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Register a new userThree-particle coincidence of the long range pseudorapidity correlation in high energy nucleus-nucleus collisions
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We report the first three-particle coincidence measurement in pseudorapidity ($Deltaeta$) between a high transverse momentum ($p_{perp}$) trigger particle and two lower $p_{perp}$ associated particles within azimuth $mid$$Deltaphi$$mid$$<$0.7 in $sqrt{{it s}_{NN}}$ = 200 GeV $d$+Au and Au+Au collisions. Charge ordering properties are exploited to separate the jet-like component and the ridge (long-range $Deltaeta$ correlation). The results indicate that the particles from the ridge are uncorrelated in $Deltaeta$ not only with the trigger particle but also between themselves event-by-event. In addition, the production of the ridge appears to be uncorrelated to the presence of the narrow jet-like component.
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The number of particles detected in a nucleus-nucleus collision strongly depends on the impact parameter of the collision. Therefore, multiplicity fluctuations, as well as rapidity correlations of multiplicities, are dominated by impact parameter fluctuations. We present a method based on Bayesian inference which allows for a robust reconstruction of fluctuations and correlations at fixed impact parameter. We apply the method to ATLAS data on the distribution of charged multiplicity and transverse energy. We argue that multiplicity fluctuations are smaller at large rapidity than around central rapidity. We suggest simple, new analyses, in order to confirm this effect.
In the 1930s, nuclear physicists developed the first realistic atomic models, showing that nuclei were made up of protons and neutrons. In the 1960s, Deep Inelastic Scattering experiments showed that protons and neutrons had internal structure: quarks and gluons (collectively, partons), and later experiments showed that the parton momentum distributions are different in heavy nuclei, compared to those in free nucleons. This difference is not surprising; partons are sensitive to their environment, and two gluons from different nucleons may fuse together, for example. Understanding how quarks and gluons behave in the nuclear environment is a significant focus of modern nuclear physics. Recent measurements have provided us with an improved understanding of how quark and gluon densities are altered in heavy nuclei. We have also begun to make multi-dimensional pictures of the nucleus, exploring how these alterations are distributed within heavy nuclei. We naturally expect these modifications to be largest in the core of a nucleus, and smaller near its periphery; this can change the effective shape of the nucleus. We have also started to explore the transverse momentum distribution of the partons in the nuclei, and, using incoherent photoproduction as a probe, study event-by-event fluctuations in nucleon and nuclei parton densities. This article will explore recent progress in measurements of nuclear structure at high energy, with some emphasis on these multi-dimensional pictures. We will also discuss how a future electron-ion collider (EIC) with high luminosity and center-of-mass energy will make exquisitely detailed images of partons in a nucleus.
The propagation of the heavy quarks produced in relativistic nucleus-nucleus collisions at RHIC and LHC is studied within the framework of Langevin dynamics in the background of an expanding deconfined medium described by ideal and viscous hydrodynamics. The transport coefficients entering into the relativistic Langevin equation are evaluated by matching the hard-thermal-loop result for soft collisions with a perturbative QCD calculation for hard scatterings. The heavy-quark spectra thus obtained are employed to compute the differential cross sections, the nuclear modification factors R_AA and the elliptic flow coefficients v_2 of electrons from heavy-flavour decay.
High-energy nucleus-nucleus collisions are studied in multi-chain model with successive collision. Analytic forms for single-particle distribution are derived.
The space-time structure of the multipion system created in central relativistic heavy-ion collisions is investigated. Using the microscopic transport model UrQMD we determine the freeze-out hypersurface from equation on pion density n(t,r)=n_c. It turns out that for proper value of the critical energy density epsilon_c equation epsilon(t,r)=epsilon_c gives the same freeze-out hypersurface. It is shown that for big enough collision energies E_kin > 40A GeV/c (sqrt(s) > 8A GeV/c) the multipion system at a time moment {tau} ceases to be one connected unit but splits up into two separate spatial parts (drops), which move in opposite directions from one another with velocities which approach the speed of light with increase of collision energy. This time {tau} is approximately invariant of the collision energy, and the corresponding tau=const. hypersurface can serve as a benchmark for the freeze-out time or the transition time from the hydrostage in hybrid models. The properties of this hypersurface are discussed.
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