1. Transverse Single Spin Asymmetries in Large-x F Production at STAR A Review of Several Mysteries L.C. Bland Brookhaven National Laboratory 31 July 2007

2. 2 Kinematics large- p T physics in p+p collisions p beam -p beam large p T or jet or or … Largest p T reached by detecting produced particles at ~ 90 (midrapidity, ~0)

3. 3 Kinematics large- x F (with sufficient p T ) physics in p+p collisions p beam -p beam large p L or jet or or … Large p L (produced particle at large ) is required to reach large Feynman- x, x F = p L / p beam = 2 p L / s

4. 4 RHIC Spin Goals Understanding the Origin of Proton Spin Transverse Spin PRD 70 (2004)114001 Spin Sum Rules Longitudinal Spin Understanding the origin of proton spin helps to understand its structure

5. 5 Transverse Single-Spin Asymmetries (A N ) Probing for orbital motion within transversely polarized protons

6. 6 Expectations from Theory What would we see from this gedanken experiment? F 0 as m q 0 in vector gauge theories, so A N ~ m q / p T or, A N ~ 0.001 for p T ~ 2 GeV/c Kane, Pumplin and Repko PRL 41 (1978) 1689

7. 7 s=20 GeV, p T =0.5-2.0 GeV/c 0 – E704, PLB261 (1991) 201. +/- - E704, PLB264 (1991) 462. QCD theory expects very small (A N ~10 -3 ) transverse SSA for particles produced by hard scattering. A Brief and Incomplete History… The FermiLab E-704 experiment found strikingly large transverse single- spin effects in p +p fixed-target collisions with 200 GeV polarized proton beam ( s = 20 GeV).

8. 8 Two of the Explanations for Large Transverse SSA Spin-correlated k T Require experimental separation of Collins and Sivers contributions Collins/Hepplemann mechanism requires transverse quark polarization and spin-dependent fragmentation Sivers mechanism requires spin-correlated transverse momentum in the proton (orbital motion). SSA is present for jet or final state initial state

9. 9 RHIC Polarized Collider BRAHMS & PP2PP STAR PHENIX AGS LINAC BOOSTER Pol. H - Source Spin Rotators (longitudinal polarization) Siberian Snakes 200 MeV Polarimeter RHIC pC Polarimeters Absolute Polarimeter (H jet) AGS pC Polarimeter Strong AGS Snake Helical Partial Siberian Snake PHOBOS Spin Rotators (longitudinal polarization) Siberian Snakes 2006: 1 MHz collision rate; Polarization=0.6

10. 10 Transverse Single-Spin Asymmetries World-wide experimental and theoretical efforts Transverse single-spin asymmetries are observed in semi-inclusive deep inelastic scattering with transversely polarized proton targets HERMES ( e ); COMPASS ( ); and planned at JLab Transverse single spin asymmetries are observed in hadron-pair production in e + e collisions (BELLE) Intense theory activity underway

11. gluon quark pion or jet quark RHIC Spin Probes Polarized proton collisions / hard scattering probes of G Describe p+p particle production at RHIC energies ( s 62 GeV) using perturbative QCD at Next to Leading Order, relying on universal parton distribution functions and fragmentation functions

12. 12 s=23.3GeVs=52.8GeV Do we understand forward production in p + p? At s < 200 GeV, not really… 2 NLO collinear calculations with different scale: p T and p T /2 Bourrely and Soffer [Eur. Phys. J C36 (2004) 371], data references therein to ISR and fixed target results data / pQCD appears to be function of, s in addition to p T Collinear NLO pQCD underpredicts the data at s < 200 GeV xFxF Ed 3 dp 3 [ b/GeV 3 ] Data-pQCD difference at p T =1.5GeV xFxF Ed 3 dp 3 [ b/GeV 3 ]

13. 13 Does pQCD describe particle production at RHIC? Compare cross sections measured for p+p +X at s=200 GeV to next-to-leading order pQCD calculations S.S. Adler et al. (PHENIX), PRL 91 (2003) 241803 J. Adams et al. (STAR), PRL 92 (2004) 171801; and PRL 97 (2006) 152302 Cross sections agree with NLO pQCD down to p T ~2 GeV/c over a wide range, 0 < 3.8, of pseudorapidity ( = -ln tan /2) at s = 200 GeV.

14. 14 STAR-FPD Cross Sections Similar to ISR analysis J. Singh, et al Nucl. Phys. B140 (1978) 189. Expect QCD scaling of form: Require s dependence to disentangle p T and x T dependence

15. 15 <z><z> <xq><xq> <xg><xg> Large rapidity production >4 probes asymmetric partonic collisions Mostly high-x valence quark + low-x gluon 0.3 < x q < 0.7 0.001< x g < 0.1 nearly constant and high 0.7 ~ 0.8 Large-x quark polarization is known to be large from DIS Directly couple to gluons probe of low x gluons NLO pQCD Jaeger,Stratmann,Vogelsang,Kretzer Forward production in hadron collider pdpd p Au q g ENEN xqpxqp xgpxgp ENEN (collinear approx.)

16. 16 Spin Effects in the Forward Direction STAR collaboration Phys. Rev. Lett. 92 (2004) 171801 Can be described by several models available as predictions: Sivers: spin correlated k in the proton (orbital angular momentum) Collins/Heppelmann: spin and k correlation in quark fragmentation Qiu/Sterman (initial state) / Koike (final state): twist-3 pQCD multi-parton correlations s=200 GeV, = 3.8 D. Morozov, for STAR [hep-ex/0512013] Spin effects initially observed in RHIC run 2 confirmed by measurements in runs 3,5. Transverse SSA persist at large x F at RHIC energies

17. 17 Overview of transverse spin runs at STAR with forward calorimetry: 20012006 Run2Run3Run5Run6 detector EEMC and FPD prototypes 6 matrices of FPD full FPD (8 matrices) East FPD West FPD++ ~15~30~45~60 0.150.250.16.8 3.8±3.3/±4.0±3.7/±4.0-3.7/3.3 sampled FOM (P 2 L) in Run 6 is ~50 times larger than from all the previous STAR runs, and ~ 725 times larger than for Run 2

18. 18 FPD =2 = 1 FPD++ East-side West-side Inclusive 0 in forward region: 4< < 3 (FPD), 2.5< <4 (FPD++) RUN6 configuration x z y

19. 19 FPD++ Physics for Run6 Run-5 FPD We staged a large version of the FPD to prove our ability to detect jet-like events, direct photons, etc. with the STAR FMS The center annulus of the run-6 FPD++ is similar to arrays used to measure forward SSA. The FPD++ annulus is surrounded by additional calorimetry to increase the acceptance for jet-like events and direct events.

20. 20 Acceptance of FPDSTAR Inclusive 0 Strong x F - p T correlation because of limited acceptance FPD xFxF 0 0.2 0.4 0.6 0.8 4 5 0 2 3 1 6 p T GeV/c

21. 21 Study of the p T dependence needs large acceptance Acceptance of FPD and FPD++STAR Inclusive 0 FPD++ FPD xFxF 0 0.2 0.4 0.6 0.8 4 5 0 2 3 1 6 p T GeV/c

22. 22 Identification and Spin Dependence Large rapidity measurements require careful calibration Left/right symmetric detectors cancels many sources of systematic errors Spin effect is visible in the raw spin-sorted yields

23. 23 π 0 A N at s=200 GeV – x F -dependence A N at positive x F grows with increasing x F A N at negative x F is consistent with zero Run 6 data at =3.7 are consistent with the existing measurements Small errors of the data points allow quantitative comparison with theory predictions STAR hep-ex/0612030

24. 24 Estimates of Systematic Errors Consistency of repeated measurements Analyzing power is measured by cross-ratio method with two-arms (left-right) detector: Run-by-run comparison with mean is consistent with statistics, except right near the threshold =3.7 =3.3

25. 25 A N (p T ) at x F > 0.4 Run3+Run5 data (hep-ex/0512013): Run6 data (hep-ex/0612030): more precise measurements consistent with the previous runs in the overlapping p T region complicated dependence on p T, but not in agreement with theoretical predictions Online calibration of CNI polarimeter Hint of A N decrease with increasing p T at p T ~1-2 GeV/c residual x F -dependence? => A N mapping in (x F,p T ) plane is required

26. 26 A N (p T ) in x F -bins Combined data from three runs at =3.3, 3.7 and 4.0 In each x F bin, does not significantly changes with p T Measured A N is not a smooth decreasing function of p T as predicted by multiple theoretical models (hep-ex/0612030) DAlesio & Murgia PRD 70 (2004) 074009 Kouvaris, Qiu, Vogelsang, Yuan PRD 74 (2006) 114013 STAR

27. 27 Summary Firmly established that large transverse single spin asymmetries are observed at s = 200 GeV, where generally cross sections agree with pQCD calculations. Large transverse single spin asymmetries are observed only at large x F ; midrapidity asymmetries are small. Large x F spin asymmetries show the same pattern for 20 s 200 GeV First observation of p T dependence at fixed-x F, enabled by the run-6 luminosity/performance Some aspects of the theory are still not understood Intense theory activity is underway to understand these spin effects. Most theorists agree the Sivers mechanism is responsible for the dynamics evidence for partonic orbital angular momentum?

28. 28 FMS construction completed installation and commissioning during Run 7 (NOW) FMS ½ Wall Pb. Glass FMS Wall FMS for Run 7 NOW!! Near full EM coverage -1< <4 Pairs of Forward Pions same side correlations (Fragmentation – Collins) Event by event x measurement from two jets. Opposite side correlated pions (dijets) – Sivers effect –d-Au (Gluon saturation in Nuclei) Other future objectives –Forward Lepton pairs –Charm PHYSICS OBJECTIVES Outlook Forward Meson Spectrometer Installation completed 2007 d-Au gold nuclei 0.001< x <0.1 1.A d-Au measurement of the parton model gluon density distributions x g(x) in gold nuclei for 0.001< x <0.1. For 0.01<x<.1, this measurement tests the universality of the gluon distribution. macroscopic gluon fields. (again d-Au) 2.Characterization of correlated pion cross sections as a function of Q 2 (p T 2 ) to search for the onset of gluon saturation effects associated with macroscopic gluon fields. (again d-Au) transversely polarized protons resolve the origin of the large transverse spin asymmetries forward production. (polarized pp) 3.Measurements with transversely polarized protons that are expected to resolve the origin of the large transverse spin asymmetries in reactions for forward production. (polarized pp) Au Au FMS Commissioning April 2007 Summed Energy (ADC cnts) Summed Energy (ADC cnts) Cell multiplicity Cell multiplicity