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Experimental Issues in Low-x Physics and Transverse Single Spin Asymmetries

Experimental Issues in Low-x Physics and Transverse Single Spin Asymmetries. EIC Meeting at Stony Brook L.C. Bland Brookhaven National Laboratory 7 December 2007. Questions to be Addressed A naïve experimental view.

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Experimental Issues in Low-x Physics and Transverse Single Spin Asymmetries

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  1. Experimental Issues inLow-x PhysicsandTransverse Single Spin Asymmetries EIC Meeting at Stony Brook L.C. Bland Brookhaven National Laboratory 7 December 2007

  2. Questions to be AddressedA naïve experimental view • Are phenomena observed in inclusive/semi-inclusive deep inelastic electron scattering universal, meaning related to phenomena in hard scattering particle production in hadronic interactions? • What is the justification to use particle production in hadronic interactions as a tool to probe for universality? • Why are transverse single spin asymmetries interesting? And, are there universal aspects between semi-inclusive deep inelastic scattering and hadronic interactions? • Why is the gluon density, especially at low Bjorken x, interesting? And, how can universal aspects of the gluon density be explored in hadronic interactions? • What is on the horizon for addressing these questions?

  3. pbeam -pbeam large pT por jet or g or … Largest pT reached by detecting produced particles at  ~ 90 (midrapidity, h~0) Kinematicslarge-pT physics in p+p collisions

  4. pbeam -pbeam large pL por jet or g or … Large pL (produced particle at large h) is required to reach large Feynman-x, xF = pL/ pbeam= 2 pL/ s Kinematicslarge-xF (with sufficient pT) physics in p+p collisions

  5. 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

  6. pion or jet quark quark gluon RHIC Spin/Low-x ProbesPolarized proton collisions / d+Au collisions 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

  7. 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 < 60 GeV

  8. 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.

  9. STAR-Forward Cross Sections Similar to ISR analysis J. Singh, et al Nucl. Phys. B140 (1978) 189. Expect QCD scaling of form:  Require s dependence (e.g., measure p0 cross sections at s = 500 GeV) to disentangle pT and xT dependence

  10. <z> <xq> <xg> Forwardp0production in a 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

  11. Q: What is the justification to use particle production in hadronic interactions as a tool to probe for universality? A: Particle production cross sections agree with next-to-leading order pQCD calculations at central and forward rapidities at s = 200 GeV.To do: Measure at s = 500 GeV to establish xF, xT and pT scaling

  12. Transverse Single-Spin Asymmetries (AN) Probing for orbital motion within transversely polarized protons

  13. Expectations from Theory What would we see from this gedanken experiment? F0 as mq0 in vector gauge theories, so AN ~ mq/s or, AN ~ 10-5 for s = 200 GeV Kane, Pumplin and Repko PRL 41 (1978) 1689

  14. 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.

  15. Sivers mechanism requires spin-correlated transverse momentum in the proton (orbital motion) and color-charge interaction. 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 SSASpin-correlated kT initial state final state Require experimental separation of Collins and Sivers contributions

  16. 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

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

  18. 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

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

  20. p0 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

  21. STAR π0AN at √s=200 GeV – xF-dependence • AN at positive xF grows with • increasing xF • AN at negative xF is consistent • with zero • Small errors of the data points • allow quantitative comparison • with theory predictions Preliminary results: hep-ex/0612030 Final results presently under review by STAR

  22. Evidence for Universality?Uses phenomenological fit to Cahn effect to get kT dependence;  Sivers moments from HERMES ;  generalized parton model

  23. 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

  24. Q: Why are transverse single spin asymmetries interesting? A: Leading-twist, collinear pQCD expects them to be zero. Experimentally, they are non-zero and large, at s=200 GeV, where cross sections are described by pQCD. They are also large at lower s, where cross sections are not described by pQCD. Transverse single spin asymmetries may provide a window onto orbital motion of partons within the proton.

  25. Q. Are there universal aspects for transverse single spin asymmetries between semi-inclusive deep inelastic scattering and hadronic interactions? • Assuming existing p+pp+X data has contributions only from the Sivers effect (related to twist-3 collinear approach), then maybe, since the xF dependence can be described. But, the fixed-xF, pT dependence is not described by theory. Also, experimental separation of Sivers/Collins contributions are not possible in p+pp+X  must go to particle correlations, or to produced particles (e.g., real/virtual g) for which fragmentation (Collins) contributions are not allowed

  26. Low-x

  27. Deep Inelastic Scattering from Nuclear TargetsKinematic Coverage Restricted to Fixed Target Experiments (no EIC, yet) From Hirai, Kumono, Nagai PRC 70 (2004) 044905, and references therein • Growth of gluon distribution at low-x within the proton cannot continue forever • Gluon density in nucleus only known to x~0.02 since g(2x)~F2(x,Q2)/ln(Q2)

  28. Gluon Saturation and the Color Glass Condensate t = ln(1/x) Iancu, Venugopalan hep-ph/0303204 • Does the low-x gluon density saturate, and is this a high-energy phase of matter? • Would a Color Glass Condensate be universal for both nuclear DIS and hadronic probes of nuclei at high energy?

  29. Hints of Gluon Saturation from Large-Rapidity Particle Production in d+Au Collisions at RHIC? d+Aup0+X cross sections at sNN = 200 GeV and <h>=4.0 [STAR, Phys.Rev.Lett. 97 (2006) 152302] NLO pQCD calculations using gluon shadowing [Guzey, Strikman and Vogelsang PLB 603 (2004) 173] CGC model calculation [Dumitru, Hayashigaki, Jalilian-Marian, Nucl.Phys. A770 (2006) 57] • Large-rapidity d+Au cross sections are suppressed • Data are best described by CGC model calculation • Many other possible explanations of suppression

  30. Can Particle Correlations Quantify a CGC? p+p: Di-jet d+Au: Mono-jet? Dilute parton system (deuteron) PT is balanced by many gluons Dense gluon field (Au) Kharzeev, Levin, McLerrangives physics picture (NPA748, 627) Color glass condensate predicts that back-to-back particle correlations in d+Au should be suppressed relative to p+p

  31. Fixed h, as E & pT grows An initial glimpse: correlations in d+Au PRL 97, 152302 (2006) • are suppressed at small <xF> and <pT,π> • consistent with CGC picture • are similar in d+Au and p+p at larger <pT,π> (<xF>) • as expected by HIJING <pT,π> ~ 1.0 GeV/c 25<Ep<35GeV <pT,π> ~ 1.3 GeV/c π0:|<η>| = 4.0 h±: |η| < 0.75; pT > 0.5 GeV/c As expected by HIJING

  32. Q: Why is the gluon density, especially at low Bjorken x, interesting? A: At sufficiently low-x gluon splitting and gluon recombination should balance each other, resulting in gluon saturation. Present-day capabilities in DIS are not conclusive whether the gluon density saturates. An EIC will address this question. Even with an EIC, establishing the universal aspects of a saturated low-x gluon density is important to establish its reality. This may be possible at RHIC and should be pervasive at the LHC, if the CGC is real.

  33. The Future(starts now…)

  34. Outlook Forward Meson Spectrometer Installation completed 2007 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 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

  35. STAR Forward Meson Spectrometer Les Bland1, Ermes Braidot2, Hank Crawford3, Anatoli Derevschikov4, Jim Drachenberg5, Jack Engelage3, Len Eun6, Carl Gagliardi5, Andrew Gordon1, Steve Heppelmann6, Eleanor Judd3, Dmitri Morozov4, Larissa Nogach4, Akio Ogawa1, Hiromi Okada1, Chris Perkins3, Nikola Poljac6, Alexander Vasiliev4 Brookhaven National Laboratory Utrecht University University of California (Berkeley) / Space Sciences Institute IHEP, Protvino Texas A&M University Penn State University Zagreb University

  36. 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. low-x Universality check Pythia Simulation

  37. Transverse Spin and FMS • Acceptance of FMS and projected RHIC performance will enable… • further reach for inclusive p0 and heavy mesons • spin-dependent near-side correlations (p0-p0)  separation of Sivers and Collins effects • spin-dependent away-side correlations (p0-jet)  isolation of Sivers effect • embark on spin-dependent inclusive g and g+jet • Note that entire delivered luminosity is useful for measurements based on run-6 operations. Projections for run-8, assuming split of longitudinal:transverse=2:1 at STAR

  38. Transverse Spin Direct g Theory expects repulsive color charge interactions to result in an opposite sign to spin-correlated momentum imbalance for g+jet. Magnitude of effect requires >105 events to see significant effect. Bacchetta et al.,Phys. Rev. Lett. 99, 212002 (2007) Comparison of run-6 data to simulation provides indication that prompt g can be extracted. The large acceptance and dynamic range of the FMS is needed to veto daughter g from p0,h,… decays. Expect >0.5M g events into small fiducial volume in 30 pb-1 sample with Eg>25 GeV.

  39. The Future is later, too

  40. Sivers in SIDIS vs Drell Yan Important test at RHIC of the fundamental QCD prediction of thenon-universality of the Sivers effect! requires very high luminosity (~ 250pb-1) Transverse-Spin Drell-Yan Physics at RHIC L. Bland, S.J. Brodsky, G. Bunce, M. Liu, M. Grosse-Perdekamp, A. Ogawa, W. Vogelsang, F. Yuan http://spin.riken.bnl.gov/rsc/write-up/dy_final.pdf

  41. Non-universality of Sivers Asymmetries: Unique Prediction of Gauge Theory ! Simple QED example: Drell-Yan: repulsive DIS: attractive Same inQCD: As a result:

  42. Experiment SIDIS vs Drell Yan: Sivers|DIS= − Sivers|DY *** TestQCD Prediction of Non-Universality *** HERMES Sivers Results RHIC Drell Yan Projections 0 Sivers Amplitude Markus Diefenthaler DIS Workshop Műnchen, April 2007 0 0.1 0.2 0.3 x

  43. Rapidity and Collision Energy Large rapidity acceptance required to probe valence quark Sivers function

  44. Backups

  45. Benchmarking Simulations p+p  J/+X  l+l-+X, s=200 GeV PHENIX, hep-ex/0611020 m+m- 1.2<|h|<2.2 e+e- |h|<0.35 J/ is a critical benchmark that must be understood before Drell-Yan

  46. cc bb Dilepton Backgrounds Drell-Yan J/  ’ Isolation needed to discriminate open heavy flavor from DY

  47. 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

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

  49. 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 xF; midrapidity asymmetries are small. • Large xF spin asymmetries show the same pattern for 20 s  200 GeV • First observation of pT dependence at fixed-xF, 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?

  50. Acceptance of FPD 6 5 4 3 2 1 0 STAR xF Inclusive 0 pT GeV/c FPD 0 0.2 0.4 0.6 0.8 Strong xF - pT correlation because of limited acceptance

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