RHIC Facility Research Articles

Recent progress on few-body hypernuclei

Few-body Λ hypernuclei provide valuable information towards understanding strange matter. Recent experimental progress by the STAR Collaboration at the RHIC facility and by the ALICE Collaboration at the LHC has been matched by theoretical progress reviewed here: (i) lifetimes of the hypertritonΛ3H,Λ3n if particle-stable,Λ4H andΛ4He and their charge symmetry breaking, and (ii) the onset of ΛΛ hypernuclear binding.

Electromagnetic & Weak Probes: Experimental Overview

Electromagnetic and electroweak probes are the most versatile probes in the study of heavy-ion collisions. Produced at every stage in the evolution of QCD matter, its messengers are practically inert to the strongly interacting medium they travel through. In this contribution, I will discuss a selection of new results from experiments at the LHC, RHIC, and SIS facilities, spanning almost four orders of magnitude in beam energy. I will conclude with a brief overview of the experimental landscape in the near future.

First-order phase transition from hypernuclear matter to deconfined quark matter obeying new constraints from compact star observations

We reconsider the problem of the hyperon puzzle and its suggested solution by quark deconfinement within the two-phase approach to hybrid compact stars with recently obtained hadronic and quark matter equations of state. For the hadronic phase we employ the hypernuclear equation of state from the lowest order constrained variational method and the quark matter phase is described by a sufficiently stiff equation of state based on a color superconducting nonlocal Nambu-Jona-Lasinio model with constant (model nlNJLA) and with density-dependent (model nlNJLB) parameters. We study the model dependence of the phase transition obtained by a Maxwell construction. Our study confirms that also with the present set of equations of state quark deconfinement presents a viable solution of the hyperon puzzle even for the new constraint on the lower limit of the maximum mass from PSR J0740+6620. In this work we provide with model nlNJLB for the first time a hybrid star EoS with an intermediate hypernuclear matter phase between the nuclear and color superconducting quark matter phases, for which the maximum mass of the compact star reaches $2.2~M_\odot$, in accordance with most recent constraints. In model nlNJLA such a phase cannot be realised because the phase transition onset is at low densities, before the hyperon threshold density is passed. We discuss possible consequences of the hybrid equation of state for the deconfinement phase transition in symmetric matter as it will be probed in future heavy-ion collisions at FAIR, NICA and corresponding energy scan programs at the CERN and RHIC facilities.

Holography of low-centrality heavy ion collisions

Large vorticities in the Quark–Gluon Plasma produced in peripheral collisions studied by the STAR collaboration at the RHIC facility have been deduced from observations of polarizations of [Formula: see text] and [Formula: see text] hyperons. Recently, the STAR collaboration has reported on the dependence of these polarizations on centrality, at impact energy 200 GeV and relatively large centralities [Formula: see text]. The polarizations increase slowly with centrality, up to perhaps [Formula: see text]. Here we use a holographic model of the vortical QGP to make predictions regarding these polarizations for smaller centralities, ranging from 5–20%. The model predicts that as one moves downwards from 20%, [Formula: see text] polarizations at first decrease but then increase sharply, in a characteristic pattern which should be readily detected if collisions can be studied at impact energies below 200 GeV and centrality as low as 5–10%. The effect should be most evident at moderate impact energies below 200 GeV, so we give predictions for impact energy 27 GeV.

Strangeness at Intermediate Baryon Density

Exploration of the QCD phase diagram has been one of the main programs of contemporary nuclear physics. The intermediate baryon density region covers a broad range of the baryon chemical potential, between 100 and 700 MeV, and is expected to include a possible critial point at the end of a phase equilibrium curve between the hadron gas and quark gluon plasma phases. Experimental programs at the SPS and RHIC facilities have provided valuable insights in this range. These proceedings motivate the exploration of the QCD phase diagram through the use of strangeness. A selection of relevant experimental results from RHIC and SPS beam energy scan programs with associated theoretical predictions is presented along with a discussion of possible physical conclusions and future plans.

Resonance Production in Heavy-Ion Collisions

Hadronic resonances are unique probes that allow the properties of heavyion collisions to be studied. Topics that can be studied include modification of spectral shapes, in-medium energy loss of parsons, vector-meson spin alignment, hydrodynamic flow, recombination, strangeness production, and the properties of the hadronic phase. Measurements of resonances in p+p, p+A, and d+A collisions serve as baselines for heavy-ion studies and also permit searches for possible collective effects in these smaller systems. These proceedings present a selection of results related to these topics from experiments at RHIC, LHC, and other facilities, as well as comparisons to theoretical models.

Holography of the QGP Reynolds number

The viscosity of the Quark–Gluon Plasma (QGP) is usually described holographically by the entropy-normalized dynamic viscosity η/s. However, other measures of viscosity, such as the kinematic viscosity ν and the Reynolds number Re, are often useful, and they too should be investigated from a holographic point of view. We show that a simple model of this kind puts an upper bound on Re for nearly central collisions at a given temperature; this upper bound is in very good agreement with the observational lower bound (from the RHIC facility). Furthermore, in a holographic approach using only Einstein gravity, η/s does not respond to variations of other physical parameters, while ν and Re can do so. In particular, it is known that the magnetic fields arising in peripheral heavy-ion collisions vary strongly with the impact parameter b, and we find that the holographic model predicts that ν and Re can also be expected to vary substantially with the magnetic field and therefore with b.

On the existence of a holographic description of the LHC quark–gluon plasmas

Peripheral collisions of heavy ions can give rise to extremely intense magnetic fields. It has been suggested that these fields might invalidate the holographic description of the corresponding quark–gluon plasmas, assuming that these can be modelled by strongly coupled field theories. In the case of the plasmas produced in collisions at the RHIC facility (including in the beam energy scans), it is known how to deal with this problem: one has to take into account the large angular momenta generated in these plasmas, and the effects of the baryonic chemical potential. But this does not work for the plasmas produced in peripheral collisions at the LHC. However, these results neglect some (less significant) aspects of bulk physics; could it be that the problem is resolved by taking into account these lower-order effects? Here we use a bulk dilatonic field (fully compatible with boundary data, as well as with the asymptotically AdS character of the bulk geometry) as a model of these effects, and show that this is unlikely to be the solution. Thus, the existence of a consistent holographic description of the most extreme LHC plasmas remains open to question.

Exploring the QCD Phase Structure with Beam Energy Scan in Heavy-ion Collisions

Beam energy scan programs in heavy-ion collisions aim to explore the QCD phase structure at high baryon density. Sensitive observables are applied to probe the signatures of the QCD phase transition and critical point in heavy-ion collisions at RHIC and SPS. Intriguing structures, such as dip, peak and oscillation, have been observed in the energy dependence of various observables. In this paper, an overview is given and corresponding physics implications will be discussed for the experimental highlights from the beam energy scan programs at the STAR, PHENIX and NA61/SHINE experiments. Furthermore, the beam energy scan phase II at RHIC (2019–2020) and other future experimental facilities for studying the physics at low energies will be also discussed.

Interaction region design and auxiliary detector systems for an EIC

There are a number of exciting physics opportunities at a future electron-ion collider facility. One possible design for such a facility is eRHIC, where the current RHIC facility located at Brookhaven National Lab would be transformed into an electron-ion collider. It is imperative for a seamless integration of auxiliary detector systems into the interaction region design to have a machine that meets the needs for the planned physics analyses, as well as take into account the space constraints due to the tunnel geometry and the necessary beam line elements. In this talk, we describe the current ideas for integrating a luminosity detector, electron polarimeter, roman pots, and a low Q2 -tagger into the interaction region for eRHIC.

Nucleon-gold collisions at200AGeV using taggedd + Auinteractions in the PHOBOS detector

Forward calorimetry in the PHOBOS detector has been used to study charged hadron production in d+Au, p+Au and n+Au collisions at sqrt(s_nn) = 200 GeV. The forward proton calorimeter detectors are described and a procedure for determining collision centrality with these detectors is detailed. The deposition of energy by deuteron spectator nucleons in the forward calorimeters is used to identify p+Au and n+Au collisions in the data. A weighted combination of the yield of p+Au and n+Au is constructed to build a reference for Au+Au collisions that better matches the isospin composition of the gold nucleus. The p_T and centrality dependence of the yield of this improved reference system is found to match that of d+Au. The shape of the charged particle transverse momentum distribution is observed to extrapolate smoothly from pbar+p to central d+Au as a function of the charged particle pseudorapidity density. The asymmetry of positively- and negatively-charged hadron production in p+Au is compared to that of n+Au. No significant asymmetry is observed at mid-rapidity. These studies augment recent results from experiments at the LHC and RHIC facilities to give a more complete description of particle production in p+A and d+A collisions, essential for the understanding the medium produced in high energy nucleus-nucleus collisions.

Angular momentum in QGP holography

The quark chemical potential is one of the fundamental parameters describing the quark–gluon plasma produced by sufficiently energetic heavy-ion collisions. It is not large at the extremely high temperatures probed by the LHC, but it plays a key role in discussions of the beam energy scan programmes at the RHIC and other facilities. On the other hand, collisions at such energies typically (that is, in peripheral collisions) give rise to very high values of the angular momentum density. Here we explain that holographic estimates of the quark chemical potential of a rotating sample of plasma can be very considerably improved by taking the angular momentum into account.

Spin effects in forward high energy proton scattering

The relation between elastic amplitudes for polarized proton collisions and spin-dependent scattering observables is studied near the forward direction with a view to providing information on particular amplitudes at low values of momentum transfer. Such information has been singularly successful, for instance, in evaluating the amount of beam polarization achieved by the RHIC facility for its spin programme. A more extensive deployment of polarized facilities at a number of accelerators would permit a much deeper analysis of spin physics in a range of kinematic domains.

HEAVY-FLAVOR MESON PRODUCTION AT RHIC

Collisions of heavy atomic nuclei at very high beam energies allow to create and study hot QCD matter under laboratory-controlled conditions. Measurements at the SPS and RHIC facilities have yielded compelling evidence for the formation of this novel state of matter, the so-called Quark-Gluon Plasma (QGP). Due to their large mass, heavy quarks (charm and bottom) are believed to be predominantly produced in the initial state of the collision so that they probe the entire lifetime of the QGP. Thus, the investigation of heavy-flavor meson production in nucleus-nucleus collisions provides profound insight into the QGP properties. In this paper we review recent results from the RHIC facility on heavy-flavor meson production and their interaction with the QCD matter on the partonic level.

Single-inclusive hadron production in transversely polarizedppandp¯pcollisions with threshold resummation

We investigate the resummation of large logarithmic perturbative corrections to the partonic cross sections for single-inclusive high-${p}_{T}$ hadron production in collisions of transversely polarized hadrons. We perform the resummation to next-to-leading logarithmic accuracy. Phenomenological results are given for $\overline{p}p$ collisions at center-of-mass energy $\sqrt{S}=14.5\text{ }\text{ }\mathrm{GeV}$ and for $pp$ collisions at $\sqrt{S}=62.4\text{ }\text{ }\mathrm{GeV}$ and at $\sqrt{S}=10\text{ }\text{ }\mathrm{GeV}$, which are relevant for possible experiments at the GSI-FAIR, RHIC and J-PARC facilities, respectively. We find significant enhancements of the spin-dependent and spin-averaged cross sections, but a decrease of the double-spin asymmetry ${A}_{TT}^{\ensuremath{\pi}}$.

HEND: A DATABASE FOR HIGH-ENERGY NUCLEAR DATA

We propose to develop a high-energy heavy-ion experimental database and make it accessible to the scientific community through an on-line interface. The database will be searchable and cross-indexed with relevant publications, including published detector descriptions. It should eventually contain all published data from older heavy-ion programs such as the Bevalac, AGS, SPS and FNAL fixed-target programs, as well as published data from current programs at RHIC and new facilities at GSI (FAIR), KEK/Tsukuba and the LHC collider. This data includes all proton-proton, proton-nucleus to nucleus-nucleus collisions as well as other relevant systems and all measured observables. Such a database would have tremendous scientific payoff as it makes systematic studies easier and allows simpler benchmarking of theoretical models to a broad range of experiments. To enhance the utility of the database, we propose periodic data evaluations and topical reviews. These reviews would provide an alternative and impartial mechanism to resolve discrepancies between published data from rival experiments and between theory and experiment. Since this database will be a community resource, it requires the high-energy nuclear physics community's financial and manpower support.

Hadron Production at Forward and Backward Rapidities in √{ NN </i≯} = 200 GeV d+Au Collisions

Understanding normal nuclear medium effects present in heavy ion collisions is essential for understanding the following dynamics of the high density matter produced in the collision. Asymmetric collisions, such as deuteron + gold, provide a key tool for studying these effects since particles produced in the forward and backward directions may be subject to different phenomena. Particle production has been studied in d+Au collisions for various kinematic regions at the RHIC facility. PHENIX has measured charged hadron production as a function of pT for different centrality classes using the PHENIX muon spectrometers for d+Au collisions at \(\sqrt {s_{NN} } = 200\) GeV. The PHENIX muon spectrometers have coverage in both forward and backward directions in the rapidity range 1.2 |η| < 2.4. The Rcp measurement, the ratio of central to peripheral production, is presented and discussed. Comparisons are also made with some relevant theoretical calculations.

The RHIC upgrades program

Over the past years plans have developed to extend the physics reach of the RHIC facility by upgrading the accelerator and the physics experiments. With the extended physics program RHIC will remain competitive beyond 2008 when the turn on of LHC is expected, as well as advance our understanding of QCD by fully exploiting RHIC's unique spin physics capabilities. In this talk I present a summary of the tentative plans for future RHIC operation, the physics goals of the upgrades program and the necessary detector upgrades of PHENIX and STAR to achieve these goals.

Radiative corrections to observables of W-boson production in collisions of polarized hadrons

Ascheme is proposed for studying the nucleon spin with the aid of observables in the production of single W bosons in hadron-hadron collisions. Special attention is given to determining the distributions of polarized quarks in the low-x region. Lowest order electroweak radiative corrections to observables are calculated. For the conditions of future experiments at the RHIC facility, the relevant cross sections and single and double asymmetries are estimated numerically with allowance for radiative corrections.

Quenching of highp⊥hadron spectra by hadronic interactions in heavy ion collisions at relativistic energies

Typically the materialization of high energetic transverse partons to hadronic jets is assumed to occur outside the reaction zone in a relativistic heavy ion collision. In contrast, a quantum mechanical estimate yields a time on the order of only a few fm/c for building up the hadronic wavefunction for jets with typical transverse momenta of pT < 10 GeV as accessible at RHIC facilities. The role of possible elastic or inelastic collisions of these high pT particles with the bulk of hadrons inside the fireball is addressed by means of an opacity expansion in the number of collisions. This analysis shows that the hadronic final state interactions can in principle account for the modification of the (moderate) high pT spectrum observed for central collisions at RHIC.