CERN Courier January/February 2012 Heavy ions Pursuit of deconfinement returns to the SPS Marek Gazdzicki and Peter Seyboth Heavy Ion Collider (RHIC) and seems to be repeated at the LHC. Despite many arguments in favour of the creation of QGP at these follow the story of the discovery of the onset energies, its discovery cannot be claimed from these data alone. A different strategy for identifying the creation of QGP was of deconfinement in heavy-ion collisions as followed by the NA49 experiment at the SPS and is now being it moved from the SPS to RHIC and the LHC continued by its successor NA61/SHINE, as well as by the STAR experiment at RHIC. The idea is to measure quantities that are – and now heads back to the SPS with the sensitive to the state of strongly interacting matter as a function of collision energy in, for example, central lead–lead collisions. NA61/SHINE experiment. The reasoning is based on simple arguments. First, the energy density of matter created at the early stage of heavy-ion collisions increases monotonically with collision energy. Thus, if two phases The quark model of hadron classification proposed by Murray of matter exist, the low-energy density phase is created in collisions Gell-Mann and George Zweig in 1964 motivated the opinion that a at low energies and the high-energy density phase in collisions at new state of matter, namely strongly interacting matter composed high energies. Second, the properties of both phases differ signifi- of subhadronic constituents, may exist. Soon thereafter, quantum cantly, with some of the differences surviving until the freeze-out chromodynamics (QCD) was formulated as the theory of strong to hadrons and so can be measured in experiments. The search interactions, with quarks and gluons as elementary constituents. strategy is therefore clear: look for a rapid change of the energy As a natural consequence, the existence of a state of quasi-free dependence of hadron production properties that are sensitive to quarks and gluons – the QCD quark–gluon plasma (QGP) – was QGP, because these will signal the transition to the new state of suggested by Edward Shuryak in 1975. These events, together matter and indicate its existence. with the rapid development of particle-accelerator and detector This strategy, and the corresponding NA49 energy-scan pro- techniques, mark the beginning of the experimental search for this gramme, were motivated in particular by a statistical model of the hypothetical, subhadronic form of matter in nature. early stage of nucleus–nucleus collisions (Gazdzicki and Goren- stein 1999). It predicted that the onset of deconfinement should First indications lead to rapid changes of the collision-energy dependence of bulk The experimental efforts received a boost from the first accelera- properties of produced hadrons, all appearing in a common energy tion of oxygen and sulphur nuclei at CERN’s Super Proton Syn- domain. Data from 1999 to 2002 on central Pb+Pb collisions at chrotron (SPS) in 1986 (√sNN ≈ 20 GeV) and of lead nuclei in 1994 20A, 30A, 40A, 80A and 158A GeV were recorded and the pre- (√sNN ≈ 17 GeV). Measurements from an array of experiments were dicted features were observed at low SPS energies (CERN Courier surprisingly well described by statistical and hydrodynamical September 2003 p17; Alt et al. models. They indicated that the created system of strongly interact- 2008, Gazdzicki et al. 2011). ing particles is close to at least local equilibrium (Heinz and Jacob An independent verification 2000). Thus, a necessary condition for QGP creation in heavy-ion The strategy is of NA49’s discovery is vital and collisions was found to be fulfilled. The “only” remaining problem clear: look for a calls for further measurements was the identification of unique experimental signatures of QGP. rapid change of in the SPS energy range. Two Unfortunately, precise quantitative predictions are currently new experimental programmes impossible to calculate within QCD and predictions of phenom- energy dependence are already in operation: the ion enological models suffer from large uncertainties. Therefore, the of hadron production programme of NA61/SHINE at results of the measurements were only suggestive of the production properties the SPS; and the beam-energy of QGP in heavy-ion collisions at the top SPS energy. The same scan at RHIC. Elsewhere, s situation persisted at the top energies of Brookhaven’s Relativistic the construction of the Image left: Be+Be collision at 150A GeV/c recorded by NA61 in the 2011 run. 17 CCJanFeb12-NA61.indd 17 11/01/2012 09:53 CERN Courier January/February 2012 Heavy ions 120 1.2 25 Pb+Pb Au+Au a) AGS b) – c) d) 20 400 K SPS(NA49) 15 RHIC LHC (based on ALICE) 0.2 ) 80 10 w 5 (Landau) y (y = 0) /(N 1.0 + 0 σ π 0 5 10 15 T (MeV) )/ 〈π〉 / – + 0.1 Pb+Pb Au+Au 40 Pb+Pb Au+Au Pb+Pb Au+Au K AGS AGS π AGS 200 ( SPS(NA49) SPS(NA49) y SPS(NA49) σ RHIC FIT (3.2 < F < 15) RHIC RHIC LHC(ALICE) FIT (F < 1.85) LHC(ALICE) 0 0 0 0.8 1 102 104 0 20 40 60 1 102 104 1 102 104 1/2 √sNN (GeV) F (GeV ) √sNN (GeV) √sNN (GeV) Fig. 1. Energy dependence of hadron production properties in central Pb+Pb (Au+Au) collisions, see text for details. Nuclotron-based Ion Collider at JINR, Dubna, is in preparation. central Pb+Pb (Au+Au) collisions are shown in figure 1c. The strik- The basic goals of this experimental effort are the confirmation and ing features of these data can be summarized and interpreted as fol- the study of the details of the onset of deconfinement and the inves- lows (Gorenstein et al. 2003). The T parameter increases strongly tigation of the transition line between the two phases of strongly with collision energy up to low SPS energies, where the creation of interacting matter. In particular, the discovery of the hypothesized confined matter at the early stage of the collisions takes place. In a second-order critical end-point would be a milestone in uncovering pure phase, increasing collision energy leads to an increase of the properties of strongly interacting matter. early-stage temperature and pressure. Consequently the transverse momenta of produced hadrons, measured by the inverse slope Four pointers parameter, increase with collision energy. This rise is followed Last year rich data from the RHIC beam-energy scan programme by a region of approximately constant value of the T parameter in were released (Kumar 2011, Mohanty 2011). Furthermore, the first the SPS energy range, where the transition between confined and results from Pb+Pb collisions at the LHC were revealed (Schukraft deconfined matter with the creation of a mixed phase is located. et al. 2011, Toia et al. 2011). It is therefore time to review the sta- The resulting softening of the equation of state “suppresses” the tus of the observation of the onset of deconfinement. The plots in hydrodynamical transverse expansion and leads to the observed figure 1 summarize relevant results that became available in June plateau or even a minimum structure in the energy dependence of 2011. They show the energy dependence of four hadron-production the T parameter. At higher energies (RHIC data), T again increases properties measured in central Pb+Pb (Au+Au) collisions, which with collision energy. The equation of state at the early stage again reveal structures referred to as the “horn”, “kink”, “step” and becomes stiff and the early-stage pressure increases with collision “dale” – all located in the SPS energy range. energy, resulting in a resumed increase of T. The horn. The most dramatic change of the energy dependence The dale. As discussed above, a weakening of the transverse is seen for the ratio of yields of kaons and pions (figure 1a). The expansion is expected to result from the onset of deconfinement steep threshold rise of the ratio characteristic for confined matter because of the softening of the equation of state at the early stage. changes at high energy into a constant value at the level expected Clearly, the latter should also weaken the longitudinal expansion for deconfined matter. In the transition region (at low SPS energies) (Petersen and Bleicher 2006). This expectation is confirmed in fig- a sharp maximum is observed, caused by the higher production ure 1d, where the width of the π− rapidity spectra in central Pb+Pb ratio of strangeness-to-entropy in confined matter than in decon- collisions relative to the prediction of ideal Landau hydrodynamics fined matter. is plotted as a function of the collision energy. In fact, the ratio has The kink. Most particles produced in high-energy interactions a clear minimum at low SPS energies. are pions. Thus, pions carry basic information on the entropy The results shown in figure 1 include new results on central Pb+Pb created in the collisions. On the other hand, entropy production collisions at the LHC and data on central Au+Au collisions from depends on the form of matter present at the early stage of colli- the RHIC beam-energy scan. The RHIC results confirm the NA49 sions. Deconfined matter is expected to lead to a final state with measurements at the onset energies while the LHC data demonstrate higher entropy than confined matter. Consequently, the entropy that the energy dependence of increase at the onset of deconfinement is expected to lead to a hadron-production properties steeper increase with collision energy of the pion yield per par- A smooth evolution shows rapid changes only at low ticipating nucleon.