Understanding Transverse Single Spin Asymmetry

1. STAR Apr 07 Forward Spin Physics at STARRHIC, BNL Understanding Transverse Single Spin Asymmetry Len EunPenn State University(For the STAR Collaboration)

2. First Detector for STAR Forward Spin STAR Apr 07 1 Beam Pipe Forward Pion Detector • 2 5x5 arrays of small Pb glass cells (up/down) • 2 7x7 arrays of small Pb glass cells (North/South) • Initially installed on both east and west sides. • For Run 6, west side was expanded to FPD++. Len Eun

3. Increased Acceptance in Run 6 STAR Apr 07 2 Forward Pion Detector ++ • Hybrid grid consisted of 6X6 small cells at the center and 168 larger cells surrounding them. • Covers wider range of rapidity than FPD • Prototype for the upcoming FMS Len Eun

4. STAR Apr 07 3 Transverse Single Spin Physics • Single Spin Analyzing Power (measure of asymmetry): • dσ↑(↓) – differential cross section of p0 when incoming proton has spin up(down) • Neutral Pion cross section at forward region shows left-right asymmetry depending on the transverse polarization of the incoming Proton. (measured on the left side) SP = +1/2 SP = -1/2 y p x π0 Left side: dσ↑ > dσ↓ P (any polarization) z π0 Right side: dσ↑ < dσ↓ Len Eun

5. AN and p0 cross section for RHIC Run 2 STAR Apr 07 4 PRL 97, 152302 (2006) PRL 92, 171801 (2004) √s=200 GeV, <η> = 3.8 Analyzing power vs. XF for polarized p+p collisions Inclusive p0 cross section for p+p collisions vs. the leading p0 energy. • At RHIC energy, p0cross section is consistent with the NLO pQCD calculation • The transverse spin asymmetry observed in lower energy persists at RHIC energy Len Eun

6. XF dependence of AN for RHIC Run 3, 5, and 6 STAR Apr 07 5 • AN increases with XF at positive XF • AN is consistent with 0 at negative XF • Run 6 added data at η = 3.3  The first map of the XF and PT dependence Len Eun

7. PT dependence of AN in XF bins STAR Apr 07 6 • In order to rule out the residual XF dependence, we need to look at the PT dependence in each XF bin. • Each graph represents a relatively small range of XF. • With the possible exception of the highest XF bin, the result does not show the predicted decrease of analyzing power by PT. Len Eun

8. Forward Meson Spectrometer (Run 7 ~) STAR Apr 07 7 Run 6: FPD++ • FMS, along with EEMC and BEMC, provides nearly complete EM coverage from -1 < η < +4 • FMS allows the detection of nearside p pair, and Jet-like reconstruction  Collins effect • In conjunction with the EEMC and BEMC, awayside jet can be identified  Sivers effect • Other physics objectives include • Small x gluon saturation • Prompt photon • Drell Yan Run 2 ~ 6: FPD Inner Calorimeter HV system Len Eun

9. Penn State Phototube Base STAR Apr 07 8 Dual Pump Cockroft-Walton Voltage Multiplier • 22-stage CW with Vmin ~ 1200V and Vmax ~ 1800V • No measurable ripple • Vdrop ~ 1% of the output voltage • High stability  No need for feedback circuitry • Improved linearity over the resistive divider base it replaces • Less than 1 pC pedestal for 60nS gate On-board intelligence via I2C serial bus • 2 line serial bus • 8-bit Digital Potentiometer with EEPROM  Non-volatile HV control • 8-bit 4 channel ADC  HV read back, voltage regulation diagnostics Integration required! Supply Voltages / Connections • +9V and +30V DC input  Total power consumption ~200mW • Cat5e cable and connector Polarized, locking, reliable, and low cost Len Eun

10. FMS Inner Calorimeter HV system STAR Apr 07 9 PC Light-tight Enclosure USB to I2C Provide I2C front end  Allows us to control the Yale bases the same way that we control Penn State bases Voltage conversion / Over-current protection I2C Multiplexing Voltage switch for each controller Over-voltage / over-current protection Supports one half of the inner FMS with maximum 256 bases Len Eun

11. Conclusion STAR Apr 07 10 FMS commissioning underway during RHIC run-7 Au+Au collisions First installation of new electronics from Berkeley complete Run 8 will provide high luminosity for polarized proton runs. Many exciting opportunities for spin physics! Len Eun

12. BACK UP

13. How can the p0 cross section depend on the proton transversity? • Proton  quark scattering is insensitive to transverse spin. However, the quark retains its initial spin after a hard scattering, and thequark  π0fragmentation can have azimuthal dependence on thetransverse spin of thequark. This process is referred to as theCollins Effect. [Nucl. Phys. B396, 161 (1993)] • A quark inside a proton may have orbital angular momentum that is correlated to the spin of the proton. If two quarks with opposite transverse momentum contribute different scattering amplitudes to the same final state, a case can be made where the proton  quarkscattering is sensitive to thetransverse spin of the proton.This process is referred to as theSivers Effect. [Phys. Rev. D 41, 83 (1990); 43, 261 (1991)]

14. Collins Effect sq = Spin of the struck quark pq = Momentum of the struck quark kTπ= Transverse momentum of the neutral pion y SP The spin of the scattered quark is correlated with the spin of the proton x sq The fragmentation of the quark to p0 has sq dependence p z kTπ pq P (any polarization) p + p  p0 + X π0 Spin of the proton affects the scattering angle through the spin of the large x quark π0

15. Sivers Effect Sp = Spin of the proton Pp = Momentum of the proton kTq = Transverse momentum of the quark inside the proton Quark transverse momentum is correlated with the spin of the proton y SP x kTq Pp Quark Parton Distribution Function has kTq dependence z pq P (any polarization) π0 p + p  p0 + X Spin of the proton affects the scattering angle through the quark transverse momentum π0

16. Angular motion of quarks inside a proton • If quarks inside a proton have orbital angular motion correlated to the spin of the proton, • And if the back side quark has different contribution to the final state than the front side quark, • Then the transverse momentum of the quark would not average out, and the jet axis would have left-right bias depending on the transverse spin of the proton. Phase Difference

17. PT dependence of AN for RHIC Run 3, 5, and 6 Theoretical models predict analyzing power to be suppressed by PT with ~1/PT dependence. Our result shows no such suppression. Analyzing power does not drop significantly until ~3.6 GeV of PT.

18. Previous graph had residual XF dependence due to limited area of coverage on PT – XF plane.