DSPIN-2009, September 1-5 1

L. Nogach, IHEP, Protvinofor the STAR Collaboration

Measurements of transverse spin effects in the forward region with

STAR detector

STASTARR

Outline:• Motivation • STAR detector• Inclusive 0 measurements• More possibilities with the FMS• 2009 run at √s=500 GeV• Summary and outlook

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Motivation

• Contrary to simple pQCD predictionsContrary to simple pQCD predictions,, f first measurements of production in production in pp↑p↑p collisions found collisions found large transverse single spin asymmetries. Similar large large transverse single spin asymmetries. Similar large effects have recently been found in effects have recently been found in ee++ee-- and semi- and semi-inclusive deep-inelastic scattering.inclusive deep-inelastic scattering.

• Significant developments in theory in the past few years Significant developments in theory in the past few years suggest common origins for these effects, but suggest common origins for these effects, but large large transverse spin asymmetries intransverse spin asymmetries in pp↑+p↑+p → → + X+ X productionproduction are not yet fully understood.are not yet fully understood.

• More fundamental question: More fundamental question: what is the underlying dynamics in kinematics where transverse spin effects are observed?

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Polarized pp collisions at RHIC

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

Spin Rotators

Siberian Snakes

pp run Year 2002 2003 2005 2006

2008

<Polarization>, % 15 30 45 55 50

Lint [pb-1] at STAR (T) 0.15 0.25 0.1 6.8 ~7.8

FOM=P2L

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STAR detector layout

FPD

FPD• Modular detector

• Small xF-pT range compared to FMS

FMS• 20x acceptance of previous forward detectors at STAR• Full azimuthal coverage for 2.5 < η < 4.0• Array of 1264 Pb-glass cells

FPD

FMS

TPC

BBC

FMS

BEMC

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PRL 92, 171801 (2004)

√s=200 GeV, <η> = 3.8

Asymmetry revealed at lower energiespersists at √s=200 GeV

PRL 101, 222001 (2008)

• AN at positive xF grows with increasing xF

• xF dependence matches theoretical model expectations qualitatively

• pT dependence at fixed xF is not consistent with 1/pT expectation of theoretical model calculations

U.D’Alesio, F.MurgiaPhys. Rev. D 70, 074009 (2004)arXiv:0712.4240

C.Kouvaris, J.Qiu, W.Vogelsang, F.Yuan, Phys. Rev. D 74, 114013 (2006)

Published measurements with the FPD (20022006)

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InclusiveAN measurements (2006)

Asymmetry for mesonsis larger than for 0 (similar tothe E704 measurements): (AN) = 0.361 ± 0.064for 0.55<xF<0.75

arXiv:0905.2840

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Inclusive 0 AN with the FMS (2008)

Octant subdivision of FMS for inclusive spin sorting

arXiv:0901.2828

• Azimuthal dependence appears to be as expected

• AN is comparable to prior measurements with the FPD

x

y

P

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negative xF

arXiv:0901.2763 +A.Ogawa @CIPANP09

positive xF

AN(pT) at |xF|>0.4 with the East FPD/FMS (2008)

• AN for negative xF consistent with zero• Indication of AN for positive xF persists up to pT ~5 GeV/c• Needs more transverse spin running

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High xF vector mesons

Background only MCRun8 FMS dataFit is Gaussian + p3

μ=0.784±0.008 GeV σ=0.087±0.009 GeV Scale=1339±135 Events

3 photon events to look for0BR

• pT(triplet)>2.5 GeV/c • E(triplet)>30 GeV• pT(photon cluster)>1.5 GeV/c • pT(0)>1 GeV/c

Significant (10) 0 signal seen in the data

arXiv:0906.2332 possibility to measure spin-1 meson AN

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First look at “jet-like” events in the FMSEvent selection:

• “Jet shape” in data matches simulation well• Reconstructed mass does not match as well• High-tower trigger used in Run 8 biases jets

• >15 cells with energy > 0.4GeV in the event

(no single pions in the event)• cone radius = 0.5 (η-φ space)

• “Jet-like” pT > 1 GeV/c, xF > 0.2

• 2 perimeter fiducial volume cut (small/large cells)

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

arXiv:0901.2828

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Forward 0-0 azimuthal correlations

• Possible back-to-back di-jet/di-hadron Sivers measurement• Possible near-side hadron correlation for Collins fragmentation function/ Interference fragmentation function + Transversity

• Low-x / gluon saturation study – accessing lowest xBjgluon

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High-xF, high mass dilepton pairs are difficult to reconstruct

Benchmark for Drell-Yan:First look at J/ψ → e+ + e- in the FMS

Reconstructed 2-cluster invariant mass Fit with Gaussian + Offset

Gaussian fit parameters:– μ = 3.080 ± 0.020 GeV/c2

– σ = 0.082 ± 0.026 GeV/c2

– χ2/d.o.f. = 20.83/26– Significance from the fit 4.5 σ

Cuts applied:– E_pair > 60.0 GeV– zγγ < 0.7– Isolation radius: 0.4 η-φ – pT_cluster > 1.0 GeV/c

First high-xF J/ψ measurement at √s > 62 GeV

arXiv:0907.4396

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2009 – first physics run at √s=500 GeV

First look atevent reconstruction in the FPD:• using matrix+preshower (no SMD data)• 20 GeV < Etotal < 80 GeV• fixed vertex (z=0), no minbias condition• N=2

Luminosity: 56x56 bunches with 1.8x1011 intensity; 10 pb-1 integratedPolarization: ~30% (longitudinal)

Forward Pion Detector module schematic:

7x7 matrix of lead glass cells

Shower Maximum Detector

preshower(7 Pb-glass cells)

lead converter

fit by Gaussian+”gamma” function

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2009 data at √s=500 GeV

FPD measures energy up to ~200 GeV ═

SMD information is required to reconstruct pions above ~60 GeV

Example of 2-photon event when two clusters significantlyoverlap in the matrix, but are clearly separated in the SMD

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Forward Hadron Calorimeter• Proposed to be staged at a minimal z from the FMS symmetrically of DX magnet• two matrices of 9x12 Pb-scintillator detectors

• real jets physics with FMS+FHC• polarization transfer coefficients through measurement of polarization in the n0 channel

first simulations of →n0

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Summary

• Large transverse single spin asymmetries at large are observed up to √s=200 GeV.

• High precision inclusive AN measurements with the FPD allow for a quantitative comparison with theoretical models.

• FMS allows to look at heavier mesons, “jet-like” events and particle correlations.

• Essential to go beyond inclusive production to disentangle dynamical origins.

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Outlook

• Measurements of AN in inclusive 0 production at √s=500 GeV (Run 11?).• Extend forward calorimetry to have hadronic capability (FHC) to

measure full jets and to study inclusive production.

• Extend measurements of transverse single spin asymmetries from hadron production to prompt photon and jets.

• Develop RHIC experiment for a future measurement of transverse single spin asymmetries for Drell-Yan production of dilepton pairs.

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Backups

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Single Spin Asymmetry

• Definition:

dσ↑(↓) – differential cross section of when incoming proton has spin

up(down)

Two methods of measurements: • Single arm calorimeter:

R – relative luminosity (by BBC)

Pbeam – beam polarization

• Two arm (left-right) calorimeter:

No relative luminosity needed

dd

ddAN

L

LR

RNN

RNN

PA

beamN

1

LRRL

LRRL

BeamN

NNNN

NNNN

PA

1

0, xF<0 , xF>0

Left

Right

p p

positive AN: more 0 going

left to polarized beam

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Possible mechanisms• Sivers effect [Phys. Rev. D 41, 83 (1990); 43, 261 (1991)]: Flavor dependent correlation between the proton spin (Sp), proton momentum

(Pp) and transverse momentum (kT) of the unpolarized partons inside. The unpolarized parton distribution function fq(x,kT) is modified to:

• Collins effect [Nucl. Phys. B396, 161 (1993)]: Correlation between the quark spin (sq), quark momentum (pq) and transverse

momentum (kT) of the pion. The fragmentation function of transversely polarized quark q takes the form:

According to the latest theoretical developments, both mechanisms contribute to AN

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Separating Sivers and Collins effects

Collins mechanism: asymmetry in the forward jet fragmentation

Sivers mechanism: asymmetry in the forward jet or production

SPkT,q

p

p

SP

p

p

Sq kT,π

To discriminate between the two effects we need to go beyond 0 detection to direct photons or jet-like events

Sensitive to proton spin – parton transverse motion correlations

Sensitive to transversity

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0 inclusive cross section

PRL 97, 152302 (2006)nucl-ex/0602011

Cross-section is consistent with NLO pQCD calculations

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Forward 0 – midrapidity azimuthal correlations

pQCD inspired “GSV cuts” (Guzey, Strikman and Vogelsang, hep-ph/0407201):• |ηTPC| < 0.9 ; 2.8 < ηFMS < 3.8• 2.5(2.0)GeV < pT

FMS • 1.5(1.0)GeV < pT

TPC < pTFMS

• |zγγFMS|< 0.7, 0.07 < Mγγ < 0.30 GeV • only leading particle considered, corrected for pile-up• as proposed in hep-ex/0502040

arXiv:0907.3473

• Possible back-to-back di-jet/ di-hadron Sivers measurements• Low-x / gluon saturation study• Step-stone towards transverse spin forward photon-jet

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Future runs at RHIC

Year Likely Beam Species Science Goal

FY10 Au+Au at 200, 62.4 GeV,

assorted lower energy

low-mass dilepton spectrum; early collision temp.; improved jet quenching studies; begin energy scan for critical point

FY11 Subinjection Au+Au;

500 GeV p+p;

short 200 GeV U+U

continue critical pt. search; gluon pol’n at low x + antiquark pol’n from W production; 1st characterization of deformation effects in U+U centrality distributions

FY12 Au+Au at 200 GeV;

500 GeV p+p

RHIC-II heavy ion goals: heavy flavor, jet, quarkonium, multi-particle correlations; antiquark polarization in proton

FY13 200 GeV p+p; further heavy ion running to complement earlier runs

continue RHIC-II heavy ion goals; transverse spin asymmetries for +jet; pp reference data for new subsystems

FY14 200 GeV Au+Au; low-E Au+Au dictated by Run10+11 results

continuepursuit of+jet; energy scal and identified heavy flavor