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Register a new userThe BEST framework for the search for the QCD critical point and the chiral magnetic effect
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The Beam Energy Scan Theory (BEST) Collaboration was formed with the goal of providing a theoretical framework for analyzing data from the Beam Energy Scan (BES) program at the relativistic heavy ion collider (RHIC) at Brookhaven National Laboratory. The physics goal of the BES program is the search for a conjectured QCD critical point as well as for manifestations of the chiral magnetic effect. We describe progress that has been made over the previous five years. This includes studies of the equation of state and equilibrium susceptibilities, the development of suitable initial state models, progress in constructing a hydrodynamic framework that includes fluctuations and anomalous transport effects, as well as the development of freezeout prescriptions and hadronic transport models. Finally, we address the challenge of integrating these components into a complete analysis framework. This document describes the collective effort of the BEST Collaboration and its collaborators around the world.
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We discuss the potential of light-nuclei measurement in heavy-ion collisions at intermediate energies for the search of the hypothetical QCD critical end-point. A previous proposal based on neutron density fluctuations has brought appealing experimental evidences of a maximum in a ratio involving tritons, protons and deuterons, ${cal O}_{tpd}$. However these results are difficult to reconcile with the state-of-the-art statistical thermal model predictions. Based on the idea that the QCD critical point can lead to a substantial attraction among nucleons, we propose new ratios involving $^4$He in which the maximum would be more evident. We argue that the experimental extraction is feasible by presenting actual measurements at low and high collision energies. We also illustrate the possible behavior of these ratios at intermediate energies applying the semiclassical method based on flucton paths using preliminary STAR data for ${cal O}_{tpd}$.
The chiral magnetic effect (CME) is an exact statement that connects via the axial anomaly the electric current in a system consisting of interacting fermions and gauge field with chirality imbalance that is put into a strong external magnetic field. Experimental search of the magnetically induced current in QCD in heavy ion collisions above a pseudocritical temperature hints, though not yet conclusive, that the induced current is either small or vanishing. This would imply that the chirality imbalance in QCD above $T_c$ that could be generated via topological fluctuations is at most very small. Here we present the most general reason for absence (smallness) of the chirality imbalance in QCD above Tc. It was recently found on the lattice that QCD above Tc is approximately chiral spin (CS) symmetric with the symmetry breaking at the level of a few percent. The CS transformations mix the right- and left-handed components of quarks. Then an exact CS symmetry would require absence of any chirality imbalance. Consequently an approximate CS symmetry admits at most a very small chirality imbalance in QCD above Tc. Hence the absence or smallness of an magnetically induced current observed in heavy ion collisions could be considered as experimental evidence for emergence of the CS symmetry above Tc.
We discuss the QCD phase diagram from two different point of view. We first investigate the phase diagram structure in the strong coupling lattice QCD with Polyakov loop effects, and show that the the chiral and Z_{N_c} deconfinement transition boundaries deviate at finite mu as suggested from large N_c arguments. Next we discuss the possibility to probe the QCD critical point during prompt black hole formation processes. The thermodynamical evolution during the black hole formation would result in quark matter formation, and the critical point in isospin asymmetric matter may be swept. (T,mu_B) region probed in heavy-ion collisions and the black hole formation processes covers most of the critical point locations predicted in recent lattice Monte-Carlo simulations and chiral effective models.
The evolution of non-hydrodynamic slow processes near the QCD critical point is explored with the novel Hydro+ framework, which extends the conventional hydrodynamic description by coupling it to additional explicitly evolving slow modes describing long wavelength fluctuations. Their slow relaxation is controlled by critical behavior of the correlation length and is independent from gradients of matter density and pressure that control the evolution of the hydrodynamic quantities. In this exploratory study we follow the evolution of the slow modes on top of a simplified QCD matter background, allowing us to clearly distinguish, and study both separately and in combination, the main effects controlling the dynamics of critical slow modes. In particular, we show how the evolution of the slow modes depend on their wave number, the expansion of and advection by the fluid background, and the behavior of the correlation length. Non-equilibrium contributions from the slow modes to bulk matter properties that affect the bulk dynamics (entropy, pressure, temperature and chemical potential) are discussed and found to be small.
With the measurement of several observables at SPS energies that demonstrate non-monotonic behavior as a function of centrality and $sqrt{s_{NN}}$, there is growing interest in pursuing a scan of relativistic heavy ion collisions at low energies at the Relativistic Heavy Ion Collider. The capabilities of the PHENIX experiment to take quality measurements at low RHIC energies is described and directly demonstrated with analyses of Au+Au collisions at $sqrt{s_{NN}}$ = 19.6 GeV and Cu+Cu collisions at $sqrt{s_{NN}}$ = 22.5 GeV. The contribution of upgrades to the PHENIX detector in the upcoming years will also be discussed in the context of a low energy RHIC run.
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