HighpT strange particle spectra and correlations in STAR

1
High-pT strange particle spectra and
correlations in STAR
Jana Bielcikova (Yale
University) for the STAR
Collaboration Hard
Probes, Asilomar, California
June 9-16, 2006
2

• Outline
• Baryon/meson enhancement and RCP at vsNN200 GeV
• Azimuthal correlations of h, ?, ? and K0S
• in dAu and AuAu collisions at vsNN200 GeV
• Test of recent ReCo model predictions
• (O/f ratio, O triggered correlations)
  
• Conclusions

3
Baryon/meson enhancement
a large enhancement of baryon/meson
ratio in AuAu relative to pp at vsNN200
GeV Year2 data the enhancement reaches
maximum at pT3 GeV/c Year4 data (high
statistics) ?/K0S ratio approaches pp
for pTgt 5 GeV/c jet fragmentation is not
a dominant source of particle production
for pT2-5 GeV/c
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Baryon/meson enhancement model comparisons
SOFTQUENCH Gyullassy, Levai, Vitev,
PRL 85 (2000),
Nucl.Phys. A 715, 779 (2003)
TEXAS Greco, Ko, Levai, PRL 90 (2003), DUKE
Fries, Mueller, Nonaka, Bass, PRC 68
(2003) OREGON Hwa, Yang, PRC 67, 034902 (2003)
Both, softquench and recombination type
of models, predict a peak in ?/K ratio,
however they cant reproduce the peak
location in pT, its magnitude and shape in
detail.
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RCP at vsNN200 GeV

• Year2
• - baryon/meson difference
• at intermediate pT 2-5 GeV/c
• Year4
• - extends the RCP to higher pT
• - strange and non-strange particles are
  consistent
• - appear to show a similar suppression at
  pTgt 5 GeV/c

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Identified correlations at medium/high pT

• identified correlations at high-pT
• can provide additional information on
• jet quenching
• baryon/meson enhancement at RHIC
• particle production mechanisms
• coalescence/recombination mechanisms
• and/or
• (modified) fragmentation of high-pT partons?
• pT dependence of trigger particle species
• ? from q-jet and g-jet,
• ?? from g-jet only

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Azimuthal correlations - method
near-side peak corresponds to intra-jet
correlations away-side peak corresponds to
inter-jet correlations This
talk is only about the near-side peak !
Note there are additional long-range ?
correlations (ridge) in AuAu
collisions (see talk by J. Putschke (STAR) )

JetRidge yield ??(JR) Jet yield
??(J) ??(?? lt 0.5) ??(?? gt 0.5) Ridge
yield ??(R) ??(JR) - ??(J)
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Correlation functions for strange particle
triggers in AuAu at 200 GeV
trigger baryon/meson baryon/antibaryon
STAR preliminary
STAR preliminary

• Selection criteria
• 3.0 GeV/c lt pTtrigger lt 3.5 GeV/c
• 1 GeV/c lt pTassociated lt 2 GeV/c
• h lt 1
• Corrections applied
• reconstruction efficiency
• of charged particles
• TPC sector boundaries

STAR preliminary
STAR preliminary
correlation functions before elliptic flow
subtraction
correlation functions after elliptic flow
subtraction syst. error due to v2 uncertainty
25
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Centrality dependence of near-side associated
yield in AuAu
Near-side associated yield rises with
centrality
-gt ridge yield rises with
centrality

• -gt jet yield is independent on centrality
• jet yield for ? lt K0S lt h ?, ridge yield h
  K0S lt ? ?

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pTtrigger dependence of associated yielddAu vs
AuAu
STAR preliminary
STAR preliminary

• within errors no dependence on trigger species
  (dAu and AuAu)
• yield is 3-4 times larger in AuAu than in
  dAu
• Dont forget AuAu is ridge
  dominated!
• Lets look on the jet yield
  separately

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pTtrigger dependence of jet and ridge yield

• near-side yield is ridge dominated (constant
  for pTtrigger gt 2.5 GeV/c)

• jet yield rises steeply with increasing
  pTtrigger
• jet/ridge ratio varies between 10-30 with
  increasing pTtrigger
• ?-trigger charged associations lt for
  K0S-triggers
• Does baryon trigger take more energy away from
  jet than meson ?

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pT distribution of associated particles
J/R10-15
Jetridge
Jet only
STAR preliminary
STAR preliminary

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What does a parton recombination model predict?
AuAu _at_ 200 GeV 3GeV/cltpTtriggerlt6GeV/c
STAR
R. Hwa, Z.Tan nucl-th/0503060
STAR preliminary

• central-to-peripheral ratio of near-side
  associated yield is
• - 3 at pTassoc 1 GeV/c
• - decreases slowly with pTassoc
• qualitative agreement between the parton
  recombination model and the data
• 
  BUT!
• a quantitative agreement requires same
  centrality and pT selection
• check required on how well the ridge is
  reproduced by the model

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Recent ReCo Model Predictions
The production of f and O is almost exclusively
from thermal s-quarks even out to 8
GeV/c (shower S contributions are strongly
suppressed)
Observables 1. The ratio of O/f yields should
rise linearly with pT 2. O or f di-hadron
correlations should be swamped by the
background and not observed
R. Hwa , nucl-th/0602024
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O/f ratio
QM05
SQM06

• A qualitative agreement with model for pT lt 4
  GeV/c
• Need more data to draw conclusions for pT gt 4
  GeV/c

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O-charged correlations
STAR preliminary

• Not corrected for efficiency, TPC sector
  boundaries, flow
• Within statistical errors we cant exclude
  existence of
• near-side peak in O triggered
  correlations.
• We expect 8-10 more statistics for O,
  so stay tuned ?.

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Conclusions

• Strong baryon/meson enhancement in AuAu over pp
  
• at vsNN200 GeV observed for
  pT2-5 GeV/c.
• Identified RCP shows a baryon/meson splitting for
  pT2-6 GeV/c,
• but for pTgt 6 GeV/c baryons and mesons are
  similarly suppressed.
• 
  

• Azimuthal correlations of identified strange (?
  and K0s) trigger particles associated with
  charged particles at moderately high-pT show
• - significant contribution from long
  range ? correlations,
• - ridge yield is rising with
  centrality
• - jet yield is constant with
  centrality and rises with pTtrigger,
• - indication of a lower associated
  jet yield for baryon triggers
• than for
  meson triggers.

• Within statistical errors, the O/? ratio is
  consistent with
• being constant and for O triggered
  correlations we cant
• exclude existence of
  near-side peak.
• 
  (more data to come ?)
• 
  

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BACKUP
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RCP at vsNN200 GeV

• Identified RCP from Year2 shows a clear
  difference between baryons and mesons at
  intermediate pT 2-5 GeV/c
• The F and K clearly follow the mesons and not
  the baryons,
• it is not a mass effect !

200 GeV
0-5
40-55
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Identified correlations from PYTHIA
Pythia 6.4 (untuned)
Default PYTHIA 6.4 is a leading-order
calculation does not describe well inclusive
pT-spectra of strange particles (?,?,
O) NLO contributions can be simulated by
introducing a K-factor the mixture of qq, qg
and gg processes is changed -gt it becomes
more gluon-like
Pythia 6.4 (K-factor3)
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Comparison of pTtrigger dependence of jet yield
in PYTHIA and dAu
STAR preliminary

• near-side yield rises slowly with pTtrigger
• default Pythia shows a different behaviour for
  anti-?,
• but yield agrees well with
  dAu
• Pythia with K-factor3 all particle species
  behave similarly
• as in the data, but the yield is a factor 2
  higher
• (because too many pions are generated with
  K-factor3)

• near-side yield rises slowly with pTtrigger
• default Pythia shows a different behaviour for
  anti-?,
• but yield agrees well with
  dAu

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pT distribution of associated charged particles
at near-side
J. Ulery (STAR), QM2005

• 50 ppbar
• 95 p
• charged

STAR Preliminary
1/NtriggerdN/dpT
STAR preliminary
pTassociated (GeV/c)

• spectral shape is similar for various trigger
  particle species
• within
  errors

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Strange particle identification in STAR
Identification of strange particles
V0-decay vertices ? ? p p- BR 64 ?
? p p K0S ? p p- BR 68

• Cuts on dE/dx of daughter particles
• Topological cuts
• -gt a very good S/B ratio ?