Measurement of charmonia at mid-rapidity at RHIC-PHENIX

cJ/e+e- in p+p collisions at √s=200GeV

Susumu OdaCNS, University of Tokyo

For the PHENIX collaboration

2007/09/2462nd annual JPS meeting

Hokkaido University

1

Motivation

0 mb

3 mb

Red : Au+Au |y|<0.35Magenta : Cu+Cu |y|<0.35Blue : Au+Au 1.2<|y|<2.2Aqua : Cu+Cu 1.2<|y|<2.2

peripheral central

2

A paper about J/ production in Cu+Cu collisions will be submitted in few weeks. • Quarkonia are good p

robes of QGP. • J/ is the most studie

d quarkonium in heavy ion collisions.

• Feed down from c into J/ is important.

Direct J/cJ/X’J/XBJ/X

Charmonium system3

c(1S)

c(2S)

J/(1S)

(2S)

c0(1P)c1(1P) c2(1P)hc(1P)

JPC 0-+ 1-- 0++ 1++ 1+- 2++

e+e- BR=5.94%

DDbar threshold

Fraction of J/ from c decay

2

0

1

JcJcJ JBR

JR

c

Measurement of c at RHIC is required.

RHIC energy

4

Precise measurements onlyError of Rc<=0.1

CDFHERA-BE705

E672/E706E369

WA11

E610

Theoretical model predictions

Color Evaporation ModelCSM+Comover

NRQCD Color Singlet Model

Measurement of c at RHIC is requiredto understand quarkonia production.

5

How to measure Rc

• Find J/e+e- (2.9<Mee<3.3GeV).

• Find cJ/ (M=Mee-Mee~0.44GeV).

• Correct acceptance event by event.

• Subtract background by event mixing of J/ and . – Normalization regions : 0.1-0.3GeV and 0.6-0.8GeV

• Run-5 (2005, 3.8 pb-1) and Run-6 (2006, 10.7 pb-1)

p+p 200GeV data is used.

N

i JTLVLJTeeaccJcTJ

JaccLVLeeaccJJ c cc

c pppyyNR

1 /,1/,/,detected /

/

1

1//

11111

c acceptance

Average over J/

6

c conditional efficiency if J/ is detected

J/ acceptance

e-

e+

p

p

PHENIX detector• Beam beam counter

– Collision vertex

• Drift chamber, pad chamber– Charged particle tracking

• Ring imaging Cherenkov counter – Electron identification

• Electromagnetic calorimeter– Photon identification and

energy measurement– Electron identification

7

p+p c+X J/+X e+e-+X

Cut parameters and peaksPhoton cut• Energy cut (E >0.3 GeV) • Electromagnetic shower profile• Fiducial cut (noisy EMC towers

are removed)• Charged particle veto (35cm x 35cm)

Event cut• |Zvertex|<30 cm

Electron cut • RICH nPMT>=2• pT>0.2 GeV• 0.5<Energy/momentum<2

Pair cut• 2.9<mass(e+e-)<3.3GeV

8

EMCal energy resolution (E)=58MeV (PbSc)(E)=42MeV (PbGl) @ E=500MeV

NJ/=3679Red : N+-=4040Blue : N+++N--=218

J/e+e-

Run-5+6p+p 02Run-6 p+p

0.6<pT<0.65GeV/cE>0.2GeV

mainly 02

PYTHIA simulation (c1, |y|<0.5)

J/ acceptance and c conditional efficiency if J/ is detected from

GEANT (PISA) simulation

9

pT,J/ (GeV/c) pT,c (GeV/c)

J/e+e-

~2% for |yJ/|<0.5

cJ/e+e-~10%

c acceptance~2%*10%=0.2%1/30,000 of produced c is detected by PHENIX central arm

Feasibility study using PYTHIA and GEANT simulation

Black : ForegroundBlue : BackgroundRed: Foreground-backgroundGreen : Normalization regions (0.1<M<0.3GeV and 0.6<M<0.8GeV)

Input Rc=0.32N(direct J/):N(c1J/):N(c2J/)=68%:16%:16% NJ/=3744

10

Feasibility study using simulation (continued)

11

Input Rc=0

Input Rc=0.32

Input Rc=0.68

Input Rc=1

Black : ForegroundBlue : BackgroundRed: Foreground-backgroundGreen : Normalization regions

Real data (Run-5 and Run-6 p+p 200GeV)

Black : ForegroundBlue : BackgroundRed: Foreground-backgroundGreen : Normalization regions

12

The fraction of J/ from c feed down (Rc) seems to be small.

Summary and outlook13

RHIC energy

• The contribution of c is important to understand the J/ data in heavy ion collisions.

• Search for the c meson via J/ decay in p+p collisions is ongoing.

• The fraction of J/ from c feed down (Rc) seems to be small.

• The Rc value will be obtained soon.

• More and more statistics are needed

(Run-8, 9, …) for detail study.

Backups

Acceptance of 0

Ratio of cross sections

1)(

)(

2

1

2

1

c

c

JBR

JBR

c

c

2

1

c

c

For simplicity, I assumed

Limited knowledge

Green : ABlue : pAAqua : ppbar

And I used mean of masses in simulation.(3510.66MeV+3556.20MeV)/2

Expected width of the convoluted peak (Gaussian sigma) is ~50MeV.

I neglect c0 contribution.

(c - J/) Mass (GeV/c2)

PHENIXRun 5 200GeV p+p

(c - J/) Mass (GeV/c2)

Previous result, Run-5 p+p 200GeVNJ/=960

Questions from audience

Question 1• Is it better to use muon pairs? The statistics of muon pairs are 5 tim

es larger than electron pairs.

• No. • The fraction of decays with J/ going to muon arm and gamma goin

g to central arm is small. • So, the statistics of J/+ in muon+central arms and the statistics in

central arm are almost equivalent. • But, the energy of gamma in muon+central arm configuration is low,

E~0.1(0.2)GeV. • This is worse situation than the central arm case with E~0.4GeV.

• Does the fact mean that the c measurement is not possible at RHIC-PHENIX?

• No. • It can be possible in p+p collisions as I showed, while the larger stati

stics is needed. • But, it is very hard in heavy ion collisions. • (The measurement with NCC+FVTX+muon trigger is interesting eve

n in heavy ion collisions, but the statistics are still necessary.)

Question 2

• Is it better to use the isolation cut (0 veto)?

• No.

• The acceptance of low-pT 0 is small, ~3% at 0 pT=0.6GeV/c (slide 15).

• So, the 0 veto will not be effective for the c measurement and it will introduce larger systematic error only.

Question 3

• You did not take into account the c0 contribution in your PYTHIA+GEANT simulation. Why?

• Because the branching ratio of c0J/ decay is small, only 1.3% (slide 3).

• If the production cross section of c0 is much larger than ones of c1 and c2, we cannot neglect it.

• But, we know the c0 fraction is not large in the fixed target experiment and Tevatron.

• So, the c0 contribution can be negligible.

Question 4• In the PYTHIA+GEANT simulation, the net (foreground-b

ackground) distribution is lower than zero at the lower side of the c peak. Why?

• There is correlation between J/ and in the foreground and most of come from 02 decays. The correlation leads high mass of J/ pairs. In the background, however, there is no correlation between J/ and , and the J/ pair mass is smaller. Therefore, the net distribution is lower than zero in simulation in the lower side. But, actually, this fact is not observed in the real data. Though, the J/ statistics is the same in the simulation and real data.

• Why don’t you include such effect in your background subtraction?

• I don’t believe the prediction capability of PYTHIA at such level.