New Results In Forward Physicsat the STAR experiment at RHIC

L.C. BlandBrookhaven National LaboratoryQuantum Field Theory SymposiumSaclay, 25 April 2009

BRAHMS & PP2PP

STAR

PHENIX

AGS

LINACBOOSTER

Pol. H- Source

Spin Rotators(longitudinal polarization)

Siberian Snakes

200 MeV Polarimeter

RHIC pC PolarimetersAbsolute Polarimeter (H jet)

AGS pC PolarimeterStrong AGS Snake

Helical Partial Siberian Snake

PHOBOS

Spin Rotators(longitudinal polarization)

Siberian Snakes

25 April 2009 2

What are the goals?• How does the proton gets its spin from quarks and gluons?

• Where do conventional descriptions of particle production (e.g., perturbative QCD) apply?

• Are there new things to learn about partonic substructures at large x?

• Can we learn about low-x structure functions from hadronic interactions?

Edmond Iancu and Raju Venugopalan, Review for Quark Gluon Plasma 3, R.C. Hwa and X.-N. Wang (eds.), World Scientific, 2003 [hep-ph/0303204].

25 April 2009 3

<z>

<xq>

<xg>

• Large rapidity production ~4 probes asymmetric partonic collisions

• Mostly high-x valence quark + low-x gluon

• 0.3 < xq< 0.7

• 0.001< xg < 0.1

• <z> nearly constant and high 0.7 ~ 0.8

• Large-x quark polarization is known to be large from DIS

• Directly couple to gluons probe of low x gluons

NLO pQCDJaeger,Stratmann,Vogelsang,Kretzer

GeVspp 200,8.3,0

Why do forward production in a hadron collider?

pd

pAu

q

g

Q2 ~ pT2

s 2EN

ln(tan(2

))

xq xF / zEN

xqpxgp

xF 2E

s

z E

Eq

xg pT

se g

EN

(collinear approx.)

25 April 2009 4

STAR

Poletips provide unique windows for forward detectors

25 April 2009 5

1Brookhaven National Laboratory2University of California- Berkeley3Pennsylvania State University4IHEP, Protvino5Stony Brook University6Texas A&M University7Utrecht, the Netherlands

8Zagreb University

STAR Forward Calorimeter ProjectsF.Bieser2, L.Bland1, E. Braidot7, R.Brown1, H.Crawford2, A.Derevshchikov4, J.Drachenberg6, J.Engelage2, L.Eun3, M.Evans3, D.Fein3, C.Gagliardi6, A. Gordon1, S.Hepplemann3, E.Judd2, V.Kravtsov4, J. Langdon5, Yu.Matulenko4, A.Meschanin4, C.Miller5, N. Mineav4, D.Morozov4, M.Ng2, L.Nogach4, S.Nurushev4, A.Ogawa1, H. Okada1, J. Palmatier3, T.Peitzmann7, S. Perez5, C.Perkins2, M.Planinic8, N.Poljak8, G.Rakness1,3, A.Vasiliev4, N.Zachariou5

These people built the Forward Meson Spectrometer (FMS) and/or its components

25 April 2009 6

Early Results from STAR Forward Pion Detector (FPD)

View along beam from interaction point of left/right symmetric modular arrays

of lead-glass detectors

View along along axis of single lead-glass detector looking at

phototube

25 April 2009 7

Data: B.I. Abelev et al. (STAR), PRL 101 (2008) 222001Theory (red): M. Boglione, U. D’Alesio, F. Murgia, PRD 77 (2008) 051502Theory (blue): C. Kouvaris, J. Qiu, W. Vogelsang, F. Yuan, PRD 74 (2006) 114013

Spin Effects for Large-xF Neutral Pion Production

dσ↑(↓)=differential cross section when proton has spin up (down).

AN =d - d d + d

Run 6 Data

Even though the kinematics of the SIDIS measurement and the forward data have little overlap, it is possible to account for most of the features of the RHIC data by calculations based on phenomenological fits to the semi-inclusive DIS data

25 April 2009 8

Does pQCD describe particle production at RHIC?Compare cross sections measured for p+p +X at s=200 GeV

to next-to-leading order pQCD calculations

J. Adams et al. (STAR), PRL 97 (2006) 152302

Particle production cross sections for p+p collisions agree with NLO pQCD down to pT~2 GeV/c over a wide range, 0 < < 3.8, of pseudorapidity ( = -ln tan /2).

25 April 2009 9

Inclusive production from dAu

J. Adams et al. (STAR), PRL 97 (2006) 152302

Particle production cross sections for d+Au collisions are smaller than expectations from only shadowing at < =4.0, and the energy dependence is best described by CGC calculations (A. Dumitru et al. Nucl Phys A765 (2006) 464)

25 April 2009 10

Dependence of RdAu

• From isospin considerations, p + p h is expected to be suppressed relative to d + nucleon h at large [Guzey, Strikman and Vogelsang, Phys. Lett. B 603, 173 (2004)]

• Observe significant rapidity dependence similar to expectations from a “toy model” of RpA within the Color Glass Condensate framework.

y=0

As y grows

Kharzeev, Kovchegov, and Tuchin, Phys. Rev. D 68 , 094013 (2003)

See also J. Jalilian-Marian, Nucl. Phys. A739, 319 (2004)

J. Adams et al. (STAR), PRL 97 (2006) 152302

pp

dAu

pp

dAuinelasticdAubinary

inelasticpp

dAu

dpEd

dpEd

NR

1972

1

3

3

3

3

25 April 2009 11

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

consistent with CGC picture

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

as expected by HIJING

25<E<35GeV

Fixed as

E & pT grows

Forward + mid-rapidity correlations in d+Au

J. Adams et al. (STAR), PRL 97 (2006) 152302

25 April 2009 12

Run-8 Results from STAR Forward Meson Spectrometer

(FMS)

Full azimuth spanned with nearly contiguous electromagnetic calorimetry from -1<<4

approaching full acceptance detector

25 April 2009 13

Guzey, Strikman and Vogelsang Guzey, Strikman and Vogelsang Phys. Lett. B603 (2004) 173Phys. Lett. B603 (2004) 173

PYTHIA Simulation PYTHIA Simulation

• constrain x value of gluon probed by high-x quark by detection of second hadron serving as jet surrogate.

• span broad pseudorapidity range (-1<<+4) for second hadron span broad range of xgluon

• provide sensitivity to higher pT for forward reduce 23 (inelastic) parton process contributions thereby reducing uncorrelated background in correlation.

25 April 2009 14

d+Au d+Au +++X, pseudorapidity correlations with forward +X, pseudorapidity correlations with forward HIJIING 1.381 Simulations HIJIING 1.381 Simulations

• increased pT for forward over run-3 results is expected to reduce the background in correlation.

• detection of in interval -1<<+1 correlated with forward (3<<4) is expected to probe 0.01<xgluon<0.1 provides a universality test of nuclear gluon distribution determined from DIS.

• detection of in interval 1<<4 correlated with forward (3<<4) is expected to probe 0.001<xgluon<0.01 smallest x range until eRHIC.

• shadowing of gluon structure function is not expected to modify width of away-side peak in distribution caused by elastic parton scattering.

25 April 2009 15

STAR Forward Meson Spectrometer

Phys. Rev. Lett. 101:222001 (2008)

• 50 larger acceptance than the run-3 forward pion detector (FPD).

• azimuth for 2.5<<4.0

• Discriminate single from up to ~60 GeV

Runs 3-6

FPD

Run8

FMS

North half of FMS before closing

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“Jet-like” events in the FMSCone-based clustering of energy

near-side correlations

“Jet-shape” distribution of energy within jet-like objects in the FMS as a function of distance from the jet axis.

Caveat for Run 8: FMS data was acquired with a “high tower” trigger, which creates a bias towards “jets” that derive from a small number of high-z initial fragments

N. Poljak, SPIN08Plot from hep-ex/0901.2828

STAR Preliminary

Simulation and Data agree well for “jet-like” events

25 April 2009 17

STAR Detector• Large rapidity coverage for electromagnetic calorimetry (-

1<<+4) spanning full azimuth azimuthal correlations

• Run-8 was the first run for the Forward Meson Spectrometer (FMS)

25 April 2009 18

FMS-FPD comparison• Emulate FPD from run-8 FMS:

FMS photons: x>0cm

|TPC|<0.725 ; 3.8<FMS<4.1

pT(TPC)>0.5 GeV/c

30<EFMS<55 GeV

leading particle analysis

• Normalization requires further systematic studies

pile-up correction

vertex efficiency

run-3/run-8 trigger

• Reproduces Gaussian width and many other similarities

=0.949±0.121

=0.965±0.198

=1.073±0.335

=1.161±0.132

J. Adams et al. (STAR), PRL 97 (2006) 152302

Run-8 FMS preliminary results/E. Braidot QM09

25 April 2009 19

Azimuthal Correlations (3)

Unc

orre

cted

Coi

ncid

ence

P

roba

bilit

y (r

adia

n-1)

p+p+h±+X, s=200 GeV

requirements:

pT,>2.5 GeV/c

2.8<<3.8

h± requirements:

1.5<pT,h<pT,

h<0.9

• clear back-to-back peak observed, as expected for partonic 22 processes

• fixed and large trigger, with variable h map out Bjorken-x dependence

• of greatest interest for forward direct- trigger

E. Braidot, QM09

25 April 2009 20

Azimuthal Correlations (=0)

leading subleading

• Near-side and away-side correlations observed for forward- pair

• Clear increase of width of away-side peak as increases

Preliminary

25 April 2009 21

Azimuthal Correlations (3)

++X

+h±+X

Un

corr

ect

ed C

oin

cid

ence

Pro

bab

ilit

y (r

adia

n-1)

-lead

p+p d+Au

“GSV” Selection “GSV” Selection

2.5 GeV/c<pT(=3)

1.5 GeV/c<pT(||<0.9)<pT

(=3)

dAu – pp = 0.09±0.04

“GSV” selection leads to clear back-to-back peak with similar pp/dAu widths as expected by pQCD

E. Braidot, QM09

25 April 2009 22

Azimuthal Correlations (3)

++X

+h±+X

Un

corr

ect

ed C

oin

cid

ence

Pro

bab

ilit

y (r

adia

n-1)

-lead

p+p d+Au

“lower-pT” Selection “GSV” Selection

2.5 GeV/c<pT(=3)

1.5 GeV/c<pT(||<0.9)<pT

(=3)

dAu – pp = 0.09±0.04

“lower-pT” Selection

2.0 GeV/c<pT(=3)

1.0 GeV/c<pT(||<0.9)<pT

(=3)

dAu – pp = 0.19±0.03

Evidence of pT dependent azimuthal broadening of signal

E. Braidot, QM09

25 April 2009 23

Au-Side Multiplicity Dependence

FMS

bEMC

east BBCmultiplicity

d Au

Select events on east BBC phototube charge sum

• -5.0 < BBC < -3.4 (Au side)

• 18 counts on average from minimum ionizing particle

25 April 2009 24

Au-Side Multiplicity DependenceFMS+EMC events

Q FMS-bEMC

low

low Q

mid

mid Qhigh

high Q

p+p

d+Au

“lower-pT” Selection: 2.0 GeV/c<pT(=3), 1.0 GeV/c<pT

(||<0.9)<pT(<>=3)

• No forward multiplicity dependence of p+p (FMS-EMC) correlations

• Multiplicity dependence of d+Au (FMS-EMC) correlations is observed

Un

co

rrec

ted

Co

inc

ide

nc

e P

rob

ab

ility (ra

dia

n-1)

25 April 2009 25

Run 8 dataPythia+GEANT (background only) (simulation generation ongoing)

STAR Preliminary

Near-Side Correlations in FMSObservation of large-xF production

For this plot simulation is background only

Fit is gaussian + cubic polynomial μ=0.784±0.008 GeV σ=0.087±0.009 GeV Scale=1339±135 Events

Significant (10) 0 signal seen in the data.

Comparison to Run 8 dAu data and transverse SSA may be interesting.

p+p+X, s = 200 GeV

Etrip>30 GeV

pT,trip>2.5 GeV/c

pT( cluster)>1.5 GeV/c

pT()>1 GeV/c

A. Gordon, Moriond 2009

25 April 2009 26

Large-xF J/ ObservationC. Perkins (for STAR), Quark Matter 2009

2.5<clus<4.0

pT,clus>1 GeV/c

Epair > 60 GeV

p+pJ/+Xe+e¯ + X, s=200 GeV

4.5 significance

p+pJ/++Xe+e¯++X, s=200 GeV

2.9 significance

• high-xF J/ may have implications for intrinsic charm at large Bjorken-x in proton

• use to benchmark simulations for future transverse-spin Drell-Yan experiment

25 April 2009 27

Conclusions

• RHIC run 8 (dAu/pp) was a big success with 20x increase of forward acceptance at STAR enabling the study of pT depenence of azimuthal correlations.

• Run-8 FMS results reproduce run-3 FPD Gaussian widths and other features.

• Comparison of (FMS)+(EMC) for pp and dAu indicates azimuthal broadening in dAu.

• Data are qualitatively consistent with a pT dependent picture of gluon saturation of the gold nucleus.

25 April 2009 28

Outlook

• Extract for two forward for d+Au collisions

• Complete scan of pT range (from GSV to run-3)

• Scan for elastic parton scattering x dependence of nuclear gluon density

• Clustering: towards +jet or jet+jet

• Complete absolute normalizations and systematics studies