Betty AbelevUI Chicago
Testing the Recombination Model at RHIC using multi-
strange baryon correlations
for the STAR Collaboration
23rd Winter Workshop on Nuclear Dynamics, Big Sky, MT
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Understanding the medium Measure
probes after Thermal freeze-out
Look for probes that traverse the entire medium – Initial
state hard scattering jets
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Medium Probes: Strangeness Partonic system vs.
hadron gas
Canonical ensemble vs. grand-canonical– Exact q.n. conservation– Overall q.n.
conservation & chemical potentials
q q s s g g s s
N K
K NEthres 530 MeVEthres 1420 MeV
Ethres = 2ms ≈ 200 MeV
R. Stock on strangeness enhancement:
“Fading away of small volume canonical constraints” hep-ph/0312039
Hamieh et al.: Phys. Lett. B486 (2000) 61
Enha
ncem
ent o
f stra
nge
bary
ons
(sss)
(dss)
(uds)
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Strangeness at intermediate pT Strange particle RCP
(Central/peripheral)– Mesons suppressed more than baryons at
intermediate pT
Mesons suppressed more than baryons– Baryon/meson ratio increases with centrality
/K0 S
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1 2 3 4 5 [GeV/c]
10-1
10-2
10-3
10-4
dN/p
Tdp T
dyCoalescence/Recombination of
partons
Exponential (thermal) parton spectrum qualitatively explains baryon excess at mid pT
mesons baryons
TT TTTSS SSSTS TSS
STT
Recombination:– Shower (S)– Thermal (T)
S & T can mix– Particle spectra
are a sum of various components
3 GeV/c hadron can be
1. Produced via fragmentation
2. If meson: coalescence of 2 1.5 GeV/c partons
3. If baryon: 3 1 GeV/c partons (more abundant!)
R.Hwa et al, Phys.Rev.C70 (024904) 2004.
ReCo cartoon
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Prediction from Rec. Model by Hwa et al
R.C Hwa & C.B. Yang nucl-th/0602024
Shower s-quarks are suppressed w.r.t u & d quarks -- Ω: sss at intermediate pT comes mostly from TTT ! Differ significantly from Λ, which includes non-strange quarks
(STAR) •(STAR)
Use Azimuthal correlations
measure fragmentation (shower-quark contribution) ifif Ω: sss at intermediate pT come mostly from TTT
no Ω correlation partners!
: statistically challenging!
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x
y
Statistical Jet Measurement Particles from same jet closely aligned in
Use characteristic jet cone shape for a statistical jet study
High pHigh pTT tracktrack
1. Find a trigger particle (pT>2 GeV/c)2. Find an associated particle (pT
trg>pTas>1.5 GeV/c) in the same
event3. Compute at primary vertex for each 4. Compute at primary vertex
q
q
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Azimuthal Correlation Function Results in a double-peaked
correlation function – Normalize by Ntriggers
p+p and d+Au: 2 Gaussians + flat background
Au+Au: 2 Gaussians +flat background+
flow
B )2σ
π)(Δ(- expA )2σΔexp(-A )C(Δ
2
π
2π2
0
20
)]2cos(21[ 22 astrigvv
If no SSS contribution to spectrum
same-side -h would be flat in most central Au+Au
same-side away-side
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Mesons and Baryons seem to have different v2 at same pT.
If flow is collective– Should scale with
n, number of valence quarks (partons)
Scaling works!
How to get V2 for multi-strange?
Can use v2 for and
J. Adams et al (STAR), Phys. Rev. Lett. 95 (2005) 122301
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Au+Au azimuthal correlations results
A clear same-side -h peak is observed for 2.5-4.5 GeV/c baryon triggers!
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Au+Au azimuthal correlation results
The magnitude of the same-side peak is independent of s-quark content!
Uncertainty due to v2 determination methods
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Comparison to singly-strange (Λ & K0
S)
For same-side meson and baryon yields are similar No dependence on strangeness content Yields increase as a function of pT
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A reference: Correlations in d+Au
-h correlation is observed in d+Au. Same-side yield: 0.015±0.026
Away-side yield: 0.04±0.016
pTtrig>2.0 GeV/c
1.5 GeV/c <pTas<pT
trig
|h|<0.75
What kind of same-side signal to expect with no medium? (not enough statistics in p+p, not enough statistics to measure Ω. Look at Ξ-h in d+Au)
STAR Preliminary
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2-dimensional correlations: Extend analysis in 2
dimensions Same procedure as
for , only use coordinates of trigger and associated
Elongation in under the jet peak: “the ridge”
beam direction
Dhridge
ridge
jet
jet+ridge
p Ttri
gger=3
-6 G
eV/c
, 1.
5 G
eV/c
<p T
asso
ciat
ed<
p Ttri
gger
STAR, PRC73, 064907 (2006)\
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Jet and ridge as a function of centrality
Jet onlyJet + Ridge
Measuring jet only: divide the space in jet+ridge (1) and ridge-only (2) regions
Subtract (2) from (1) to obtain jet-only measurement
jet-only contribution consistent across Nch
– At this pT trig & pT assoc.: 5 x increase of jet+ridge from d+Au to Au+Au increase is all in the ridge.
Compare d+Au result with result in Au+Au:– The pT-integrated (2-6 GeV/c) yield in Au+Au (0.20±0.05) is 10 x the yield in d+Au
(0.015±0.026)! – The ridge?
Jana Bielcikova’s talk, this w
orkshop
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-h in the dip
The signal is indistinguishable from the ridge (perhaps visually only)
We observe a dip in the signal in the =0 region This dip (though to be a detector effect) is still under
investigation Expect a significant loss of signal due to the dip
2.5<pTtrig<4.5 GeV/c
1.5<pTas<pT
trig
STAR Preliminary
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2-D correlation results • The point-by-point subtraction of the two regions: result consistent with 0
We don’t yet have the statistics to subtract the ridge
• Subtracting the fits: the result is encouraging, but is qualitative – large errors.
STAR Preliminary
STAR Preliminary
STAR Preliminary
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Conclusions Multi-strange baryon azimuthal correlations were
observed in d+Au and Au+Au data baryon same-side azimuthal correlation yield in
central Au+Au is ten times that in d+Au data Omega baryon same-side peak was observed in most
central Au+Au, contrary to predictions The ridge was observed in - baryon
correlations in Au+Au, and the excess Au+Au yield is likely due to the ridge
The statistics are not yet sufficient to separate jet and ridge contributions for multi-strange
Outlook:– Omega spectrum measured to a higher pT to detect onset of
fragmentation– Study of the ridge: its composition and dependence on
kinematic and geometrical variables– Study of the away-side