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Transverse Spin Asymmetries at RHIC Access to transverse momentum dependent distributions

Transverse Spin Asymmetries at RHIC Access to transverse momentum dependent distributions. L.C. Bland Brookhaven National Laboratory 11 June 2007. Relativistic Heavy Ion Collider. 3 Spin Experiments PHENIX STAR BRAHMS PHOBOS (heavy-ion). Characteristics 2 counter-circulating rings

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Transverse Spin Asymmetries at RHIC Access to transverse momentum dependent distributions

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

  2. Relativistic Heavy Ion Collider 3Spin 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 RHIC pC Polarimeters Absolute Polarimeter (H jet) BRAHMS & PP2PP PHOBOS Siberian Snakes Siberian Snakes PHENIX STAR Spin Rotators (longitudinal polarization) Spin Rotators (longitudinal polarization) Pol. H- Source LINAC BOOSTER Helical Partial Siberian Snake AGS 200 MeV Polarimeter AGS pC Polarimeter Strong AGS Snake 2006: 1 MHz collision rate; Polarization=0.6

  4. Definition: dσ↑(↓) – differential cross section of p0 when incoming proton has spin up(down) One way to measure: Single arm detector: R – relative luminosity PBeam – beam polarization Left p0,xF<0 p0 , xF>0 pT,p p  p pL,p=xFp Transverse Single Spin Asymmetry (SSA)Definitions: how and what to measure in an experiment • positive AN: more p0 from spin up() than from spin down() • Accuracy,dAN  1/(N+N)½ many events required for good accuracy In general, AN is a function of both xF and pT

  5. Expectations from Theory What would we see from this gedanken experiment? F0 as mq0 in vector gauge theories, so AN ~ mq/pT or,AN ~ 0.001 for pT ~ 2 GeV/c Kane, Pumplin and Repko PRL 41 (1978) 1689

  6. A Brief and Incomplete History… s=20 GeV, pT=0.5-2.0 GeV/c • QCD theory expects very small (AN~10-3) transverse SSA for particles produced by hard scattering. • 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). • 0 – E704, PLB261 (1991) 201. • +/- - E704, PLB264 (1991) 462.

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

  8. Transverse Single-Spin AsymmetriesWorld-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 (m); 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 ScatteringHard scattering hadroproduction p Factorization theorems state that the inclusive cross section for p+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 p production cross section.

  10. Ed3s/dp3[mb/GeV3] q=6o q=10o q=53o xF Do we understand forward p0 production in p + p?At s < 200 GeV, not really… √s=23.3GeV √s=52.8GeV Data-pQCD difference at pT=1.5GeV 2 NLO collinear calculations with different scale: pT and pT/2 Bourrely and Soffer [Eur. Phys. J C36 (2004) 371], data references therein to ISR and fixed target results Ed3s/dp3[mb/GeV3] q=15o q=22o xF sdata/spQCD appears to be function of q, √s in addition to pT Collinear NLO pQCD underpredicts the data at s < 200 GeV

  11. Does pQCD describe particle production at RHIC?Compare cross sections measured for p+pp0 +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 pT~2 GeV/c over a wide range, 0 < h< 3.8, of pseudorapidity (h = -ln tan /2) at s = 200 GeV.

  12. Accepted for publication in Phys. Rev. Lett. hep-ex/0701041 K+  0 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 pT GeV/c pp collision at s=200GeV (3) Cross sections at forward rapidity y=2.95 are consistent with NLO pQCD.

  13. Expect QCD scaling of form:  Require s dependence to disentangle pT and xT dependence STAR-FPD Cross Sections Similar to ISR analysis J. Singh, et al Nucl. Phys. B140 (1978) 189.

  14. <z> <xq> <xg> Forwardp0production in hadron collider Ep p0 p d EN qq qp p Au xgp xqp qg EN (collinear approx.) • Large rapidity p production (hp>4) probes asymmetric partonic collisions • Mostly high-x valence quark + low-x gluon • 0.3 < xq< 0.7 • 0.001< xg < 0.1 • <z> 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

  15. First Transverse SSA at RHICprior to RHIC run 6 p+p collisions , s = 200 GeV S.S. Adler et al. Phys. Rev Lett. 95 (2005) 202001 Particle production at ~90 (midrapidity) relative to the colliding beams have zero tranverse single spin asymmetry.

  16. Spin Effects in the Forward Direction √s=200 GeV, <η> = 3.8 Spin effects initially observed in RHIC run 2 confirmed by measurements in runs 3,5. STAR collaboration Phys. Rev. Lett. 92 (2004) 171801 D. Morozov, for STAR [hep-ex/0512013] 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  p fragmentation • Qiu/Sterman (initial state) / Koike (final state): twist-3 pQCD  multi-parton correlations  Transverse SSA persist at large xF at RHIC energies

  17. Particle Identification using RICH e   K p,K identification < 30 GeV/c pbar > 17 GeV/c with efficiency ~ 97%

  18.  SSAs at 2.3 and 4 deg. at s = 200 GeV SSA 0.1 + 0  4 2.3 0.1 • SSA(+): positive SSA(): negative 4-6% in 0.15 <xF< 0.3. • SSA () survive.

  19. SSAs at 2.3 deg. at √s = 200 GeV SSA 0.1 0 0.1 K p • SSA(K+), SSA(K-): positive 2-5% for 0.15 <xF <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.

  20. sampled Overview of transverse spin runs at STAR with forward calorimetry: 2001→2006 FOM (P2L) in Run 6 is ~50 times larger than from all the previous STAR runs, and ~725 times larger than for Run 2

  21. Polarization Measurements 2006 Run

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

  23. y x z h= 1 RUN6 configuration h=2 FPD++ 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++)

  24. STAR Di-jets at STARp+p, s=200 GeV Large acceptance of STAR ideal for di-jet detection. arXiv:0705.4629v1 [hep-ex]

  25. STAR Results vs. Di-Jet Pseudorapidity Sum Run-6 Result VY 1, VY 2 are calculations by Vogelsang & Yuan, PRD 72 (2005) 054028 AN pbeam  (kT  ST) jet Emphasizes (50%+ ) quark Sivers Boer & Vogelsang, PRD 69 (2004) 094025 pbeam into page jet Idea: directly measure kT by observing momentum imbalance of a pair of jets produced in p+p collision and attempt to measure if kT is correlated with incoming proton spin STAR • AN consistent with zero • ~order of magnitude smaller in pp  di-jets than in semi-inclusive DIS quark Sivers asymmetry! arXiv:0705.4629v1, submitted to PRL

  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 p0 SSA. The FPD++ annulus is surrounded by additional calorimetry to increase the acceptance for jet-like events and direct g events.

  27. Acceptance of FPD and FPD++ 6 5 4 3 2 1 0 STAR xF pT GeV/c FPD++ p+p0X s = 200 GeV FPD 0 0.2 0.4 0.6 0.8 • Single <h>  limited acceptance  strongxFand pTcorrelation • Study of pTdependence needs large acceptance.

  28. STAR π0AN at √s=200 GeV – xF-dependence • AN at positive xF grows with • increasing xF • AN at negative xF is consistent • with zero • Run 6 data at <η>=3.7 are • consistent with the existing • measurements • Small errors of the data points • allow quantitative comparison • with theory predictions hep-ex/0612030

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

  30. STAR AN(pT) in xF-bins • Combined data from three runs at <η>=3.3, 3.7 and 4.0 • In each xF bin, <xF> does not significantly changes with pT • Measured AN is not a smooth decreasing function of pT as predicted by multiple theoretical models • (hep-ex/0612030) D’Alesio & Murgia PRD 70 (2004) 074009 Kouvaris, Qiu, Vogelsang, Yuan PRD 74 (2006) 114013

  31. Brahms • Transvers beam pol • Particle ID • BRAHMS measured AN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

  32. SOUTH PHENIX Muon Piston Calorimeter 2.22.2 18 cm3 • 192 PbWO4 crystals with APD readout • Better than 80% of the acceptance is okay

  33. PHENIX Goes ForwardFirst results with muon piston calorimeter from run 6p+pp0+X, s = 62 GeV Transverse SSA persists with similar characteristics over a broad range of collision energy (20 < s < 200 GeV)

  34. SummaryFrom 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 xF; midrapidity asymmetries are small. • Large xF spin asymmetries show the same pattern for 20 s  200 GeV • First observation of pT 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?

  35. (hep-ex/0602012) • Ng>3 requirement should allow p0-p0 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 h,f space from thrust axis  events are expected to exhibit similar jet characteristics as found at h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 Etrig condition, enables • integration over the Collins angle  isolating the Sivers effect, or • dependence on the Collins angle  isolating the Collins/Heppelmann effect Expect that jet-like events are ~15% of p0 events OutlookStill More RHIC Run 6 Results to Come • Is the single spin asymmetry observed for p0also present for the jet the p0 comes from? • Answer discriminates between Sivers and Collins contributions • Trigger on energy in small cells, reconstruct p0 andmeasure the energy in the entire FPD++ • Average over the Collins angle and define a new xF for the event, then measure analyzing power versus xF

  36. OutlookRHIC Run 8, polarized p+p collisions at s=200 GeVProject 95 / pb of Integrated Luminosity http://spin.riken.bnl.gov/rsc/report/RHIC_spin_LRP07.pdf

  37. Au Au FMS Commissioning April 2007 • Summed Energy (ADC cnts) • Cell multiplicity 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 Forward Meson Spectrometer Installation completed 2007 FMS for Run 7 NOW!! PHYSICS OBJECTIVES • A d-Au measurement of the parton model gluon density distributions x g(x) in gold nucleifor0.001< x <0.1. For 0.01<x<.1, this measurement tests the universality of the gluon distribution. • Characterization of correlated pion cross sections as a function of Q2 (pT2) to search for the onset of gluon saturation effects associated with macroscopic gluon fields. (again d-Au) • Measurements withtransversely polarized protonsthat are expected toresolve the origin of the large transverse spin asymmetriesin reactions for forward  production. (polarized pp) FMS construction completed installation and commissioning during Run 7 (NOW) FMS Wall FMS ½ Wall Pb. Glass

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