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

8/28/20142 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, experiments never 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.

8/28/20143 A N DY Schematic of Measurement Apparatus RHIC for Spin 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

8/28/20144 Measured Quantity STAR arXiv: Helicity asymmetry for inclusive jet production is measured as a function of p T. Measurements are sensitive to ~2p T /  s

8/28/20145 gluon quark pion or jet quark RHIC Spin Probes Proton 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

8/28/20146 Implications of Measurement de Florian, Sassot, Stratmann, Vogelsang PRL 113 (2014) / arXiv: 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

8/28/20147 Why Study Hadronic Collisions? Many examples where new particles are first observed in the interactions of hadrons: J/  (hidden charm);  (hidden beauty); W ,Z 0 (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 [A N ] for  production and induced polarization [P] for  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?

8/28/20148 What Measurements Can Be Sensitive to OAM? 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

8/28/20149 Transverse SSA in p   +p Collisions Inclusive  0 A N persists to large  s J. Adams et al. (STAR), PRL 92 (2004) ; and PRL 97 (2006) Even though the kinematics of the SIDIS measurement and the forward    data 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 Data: B.I. Abelev et al. (STAR), submitted to PRL [arXiv:hep-ex/ ] Theory (red): M. Boglione, U. D’Alesio, F. Murgia PRD 77 (2008) Theory (blue): C. Kouvaris, J. Qiu, W. Vogelsang, F. Yuan, PRD 74 (2006)

8/28/ Inclusive  0 A N at  s=62.4 GeV Muon piston calorimeter provides PHENIX access to large rapidity Observe large A N that is iincreasing with x F, as seen at higher and lower  s arXiv:

8/28/ Inclusive   Transverse SSA at Midrapidity p T range of midrapidity   production comparable to p T values for large-x F   production. Why is large x F so important for transverse SSA? arXiv: PRL 91 (2003)

8/28/ Inclusive   Transverse SSA at Midcentral Rapidity Mid-central   cross section consistent with NLO pQCD within scale uncertainties Mid-central   transverse SSA are consistent with zero, as expected by twist-3 model. STAR arXiv:

8/28/ x F and p T dependence of A N for p  +p  ± +X,  s=62 GeV A N (  +) ~ -A N (  - ), consistent with results at lower  s and u,d valence differences At fixed x F, evidence that A N grows with p T I. Arsene, et al. PRL101 (2008)

8/28/ Forward Pion Transverse SSA Versus  s RHIC  s=62.4 GeV FNAL  s=19.4 GeV BNL  s=6.6 GeV ANL  s=4.9 GeV Forward pion analyzing power in p+p collisions exhibits similar x F dependence over a broad range of  s Aidala, Bass, Hasch, Mallot RMP 85 (2013) 655 / arXiv:

8/28/ What are the issues?

8/28/ Issue 1 – Inclusive  production does not distinguish initial-state versus final-state k T Other mechanisms have been suggested in recent years Collins mechanism requires transverse quark polarization and spin-dependent fragmentation Sivers mechanism requires spin-correlated transverse momentum in the proton (orbital motion) and color-charge interaction. SSA is present for jet or  final state initial state

8/28/ PRL 99 (2007) 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) 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 Midrapidity Di-Jet Production STAR

8/28/ 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

8/28/ Issue 3 – Initial-State versus Final- State Interactions DIS: attractive Drell-Yan: repulsive Simple QED example: Same in QCD: 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: ]

8/28/ Summary Transverse SSA for Inclusive Pion Production Large  s pion production cross sections are consistent with NLO pQCD Large A N is found to increase with x F over a very broad range of  s x F and p T dependences are now disentangled, with A N increasing with p T 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

8/28/ From Pions to Jets 21 Left/right symmetric HCal Left/right symmetric ECal Left/right symmetric preshower Trigger/DAQ electronics Blue-facing BBC Beryllium vacuum pipe Forward Jets A N DY at IP2

8/28/ Data/Simulation Comparison Jet p T and x F are calculated ignoring mass good agreement between data and simulations above trigger threshold Jet data is well described by simulation Tower multiplicityJet pT 30-50GeV 50-70GeV 70-90GeV Jet shape

8/28/ Forward Jets arXiv: Forward jet cross section is consistent with NLO pQCD [Mukerjee & Vogelsang, PRD 86 (2012) / arXiv: ] Forward jet A N is consistent with Sivers effect from SIDIS [twist-3 (Gamberg, Kang, Prokudin, PRL 110, (2013) / arXiv: ) and generalized parton model (M. Anselmino, et al PRD / arXiv: )]

8/28/ Jet-like versus Jet Caveat emptor ECal triggered jet biases towards EM rich jets more   like A N bias extends well beyond the trigger threshold Run11 data only cone-jet algorithm

8/28/ Regarding Bias… (and calibrations) Pure EM response of HCal has nearly all aspects accounted for in full simulation relative to data [    ] 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

8/28/ 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…

8/28/ Attractive vs Repulsive Sivers Effects Unique Prediction of Gauge Theory ! DIS: attractive Drell-Yan: repulsive Same in QCD: As a result: Transverse Spin Drell-Yan Physics at RHIC (2007) 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 A N DY, detailed simulation studies are still required

8/28/ 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 Existing E864 calorimeter from IP2 FPS (preshower) TPC bEMC Existing forward GEM tracker (redistributed in z) 0.5 T solenoid What are prospects for polarized Drell-Yan with such a forward detector?

8/28/ Status of Run-14 Test 6x6 stack of calorimeter cells mounted at 730 cm from interaction point and 35 cm from beam Pair-mass from forward calorimeter for 3 He+Au collisions [online reconstructions / offline calibration in progress]

8/28/ Prospects for Polarized DY in 2016 When 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: ]

8/28/ 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