High-energy Heavy-ion Experiments Research Articles

Charge-dependent anisotropic flow in high-energy heavy-ion collisions from a relativistic resistive magneto-hydrodynamic expansion

We have investigated the charge-dependent anisotropic flow in high-energy heavy-ion collisions, using relativistic resistive magneto-hydrodynamics (RRMHD). We consider the optical Glauber model as an initial model of the quark-gluon plasma (QGP) and the solution of the Maxwell equations with source term of the charged particles in two colliding nuclei as initial electromagnetic fields. The RRMHD simulation is performed with these initial conditions in Au-Au and Cu-Au collisions at $\sqrt{s_{\mathrm{NN}}} = 200$ GeV. We have calculated the charge-odd contribution to the directed flow $\Delta v_1$ and elliptic flow $\Delta v_2$ in both collisions based on electric charge distributions as a consequence of RRMHD. Our results show that the $\Delta v_1$ and $\Delta v_2$ are approximately proportional to the electrical conductivity ($\sigma$) of the medium. In the $\sigma=0.023~\mathrm{fm}^{-1}$ case, our result of $\Delta v_1$ is consistent with STAR data in Au-Au collisions. Furthermore, in Cu-Au collisions, $\Delta v_1$ has a non-zero value at $\eta = 0$. We conclude that the charge-dependent anisotropic flow is a good probe to extract the electrical conductivity of the QGP medium in high-energy heavy-ion experiments.

Analytic Model Studies of Polarized Baryon Production

We investigate known exact solutions of hydrodynamics and derive analytic formulas for the polarization of baryons produced at freeze-out. Such polarization (observed in high energy heavy-ion experiments) carries information on the time evolution of the quark-gluon plasma (sQGP), and our results give first analytic insight into the connection between this type of measurements and dynamical properties of the sQGP (e.g. vorticity). We present results for a rotating and acceleratingly expanding solution and also give hints on how to calculate the polarization using a rotating extension of the Buda--Lund parameterization.

Medium-induced optical effects for prompt photons

Electromagnetic aspects of a QCD matter have been a hot topic in recent years. High-energy heavy-ion experiments revealed that flow harmonics of direct photons are not well-explained by most hydrodynamic models. In this work, I discuss possible effects of refraction by the hot medium since it can work as a lens which converts its geometrical anisotropy into momentum anisotropy of photons. Flow harmonics of prompt photons are estimated numerically. The results indicate that the effect of refraction is small but phenomenologically non-trivial, exhibiting sensitivity to the QCD refractive index.

An equation-of-state-meter of quantum chromodynamics transition from deep learning

A primordial state of matter consisting of free quarks and gluons that existed in the early universe a few microseconds after the Big Bang is also expected to form in high-energy heavy-ion collisions. Determining the equation of state (EoS) of such a primordial matter is the ultimate goal of high-energy heavy-ion experiments. Here we use supervised learning with a deep convolutional neural network to identify the EoS employed in the relativistic hydrodynamic simulations of heavy ion collisions. High-level correlations of particle spectra in transverse momentum and azimuthal angle learned by the network act as an effective EoS-meter in deciphering the nature of the phase transition in quantum chromodynamics. Such EoS-meter is model-independent and insensitive to other simulation inputs including the initial conditions for hydrodynamic simulations.

Beam test results of STS prototype modules for the future accelerator experiments FAIR/CBM and NICA/MPD projects

High-energy heavy-ion collision experiments provide the unique possibility to create and investigate extreme states of strongly-interacted matter and address the fundamental aspects of QCD. The experimental investigation the QCD phase diagram would be a major breakthrough in our understanding of the properties of nuclear matter. The reconstruction of the charged particles created in the nuclear collisions, including the determination of their momenta, is the central detection task in high-energy heavy-ion experiments. It is taken up by the Silicon Tracking System in CBM@FAIR and by Inner Tracker in MPD@NICA currently under development. These experiments requires very fast and radiation hard detectors, a novel data read-out and analysis concept including free streaming front-end electronics. Thermal and beam tests of prototype detector modules for these tracking systems showed the stability of sensors and readout electronics operation.

Jet-track correlations in PbPb and pp collisions at 2.76 TeV

Jet quenching is one of the established signatures of the Quark Gluon Plasma, and this phenomenon has been studied extensively in high energy heavy ion experiments over the last decade. Despite significant progress made in those studies, the specifics of the energy loss mechanisms and the details of jet-medium interactions require further quantitative understanding. Studies of two-dimensional angular correlations of charged hadrons with respect to reconstructed jets make it possible to experimentally assess the energy radiation patterns of an energetic parton traversing the medium as well as its fragmentation function. In these proceedings, we present new differential measurements of charged particle densities and energy flow about the jet direction as a function of relative azimuth and relative pseudorapidity from 2.76 TeV PbPb and pp collisions recorded by the CMS Collaboration. With a sample of back-to-back dijets, previously used to reconstruct the event-wise momentum imbalance (missing pT), we explore modifications to correlated charged hadron distributions for both the leading and the subleading sides of the dijet. With this technique, we can individually assess the contribution of the medium-induced modifications to each side of the dijet and explore the extent of potential medium response to the jet propagation, while also extending these measurements to large angular and radial distances.

Picosecond Cherenkov detectors for high-energy heavy ion experiments at LHEP/JINR

The modular Cherenkov detectors based on MCP-PMTs are developed for study Au+Au collisions in MPD and BM@N experiments with beams of Nuclotron and future collider NICA in Dubna. The aim of the detector is fast and effective triggering nucleus–nucleus collisions and generation of start signal for TOF detectors. The detector performance is studied with MC simulation and test measurements with a beam of Nuclotron.

Development and study of picosecond start and trigger detector for high-energy heavy ion experiments

Two modular Cherenkov detectors based on MCP-PMTs XP85012 are developed for study Au+Au collisions in future experiments with beams of Nuclotron and collider NICA at JINR, Dubna. The detector arrays register high-energy photons and relativistic charged particles with aim of fast triggering the collisions and generation of start signal with time resolution better then 50ps for TOF detector. The MC simulation showed the trigger efficiency of 100% for impact parameter range b<10fm. The best time resolution for single detector channel σt≈21ps was obtained with DRS4 digitizer in TOF measurements with a deuteron beam.

Photon–hadron discrimination with improved clustering for a preshower detector in high energy heavy ion experiments

The fuzzy c-mean clustering algorithm has been applied to the data set consisting of hits in a highly granular photon multiplicity detector installed in the ALICE experiment at the LHC. The clusters obtained using a modification of the algorithm based on the intensity of cells (called weighted fuzzy c-mean algorithm) are used as input in an artificial neural network formalism for photon–hadron discrimination. Results are discussed in terms of the photon reconstruction efficiency and the purity of photon sample and their centrality and pseudorapidity dependence at the LHC energy.

Application of fuzzy-based pattern recognition techniques for cluster finding in a preshower detector in high energy heavy ion experiments

We apply the Fuzzy c-mean clustering algorithm for the data set consisting of hits in a highly granular preshower detector in the case of collision of heavy nuclei at high energy. A set of validity indexes are examined for optimum clustering performances suitable for the particular data set. The performance is also studied for different densities of hit pattern. Using information from one of the validity indexes giving good performance, FCM clustering is also studied for fixed set of parameters. The results compare well with those obtained using iterative optimization.

Spectroscopy of resonance decays in high-energy heavy-ion experiments

Invariant mass distributions of the hadronic decay products from resonances formed in relativistic heavy ion collision (RHIC) experiments are investigated with a view to disentangle the effects of thermal motion and the phase space of decay products from those of intrinsic changes in the structure of resonances at the freeze-out conditions. Analytic results of peak mass shifts for the cases of both equal and unequal mass decay products are derived. The shift is expressed in terms of the peak mass and width of the vacuum or medium-modified spectral functions and temperature. Examples of expected shifts in meson (e.g., rho, omega, and sigma) and baryon (e.g., Delta) resonances that are helpful to interpret recent RHIC measurements at BNL are provided. Although significant downward mass shifts are caused by widened widths of the $\rho-$meson in medium, a downward shift of at least 50 MeV in its intrinsic mass is required to account for the reported downward shift of 60-70 MeV in the peak of the rho-invariant mass distribution. An observed downward shift from the vacuum peak value of the Delta distinctively signals a significant downward shift in its intrinsic peak mass, since unlike for the rho-meson, phase space functions produce an upward shift for the Delta isobar.

Multiboson Effects in Bose–Einstein Interferometry and the Multiplicity Distribution

Multiboson symmetrization effects on two-particle Bose–Einstein interferometry are studied for ensembles with arbitrary multiplicity distributions. This generalizes the previously studied case of a Poissonian input multiplicity distribution. In the general case we find interesting residual correlations which require a modified framework for extracting information on the source geometry from two-particle correlation measurements. In sources with high phase-space densities, multiboson effects modify the Hanbury Brown–Twiss (HBT) radius parameters and simultaneously generate strong residual correlations. We clarify their effect on the correlation strength (intercept parameter) and thus explain a variety of previously reported puzzling multiboson symmetrization phenomena. Using a class of analytically solvable Gaussian source models, with and without space-momentum correlations, we present a comprehensive overview of multiboson symmetrization effects on particle interferometry. For event ensembles of (approximately) fixed multiplicity, the residual correlations lead to a minimum in the correlation function at nonzero relative momentum, which can be practically exploited to search, in a model-independent way, for multiboson symmetrization effects in high-energy heavy-ion experiments.

Signatures of quark–gluon plasma phase transition in high-energy nuclear collisions

In high-energy nuclear collisions, the new phase of the quark-gluon plasma is indicated by an anomalous increase in pressure, an excess of direct photon production, an excess of strangeness production, and an anomalous J/psi suppression. We review these signatures and discuss how recent high-energy heavy-ion experiments at CERN are consistent with the production of the quark-gluon plasma in high-energy Pb+Pb collisions.

A forward magnetic spectrometer system for high-energy heavy-ion experiments

A small aperture magnetic spectrometer has been built to study hadron production in 197 Au+ 197 Au collisions at the AGS energy of 11.6A GeV/ c. It operates in the forward angular range from 6 to 30° with respect to the incident beam axis and covers the mid-rapidity region for heavy particles such as protons. The detector components of the spectrometer system include two time projection chambers, four drift chamber modules and a time-of-flight scintillation counter wall. A few new technologies are implemented in the design of the system to achieve the performance goals. The spectrometer has proved to function properly under the high particle-density environment encountered in experiments with the heavy-ion colliding system. The achieved momentum resolution is 1.3% in r.m.s. for pions at 1 GeV/ c and 1.6% for protons at the same momentum. With the time-of-flight resolution of 76 ps in r.m.s., the particle identification momentum limit extends to 4 GeV/ c for pions, 3 GeV/ c for kaons, 5 GeV/ c for protons, and 4.5 GeV/ c for anti-protons. The tracking efficiency stays above 86% for tracks up to 5 GeV/ c with as many as 10 tracks in the spectrometer aperture.

Photon production from a quark-gluon plasma.

In-medium interactions of a particle in a hot plasma are considered in the framework of thermal field theory. The formalism to calculate gauge invariant rates for photon and dilepton production from the medium is given. In the application to a QED plasma, astrophysical consequences are pointed out. The photon production rate from strongly interacting quarks in the quark--gluon plasma, which might be formed in ultrarelativistic heavy ion collisions, is calculated in the previously unaccessible regime of photon energies of the order of the plasma temperature. For temperatures below the chiral phase transition, an effective field theory incorporating dynamical chiral symmetry breaking is employed, and perturbative QCD at higher temperatures. A smooth transition between both regions is obtained. The relevance to the soft photon problem and to high energy heavy ion experiments is discussed.

High-energy heavy-ion-induced charge transport across multiple junctions

High-energy heavy ion experiments and numerical simulations show that two or more junctions bridged by an ion track respond in a coupled manner. The potential difference across the structure strongly affects the amount of charge collected at each of the junctions. Experiments with 1.7 GeV /sup 197/Au ions indicate that, for emitter biases less than or equal to 0.0 volts and potential differences of 5 or more volts, more charge may be collected at a junction than was initially deposited by the ion. Simulations show that, when an ion track intersects multiple junctions of a device, the responses of the individual junctions cannot be modeled independently of each other. Simulation and experimental results indicate that charge transport in a multiple junction structure cannot be modeled using simple geometry or funneling assumptions.

Gluon multiplication in high energy heavy ion collisions.

Hot gluons are the dominant components of the QCD plasma to be formed in future high energy heavy ion experiments. In this paper we study the elementary processes in the plasma medium for gluon multiplication based on all orders of the tree diagrams in perturbative QCD. When applying to the chemical equilibration in the expanding system, we found that the gluon reaches chemical equilibrium well within its plasma phase. The inclusion of all the next-to-leading-order processes makes the equilibration considerably faster than the simple gg\ensuremath{\leftrightarrows}ggg one considered previously.

Final state [formula omitted] suppression in nuclear collisions

The hadroproduction of J ψ , ψ′ and ϒ on nuclear targets shows two “initial state” effects: a modification of the structure functions in nuclei leads a reduced production relative to pp interactions, and parton scattering in nuclear matter broadens the transverse momentum spectrum in comparison to that in pp collisions. If charmonium or bottonium states are to be used as probes for the nature of dense matter formed in nucleus-nucleus collisions, both these effects must first be taken into account. We do this and discuss the resulting final state features of J ψ and ψ′ production in high energy heavy ion experiments.

A single arm spectrometer detector system for high-energy heavy-ion experiments

The recent availability of 14.6 GeV/ c per nucleon 16O and 28Si ions from the Brookhaven National Laboratory Tandem-AGS accelerator facility has prompted the design, construction and operation of a large-solid-angle (25 msr) magnetic spectrometer with particle identification from ∼0.5 to ∼4.7 GeV/ c. A small-solid-angle Cherenkov counter complex views the target through the magnet and extends the particle identification up to ∼15 GeV/ c. This experiment (E-802) employs event characterization detectors, a charged-particle multiplicity array, a highly segmented lead-glass detector, and a zero degree calorimeter. The facility measures momentum spectra of identified heavy-ion-produced hadrons with high resolution ( Δp/ p≤0.005) as a function of collision centrality given by triggers from the event characterization detectors. Construction and performance details of the spectrometer components and auxiliary detectors are described.

New Scheme for Spontaneous Symmetry Breaking of Color SU(3)

A new spontaneous-symmetry-breaking mechanism is formulated for SU(3), which is used to describe the formation of bags around quarks. The Higgs field is replaced by the scalar product of two colored fermion fields. This model gives mass only to one gluon (equivalent to ${{A}_{\ensuremath{\mu}}}^{8}$) when spontaneously broken. The consequences of this scheme are discussed, and it is argued that it can explain several puzzling high-energy heavy-ion experiments.