System size dependence

1
System size dependence

• Comments on...
• pp and pA collisions
• AA with different size

• Karel afarík, CERN
• http//home.cern.ch/karel/fr.ppt
• Karel.Safarik_at_cern.ch

2
Outlook (1/2)

• Enhancement in pp and pA collisions
• strangeness enhancement in hh collisions
• pA data from 80s
• do we have some knowledge ?
• pp versus pA enhancement
• trivial reason - isospin (pp and pn)
• re-scattering
• recent results
• do we contradict to previous experiments ?
• System size dependence for AA
• different centrality measures

3
Outlook (2/2)

• Multi-strange particle enhancement
• Compatibility of strangeness data among SPS
  experiments
• ? production in WA97 and NA49
• Microscopic model explanations
• prediction from QGP
• ropes, droplets
• string models final state re-scattering
• multi-particle (de-)nihilation
• Conclusions

4
Strangeness in hh

• Data on K/p in hadron-hadron interactions show
  steady (slow) increase with energy

• with multiplicity
• (CERN SppS and FNAL Tevatron)
• Possible explanations
• experimental bias, due to high pt cut-off
• increasing role of gluons for particle
  production
• It is a physical effect
• does this mean deconfinement ?

5
pA data from 80s

• pA collisions (at that time high energies) were
  extensively studied at the beginning of 80s
• predictions for nuclear densities that may be
  achieved in head-on collisions of large nuclei
• A.S.Goldhaber, Nature 275 (1978) 114
• among other also strange particle yields was
  measured
• P.Skubic et al., Phys.Rev. D18 (1978) 3115
• D.Antreasyan et al., Phys.Rev. D19 (1979) 764
• D.S.Barton et al., Phys.Rev. D27 (1983) 2580
• M.G.Abreu et al., Z.Phys. C25 (1984) 115
• W.Busza, R.Ledoux, Ann.Rev.Nucl.Part.Sci. 38
  (1988) 119
• D.H.Bricket al., Phys.Rev. D39 (1989) 2484, 45
  (1992) 734
• C.De Marzo et al., Phys.Rev. D26 (1982) 1019
• I.Derado et al., Z.Phys. C50 (1991) 31 ...

6
Centrality measures

• In pA
• usually
• n A shp / shA (? A1/3)
• assumes that incident nucleon has constant
  x-section during re-scattering
• or just A
• In AA
• number of wounded nucleons (Nw n 1)
• number of participants Np
• sometimes identical with Nw
• sometimes includes nucleons kicked-out in
  secondary re-scattering

7
A-dependence (1/3)

• pion production in central region in pp and pA

8
A-dependence (2/3)

• kaon production in central region in pp and pA

9
A-dependence (3/3)

• K/p ratio in central region in pp and pA

10
4p data

• Much lower statistics
• bubble chamber or streamer chamber
• Gray particle identification (slow protons np)
• number of interaction np ???np event-by-event
• total number of re-scattering nt from net charge
• number of secondary interaction ns nt - np
• pA 200 GeV/c (D.H.Brick et al.)
• L large (factor 2) increase compare to pp
• yield proportional to np and ns
• K0 smaller increase, saturation with np and ns
• conclusion L excess due to secondary
  re-scattering in target

11
4p data

• pA data at 200 GeV/c (C.De Marzo et al.)
• rapidity distribution RA(y) (dN/dy)hA /
  (dN/dy)hp
• excess in target fragmentation region
• plateau between 2 - 4 in y
• above goes down, even below 1
• RA in central region increases with A slower than
  nA
• RXe/RAr 1.21 nXe/nAr 1.40 same for
  charged, negative, similar for L and K0 (small
  statistics)
• energy attenuation
• Conclusion no increase in projectile region

12
pA versus pp

• all experiments found consistently an increase in
  yield of any secondaries between pp and
  back-extrapolation from pA
• this enhancement sets already at first measured
  nuclei (usually C but is there already for Be !)
• conclusions (from D.S.Barton et al.)
• the A dependence of the inclusive cross-sections
  in projectile fragmentation region exhibits a
  remarkable simplicity
• the A dependence is an universal, independent of
  the outgoing particle, its transverse momentum,
  and the incident energy

13
pA versus pp

• Ratio (strange) / (non-strange) in pA collisions
  independent on A
• however different (higher) than in pp

14
pp vs. pA enhancement

• trivial reason - difference in pp and pn
  collisions
• there is a difference in isospin which can affect
  yields
• an exercise (just for fun)
• PYTHIA pn versus pp at 100 GeV increses
• ? 1
• anti ? 2
• ? 28
• anti ? 14
• second reason - re-scattering
• increases yield of all produced particles again
  in different way and mostly in target
  fragmentation

15
Recent pA data (1/3)
16
Recent pA data (2/3)

• K/p ratio in forward hemisphere as a function of
  centrality in pPb
• increase for K/p by about 50
• K-/p- independent on centrality (and anti-p/p-
  also)

• Why is this ?
• pA at definite centrality is something
  completely different than minimum bias pA for
  various A
• trigger bias ?
• pA centrality triggered by gray particles

17
Recent pA data (3/3)

• K/p for central pPb as a function of xF (!?)
• an increase up to a factor ?3 compare to pp data
  !
• Comments
• part of such increase is trivial
• pp is not the same as pA
• if K and p have the same momenta (xF)
• p has substantially larger rapidity (velocity)
• for large number of collisions n particle yields
  at high rapidities are suppressed
• K is still not at high rapidity
• this seems more as p suppression than K
  enhancement
• data on K and p separately are needed !

18
Strangeness PbPb data

• Data from SPS Pb-beam era (1996-) on strangeness
  are in unprecedented agreement
• K yields from
• NA44 (charged)
• NA49 (charged and neutral)
• WA97 (neutral)
• in central region for central PbPb collisions
  agree within one standard deviation !
• ? from WA97 and NA49 as well

19
Strangeness PbPb data

• Centrality dependence of K yields
• apparent disagreement between NA49 and WA97
• two main differences
• 4p (NA49) versus central (WA97) yields
• small effect (about 7 for most central
  collisions)
• definition of Npart
• number of nucleons participating in initial
  collisions among themselves (WA97)
• include on top of that also knocked-out nucleons
  (NA49)
• at both ends of the scale the two definitions
  coincides but in the middle Npart WA97 lt Npart
  NA49
• affects both axes vertical and horizontal

20
K centrality dependence

• now clarified

21
K/? centrality dependence (1/2)
22
Collision geometry (1/2)

• Pb - Pb 60 participants (NA57 class 0)

23
Collision geometry (2/2)

• S - S 60 participants

24
Small vs. large system

• What is the good centrality measure
• Few proposals
• transverse density Npart / (transverse overlap)
  (Nardi)
• doesnt work too well
• longitudinal thickness R - b/2 (volume to
  surface ratio, NA49)
• works surprisingly well
• density of produced particles vs. energy density
  (S.Kabana)
• it works, but large uncertainties ...

25
K/? centrality dependence (2/2)
26
Particle vs. energy density
27
Multi-strangeness data

• Agreement between WA97 and NA49 results
  concerning ? production in pA and AA collisions
• WA97 published data for minimum bias pBe and for
  minimum bias pPb interactions
• NA49 presented data for pp and for pPb with two
  different centralities
• mean number of collisions are ? 3.7 (which
  approx. corresponds to minimum bias pPb) and 5.7
• yields at first pPb point for ?, ?, ?, ? agree
  well with WA97 data
• NA49 pp yields significantly below expectations
  from pA - strangeness enhancement in pA ?
• However, pp yields for any particle are well
  below extrapolation from pA ...

28
Multi-strangeness data

• WA97 versus NA49 cascades

29
Multi-strangeness data

• WA97 versus NA49 anti-cascades

30
W enhancement

• W plus anti-W enhanced more than a factor 15

31
Multi-strange enhancement

• Strange particle yields in central PbPb
  collisions can be described in statistical
  approach
• large W enhancement achieved (apart of adjustment
  of thermodynamical variables) mainly due to
  enlargement of volume (phase space)
• canonical vs. grand-canonical ensemble
• this needs on microscopical level a very fast
  communication across large volume
• Naturally explained in deconfined phase
  (sometimes called QGP) where a movement of
  communicators with sufficient cross-section is
  allowed across large volume

32
Microscopic models

• String models
• A.Capella, C.A.Salgado, S.Vance, M.Gyulassy
• string rearrangement
• stopping effectively baryon junction
• sea diquark-antidiquark strings

33
Microscopic models

• This effectively increase production of strange
  and multi-strange (anti-)baryons
• by factor about 2 - 3 for W and anti-W
• In order to go further binary final state
  interaction are used
• this eventually have to lead chemical equilibrium
  and hence agreement with the data
• but, the cross section
• (A.C, C.A.S) use for velocitycross-section a
  mean value between 0.14 and 0.20 mb, this gives
  for the equilibration time of the order of 100 fm
  !
• (C.M.Ko) use for strangeness exchange reaction ?
  5 mb which will give something like 30 fm
  equilibration time
• Anyway, most of the Ws are produced in
  re-interaction
• therefore they have to participate in radial flow
  !

34
Microscopic models

• Antibaryon (de-)nihilation (inverse of
  annihilation)
• C.Greiner, S.Leupold, W.Cassing
• now we have a huge cross-section, of about 50 mb
• this leads to short equilibration time ? 3 fm
• this can work, however
• asymmetry between baryons and anti-baryons
• we start with high baryon (p and n) density
• then using detail balance we get large rate for
  anti-hyperons
• this argument does not work for hyperons
• in other words anti-W is produced via
• np 3K ? anti-W p
• transverse momentum spectra (detailed simulation)
• two-body (de-)nihilation (pp?WW) has threshold
  3.1 GeV !

35
Conclusions

• Strangeness increases with energy and
  multiplicity in hh collisions
• Enhancement between pp and extrapolation from pA
  data has been observed for different particle
• However, particle ratios within pA data stays
  constant
• Data for pA at 200 GeV/c do not show projectile
  enhancement
• Possible difference in pp and pn and
  re-scattering in target region

36
Conclusions

• NA49 data on K/p ratio show strangeness
  enhancement in pA collisions for positive charge
• Is there contradiction to other experiments from
  80s ?
• Needs experimental investigation
• Different centrality behavior for different size
  systems
• few attempts to redefine axis, best works R -b/2
  (NA49)
• Agreement in pA data for X production between
  NA49 and WA97

37
Conclusion (cont.)

• Multi-strange particles enhanced up to factor of
  about 15 in central PbPb collisions
• one gets yields in agreement with statistical
  (maximum entropy) description
• for that we probably need fast communication
• naturally obtained in deconfined medium
• obtained in models which itroduced something like
  deconfined medium (ropes, droplets)
• string models (without final state) gives for W
  enhancement by factor of 3
• one reasonably gets to the equilibrium yields for
  anti-W by anti-baryon (de-)nihilation (but W)

38
Strangeness PbPb data

• the only unresolved issue is ?-meson data
• NA49 measures ? ? KK decay mode
• NA50 measures ? ? ?? decay mode (smaller
  acceptance)
• yield at midrapidity dN/dyNA49 lt dN/dyNA50
  (factor gt 2)
• speculation about differences in KK and ??
  channels
• mT slopes TNA49 (300 MeV) gt TNA50 (220 MeV)
• the width of rapidity distribution substantially
  larger in Pb-Pb than in pp (why ?)
• S.Margetis, K.S., O.Villalobos Baillie, Ann.Rev.
  Nucl.Part.Sci. 50 (2000) 299
• D.Röhrich, J.Phys. G 27 (2001) 355
• K.S., J.Phys. G27 (2001) 579

39
Strangeness PbPb data

• ?-meson (as well as W) has no resonance with p

?
40
Microscopic models