Carl Gagliardi – d+Au and p+Au Collisions at RHIC 1

d+Au and p+AuCollisions at RHIC

Carl A. GagliardiTexas A&M University

Outline

• Jet quenching and d+Au as the control experiment• Small-x physics and saturation• Upgrade plans for the future

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 2

Hard scattering 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 and p+Au Collisions at RHIC 3

Suppression of high-pT inclusive hadron yields

• Central Au+Au collisions: factor of 4-5 suppression • pT > 5 GeV/c: suppression ~ independent of pT

PRL 91, 172302

Au+Au relative to p+pRAA

Binary scaling

Factor of 4-5

PRL 91, 072301

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 4

Jets and two-particle azimuthal distributions

p+p di-jet

• Trigger: track with pT>4 GeV/c• distribution: 2 GeV/c<pT<pT

trigger

• Normalize to number of triggers

trigger

PRL 90, 082302

At 200 GeV, p+p collisions show conventional di-jet structure expected for 2 2 partonic collisions in pQCD

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 5

Azimuthal distributions in Au+Au

Au+Au peripheral Au+Au central

pedestal and flow subtracted

PRL 90, 082302

Near side: peripheral and central Au+Au similar to p+p

Away side: strong suppression of back-to-back correlations in central Au+Au

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 6

PRL 91, 172302RCP

Theory vs. datapQCD-I: Wang, nucl-th/0305010pQCD-II: Vitev and Gyulassy, PRL 89, 252301Saturation: KLM, Phys Lett B561, 93

pT>5 GeV/c: well described by pQCD+jet quenching, but also by gluon saturation model (up to 60% central)

Final state

Initial state

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 7

Is suppression an initial or final state effect?

Initial state? gluon

saturation

Final state? partonic

energy loss

How to discriminate? Turn off final state d+Au collisions!

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 8

What RHIC found – Jet Quenching

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

Both are strongly suppressed in central Au+Au collisions at 200 GeV

STARSTAR

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 9

More Recent Focus – CorrelationsPHENIX, nucl-ex/0507004

STAR Preliminary

dN

2/d

Δφ

1dΔ

φ2/N

trig

φ1=φ1-φtrig

φ2=

φ2-φ

trig

Au+Au 10%

Obtain more detailed information regarding the underlying dynamics.

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 10

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.

yT es

px 2

~

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 11

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 and p+Au Collisions at RHIC 12

PHENIX and PHOBOS report similar effectsPRL 94, 082302 PRC 70, 061901(R)

Charged particles are suppressed in the forward direction in d+Au collsions

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 13

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 and p+Au Collisions at RHIC 14

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 and p+Au Collisions at RHIC 15

Another recent calculation

Very good description of the pT dependence of the BRAHMS d+Au → h- + X cross section at η = 3.2

(Dumitru, Hayashigaki, and Jalilian-Marian, hep-ph/0506308)

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 16

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 and p+Au Collisions at RHIC 17

Saturation physics at RHIC?

• Fundamental question regarding saturation – Where does it set in?

• Forward hadron production at RHIC samples similar x values as mid-rapidity production at the LHC– Complex interplay at the LHC– Will probably need p+p, p+Pb, and Pb+Pb – all at the

same √s – to unravel it fully

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 18

x values in saturation calculations

In CGC calculations, the BRAHMS kinematics corresponds to <xg> <~ 0.001 (Dumitru, Hayashigaki, and Jalilian-Marian, hep-ph/0506308)

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 19

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)

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 20

Comparing d+Au dN/dη to p+emulsion

PHOBOS attributes effects to limiting fragmentation

nucl-ex/0409021

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 21

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.

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., hep-ph/0501260.● Nikolaev and Schaefer, PRD 71 (2005) 014023.

Others?

A short list (probably incomplete)

● ...

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 22

<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 and p+Au Collisions at RHIC 23

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 and p+Au Collisions at RHIC 24

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

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 25

d+Au +X at 200 GeV

d+Au π0 cross section at η = 4.0 is well described by a LO CGC calculation with a K-factor of 0.8(Dumitru, Hayashigaki, and Jalilian-Marian, hep-ph/0506308)

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 26

dependence of RdAu

pp

dAuR

1972

1dAu

Observe significant rapidity dependence, similar to BRAHMS measurements and expectations from saturation framework.

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 27

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)

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 28

PYTHIA (with detector effects) predicts

• “S” grows with <xF> and <pT,>

• “s” decrease with <xF>

and <pT,>

PYTHIA prediction agrees with data

Larger intrinsic kT

required to fit data

Statistical errors only

25<E<35GeV

45<E<55GeV

STAR Preliminary

STARSTAR

FPD-TPC correlations in p+p

STAR Preliminary

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 29

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 and p+Au Collisions at RHIC 30

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 and p+Au Collisions at RHIC 31

25<E<35GeV

35<E<45GeV

HIJING predicts similar effects in d+Au as seen in p+p

Are there “trivial” differences between p+p and d+Au?

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 32

STARSTAR

Statistical errors only

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

Spp-SdAu= (9.0 ± 1.5) %

consistent with CGC picture

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

as expected by HIJING

25<E<35GeV

35<E<45GeV

Fixed as

E & pT grows

STAR Preliminary

STAR Preliminary

STAR Preliminary

STAR Preliminary

Correlations in d+Au

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 33

STAR Forward Meson Spectrometer upgrade

• 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<<+4

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 34

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 coincidence structure.

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

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 35

PHENIX Nosecone Calorimeter upgrade

PHENIX Forward Spectrometer:

Forward Silicon

charged particle tracking

Forward Calorimeter (em and hadronic)

W/Si calorimeter

0.9 < η < 3

energy and position measurements

γ/e/jet trigger

Forward muon system

muon tracking

muon trigger

Same upgrades on South side

NCC

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 36

Detection at forward / backward rapidity with Nosecone Calorimeters -

• Direct detection of neutral pions (0.9 < || <3.0).

• Large acceptance for high pT pions.• High energy photons.• Determines jet direction plus a rough energy measurement.• γ + jet coincidences and detection.

EMhad

Side view Front view

χc

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 37

PHENIX Endcap Silicon Vertex upgrade

Endcaps detect following by displaced vertex of muons:D μ + X B μ + XB J/ + X μ+ μ

IP

μ

B

Secondary vertex resolution ~133 m (endcap)

barrelendcapendcap

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 38

Gluon shadowing and spin structure function with VTX

Extracting gluon structure function– Vertex detector provides broad range in x in the predicted

shadowing region (x <= 10-2) and at larger x– Measure gluon shadowing in p+A versus p+p

GluonShadowingPredictions

coverage

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 39

Double parton correlationsCDF, PRL 79, 584

PRL 88, 031801

A-dependence of 4-jet yields in p+A collisions can be used to measure x1 – x2 momentum correlations within the proton.

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 40

Conclusions

• d+Au collisions have played a key role in understanding the physics of Au+Au collisions

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

• Both STAR and PHENIX have upgrade plans that will dramatically improve their forward capabilities

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

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 41

Carl Gagliardi – d+Au and p+Au Collisions at RHIC 42

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