Possible resolutions of theD-puzzle

Abstract

We propose possible ways of explaining the net charge event-by-event fluctuations in Au+Au collisions at the Relativistic Heavy Ion Collider within a quark recombination model. We discuss various methods of estimating the number of quarks at recombination and their implications for the predicted net charge fluctuations. We also discuss the possibility of diquark and quark-antiquark clustering above the deconfinement temperature.Fluctuations of the net electric charge of all particles emitted into a specified rapidity window have been proposed as a possible signal for the formation of deconfined quark matter in relativistic heavy ion collisions [1,2]. The argument at the basis of this proposal is that charge fluctuations in a quark-gluon plasma are expected to be significantly smaller (by a factor 3−4) than in a hadronic gas. Because the net charge contained in a given volume is locally conserved and can only be changed by particle diffusion, thermal fluctuations generated within the deconfined phase could survive hadronization and final state interactions. Quantitative estimates of the diffusion of net charge showed that the survival of these fluctuations from an early stage of the collision requires a moderately large rapidity window [3].The most widely used measure for the entropy normalized net charge fluctuations is the D measure [2]:where (∆Q) 2 denotes the event-by-event net charge fluctuation within a given rapidity window ∆y, and N ch is the total number of charged particles emitted in this window. For a free plasma of quarks and gluons D ≈ 1, while for a free pion gas D ≈ 4. For the comparison with experimental data a number of corrections for acceptance and global charge conservation must be applied to the expression for D [4]. The relation of the D-measure to other measures of net charge fluctuations has been discussed by various authors [5,6,7].Several experiments have measured net charge fluctuations in heavy ion collisions at the CERN Super-Proton Synchrotron (SPS) and at the Relativistic Heavy Ion Collider (RHIC) in Brookhaven [8,9,10,11]. The results for D are generally somewhat smaller than 4, but much larger than the value predicted for a free quark-gluon gas. For example, the STAR collaboration has measured D = 2.8 ± 0.05 in central Au+Au collisions at √ s N N = 130 GeV [8], before applying corrections for global charge conservation and other effects [4]. The PHENIX experiment measured net charge fluctuations in a limited azimuthal acceptance window around midrapidity, which extrapolate to a value D ≈ 3 [9]. These results are surprising, because many other observables indicate that a deconfined quark-gluon plasma is formed in these collisions.Bia las has argued that the measured values of D could be compatible with the net charge fluctuations in a deconfined quark phase, if hadronization proceeds according to simple valence quark counting rules [12] and if gluons do not play an active role in the hadronization. Indeed, hadron abundances measured in relativistic heavy ion collisions at the SPS ...

Hadron production in central nucleus–nucleus collisions at chemical freeze-out

“…Nevertheless, it is interesting to speculate whether the deviations from the thermal model (including rather poor-quality fits) which we have encountered for the SPS energies are a hint for the critical point. It is expected that, in the (broad) vicinity of the critical (end)point the thermal model would not work [68]. Thermal fits including fluctuations have been already performed for the top SPS energy [69].…”

SQM 2006: theory summary and perspectives

“…The reach of RFD can be extended and the problem having to terminate the calculation at a single flavor-independent and fixed freeze-out temperature can be overcome by combining the RFD calculation with a microscopic hadronic cascade model -this kind of hybrid approach (dubbed hydro plus micro) was pioneered in [58] and has subsequently been adopted by other groups [59,61,47,48]. Its key advantages are that the freeze-out occurs naturally as a result of the microscopic evolution and that flavor degrees of freedom are treated explicitly through the hadronic cross sections of the microscopic transport.…”

“…RFD has been extremely successful in describing single particle spectra and collective flow effects at RHIC [44,45,46], even though no hydrodynamical model implementation has so far attempted to address the entire array of available data in a single consistent calculation (a forthcoming publication aims to remedy this situation [48]). The shape of the spectra as well as the transverse momentum dependence of the elliptic flow for minimum bias data are generally reproduced nicely.…”

“…First implementations of these hybrid approaches were restricted to 1+1 [58] and 2+1 [59] dimensions in the hydrodynamic component of the model. However, recently new state of the art fully three-dimensional ideal hydrodynamics models have become available and are now being incorporated into the hydro+micro framework [47,60,61,48].…”

Angular hadron correlations probing the early medium evolution

“…In [5] it was shown that there is about 50% uncertainty in the extraction of the medium quenching power (given by K) from R AA even for model evolutions which describe bulk soft matter properties; a 2+1d hydrodynamical evolution [6], a 3+1d hydrodynamical evolution [7] and a parametrized evolution [8]. Greater sensitivity to the medium density evolution can hence only be gained in more differential probes such as γ-hadron correlations [2], R AA for non-central collisions [5] or back-toback hadron correlations [4], here we focus on the latter.…”