Relativistic Heavy Ion Experiments Research Articles

Recent progress on hypernuclei

Some of last year’s progress made in hypernuclear physics is reviewed as follows: (i) resolving the He overbinding problem in single-Λ hypernuclei [1]; (ii) arguing that the onset of binding double-Λ hypernuclei is most likely at A=5, with the neutral systems and unbound by a large margin [2]; and (iii) revising the calculated value of the loosely bound lifetime to a level of ∼20% shorter than the free Λ lifetime [3], given recent claims from relativistic heavy ion experiments that τ() is shorter than τΛ by as much as ≈(30±8)%. Also discussed briefly in this context is the lifetime expected for the questionable hypernucleus.

Flavor hierarchy of jet quenching in relativistic heavy-ion collisions

Relativistic heavy-ion experiments have observed similar quenching effects for (prompt) D mesons compared to charged hadrons for transverse momenta larger than 6-8 GeV, which remains a mystery since heavy quarks typically lose less energies in quark-gluon plasma than light quarks and gluons. Recent measurements of the nuclear modification factors of B mesons and B-decayed D mesons by the CMS Collaboration provide a unique opportunity to study the flavor hierarchy of jet quenching. Using a linear Boltzmann transport model combined with hydrodynamics simulation, we study the energy loss and nuclear modification for heavy and light flavor jets in high-energy nuclear collisions. By consistently taking into account both quark and gluon contributions to light and heavy flavor hadron productions within a next-to-leading order perturbative QCD framework, we obtain, for the first time, a satisfactory description of the experimental data on the nuclear modification factors for charged hadrons, D mesons, B mesons and B-decayed D mesons simultaneously over a wide range of transverse momenta (8-300 GeV). This presents a solid solution to the flavor puzzle of jet quenching and constitutes a significant step towards the precision study of jet-medium interaction. Our study predicts that at transverse momenta larger than 30-40 GeV, B mesons also exhibit similar suppression effects to charged hadrons and D mesons, which may be tested by future measurements.

Exploring the unknown Lambda-neutron interaction

No published \LambdaΛn scattering data exist. A relativistic heavy-ion experiment has suggested that a \LambdaΛnn bound state was seen. However, several theoretical analyses have cast serious doubt on the bound-state assertion. Nevertheless, there could exist a three-body \LambdaΛnn resonance. Such a resonance could be used to constrain the \LambdaΛn interaction. We discuss \LambdaΛnn calculations using nn and \LambdaΛn pairwise interactions of rank-one, separable form that fit effective range parameters of the nn system and those hypothesized for the as yet unobserved \LambdaΛn system based upon four different \LambdaΛN potentials. The use of rank-one separable potentials allows one to analytically continue the \LambdaΛnn Faddeev equations onto the second complex energy plane in search of resonance poles, by examining the eigenvalue spectrum of the kernel of the Faddeev equations. Although each of the potential models predicts a \LambdaΛnn sub-threshold resonance pole, scaling of the \LambdaΛn interaction by as little as \sim∼5% does produce a physical resonance. This suggests that one may use photo-(electro-)production of the \LambdaΛnn system from tritium as a tool to examine the strength of the \LambdaΛn interaction.

Elliptic Flow in Ultrarelativistic Collisions with Polarized Deuterons.

Predictions are made for elliptic flow in collisions of polarized deuterons with a heavy nucleus. It is shown that the eccentricity of the initial fireball, evaluated with respect to the deuteron polarization axis perpendicular to the beam direction, has a substantial magnitude for collisions of highest multiplicity. Within the Glauber approach we obtain ∼7% for the deuteron states with spin projection 0, and ∼-3% for spin projection ±1. We propose to measure the elliptic flow coefficient as the second order harmonic coefficient in the azimuthal distribution of produced charged hadrons with respect to the fixed polarization axis. Collective expansion yields a value of the order of 1% for this quantity, as compared to zero in the absence of polarization and/or collectivity. Such a vivid rotational symmetry breaking could be measured with the current experimental accuracy of the relativistic heavy-ion experiments. The effect has a fundamental significance for understanding the nature of dynamics in small systems, as its experimental confirmation would prove the presence of the shape-flow transmutation mechanism, typical of hydrodynamic expansion or rescattering in the later stages of the fireball evolution.

Heavy-flavour transport: from large to small systems

Predictions for heavy-flavour production in relativistic heavy-ion experiments provided by the POWLANG transport setup, including now also an in-medium hadronization model, are displayed, After showing some representative findings for the Au-Au and Pb-Pb cases, a special focus will be devoted to the results obtained in the small systems formed in proton(deuteron)-nucleus collisions, where recent experimental data suggest the possible formation of a medium featuring a collective behaviour.

A Review onϕMeson Production in Heavy-Ion Collision

The main aim of the relativistic heavy-ion experiment is to create extremely hot and dense matter and study the QCD phase structure. With this motivation, experimental program started in the early 1990s at the Brookhaven Alternating Gradient Synchrotron (AGS) and the CERN Super Proton Synchrotron (SPS) followed by Relativistic Heavy Ion Collider (RHIC) at Brookhaven and recently at Large Hadron Collider (LHC) at CERN. These experiments allowed us to study the QCD matter from center-of-mass energies (sNN) 4.75 GeV to 2.76 TeV. Theϕmeson, due to its unique properties, is considered as a good probe to study the QCD matter created in relativistic collisions. In this paper we present a review on the measurements ofϕmeson production in heavy-ion experiments. Mainly, we discuss the energy dependence ofϕmeson invariant yield and the production mechanism, strangeness enhancement, parton energy loss, and partonic collectivity in nucleus-nucleus collisions. Effect of later stage hadronic rescattering on elliptic flow (v2) of proton is also discussed relative to corresponding effect onϕmesonv2.

Wounded quarks and multiplicity at relativistic ion colliders

In this paper, we propose a parameterization which is based on a phenomenological model involving the wounded quarks interactions for explaining the average charged particle multiplicity $\ < n_{ch}\ >$, the central pseudo-rapidity density $\ < (dn/d\eta)_{\eta=0}\ >$ and complete rapidity dependence of $dn/d{\eta}$ in relativistic heavy-ion collider experiments. The model also interrelates nucleus-nucleus (A-A) collisions with p-A and p-p interactions. Our parameterization rests on simple assumptions regarding mean number of participating quarks and their average number of collisions. The results for $\ < n_{ch}\ >$ and their variations with the mass number of colliding nuclei, center-of-mass energy ($\sqrt{s_{NN}}$) and collision centrality are well supported by the available experimental data. Finally we give the predictions from our model for A-A collisions at the Large Hadron Collider (LHC) and Compressed Baryonic Matter (CBM) experiments. Our results indicate the existence of a possible universal production mechanism for p-p, p-A and A-A collisions.

What can we learn from electromagnetic plasmas about the quark–gluon plasma?

Ultra-relativistic electromagnetic plasmas can be used for improving our understanding of the quark–gluon plasma. In the weakly coupled regime, both plasmas can be described by transport theoretical and quantum field theoretical methods leading to similar results for the plasma properties (dielectric tensor, dispersion relations, plasma frequency, Debye screening, transport coefficients, damping and particle production rates). In particular, future experiments with ultra-relativistic electron–positron plasmas in ultra-strong laser fields might open the possibility of testing these predictions, e.g. the existence of a new fermionic plasma wave (plasmino). In the strongly coupled regime, electromagnetic plasmas such as complex plasmas can be used as models or at least analogies for the quark–gluon plasma possibly produced in relativistic heavy-ion experiments. For example, pair correlation functions can be used to investigate the equation of state and cross section enhancement for parton scattering can be explained.

Spectral Functions and Transport in High Temperature QCD

The relativistic heavy ion experiments at RHIC have made it clear that is important to understand real-time and nonequilibrium processes in the quark-gluon plasma. In this talk I focus on two developments, connected through the construction of spectral functions from lattice QCD correlators: charmonium in lattice QCD with two dynamical fermion flavours at high temperature and transport coefficients and spectral functions at small energies obtained via the Maximum Entropy Method. For more details and much more references I refer to a recent review.1)

Development of a Time Projection Chamber using CF 4 gas for relativistic heavy ion experiments

A prototype Time Projection Chamber (TPC) using pure CF 4 gas was developed for possible use in heavy ion experiments. Basic characteristics such as gain, drift velocity, longitudinal diffusion and attenuation length of produced electrons were measured with the TPC. At an electric field of 900 V/cm, the drift velocity and longitudinal diffusion for 1 cm drift were obtained as 10 cm / μ s and 60 μ m , respectively. The relatively large gain fluctuation is explained to be due to the electron attachment process in CF 4. These characteristics are encouraging for the measurement of the charged particle trajectories under high multiplicity conditions at RHIC.

Boost invariance and multiplicity dependence of the charge balance function in π+p and K+p collisions at s=22 GeV

Boost invariance and multiplicity dependence of the charge balance function are studied in $\pi^{+}\rp$ and $\rK^{+}\rp$ collisions at 250 GeV/$c$ incident beam momentum. Charge balance, as well as charge fluctuations, are found to be boost invariant over the whole rapidity region, but both depend on the size of the rapidity window. It is also found that the balance function becomes narrower with increasing multiplicity, consistent with the narrowing of the balance function when centrality and/or system size increase, as observed in current relativistic heavy ion experiments.

Search for neutral strangelets at the relativistic heavy ion experiment E864

We report on the first search for neutral strange quark matter in Au + Pb reactions at 11.6 A GeV/c in experiment E864 at the Brookhaven National Laboratory AGS. We have sampled approximately 1.3 billion 10% most central Au + Pb interactions and have observed no evidence for the production of neutral strangelet states with a baryon number in the range 3 < A < 100. We thus set production limits at the level of 8 × 10-8 for these exotic objects in the mass range 15 < A < 100, and limits between 10-4 and 10-7 in the mass range 3 < A < 15. These limits are rather insensitive to the details of production models thanks to the large acceptance of the E864 spectrometer. The limits for neutral strangelets reported in this work complement other limits reported by our collaboration for positive and negative strangelets (Xu Z 1999 J. Phys. G: Nucl. Part. Phys. 25 403, Van Buren G 1999 J. Phys. G: Nucl. Part. Phys. 25 411).

Hdibaryon in the chiral color dielectric model

The mass of the $H$ dibaryon with spin, parity ${J}^{\ensuremath{\pi}}{=0}^{+}$, and isospin $I=0$ is calculated in the framework of the chiral color dielectric model. The wave function of the $H$ dibaryon is expressed as a product of two color-singlet baryon clusters. Thus the quark wave functions within the cluster are antisymmetric. Appropriate operators are then used to antisymmetrize intercluster quark wave functions. The radial part of the quark wave functions are obtained by solving the quark and dielectric field equations of motion obtained in the color dielectric model. The mass of the $H$ dibaryon is computed by including the color magnetic energy as well as the energy due to meson interaction. The recoil correction to the $H$ dibaryon mass is incorporated by Peierls-Yoccoz technique. We find that the mass of the $H$ dibaryon is smaller than the $\ensuremath{\Lambda}\ensuremath{-}\ensuremath{\Lambda}$ threshold by over 100 MeV. The implications of our results on the present day relativistic heavy ion experiments are discussed.

A high-resolution quartz Čerenkov detector for relativistic heavy ion beams

A quartz-based Čerenkov beam detector with photomultiplier readout was constructed for relativistic heavy ion experiments. It was successfuly used to detect interactions of 2, 4 and 10.4 AGeV kinetic energy gold projectiles in Au targets. The charge resolution of 1.4 e for gold ions permits its use to form a minimum-bias first-level interaction trigger. Gain and charge resolution were very stable for a two month period, during which a total number of ∼10 11 gold ions passed through the detector.

A gas Cherenkov beam counter with a timing resolution of 30 ps for relativistic heavy ion experiments

We constructed gas a Cherenkov beam counter for a lead beam to obtain start signals of time of fight measurements for relativistic heavy ion experiments in WA98 at CERN-SPS. The intrinsic timing resolution of the gas Cherenkov counter was found to be ≤ 30 ps at a beam intensity up to 3 × 10 5 ions/s and the charge resolution ( ΔZ Z ) was 2.0% for lead ions of 160 GeV/ c per nucleon. This counter has been successfully operated for more than 1 1 2 month of heavy ion run in 1995.

THE SEARCH FOR THE QUARK-GLUON PLASMA

▪ Abstract We provide an overview of the present understanding of the transition from hadrons to a quark-gluon plasma, its signatures, and the experimental results so far. We discuss results of numerical simulations of the lattice gauge theory and critically evaluate the various observables that have been proposed as signatures of the QCD phase transition. We place the existing data from relativistic heavy-ion experiments at the Brookhaven Alternating Gradient Synchrotron (AGS) and CERN Super Proton Synchrotron (SPS) into perspective and provide an overview of the techniques and strategies that will be employed in the search for the quark-gluon plasma at heavy-ion colliders, such as the Relativistic Heavy-Ion Collider (RHIC) and the Large Hadron Collider (LHC).

Deuteron and anti-deuteron production in CERN experiment NA44

The abundances of light nuclei probe the later stages of the evolution of a system formed in a relativistic heavy-ion collision. After the system has cooled and expanded, nucleons in close proximity and moving with small relative momenta coalesce to form nuclei. Light nuclei production enables the study of several topics, including the mechanism of composite particle production, freeze-out temperature, size of the interaction region, and entropy of the system. NA44 is the only relativistic heavy-ion experiment to have both deuteron and antideuteron results in both pA and AA collisions and the first CERN experiment to study the physics topics addressed by d and d production.

Study of timing degradation and light attenuation in long plastic scintillation rods for time-of-flight counters in relativistic heavy ion experiments

We have studied the timing and light yield characteristics of long and thin scintillator samples of various sizes. Both the light yield and the time resolution degrade exponentially with respect to the distance from one end; the inverse slope parameters for the time resolutions are found to be twice those of the light yield. This is interpreted in terms of a photoelectron statistics picture, which can also explain the observed position dependence of time resolution for time-of-flight counters.

Response matrix approach to the analysis of calorimetry data

We describe a correction procedure for calorimetry data based on a response matrix approach. The response matrix is calculated using a Monte Carlo event generator coupled with a fast deposition program which simulates the response of the calorimeter to hadronic and electromagnetic showers. The data which are analyzed are transverse energy measurements from a relativistic heavy ion experiment (E814/E877) at the Brookhaven AGS. We use the method to determine the differential cross section ( dσ dE T ) as well as the spatial distribution of transverse energy ( dE T d η ) , both in bins of pseudorapidity η and as a parameterized function of η. The method is found to be insensitive to the characteristics of the event generator.

The Hadron to quark-gluon transition in relativistic heavy-ion collisions

In this paper we construct a scenario for the QCD transition from the hadron phase to the quark/gluon phase using physical models for these phases. The hadron phase is modeled by a spectrum of hadrons with masses which drop (with a common scaling factor) towards zero at chiral symmetry restoration. The number of hadronic effective degrees of freedom is limited by the number of microscopic degrees of freedom in the quark/gluon phase. This limitation can be imposed either by fiat or through the introduction of a temperature-dependent excluded volume. Given that the number of degrees of freedom in hadrons and in quarks and gluons are roughly equal, the QCD phase transition is inhibited by the bag constant. The only phase transition seen in lattice-gauge calculations, once low-mass quarks are included, is the restoration of chiral symmetry which occurs at the relatively low temperature of ~ 150 MeV. At present, lattice gauge calculations do not have the resolution to determine the properties of the higher hadronic states accurately. They do, however, demonstrate that chiral restoration takes place in the (ρ. a 1), ( N( 1 2 +)), ( N( 1 2 −)) and (π, σ) systems by yielding “screening masses” for chiral partners which are distinct for T < T x SR and identical for T> T x SR . Further, within numerical accuracy, these “screening masses” are consistent with pure thermal energies and show no evidence of remaining bare masses once chiral symmetry is restored. These, and other lattice-gauge results, will be discussed in the light of our scenario. We shall also consider the consequences of our picture for relativistic heavy-ion experiments.