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 pCollisions at high pT T (pQCD)(pQCD)

)ˆˆˆ(),(

ˆˆˆ

),(),( 22

2/

2/3

3

utszDz

s

td

dxfxfdxdx

pd

dE hhc

h

cdab

bpbapaabcd

bah

h

At sufficiently large transverse momentum, let us consider the process:

p + p hadron + x

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

• Sensitive to initial gluon density and gluon distribution

0D

D0

J/

K+

l

l

K-

e-/-

e+/+

e-/-

e+/+

• Energy loss when propagating through dense medium

• Different scaling properties in central and forward region indicate shadowing, which can be due to CGC.

• Suppression or enhancement of charmonium in the medium is a critical signal for QGP.

• Sensitive to initial gluon density and gluon distribution

Parton Distribution Function Important

Uncertainties in gluon structure function of the proton

x

CTEQ5M1

CTEQ5HJ

MRST2001

Band – experimental constraints

J. Pumplin et al, JHEP07(2002)012

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/ cc)(e+e-J/ X)

= 0.59 +0.15- 0.13 + 0.12-

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 !

K ~ 1.5

Neutral D mesons

LO QCD does not reproduce the cross sections !

K Factor !!

K ~ 4.5

Charged D mesons 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 (B.R.: 6.2% 100% () = 6.2%)– D K p (B.R.: 9.1%)– D*± D0π (B.R.: 68% 3.8% (D0 K ) = 2.6%) c p K (B.R.: 5%)

pc

xMxxP

0

0 exp)(

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

Exception D*: search for peak around m(D*)-m(D0) =0.1467 GeV/c2

D0

D0D*

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: conversions (0 ) 0, Dalitz decays , , … decays (small)– Ke3 decays (small)

mBc

MmD

c

Mpc

xMxxP

/ MeV11)( ; / MeV15)(

exp)(

00

00

~7.6M AuAu 200GeV Run IV P05ia production 0~80% Min. Bias. |Vz| < 30cm

Electrons can be separated from pions. But the dEdx resolution is worse than d+Au

Log10(dEdx/dEdxBichsel) distribution is Gaussian.

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.

2/ndf = 65/46

0.3<pT<4.0 GeV/c

TOF electron measurements

|1/-1|<0.03

2/ndf = 67/70

Mass(e+e-)<0.15 GeV/c2

Combinatorial background reconstructed by track rotating technique.

Invariant mass < 0.15 for photonic background.

γ conversion π0 Dalitz decay η Dalitz decay Kaon decay vector meson decays

Dominant source at low pT

Electron Spectrum

Charm pT Spectra

Power-law function with parameters dN/dy, <pT> and n to describe the D0 spectrum

D0 and e combined fit

Generate D0e 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

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 per NN interaction

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 PHENIX0.92+-0.15+-0.54 mb

STAR1.4+-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/CoalescenceFragmentation

p(heavy quark meson)/p(heavy quark) < 1

Recombination/Coalescencep(heavy quark meson)/p(heavy quark) >= 1

Nuclear Modification Factors

ddp

Nd

collddpNd

TAA

T

pp

T

AA NpR 2

2

/)(

Use number of binary nucleon-nucleon collisions to gauge the colliding parton flux:

N-binary Scaling RAA or RCP = 1 simple superposition of independent nucleon-nucleon collisions !

Peripheralcoll

T

Centralcoll

TTCP

NddpNd

NddpNd

pR

]/[

]/[

)( 2

2

Charm and Non-photonic Electron Spectra

1.13 0.09(stat.) 0.42(sys.) mb in 200GeV minbias Au+Au collsions

Total charm Binary Scalingsuppression at high pT

Charm Nuclear Modification Factor

STAR: Phys. Rev. Lett. 91 (2003) 172302

Suppressions!!

RAA suppression for single electron incentral Au+Au similar to charged hadrons at 1.5<pT<3.5 GeV/c

Heavy flavor production IS alsomodified by the hot and dense mediumin central Au+Au collisions at RHIC

electrons

K p d

electrons

hadrons

High pT Electron ID

dE/dx from TPC

SMD from EMC

hadrons electrons

High pT Electron ID

p/E from EMC

After all the cuts

The shape and yield at high pT

Note: FONLL – effective fragmentation functionharder than commonly used Peterson function!

STAR – difference ~ 5.5PHENIX -- ~1.7 (?)

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 !

Non-photonic electron RAA

Heavy quark energy loss: Early Expectations

k

E

M

dP

k

dkkdCdP Fs

,

)/1()(

0

2220

022

022

22

Y. Dokshitzer & D. Kharzeev PLB 519(2001)199

Radiative energy loss of heavy quarks and light quarks

--- Probe the medium property !

Heavy quark has less dE/dx due to suppression of small angle gluon radiation

“Dead Cone” effect

M. Djordjevic, et. al. PRL 94(2005)112301

J. Adams et. al, PRL 91(2003)072304

What went wrong?

Radiative Energy Loss not EnoughMoore & 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

B

D

PYTHIA Simulations of e-h correlations from p+p

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 quarksCollectivity 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 !!!

• J/ – Small: r ~ 0.2 fm

– Tightly bound: Eb ~ 640 MeV

HG

QGP Observed in

dileptons invariant mass spectrum

Other charmonia• ’ ~ 8%• ~ 32%

Color Screening

J/psi Suppression and Color Screening

cdiss

J

cdissdiss

TT

TTT

)25.1(

1.1

/

'

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/.

c c Charm quarks c-c may not bindInto J/ in high T QCD medium

The J/ yield may be increased due to charm quark coalescence at the final stage of hadronization (e.g., R.L. Thews, hep-ph/0302050)

Recent LQCD Calculation:

dirdirdirJJ SSSS '// 1.03.06.0

J/ Quark Potential Model

Lattice QCD Calculations

J/ from di-lepton Measurements

J/-

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

cannot be made from KK coalescence !

J/ Suppression or NotNuclear Absorption of J/ important at low energy important (SPS) !

Both QCD color screening and charm quarkcoalescence are interesting, which oneis more important at RHIC?

At RHIC the J/ measurement requires highluminosity running!

Centrality and pT dependence important !

pT Scales and Physical ProcessesRCP

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