Transverse Spin Physics at STAR Len K Eun

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

2 STAR Transverse Spin Physics at STAR Large Transverse Single Spin Asymmetry(SSA) in forward meson production persists up to RHIC energy. • Collins effect: asymmetry comes from the transversity and the spin dependence of jet fragmentation. SP p No asymmetry for the jet axis PRL 92, 171801 (2004) p Sq k. T, π • Sivers effect: asymmetry comes from spin-correlated k. T in the initial parton distribution SP k. T, q Di-jet, photon-jet not exactly back to back p p √s=200 Ge. V, <η> = 3. 8 Photons have asymmetry Jet vs. Photon sign flip predicted

3 STAR Forward Pion Detector (FPD) Run 6 Configuration • STAR forward calorimeters have gone through significant upgrades since run 3. • In run 6, the original FPD remained in the east, while the west FPD was expanded to FPD++. • The east FPD consists of two 7 X 7 Pb-glass modules, EN and ES. During run 6, it was placed at the “far” position. (x-offset~30 cm, < >~3. 7)

4 STAR Forward 0 Single Spin Asymmetry(SSA) At √s=200 Ge. V, 0 cross-section measured by STAR FPD is consistent with the NLO p. QCD calculation. Results at < >=3. 3 and < >=3. 8 have been included in the DSS global pion fragmentation function analysis. (Phys. Rev. D 75(2007) 114010) Phys. Rev. Lett. 101: 222001, 2008 Black Points: Phys. Rev. Lett. 101: 222001, 2008 4 From Spin 2008 talk by J. Drachenberg, ar. Xiv: 0901. 2763

5 STAR p. T Dependence of AN For Fixed XF, the asymmetry AN does not fall with p. T as predicted by models and perhaps expected on very general grounds. NLO PQCD does describe the size and shape of this forward 0 cross section. Model calculations (Sivers, Collins or twist-3) can explain the XF dependence of AN. X Flat or increasing dependence of AN on p. T is very difficult to understand within any of these frameworks! Phys. Rev. Lett. 101: 222001, 2008

STAR Previous Observation of Transverse SSA Forward Production of Eta Meson by FNAL Exp 704 1) Nominally (perhaps not significantly) larger asymmetry for Eta than Pi 0. 2) Large Uncertainty in Eta AN. proton & anti-proton 6

7 STAR Run 6 FPD Acceptance for 0 and Eta Eto Zgg Separation(FPD cell) Fast Simulator The ratio of N(reconstructed particles) to N(generated particles with Co. M within FPD) t(G p 0 Eto t( Zgg • 7 x 7 FPD has limited acceptance for Eta mesons. At 40 Ge. V, a symmetrically decaying Eta needs to point to the center of the FPD to fit in. Acceptance improves greatly at higher energy. • 0 reconstruction efficiency starts to drop over 60 Ge. V, where the separation between two photons for symmetric decay becomes on average less than 1 cell width. Separation(FPD cell) e. V) Ge V)

8 STAR Event Selection for Run 6 FPD Eta Analysis Di-Photon Center of Mass (EN&ES) 0 mass region with Center Cut in black Event Cuts • 2 photon events • Etotal>25 Ge. V • Hardware threshold nominally at 25 Ge. V • “Center Cut” for 2 g Co. M defined as mass region with Center Cut in red • 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 STAR Eta Signal in Run 6 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. 0 Mass Cut Eta Mass Cut AN(x. F) will be reported for di-photon events in these two shaded mass regions. We will not separate contributions from backgrounds under the Eta and p 0 peaks.

10 STAR Mass Dependence of AN STAR 2006 PRELIMINARY 1. 2. 3. 4. Nphoton = 2 Etotal > 40 Ge. V No Center Cut Average Beam Polarization = 56% • Forward asymmetry clearly reveals the shape of two mass resonances. • There is an “asymmetry valley” in between 0 and Eta mass regions.

11 STAR AN(x. F) in 0 and Eta Mass Regions 1. 2. 3. 4. Nphoton = 2 Center Cut ( and f) Pi 0 or Eta mass cuts Average Beam Polarization = 56% For , the asymmetry in the Eta mass region is greater than 5 sigma above zero, and about 4 sigma above the asymmetry in the 0 mass region.

12 STAR Should AN be larger for Eta than π0 ? • Gluons or η has Isospin I=0. • u quark has Isospin I=1/2 • π0 has 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 π0 meson. Differences in fragmentation could relate to: • Mass differences? • Isospin differences? • Role of Strangeness? • But Collins Effect Should be suppressed when Z~1

13 STAR Forward Meson Spectrometer (FMS) New for Run 8: New for Run 8 FMS • Stack of 1264 lead glass cells, roughly 18 X 0 in z. • Located at far West side of Hall, at the opening to RHIC tunnel. Faces blue beam. East FPD (since runs 3) • 7. 5 meters from interaction point

14 STAR FMS Greatly Enhances STAR EM Coverage • Tracking • EMC Calorimeters TPD b. EMC -1<η<1 FTPC e. EMC 1<η<2 FMS 2. 5<η<4 FPD movable With installation of FMS, STAR EM calorimeter coverage spans most of the pseudo-rapidity region from -1<h<4.

15 Forward Meson Spectrometer Lead Glass From FNAL E 831 804 cells of 5. 8 cm 60 cm Schott F 2 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 Designed and built at UC Berkeley/SSL 15

16 STAR Preliminary Run 8 FMS π0 AN Azimuthal Angle Dependence of AN AN vs. x. F Consistent with previous measurements STAR Preliminary stat. errors only Estimate stot. ≤ 1. 2 sstat. From SPIN 08 talk by N. Poljack, ar. Xiv: 0901. 2828

17 STAR p. T-dependence of π0 AN • Large solid angle of FMS allows simultaneous mapping of x. F vs p. T with greater statistics Run 3+5+6 Run 8 F. o. M. was smaller in run 8 than in run 6 More statistics needed Black: Phys. Rev. Lett. 101: 222001, 2008 Blue: From Spin 2008 talk by J. Drachenberg, ar. Xiv: 0901. 2763

18 STAR Sivers Effect with Mid-Rapidity Di-Jets spin • The Sivers effect predicts spin dependent left/right bias in k. T due to parton orbital angular momentum. k. Tx 1 di-jet bisector 2 • For di-jets at mid-rapidity, this initial state k. T 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.

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 STAR Summary 1. The STAR Forward Pion Detectors (FPD) at RHIC measured cross-section for 0 meson in < >=3. 3~4. 0 region during √s=200 Ge. V p+p collision. It was found to be consistent with p. QCD calcuations. 2. From RHIC run 3 to run 8, the FPD measured large forward single spin asymmetry, AN, for 0. The x. F dependence of AN was qualitatively consistent with theoretical predictions. p. T dependence, however, differed significantly from predictions based on all currently existing models 3. In addition to 0, Eta mesons were observed in the east FPD during RHIC run 6. We measured the single spin asymmetry in the 0 and the Eta mass regions, at < >~3. 65 and x. F above 0. 4. We found the AN in Eta mass region to be ~4 standard deviation greater than the AN in 0 mass region from 55 Ge. V to 75 Ge. V. (x. F=0. 55~0. 75) 4. Forward Meson Spectrometer (FMS), commissioned in RHIC run 8, greatly enhances EM coverage of STAR. The preliminary results from run 8 show that 0 AN(x. F) 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. p. T utilizing much improved acceptance of the FMS Len Eun

STAR Back Up

STAR Pattern from Previous Transverse SSA Measurements of Forward Pion / Eta Production with High Energy Polarized Proton / Antiproton Beams 1. Majority valence quark in polarized proton. • for proton • for antiproton. 2. Minority valence quark • for proton • for antiproton. 1. Pion containing only majority quarks: large positive AN. 2. Pion containing only minority quarks: large negative AN. 3. Pion containing both majority and minority quarks: intermediate positive AN. 6

9 STAR Eta and 0 Energy Sharing (Zgg) Distribution 0 mass region with Center Cut Nevents Eta mass region with Center Cut Zgg

7 STAR Mass Dependence of AN STAR 2006 PRELIMINARY 1. 2. 3. 4. 5. Nphoton = 2 No energy cut With Center Cut Zgg < 0. 85 Average Yellow Beam Polarization = 56% • Yellow beam asymmetry clearly reveals the shape of two mass resonances. • There is an “asymmetry valley” in between 0 and Eta mass regions.

8 STAR AN(x. F) in 0 and Eta Mass Regions 1. 2. 3. 4. 5. Nphoton = 2 Center Cut ( and f) Pi 0 or Eta mass cuts Zgg < 0. 85 Average Yellow Beam Polarization = 56% For , the asymmetry in the Eta mass region is greater than 5 sigma above zero, and about 4 sigma above the asymmetry in the 0 mass region.

20 STAR p. T-dependence of π0 AN for Negative x. F consistent with zero Run 8 FPD east result : ar. Xiv: 0901. 2763 (James Drachenberg– SPIN 08)