Transverse Spin Asymmetries at RHIC Access to transverse momentum dependent distributions L.C. Bland Brookhaven National Laboratory 11 June 2007

2 Relativistic Heavy Ion Collider 3 Spin Experiments PHENIX STAR BRAHMS PHOBOS (heavy-ion) Characteristics 2 counter-circulating rings 3.8 km in circumference Top Energies (each beam): 100GeV / Au-Au 250GeV / p-p Mixed Species (d+Au)

3 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

4 Transverse Single Spin Asymmetry (SSA) Definitions: how and what to measure in an experiment Definition: dσ ↑(↓) – differential cross section of   when incoming proton has spin up(down) One way to measure: Single arm detector: R – relative luminosity P Beam – beam polarization positive A N : more  0 from spin up(  than from spin down(  Accuracy,  A N  N   N   ½  many events required for good accuracy Left p p  p L,   x F p p T, , x F >0    x F <0 In general, A N is a function of both x F and p T

5 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 ~ for p T ~ 2 GeV/c Kane, Pumplin and Repko PRL 41 (1978) 1689

6  s=20 GeV, p T = 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).

7 Two of the Explanations for Large Transverse SSA 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 

8 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

9 Hard Scattering Hard scattering hadroproduction  Factorization theorems state that the inclusive cross section for p+p   +X can be computed in perturbative QCD using universal PDF and fragmentation functions, and perturbatively calculated hard-scattering cross sections,, for partonic process a+b  c. All such processes are summed over to yield the inclusive  production cross section.

10 √s=23.3GeV√s=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 ]

11 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) J. Adams et al. (STAR), PRL 92 (2004) ; and PRL 97 (2006) 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.

12 Cross sections at forward rapidity y=2.95 NLO pQCD. Cross sections at forward rapidity y=2.95 are consistent with NLO pQCD. pp collision at  s=200GeV (3) p T GeV/c hep-ex/ Accepted for publication in Phys. Rev. Lett.  K+K

13 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

14 <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 < < 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q gg ENEN xqpxqp xgpxgp   ENEN (collinear approx.)

15 First Transverse SSA at RHIC prior to RHIC run 6 S.S. Adler et al. Phys. Rev Lett. 95 (2005) Particle production at ~90  (midrapidity) relative to the colliding beams have zero tranverse single spin asymmetry. p  +p collisions,  s = 200 GeV

16 Spin Effects in the Forward Direction STAR collaboration Phys. Rev. Lett. 92 (2004) 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/ ] 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 p,K identification 17 GeV/c with efficiency ~ 97% e   KK Particle Identification using RICH

18   SSAs at 2.3 and 4 deg. at  s = 200 GeV SSA (   ) survive. SSA(  + ): positive SSA(   ): negative 4-6% in 0.15 <x F < 0.3.  ++ 2.3  44 0.1  SSA

19 SSAs at 2.3 deg. at √s = 200 GeV SSA(K + ), SSA(K - ): positive 2-5% for 0.15 <x F <0.3. SSA(pbar), SSA(K - ) > 0: Contribution from sea-quarks. SSA(p) ~ 0: Significant fraction of proton can be mostly from polarized beam proton, but only ones showing SSA~0. p KK 0.1  SSA

20 Overview of transverse spin runs at STAR with forward calorimetry: 2001→2006 Run2Run3Run5Run6 detector EEMC and FPD prototypes 6 matrices of FPD full FPD (8 matrices) East FPD West FPD++ ~15~30~45~ ±3.3/±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

Polarization Measurements 2006 Run

22 RHIC Luminosity Run-6 vs. Run-5 Plot by Phil Pile An extraordinary Run-6! Average Polarization 60%!

23 FPD  =2  =  1 FPD++ East-side West-side Di-jet results:  1<  <2 (Barrel EMC, Endcap EMC, 2  ) Inclusive  0 in forward region:  4<  <  3 (FPD), 2.5<  <4 (FPD++) RUN6 configuration x z y

24 Di-jets at STAR p  +p,  s=200 GeV STAR Large acceptance of STAR ideal for di-jet detection. arXiv: v1arXiv: v1 [hep-ex]

25 Idea: directly measure k T by observing momentum imbalance of a pair of jets produced in p+p collision and attempt to measure if k T is correlated with incoming proton spin Boer & Vogelsang, PRD 69 (2004) jet A N  p beam  ( k T  S T ) p beam into page STAR Results vs. Di-Jet Pseudorapidity Sum Run-6 Result STAR arXiv: v1, submitted to PRL Emphasizes (50%+ ) quark Sivers A N consistent with zero  ~order of magnitude smaller in pp  di-jets than in semi-inclusive DIS quark Sivers asymmetry! VY 1, VY 2 are calculations by Vogelsang & Yuan, PRD 72 (2005)

26 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.

27 Acceptance of FPD and FPD++STAR p  +p  0 X   s = 200 GeV Single  limited acceptance  strong x F and p T correlation Study of p T dependence needs large acceptance. FPD++ FPD xFxF p T GeV/c

28 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/

29 A N (p T ) at x F > 0.4 Run3+Run5 data (hep-ex/ ): Run6 data (hep-ex/ ): 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

30 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/ ) D’Alesio & Murgia PRD 70 (2004) Kouvaris, Qiu, Vogelsang, Yuan PRD 74 (2006) STAR

31 Brahms Transvers beam pol Particle ID BRAHMS measured A N  s=62.4 GeV and 200 GeV Large xF dependent SSAs seen for pions and kaons Collinear factorization and (NLO) pQCD describe unpolarized cross-section at RHIC in wide kinematic region

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33 PHENIX Muon Piston Calorimeter SOUTH 192 PbWO4 crystals with APD readout Better than 80% of the acceptance is okay 2.2  2.2  18 cm 3

34 PHENIX Goes Forward First results with muon piston calorimeter from run 6 p  +p   +X,  s = 62 GeV Transverse SSA persists with similar characteristics over a broad range of collision energy (20 <  s < 200 GeV)

35 Summary From RHIC run 6 (“Renaissance Run”) 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, 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?

36 Outlook Still More RHIC Run 6 Results to Come Is the single spin asymmetry observed for   also present for the jet the   comes from? Answer discriminates between Sivers and Collins contributions Trigger on energy in small cells, reconstruct   and measure the energy in the entire FPD++ Average over the Collins angle and define a new x F for the event, then measure analyzing power versus x F Expect that jet-like events are ~15% of   events (hep-ex/ ) N  >3 requirement should allow     analysis (upper left) for each event, examine PYTHIA record for final-state hard scattered partons  event selection chooses jet-like events. (upper right) event-averaged correlation between photon energy and distance in  space from thrust axis  events are expected to exhibit similar jet characteristics as found at  0 (middle) multi-photon final states enable reconstruction of parent parton kinematics via momentum sum of observed photons. (bottom) projected statistical accuracy for data sample having 5 pb -1 and 50% beam polarization. Azimuthal symmetry of FPD++ around thrust axis, selected by E trig condition, enables integration over the Collins angle  isolating the Sivers effect, or dependence on the Collins angle  isolating the Collins/Heppelmann effect

37 Outlook RHIC Run 8, polarized p+p collisions at  s=200 GeV Project 95 / pb of Integrated Luminosity

38 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 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