Single-Spin Asymmetries in Star:Single-Spin Asymmetries in Star:

Plans and GoalsPlans and Goals for Near Future and Future Future Measurements for Near Future and Future Future Measurements of forward neutral mesons.of forward neutral mesons.

Steve HeppelmannSTAR collaboration

STARSTAR

�HERMES Collab., PRD64 (2001) 097101.�HERMES Collab., PLB 535 (2002) 85.�HERMES Collab., PRL 84 (2000) 4047.

s=20 GeV, pT=0.5-2.0 GeV/c:

�0 – E704, PLB261 (1991) 201.�+/- - E704, PLB264 (1991) 462.

p + p + X The intuitive message:1) A transverse proton which interacts at

large xf consists of a leading quark (u) for , (d) for

2) The leading quark carries much of the transverse spin of the proton.

3) Because the quark level single spin asymmetry for scattering of transversely polarized quarks is very small at high energy, the measured asymmetry comes from features of the scattering process not evident in leading twist perturbative QCD with collinear partons.

4) This is interesting. Models involve things like• Transverse parton distributions

(Orbital angular momentum).• Spin dependent fragmentation

(Collins Effect).• Higher twist calculations.

0,

Xepe 'Xpp

The Opacity of STAR to Photons as Seen Upstream of the Interaction Region.

Summary of Star Forward Measurements of Transverse

Asymmetries and Cross Sections

• An vs Xf

• pp→º+X Cross Sections

3.8

4.1

BackwardAngles

3.3

3.8

4.0

STAR Forward Calorimeter: An Evolving History

• Prototype FPD Proposal Dec 2000 – Approved March 2001 – Run 2 polarized proton data (published

2004 spin asymmetry and cross section)

• FPD proposal June 2002– Review July 2002

– Run 3 data pp dAu (Preliminary An Results)

• FMS Proposal Submitted Jan 2005. Near full Forward EM Coverage.

(hep-ex/0502040).

FMS (Fall 2006 NSF??)FMS (Fall 2006 NSF??) Forward Meson Spectrometer Forward Meson Spectrometer

ObjectivesObjectives• Measure gluon distributions as small x.

• Look for evidence of gluon saturation effects. Comparison between forward pions pp and in dAu interactions.

• Full characterization of events leading to single transverse spin asymmetry in pion production.– Sivers vs Collins Effect.

Transverse Spin Short Term Transverse Spin Short Term Objectives Objectives

• 1 Week1 Week Transverse Running (Current Run (Run 5)? not in current plan?) – 20 Times “Figure of Merit” (Polarization x

Integrated Luminosity”)

– Get Pt Dependence of AN at Fixed Rapidity

– Measure kT transverse asymmetry. • 10k back to back events.

Sivers EffectSivers Effect on two pion correlations. on two pion correlations.Correlation between kCorrelation between kT T

and transverse Spin of proton.and transverse Spin of proton.

11

22

The pThe pt1 t1 + p+ pt2 t2 ~ k ~ kT T correlated with spin.correlated with spin.

( )t td p k

STAR -FPD STAR -FPD PreliminaryPreliminaryCross SectionsCross Sections

6

5

13

3

B

N

pxdp

dE B

TN

F

Similar to ISR analysisJ. Singh, et al Nucl. Phys. B140 (1978) 189.

How large is kT?

6

( ) ( )~

2 ( )

( ) ( ) ~ ( ) 1 6

~ 6 ~ 6 ~ 0.22 /

~ 70 /

t t t tN

t

tt t t t t

t

tTN

t

T

d p k d p kA

d p

kd p k p k d p

p

kkA

p GeV c

k MeV c

FPD calorimeters in near position

Current FPD Detector• Both East and West of Interaction region we have:

•2 5x5 arrays of Pb Glass (up/down)•2 7x7 arrays of Pb Glass (North/South)

Star Detector CoverageBBC – scintillation countersEMC – EM calorimetersFPD – Forward calorimetersTPC – Charged Particle Tracking

• PYTHIA prediction agrees well with the inclusive 0 cross section at 3-4• Dominant sources of large xF production from:

● q + g q + g (22) + X

● q + g q + g + g (23) + X

g+g andq+g q+g+g

q+g

Soft processes

PYTHIA: a guide to the physicsForward Inclusive Cross-Section: Subprocesses involved:

q

gg

q g

STAR FPD

Back-to-back Azimuthal Correlationswith large

Midrapidity h tracks in TPC

• -0.75 < < +0.75

Leading Charged Particle(LCP)

• pT > 0.5 GeV/c LCP

Coi

cide

nce

Prob

abil

ity

[1/

radi

an]

S = Probability of “correlated” event under Gaussian

B = Probability of “un-correlated” event under constant

s = Width of Gaussian

Fit LCP normalized distributions and with

Gaussian+constant

Beam View Top View

Trigger by forward

• E > 25 GeV

• 4

]

]

PYTHIA (with detector effects) predicts

• “S” grows with <xF>

and <pT,>

• “s” decrease with

<xF> and <pT,>

PYTHIA prediction agrees with p+p data

Larger intrinsic kT

required to fit data

Statistical errors only

25<E<35GeV

45<E<55GeV

STARSTAR

STAR Preliminary

STAR Preliminary

Frankfurt, Guzey and Strikman, Frankfurt, Guzey and Strikman, J. Phys. G27 (2001) R23 [hep-ph/0010248].J. Phys. G27 (2001) R23 [hep-ph/0010248].

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.

Back to a kT Measurement

• For pT1 (triggered º)>2.5 GeV/c

• <pT2> (leading charged Track) ~1GeV/c

• Width of (pT1 - pT2) ~ 0.5 GeV/c

• 10,000 events …. 5 MeV/c measurement of <(pT1

- pT2)>.

• With 10,000 events and 50% polarization,With 10,000 events and 50% polarization,

the transverse spin asymmetry in the the kthe transverse spin asymmetry in the the kTT

observation should be significant if the Aobservation should be significant if the ANN is is

due to initial state trigger bias effects.due to initial state trigger bias effects.

•Current FPDCurrent FPDRegion of interest 2<XF<.6Larger rapidity, Lower Pt.

Smaller rapidity Larger Pt.

• FPD++FPD++•Direct Photon Direct Photon AAN N

MeasurementMeasurement

About Direct Photon AN

• For XF >= ~.5 Direct Photon Dominates

100FPDE

GeV

º photons separated in Small cell detector up to about 60 GeV

Large Cells to guarantee “single Photon”.

If 1 week of transverse running in the next month…… this could be installed If 1 week of transverse running in the next month…… this could be installed for fall 2005.for fall 2005.

FMS Design

FPD Calorimeters

Collins Effect• Correlation between transverse spinCorrelation between transverse spinand transverse polarization of large x quarkand transverse polarization of large x quark•Scattering preserves quark transverse spin.Scattering preserves quark transverse spin.•Fragmentation azimuthal dependence givesFragmentation azimuthal dependence gives rise to Arise to ANN asymmetry. asymmetry.

Two questions about Collins Effect

• Can Transversity of the scattered quark in the proton be measured with fragmentation dependence on quark transverse spin?

• Can this effect explain the AN of forward pion production? (can this provide a big enough kT -> jT bias?)

• The Answer to the first could be yes and even if the answer to the second is no.

• It is possible that Collins has little to do with AIt is possible that Collins has little to do with AN N but but is still the best way to measure the Nucleon is still the best way to measure the Nucleon Transverse structureTransverse structure..

• We can easily answer the second question, the first We can easily answer the second question, the first will take more work.will take more work.

Near Side Two pion Correlations

• In Full FMS, two forward º’s.

• In FTPC + FPD++ (FPD) º + charged track– FTPC energy resolution ~ 10 % for Pt=2.0

GeV/c track.

• Jet Fragmentation into two energetic particles.

• Clean measurement with full FMS.

V. Guzey, M. Strikman and W. Vogelsang, V. Guzey, M. Strikman and W. Vogelsang, Phys. Lett. B603 (2004) 173 [hep-ph/0407201].Phys. Lett. B603 (2004) 173 [hep-ph/0407201].

Pythia Simulation Pythia Simulation

STARSTARResultsResults

Dependence of RdAu

●

pp3

3

dAu3

3

dAu

ppdAu

dpEd

dpEd

NR

inelasticbinary

inelastic

Observe significant rapidity dependence, similar to expectations from models which suppress gluon density in heavy nuclei

y=0

As y grows

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

Example model (CGC):

STARSTAR

RdAu

for systematically below linear extrapolation of h data to =4, consistent with expectations that p + p h is isospin suppressed at large [Guzey, Strikman and Vogelsang, Phys. Lett. B 603, 173 (2004)]

New FMS CalorimeterLead Glass From FNAL E831804 cells of 5.8cm5.8cm60cmSchott F2 lead glass

Loaded On a Rental Truck for Trip To BNL

Summary of Forward Thinking with STAR

• FPD Prototype ->FPD->FPD++->FMS• Staged options for forward calorimeter improvements in

STAR over the next two years should bring many results soon.– AN pT dependence– <kT> measurement (Sivers Effect)– Near side correlations (Collins Effect)– Direct Photon AN

– Gluon Saturation in Nuclei– ?? AN from nuclear spectator ??

• A little more transverse running in the next month could pay off with much more data sooner.