Recent Results from STAR

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Recent Results fromSTAR

• Carl A. Gagliardi
• Texas AM University
• for the Collaboration

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Time line of a relativistic heavy ion collision
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Two thin disks of quarks and gluons approach
Initial collision products of hard scattering,
including heavy flavors, created
Dense partonic medium The QGP?
Hadron gas phase
Outline Introduction to RHIC and STAR Phase
2, then 3, then 1, then 4
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RHIC the Relativistic Heavy Ion Collider
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The STAR Detector
E-M Calorimeter
Projection           Chamber
Time of    Flight
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Geometry of heavy ion collisions
Number of participants Number of
incoming nucleons in the overlap region Number
of binary collisions Number of
equivalent inelastic nucleon-nucleon
collisions Both depend on impact parameter or
centrality

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Kinematics for colliders
Pseudo-rapidity
Transverse momentum (pT) and pseudorapidity
(?) provide a convenient description
Mid-rapidity ? 0, perpendicular to the
incident beams ? 4 Scattering at ? 2.1o in
the CM (or lab) frame Terminology that has
become common at RHIC Low pT pT lt 2
GeV/c Intermediate pT 2 lt pT lt 6
GeV/c High pT 6 GeV/c lt pT
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Hard scattering at RHIC and NLO pQCD
PRL 91, 241803
hep-ex/0608030
PHENIX p0
STAR Jets
At 200 GeV, pQCD does a very good job describing
high-pT yields in pp
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Hard partonic collisions and energy lossin dense
matter
Bjorken, Baier, Dokshitzer, Mueller, Pegne,
Schiff, Gyulassy, Levai, Vitev, Zhakarov, Wang,
Wang, Salgado, Wiedemann,
Multiple soft interactions
Gluon bremsstrahlung
Opacity expansion
Strong dependence of energy loss on gluon density
?glue measure ?E ? color charge density at
early hot, dense phase
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From the first three years of RHIC
Phys. Rev. Lett. 91, 072304 (2003).

• In central AuAu collisions
• Strong suppression of inclusive hadron production
• Disappearance of the away-side jet
• dAu looks like pp
• Jet quenching in the dense medium

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Limitations of RAA
K.J. Eskola et al., NP A747, 511
RAA at 10 GeV/c
Central RAA Data
(proportional to gluon density)

• Leading hadrons preferentially arise from the
  surface
• Limited sensitivity to the region of highest
  energy density
• Need more penetrating probes

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Heavy flavor yields
See talk by A. Suaide
STAR preliminary

• Charm mid-rapidity total cross section (dominated
  by low pT) consistent with binary scaling
• At high-pT, expect less suppression for c and b
  than for light hadrons
• Can study this using non-photonic electrons
  from heavy flavor decays

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Suppression of non-photonic electrons
Gluons and light quarks
See talk by A. Suaide
Heavy quarks
STAR nucl-ex/ 0607012

• High-pT electron suppression is comparable to
  inclusive charged hadron suppression
• Data agree with c ? e predictions that assume a
  very high density
• But b ? e should be there, too!
• Do we really understand partonic energy loss?

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Di-jets at much higher pT
STAR nucl-ex/0604018
8 lt pT(trig) lt 15 GeV/c No background subtraction
Clear emergence of the away-side jet --
Comparable yields on near side in all systems
-- Strong suppression of away-side in central
AuAu
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Medium modification of di-jets
STAR nucl-ex/0604018
Scaling factors relative to dAu
8 lt pT(trig) lt 15 GeV/c
0.57 0.06
0.25 0.06

• Direct measurement of the medium modification
  (and lack thereof) of the away-side jet
• Near side appears unmodified
• Away-side yield decreased, but spectrum shape
  unchanged
• Can partonic energy loss models describe these
  results simultaneously?

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Di-jets and the medium
T. Renk, Hard Probes 06
Trigger direction
Trigger direction

• High-pT di-jets provide better sensitivity to the
  interior
• of the reaction zone

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How does the medium respond?
Di-hadron correlations in two dimensions Jet
peak sits on top of a long-ranged ridge in ??
on the near side
Dh
Df

• Jet peak
• Yield independent of centrality
• Ridge
• Yield grows dramatically with centrality
• Spectrum similar to that of the bulk, unlike the
  jet peak
• Qualitatively consistent with models including
  longitudinal or radial flow, also with
  coalescence model need detailed calculations!

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How does the medium respond?
2.5 lt pTtrig lt 4.0 GeV/c 1.0 lt pTassoc lt 2.5 GeV/c

• Measure intermediate-pT di-hadron distributions
• Yield away from ?f p grows significantly
• Mach cone? Gluon Cherenkov radiation? Deflected
  jets?

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Explore the dynamics with 3-particle correlations
12 most central AuAu collisions
pp collisions
3.0 lt pTtrig lt 4.0 GeV/c 1.0 lt pTassoc lt 2.0 GeV/c
STAR preliminary

• Evidence for enhancements on the diagonals at p
  1.4 radians
• Very sensitive to the procedure used to subtract
  the 2-particle correlations

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Azimuthal anisotropy elliptic flow
py
px
Initial coordinate-space anisotropy
Final momentum-space anisotropy
Anisotropy self-quenches, so v2 is sensitive to
early times
Elliptic term
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Elliptic flow in the hydrodynamic regime
See talk by M. Munhoz

• Expected mass splitting well reproduced up to pT
  1.5 GeV/c
• Elliptic flow saturates the hydrodynamic limit
• Are we producing a perfect liquid at RHIC?

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Constituent quark scaling
What if quarks coalesce to make hadrons?
v2 obeys constituent quark scaling --
Hadronization through coalescence -- Evidence
for flowing quarks (?)
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Anomalous baryon/meson yields at intermediate pT
Related material talk by M. Lamont
STAR nucl-ex/0606003

• Large p and p to p ratio in central AuAu at
  intermediate pT
• Above pT 6 GeV/c, ratio is the same in dAu and
  central AuAu
• Limit of the coalescence domain?

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Glauber vs Color Glass Condensate
Hirano et al, PLB 636, 299
CGC Glauber
CGC Treats the nucleus as a saturated gluon
field

• Do we have Glauber matter distribution perfect
  liquid, or Color Glass Condensate distribution
  viscous matter?
• Is the gluon field in the Au nucleus saturated?
• Forward dAu collisions provide information about
  the gluon density in Au at low gluon momentum
  fractions

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? dependence of RdAu
nucl-ex/0602011

• Observe significant rapidity dependence.
• pQCD calculations significantly over predict
  RdAu.

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Any difference between pp and dAu?
pp Di-jet
dAu Mono-jet?
Dilute parton system (deuteron)
PT is balanced by many gluons
Dense gluon field (Au)
Kharzeev, Levin, McLerran gives physics picture
(NPA748, 627)
Color glass condensate predicts that the
back-to-back correlation from pp should be
suppressed
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Forward mid-rapidity correlations in dAu
nucl-ex/0602011

• are suppressed at small ltxFgt and ltpT,pgt
• consistent with CGC picture
• are similar in dAu and pp at larger ltxFgt and
  ltpT,pgt
• as expected by HIJING

ltpT,pgt 1.0 GeV/c
25ltEplt35GeV
ltpT,pgt 1.3 GeV/c
p0 lt?gt 4.0 h ? lt 0.75 pT gt 0.5 GeV/c
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Many recent descriptions of low-x suppression in
dAu
A short (and surely incomplete) list
Saturation (color glass condensate)
Shadowing

• R. Vogt, PRC 70 (2004) 064902.
• Guzey, Strikman, and Vogelsang, PLB 603 (2004)
  173.

• Jalilian-Marian, NPA 748 (2005) 664.
• Kharzeev, Kovchegov, and Tuchin, PLB 599 (2004)
  23 PRD 68 (2003) 094013.
• Armesto, Salgado, and Wiedemann, PRL 94 (2005)
  022002.
• Dumitru, Hayashigaki, and Jalilian-Marian, NPA
  765 (2006) 464

Parton recombination

• Hwa, Yang, and Fries, PRC 71 (2005) 024902.

Leading fragmentation
Multiple scattering

• PHOBOS (several)

• Qiu and Vitev, PRL 93 (2004) 262301
  hep-ph/0410218.

Black disk limit

• Frankfurt and Strikman, nucl-th/ 0603049

Factorization breaking

• Kopeliovich et al., PRC 72 (2005) 054606.
• Nikolaev and Schaefer, PRD 71 (2005) 014023.

Others?

• ...

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Resonance production at RHIC
Hadronic phase

• Resonances like K, ?, S have very short
  lifetimes
• Many decays occur within the reaction zone
• Rescattering and regeneration can lead to
• Mass shifts
• Width broadening
• Modifications of the pT spectra
• Sensitive to the lifetime of the hadronic phase

?t
?
S
?
L
S
S measured
S lost
L
?
S
?
?
S
L
L
L
S measured
Chemical freeze-out
Thermal freeze-out
time
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Resonance vs. non-resonance yields
STAR nucl-ex/0604019

• K and ? are suppressed in central AuAu S
  isnt suppressed
• Evidence that both rescattering and regeneration
  occur during the hadronic phase
• Lifetime of the hadronic phase gt 4 fm/c

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Conclusion

• STAR has a wealth of new data!
• We are learning new information the all phases of
  a relativistic heavy ion collision
• Help us answer questions weve been asking
• Help us ask questions we werent smart enough to
  ask before
• The answers to those questions will tell us
• What is the strongly interacting matter that we
  are creating at RHIC?
• What are its properties?

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The STAR Collaboration
U.S. Labs Argonne, Lawrence Berkeley, and
Brookhaven National Labs U.S. Universities
UC Berkeley, UC Davis, UCLA, Caltech,
Carnegie Mellon, Creighton, Indiana, Kent State,
MIT, MSU, CCNY, Ohio State, Penn State, Purdue,
Rice, Texas AM, UT Austin, Washington, Wayne
State, Valparaiso, Yale Brazil
Universidade de Sao Paolo China IHEP -
Beijing, IPP - Wuhan, USTC, Tsinghua, SINAP, IMP
Lanzhou Croatia Zagreb University Czech
Republic Nuclear Physics
Institute England University of Birmingham
France Institut de Recherches Subatomiques
Strasbourg, SUBATECH - Nantes Germany Max
Planck Institute Munich University of
Frankfurt India Bhubaneswar, Jammu, IIT-Mumbai,
Panjab, Rajasthan, VECC Netherlands NIKHEF/Utrec
ht Poland Warsaw University of
Technology Russia MEPHI Moscow, LPP/LHE
JINR Dubna, IHEP Protvino South
Korea Pusan National University
Switzerland University of Bern
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Non-photonic electrons in pp
STAR nucl-ex/0607012
See talk by A. Suaide

• Fixed-order next-to-leading log calculations
  underpredict non-photonic electron yield in pp
  by a factor of 5
• Significant theoretical uncertainty regarding
  relative strength of c vs. b

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High-pT identified particles in pp and dAu
STAR, PLB 637, 161 (2006)

• pQCD calculations with recent (AKK) fragmentation
  functions give a reasonable description of pion
  and proton yields in elementary collisions
• Calculations with KKP significantly underestimate
  proton yields at high-pT
• Protons arise primarily from gluon fragmentation
  pions receive a large quark contribution at
  high-pT

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Identified particle nuclear modification factors
STAR nucl-ex/0606003
Related material talk by M. Lamont

• Rcp for protons and pions converge at pT gt 6
  GeV/c
• Dominance of fragmentation over coalescence at
  high pT

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Jet-like correlations in the soft sector
STAR J Phys G 32, L37 (2006)

• Jet-like di-hadron correlations survive in AuAu
  at low pT
• Correlation narrows in f and broadens in ?
• Coupling to a longitudinally flowing medium?
• Need detailed model calculations!

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dAu ? p0X at 200 GeV
nucl-ex/0602011
pT dependence of dAu p0 cross section at lt?gt
4.0 is best described by a LO CGC
calculation. (Dumitru, Hayashigaki, and
Jalilian-Marian, NPA 765, 464)