Where Did All Those Strange Particles Come From

1
Where Did All Those Strange Particles Come From?

• Craig Ogilvie, MIT
• 18 Nov 1998

2
Summary of Last Week

• Charmonium states screened in a QGP gt
  suppression
• pp reactions produce pre-resonant, color octet
  state gt evolves into J/y, c etc.
• Explains pp, pA and AA data for Altsulphur
• NA50 PbPb data has fewer J/y
• smooth dependence with Et, no discontinuity
• hadronic model reproduces data
• early role of heavy hadrons, formation time of
  hadrons?
• QGP model reproduces data
• can plasma be opaque to J/y, no e-loss of hard
  partons ?
• resolve alternatives measure pt spectra of J/y

3
Today

• Strangeness
• logic of a strangeness signature
• existing data
• RHIC capabilities

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Logic of Strangeness Signature

• Strange quark pairs are produced in
• initial parton scattering, collisions during QGP
• breaking of strings - color field between quarks
• hadronization, e.g. u-quark pulls s from vacuum
• in scattering of hadrons after hadronization

Pro Start with zero strangeness, rarely
destroyed once created. Con Is strangeness
different in AA than pp AND difference can
be ascribed to creation of strange pairs within
QGP? Enhancement is key
5
Strange Mesons

• Six of the nine pseudo-scalar mesons, JP0- have
  significant strange content
• Five of the vector mesons, JP1-
• K (892 MeV),

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Strange Baryons

• Name N D L S X W
• of s quarks 0 1 2
  3
• e.g. uud uds uss sss
• Isospin 1/2 3/2 0 1 1/2 0
• mass (GeV) 1 1.1 1.3 1.7

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QGP Chemical Equilibrium

• rate of strange production rate of annihilation
• average population governed by partition function
• e.g. grand canonical ensemble
• Nq expressible in terms of T, mq

F.Becattini hep-ph 9701275
8
Rate of Strangeness Production in a QGP
ns(t0) 0 !!!

• gluon collisions dominate production of strange
  pairs
• kinetic distribution of gluons, quarks, not
  chemical equil. T.Altherr PRC49 (1994), 1684

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Production Rates in a QGP

• Large production rate of ss gt short time to
  chemical equilibrium, ts1/R
• T200 MeV, ts10fm/c gt strangeness may not reach
  chemical equilibrium during lifetime of plasma
• partial equilibrium?

200
T (MeV)
300
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Is this different than strangeness from
hadronization?

• e e- gt gt hadrons
• what is the population of hadrons, p, K etc.
• soft, non-perturbative physics
• hadronization, fragmentation of strings.
• statistical population of thermal states.
• not thermal driven by multiple re-scattering
• but a decay according to available phase space

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Thermal Fit
F.Becattini hep-ph 9701275
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Plus Production During Hadronic Stage
m
Also absorption gt possibly re-establishes chemica
l equilibrium appropriate to hadron phase
13
Repeat Logic of Strangeness

• Strange quark pairs are produced in
• initial parton scattering, collisions during QGP
• breaking of strings - color field between quarks
• hadronization, e.g. u-quark pulls s from vacuum
• in scattering of hadrons after hadronization

Pro Start with zero strangeness, rarely
destroyed once created. Con Is strangeness
different in AA than pp AND difference can
be ascribed to creation of strange pairs within
QGP? Enhancement is key
14
Existing Data

• Dual goals for existing expts.
• search for QGP at AGS, SPS
• develop understanding of strangeness production
  in exit, hadronic stage at RHIC

15
Non-linear Increase of Kaons with Number of
Participants
AuAu 11.6AGeV/c
NN gt K

• K /Npp larger than initial N-N collisions,
  steadily increases
• suggests multiple, secondary collisions increase
  K
• qualitatively similar 1-160 AGeV
• p proportional to of participants gt absorption
  (also at 10AGeV)

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Ebeam Dependence of Kaon Enhancement
W
X
K, L
KaoS, E866 WA97 CERN-EP/98-64

• Exponent, a, increases _at_ lower Ebeam
• secondary collisions more important as approach
  K threshold
• A heavy-ion excitation function of strangeness is
  different than pp
• multi-strange more enhanced than singly-strange
• recently E896 _at_ AGS measured multi-strange

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K/p at Mid-Rapidity in Central AuAu
prelim
prelim
2
2

• K/p in AA increases with Ebeam, SPS similar to
  AGS
• Divide by K/p (pp) enhancement largest at
  lower Ebeam
• secondary collisions more important in K _at_ lower
  Ebeam
• role played by change in p yield?

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Transport Models Hadronic Degrees of Freedom
AuAu central 8 QM97
RQMD 2.3
G.Odyniec QM97

• Hadronic models do not reproduce strangeness
  systematics
• underprediction at SPS, but trend already present
  at AGS
• if a model could reproduce 1-10AGeV
• then deviation at SPS could be onset of new
  physics, QGP

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Partial Chemical-Equilibrium

• 90s excitement - QGP drives gAA _at_ SPS gt gpp
• but higher g at AGS suggests hadronic scattering

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Summary

• Strangeness enhancement largest at lower Ebeam
• hadronic mechanisms dominate
• Any QGP signal would sit on this baseline
• strange-yields not reproduced by hadronic
  transport models
• but without a definitive smoking gun, how bad a
  failure before you need a QGP
• key is excitation function 1-10, 40, 160 AGeV
• ideal would be a minimum in enhancement
• imply a QGP-driven enhancement above minimum

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Hypothetical Minimum
hypothetical QGP rise
excitation function also at RHIC
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RHIC

• Brahms, PHOBOS, STAR, Phenix
• all will measure singly strange particles
• STAR has good multi-strange acceptance
• PHOBOS?
• excitation function is key
• STAR will measure K/p ratio in each event
• do events with large strangeness also have small
  J/y ?

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Next Week

• HBT