Physics Results of the NA49 exp. on Nucleus – Nucleus Collisions at SPS Energies P. Christakoglou, A. Petridis, M. Vassiliou Athens University HEP2006, Ioannina April 2006

Outline Introduction Energy dependence of Pb+Pb collisions transverse mass spectra particle yields yield ratios Electric charge correlations the Balance Function system size and centrality dependence rapidity dependence energy dependence Conclusions

Introduction Pb+Pb collisions at top SPS energy: Initial energy density exceeds the critical value predicted by lattice QCD Strong collective behavior Proposed signatures for deconfinement observed Signatures not specific for deconfinement SPS energy scan : 20, 30, 40, 80, 158 GeV/nucleon To search for structure in the energy dependence of hadron production characteristics indicating the onset of deconfinement

dE/dx forward rapidity TOF+dE/dx midrapidity The NA49 Detector VCAL detects projectile spectators dE/dx resolution 3-6% TOF resolution ~60ps

Transverse mass spectra at mid-rapidity Kinetic freeze-out at T~ 90–110 MeV, βτ ~ 0.45c 20 AGeV 40 AGeV158 AGeV

Energy dependence of average transverse mass Increase of for final state particles (π,Κ,p) slows sharply at the lowest SPS energy

Energy dependence of the effective temperature parameter of Kaons consistent with approximately constant pressure and temperature in a mixed phase system

Rapidity Spectra

Energy dependence of antibaryon/baryon ratios The effect of the increasing baryon density is seen in the strong decrease of antibaryon/baryon ratios towards lower SPS energies.

Global view – Phase diagram Statistical model describes yields from AGS to RHIC energies T of “hadrochemical” freeze out increases decreases hadrochemical freeze out points at SPS energies approach the phase boundary

Energy dependence – pion yields Increase of ratio with energy gets steeper in the SPS range Pion deficit changes to enhancement compared to p+p SMES: statistical model of the early stage Increase of initial d.o.f. between AGS and SPS

Energy dependence –ratio of K, Λ yields to pions anti-strange quark carriers: sharp peak of yield small hadronic models do not reproduce the sharp peak strange quark cariers: similar peak in Λ/π structure in

Energy dependence – ratio of strange hadrons to pions Strangeness to pion ratio peaks sharply at the SPS The peak is not seen in p+p collisions SMES reproduces the data Suggests ONSET of deconfinement at SPS

Electric charge correlations in A+A collisions Oppositely charged particles are created at the same location of space – time. Charge – anticharge particles that were created earlier (early stage hadronization) are separated further in rapidity. Particle pairs that were created later (late stage hadronization) are correlated at small Δy. The Balance Function quantifies the degree of this separation and relates it with the time of hadronization.

The Balance Function The Balance function is defined as a correlation in y of oppositely charged particles, minus the correlation of same charged particles, normalized to the total number of particles. where P 1 : any rapidity interval in the detector P 2 : relative rapidity interval Bass-Danielewicz-Pratt, Phys.Rev.Lett.85, 2000 D. Drijard et al, Nucl. Phys. B(155), 1979

Due to cooling the width falls with time (σ therm ). The width of the Balance Function The overall width of the Balance Function (BF) in relative rapidity is a combination of the thermal spread and the effect of diffusion. The effect of diffusion stretches the BF (σ δn ). If the hadronization occurred at early times then the effect of collisions is to broaden the BF. On the other hand late stage hadronization suggests narrower BF.

System size and centrality dependence - √s NN = 17.2 GeV The width takes its maximum value for p+p interactions. Data show a strong system size and centrality dependence. Neither HIJING nor shuffled data show any sign of system size or centrality dependence. C. Alt et al. [NA49 collaboration], Phys.Rev. C71, (2005).

Comparison NA49 – STAR NA49 data show a strong centrality dependence of the order of (17 ± 3)%. STAR data show also a strong centrality dependence of the order of (14 ± 2)%. J. Adams et al., (STAR Collaboration) Phys. Rev. Lett. 90, (2003) √s NN = 130 GeV

Rapidity dependence - √s NN = 17.2 GeV Mid – rapidity (2.5 < η < 3.9)Forward rapidity (4.0 < η < 5.4) Acceptance filter OFF Acceptance filter ON

Energy dependence

Conclusions Central Pb+Pb collisions were studied in the SPS energy range. At 30A GeV : 1.The ratio of strangeness to pion production shows a sharp maximum. 2.The rate of increase of the produced pion multiplicity per wounded nucleon increases. 3.The effective temperature of pions and kaons levels to a constant value. These features are not reproduced by hadronic models but find a natural explanation in a reaction scenario with the onset of deconfinement in the early stage of the reaction at low SPS energy.

Conclusions (cont.) Results from a study on : 1.system size and centrality dependence 2.rapidity dependence 3.energy dependence of charged particles correlations, performed with the Balance Function, are consistent with the predicted delayed hadronization.

BACK UP

√s NN = 8.8 GeV – RAPIDITY DEPENDENCE Mid – rapidity (1.8 < η < 3.2 ) Forward rapidity (3.3 < η < 4.7) Acceptance filter OFF Acceptance filter ON