Overview* of experimental results in heavy ion collisions

Transcription

1 Overview* of experimental results in heavy ion collisions Dipartimento di Fisica Sperimentale dell Universita di Torino and INFN Torino * The selection criteria of the results presented here are (to some extent)..speaker dependent!!!

2 Outline Part one: past and present Introduction Experimental considerations SPS: experiments and results RHIC: experiments and results Part two: future ALICE at LHC: detector and Physics performance CBM at GSI: a short overview

3 Introduction -Few words on QGP -Why heavy ion collisions to study QGP

4 Lattice QCD calculations In lattice QCD, non-perturbative problems are treated by discretization on a space-time lattice. Indication of trans. from HG to QGP at T c 170 MeV ε c 1 GeV/fm 3 QGP? true phase trans. or crossover? intermediate phase of strongly interacting QGP? Chiral symmetry restoration? Constituent mass current mass HG?

5 Why ultrarelativistic heavy-ion collisions? High energy p-p p p collisions (e.g. (e.g. p lab =100GeV/c, E cm =14 GeV About ½ of c.m. energy carried by leading particles About ½ of c.m. energy used to produce (about 10) π/k of low p t High energy A-A A A collisions Due to multiple N-N N N interaction, even more than ½ of the c.m. energy is useful for particle production Pb-Pb collisions: about 200 pairs of participant nucleons Interaction volume : ~ 1000 fm 3 Energy densities ~ 1 GeV/ / fm 3 can be reached!!! This is just a very crude estimate, to be taken with caution; but t the enrgy density order of magnitude is the one needed to form QGP

6 Heavy-ion collisions QCD predicts that hadronic matter undergoes a phase transition at a critical temperature of T~ MeV, giving a new deconfined state of quark and gluons: the Quark Gluon Plasma (QGP). The energy density required is quite high and can be reached only through central heavy ions collisions SPS, RHIC and LHC can reach such energies, but then we have to search for observables

7 Heavy-ion collisions: evolution of the system fireball

8 Space-time Evolution of Collisions time γ γ e φ Open and hidden charm e jet p K π μ Open and hidden beauty Λ Freeze-out QGP Expansion Hadronization Pre-equilibrium space Pb Pb J.Harris

9 Diagnostic tools experimental challenge: to observe in the final state the signatures of the phase transition Low-p t soft probes thermal particle production from QGP single particle spectra two particle correlations particle abundances and ratios flow patterns E t High-p t hard probes during formation phase parton scattering processes with large Q 2 create high mass or high momentum objects that penetrate hot and dense matter and are sensitive to the nature of the medium Caveat: pure hadronic effects can mimic expected QGP signaturures Therefore one needs: to establish experimentally a solid baseline studying systems where no QGP is expected (e.g. pp, pa) and use these data as a reference beams of hard probes: jets, J/ψ. matter box vacuum QGP????

10 History of High-Energy A+B Beams BNL-AGS AGS: mid 80 s, early 90 s O+A, Si+A 15 AGeV/c s NN ~ 6 GeV Au+A 11 AGeV/c s NN ~ 5 GeV CERN-SPS SPS: mid 80 s, 90 s O+A, S+A 200 AGeV/c s NN ~ 20 GeV Pb+A 160 AGeV/c s NN ~ 17 GeV BNL-RHIC RHIC: early 00 s Au+Au s NN ~ 130 GeV Au+Au, p+p, d+au s NN ~ 200 GeV

11 Exploring the phase diagram of nuclear matter with heavy-ion collisions

12 Experimental considerations A couple of experimental issues typical of ultrarelativistic heavy ion collisions: - the high multiplicity challenge - event geometry determination

13 The high multiplicity challenge - I

14 The high multiplicity challenge - II Simulated ALICE event High hadron mult.: High granularity of the tracking detectors (TPCs or silicon det.) peculiar of heavy ions Hadron blind detectors, only leptons are detected

15 Event geometry: participants vs. spectators spectators participants b spectators y=0 rapidity The number of projectile participants (N part ) decreases with b The number of projectile spectators (N spec ) increases with b A proj =N spec +N part

16 Zero-Degree Calorimeters ZDC Measure the energy carried by spectators ( individual neutrons, protons and fragments) Spectators have in average the same energy per nucleon E N of the beam Number of projectile spectators directly measured: N spec =E ZDC /E N Impact parameter derived from N spec

17 Charged particle multiplicity Minimum bias distr. Charged particle multiplicity dn/dη is correlated with N part and b: the centrality can be selected by dividing the event sample in multiplicity bins N part or b can be derived in a model-dependent way ALICE Physics Performance Report (in preparation)

18 Neutral transverse energy E T The same considerations concerning charged particle multiplicity also hold for neutral transverse energy. As charged particle multiplicity, also ET is (anti) correlated with E ZDC E T (GeV) E ZDC (GeV) NA50 collaboration, Phys. Lett. B 450 (1999) 456

19 Energy density determination The initial energy density ε 0 reached in heavy ion collisions can be estimated within the Bjorken model from the measured transverse energy E T as: where the initially produced collision fireball is considered as a cylinder of length dz = τ 0 dy and transverse radius R ; πr 2 is the overlap area of colliding nuclei and τ 0 is the initial time (typically τ 0 ~ 1 fm/c) Caveat: implicit hypothesis of Bjorkens formula is vanishing net barion density at mid rapidity (not completely fulfilled at SPS)

20 RECENT PAST (and present): SPS Global observables & particle ratios Probes»Multi-strange enhancement»low-mass dileptons»heavy quarkonia

21 Global observables - I Energy density Transverse energy measured for the most central central Pb-Pb collisions ~ 400 GeV From Bjorken formula (with τ 0 =1 fm/c): ε=3.5 ± 0.5 GeV Other methods give even higher values of ε ε well above the expect. critical value of ~1 GeV (Partial) nuclear stopping Fireball is not baryon-free (see below for more details)

22 Global observables - II Temperature at kinetic freezeout inferred from inverse slope of transverse mass distibutions Different particles show different slopes!! Heavier particle larger inverse slope larger T NA44 data, central Pb-Pb coll. at SPS

23 Global observables - III The dependence of the inverse slope on the particle mass is interpreted as thermal motion + collective expansion of fireball (which occurs with the same velocity for all particles E kin m ) At SPS: T fo ~145 MeV B ~ 0.4

24 Temperature from m t distribution Data: indication of collective directed flow (@ SPS β ~ 0.4 ) superposed to thermal motion Some particle (e.g. Ω) do not participate to collective flow due to their small cross section and creation in early stage of the collision

25 Particle ratios Yields (and hence ratios) of particles of different species measured at SPS by different experiments: NA49 NA44 WA97/NA57 WA98 NA50

26 .NA49 Pb-Pb event

27 Particle ratios from thermal model Based on grand canonical ensamble to describe the density n i of the particle species i in an equilibrated fireball: g i =spin degeneracy P. Braun-Munzinger, I. Heppe and J. Stachel, Phys. Lett. B 465 μ i =μ B B i -μ s S i -μ I I i =chemical potential B, S, I = baryon, strangeness and isospin quantum numbers Free parameters: T and μ B Caveats: Underlying hypothesis: equilibrium is reached The temperature T is the one at chemical freezeout

28 Thermal model vs. SPS data - Agreement between data and predictions indicates that equilibrium is at least closely approached - Resulting values of the free parameters of the fit: T=168 MeV, μ B =266 MeV; very close to the expected boundary between hadronic matter and QGP Villa the Gualino system Oct. 05 crosses this boundary before chemical freeze out.

29 Strangeness enhancement QGP formation expected to lead to an enhancement of strange particles (Rafelsky,, 1982) because: Pauli Blocking Decrease of s quark mass Strangeness enhancement of single-strange strange particle observed in A-A A A collisions already at rather low energies Multi-strange enhancement observed at SPS More difficult to explain with non QGP-inspired models

30 Multi-strange baryons: NA57 exp. at SPS B=1.4 T Multi-strange baryons detected at midrapidity Centrality (top 60% of inelastic cross section) measured by microstrip multiplicity

31 The following decay channels (and the corresponding ones for antiparticles) are studied Λ π - p Ξ - Λπ - (with Λ π - p) Ώ - ΛΚ - (with Λ π - p) Particle selection based on kinematical cuts NA57 results at 158 GeV Large enhancement, increasing with centrality, more pronounced for: -Hyperons(or antihyperons) with higher content of strangeness - Hyperons w.r. to antihiperons for the same content of strangeness

32 NA57 results at 40 and 158 GeV Evolution of the antihyperon to hiperon ratios with sqrt(s) : The ratios increase with energy with a stronger dependence for particles with smaller strangeness content. Pattern consistent with a decrease of the baryon density in the central region with increasing energy Question: which mechanisms can account for the observed hyperon and antihyperon enhancement in Pb-Pb collisions?

33 NA45 Layout Good acceptance in the low-mass and low pt region

34 NA45 results: p-ap Data in agreement with a Cocktail including all known sources of dielectrons (e.g. π and η Dalitz, ρ/ω and φ)

35 NA45 results: S-Au 200 AGeV/c The The Cocktail underestimates the experimental dielectron mass spectrum by about one order of magnitude in the mass region between 200 MeV/c 2 and 800 MeV/c 2 The The ρ/ω peak is washed out

36 NA45 results: 158 AGeV/c The effect is also observed in Pb-Au at 158 AGeV/c

37 NA45 results: 158 AGeV/c Mass: excess mainly present in the mass region between η and ρ/ω. Centrality: the excess increases with the centrality. Excess vanishes for periph.. collisions with N part 150 (i.e. b 8 b 8 fm). Data for more peripheral collission missing!!!!