Heavy%20Flavor%20Physics%20at%20STAR

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Heavy Flavor Physics at STAR

• Pibero Djawotho
• Indiana University Cyclotron Facility
• for the STAR Collaboration
• Hard Probes 2006, Asilomar, CA
• June 12, 2006

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Outline

• Motivation
• Open charm production at STAR
• Quarkonia at STAR
• Other heavy flavor topics at STAR
• Summary

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Motivation charm production

• Final goal ? understand the properties of the hot
  and dense matter
• charm is mainly produced in initial collisions
  via gluon fusion (e.g. Gyulassy and Lin, PRC 51
  (1995) 2177).
• charm total cross section should follow Nbin in
  AuAu.
• c,b smaller energy loss than light quarks
• Dead cone effect. Dokshitzer and Kharzeev, PLB
  519 (2001) 199.
• Important to measure the charm total cross
  section in AuAu and compare with pp and dAu.
• Important question do we understand charm and
  bottom production mechanism?
• initial parton fusion, gluon splitting, flavor
  excitation?

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Motivation charm energy loss

• In 2001, Dokshitzer and Kharzeev, PLB 519 (2001)
  199, proposed the dead cone effect ? charm
  quark has smaller energy loss.
• Recently, heavy quark energy loss in medium
  Armesto et al., PRD 71 (2005) 054027 Djordjevic
  et al., PRL 94 (2005) 112301.
• Mechanisms other than gluon emission may play an
  important role in heavy quark energy loss
  (collisional energy loss?)

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Open charm measurements at STAR
TPC ? lt 1, 0 lt ? lt 2? ToF -1 lt ? lt 0, ??
0.1 EMC 0 lt ? lt 1 (now ? lt 1), 0 lt ? lt 2?

• Hadronic decay D0?K-p (BR3.8) ? TPC
• Semileptonic decay c?lanything (BR9.6)
• D0?eanything (BR6.87) ? TPCToF/EMC
• D0?µanything (BR6.5) ? TPCToF

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D0 analysis

• Select K and p tracks from dE/dx in TPC
• Combine all pairs from same events ?
  signalbackground
• Combine all pairs from different events ?
  background (event-mixing technique)
• Subtract background from signalbackground ?
  signal

PRC 71 (2005) 064902 PRL 94 (2005) 062301
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D0 signal at STAR
QM2005 nucl-ex/0510063
PRL 94 (2005) 062301
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TPC-ToF e Measurement
clean e identification with TPC-ToF up to 2.5
GeV/c (the better the lower pT)
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Photonic electrons

• Dominant sources of background electrons
• ??ee- conversion in material of the detector
• ?0/???ee- Dalitz decay
• Kaon and vector meson decays
• Measuring the background
• Combine all electron pairs in event
• Mass distribution
• Photonic electron pairs
• Accidental combinations
• Nphotonic (Nlike Nunlike)/eB
• Non-photonic electron spectrum inclusive
  photonic

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Photonic Single Electron Background Subtraction
Excess over background
pT-dependent hadron contamination (5-30)
subtracted (year 2 data, no EMC)
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TPC-ToF µ Measurement
0.17 lt pT lt 0.21 GeV/c 0-12 AuAu
STAR Preliminary
?
?
M2 (GeV2/c4)
1) Data 2) Primary track

• Low-pT ? from TPCToF
• Differentiate between m from
• K, p decay
• c decay
• Use DCA distribution

3) B.G. (?, K decay) 4) Sum of 2),3)
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Combined fit

• D0, e, and µ combined fit
• Power-law function with parameters dN/dy, ?pT?
  and n adjusted to describe D0 spectrum
• Generate D0?e decay kinematics with parameters
  above
• Vary parameters to get minimum ?2 by comparing
  power-law to D0 data and the decayed e shape to
  e and µ data
• Advantage D0 and µ constrain low pT, e
  constrains high pT
• Covering 95 of cross section

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Charm total cross section

• ?1.40.20.4 mb in minimum bias dAu collisions
  at vsNN200 GeV
• ?1.260.090.23 mb in minimum bias AuAu
  collisions at vsNN200 GeV
• ?1.330.060.18 mb in 0-12 AuAu collisions at
  vsNN200 GeV
• Charm total cross section obeys Nbin scaling from
  dAu to AuAu within error bars
• Supports conjecture that charm is exclusively
  produced in initial scattering
• However, the total charm cross section is 5
  larger than NLO (and FONLL) predictions!!!

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Blast-Wave Fit Charm Freeze-Out
STAR Preliminary
STAR Preliminary
Blast-wave fit combining D0, muons, and electrons
at pT lt 2 GeV/c Charm hadrons may freeze-out
earlier T gt 140 MeV Charm hadron collective
velocity ??T? less than that of ? and ? - charm
flow? Caveat Charm not 100 thermalized Interpre
tation of Blast Wave fit questionable.
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Nuclear modification factor ToF
STAR Preliminary
STAR Preliminary
ToF non-photonic electrons suppressed in 0-12
AuAu at vsNN200 GeV
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Extending the pT reach EMC electrons

• Acceptance 0 lt ? lt 1 , 0 lt ? lt 2?
• ? lt 1 since 2006
• PID with EMC detectors
• tower (energy) ? p/E
• shower maximum detector (SMD) ? shower
  shape/hadron discrimination
• High-energy tower trigger ? enhance high-pT
  sample (luminosity limited!)

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Non-photonic electron pT spectra

• ToF measurements from pp, dAu, AuAu minimum
  bias 0-12, 0-20, 20-40, 40-80
• EMC measurements from pp, dAu, AuAu 0-5,
  10-40, 40-80
• Non-photonic electron pT spectra measured by ToF
  and EMC are consistent with proper Nbin scaling

STAR Preliminary
(ee-)/2 vsNN200 GeV AuAu
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Nuclear Modification Factor EMC

• RdAu consistent with slight Cronin enhancement
• RAuAu suppressed to 0.6 in 40-80
• RAuAu suppressed to 0.5 in 10-40
• RAuAu strongly suppressed to 0.2 in 0-5 at high
  pT (4-8 GeV/c)

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pQCD calculations for pp vs. data

• All suppression predictions use the most recent
  pQCD calculations as starting point (pp
  reference).
• Where does bottom start to dominate?
• Relative contribution of charm and bottom?
• Large uncertainty in the crossing point
• From 3 to 10 GeV/c
• From shape significant b contribution at high-pT?

STAR Preliminary
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Heavy Quark Energy Loss theory vs. data
radiative only (cb)?e
radiative collisional (cb)?e
radiative collisional c?e
Where is the contribution from beauty? Join
discussion session on parton energy loss
tonight!!!
STAR Preliminary
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Quarkonia in STAR

• Slowly getting started with J/?
• Signal in 200 GeV pp from 2005
• Tested and working trigger in pp
• No trigger for AuAu until full ToF in 2009
• Much more from 2006 in the works
• Also signal in AuAu with TPC only
• Large hadron contamination
• Need full EMC

STAR Preliminary
J/??ee- pp vs200 GeV
STAR Preliminary
J/??ee- AuAu vsNN200 GeV
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? in STAR

• Cannot resolve different S states ? ?(1S2S3S) ?
  ee-
• STAR
• Large acceptance (h lt 1, full EMC)
• PID for electrons (EMC, TPC)
• Trigger
• Very efficient gt 80
• Luminosity limited

STAR Preliminary

• First look in 2004 ½ EMC, little statistics
• 90 C.L. signal lt 4.91
• Bds/dy C.L. lt 7.6 mb

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Summary

• FONLL calculations underpredict STAR pp data
• Charm total cross section follows Nbin scaling ?
  charm produced via initial hard scattering
• Strong suppression of non-photonic electrons at
  high pT observed in central AuAu collisions ?
  Challenge to existing energy loss models
• Contribution from bottom?
• Charm pT distribution has been modified by the
  hot and dense medium in central AuAu collisions
• Role of collisional energy loss
• Are there other mechanisms?
• Work in progress
• J/y and ?
• e-h correlations
• Non-photonic electrons in 62 GeV AuAu

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Other must see heavy flavor talks from STAR

• High-pT electrons Jaroslav Bielcik
• Wednesday parallel session III
• e-h correlations Xiaoyan Lin
• Monday parallel session I
• Charm cross section Yifei Zhang
• Monday parallel session I
• STAR upgrades for Heavy Flavor Nu Xu
• Monday plenary

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Extra Slides
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RHIC Run Overview
Run Year Species vsNN GeV ?Ldt
III 2002/2003 dAu pp 200 200 73 nb-1 5.5 pb-1
IV 2003/2004 AuAu AuAu pp 200 62 200 3740 µb-1 67 µb-1 7.1 pb-1
V 2004/2005 CuCu CuCu CuCu pp pp 200 62 22 200 410 42.1 nb-1 1.5 nb-1 0.02 nb-1 29.5 pb-1 0.1 pb-1
VI 2006 pp pp pp 200 62 500
http//www.agsrhichome.bnl.gov/RHIC/Runs/
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STAR coverage
TPC SVT CTB BEMC
EEMC
p
FTPC West
FTPC East
? f
0
-p
0
1
-1
2
-4
-2.5
4
2.5
? ?
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Time Projection Chamber (TPC)

• Main STAR detector
• 4 m in diameter
• 4.2 m in length
• ? lt 1, 0 lt f lt 2p
• 100 MeV/cltplt 30 GeV/c
• Magnetic field 0, 0.25 T, 0.5 T
• Drift gas P10 (10 methane, 90 argon)
• Cathode potential 28 kV
• Pressure 1 atm
• Drift velocity 5.45 cm/µs

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Barrel EMC

• Coverage
• ? lt 1
• 0 lt ? lt 2?
• 4800 towers
• Tower
• 21X0
• ???f 0.050.05
• dE/E16/vE
• Shower Maximum Detector (SMD)
• 5X0
• ???f 0.0070.007

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Central Trigger Barrel (CTB)

• 240 scintillator slats
• ? lt 1
• 0 lt f lt 2p
• Measures charged particles multiplicity

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Relativistic Heavy Ion Collider (RHIC)Brookhaven
National Laboratory (BNL), Upton, NY
1 km
v 0.99995?c 186,000 miles/sec
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Solenoidal Tracker At RHIC (STAR) 12 countries
51 institutions 545 collaborators
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The STAR detector
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STAR Main Detector
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Motivation charm production mechanism

• Our final goal is to understand the properties of
  the hot and dense matter produced in relativistic
  heavy-ion collisions.
• Charm provides a unique tool to study properties
  of this new matter.
• However, we need to first understand the charm
  production mechanism initial parton fusion,
  flavor excitation, etc.
• Theorists believe charm is mainly produced in
  initial collisions via gluon fusion in
  relativistic heavy-ion collisions. Gyulassy and
  Lin, PRC 51 (1995) 2177. ? charm total cross
  section should follow Nbin scaling from pp to
  AuAu.
• Its important to measure the charm total cross
  section in AuAu and compare with pp and dAu.

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Motivation charm vs. thermalization

• Charm (Moore and Teaney, PRC 71 (2005) 064904)
  and charm resonance (Hees and Rapp, PRC 71
  (2005) 034907) interact with the medium via
  scattering
• Its phase space shape may change at low pT (lt 3-5
  GeV/c)
• Charm could pick up elliptic flow from the
  medium.
• Measurements of charm pT spectra and elliptic
  flow may give us hint that the partonic matter
  could be thermalized.

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Identifying low-pT electrons with ToF
TPC
ToF 1/?-10.03
p
K
p
K
e
?
?
e
TPCToF

• Time-of-flight (ToF) detector measures timing
  between collision point and detector ? particle
  velocity ?v/c
• Identify particles by combining energy loss dE/dx
  in TPC with velocity ? from ToF

e
?
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Nuclear modification factor EMC
PRL 91 (2003) 172302

• Charm is suppressed at high pT as strongly as
  light flavors!!!
• Caution comparing decay electrons and hadrons
  only sensible when RAA is flat at high pT.

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Nuclear modification factor EMC

• Charm high-pT suppression is as strong as light
  hadrons!!!
• Present theories do not describe the data
  adequately.
• Only charm contribution would describe the RAA
  but not the pp spectrum.
• However, the amount of beauty contribution to the
  electrons is still uncertain.
• We need to measure RAA from D directly to clarify.

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Motivation Quarkonia
Kaczmarek et al., Nucl. Phys. Suppl. B 129 (2004)
560-562

• The raison dêtre of RHIC is to study properties
  of the quark-gluon plasma (QGP) produced in
  relativistic heavy-ion collisions.
• J/? suppression provides an unambiguous signature
  of the QGP ? Debye screening of quark color
  charge weakens the c-cbar potential. Matsui and
  Satz, PLB 178 (1986) 416.
• Lattice calculations demonstrate weakening of
  potential at higher temperatures.
• Measurements of ? production in relativistic
  heavy-ion collisions could help rule out some
  predictions on quarkonia suppression in QGP.

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Identifying high-pT electrons with the EMC

• TPC dE/dx for p gt 1.5 GeV/c
• Only primary tracks (reduces effective radiation
  length)
• Electrons can be discriminated well from hadrons
  up to 8 GeV/c
• Allows to determine the remaining hadron
  contamination after EMC
• EMC
• Tower E ? p/E
• Shower Max Detector (SMD)
• Hadrons/Electron shower develop different shape
• Use shower shape to discriminate
• 85-90 purity (pT dependent)
• h discrimination power 103-104