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

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

3 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. Colliding-Beam Spin Physics: Only at RHIC

4 Located at far West side of Hall, at the opening to RHIC tunnel. Faces blue beam. 7.5 meters from interaction point. New for Run 8: Forward Meson Spectrometer (FMS) Stack of 1264 lead glass cells, roughly 18 X 0 in z. FPD (runs 3+) STAR Detector

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

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

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

8 (STAR), PRL 92 (2004) Data from Jan 2002 Suggests that asymmetry data can be described within the context of pQCD. Cross sections well described within pQCD framework at √s=200 GeV Distributions steeply falling with P T and X F. “Forward Neutral Pion Transverse Single Spin Asymmetries in p+p Collisions at sqrt(s)=200 GeV,” PRL 101 (2008) Runs 2, 3, 5, and 6 forward  0 asymmetry from FPD FPD set at different distances from beam for the different <  ranges. P T vs X F coverage of data

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

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

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

12 “Jet-like” events in the FMS can potentially allow integration over fragmentation products to measure Sivers effect. Inclusive  0 asymmetries can not distinguish these. “Collins effect”: asymmetry in fragmentation “Sivers effect”: asymmetry from initial state K T - spin 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 Plot from hep-ex/ STAR Preliminary Simulation and Data agree well for “jet-like” events

13 STAR 2006 PRELIMINARY Heavier mesons also accessible at high X F 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 Z  <0.85 Average Yellow Beam Polarization=56% S. Heppelmann, PANIC 2008

14 Inclusive  0 asymmetries include feed-down from higher mass decays. One source of  0 s is the spin 1  (782 MeV)   decays are accessible in the FMS through their  0  (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. Measurement of vector mesons at large x F

15 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 π 0 from the ω decay and one with  Choose π 0 to be pair with mass closest to 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→π 0 γ decays M(π 0 clusters) (GeV/c 2 ) Run 8 data STAR Preliminary All cuts applied (see below) Require that M(π 0 ) be within 0.1 GeV of GeV.  Selection Pair mass for all triples Kinematic cuts to reduce QCD background (real π 0 decays with a third EM-rich hadronic cluster in the FMS): P T (triplet)>2.5 GeV E(triplet)>30 GeV P T (photon cluster)>1.5 GeV P T (π 0 )>1 GeV.

16 Run 8 data Pythia+GEANT (background only) (simulation generation ongoing) STAR Preliminary Mass distribution of all triples 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 may be interesting.

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  0 s Outlook Run 9 (running now) should allow STAR to extend constraints on  G and also to begin W measurements. Possibility of future direct-  A N measurements with FMS. Possibility of future Drell-Yan A N with FMS. Run 8 measurements of A N vs P T for inclusive   s at high X F. Run 8 pp and dAu comparisons.

18 End

19 Kinematic cuts to reduce QCD background (real π 0 decays with a third EM-rich hadronic cluster in the FMS): P T (triplet)>2.5 GeV E(triplet)>30 GeV P T (photon cluster)>1.5 GeV P T (π 0 )>1 GeV. STAR Preliminary Comparison of data (black) to simulation (red) Run 8 data Pythia+GEANT Run 8 data Pythia+GEANT

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

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

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