Exploring Transverse Spin Physics at STAR Collaboration

1. STAR Transverse Spin Physics at STAR Len K. Eun For Collaboration 26th Winter Workshop on Nuclear Dynamics Ochos Rios, Jamaica, January 2010

2. 2 STAR Transverse Spin Physics at STAR • Collins effect: asymmetry comes from the transversity and the spin dependence of jet fragmentation. • Sivers effect: asymmetry comes from spin-correlated kT in the initial parton distribution Large Transverse Single Spin Asymmetry(SSA) in forward meson production persists up to RHIC energy. PRL 92, 171801 (2004) SP SP No asymmetry for the jet axis kT,q p p p p Sq kT,π Di-jet, photon-jet not exactly back to back √s=200 GeV, <η> = 3.8 Photons have asymmetry Jet vs. Photon sign flip predicted

3. 3 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 consists of two 7X7 Pb-glass modules, EN and ES. During run6, it was placed at the “far” position. (x-offset~30cm, <h>~3.7)

4. 4 STAR Forward 0 Single Spin Asymmetry(SSA) At √s=200GeV, 0 cross-section measured by STAR FPD is consistent with the NLO pQCD calculation. Results at <>=3.3 and <>=3.8 have been included in the DSS global pion fragmentation function analysis. (Phys.Rev.D75(2007) 114010) Phys.Rev.Lett.101:222001,2008  Black Points: Phys.Rev.Lett.101:222001,2008 4 From Spin2008 talk by J.Drachenberg, arXiv:0901.2763

5. 5 STAR pT Dependence of AN For Fixed XF, the asymmetry AN does not fall with pT as predicted by models and perhaps expected on very general grounds. • NLO PQCD does describe the size and shape of this • forward p0 cross section. • Model calculations (Sivers, • Collins or twist-3) can explain the XF dependence of AN. • X Flat or increasing dependence • of AN on pTis very difficult • to understand within any of • these frameworks! Phys.Rev.Lett.101:222001,2008

6. 6 STAR Previous Observation of Transverse SSA Forward Production of Eta Meson by FNAL Exp 704 • Nominally (perhaps not significantly) larger asymmetry for Eta than Pi0. • Large Uncertainty in Eta AN. proton & proton proton & anti-proton

7. 7 STAR Separation(FPD cell) Zgg p0 Separation(FPD cell) Etot(GeV) Zgg Etot(GeV) Run6 FPD Acceptance for p0 and Eta Fast Simulator The ratio of N(reconstructed particles) to N(generated particles with CoM within FPD) Eta • 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. • p0 reconstruction efficiency starts to drop over 60GeV, where the separation between two photons for symmetric decay becomes on average less than 1 cell width.

8. 8 Di-Photon Center of Mass (EN&ES) STAR h mass region with Center Cut in red p0 mass region with Center Cut in black Event Selection for Run6 FPD Eta Analysis • 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. 9 STAR p0 Mass Cut Eta Mass Cut Eta Signal in Run6 FPD 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 separate contributions from backgrounds under the Eta and p0 peaks.

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

11. 11 STAR For , the asymmetry in the Eta 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 Beam Polarization = 56%

12. 12 STAR Should AN be larger for Eta than π0 ? • Gluons or η has Isospin I=0. • u quark has Isospin I=1/2 • π0has Isospin I=1. • But we expect both 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. • For Collins Effect: Asymmetry reflects fragmentation of the quark jet into a leading η or π0meson. Differences in fragmentation could relate to: • Mass differences? • Isospin differences? • Role of Strangeness? • But Collins Effect Should be suppressed when Z~1

13. 13 STAR STAR Forward Meson Spectrometer (FMS) New for Run 8: 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. • 7.5 meters from interaction point East FPD (since runs 3)

14. 14 • EMC Calorimeters • bEMC -1<η<1 • eEMC 1<η<2 • FMS 2.5<η<4 • FPD movable STAR FMS Greatly Enhances STAR EM Coverage • Tracking • TPD • FTPC With installation of FMS, STAR EM calorimeter coverage spans most of the pseudo-rapidity region from -1<h<4.

15. 15 Forward Meson Spectrometer Lead Glass From FNAL E831 804 cells of 5.8cm5.8cm60cm Schott F2 lead glass Small Cell PSU Type 224 of 476 Cockcroft-Walton HV bases with computer control through USB. Designed/built in house for FEU-84. Designed and built at Penn State University QT board • Readout of 1264 channels of FMS provided by QT boards. Each board has • 32 analog inputs • 5-bit TDC / channel • Five FPGA for data and trigger • Operates at 9.38 MHz and higher harmonics • Produces 32 bits for each RHIC crossing for trigger • 12-bit ADC / channel 15 Designed and built at UC Berkeley/SSL

16. 16 STAR Preliminary Run8 FMS π0 AN AN vs. xF  Consistent with previous measurements Azimuthal Angle Dependence of AN STAR Preliminary stat.errors only Estimate stot. ≤1.2 sstat. From SPIN08 talk by N. Poljack, arXiv:0901.2828

17. 17 STAR pT-dependence of π0 AN • Large solid angle of FMS allows simultaneous mapping of xF vs pT with greater statistics Run3+5+6 Run8  Black: Phys.Rev.Lett.101:222001,2008 F.o.M. was smaller in run8 than in run6  More statistics needed Blue: From Spin2008 talk by J.Drachenberg, arXiv:0901.2763

18. 18 STAR Sivers Effect with Mid-Rapidity Di-Jets • The Sivers effect predicts spin dependent left/right bias in kT due to parton orbital angular momentum. • For di-jets at mid-rapidity, this initial state kT asymmetry leads to a spin dependent shift in the azimuthal opening angle for the jets. • In contrast, di-jet measurements are insensitive to the Collins effect, which produces an asymmetry within a jet. spin 1 di-jet bisector kTx 2

19. 19 STAR Sivers Effect with Mid-Rapidity Di-Jets PRL 99 (2007) 142003 • The observed AN is an order of magnitude smaller than what was seen in SIDIS by HERMES • Might be due to cancellations between initial state and final state, u and d quark, and/or very small gluon Sivers effect.

20. 20 STAR Summary • The STAR Forward Pion Detectors (FPD) at RHIC measured cross-section for p0 meson in <h>=3.3~4.0 region during √s=200GeV p+p collision. It was found to be consistent with pQCD calcuations. • From RHIC run3 to run8, the FPD measured large forward single spin asymmetry, AN, for p0. The xF dependence of AN was qualitatively consistent with theoretical predictions. pT dependence, however, differed significantly from predictions based on all currently existing models • In addition to p0, Eta mesons were observed in the east FPD during RHIC run6. We measured the single spin asymmetry in the p0 and the Eta mass regions, at <h>~3.65 and xF above 0.4. We found the AN in Eta mass region to be ~4 standard deviation greater than the AN in p0 mass region from 55GeV to 75GeV. (xF=0.55~0.75) • Forward Meson Spectrometer (FMS), commissioned in RHIC run8, greatly enhances EM coverage of STAR. The preliminary results from run8 show that p0 AN(xF) is consistent with previous measurement, while the azimuthal angle dependence of AN is as expected. With additional transverse running, we can also significantly improve our measurement of AN vs. pT utilizing much improved acceptance of the FMS Len Eun

21. STAR Back Up

22. 6 STAR Pattern from Previous Transverse SSA 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: large positive AN. • Pion containing only minority quarks:large negative AN. • Pion containing both majority and minority quarks: intermediate positive AN.

23. 9 STAR Eta and p0 Energy Sharing (Zgg) Distribution p0 mass region with Center Cut Eta mass region with Center Cut Nevents Zgg Zgg

24. 7 STAR Mass Dependence of AN • Nphoton = 2 • No energy cut • 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 Eta mass regions.

25. 8 STAR For , the asymmetry in the Eta 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 • Zgg < 0.85 • Average Yellow Beam Polarization = 56%

26. 20 STAR pT-dependence of π0 AN for Negative xF Negative xF consistent with zero Run8 FPD east result : arXiv:0901.2763(James Drachenberg– SPIN08)