Carl Gagliardi – d+Au Collisions at STAR – DIS’06 1 d+Au Collisions at STAR Outline d+Au collisions and saturation physics at RHIC Recent STAR results.

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 1

d+Au Collisions at STAR

Outline

• d+Au collisions and saturation physics at RHIC• Recent STAR results• STAR plans for the future

Carl A. GagliardiTexas A&M University

for the CollaborationSTARSTAR

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 2

E-M Calorimeter

Time of    Flight

Projection           Chamber

STAR detector

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 3

Mid-rapidity p+p at RHIC and NLO pQCDPRL 91, 241803

PHENIX π0

Calculations byW. Vogelsang

At 200 GeV, pQCD does a very good job describing mid-rapidity yields

STAR (h++h-)/2BRAHMS (h++h-)/2

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 4

Mid-rapidity d+Au

PRL 91, 072304

Pedestal&flow subtracted

Inclusive yields and back-to-back di-hadron correlations are very similar in p+p and d+Au collisions

In contrast, Au+Au collisions are very different from p+p and d+Au – but that’s not the subject of this talk

STARSTAR

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 5

Mid-rapidity vs. forward rapidity

MidRapidity

ForwardRapidity

CTEQ6M• Gluon density can’t grow forever.• Saturation may set in at forward rapidity when gluons start to overlap.• Can be explored by comparing p(d)+A to p+p.

yT es

px 2

~

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 6

Forward particle production in d+Au collisionsBRAHMS, PRL 93, 242303

Sizable suppression in charged hadron production in d+Au collisions relative to p+p collisions at forward rapidity

BRAHMS

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 7

ln 1x related to rapidity of

produced hadrons.

As y grows

Expectations for a color glass condensate

D. Kharzeev, hep-ph/0307037

Iancu and Venugopalan, hep-ph/0303204

Are the BRAHMS data evidence for gluon saturation at RHIC energies?

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 8

Recent saturation model calculation

Very good description of the pT dependence of the BRAHMSd+Au → h− + X cross section at η = 3.2

(Dumitru, Hayashigaki, and Jalilian-Marian, NP A765, 464)

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 9

Is saturation really the explanation?

Difficult to explain BRAHMS results with standard shadowing, but in NLO pQCD calculations <xg> ~ 0.02 is not that small

(Guzey, Strikman, and Vogelsang, PL B603, 173)

In contrast, <xg> <~ 0.001 in CGC calculations(Dumitru, Hayashigaki, and Jalilian-Marian, NP A765, 464 )

Basic difference:

pQCD: 2 2 CGC: 2 1

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 10

Do we understand forward π0 production in p + p?

Bourrely and Soffer, EPJ C36, 371: NLO pQCD calculations underpredict the data at low s from ISR Ratio appears to be a function of angle and √s, in addition to pT

√s=23.3GeV √s=52.8GeV

xF xF

Ed

3 d

p3 [b

/GeV

3 ]

Ed

3 d

p3 [b

/GeV

3 ]

NLO calculations with different

scales:

pT and pT/2

Data-pQCD differences

at pT=1.5GeV

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 11

The error bars are statistical plus point-to-point systematic

Consistent with NLO pQCD calculations at 3.3 < η < 4.0

Data at low pT trend from KKP fragmentation functions toward Kretzer. PHENIX observed similar behavior at mid-rapidity.

p+p +X at 200 GeV

nucl-ex/0602011

STARSTAR

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 12

d+Au +X at 200 GeV

pT dependence of d+Au π0 cross section at <η> = 4.0 is best described by a LO CGC calculation.

(Dumitru, Hayashigaki, and Jalilian-Marian, NPA 765, 464)

nucl-ex/0602011

STARSTAR

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 13

dependence of RdAu

nucl-ex/0602011

STARSTAR

pp

dAudAuR

197*2

1

• Observe significant rapidity dependence.

• pQCD calculations significantly over predict RdAu.

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 14

Any difference between p+p and d+Au?

Kharzeev, Levin, McLerran gives physics picture (NPA748, 627)

d+Au: Mono-jet?

PT is balanced by many gluons

Dilute parton system

(deuteron)

Dense gluon

field (Au)

Color glass condensate predicts that the back-to-back correlation from p+p should be suppressed

p+p: Di-jet

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 15

Back-to-back correlations with the color glass

(Kharzeev, Levin, and McLerran, NP A748, 627)

The evolution between the jets makes the correlations disappear.

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 16

25<E<35GeV

35<E<45GeV

• HIJING predicts similar correlations in d+Au as PYTHIA predicts for p+p.

• Only significant difference is combinatorial background level.

Forward + mid-rapidity di-hadron correlations

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 17

STARSTAR• are suppressed at small <xF> and <pT,π>

consistent with CGC picture

• are similar in d+Au and p+p at larger <xF> and <pT,π>

as expected by HIJING

25<E<35GeV

Fixed as

E & pT grows

Forward + mid-rapidity correlations in d+Au

nucl-ex/0602011

π0: |<η>| = 4.0h±: |η| < 0.75; pT > 0.5 GeV/c

<pT,π> ~1.0 GeV/c

<pT,π> ~1.3 GeV/c

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 18

STAR Forward Meson Spectrometer

• FMS increases areal coverage of forward EMC from 0.2 m2 to 4 m2

• Addition of FMS to STAR provides nearly continuous EMC from -1<η<+4

• Available for Run 7

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 19

p+p and d+Au ++X correlations with forward

hep-ex/0502040p+p in PYTHIA d+Au in HIJING

Conventional shadowing will change yield, but not angular correlation. Saturation will change yield and modify the angular correlation.

Sensitive down to xg ~ 10-3 in pQCD scenario; few x 10-4 in CGC scenario.

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 20

Conclusions

• Forward rapidity inclusive π0 production at RHIC is well described by pQCD calculations

• d+Au results at RHIC provide hints that saturation effects are becoming important

• Future STAR measurements will elucidate the dynamics underlying forward inclusive particle suppression at RHIC

• RHIC may be the ideal accelerator to explore the onset of saturation

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 21

The STARSTAR CollaborationU.S. Labs:

Argonne, Lawrence Berkeley, and Brookhaven National Labs

U.S. Universities: UC Berkeley, UC Davis, UCLA, Caltech, Carnegie Mellon, Creighton, Indiana, Kent State, MIT, MSU, CCNY, Ohio State, Penn State, Purdue, Rice, Texas A&M, UT Austin, Washington, Wayne State, Valparaiso, Yale

Brazil: Universidade de Sao Paolo

China: IHEP - Beijing, IPP - Wuhan, USTC,Tsinghua, SINAP, IMP Lanzhou

Croatia: Zagreb University

Czech Republic: Nuclear Physics Institute

England:University of Birmingham

France: Institut de Recherches Subatomiques Strasbourg, SUBATECH - Nantes

Germany: Max Planck Institute – Munich University of Frankfurt

India:Bhubaneswar, Jammu, IIT-Mumbai, Panjab, Rajasthan, VECC

Netherlands:NIKHEF/Utrecht

Poland:Warsaw University of Technology

Russia: MEPHI – Moscow, LPP/LHE JINR – Dubna, IHEP – Protvino

South Korea:Pusan National University

Switzerland:University of Bern

STARSTAR

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 22

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 23

Pseudo-rapidity yield asymmetry vs pT

Back/front asymmetry in 200 GeV d+Au consistent with general expectations of saturation or coalescence; doesn’t match pQCD prediction.

Au direction / d direction

PRC 70, 064907STARSTAR

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 24

Nuclear Gluon Density

World data on nuclear DIS constrains nuclear modifications to gluon density only for xgluon > 0.02

e.g., see M. Hirai, S. Kumano, T.-H. Nagai, Phys. Rev. C70 (2004) 044905 and data references therein

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 25

One calculation within the saturation picture

RdAu

RCP

Saturation model calculation with additional valence quark contribution (Kharzeev, Kovchegov, and Tuchin, PL B599, 23)

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 26

x values in saturation calculations

In CGC calculations, the BRAHMS kinematics corresponds to <xg> <~ 0.001 (Dumitru, Hayashigaki, and Jalilian-Marian, NP A765, 464 )

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 27

Many recent descriptions of low-x suppression

Saturation (color glass condensate)● Jalilian-Marian, NPA 748 (2005) 664.● Kharzeev, Kovchegov, and Tuchin, PLB 599 (2004) 23; PRD 68 (2003) 094013.● Armesto, Salgado, and Wiedemann, PRL 94 (2005) 022002.● Dumitru, Hayashigaki, and Jalilian-Marian, NPA 765 (2006) 464

Multiple scattering● Qiu and Vitev, PRL 93 (2004) 262301; hep-ph/0410218.

Shadowing● R. Vogt, PRC 70 (2004) 064902.● Guzey, Strikman, and Vogelsang, PLB 603 (2004) 173.

Parton recombination● Hwa, Yang, and Fries, PRC 71 (2005) 024902.

Factorization breaking● Kopeliovich et al., PRC 72 (2005) 054606.● Nikolaev and Schaefer, PRD 71 (2005) 014023.

Others?

A short list (probably incomplete)

● ...

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 28

<z>

<xq>

<xg>

NLO pQCD

S. Kretzer

p p 0, 3.8, s 200GeV

Forward π0 production at a hadron collider

• Large rapidity π production (η~4) probes asymmetric partonic collisions

• Mostly high-x quark + low-x gluon

• 0.3 < xq< 0.7

• 0.001< xg < 0.1

• <z> nearly constant and high ~ 0.7-0.8

• A probe of low-x gluons

N

N

q

g

s 2EN

ln(tan(2

))

xq xF / zEN

xqpxgp

xF 2E

s

z E

Eq

xg pT

se g

EN

(collinear approx.)

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 29

Constraining the x-values probed in hadronic scatteringGuzey, Strikman, and Vogelsang, Phys. Lett. B 603, 173

Measure two particles in the final state to constrain the x-values probed

Log

10(x

Glu

on)

Gluon

TPC

Barrel EMC

FTPC FTPC

FPDFPD

For 22 processes

• FPD: || 4.0

• TPC and Barrel EMC: || < 1.0

• Endcap EMC: 1.0 < < 2.0

• FTPC: 2.8 < < 3.8

Collinear partons:

● x+ = pT/s (e+1 + e+2)

● x = pT/s (e1 + e2)

Log10(xGluon)