Charm and Electrons in
Thomas Ullrich, STAR/BNL
International Workshop on
Electromagnetic Probes of Hot and Dense
Matter
ECT, Trento
June 8, 2005
Outline

STAR’s Heavy Flavor Program



Open Charm (and Beauty) Production




Non-photonic electrons
 p+p: the reference
 d+Au: cold nuclear matter effects
 Au+Au: ( QM’05)
D mesons
 d+Au: charm cross-section
 Au+Au: ( QM’05)
Thermalization of heavy quarks ?


Detector capabilities
Experimental techniques
Au+Au: v2 of non-photonic electrons
Quarkonia: J/Y and 
Summary and Outlook
2
Detecting D-Mesons via Hadronic Decays
Hadronic Channels:
 D0  K 

 D  K p
 D*±  D0π
 D0  K  r
 Lc  p K 
(B.R.: 3.8%)
(B.R.: 9.1%)
(B.R.: 68%  3.8% (D0  K  ) = 2.6% )
(B.R.: 6.2%  100% (r+-) = 6.2%)
(B.R.: 5%)
3
Detecting D-Mesons via Hadronic Decays
TPC:

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High tracking efficiency for tracking
hadrons (~90%)
dp/p ~ 1% at 1 GeV/c
large acceptance |h|<1
PID (dE/dx) limits:


p up to 1 GeV/c
K,  up to 0.7 GeV/c
SVT:

Hadrons in STAR:
TPC: tracking, PID
SVT: vertex’ing, PID
ZDC/CTB: centrality/trigger
current vertex’ing performance not
sufficient to resolve typical charm
secondary vertices (ct ~ 120(D0) 315(D) mm)  background 
 Current analyses are based
on TPC alone
4
General Techniques for D Reconstruction
1. Identify charged daughter tracks through
energy loss in TPC
2. Alternatively at high pT use h and assign
referring mass (depends on analysis)
3. Produce invariant mass spectrum in same event
4. Obtain background spectrum via mixed event
5. Subtract background and get D spectrum
6. Often residual background to be eliminated by
fit in region around the resonance
D0
D*
D0
Exception D*: search for peak around
m(D*)-m(D0) =0.1467 GeV/c2
5
Detecting Charm/Beauty via Semileptonic D/B Decays
Semileptonic Channels:
 c  e+ + anything


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D0  e+ + anything
D  e + anything
b  e+ + anything

B  e + anything
(B.R.: 9.6%)
(B.R.: 6.87%)
(B.R.: 17.2%)
(B.R.: 10.9%)
(B.R.: 10.2%)
 single “non-photonic” electron continuum
“Photonic” Single Electron Background:
0
 g conversions (  gg)
 0, h, h’ Dalitz decays
 r, f, … decays (small)
 Ke3 decays (small)
6
Detecting Charm/Beauty via Semileptonic D/B Decays
Electrons in STAR:
EEMC (tower, SMD): PID
ToF patch: PID
TPC: tracking, PID
BEMC (tower, SMD): PID
7
Electron ID in STAR – EMC
electrons
1. TPC for p and dE/dx
● e/h ~ 500 (pT dependent)
2. Tower E  p/E
● e/h ~ 100 (pT dependent)
3. Shower Max Detector (SMD)
shape to reject hadrons
● e/h ~ 20
hadrons
4. e/h discrimination power ~ 105
Works for pT > 1.5 GeV/c
8
Electron ID in STAR – ToF Patch
MRPC – ToF (prototype):
Df  /30
-1 < h < 0
electrons
Electron identification:
TOF |1/ß-1| < 0.03
TPC dE/dx electrons
9
Inclusive Single Electrons p+p/d+Au
ToF + TPC:
0.3 GeV/c < pT < 3 GeV/c
TPC only:
2 < pT < 3.5 GeV/c
EMC + TPC:
pT > 1.5 GeV/c
Inclusive  non-photonic spectra : How to assess photonic background?
PHENIX 1: cocktail method
PHENIX 2: converter method
STAR: measurement of main background sources
10
Photonic Single Electron Background Subtraction in pp and dAu
Method:
1. Select an primary electron/positron
(tag it)
2. Loop over opposite sign tracks
anywhere in TPC
3. Reject tagged track when
m < mcut ~ 0.1 – 0.15 MeV/c2
4. Cross-check with like-sign
Opening Angle
Rejection Efficiency:
• Simulation/Embedding
Invariant Mass Square
•
•
•
•
background flat in pT
weight with measured 0 spectra
(PHENIX)
g conversion and 0 Dalitz decay
reconstruction efficiency ~60%
Rejected
Relative contributions of remaining
sources: PYTHIA/HIJING + detector
simulations
11
g conversion and 0 Dalitz decay
Signal
reconstruction efficiency :
~60% at pT>1.0 GeV/c
Photonic Single Electron Background Subtraction
Excess over
background
pT dependent hadron contamination (5-30%) subtracted
12
Non-Photonic Single Electron Spectra in p+p and d+Au
13
Nuclear Effects RdAu ?
Nuclear Modification Factor:
d 2 N dAu /dpT dh
dAu
inel
RdAu 
;
where
T

N
/

dAu
bin
pp
TdAud 2 pp /dpT dh
hadrons
Within errors compatible with RdAu = 1 …
… but also with RdAu(h)
NOTE: RdAu for a given pT comes from heavy mesons from a wide pT range
p(D) > p(e) (~ 1.5-3)  makes interpretation difficult
14
D0 Mesons in d+Au
Mass and Width consistent
with PDG values considering
detector effects:
• mass=1.867±0.006 GeV/c2;
• mass(PDG)=1.8645±0.005
GeV/c2
dN Dd +0 Au
• mass(MC)=1.865 GeV/c
0.20280.0040.008
• width=13.7±6.8
dy yMeV
0
• width(MC)=14.5 MeV
pT 1.320.08 GeV /c
15
Obtaining the Charm Cross-Section cc
From D0 mesons alone:


ND0/Ncc ~ 0.540.05
Fit function from exponential fit to mT spectra
Combined fit:



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Assume D0 spectrum follows a power law function
Generate electron spectrum using particle composition from PDG
Decay via routines from PYTHIA
Assume: dN/dpT(D0, D*, D, …) have same shape only
normalization
In both cases for d+Au  p+p:

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ppinel = 42 mb
Nbin = 7.5  0.4 (Glauber)
|y|<0.5 to 4: f = 4.70.7 (PYTHIA)
RdAu = 1.3  0.3  0.3
16
Charm Cross-Section cc
pp Charm Cross-Section
From D0 alone:
cc = 1.3  0.2  0.4 mb
From combined fit:
cc = 1.4  0.2  0.4 mb
17
Discrepancy between STAR and PHENIX ?
STAR from d+Au: cc = 1.4  0.2  0.4 mb (PRL94,062301)
PHENIX from p+p (preliminary): cc = 0.709  0.085 + (+0.332,-0.281) mb
PHENIX from min. bias Au+Au: cc = 0.622  0.057  0.160 mb (PRL94,082301)
Reality check: 1.4  0.447 mb and 0.71  0.343 mb are not so bad given the
currently available statistics (soon be more!)
p
pp
SPS, FNAL (fixed target) and ISR (collider) experiments
18
Discrepancy between STAR and PHENIX ?
Combined fit of STAR D0 and PHENIX electrons:
No discrepancy: cc=1.1 0.1  0.3 mb
90%
15%
STAR: PRL 94, 062301 (2005)
PHENIX p+p (QM04): S. Kelly et al. JPG30(2004) S1189
19
Consequences of High Cross-Section: J/Y Recombination
Statistical model (e.g. A. Andronic et. al. PLB 571,36(2003)) :
Largecc yield in heavy ion collisions




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J/Y production through recombination
possible J/Y enhancement
In stat models: cc typically
from pQCD calculations
(~390 mb)
STAR cc  much larger
enhancement (~3-4) for J/Y
production in central Au+Au
collisions
PHENIX’s upper limit would
invalidate the expectation from
large cc ?!
Δy = 1
Δy = 2
Δy = 3
Δy = 4
20
NLO/FONLL
Recent calculations in NLO (e.g. R. Vogt et al. hep-ph/0502203)

Calculations depend on:
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quark mass mc
factorization scale mF (typically mF = mc or 2 mc)
renormalization scale mR (typically mR = mF)
parton density functions (PDF)
Hard to obtain large  with mR = mF (which is used in PDF fits)
Fixed-Order plus Next-to-Leading-Log (FONLL)
 designed to cure large logs for pT >> mc where mass is not relevant
K factor (NLO  NNLO) ?

400
+381
 cFONLL
256+-146
mb;  cNLO
c
c 244 -134 mb
99
 bbFONLL 1.87+-00..67
mb
from hep-ph/0502203
21
NLO/FONLL

For pT spectra m2 mT2

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for  calculations m2 m2
pT integrated  < direct calculated 
FONLL higher over most pT than NLO
Choice of FF plays big role
Uncertainty bands:

reflect uncertainties in m and mc
22
Charm Total Cross Section
Can we confirm or
rule out Cosmic Ray
experiments?
(Pamir, Muon, Tian
Shan) under similar
conditions?
NPB (Proc. Suppl.) 122
(2003) 353
Nuovo Ciment. 24C (2001)
557
X. Dong USTC


PHENIX,STAR:
stat. error only
NLO calculations under-predict current cc at RHIC
More precise data is needed  high statistics D mesons in pp
23
Comparison: Non-Photonic Electrons with NLO
FONLL calculations:
Charm:
scaled by STAR/FONLL
Bottom:
Can be estimated from fit
of sum to data (numbers
soon)
Errors used: data + FONLL
uncertainty bands
Plenty of room for bottom !!!
24
High-pT D0-Meson Spectra in d+Au
How is it done ?
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Assumptions: same shape of D0,
D*, D spectra
D0  K  defines low pT points
D0  K  r defines one high-pT
point
Combined allow power law fit
Allows to move D* and D spectra
into place
Cross-check with known ratios
Problem: D*/D0 and D/ D0 not well
known (pT, s dependent ?)
Note: spectrum depends on
one point: D0  K  r
25
High-pT D-Meson Spectra in d+Au
Headache: Spectra very hard (too hard)

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NLO: fragmentation function  d function (Peterson FF needs ec = eb) ?
Yield at 10 GeV/c only factor 3 below CDF (LO/NLO ~ 10) ?
Intensive systematic studies of D0  K  r of many people over many month …
26
High-pT D-Meson Spectra in d+Au
Until we found the problem …
 subtle effect  after correction no significant signal D0  K  r
0
  “combined” low to high-pT D spectra is gone
Note: D* itself is still valid!!! Now
a “standalone” spectra. Doesn’t
affect possibility of studying RAA
in Au+Au
Upper limits from D0  K  (90% CL)
27
Strong Elliptic Flow at RHIC
Strong elliptic flow at RHIC (consistent with hydro limit ?)

Scaling with Number of Constituent Quarks (NCQ)
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partonic degrees of freedom !?
(v2/n) vs. (pT/n) shows no mass and flavor dependence
Strong argument for partonic phase with thermalized light quarks
What’s about charm?


Naïve kinematical argument: need Mq/T ~ 7 times more collisions to thermalize
v2 of charm closely related to RAA
28
Charm Elliptic Flow from the Langevin Model
AMPT:
(C.M. Ko)
← =10 mb
← =3 mb
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Diffusion coefficient in QGP: D = T/Mh (h momentum drag coefficient)
Langevin model for evolution of heavy quark spectrum in hot matter
Numerical solution from hydrodynamic simulations
pQCD gives D(2T)  6(0.5/as)2
29
Charm Elliptic Flow through Resonance Effects
Van Hees & Rapp, PRC 71, 034907 (2005)
 Assumption: survival of resonances in the QGP
 Introducing resonant heavy-light quark scattering
 heavy particle in heat bath of light particles (QGP) + fireball evolution
time-evolved c pT spectra in local rest frame
“Nearly” thermal:
T ~ 290 MeV
Including scalar, pseudoscalar, vector, and
axial vector D-like-mesons gives:
σcq→cq(s1/2=mD)≈6 mb
Cross-section is isotropic  the transport cross section is 6 mb, about 4
times larger than from pQCD t-channel diagrams
30
How to Measure Charm v2
Best: D mesons  need large statistics, high background  not yet
Alternative: Measure v2 of electrons from semileptonic charm decays


Emission angles are well preserved above p = 2 GeV/c
2-3 GeV Electrons correspond to ≈3-5 GeV D-Mesons
31
Analysis: v2 of Non-Photonic Electrons
v2 = cos(2[Φ-Ψ]) / Ψres

Same procedures as for single electrons (incl. background subtraction)

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But much harder cuts (plenty of statistics)
Special emphasis on anti-deuteron removal
γ-conversions, π0-Dalitz electrons removed via invariant mass
Remaining 37% photonic electron background subtracted with v2max=17%
Reaction plane resolution Yres ~ 0.7
Consistency check: PYTHIA + MEVSIM
(v2 generator) + analysis chain  OK
32
v2 of Non-Photonic Electrons
Phenix : Min. Bias
Star:
0-80%
STAR: stat. errors only
Phenix:
nucl-ex/0404014 (QM2004)
nucl-ex/0502009 (submitted to PRC)
Star:
J. Phys. G 190776 (Hot Quarks 2004)
J. Phys. G 194867 (SQM 2004)
 Indication of strong non-photonic electron v2
 consistent with v2(c) = v2(light quark)
 smoothly extending from PHENIX results
 Teany/Moor  D (2T) = 1.5 (as = 1?)  expect substantial suppression RAA
 Greco/Ko  Coalescence model (shown above) appears to work well
Quarkonia in STAR
STAR:
 Large acceptance |h|<1
 High tracking efficiency (90%)
 J/Y: acceptance  efficiency (pTe > 1.2 GeV/c) ~ 10%
 : Acceptance  efficiency (pTe > 3.5 GeV/c) ~ 14%
 Without Trigger (min. bias running):


Min bias (100 Hz): 18 J/Y and 0.02  per hour running
Signal-to-Background Ratios


S/B > 1: 1 for 
S/B = 1:25 – 1:100 for J/Y
 Seff = S/(2(B/S)+1)  significance close to that of J/Y
 STAR needs quarkonia triggers
34
Quarkonia Trigger in STAR
J/Y  e+ e- :
 L0-trigger: 2 EMC tower with E > 1.2 GeV (~60° apart)
 L2-trigger (software): veto g, better E, 2.5 < Minv < 3.5 GeV/c2
 Efficiency currently too low in Au+Au (pp only)  need full ToF
  e+ e- :
 L0-trigger: 1 EMC tower with E > 3.5 GeV
 L2-trigger (software): Minv > 7 GeV/c2
 High Efficiency (80%) – works in Au+Au




Tests in Au+Au show it works
small background
counts = expectations
Need full EMC for that


trigger
threshold
2004 ½ barrel EMC
2005 ½ - ¾ barrel EMC
35
No N+++N-subtracted
Summary and Outlook
Heavy Flavor Production in RHI is the next big topic that needs to be addressed
 STAR has solid baseline measurements in pp and d+Au

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

Measurements indicate a large cc in pp at RHIC

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

d/dy|y=0 = 0.300.04(stat)0.09(sys) mb
NLO pQCD calculations under predict this value (~ a factor of 3-5)
Large cc appear to rule out expectation of J/ψ enhancement from some
charm coalescence and statistical models
Preliminary results on v2 of non-photonic electrons indicate
substantial elliptic flow of charm in Au+Au collisions at RHIC




D0 in d+Au from pT = 0 - 3 GeV/c
D* in d+Au mesons from pT = 1.5 – 6 GeV/c
Non-photonic single electrons in p+p and d+Au from 1.5 – 10 GeV/c
consistent with v2c = v2light-q theory calculations
consistent (smoothly extending) with PHENIX results
try to extend to higher pT range (possibly b dominated)
First Results on J/Y and  soon
36
STAR Collaboration
545 Collaborators from 51 Institutions
in 12 countries
Argonne National Laboratory
Institute of High Energy Physics - Beijing
University of Bern
University of Birmingham
Brookhaven National Laboratory
California Institute of Technology
University of California, Berkeley
University of California - Davis
University of California - Los Angeles
Carnegie Mellon University
Creighton University
Nuclear Physics Inst., Academy of Sciences
Laboratory of High Energy Physics - Dubna
Particle Physics Laboratory - Dubna
University of Frankfurt
Institute of Physics. Bhubaneswar
Indian Institute of Technology. Mumbai
Indiana University Cyclotron Facility
Institut de Recherches Subatomiques de Strasbourg
University of Jammu
Kent State University
Institute of Modern Physics. Lanzhou
Lawrence Berkeley National Laboratory
Massachusetts Institute of Technology
Max-Planck-Institut fuer Physics
Michigan State University
Moscow Engineering Physics Institute
City College of New York NIKHEF
Ohio State University
Panjab University
Pennsylvania State University
Institute of High Energy Physics - Protvino
Purdue University
Pusan University
University of Rajasthan
Rice University
Instituto de Fisica da
Universidade de Sao Paulo
University of Science and Technology of China USTC
Shanghai Institue of Applied Physics - SINAP
SUBATECH
Texas A&M University
University of Texas - Austin
Tsinghua University
Valparaiso University
Variable Energy Cyclotron Centre. Kolkata
Warsaw University of Technology
University of Washington
Wayne State University
Institute of Particle Physics
Yale University
University of Zagreb
37