Recent Spin and FMS Results at STAR

1. Recent Spin and FMS Results at STAR STAR Andrew Gordon Brookhaven National Laboratory Moriond-QCD March 14-21, 2009

2. Colliding-Beam Spin Physics: Only at RHIC 1) Longitudinal spin program at STAR Inclusive Jets s=200 GeV Strong constraint on the size of Δg from RHIC data for 0.05<x<0.2. STAR data contributes strongly to global fits as in D. deFlorian et al., PRL 101 072001, 2008. Run 9 currently running polarized protons at s=500 GeV for the first time. This will allow start of measurements of W bosons.

3. Colliding-Beam Spin Physics: Only at RHIC 2) Transverse Spin program at STAR Run 8 (2008) and earlier runs show large asymmetries in the forward region. New detector will allow higher kinematic reach and extensions beyond inclusive 0 data.

4. STAR Detector New for Run 8: Forward Meson Spectrometer (FMS) Stack of 1264 lead glass cells, roughly 18 X0 in z. Located at far West side of Hall, at the opening to RHIC tunnel. Faces blue beam. FPD (runs 3+) 7.5 meters from interaction point.

5. North-half, view from the hall Nearly contiguous coverage for 2.5<h<4.0. Run 8 FMS FMS provides nearly 20x the coverage of previous forward detectors at STAR FMS newly commissioned for Run 8 Run 5 FPD

6. Bunch-by-bunch polarization from colliding beams at STAR Run 8 Data STAR Preliminary Polarization x Asymmetry (uncalibrated) Bunch number at STAR East BBC (yellow) Expected spin up Expected spin down West BBC (blue) ~3.5 s (statistical) measurement of polarization per bunch per hour See also J. Kiryluk (STAR) ArXiv:hep-ex/0501072v1, 28 Jan 2005 Statistical uncertainties only

7. Why high XF at a Collider? High rapidity p’s (hp~4) from asymmetric partonic collisions p+p p0, hp=3.8, √s=200GeV Mostly high-x valence quark on low-x gluon <z> Fragmentation z nearly constant and high 0.7 ~ 0.8 <xq> NLO pQCD Jaeger, Stratmann, Vogelsang, Kretzer <xg> Which beam is polarized (not averaged over) probes different regions p0 Detector p p If Polarization here: XF>0 Valence quark spin effects If Polarization here: XF<0 Low x gluons and other partons

8. Data from Jan 2002 (STAR), PRL 92 (2004) 171801 Runs 2, 3, 5, and 6 forward p0 asymmetry from FPD Distributions steeply falling with PT and XF. PT vs XF coverage of data Cross sections well described within pQCD framework at √s=200 GeV FPD set at different distances from beam for the different <h> ranges. Suggests that asymmetry data can be described within the context of pQCD. “Forward Neutral Pion Transverse Single Spin Asymmetries in p+p Collisions at sqrt(s)=200 GeV,” PRL 101 (2008) 222001

9. Run 3, 5, and 6 asymmetry data: Theory can predict XF dependence based on Sivers function fits to p+/p- asymmetries… Run 6 Data L=6.8 pb-1 Yellow beam polarization = 562.6 % ds - ds AN = ds +ds dσ↑(↓)=differential cross section when proton has spin up (down). Data: B.I. Abelev et al. (STAR), PRL 101 (2008) 222001 Theory (red): M. Boglione, U. D’Alesio, F. Murgia [arXiv:hep-ph/0712.4240] Theory (blue): C. Kouvaris, J. Qiu, W. Vogelsang, F. Yuan, PRD 74 (2006) 114013

10. …but rising PT dependence is not predicted by the same fits XF>0.4 B.I. Abelev et al. (STAR) PRL 101 (2008) 222001 Data broken out in XF bins

11. FMS Acceptance allows azimuthal dependence to be measured Run 8 data Run 8 data P(blue)=45.5±3.3% L~6.2 pb-1 in plot AN versus <cos φ> for positive (blue beam) and negative (yellow beam) XF AN as a function of xF integrated over the FMS acceptance. Important confirmation of previous data Plots from Nikola Poljak, for STAR collaboration, “Spin-dependent Forward Particle Correlations in p+p Collisions at s = 200 GeV,” hep-ex/0901.2828, to be published as Spin 2008 conference proceedings.

12. “Jet-like” events in the FMS can potentially allow integration over fragmentation products to measure Sivers effect. “Collins effect”: asymmetry in fragmentation “Sivers effect”: asymmetry from initial state KT-spin correlations STAR Preliminary Inclusive p0 asymmetries can not distinguish these. 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 “Jet-shape” distribution of energy within jet-like objects in the FMS as a function of distance from the jet axis. Simulation and Data agree well for “jet-like” events Plot from hep-ex/0901.2828.

13. STAR 2006 PRELIMINARY Heavier mesons also accessible at high XF Di-photons in FPD with E(pair)>40 GeV No “center cut” (requirement that two-photon system point at middle of an FPD module) With center cut and Zgg<0.85 Average Yellow Beam Polarization=56% S. Heppelmann, PANIC 2008

14. Measurement of vector mesons at large xF Inclusive p0 asymmetries include feed-down from higher mass decays. One source of p0s is the spin 1 w (782 MeV). w decays are accessible in the FMS through their p0g (BR=9%) decay channel. These events are closer to the original fragmentation product. Spin-1 production is potentially interesting, and explictly occurs in some models of string fragmentation that provide an intuitive picture of asymmetry in fragmentation.

15. w Selection Look at all triplets of clusters with E>6 GeV. Apply fiducial cuts of 1/2 cell from all module boundaries Associate two of clusters with π0 from the ω decay and one with g Choose π0 to be pair with mass closest to 0.135 GeV, and photon from ω decay to be the third cluster. A fast, photons-only simulation indicates that this produces the correct identification for nearly 100% of ω→π0γ decays Require that M(π0) be within 0.1 GeV of 0.135 GeV. STAR Preliminary Run 8 data Pair mass for all triples • Kinematic cuts to reduce QCD background (real π0 decays with a third EM-rich hadronic cluster in the FMS): • PT(triplet)>2.5 GeV • E(triplet)>30 GeV • PT(photon cluster)>1.5 GeV • PT(π0)>1 GeV. All cuts applied (see below) M(π0 clusters) (GeV/c2)

16. Mass distribution of all triples STAR Preliminary For this plot simulation is background only Run 8 data Pythia+GEANT (background only) (simulation generation ongoing) Fit is gaussian + cubic polynomial  μ=0.784±0.008 GeV σ=0.087±0.009 GeV Scale=1339±135 Events Comparison to Run 8 dAu data may be interesting. Significant (10s) wp0g signal seen in the data.

17. Conclusions Large-acceptance FMS is online “jet-like” events hold the promise of a direct measurement of the Siver’s effect. Data should allow asymmetry measurements beyond inclusive p0s Outlook Run 8 measurements of AN vs PT for inclusive p0s at high XF. Run 8 pp and dAu comparisons. Run 9 (running now) should allow STAR to extend constraints on DG and also to begin W measurements. Possibility of future direct-g AN measurements with FMS. Possibility of future Drell-Yan AN with FMS.

18. End

19. Kinematic cuts to reduce QCD background (real π0 decays with a third EM-rich hadronic cluster in the FMS): • PT(triplet)>2.5 GeV • E(triplet)>30 GeV • PT(photon cluster)>1.5 GeV • PT(π0)>1 GeV. Comparison of data (black) to simulation (red) STAR Preliminary STAR Preliminary Run 8 data Run 8 data Pythia+GEANT Pythia+GEANT

20. Sizeable asymmetries can be used to measure beam polarization STAR BBC Inner tiles cover ~3.5<h<5 Can use large asymmetries to measure (relative) bunch-by-bunch polarization See also J. Kiryluk (STAR) ArXiv:hep-ex/0501072v1, 28 Jan 2005

21. Inner tiles of BBC accumulated every clock cycle { Scaler Boards } Discriminated phototube outputs 24-bit word is histogrammed every clock cycle { East-West Coincidence } Bunch Crossing (7-bits)

22. Inclusive Jets Jets at each pT are a different mix of subprocesses Smeared x-range for jets at a few pTs Phys. Rev. Lett. 97 (2006) 252001