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

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 k T in the initial parton distribution 2 PRL 92, (2004) √s=200 GeV, = 3.8 SPSP k T,q p p SPSP p p SqSq k T, π Large Transverse Single Spin Asymmetry(SSA) in forward meson production persists up to RHIC energy. Photons have asymmetry Jet vs. Photon sign flip predicted No asymmetry for the jet axis Di-jet, photon-jet not exactly back to back

STAR STAR Forward Pion Detector (FPD) 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, ~3.7) 3 Run6 Configuration

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) ) 4 Phys.Rev.Lett.101:222001, Black Points: Phys.Rev.Lett.101:222001,2008 From Spin2008 talk by J.Drachenberg, arXiv:

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

1) Nominally (perhaps not significantly) larger asymmetry for Eta than Pi0. 2) Large Uncertainty in Eta A N. STAR Previous Observation of Transverse SSA Forward Production of Eta Meson by FNAL Exp proton & protonproton & anti-proton

STAR Run6 FPD Acceptance for  0 and 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.  0 reconstruction efficiency starts to drop over 60GeV, where the separation between two photons for symmetric decay becomes on average less than 1 cell width. Fast Simulator The ratio of N(reconstructed particles) to N(generated particles with CoM within FPD) Etot(GeV) Z  Separation(FPD cell) Eta 00 Etot(GeV) Z  Separation(FPD cell) 7

Event Cuts 2 photon events E total >25GeV Hardware threshold nominally at 25GeV “Center Cut” for 2  CoM defined as STAR Event Selection for Run6 FPD Eta Analysis Etot: Detector summed energy Z  and photon separation: Fitted photon energy/locations Reconstructs on the entire FPD Vertex set at zero for all events  0 mass region with Center Cut in black  mass region with Center Cut in red Di-Photon Center of Mass (EN&ES) 8

STAR Eta Signal in Run6 FPD  0 Mass Cut Eta Mass Cut Center Cut 3 columns for 3 energy bins Each column shows a single plot in log and linear scale. Di-Photon Invariant Mass Spectra in 3 Energy Bins 9 A N (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  0 peaks.

STAR Mass Dependence of A N Forward asymmetry clearly reveals the shape of two mass resonances. There is an “asymmetry valley” in between  0 and Eta mass regions. 1.N photon = 2 2.E total > 40GeV 3.No Center Cut 4.Average Beam Polarization = 56% STAR 2006 PRELIMINARY 10

STAR A N (x F ) in  0 and Eta Mass Regions 1. N photon = 2 2. Center Cut (  and  ) 3. Pi0 or Eta mass cuts 4. 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. 11

*Assume mixing angle: 12 STAR Should A N be larger for Eta than π 0 ? Gluons or η has Isospin I=0. u quark has Isospin I=1/2 π 0 has Isospin I=1. But we expect both mesons to come from fragmentation of quark jets. 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 π 0 meson. Differences in fragmentation could relate to: Mass differences? Isospin differences? Role of Strangeness? But Collins Effect Should be suppressed when Z~1

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. 7.5 meters from interaction point East FPD (since runs 3) STAR STAR Forward Meson Spectrometer (FMS) 13

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

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

From SPIN08 talk by N. Poljack, arXiv: Estimate  tot. ≤ 1.2  stat. stat.errors only STAR Preliminary 16 STAR Preliminary Run8 FMS π 0 A N Azimuthal Angle Dependence of A N A N vs. xF  Consistent with previous measurements

Run STAR p T -dependence of π 0 A N Run8 Large solid angle of FMS allows simultaneous mapping of x F vs p T with greater statistics Blue: From Spin2008 talk by J.Drachenberg, arXiv: Black: Phys.Rev.Lett.101:222001,2008 F.o.M. was smaller in run8 than in run6  More statistics needed

The Sivers effect predicts spin dependent left/right bias in k T due to parton orbital angular momentum. 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. STAR Sivers Effect with Mid-Rapidity Di-Jets 11 22 spin di-jet bisector kTxkTx 18

STAR Sivers Effect with Mid-Rapidity Di-Jets PRL 99 (2007) The observed A N 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. 19

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=200GeV p+p collision. It was found to be consistent with pQCD calcuations. 2.From RHIC run3 to run8, the FPD measured large forward single spin asymmetry, A N, for  0. The x F dependence of A N 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 run6. 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 A N in Eta mass region to be ~4 standard deviation greater than the A N in  0 mass region from 55GeV to 75GeV. (x F =0.55~0.75) 4.Forward Meson Spectrometer (FMS), commissioned in RHIC run8, greatly enhances EM coverage of STAR. The preliminary results from run8 show that  0 A N (x F ) is consistent with previous measurement, while the azimuthal angle dependence of A N is as expected. With additional transverse running, we can also significantly improve our measurement of A N vs. p T utilizing much improved acceptance of the FMS Len Eun 20

STAR Back Up

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 A N. 2.Pion containing only minority quarks: large negative A N. 3.Pion containing both majority and minority quarks: intermediate positive A N. Pattern from Previous Transverse SSA Measurements of Forward Pion / Eta Production with High Energy Polarized Proton / Antiproton Beams STAR 6

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

STAR Mass Dependence of A N Yellow beam asymmetry clearly reveals the shape of two mass resonances. There is an “asymmetry valley” in between  0 and Eta mass regions. 1.N photon = 2 2.No energy cut 3.With Center Cut 4.Z  < Average Yellow Beam Polarization = 56% STAR 2006 PRELIMINARY 7

STAR A N (x F ) in  0 and Eta Mass Regions 1.N photon = 2 2.Center Cut (  and  ) 3. Pi0 or Eta mass cuts 4.Z  < 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. 8

Run8 FPD east result : arXiv: (James Drachenberg– SPIN08) Negative x F consistent with zero 20 STAR p T -dependence of π 0 A N for Negative xF