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Overview of Orbital Angular Momentum via TMD Measurements in Hadronic Collisions

Overview of Orbital Angular Momentum via TMD Measurements in Hadronic Collisions. OUTLINE General comments Transverse SSA for p  pX Transverse SSA for p  pjetX Outlook Conclusions. L.C. Bland Brookhaven National Lab ECT*, Trento 28 August 2014. Introductory Comments….

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Overview of Orbital Angular Momentum via TMD Measurements in Hadronic Collisions

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  1. Overview of Orbital Angular Momentum via TMD Measurements in Hadronic Collisions • OUTLINE • General comments • Transverse SSA for ppX • Transverse SSA for ppjetX • Outlook • Conclusions L.C. Bland Brookhaven National Lab ECT*, Trento 28 August 2014

  2. Introductory Comments… • This is at least the second ECT* workshop on OAM. There has been progress, but there remain many open questions for both experiment and theory • The role of polarized proton collisions, and more generally hadronic interactions with polarization, needs to be continually emphasized [see below]. • As an experimentalist, “to measure” has special meaning… In my opinion, experimentsnever measure distribution functions. Instead, experiments measure [spin-dependent] cross sections that are interpreted by theory, ideally to provide universal distribution [or fragmentation] functions. Comparing measurements to theory is the way we understand hadronic structure.

  3. Schematic of Measurement ApparatusRHIC for Spin RHIC pC Polarimeters Absolute Polarimeter (H jet) ANDY 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

  4. STAR arXiv:1405.5134 Measured Quantity • Helicity asymmetry for inclusive jet production is measured as a function of pT. • Measurements are sensitive to <x>~2pT/s

  5. pion or jet quark quark gluon RHIC Spin ProbesProton collisions / collinear factorization 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

  6. Implications of Measurement Evidence for polarization of gluons from global NLO fit to preliminary version of inclusive jet data from STAR, neutral pion data from PHENIX and polarized deep inelastic scattering de Florian, Sassot, Stratmann, Vogelsang PRL 113 (2014) 012001 / arXiv:1404.4293

  7. Why Study Hadronic Collisions? • Many examples where new particles are first observed in the interactions of hadrons: J/ (hidden charm);  (hidden beauty); W,Z0 (weak bosons); H (Higgs boson), as just a few well-known examples. • Hadro-production has provided many examples of first observation of unexpected phenomena: transverse single-spin asymmetries (SSA) - both analyzing power [AN] for p production and induced polarization [P] for L production; Lam-Tung violation in Drell-Yan [see backup for details/references]; … • Hadro-production provides direct sensitivity to gluons: unpolarized gluon PDF, especially to low-x ; gluon spin contribution (G) • Hadro-production is pursued because of emergent phenomena: quark-gluon plasma ; onset of gluon saturation? • Hadro-production can provide an important test of universality – are the quantities measured in hard scattering processes really telling us about the structure of the proton?

  8. 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 What Measurements Can Be Sensitive to OAM?

  9. Data: B.I. Abelev et al. (STAR), submitted to PRL [arXiv:hep-ex/0801.2990] Theory (red): M. Boglione, U. D’Alesio, F. Murgia PRD 77 (2008) 051502. Theory (blue): C. Kouvaris, J. Qiu, W. Vogelsang, F. Yuan, PRD 74 (2006) 114013 Transverse SSA in p+p CollisionsInclusive 0 AN persists to large s J. Adams et al. (STAR), PRL 92 (2004) 171801; and PRL 97 (2006) 152302 Even though the kinematics of the SIDIS measurement and the forward p0data have little overlap, it was possible to account for most of the features of the RHIC data by calculations based on phenomenological fits to the SIDIS data

  10. Inclusive 0 AN at s=62.4 GeV • Muon piston calorimeter provides PHENIX access to large rapidity • Observe large AN that is iincreasing with xF, as seen at higher and lower s arXiv:1312.1995

  11. Inclusive p0 Transverse SSA at Midrapidity PRL 91 (2003) 241803 arXiv:1312.1995 • pT range of midrapidity p0 production comparable to pT values for large-xFp0 production. • Why is large xF so important for transverse SSA?

  12. STAR Inclusive p0 Transverse SSA at Midcentral Rapidity arXiv:1309.1800 • Mid-central p0 cross section consistent with NLO pQCD within scale uncertainties • Mid-central p0 transverse SSA are consistent with zero, as expected by twist-3 model.

  13. xF and pT dependence of AN for p+pp±+X, s=62 GeV I. Arsene, et al. PRL101 (2008) 042001 • AN(p+) ~ -AN(p-), consistent with results at lower s and u,d valence differences • At fixed xF, evidence that AN grows with pT

  14. RHIC s=62.4 GeV Forward Pion Transverse SSA Versus s ANL s=4.9 GeV BNL s=6.6 GeV FNAL s=19.4 GeV Aidala, Bass, Hasch, Mallot RMP 85 (2013) 655 / arXiv:1209.2803 Forward pion analyzing power in p+p collisions exhibits similar xF dependence over a broad range of s

  15. What are the issues?

  16. Issue 1 – Inclusive p production does not distinguish initial-state versus final-state kT Sivers mechanism requires spin-correlated transverse momentum in the proton (orbital motion) and color-charge interaction. SSA is present for jet or g Collins mechanism requires transverse quark polarization and spin-dependent fragmentation initial state final state Other mechanisms have been suggested in recent years

  17. Midrapidity Di-Jet Production 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! PRL 99 (2007) 142003

  18. Issue 2 - Factorization • Factorization is used for inclusive particle production for collinear distribution and fragmentation functions, and in general, works well for RHIC energies • TMD factorization is not proven for inclusive (or, more complicated) hadro-production, although has been proven for Drell-Yan production. • Twist-3 collinear calculations are based on factorization. Moments of qg correlators from twist-3 analyses are related to the Sivers function

  19. Simple QED example: Drell-Yan: repulsive DIS: attractive Same inQCD: Issue 3 – Initial-State versus Final-State Interactions In general, particle production in p+p collisions will mix initial-state (DY-like) and final-state (SIDIS-like) interactions. Present understanding is that p+pX is DY-like  “sign-mismatch” between SIDIS and p+pX transverse SSA [arXiv:1103.1591]

  20. SummaryTransverse SSA for Inclusive Pion Production • Large s pion production cross sections are consistent with NLO pQCD • Large AN is found to increase with xF over a very broad range of s • xF and pT dependences are now disentangled, with AN increasing with pT to a plateau • Pion asymmetries are consistent with zero at central and mid-central rapidities Go beyond inclusive pion production to test present understanding  jets, Drell-Yan production, direct photons

  21. Forward JetsANDY at IP2 Trigger/DAQ electronics Left/right symmetric HCal Left/right symmetric ECal Blue-facing BBC Left/right symmetric preshower Beryllium vacuum pipe From Pions to Jets 21

  22. Data/Simulation Comparison 30-50GeV 50-70GeV 70-90GeV Tower multiplicity Jet pT Jet shape Jet pT and xF are calculated ignoring mass good agreement between data and simulations above trigger threshold Jet data is well described by simulation

  23. Forward Jets arXiv:1304.1454 • Forward jet cross section is consistent with NLO pQCD [Mukerjee & Vogelsang, PRD 86 (2012) 094009 / arXiv:1209.1775] • Forward jet AN is consistent with Sivers effect from SIDIS [twist-3 (Gamberg, Kang, Prokudin, PRL 110, 232301(2013) / arXiv:1302.3218) andgeneralized parton model (M. Anselmino, et al PRD 88.054023 / arXiv:1304.7691)]

  24. Jet-like versus JetCaveat emptor Run11 data only cone-jet algorithm • ECal triggered jet biases towards EM rich jets • more p0 like AN • bias extends well beyond the trigger threshold 24

  25. Regarding Bias…(and calibrations) • Pure EM response of HCal has nearly all aspects accounted for in full simulation relative to data [0] • Pure hadronic response of HCal has most aspects accounted for in full simulation relative to data [p- and conjugate] • Pure hadronic response (that is sensitive to hadronic energy calibration) has most aspects accounted for in full simulation relative to data [K*Kp and conjugate] Jets include both electromagnetic and hadronic components There are many canceling effects for jet-related observables Control of detector biases are critical

  26. Outlook-I • RHIC remains the only accelerator with polarized proton beams. The large s of the collider has established via cross section measurements that particle production can be explained by NLO pQCD. • Large acceptance RHIC experiments (STAR and PHENIX) are well instrumented at midrapidity. Spin asymmetries, in general, increase in the forward direction (valence quark phenomena?) • Efforts are underway at both STAR and PHENIX to improve forward detector capabilities, because of the important path to a future electron-ion collider • Economic realities may require clever reuse of existing equipment to start on the path…

  27. Attractive vs Repulsive Sivers Effects Unique Prediction of Gauge Theory ! Drell-Yan: repulsive DIS: attractive Same inQCD: As a result: Transverse Spin Drell-Yan Physics at RHIC (2007) http://spin.riken.bnl.gov/rsc/write-up/dy_final.pdf • For now, RHIC is the only accelerator with polarized beams • RHIC should pursue polarized DY, in kinematics that match as closely as possible those from SIDIS  forward • Although STAR-forward is similar to ANDY, detailed simulation studies are still required

  28. 0.5 T solenoid Existing forward GEM tracker (redistributed in z) Existing E864 calorimeter from IP2 FPS (preshower) TPC bEMC Forward Instrumentation for 2016?“STAR-Forward” • Some caveats: • Suitability of E864 calorimeter was investigated in the 2014 RHIC run • Radiation resistance of silicon photomultipliers was investigated in the 2014 RHIC run • Suitability of GEM tracker to be demonstrated by efficiency measurements • Future efforts are pending review eEMC What are prospects for polarized Drell-Yan with such a forward detector?

  29. 6x6 stack of calorimeter cells mounted at 730 cm from interaction point and 35 cm from beam Status of Run-14 Test Pair-mass from forward calorimeter for 3He+Au collisions [online reconstructions / offline calibration in progress]

  30. Prospects for Polarized DY in 2016When STAR-Forward is Realized… • Accounts for acceptance of STAR-forward calorimeter • Assumes data sample of 400 pb-1 in 2016 • Background evaluations are in progress… • QCD backgrounds to DY production are constrained from existing di-jet measurements [arXiv:1308.4705]

  31. Conclusions • Polarized proton collisions have provided important information regarding proton structure • Transverse SSA asymmetries for p+pX are large in the forward direction over a broad range of s. Spin-averaged cross sections can be described by NLO pQCD at RHIC energies where transverse SSA are large. • There are many issues of interpreting p+pX transverse SSA, particularly in regard to the role of the Sivers function which is related to partonic OAM • Forward p+pjet+X have small, positive analyzing powers. Cross sections are in agreement with NLO pQCD. This addresses some, but not all, issues regarding OAM • Forward p+p*+X may still be possible at RHIC on a time scale relevant to address the predicted sign change

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