ismd 2005, kromeriz czech republic aug.9-15 1 measurement of identified particle production at rhic...

ISMD 2005, Kromeriz Czech Republic Aug.9-15 ISMD 2005, Kromeriz Czech Republic Aug.9-15 1

Measurement of identified Measurement of identified particle production at RHICparticle production at RHIC

An TaiUniversity of California at Los Angeles

For the STAR CollaborationFor the STAR Collaboration


Outline

• Introduction

• Results from in 200 GeV Au+Au collisions at RHIC new

mechanism for hadronization

• Results from in 200 GeV dAu collisions at RHIC an

important reference

• Heavy flavor production

• Summary


A Pictorial View of Micro-Bangs at RHIC

Thin PancakesLorentz =100

Nuclei pass thru each other

< 1 fm/c

Huge StretchTransverse ExpansionHigh Temperature (?!)

The Last Epoch:Final Freezeout--

Large Volume

What are properties of matter formed at RHIC ?


J/D

K*

K

p

d, HBT

vv22

saturatessaturates

TT

saturatessaturates

Q2

time

Identified particles probing the matter properties

PID in STAR:TPC: tracking p,K,π ….BEMC electron …

*ToF patch p,K,π, electron.

Δφ π/30, -1 < η < 0


STAR Particle Identification

Electron identification: TOFr |1/ß-1| < 0.03 TPC dE/dx electrons!!!

electrons


Examples of mass plots

|y|<1

0.4 <pt< 6.0

|y|<1

0.4 <pt< 6.0|y|<1

0.6 <pt< 5.0

K0s, , : topology

cuts, like decay length,

DCA of V0-primV.

: event mixing

technique

|y|<0.5

0.4 <pt< 1.3


Identified Spectra in Simpler Systems

• Theoretical understanding of p+p reference not completely under control• NLO calculations fail, especially for baryons: poorly constrained

fragmentation functions? Other hadronization schemes ?

fragmentation functions from e+e- data, hep-ph/0010289


/K0s in Au+Au and pp –baryon

abnormally

(1) Baryon is enhanced at intermediate pt region with respective to meson in AA

(2) At higher pt, the ratios seem to approach that in pp

(3) Recombination/coalescence models at intermediate pt vs fragmentation at higher pt

Central0-5%

Peripheral60-80%


How recombination/coalescence works

Efficient way to produce baryons at intermediate pt when parton density is high


Study of nuclear effects

dydpdN

dydpdN

N

NpR

Tperipheral

Tcentralcentralcoll

peripheralcoll

TCP /

/)(

Number of Participants

Impact Parameter

Npart – No of participant nucleonsNcoll – No of binary nucleon-nucleon collisions


Nuclear effects in AA

Nuclear suppression Nuclear suppression seen for both baryon seen for both baryon and mesonand meson

The suppression is The suppression is stronger for mesons stronger for mesons than for baryonsthan for baryons

The effect is grouped The effect is grouped by particle type by particle type (baryon vs meson), (baryon vs meson), supporting supporting recombination/coalescrecombination/coalescence pictureence picture

STAR Preliminary

Au+Au

Energy Loss: Partonic or hadronic interaction ?

Suppression:

Initial state or final state effect ?

qq


Φ production probing early state

mΦ~1019 MeV/c2 ; mΛ~1116 MeV/c2; mKs~498 MeV/c2

May determine whether the particle dependence of the nuclear modification factor is grouped by the particle mass or particle typePossible production mechanism:

(1) ggg -> (2) s sbar -> (3) K+K- -> Small cross section for scattering with hadronic medium

sensitive to source properties of early time.


Φ production dynamics

N()/K- independent of collision centralities.

K+K- Φ is not a dominant channel

Φ does not pick up as much as transverse flow as proton


Rcp of Φ meson in AA

Au+Au

STAR preliminary

Species dependence Species dependence at intermediate pt at intermediate pt region is verifiedregion is verified

The suppression can The suppression can not be due to later not be due to later hadronic processeshadronic processes


K0s, , : 0.4 – 6 GeV/c; : 0.6 – 5 GeV/c. Statistical errors only.

Double exponential fit function

d+Au as a reference


Rcp in dAu

• ‘Cronin effect’ is seen in dAu system

•Same baryon/meson separation in dAu as in AuAu collisions Recombination/coalescence modes work in d+Au ?

•Rcp in AA is lower than that in dAu the suppression is due to final state effect

dAu 200 GeV

STAR Preliminary

AuAu 200 GeV


Heavy flavor measurements

D0 K (B.R=3.8%)

D*± D0π (B.R=68% 3.8% (D0 K ) = 2.6% )

D*±

2.4<pt<3.5 GeV/c

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:

Mainly from Photon conversion and π0 Dalitz decays

D0


Charm cross section at RHIC

(1) NLO pQCD calculations tuned for low energy points under-predict the ccbar production cross section at RHIC(2) The Pythia prediction with the Peterson fragmentation function is softer than the measured electron spectrum

Phys. Rev. Lett. 94 (2005) 062301

π+A 350 GeV/c


Heavy Flavor RAA ---Challenge to radiative picture?

Suppression is approximately the same as for hadrons

Where is b contribution ?

M. Djordjevic, et. al. nucl-th/0507019


Summary

• STAR has a vigorous program which is pushing our measured probes more and more sensitive to the early stage of the formed medium

• Nuclear effects at intermediate pt show particle type dependence, supporting recombination/coalescence as dominant processes for hadronization

• Strong suppression of high pt hadron production is observed for both light and heavy quarks, which demonstrates the formation of strongly-interacting dense medium at RHIC.


Dead cone effect---Radiative Energy Loss of Heavy

Quarks

See also Armesto et al, Phys. Rev. D71 (2005) 054027

• Coupling of heavy quarks to the medium reduced due to mass

• Expectation: even for high medium density, higher RAA for single electrons from heavy flavor than for light hadrons


/2K0s in d+Au

Close to Au+Au most peripheral ratio (60-80%)

No significant centrality dependence in d+Au


Electron ID in STAR – EMC

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

4. e/h discrimination power ~ 105

Works for pT > 1.5 GeV/c

electrons

hadrons


Inclusive Single Electrons p+p/d+Au

•Inclusive non-photonic spectra : How to assess the background?

•PHENIX 1: cocktail method

•PHENIX 2: converter method

•STAR: measurement of main background sources (TPC !!!)

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


Photonic Single Electron Background Subtraction in pp and dAuMethod:

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

Rejection Efficiency: • Simulation/Embedding

• background flat in pT

• weight with measured 0 spectra (PHENIX)

conversion and 0 Dalitz decay reconstruction efficiency ~60%

• Relative contributions of remaining sources: PYTHIA/HIJING + detector simulations

Invariant Mass Square

Rejected

Signal

Opening Angle

conversion and 0 Dalitz decay reconstruction efficiency :~60% at pT>1.0 GeV/c


Photonic Single Electron Background Subtraction

pT dependent hadron contamination (10-30%) subtracted

Excess overbackground


Non-Photonic Single Electron Spectra in p+p and d+Au


Nuclear Effects RdAu ?•Nuclear Modification Factor: inel

ppdAubindAu

TppdAu

TdAudAu NT

ddpdT

ddpNdR

/ where;/

/2

2

•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) (~ 2-3) makes interpretation difficult

hadrons


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

• mass(MC)=1.865 GeV/c2

• width=13.7±6.8 MeV• width(MC)=14.5 MeV

cp

dy

dN

T

y

Aud

D

/GeV 08.032.1

008.0004.0028.00

0


Obtaining the Charm Cross-Section cc

•D0 Mesons– Requires fit to data points for extrapolation

• functional form?• error on extrapolation

– Requires the knowledge of ND0/Ncc

•Non-Photonic Single Electrons– What functional form ?

• PYTHIA does not describe data – tweaking it is not satisfactory

• NLO/FONLL not reliable for pT < F = R = mc pTD

•STAR Combine both measurements– Fraction of covered ?


Obtaining the Charm Cross-Section cc• Combined fit:

– Assume D0 spectrum follows a power law function– Generate electron spectrum using particle composition from PDG

• ND0/Ncc ~ 0.540.05• Decay via routines from PYTHIA

– Assume that only normalization scale different between the various D meson pT spectra is different (D0, D*, D, …)

• From D0 mesons alone:– ND0/Ncc ~ 0.540.05– Fit function from exponential fit to mT spectra

• In both cases for d+Au p+p: pp

inel = 42 mb– Nbin = 7.5 0.4 (Glauber) – |y|<0.5 to 4: f = 4.70.7 (simulations)– RdAu = 1.3 0.3 0.3


Obtaining the 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


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) mbPHENIX 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 thecurrently available statistics (soon be more!)

pp p

SPS, FNAL (fixed target) and ISR (collider) experiments


Discrepancy between STAR and PHENIX ?

90%

15%

Combined fit of STAR D0 and PHENIX electrons:No discrepancy: cc=1.1 0.1 0.3 mb

STAR: PRL 94, 062301 (2005)PHENIX p+p (QM04): S. Kelly et al. JPG30(2004) S1189


Consequences of High Cross-Section: J/ Recombination Statistical model (e.g. A. Andronic et. al. PLB 571,36(2003)) :

Largecc yield in one heavy ion collision J/ production through recombination possible J/ enhancement

Statistical model

In stat models: cc typically from pQCD calculations Consequence of STAR cc (from d+Au) much larger enhancement (~3-

10) for J/ production in central Au+Au collisions PHENIX’s upper limit would invalidate the expectation from large cc ?!


NLO/FONLL•Recent calculations in NLO (e.g. R. Vogt et al. hep-ph/0502203)

– Since m≠0 heavy quark production is a hard process– Calculations depend on:

• quark mass mc

• factorization scale F (typically F = m or 2m)

• renormalization scale R (typically R = F)

• parton density functions (PDF)

– Total cross-section depends only on mc, not kinematic quantities

– Hard to obtain large with R = F (which is used in PDF fits)

– For pT spectra m(for calculations m

• pT integrateddirect calculated

•Fixed-Order plus Next-to-Leading-Log (FONLL)– designed to cure large logs for pT >> mc where mass is not relevant

– FONLL higher over most pT than NLO (also tot)

•K factor (NLO NNLO) ?


NLO/FONLL

b

bb

FONLLbb

NLOcc

FONLLcc

99.067.0

381134

400146

87.1

244 ;256

from hep-ph/0502203


Charm Total Cross SectionCan we confirm or rule out Cosmic Ray experiments? (Pamir, Muon, Tian Shan) under similar conditions?NPB (Proc. Suppl.) 122 (2003) 353Nuovo Ciment. 24C (2001) 557

X. Dong USTC

– NLO calculations under-predict current cc at RHIC

– More precise data is needed high statistics D mesons in pp


Comparison: Non-Photonic Electrons with NLO

•FONLL calculations:•Charm:•scaled by STAR/FONLL

•Bottom:•derived from fit of sum to data

•Errors used: data + FONLL uncertainty bands

Plenty of room for bottom !!!


Can We Disentangle Charm from Bottom ?

•Method I: Finding the Secondary Vertex – Requires excellent resolution of vertex tracker (< 50 m)– Survival probability P(x) depends on M/c

• D ~ 15 MeV/m• B ~ 11 MeV/m (B lives long but is heavy)

– Works only at high p but bb is dominated by pTe < 4 GeV/c

•Method II: Identifying the Ks from the semileptonic decay– If a K is present within certain kinematical region around the e the ratio of opposite/same sign pairs

(K+e- + K-e+)/(K-e- + K+e+) is– charm (ce K anything): ~ 55:1 (PYTHIA) – bottom (be K anything): ~ 1:6 (PYTHIA) – works only in pp and requires large acceptance + high pT PID

•Method III: Subtracting e evaluated from measured D spectra– Requires knowledge of D spectra out to very high pT

• Need D spectra out to 11 GeV/c to describe electrons at pT ~ 5 GeV/c


High-pT D0-Meson Spectra in d+Au•How is it done ?

– Assumptions:• pT spectra of D0, D*, D

• same shape

– D0 K defines low pT points

– D0 K defines one high-pT point

– Combined allow power law fit– functional form allows to move

D* and D spectra into place– cross-check with known ratios

OK– Problem: D*/D0 and D/ D0 not

well known (pT, s dependent ?)Note: spectrum depends onone point: D0 K


High-pT D-Meson Spectra in d+Au•Headache: Spectra very hard (too hard)

– Fragmentation function function (Peterson FF needs c = b) ?

– Yield at 10 GeV/c only factor 3 below CDF (LO/NLO ~ 10) ?

Intensive systematic studies of D0 K of many people over many month …


High-pT D-Meson Spectra in d+Au•Until we found the problem …

– very, very, subtle effect – Downside: combined low to high-pT D0 spectra is gone

• ratios not well enough known• cannot normalize D*, D to D0 appropriately any more

Upper limits from D0 K (90% CL)Note: D* itself is still valid!!! Now a “standalone” spectra. Doesn’t affect possibility of studying RAA in Au+Au


Thermalization of heavy quarks ?

v2 of non-photonic electrons in

Au+Au


Strong Elliptic Flow at RHIC

•Strong elliptic flow at RHIC (consistent with hydro limit ?)– scaling with Number of Constituent Quarks (NCQ)

• partonic degrees of freedom !?

– v2/n(pT/n) shows no mass and flavor dependence– Strong argument for partonic phase with thermalized quarks

•What’s about charm?– Naïve kinematical argument: need Mq/T ~ 7 times more collisions to thermalize – v2 of charm closely related to RAA


Charm Elliptic Flow from the Langevin Model

– Diffusion coefficient in QGP: D = T/M momentum drag coefficient)

– Langevin model for evolution of heavy quark spectrum in hot matter

– Relates collisional energy loss and elliptic flow v2

– pQCD gives D(2T) 6(0.5/s)2

AMPT:(C.M. Ko)

← =10 mb

← =3 mb


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 interactions– heavy particle in heat bath of light particles (QGP) +

fireball evolutiontime-evolved c pT spectra in local rest frame

“Nearly” thermal: T ~ 290 MeV

Including scalar, pseudoscalar, vector, and axial vector D 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


How to Measure Charm v2

•Best: D mesons need large statistics, high background not now•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


Measuring v2 of Non-Photonic Electrons

•Analysis– Same procedures as for single electrons (incl. background subtraction)– … but with much harder cuts (plenty of statistics)– special emphasis on anti-deuteron removal– reaction plane resolution ~ 30%Consistency check: PYTHIA + MEVSIM (flow generator) through analysis chain

● D0 (input)○ e±

● 0 ee (input)○ e±

Phenix : Min. BiasStar: 0-80%STAR: stat. errors onlyCorrected for residual e± contaminations from π decays with v2

max=17%

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)

v2 of Non-Photonic Electrons

Indication of strong non-photonic electron v2

consistent with v2(c) = v2(light quark)smoothly extending from PHENIX resultsTeany/Moor D (2T) = 1.5 expect substantial suppression RAA

Greco/Ko Coalescence model (shown above) appears to work well


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 PhysicsMichigan 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 UniversityPusan 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

545 Collaborators from 51 Institutionsin 12 countries

STAR Collaboration


Motivation II

the Cronin effect in pA, d+Au collisions

Nuclear Modification factor RAA or RCP

Should the Cronin effect be influenced by the final state particle Should the Cronin effect be influenced by the final state particle formation dynamics?formation dynamics? Recombination models! Recombination models!

p

q

h

A

traditional models :

Multiple parton/hardon scatterings in initial state

Recombination/Coalescence:

Final state effect


STAR Detector at RHIC

Time Projection Chamber

Forward TPCs

• pion, kaon, proton and electron : identified using ionization energy loss technique.

• Other particles are reconstructed from them.


Data Set and Cuts

STAR d+Au 200GeV Minimum Bias data

Event Selection:

|VertexZ| < 50cm, Primary vertex found, good run ~ 10M events after the cuts.

Centrality definition in STAR: 0-20%(Nch>=17), 20-40%(17>Nch>=10), 40-100%(Nch<10) Nch (Uncorrected # of charged

particles @ Forward TPC-Au

side, -3.8<<-2.8)


Invariant mass plots

|y|<1

0.4 <pt< 6.0

|y|<1

0.4 <pt< 6.0

|y|<1

0.6 <pt< 5.0

K0s, , : topology

cuts, like decay length,

DCA of V0-primV.

: event mixing.

These particles can be

identified at much

higher pT ( up to 6 GeV/c

).

|y|<0.5

0.4 <pt< 1.3


K0s, , : 0.4 – 6 GeV/c; : 0.6 – 5 GeV/c. Statistical errors only.

Double exponential fit function

better than the exponential function ( low pT )and the power-law ( high pT ).

mT Spectra and fits in d+Au


Compare various fit functions


The mean pTs are little

dependence of Number

of Charged Particles.

Themean pTs

increase with Number of

Charged Particles

<PT> vs. Number of Charged Particles

dAu Minbias


/K0s in Au+Au and pp

Au+Au and pp @ 200 GeV

Au+Au most peripheral

Recombination models

Central0-5%

Peripheral60-80%

R.J.Fries et al. 2003 Phys. Rev. C 68 044902

V.Greco et al. 2003 Phys. Rev. C 68 2537


/2K0s in d+Au

No significant centrality dependence in d+Au

Close to Au+Au most peripheral ratio (60-80%)

Soft+Hard Reco may work in d+Au


Rcp in Au+Au 200 GeV

Suppression

Rcp’s are grouped into Mesons (K0

s, ) and Baryons (, ).

Particle-type dependence!

STAR Preliminary


RAB charged hadrons in d+Au STAR

d+Au : Cronin enhancement

Au+Au : Suppression


Rcp of KK00ss/// in d+Au STAR

Rcp’s are grouped into Mesons (K0

s, ) and Baryons (, ).

Particle-type dependence again!

Cronin effect

Final state formation dynamics (Recombination model ) Rcp difference between BM.


Rcp of K//p in d+Au

D. Kotchetkov, QM2004PHENIX B-M dependence:

(, K) vs. (p, )

STAR TOF measurement

B-M dependence: (, K) vs. (p)


RAA in low energy p+A collisions

s =27.4GeV

P.B Straub,PRL 68, 452(1992)Rw/Be in p+A collisions

W: tungsten Be: beryllium

s =38.8GeV

RRw/Bew/Be : : Mesons Mesons (2 quarks):(2 quarks):

kaon and pionkaon and pion ~ 1.5; ~ 1.5; Baryons Baryons (3 quarks):(3 quarks):

protonproton ~ 2.5 ~ 2.5

Particle-type Particle-type dependence ??dependence ??

~1.4

~1.5

~2.5


Summary

• Measured productions for 4 identified particles(K0s, , ,

) in d+Au collisions at RHIC;

• /K0s ratio increases with multiplicity.

– dAu ratio is close to AuAu most peripheral (60-80%)– Recombination models are in qualitative agreement with the data.

• pA Nuclear modification factor, RAB, shows Cronin effect– Baryon–Meson(B-M) dependence?.

• Au+Au Nuclear modification factor, RCP, shows B-M difference and suppression.

– Consistent with parton recombination + jet quenching.

• d+Au Nuclear modification factor, also shows B-M difference and Cronin effect.

– PHENIX : (K/ vs. p/), STAR : (K0s/vs. )

– Cronin: Final state particle formation dynamics (recombination)


<p<pTT> centrality > centrality dependencedependence

1) , K, p mean transverse momentum <pT> increase in more central collisions;2) Heavier mass particle <pT> increase faster than lighter ones as expected from hydro type collective flow;

1) , K, p mean transverse momentum <pT> increase in more central collisions;2) Heavier mass particle <pT> increase faster than lighter ones as expected from hydro type collective flow;3) -meson seems flow differently.

ismd 2005, kromeriz czech republic aug.9-15 1 measurement of identified particle production at rhic...

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