeV/c, and interactions at 4.5 AGeV/c. The experimental results have been compared with the results obtained from the analysis of event sample simulated (MC-RAND) by generating random numbers and also with the analysis of events generated by the UrQMD and AMPT model. Our study confirms the presence of strong short-range correlations among the produced particles in the forward and the backward pseudo-rapidity region. The analysis of the simple Monte Carlo-simulated (MC-RAND) events signifies that the observed correlations are not due to mere statistics only; explanation of such correlations can be attributed to the presence of dynamical fluctuations during the production of charged pions. Comparisons of the experimental results with the results obtained from analyzing the UrQMD data sample indicate that the UrQMD model cannot reproduce the experimental findings. The AMPT model also cannot explain the experimental results satisfactorily. Comparisons of our experimental results with the results obtained from the analysis of higher energy emulsion data and with the results of the RHIC data have also been presented.

Forward-backward multiplicity correlations in relativistic heavy-ion collisions in a superposition approach

We analyze the multiplicity correlations between distant forward and backward rapidity regions in relativistic heavy-ion collisions in a superposition framework, where the particle production occurs through independent emission from correlated sources. This in principle allows for inferring information on the long-range forward-backward correlations of the sources in the earliest phase of the collision, based solely on the experimental information on the statistical features of the observed particle distributions. Our three-stage study incorporates the fluctuations of parton production in the early phase, the effect of intermediate hydrodynamic evolution, as well as fluctuations in the production of particles at freeze-out. We investigate the dependence of the results on the features of the overlaid distributions and hydrodynamics, as well as the centrality dependence. We analyze the existing data from the STAR Collaboration. Predictions for the forward-backward multiplicity correlations in Pb+Pb collisions to be analyzed at the LHC are also made.

Method for the analysis of forward-backward multiplicity correlations in heavy-ion collisions

In heavy-ion ({\it A-A}) collisions, the correlations among the particles produced across wide range in rapidity, probe the early stages of the reaction. The analyses of forward-backward multiplicity correlations in these collisions are complicated by several effects, which are absent or minimized in hadron-hadron collisions. This includes effects, such as the centrality selection in the {\it A-A} collisions, which interfere with the measurement of the dynamical correlations. A method, which takes into account the fluctuations in centrality selection, has been utilized to determine the forward-backward correlation strength {$b_{\rm corr}$} in {\itA-A} collisions. This method has been validated by using the HIJING event generator in case of Au-Au collisions at $\sqrt{s_{NN}}$= 200 GeV and Pb-Pb collisions at $\sqrt{s_{NN}}$= 2.76 TeV. It is shown that the effect of impact parameter fluctuations is to be considered properly in order to obtain meaningful results.

The study of non-collectivity by the forward–backward multiplicity correlation function

We propose a forward–backward multiplicity correlation function CNFB, which is experimentally accessible, to measure the non-collectivity contribution. It is found that the function is sensitive to both the jet contribution and small number statistics. We point out that the effect of the latter is also involved in the previous studies of the forward–backward elliptic correlation function but was misinterpreted as a contribution from jets. We study the CNFB in Au+Au collision at GeV with a multiphase transport model (AMPT). The result shows that the estimated jet contribution is much smaller than in the previous study due to the finite number statistics which has not been noticed before. The connection between this study and the forward–backward elliptic correlation function is also discussed.

Parton Rescattering Effect on the Charged Hadron Forward-Backward Multiplicity Correlation in pp Collisions at √s = 200 GeV

The parton rescattering effect on the charged hadron forward-backward multiplicity correlation in pp collisions at √s = 200 GeV is studied by a parton and hadron cascade model, PACIAE, based on the PYTHIA model. The calculated multiplicity and pseudorapidity distribution of the final state charged hadrons are well compared with the experimental data. It is found that the final state charged hadron pseudorapidity distribution is different from the initial state charged partons. The parton rescattering effect on the charged hadron forward-backward multiplicity correlation increases with the increasing parton rescattering strength in the center pseudorapidity region (|η| < 1). However, this effect becomes weaker in the outer pseudorapidity region (|η| > 1).

Symmetric correlations as seen in central Au+Au collisions ats=200AGeV

We analyze the forward-backward multiplicity correlation coefficient as measured by STAR. We show that in the most central Au $+$ Au collisions bins located symmetrically around $\ensuremath{\eta}=0$ with large separation in pseudorapidity are more strongly correlated than bins located asymmetrically with smaller separation. In proton-proton collisions the opposite effect is observed. It suggests a qualitatively different behavior of the two-particle correlation as a function of pseudorapidity sum in $p+p$ and Au+Au collisions.

Probing the quark-gluon phase transition using energy and system-size dependence of long-range multiplicity correlations in heavy ion collisions from the STAR experiment

Long-range forward-backward multiplicity correlations have been measured in heavy ion collisions at RHIC. Results for short and long-range multiplicity correlations (forward-backward) are presented for several systems (Au+Au, Cu+Cu, and pp) and energies (e.g. \(\sqrt {s_{NN} } \) = 200 and 62.4 GeV). A strong, long-range correlation is seen for central heavy ion collisions at \(\sqrt {s_{NN} } \) = 200 GeV that vanishes in semi-peripheral events and pp collisions. There is no apparent scaling with the number of participants (N part) involved in the collision. These correlations provide information about the longitudinal behavior of the system formed in heavy ion collisions. To access the transverse behavior, the clusters produced in the same heavy ion collisions have been characterized by a study of the energy and system size dependence of the percolation density parameter (ρ). The relationship between the long-range correlation and percolation has been explored to characterize the hadron-quark/gluon phase transition and rapid thermalization of the system.

Forward–backward multiplicity correlations at STAR

Forward–backward multiplicity correlations measured at STAR are discussed. A strong long-range correlation for inclusive charged hadrons was measured as a function of pseudorapidity separation (Δ η) in Au+Au collisions at s NN = 200 GeV , and were shown to decrease with decreasing centrality. STAR preliminary results show the correlation strength for pions, kaons, (anti)protons as a function of rapidity separation (Δ y) in 0–10% and 10–20% Au+Au collisions at s NN = 200 GeV . The Color Glass Condensate picture, which describes particle sources as longitudinal flux tubes, predicts that the correlation will grow with centrality. Fluctuations in the number of gluons at early times will produce a long-range correlation strength larger for pions than for baryons. A strong, long-range correlation is measured for pions in central collisions, which decreases for 10–20% collisions.

Forward-backward multiplicity correlations for identified particles in Au+Au 200 GeV collisions

Preliminary STAR measurements of the forward-backward multiplicity correlation as a function of rapidity separation for pions and (anti)protons in Au+Au 200 GeV collisions are presented for 0–10% and 10–20% centralities. A strong, uniform correlation is shown for pions, and is smaller for (anti)protons. The Color String Percolation Model predicts the long-range correlation, and suggests a possible measurement to study a transition from percolation to the quark-gluon plasma.

Recent results of fluctuation and correlation studies from the STAR experiment

Enhanced fluctuations and correlations have been observed in the phase transitions of many systems. Their appearance at the predicted QCD phase transition (especially near the expected critical point) may provide insight into the nature of the phase transition. Recent results from the STAR experiment will be presented with a focus on particle ratio (K/π and p/π) fluctuations, forward-backward multiplicity correlations, and balance functions. Also discussed will be possibilities for measuring these correlations and fluctuations in the recently initiated beam energy scan at RHIC.

Centrality dependence of forward-backward multiplicity correlation in Au+Au collisions atsNN=200GeV

We have studied the centrality dependence of charged particle forward-backward multiplicity correlation strength in Au+Au collisions at $\sqrt{s_{NN}}$=200 GeV with a parton and hadron cascade model, PACIAE, based on PYTHIA. The calculated results are compared with the STAR data. The experimentally observed correlation strength characters: (1) the approximately flat pseudorapidity dependence in central collisions and (2) the monotonous decrease with decreasing centrality are well reproduced. However the theoretical results are bigger than the STAR data for the peripheral collisions. A discussion is given for the comparison among the different models and STAR data.

Forward-backward multiplicity correlations in pp, [formula omitted] and Au+Au collisions at RHIC energy

The strength of charged particle forward-backward multiplicity correlation in p ¯ p and pp collisions at s = 200 GeV is studied by the PYTHIA model and compared with the UA5 and STAR data correspondingly. It turns out that a factor of 3-4 apparent discrepancy between UA5 and STAR data can be attributed to the differences in detector acceptance and observing bin width. We have also studied the centrality bin size dependence of the correlation strength in 0-10 to 0-5 and to 5% most central Au+Au collisions at s NN = 200 GeV with the PACIAE model. It turns out that the correlation strength decreases with decreasing centrality bin size monotonously.