STAR

1. STAR Measurement of Transverse Single Spin Asymmetry AN in Eta Mass Region at Large Feynman XF with the STAR Forward Pion Detector Len Eun BNL Mar 09

2. 1 STAR , Previous observation of Single Spin Transverse Asymmetry for Forward Production of Eta Meson by FNAL Exp 704. They reported: • Nominally (perhaps not significantly) larger asymmetry for Eta than Pi0. • Large Uncertainty in Eta AN. 2

3. 2 STAR Should AN be larger for Eta production than Pi0 production? • Gluons have Isospin I=0. Fragmentation to I=0 Eta mesons at large fragmention fraction Z may be enhanced for gluon jets. • But in PQCD (both Collins or Sivers models), we expect these mesons to come from fragmentation of quark jets. *Assume mixing angle: • For Sivers Effect: Asymmetry is in the jet and should not depend on the details of fragmentation. Do Eta’s and Pi0’s come from different kinds of jets? • For Collins Effect: Asymmetry reflects transverse quark polarization through transverse momentum dependence in the fragmentation of the quark jet to a leading Pi0 or Eta meson. Differences in fragmentation could relate to: • Mass differences? • Isospin differences? • Role of Strangeness?

4. 3 STAR Pattern from Previous Transverse Single Spin Asymmetry Measurements of Forward Pion/Eta Production with High Energy Polarized Proton/Antiproton Beams. • Majority valence quark in polarized proton. • for proton • for antiproton. • Minority valence quark • for proton • for antiproton. • Pion containing only majority quarks gets large positive AN. • Pion containing only minority quarks gets large negative AN. • Pion containing both majority and minority quarks (neutral pions) have intermediate positive AN.

5. 4 STAR STAR Forward Pion Detector (FPD) Run6 Configuration • STAR forward calorimeters have gone through significant upgrades since run3. • In run6, the original FPD remained in the east, while the west FPD was expanded to FPD++. • The east FPD is consisted of two 7X7 Pb-glass modules, EN and ES. During run6, it was placed at x-offset~30cm, <h>~3.7.

6. 5 STAR Forward p0 Single Spin Asymmetry At √s=200GeV, p0 cross-section measured by STAR FPD is consistent with the NLO pQCD calculation. Results at <h>=3.3 and <h>=3.8 have been included in the DSS global pion fragmentation function analysis. (Phys.Rev.D75(2007) 114010) We find that AN increases with xF, roughly consistent with theoretical predictions. Contrary to predictions, however, AN does not fall as a function of pT at fixed xF in this kinematic region. From Spin2008 talk by J.Drachenberg Accepted by PRL

7. 6 STAR Separation(FPD cell) Zgg p0 Separation(FPD cell) Etot(GeV) Zgg Etot(GeV) Acceptance and Reconstruction Fast Simulator The ratio of N(reconstructed particles) to N(generated particles whose center of mass falls within Forward Pion Detector) Eta Particle Energy (GeV) • 7x7 FPD has limited acceptance for Eta mesons. At 40GeV, a symmetrically decaying Eta needs to point to the center of the FPD to fit in. Acceptance improves greatly at higher energy. • The reconstruction efficiency for p0 starts to drop at E>60GeV, where the separation between two photons for symmetric decay becomes ~1 cell width.

8. 7 STAR Event Selection and Mass Reconstruction Di-Photon Center of Mass Distributions h mass region with Center Cut in red p0 mass region with Center Cut in black • Event Cuts • 2 photon events • Etotal>25GeV • Hardware threshold nominally at 25GeV • “Center Cut” for 2g CoM defined as • Etot: Detector summed energy • Zgg and photon separation: Fitted photon energy/locations • Reconstructs on the entire FPD • Vertex set at zero for all events

9. 8 STAR p0 mass region with Center Cut Eta mass region with Center Cut Mgg Mgg Nevents Zgg Zgg h and p0 Energy Sharing (Zgg) Distribution

10. 9 STAR p0 Mass Cut Eta Mass Cut Observation of Eta Signal Di-Photon Invariant Mass Spectra in 3 Energy Bins • Center Cut • 3 columns for 3 energy bins • Each column shows a single plot in log and linear scale. AN(xF) will be reported for di-photon events in these two shaded mass regions. We will not here separate possible contributions from backgrounds under the Eta and p0 peaks.

11. 10 STAR STAR 2006 PRELIMINARY Mass Dependence of AN • Nphoton = 2 • Etotal > 40GeV • No Center Cut • Average Yellow Beam Polarization = 56% • Yellow beam asymmetry clearly reveals the shape of two mass resonances. • There is an “asymmetry valley” in between p0 and h mass regions.

12. 10 STAR Mass Dependence of AN • Nphoton = 2 • Etotal > 40GeV • With Center Cut • Zgg < 0.85 • Average Yellow Beam Polarization = 56% STAR 2006 PRELIMINARY • Yellow beam asymmetry clearly reveals the shape of two mass resonances. • There is an “asymmetry valley” in between p0 and h mass regions.

13. 11 STAR For , the asymmetry in the h mass region is greater than 5 sigma above zero, and about 4 sigma above the asymmetry in the p0 mass region. AN(xF) in p0 and Eta Mass Regions • Nphoton = 2 • Center Cut (h and f) • Pi0 or Eta mass cuts • Average Yellow Beam Polarization = 56%

14. 12 STAR Epair>30GeV, Zgg<0.5 Epair>50GeV, Zgg<0.5 Eta Signal from FMS in Run 8 Event selection: one and only one 2-photon cluster, where a cluster is defined within a circle of radius 0.85 in h-j plane. (Over 2X Larger than an FPD module)

15. STAR Eta Signal from FMS in Run 8 Two photon cluster + one or more photon(s) Epair>30GeV, Zgg<0.5

16. 13 STAR Run 8 FMS Trigger Analysis In run 8, a simple high tower algorithm was used for trigger. The thresholds were 400ADC counts for the small cells, and 200ADC counts for the large cells. The nominal gain was targeted at (as I understand) 40ADC counts per GeV. The actual gains varied greatly from the nominal value, and the trigger ended up coming largely from relatively small number of cells.

17. 14 STAR p0Distributions in the FMS The following plots are in polar coordinates, r=(5-h) and q=j

18. STAR Summary • Previously, the STAR Forward Pion Detectors at RHIC (Brookhaven National Laboratory) have been used to successfully measure the forward single spin asymmetry, AN, for p0 meson in <h>=3.3~4.0 region. • In RHIC run 6, during √s=200 GeV p+p collisions, p0 and Eta mesons were observed in the east FPD. We measured and compared the single spin asymmetry in the p0 and the Eta mass regions, at <h>~3.65 and xF above 0.4. • AN as a function of the invariant mass reveals p0 and Eta resonance peaks. • From 55GeV to 75GeV, (xF=0.55~0.75) the average transverse single spin asymmetry in the Eta mass region was measured to be AN = 0.361 ± 0.064, about 4 standard deviations greater than the average AN in the p0 mass region. • Preliminary estimates of possible systematic effects show that the systematic uncertainties are considerably smaller than the statistical uncertainties. • Analysis of Run 8 data from the FMS is in progress. We have a very good Eta signal, but there are also features that need more work to fully understand. Len Eun

19. STAR A Note on Systematics and Background • Based on our recently published PRL on p0 AN, the systematic errors are well under 1% for all xF bins. This result would apply to high energy etas as well as p0s. Considering the size of statistical error for these high xF bins, we see that the statistical errors dominate the systematics. • Our published result for p0 asymmetry goes up to <xF>~0.6 ( <Ep0>=60 GeV). Beyond about 65 GeV, mis-reconstructed single photons could begin to contribute to the p0 background. These studies are ongoing. However, there is no such issue for Eta's at this energy. Based on the shape of the the mass distributions, one could assume that the scale of the backgrounds to Eta's in the( E_eta>50 GeV) region is no more than about 20%. However we do not make a claim about backgrounds! We are careful to restrict our presentation to analysis of reconstructed two photon events in the p0/h mass regions, which includes both the p0/h signals and unidentified backgrounds. We have not attempted to separate the two yet, and (I hope) we made that clear in slide#7. • We use the "cross ratio" method to calculate the asymmetry. This method is extremely insensitive to the changes in acceptance.