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Single-Spin Asymmetries in Star: Plans and Goals for Near Future and Future Future Measurements

STAR. Single-Spin Asymmetries in Star: Plans and Goals for Near Future and Future Future Measurements of forward neutral mesons. Steve Heppelmann STAR collaboration. p  + p   + X. The intuitive message :

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Single-Spin Asymmetries in Star: Plans and Goals for Near Future and Future Future Measurements

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  1. STAR Single-Spin Asymmetries in Star: Plans and Goals for Near Future and Future Future Measurements of forward neutral mesons. Steve Heppelmann STAR collaboration

  2. p + p  + X • The intuitive message: • A transverse proton which interacts at large xf consists of a leading quark (u) for , (d) for • The leading quark carries much of the transverse spin of the proton. • Because the quark level single spin asymmetry for scattering of transversely polarized quarks is very small at high energy, the measured asymmetry comes from features of the scattering process not evident in leading twist perturbative QCD with collinear partons. • This is interesting. Models involve things like • Transverse parton distributions (Orbital angular momentum). • Spin dependent fragmentation (Collins Effect). • Higher twist calculations. • 0 – E704, PLB261 (1991) 201. • +/- - E704, PLB264 (1991) 462. • HERMES Collab., PRD64 (2001) 097101. • HERMES Collab., PLB 535 (2002) 85. • HERMES Collab., PRL 84 (2000) 4047. s=20 GeV, pT=0.5-2.0 GeV/c:

  3. The Opacity of STAR to Photons as Seen Upstream of the Interaction Region.

  4. Summary of Star Forward Measurements of Transverse Asymmetries and Cross Sections • An vs Xf • pp→º+X Cross Sections

  5. STAR Forward Calorimeter: An Evolving History • Prototype FPD Proposal Dec 2000 • Approved March 2001 • Run 2 polarized proton data (published 2004 spin asymmetry and cross section) • FPD proposal June 2002 • Review July 2002 • Run 3 data pp dAu (Preliminary An Results) • FMS Proposal Submitted Jan 2005. Near full Forward EM Coverage.(hep-ex/0502040).

  6. FMS (Fall 2006 NSF??) Forward Meson Spectrometer Objectives • Measure gluon distributions as small x. • Look for evidence of gluon saturation effects. Comparison between forward pions pp and in dAu interactions. • Full characterization of events leading to single transverse spin asymmetry in pion production. • Sivers vs Collins Effect.

  7. Transverse Spin Short Term Objectives • 1 Week Transverse Running (Current Run (Run 5)? not in current plan?) • 20 Times “Figure of Merit” (Polarization x Integrated Luminosity”) • Get Pt Dependence of AN at Fixed Rapidity • Measure kTtransverse asymmetry. • 10k back to back events.

  8. Sivers Effect on two pion correlations. Correlation between kT and transverse Spin of proton. 1 The pt1 + pt2 ~ kT correlated with spin. 2

  9. STAR -FPD Preliminary Cross Sections Similar to ISR analysis J. Singh, et al Nucl. Phys. B140 (1978) 189.

  10. How large is kT?

  11. Current FPD Detector • Both East and West of Interaction region we have: • 2 5x5 arrays of Pb Glass (up/down) • 2 7x7 arrays of Pb Glass (North/South) FPD calorimeters in near position

  12. Star Detector Coverage BBC – scintillation counters EMC – EM calorimeters FPD – Forward calorimeters TPC – Charged Particle Tracking

  13.  q g  q g g PYTHIA: a guide to the physics Subprocesses involved: Forward Inclusive Cross-Section: q+g g+g and q+g  q+g+g STAR FPD Soft processes • PYTHIA predictionagrees well with the inclusive 0 cross section at 3-4 • Dominant sources of large xF production from: • q + g  q + g (22) + X • q + g  q + g + g (23)+ X

  14. Back-to-back Azimuthal Correlationswith large  Fit LCP normalized distributions and with Gaussian+constant Beam View Top View Trigger by forward   ] • E > 25 GeV •  4 ] Coicidence Probability [1/radian] Midrapidity h tracksin TPC • -0.75 < < +0.75 Leading Charged Particle(LCP) • pT > 0.5 GeV/c LCP S = Probability of “correlated” event under Gaussian B = Probability of “un-correlated” event under constant s = Width of Gaussian

  15. STAR STAR Preliminary STAR Preliminary PYTHIA (with detector effects) predicts • “S” grows with<xF> and <pT,> • “s” decrease with <xF> and <pT,> PYTHIA prediction agrees with p+p data Larger intrinsic kT required to fit data 25<E<35GeV 45<E<55GeV Statistical errors only

  16. Frankfurt, Guzey and Strikman, J. Phys. G27 (2001) R23 [hep-ph/0010248]. • constrain x value of gluon probed by high-x quark by detection of second hadron serving as jet surrogate. • span broad pseudorapidity range (-1<h<+4) for second hadron  span broad range of xgluon • provide sensitivity to higher pT for forward p0 reduce 23 (inelastic) parton process contributions thereby reducing uncorrelated background in Df correlation. Pythia Simulation

  17. Back to a kT Measurement • For pT1 (triggered º)>2.5 GeV/c • <pT2> (leading charged Track) ~1GeV/c • Width of (pT1 -pT2) ~ 0.5 GeV/c • 10,000 events …. 5 MeV/c measurement of <(pT1 -pT2)>. • With 10,000 events and 50% polarization, the transverse spin asymmetry in the the kT observation should be significant if the AN is due to initial state trigger bias effects.

  18. Figure 5 STAR published measurement of transverse asymmetry for 0 production. • Current FPD Region of interest 2<XF<.6 Larger rapidity, Lower Pt. Smaller rapidity Larger Pt. • FPD++ • Direct Photon AN Measurement

  19. About Direct Photon AN For XF >= ~.5 Direct Photon Dominates º photons separated in Small cell detector up to about 60 GeV Large Cells to guarantee “single Photon”. If 1 week of transverse running in the next month…… this could be installed for fall 2005.

  20. FMS Design FPD Calorimeters

  21. Collins Effect • Correlation between transverse spinand transverse polarization of large x quark • Scattering preserves quark transverse spin. • Fragmentation azimuthal dependence gives • rise to AN asymmetry.

  22. Two questions about Collins Effect • Can Transversity of the scattered quark in the proton be measured with fragmentation dependence on quark transverse spin? • Can this effect explain the AN of forward pion production? (can this provide a big enough kT -> jT bias?) • The Answer to the first could be yes and even if the answer to the second is no. • It is possible that Collins has little to do with AN but is still the best way to measure the Nucleon Transverse structure. • We can easily answer the second question, the first will take more work.

  23. Near Side Two pion Correlations • In Full FMS, two forward º’s. • In FTPC + FPD++ (FPD) º + charged track • FTPC energy resolution ~ 10 % for Pt=2.0 GeV/c track. • Jet Fragmentation into two energetic particles. • Clean measurement with full FMS.

  24. V. Guzey, M. Strikman and W. Vogelsang, Phys. Lett. B603 (2004) 173 [hep-ph/0407201]. STAR Results Pythia Simulation

  25. STAR Example model (CGC): y=0 As y grows Kharzeev, Kovchegov, and Tuchin, Phys. Rev. D 68 , 094013 (2003)  Dependence of RdAu Observe significant rapidity dependence, similar to expectations from models which suppress gluon density in heavy nuclei RdAu for p0systematically below linear extrapolation of h data to =4, consistent with expectations that p + p  h is isospin suppressed at large [Guzey, Strikman and Vogelsang, Phys. Lett. B 603, 173 (2004)]

  26. New FMS Calorimeter Lead Glass From FNAL E831 804 cells of 5.8cm5.8cm60cm Schott F2 lead glass Loaded On a Rental Truck for Trip To BNL

  27. Summary of Forward Thinking with STAR • FPD Prototype ->FPD->FPD++->FMS • Staged options for forward calorimeter improvements in STAR over the next two years should bring many results soon. • AN pT dependence • <kT> measurement (Sivers Effect) • Near side correlations (Collins Effect) • Direct Photon AN • Gluon Saturation in Nuclei • ?? AN from nuclear spectator ?? • A little more transverse running in the next month could pay off with much more data sooner.

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