Freeze-out conditions in nuclear collisions at the SPS

1
Freeze-out conditions in nuclear collisions at
the SPS

• Federico Antinori
• INFN Padova - Italy

2
Contents

• Introduction
• Particle spectra (kinetic freeze-out)
• Particle rates (chemical freeze-out)
• Messages from freeze-out
• (and other special features at the SPS...)
• So, where do we stand? (my take...)
• So, what next? (my take...)
• Conclusions

3
Freeze-out?
4
Freeze-out?

• to speak of freeze-out one needs
• a collective system
• with some degree of uniformity in its space-time
  evolution
• freeze-out ? interactions cease
• inelastic interactions ? chemical freeze-out ?
  particle rates
• elastic interactions ? kinetic freeze-out ?
  particle spectra
• can we describe the freeze-out conditions using a
  relatively small number of parameters?

5
Particle spectra
6
Transverse mass spectra

• Indeed, transverse mass spectra are close to
  thermal
• e.g. NA57 53 central
• ...but thermal spectra, by themselves, do not
  imply thermalization or collectivity!

7
Inverse slope systematics

• The inverse slope, or apparent temperature T
  increases with the particle mass
• does not happen in pp
• Collective transverse flow?
• for a common transverse flow velocity vT, and
  for mT lt 2m, one indeed expects

NA44, QM96
8
Blast wave model

• more refined treatment mT distribution described
  as combined result of thermal motion (T) and
  collective transverse expansion (b)

• e.g. NA49 158 GeV, 10 centy (20 for W)
  
• constant velocity profile (n0)

Schnedermann et al. Phys. Rev. C48 (1993) 2462
9
Blast wave vs energy
NA49 7 10 most central
40 GeV
158 GeV

• Freeze-out independent of ?s
• T 120 130 MeV
• b 0.45

30 GeV
20 GeV
10
Blast wave vs centrality

• NA57 158 GeV
• Centrality classes
• 0 ? 40 to 53 most central
• 1 ? 23 to 40 most central
• 2 ? 11 to 23 most central
• 3 ? 4.5 to 11 most central
• 4 ? 4.5 most central
• With increasing centrality
• Transverse flow velocity increases
• Freeze-out temperature decreases
• ? careful when combining different centralities!

NA57 J. Phys. G 30 (2004) 823
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p?nta ?e? ...?

• So, what happens
• multi-strange particles freeze out earlier?
• everything flows the same way?

n1
NA57
NA57
Fit to singly strange particles
12
Is the W special?
Inverse slope depends on mT range used to fit
the spectrum
Measured W slopes below blast wave expectation
13
Kinetic freeze-out from HBT

• HBT h-, d spectra

• Blast wave analysis of spectra

1s contours n1
central Pb-Pb
NA49 Eur. Phys. J. C 2 (1998) 661 B.Tomášik
et al., nucl-th/9907096

• T 120 MeV
• b .5

(central Pb-Pb)
14
v2?

• some discrepancy between NA45 and NA49
• v2 not at hydro limit?
• v2 well below hydro limit for Tf 120 MeV
• but NA57 Tf increases for peripheral events

Snellings et al. nucl-ex/0305001

• Tf 146 17 MeV for (11-23) centrality NA45
  data (13-26)
• so hydro limit perhaps not too far...

NA45 Phys. Rev. Lett. 92 (2004) 032301
15
Kinetic freeze-out

• collective expansion thermal motion
• independent of ?s for central collisions
• but pronounced centrality dependence
• some room for deviation from kinetic freeze-out
  systematics for multi-strange particles,
  expecially W (early freeze-out?)
• v2 not too far from hydro limit?

16
Particle rates
17
Collective behaviour?

• The particle composition of the system in Pb-Pb
  is very different from that of proton induced
  collisions!

18
Thermal fits

• if abundances determined by thermodynamical
  equilibrium ? particle ratios
  described by two parameters T, µB
• two chemical potentials eliminated using
  strangeness neutrality and isospin conservation
• strong resonance decays, account for products
  of weak decays, finite size and excluded volume
  corrections, ...
• does this work with the data?

19
Thermal fits

• not too bad!
• e.g. Braun-Munzinger Stachel _at_ top SPS

• high degree of thermal equilibration
  even for rare, multi-strange
  particles

20

• particle species mix indeed essentially described
  by two parameters
• remarkable!
• but lets have a closer look...

21
gs

• 4p yield ratios sometimes preferred for thermal
  fits
• to be safe in case strangeness ends up too far
  away in rapidity from where originally produced
• use one set of (T, µB) for full rapidity...
• in this case gs 0.7 - 0.8 must be introduced
• strangeness undersaturation factor
• for each particle, a factor gsN(ss)
• e.g. Becattini et al. hep-ph/0310049

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gq

• see e.g. Becattini et al. hep-ph/0310049
• gq controls overall abundance of light q, q
• w.r.t. previous, additional factor gqB for
  baryons
• c2/dof comes very close to one
• (but with an additional parameter...)
• same at RHIC see Rafelski Letessier
  hep-ph/0309030
• 158 A GeV/c Pb-Pb
• gq 1.6 (fixed to best) gs 0.929 0.027
• pentaquark (if ) production (if statistical)
  would be particularly sensitive to need for gq!
• additional gq2 factor for T w.r.t. other models
• e.g. Letessier et al. hep-ph/0310188,
  Becattini et al. hep-ph/0310049

E
23
Resonances?

• e.g. ?(1520) deviates from sistematics ( 1/2)
  Markert J. Phys. G 28 (2002) 1753
• rescattering of decay prods?
• regeneration of resonances also possible?
• chemical composition changes after chemical
  freeze-out ... ?
• actually, how is this accounted for in thermal
  models?

Fit to NA49 data Becattini et al.
hep-ph/0310049
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T systematics
filled AA open elementary
Satz Nucl.Phys. A715 (2003) 3c

• we dont seem to be able to get a system of
  hadrons with a temperature beyond Tmax 170 MeV
• sounds like Hagedorn...

25
T vs µB systematics

• the extracted freeze-out points at SPS (and RHIC)
  lay very close to the predicted QGP phase boundary

26
Freeze-out vs centrality?

• how does the (T, µB) point move with centrality?
• that would be an interesting analysis!

27
Canonical vs Grand Canonical

• Energy penalty to create a strange particle
• Canonical
• computed taking into account also energy to
  create companion to ensure conservation of
  strangeness
• Grand Canonical limit
• just due to creation of particle itself. The
  rest of the system acts as a reservoir and picks
  up the slack

• removal of canonical suppression in
  nucleus-nucleus
• increases with strangeness
• observed enhancements
• detailed centrality dependence not reproduced
  (but very crude modelling...)
• Hamieh et al. Phys. Lett. B486 (2000) 61

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Does this explain the observed enhancement
pattern?

• a system in eq., if it is large enough, is in GC
  eq., but being large in
  itself is not a sufficient condition for being
  GC!
• if AA colls. were just a superposition of pp,
  they would have to be treated canonically all the
  same!
• the system must also know it is large...
• it must know that an O generated here can be
  compensated by, say, an O- on the other side of
  the fireball!
• what counts is the correlation volume
• Canonical Suppression is removed!
• an observation, not an explanation

29
Hadronic transport

• Hadronic transport codes
• do reasonably well on singly strange particles
• but fail to reproduce the production of
  multi-strange particles at SPS and RHIC
• see for instance
• Soff et al. Phys. Lett. B471 (1999) 89,
• C.Greiner nucl-th/0208080 and references
  there,
• STAR nucl-ex/0307024,
• Huovinen Kapusta nucl-th/0310051
• they get closer if
• masses are reduced towards chiral values
• or string tension is enhanced with respect to
  ee/pp/pA
• enhanced contribution from inelastic scattering
  during expansion
• Activity is still ongoing
• e.g. Capella nucl-th/0303045 Subrata Pal et
  al. nucl-th/0106074
• hadronic models are getting more and more
  exotic...

Huovinen Kapusta nucl-th/0310051
Huovinen Kapusta nucl-th/0310051
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Freeze-out messages

• From particle spectra
• collective behaviour
• collective transverse flow
• elliptic flow
• From particle rates
• collective behaviour
• strangeness conserved over large volume
• chemical composition as expected for chemical
  equilibrium
• even for rare multi-strange particles
  (order-of-magnitude
  enhancement!)
• historic QGP signature
• not reproduced by hadronic transport

31
More special features of HIC _at_ SPS
Peripheral
NA50
NA50

• J/y suppression the other historic QGP
  signature

• enhancement of intermediate-mass dileptons

• enhancement of low-mass dileptons

32
Away-side?

• RHIC disappearance of the away-side jet in
  central collisions
• SPS?
• qualitatively similar pattern
• quantitatively, within these errors
• consistent with acoplanarity broadening
• no evidence for suppression of away-side yield

NA45 peripheral
NA45 central
33
RAA at the SPS?

• Original WA98 result for p0
• WA98 EPJC 23 (2002) 225
• no pp data used compiled reference
• New analysis by D.dEnterria QM04 and
  nucl-ex/0403055
• used different pp reference, including more data
  samples ?

34

• RAA from WA98 with new reference for 3
  centralities
• suppression in central collisions at SPS?

• yellow band Cronin and shadowing, but no
  quenching
• Vitev Gyulassy Phys. Rev. Lett. 89 (2002)
  252301

35
My own take on all this...

• - where do we stand?- what next?

36
Where do we stand?

• SPS
• first pieces of evidence for deconfinement
• hyperon enhancements
• J/y suppression
• large deviation from pp behaviour in dilepton
  spectra
• what does this mean?
• RHIC
• new impressive pieces of evidence
• strong collectivity (v2 _at_ hydro limit)
• jet quenching
• indications of recombination phenomenology
• no evidence RHIC matter different from SPS matter

37
what next i) investigate onset

• SPS and RHIC
• evidence for a collective, strong interacting,
  system
• signatures of deconfinement
• AGS
• also signs of collective behaviour
• but no evidence of deconfinement
• (of course absence of evidence ? evidence of
  absence!)
• hints for interesting behaviour at low SPS E

38

• a) confirm NA49 results
• (independent confirmation is a must in our
  business!)
• further SPS running?
• FAIR?
• FT-RHIC?
• b) correlate excitation of strangeness with that
  of other QGP observables
• fluctuations?
• dileptons?
• J/y?
• ...
• easier said than done! ? a very tall order for
  FAIR!

39
what next ii) probe the medium

• what are the properties of the system created in
  the collision?
• ? study medium with high energy probes
• parton energy loss
• in-medium fragmentation
• ? high ?s frontier RHIC, LHC!
• my favourite observable heavy flavour!

40
Heavy Flavours

• c, b produced in early stages of collision, then
  conserved (neglecting annihilation)
• ? ideal probes of bulk, strongly interacting
  phase
• energy loss?
• thermal production?
• no extra production at hadronization
• ? ideal probes of fragmentation
• independent string fragmentation vs recombination

QUARK-GLUON PLASMA AND PRODUCTION OF
LEPTONS, PHOTONS AND PSIONS IN HADRONIC
COLLISIONS E.Shuryak, Yadernaya Fizika 28 (1978)
403
41
Heavy flavour energy loss?

• Energy loss for heavy flavours is expected to be
  reduced
• i) Casimir factor
• light hadrons originate predominantly from gluon
  jets, heavy flavoured hadrons
  originate from heavy quark jets
• CR is 4/3 for quarks, 3 for gluons
• ii) dead-cone effect
• gluon radiation expected to be suppressed for q lt
  MQ/EQ
• Dokshitzer Karzeev, Phys. Lett. B519 (2001)
  199
• Armesto et al., Phys. Rev. D69 (2004) 114003

average energy loss
distance travelled in the medium
Casimir coupling factor
transport coefficient of the medium
? R.Baier et al., Nucl. Phys. B483 (1997) 291
(BDMPS)
42
e.g. D0 ? K-p in ALICE

• expected performance
• S/B 10
• S/?(SB) 40 (1 month
  Pb-Pb running)

• ? similar performance in pp
• (wider primary vertex spread)

pT - differential
43
RAA(D0) in ALICE

• expected performance (1 month Pb-Pb, 9 months
  p-p)

• ? such measurements allow to study medium
  properties!

44
Conclusions

• Freeze out is interesting!
• collective system, strong expansion, chemical
  equilibrium
• SPS
• indications of partonic behaviour
• hyperon enhancements
• J/y suppression
• threshold behaviour at low energy SPS limit?
• RHIC further evidence of partonic behaviour
• parton energy loss
• indication for kinetic parton recombination
• Next steps
• investigate onset (ambitious new programme!)
• probe medium (my bet heavy flavour will be a
  fundamental tool)