Transcript AGS-RHIC User’s Workshop -- Heavy Quark Production at RHIC

Heavy Quark Probes of
Hadronization of Bulk Matter
at RHIC
Huan Zhong Huang
Department of Physics and Astronomy
University of California at Los Angeles
Department of Engineering Physics
Tsinghua University
Collisions at high pT (pQCD)
At sufficiently large transverse momentum, let us consider the process:
p + p  hadron + x
d 3
dˆ
Eh 3    dxa dxb f a / p ( xa ,  2 ) f b / p ( xb ,  2 )
d ph abcd
dtˆ
abcd
sˆ
2
D
(
z
,

) (sˆ  tˆ  uˆ )
ch
h
2
 zh
1) f(x,2) – parton structure function
2) ab->cd – pQCD calculable at large 2
3) D(zh,2) – Fragmentation function
To produce heavy quark pairs, the CM energy must>2m
Heavy Quark Production Mechanism
K+
D
0
l
K-
D0
e-/-
e-/J/y
e+/+
e+/+
l
• Sensitive to initial
gluon density and
gluon distribution
• Energy loss when
propagating through
dense medium
• Different scaling
properties in central
and forward region
indicate shadowing,
which can be due to
CGC.
• Sensitive to initial
gluon density and
gluon distribution
• Suppression or
enhancement of
charmonium in the
medium is a critical signal
for QGP.
Parton Distribution Function Important
Uncertainties in gluon structure function of the proton
J. Pumplin et al, JHEP07(2002)012
MRST2001
CTEQ5M1
CTEQ5HJ
Band – experimental constraints
x
Fragmentation Functions
Fragmentation Functions from e+e Collisions
Belle Data
Charm Mesons from Hadronic Collisions
Charm meson pT ~ follow the NLO charm quark pT
-- add kT kick
-- harder fragmentation ( func or recombination scheme)
kT Kick? What about kL?
The xF distribution matches the NLO charm quark xF !
Belle Puzzle !
PRL 89, 142001 (2002)
(e+e-J/y cc)
+0.15 + 0.12
=
0.59
- 0.13 (e+e-J/y X)
An order of magnitude higher than theoretical predictions
-- B.L. Ioffe and D.E. Kharzeev, PRD 69, 014016 (2004)
These results challenge our current understanding of how charm
quarks/mesons are produced.
We may question our view for the underlying charm production
process, e.g., the universality of fragmentation process and the
fragmentation schemes !
Neutral D mesons
K ~ 1.5
LO QCD
does not
reproduce
the cross
sections !
K Factor !!
Charged D mesons
K ~ 4.5
K factor
energy,
particle
dependent !
Charm-Beauty
different !
We don’t know
the production
mechanism
at all !
Detecting D-Mesons via Hadronic Decays
• Hadronic Channels:
– D0  K 
(B.R.: 3.8%)
– D0  K  r
(B.R.: 6.2%  100% (r-) = 6.2%)
– D  K  p
(B.R.: 9.1%)
– D*±  D0π
(B.R.: 68%  3.8% (D0  K  ) = 2.6% )
 Lc  p K 
(B.R.: 5%)
 M  x0 

P( x  x0 )exp 
c


p


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
D*
Exception D*:
search for
peak around
m(D*)-m(D0)
=0.1467
GeV/c2
D0
D0
Detecting Charm/Beauty via Semileptonic D/B Decays
• Semileptonic Channels:
– D0  e+ + anything
(B.R.: 6.87%)
– D  e + anything
(B.R.: 17.2%)
– B  e + anything
(B.R.: 10.2%)
 single “non-photonic” electron continuum
• “Photonic” Single Electron Background:
 g conversions (0  gg)
 0, h Dalitz decays
 r, f, … decays (small)
– Ke3 decays (small)
 M  x0 

P( x  x0 )exp 
 c  p 
M 0
M
( D )15 MeV/ m ; ( B 0 ) 11 MeV/ m
c
c
TOF electron measurements
~7.6M AuAu 200GeV
Run IV P05ia production
0~80% Min. Bias.
|Vz| < 30cm
2/ndf = 65/46
2/ndf = 67/70
Electrons can be separated from
pions. But the dEdx resolution is
worse than d+Au
Log10(dEdx/dEdxBichsel)
distribution is Gaussian.
0.3<pT<4.0 GeV/c
|1/-1|<0.03
 2 Gauss can not describe
the shoulder shape well.
 Exponential + Gaussian fit is
used at lower pT region.
 3 Gaussian fit is used at
higher pT region.
Electron Spectrum
γ conversion
Dominant source
π0 Dalitz decay
at low pT
η Dalitz decay
Kaon decay
vector meson decays
Mass(e+e-)<0.15 GeV/c2
 Combinatorial background reconstructed by track
rotating technique.
 Invariant mass < 0.15 for photonic background.
Charm pT Spectra
D0 and e combined fit
Power-law function with parameters
dN/dy, <pT> and n to describe the D0
spectrum
Generate D0e decay kinematics
according to the above parameters
Vary (dN/dy, <pT>, n) to get the min.
2 by comparing power-law to D0
data and the decayed e shape to e
data
<pT>=1.20  0.05(stat.) GeV/c in
minbias Au+Au
<pT>=1.32  0.08(stat.) GeV/c in d+Au
Charm Total Cross Section
Charm total cross section per NN
interaction
1.13  0.09(stat.)  0.42(sys.) mb in
200GeV minbias Au+Au collsions
1.4  0.2(stat.)  0.4(sys.) mb in
200GeV minbias d+Au collisions
Charm total cross section follows
roughly Nbin scaling from d+Au to
Au+Au considering errors
Indication of charm production in initial
collisions
Systematic error too large !
Experimental Statistical and Systematic Errors
c-cbar production CS
PHENIX
0.92+-0.15+-0.54 mb
STAR
1.4+-0.2+-0.4 mb
Errors taken seriously
High pT region does
not contribute to total
CS much. STAR data
need to be compared
with PHENIX data!
Heavy Quarks Unique
Heavy Quark Flavors (Charm or Beauty)
Heavy Flavors once produced –
do not change to light flavor easily
heavy quark production can be calculated from
pQCD approach more reliably than light quarks
Trace heavy quark flavors in nuclear collisions
-- collision dynamics and hadronization mechanism
Fragmentation versus Recombination/Coalescence
Fragmentation
p(heavy quark meson)/p(heavy quark) < 1
Recombination/Coalescence
p(heavy quark meson)/p(heavy quark) >= 1
Nuclear Modification Factors
Use number of binary nucleon-nucleon collisions to
gauge the colliding parton flux:
RAA ( pT ) 
d 2 N AA
dpT dh
/ N coll
d 2 N pp
dpT dh
d 2N
[
/ N coll ]Central
dpT dh
RCP ( pT ) 
d 2N
[
/ N coll ]Peripheral
dpT dh
N-binary Scaling  RAA or RCP = 1
simple superposition of independent nucleon-nucleon collisions !
Charm and Non-photonic Electron Spectra
1.13  0.09(stat.)  0.42(sys.) mb in
200GeV minbias Au+Au collsions
Total charm  Binary Scaling
suppression at high pT
Charm Nuclear Modification Factor
RAA suppression for single electron in
central Au+Au similar to charged
hadrons at 1.5<pT<3.5 GeV/c
Heavy flavor production IS also
modified by the hot and dense medium
in central Au+Au collisions at RHIC
Suppressions!!
STAR: Phys. Rev. Lett. 91 (2003) 172302
K
p
High pT Electron ID
d

dE/dx from TPC
electrons
SMD from EMC
hadrons
electrons
High pT Electron ID
p/E from EMC
After all the cuts
hadrons
electrons
The shape and yield at high pT
Note:
FONLL – effective
fragmentation function
harder than commonly
used Peterson function!
STAR – difference ~ 5.5
PHENIX -- ~1.7 (?)
Non-photonic electron RAA
Non-photonic
electrons RAA
-- similar magnitude
as light hadrons
-- STAR-PHENIX
data consistent
in the overlapping
region
The high pT region
n-p electron RAA
surprising !
Heavy quark energy loss: Early Expectations
Heavy quark has less dE/dx due
to suppression of small angle
gluon radiation
“Dead Cone” effect
Y. Dokshitzer & D. Kharzeev PLB 519(2001)199
dP 
 sCF d
k2 dk2
dP0

2
2 2 2

 (k    0 )
(1   02 /  2 ) 2
0 
M
k
,  
E

M. Djordjevic, et. al. PRL 94(2005)112301
J. Adams et. al, PRL 91(2003)072304
Radiative energy loss of heavy
quarks and light quarks
--- Probe the medium property !
What went wrong?
Radiative Energy Loss not Enough
Moore & Teaney, PRC 71, 064904 (2005)
Large collisional (not radiative) interactions also
produce large suppression and v2
Charm Quark in Dynamical Model (AMPT)
Large scattering cross sections needed !
Does Charm Quark Flow Too ?
Reduce Experimental Uncertainties !!
Suppression in RAA  Non-zero azimuthal anisotropy v2 !
B and D contributions to electrons
Experimental
measurement of
B and D
contributions to
non-photonic
electrons !
Direct measurement
of D and B mesons
Poor (Wo)Man’s Approach to Measure B/D
Contributions to Electrons – e-h correlations
PYTHIA Simulations of e-h correlations from p+p
B
D
X. Lin hep-ph/0602067
B does not seem dominant at pT 4.5 GeV/c
Preliminary STAR Data
Xiaoyan Lin – STAR presentation at Hard Probe 2006
Open Issues
Phenix and STAR results  Converge?!
Systematic errors on non-photonic
electrons under control !
Quantitative description for energy loss
and pT spectra for light/heavy quarks
Collectivity for heavy quarks?
Recombination  DS/D0
PYTHIA Prediction
Charm quark recombines with a light (u,d,s) quark from
a strangeness equilibrated partonic matter  DS/D0 ~ 0.4-0.5
at intermediate pT !!!
Color Screening
• J/
– Small: r ~ 0.2 fm
– Tightly bound: Eb ~ 640 MeV
HG
QGP

Observed in
dileptons invariant
mass spectrum

Other charmonia
• ’ ~ 8%
•  ~ 32%
J/psi Suppression and Color Screening
QCD Color Screening: (T. Matsui and H. Satz, Phys. Lett. B178, 416 (1986))
A color charge in a color medium is screened similar to Debye
screening in QED  the melting of J/y.
c
Charm quarks c-c may not bind
Into J/y in high T QCD medium
c
The J/y yield may be increased due to charm quark coalescence at
the final stage of hadronization (e.g., R.L. Thews, hep-ph/0302050)
SJ /y  0.6S
dir
J /y
Recent LQCD Calculation:
 0.3S  0.1S
dir
dir
y'
diss
y'
 T
diss
J /y
 (1.5 - 2)Tc
T
T
diss
 1.1Tc
J/y Quark Potential Model
Lattice QCD Calculations
J/y from di-lepton Measurements
J/yPHENIX Data
Branching ratios: e+e- 5.93%; - 5.88%
J/psi is suppressed in central Au+Au Collisions !
Factor ~ 3
the same as that at SPS
Satz: Only  states are
screened both at
RHIC and SPS.
Alternative:
Larger suppression
in J/psi at RHIC due to
higher gluon density,
but recombination
boosts the yield up !
V2 of J/psi
V2 of J/psi can differentiate scenarios !
pQCD direct J/psi should have no v2 !
Recombination J/psi can lead to
non-zero v2 !
The case for partonic DOF/Deconfinement
can be made with strange vector meson
f cannot be made from KK coalescence !
J/y Suppression or Not
Nuclear Absorption of J/y important at low energy
important (SPS) !
Both QCD color screening and charm quark
coalescence are interesting, which one
is more important at RHIC?
At RHIC the J/y measurement requires high
luminosity running!
Centrality and pT dependence important !
pT Scales and Physical Processes
RCP
Three PT Regions:
-- Fragmentation
-- multi-parton dynamics
(recombination or
coalescence or …)
-- Hydrodynamics
(constituent quarks ?
parton dynamics
from gluons to
constituent quarks? )
Where does heavy quark fit?
The End