Strangeness, charm and beauty in quark matter: SQM 2007 experimental overview

Abstract

This paper aims at providing an experimental overview of the Strangeness in Quark Matter 2007 Conference. StrangenessStrangeness is one of the "historic" observables in ultrarelativistic nucleus-nucleus collisions. In elementary collisions, strangeness production is suppressed relative to the production of light flavours. For a large system, this suppression is expected to be removed -i.e. strangeness production is expected to be enhanced -provided the system allows for a thermodynamical description in which strangeness conservation can be treated over a large volume and does not need to be enforced locally at each collision, or, in other words, provided the correlation volume is also large, as expected if partons are freed in a Quark-Gluon Plasma (QGP) [1,2]. This is non-trivial: if, for instance, nucleusnucleus collisions were just a simple superposition of independent nucleon-nucleon collisions, no strangeness enhancement would be expected, no matter how large the system size. In such a hypothetical case, there would be no mechanism of long-range communication within the system, and the correlation volume would still be that of each individual nucleon-nucleon collision: the system would be large, but it wouldn't "know". In other words a large system size, by itself, is not sufficient to get enhancement; a mechanism of long-range communication, sufficiently "fast" on the timescale of the collision (a few fm/c) -as would be provided by colour deconfinement -is also needed. A pattern of strangeness enhancement increasing with the particle's strangeness content was indeed observed in Pb-Pb collisions at the SPS [3]. The most recent data on hyperon enhancements from the SPS were presented at this conference by both NA49 [4] and NA57 [5]. Reasonable agreement is found between the two experiments. Hadronic transport models are unable to reproduce these data (see for instance [6]), while they have an obvious explanation within a deconfined scenario. The centrality dependence of the enhancements is also an important piece of information in order to constrain the models. The "canonical suppression model" [7] -for instancepredicts a fast saturation of the enhancements with the event centrality, at odds with the data. The energy dependence is also of primary interest in this respect. For the most central collisions, the enhancements do show an energy dependence, decreasing as the energy is increased from low SPS energy (√s NN = 8.73 GeV) to top SPS energy (√s NN = 17.3 GeV) [5] and to RHIC (√s NN = 200 GeV) [8]; this dependence, however, is much weaker than had been predicted [9]. I think it is fair to say that neither the centrality dependence nor the energy dependence of hyperon enhancements is yet well understood. New results from STAR where shown at this conference on the hyperon enhancements in Cu-Cu collisions at RHIC (figure 1) [10] -found to be very similar to those measured in Au-Au collisions at corresponding values of the number of participants -and on the φ enhancement in Au-Au and Cu-Cu collisons [11] -found ...

Strangeness in quark matter 2008—experimental summary

“…The questions regarding the relative contributions from charm and bottom to the observed heavy flavor flow and energy loss will be resolved by the higher luminosities of RHIC II and the ongoing upgrades of PHENIX and STAR. Compelling questions about nuclear matter at truly unprecedented energy densities eagerly await the beginning of heavy ion collisions at the LHC [51,52]. The prospects for new data from these varied facilities suggest that future experimental summary talks at this conference series will continue to be challenging in the most pleasant of ways.…”

ALICE Masterclass on strangeness

“…Strangeness is one of the "historic" observables in ultrarelativistic nucleus-nucleus collisions [10][11][12]. In elementary collisions, strangeness production is suppressed relative to the production of light flavors.…”

Future experiments in relativistic heavy ion collisions

“…Federico has summarized the experimental results of the past years in his contribution [10], so here I will only touch on particle identified measurements that are of particular relevance to future experiments.…”