Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 1 Transverse Spin Studies with STAR at RHIC Carl A. Gagliardi Texas A&M University for the Collaboration STAR Outline Previous results Separating the underlying effects Current status Future plans

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 2

3 STAR detector layout

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 4 Transverse spin runs at STAR RHIC Run 2 (’01-’02) –First polarized pp run –Used prototype Endcap EMC for forward π 0 detection –~0.15 pb -1 with P beam ~ 16% –A N measurement at ~ 3.8 Run 3 (’03) –Forward π 0 detectors (FPD) installed –~0.25 pb -1 with P beam ~ 27% –Measurements at ~ ±3.7 and ~ ±4.0 Run 5 (’05) –25 hours of transverse data during June, ’05 –~0.07 pb -1 and P beam ~ 46% Run 6 (’06) –Started transverse data taking on April 7 th –FPD++ and full Barrel+Endcap EMCs installed –Measurements underway at both forward and mid-rapidities

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 5 First A N Measurement at STAR PRL 92, (2004) Sivers: spin and k  correlation in parton distribution functions (initial state) Collins: spin and k  correlation in fragmentation function (final state) Qiu and Sterman (initial state) / Koike (final state): twist-3 pQCD calculations, multi-parton correlations Can be described by several models: Similar to result from E704 experiment (√s=20 GeV, 0.5 < p T < 2.0 GeV/c) √s=200 GeV, = 3.8 STAR

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 6 Forward π 0 production at ISR energies 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 p T √s=23.3GeV√s=52.8GeV xFxF xFxF           Ed 3  dp 3 [  b/GeV 3 ] NLO calculations with different scales: p T and p T /2 Data-pQCD differences at p T =1.5GeV

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 7 pp  π 0 + X cross sections at 200 GeV NLO pQCD calculations by Vogelsang, et al. nucl-ex/ STAR The error bars are statistical plus point-to-point systematic Consistent with NLO pQCD calculations at 3.3 < η < 4.0 Data at low p T trend from KKP fragmentation functions toward Kretzer. PHENIX observed similar behavior at mid-rapidity.

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 8 Inclusive π 0 A N from runs 3+5 Sizable asymmetries for x F > 0.4 Back angle data consistent with A N ~ 0 ~ 6 σ from 0 for x F >0.4

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 9 Combined A N (p T ) for x F > 0.4 from runs 3+5 Online calibration of CNI polarimeter Data are consistent with the 1/p T expectation of pQCD

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 10 Separating Sivers and Collins effects Collins mechanism: asymmetry in the forward jet fragmentation Sivers mechanism: asymmetry in the forward jet or γ production SPSP k T,q p p SPSP p p SqSq k T, π Need to go beyond π 0 detection to jets and direct photons Sensitive to proton spin – parton transverse motion correlations Sensitive to transversity

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 11 Event yield with FPD when N γ > 1 ~16% arises from “jet-like” events with ≥3 observed photons Also expect γ/π 0 > 1 for single photons with x F > 0.5 Need to separate these contributions reliably

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 12 FPD  FPD++ for run 6 West end of the STAR interaction region Runs 3-5

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 13 Run 6 FPD  FPD++ for run 6 West end of the STAR interaction region

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 14 Run 6 FPD  FPD++ for run 6 West end of the STAR interaction region

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 15 FPD++ enables jet and direct photon detection Trigger on EM energy in small cells Large enough to integrate over a jet cone Large enough for direct photon isolation cuts

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 16 Projected sensitivity PYTHIA simulations Trigger on small cells; sum over entire side Vector sum of photon momenta reproduces most forward hard- scattered parton Expect 4~5 σ effect for 5 pb -1 if observed π 0 A N is due to Sivers effect L.C. Bland, hep-ex/

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 17 D. Boer and W. Vogelsang, Phys. Rev. D 69, (2004) Back-to-back di-jets: access to gluon Sivers function 5 pb -1 measurement will be sensitive at level of model predictions Measurements near mid-rapidity with STAR – search for spin-dependent deviation from back-to-back alignment > 7 GeV trigger jet > 4 GeV away side jet

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 18 Level-2 trigger monitoring output after ~0.84 pb -1 Clear back-to-back di-jet signature 6-fold symmetry from Level-0 (single jet) trigger condition Level-2 filter is insensitive to the granularity Occasional high- order bit problem in 1 of 5520 EMC towers

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 19 Di-jet k Tx >0Di-jet k Tx <0 Related to di-jet Δφ Level-2 trigger monitoring output after ~0.84 pb -1 Clear back-to-back di-jet signature 6-fold symmetry from Level-0 (single jet) trigger condition Level-2 filter is insensitive to the granularity

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 20 Run 7 and beyond

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 21 Run 7 and beyond Addition of FMS to STAR provides nearly continuous EMC from -1< η <+4 STAR Forward Meson Spectrometer

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 22 Conclusions STAR finds sizable A N for inclusive π 0 at forward rapidity Separating the Sivers contribution from the Collins contribution requires going beyond π 0 measurements Measurements are underway at both forward and mid- rapidity Stay tuned!

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 23 STAR The STAR Collaboration U.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 STAR

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Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 25 Possible mechanisms Sivers effect [Phys Rev D 41, 83 (1990); 43, 261 (1991)]: Flavor dependent correlation between the proton spin (S p ), proton momentum (P p ) and transverse momentum (k T ) of the unpolarized partons inside. The unpolarized parton distribution function f q (x,k T ) is modified to: Collins effect [Nucl Phys B396, 161 (1993)]: Correlation between the quark spin (s q ), quark momentum (p q ) and transverse momentum (k T ) of the pion. The fragmentation function of transversely polarized quark q takes the form:

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 26 NLO pQCD S. Kretzer Forward π 0 production at a hadron collider Large rapidity π production ( η ~4) probes asymmetric partonic collisions Mostly high-x quark + low-x gluon 0.3 < x q < < x g < 0.1 nearly constant and high ~ A probe of low-x gluons N N  qq gg ENEN xqpxqp xgpxgp   ENEN (collinear approx.)

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 27 x F and p T range of existing FPD data STAR

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 28 Separated x F and p T dependence Similar to ISR analysis J. Singh et al., Nucl. Phys. B140, 189 (1978).

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 29 Single Spin Asymmetry Definitions Definition: dσ ↑(↓) – differential cross section of   when incoming proton has spin up(down) Two measurement techniques Single arm calorimeter: R – relative luminosity (by BBC) P beam – beam polarization Two arms (left-right) calorimeter: No relative luminosity needed π 0, x F <0 π 0, x F >0 Left Right p p positive A N : more  0 going left to polarized beam