Correlations In Heavy Ion Collisions Research Articles

A study of pion fluctuation and correlation in heavy-ion collisions

A study of pion correlation and fluctuation has been carried out in terms of normalised factorial cumulant moment of second and third order in 16 O-AgBr, 28 Si-AgBr and 32 S-AgBr interactions at 4.5AGeV/c. Two-particle pion correlations were found to be stronger than the three-particle correlation. Experimental analysis has been compared with the results obtained from the analysis of events simulated by generating random numbers (Monte Carlo Simulation). A comparison of our results with the UrQMD, modified FRITIOF and AMPT models has also been performed. The UrQMD model completely failed to reproduce the experimental results. The modified FRITIOF model also cannot reflect all the features of the experimental outcome. The AMPT model in comparison to the other two models can qualitatively reproduce the experimental results although the strength of the observed correlation in the case of the AMPT model is less than the experimental values.

Background subtraction methods for precision measurements of di-hadron and jet-hadron correlations in heavy ion collisions

Di-hadron correlations and jet-hadron correlations are frequently used to study interactions of the Quark Gluon Plasma with the hard partons that form jets. The existing background subtraction methods for these studies depend on several assumptions and independent measurements of the Fourier coefficients of the combinatorial background. In this paper, we present two background subtraction methods which use the observation that the near-side correlation signal is negligible for large separations between the associated particle and the trigger. The combinatorial background is fit in this background-dominated region. Including the reaction plane dependence of the raw correlation improves these results. To test the accuracy of these methods, a simple model is used to simulate di-hadron and jet-hadron correlations with a combinatorial background similar to that observed in the data. The true signal is compared to the extracted signal. The results are compared to results from two variants of the Zero-Yield-At-Minimum (ZYAM) method. We test these methods for mid-peripheral and central collisions and for di-hadron and jet-hadron correlations. These methods are more precise than the ZYAM method with fewer assumptions about the shape and level of the combinatorial background, even in central collisions where the experimental resolution on the measurement of the reaction plane dependence is poor. These methods will allow more accurate studies of modifications of the away-side jet and will be particularly useful for studies of jet-hadron correlations, where the combinatorial background is poorly constrained from previous studies.

Pseudorapidity correlations in heavy ion collisions from viscous fluid dynamics

We demonstrate by explicit calculations in 3+1 dimensional viscous relativistic fluid dynamics how two-particle pseudorapidity correlation functions in heavy ion collisions at the LHC and RHIC depend on the number of particle producing sources and the transport properties of the produced medium. In particular, we present results for the Legendre coefficients of the two-particle pseudorapidity correlation function, an,m, in Pb+Pb collisions at 2760 GeV and Au+Au collisions at 200 GeV from viscous hydrodynamics with three dimensionally fluctuating initial conditions. Our results suggest that the an,m provide important constraints on initial state fluctuations in heavy ion collisions.

Effects of the kinetic symmetry energy reduced by short-range correlations in heavy-ion collisions at intermediate energies

Besides earlier predictions based on both phenomenological models and modern microscopic many-body theories, circumstantial evidence was recently found for a reduced kinetic symmetry energy of isospin-asymmetric nucleonic matter compared to the free Fermi gas model prediction due to the short-range correlation of high-momentum neutron-proton pairs. While keeping the total symmetry energy near the saturation density of nuclear matter consistent with existing experimental constraints, we examine the correspondingly enhanced role of the isospin degree of freedom in heavy-ion collisions at intermediate energies due to the reduced (enhanced) kinetic (potential) symmetry energy. Important observable consequences are investigated.

Hard Probes in Heavy Ion Collisions: Current Status and Prospects for Application of QCD Evolution Techniques

In the past decade the observation of cross section modification for leading hadrons, heavy flavor and two particle correlations in heavy ion collisions has provided important insights into the dynamics of parton propagation in dense strongly-interacting matter. The development of the theory of reconstructed jets and related experimental measurements have further shed light on the characteristics of in-medium parton showers. So far, experimental results from ultra-relativistic nuclear collisions at RHIC and LHC have been analyzed in the framework of parton energy loss, where the precision of the theoretical predictions cannot be systematically improved. Only recently have higher order calculations and applications of resummation and evolution to heavy ion collisions begun to emerge. Several examples of such advances are discussed in these proceedings.

Baryon-antibaryon flavor correlation in quark-combination models in heavy-ion collisions

We extend the study of hadron yield correlation in quark-combination models in a recent publication to baryon-antibaryon flavor correlations in heavy-ion collisions. We show that the universal behaviors of the antibaryon-to-baryon yield ratios as functions of that of ${K}^{\ensuremath{-}}$ to ${K}^{+}$ are naturally explained. We study also the ``mixed ratios'' and propose other measurements that might be sensitive to the hadronization mechanism.

Heavy-flavor azimuthal correlations of D mesons

Observables of heavy-quark azimuthal correlations in heavy-ion collisions are new and promising probes for the investigation of the in-medium energy loss. We explore the potential of these observables to discriminate the collisional and radiative contributions within a hybrid EPOS+MC@sHQ transport approach.

Numerical integration of thermal noise in relativistic hydrodynamics

Thermal fluctuations affect the dynamics of systems near critical points, the evolution of the early universe, and two-particle correlations in heavy-ion collisions. For the latter, numerical simulations of nearly ideal, relativistic fluids are necessary. The correlation functions of noise in relativistic fluids are calculated, stochastic integration of the noise in $3+1$-dimensional viscous hydrodynamics is implemented, and the effect of noise on observables in heavy-ion collisions is discussed. Thermal fluctuations will cause significant variance in the event-by-event distributions of integrated ${v}_{2}$ while changing average values even when using the same initial conditions, suggesting that including thermal noise will lead to refitting of the hydrodynamical parameters with implications for understanding the physics of hot QCD.

The FARCOS project. First characterization of CsI(Tl) crystals of the FARCOS array using charged particle beams at LNS

The construction of a new array to study femtoscopy and multi-particle correlations in heavy ion collisions at intermediate energies (E=20-1000 AMeV) has been started at the INFN. The project, named FARCOS (Femtoscope ARray for COrrelations and Spectroscopy) is aimed at developing of a detection system with high pixelation capabilities in order to perform precision measurements of particle correlations for nuclear dynamics and spectroscopy. We present first detection simulations for FARCOS and first results related to the commissioning of CsI(T1) crystals, an important detection stage of each telescope.

Accessing Dynamical Emissions with Two-particle Correlations in Heavy-ion Collisions

Accessing Dynamical Emissions with Two-particle Correlations in Heavy-ion Collisions

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.

Study on initial geometry fluctuations via participant plane correlations in heavy ion collisions: part II

Further investigation of the participant plane correlations within a Glauber model framework is presented, focusing on correlations between three or four participant planes of different order. A strong correlation is observed for $\cos(2\Phi_{2}^*+3\Phi_{3}^*-5\Phi_{5}^*)$ which is a reflection of the elliptic shape of the overlap region. The correlation between the corresponding experimental reaction plane angles can be easily measured. Strong correlations of similar geometric origin are also observed for $\cos(2\Phi_{2}^*+4\Phi_{4}^*-6\Phi_{6}^*)$, $\cos(2\Phi_2^*-3\Phi_3^*-4\Phi_4^*+5\Phi_5^*)$, $\cos(6\Phi_2^*+3\Phi_3^*-4\Phi_4^*-5\Phi_5^*)$, $\cos(\Phi_1^*-2\Phi_2^*-3\Phi_3^*+4\Phi_4^*)$, $\cos(\Phi_1^*+6\Phi_2^*-3\Phi_3^*-4\Phi_4^*)$, and $\cos(\Phi_1^*+2\Phi_2^*+3\Phi_3^*-6\Phi_6^*)$, which are also measurable. Experimental measurements of the corresponding reaction plane correlators in heavy ion collisions at RHIC and the LHC may improve our understanding of the physics underlying the measured higher order flow harmonics.

Charged-particle pseudorapidity density and anisotropic flow over a wide pseudorapidity range using ALICE at the LHC

The pseudorapidity density and anisotropic flow of charged-particles provide fundamental information about global variables and correlations in heavy-ion collisions. The pseudorapidity density is related to the energy available for particle production while the anisotropic flow is related to collective effects from interactions between these particles. Extending these measurements to very forward pseudorapidities yields information about the longitudinal expansion of the system. The first measurements performed at the LHC over more than 8 units of pseudorapidity are presented. The longitudinal scaling of the measurements is analyzed and comparison to models is performed.

A method for studying initial geometry fluctuations via event plane correlations in heavy ion collisions

A method is proposed to measure the relative azimuthal angle distributions involving two or more event planes of different order in heavy ion collisions using a Fourier analysis technique. The analysis procedure is demonstrated for correlations involving two and three event planes (Phi_n, Phi_m and Phi_h). The Fourier coefficients of these distributions are found to coincide with previously proposed correlators, such as cos(6Phi_2-6Phi_3) and cos(Phi_1+2Phi_2-3Phi_3) etc, hence the method provides a natural framework for studying these correlators at the same time. Using a Monte Carlo Glauber model to simulate Au+Au collisions with fluctuating initial geometry, we are able to identify several new two- or three-plane correlators that have sizable magnitudes and should be measured experimentally.

Universal stress-tensor correlation functions of strongly coupled conformal fluids

We have shown in two accompanying papers that, for Einstein gravity, the graviton multi-point functions are universal in a particular kinematic region and depend only on the (generalized) Mandelstam variable s. The effects of the leading corrections to Einstein gravity were shown to be similarly universal, inducing a specific difference in the angular dependence. Here we show, relying on the gauge–gravity duality, that the stress-tensor correlation functions of conformal fluids whose gravitational dual is either Einstein gravity or its leading correction are also universal. We discuss the possible significance of these results to multi-particle correlations in heavy-ion collisions. We show that, to test our ideas, the stress-energy correlation functions have to measured rather than the standard multiplicity correlation functions. We then discuss schematically how stress-energy correlations in heavy-ion collisions can be used to test our findings. We argue that, if these correlations can be measured precisely enough, they will provide a unique way to probe the existence of a gravitational dual to the quark–gluon plasma and to determine its universality class.

Probing early-time correlations in heavy ion collisions

We investigate the common influence of correlations arising from initial state parton density fluctuations on measured multiplicity and momentum fluctuations as well as flow fluctuations. We calculate both a universal correlation scale in an initial stage Glasma flux tube picture and the modification to these correlations from later stage hydrodynamic flow. We find quantitative agreement with measurements over a range of collision systems and energies.

Charged Particle Multiplicity and Correlations in Heavy Ion Collisions in the ATLAS Experiment

Charged Particle Multiplicity and Correlations in Heavy Ion Collisions in the ATLAS Experiment

Soft Particle Production, Flow and Correlations in Heavy Ion Collisions at the LHC

In this paper we discuss recent results on soft particle production, correlations and flow in Pb-Pb collisions at p sNN = 2.76 TeV.

Genuine pion–pion correlations in heavy-ion collisions

The search for genuine two-particle dynamical correlations among the pions produced in 16O–AgBr interactions at 60 AGeV and 32S–AgBr interactions at 200 AGeV (super proton synchrotron data) has been carried out by the method of normalized factorial cumulant moments in pseudo-rapidity (η), azimuthal angle (ϕ) and two-dimensional anisotropic (η–ϕ) phase space. The analysis reveals the presence of genuine two-particle dynamical correlations among the pions in 16O–AgBr and 32S–AgBr interactions in both one and two dimensions. Stronger correlation has been observed in higher dimensional space. Correlations between the pions are found to decrease with the increase of projectile mass and energy. Experimental results have been compared with the FRITIOF data sample in the pseudo-rapidity space. In azimuthal angle space, events simulated by generating random numbers (Monte Carlo Simulation) have been used for the purpose of comparison. The FRITIOF data and the data of the simulated interactions have failed to reproduce the experimental results. The experimental results have also been compared with the event sample generated by the UrQMD model in both pseudo-rapidity and azimuthal angle space. Like the FRITIOF model, the UrQMD model is also unable to replicate the experimental results. The variation of normalized cumulant moment (K2) against the number of bin M has been fitted with a function K2 = aMb in both one and two dimensions. No fitting of this kind has been possible in the case of the FRITIOF data, UrQMD data and the MC simulated data.

Flux tube model signals for baryon correlations in heavy ion collisions

The flux tube model offers a pictorial description of what happens during the deconfinement phase transition in QCD. The 3-point vertices of a flux tube network lead to formation of baryons upon hadronisation. Therefore, correlations in the baryon number distribution at the last scattering surface are related to the preceding pattern of the flux tube vertices, and provide a signature of the nearby deconfinement phase transition. I discuss the nature of the expected signal, which should be observable in heavy ion collisions at RHIC and LHC.