Forward Physics with Polarized proton-proton Collisions at the experiment.

1. Forward Physics with Polarized proton-proton Collisionsat the experiment. John KosterRIKEN2012/07/25

2. Motivation: Structure of Matter • Structure of the proton • 1955 Hofstadter: Radius 0,8 fm Nobel Prize 1961 • 1968 Friedman, Kendall, Taylor: quarks in the proton Nobel Prize 1990 • Highest Q²: quarks, gluons elementary e, electron p, proton g, photon Q² = negative momentum transfer squared

3. Parton Distribution Functions The three leading order, collinear PDFs unpolarized PDF quark with momentumx=pquark/pprotonin a nucleon q(x) helicity PDF quark with spin parallel to the nucleon spinin a longitudinally polarized nucleon Dq(x) transversity PDF quark with spin parallel to the nucleon spin in a transversely polarized nucleon DTq(x)

4. Probes to Study Polarized Proton Structure Deep inelastic scattering (DIS) and Semi-inclusive DIS (SIDIS) ppcollisions • Most probes integrate over x and Q2 • +/- Theoretical interpretation of results often requires additional effort. Typically, several effects contribute to one measurement. • + Direct access to gluons • + High scales accessible with RHIC (collider) • + Kinematics are “simple” (x,Q2) • + Underlying theory is well understood • Each angular moment accesses different proton structure. • Indirect access to gluons • Highest scales not accessible with existing facilities. Collider project (EIC) in design stage. Figures from DSSV: Prog.Part.Nucl.Phys. 67 (2012) 251-259

5. Current Status of Distribution Functions Selected experimental inputs: • What do we learn? • Proton momentum: carried ½ by gluons, ½ by quarks • ∫ x q(x) dx • Gluon distribution continues to rise at low-x. • Sea is not symmetric between u and d. D0: Phys.Rev.Lett.101:062001,2008 E866: Phys. Rev. D 64 (2001) 052002 MSTW 2008 NLO PDFsEur.Phys.J.C63:189-285,2009 F2 from Zeus

6. Current status of helicity distributions What do we learn? Decompose proton spin: Gluon Spin+so far: small in limited xBjrange Quark Spin +~0.24 Orbital Angular Momentum All plots from DSSV: PRD 80, 034030 (2009), experimental results from respective collaborations

7. h Current Status of Transverse Spin Early measurements in transverse spin indicated deeper structure when proton transversely polarized. Left Right Early Theory Expectation:Small asymmetries at high energies (Kane, Pumplin, Repko, PRL 41, 1689–1692 (1978) ) Vanishing asymmetry Z.Phys., C56, 181 (1992) IP Conf. Proc., vol. 915 (2007) PRL 101, 222001 (2008)

8. Possible AN Explanations: Transverse Momentum Dep. Distributions Sivers Effect:Introduce transverse momentum of parton relative to proton. Collins Effect:Introduce transverse momentum of fragmenting hadron relative to parton. SP SP kT,p p p p p Sq kT,π Correlation between Proton spin (Sp) and quark spin (Sq) + spin dep. frag. function Correlation between Proton spin (Sp) and parton transverse momentum kT,p Graphics from L. Nogach (2006 RHIC AGS Users Meeting)

9. Possible AN Explanations: Higher Twist Correlation Functions PB • No kT (collinear partons) • Additional interactions between proton and scattering partons • Goes beyond leading twist (two free colliding quarks) Higher twist interaction contributions expected to drop like 1/pT So far, 1/pT has not been observed in proton-proton collisions PA↑ What is expected AN dependence on pT? pT=0  AN=0 pT large, AN ~ 1/pT Low pT (TMD regime) Graphic from Zhongbo Kang

10. Selected Extractions of Transverse Structure Sivers Sivers Collins Torino09 Transversity

11. Connection to Partons in pp Collisions Detectedhadron Detectedhadron • Forward rapidity: • Selects large-x1, small x2 • Dominant process: quark-gluon scattering. Proton 1 Proton 2 Mid-rapidity Forward-rapidity Large contribution from gg scattering Symmetric x1,x2 distribution

12. Forward Rapidity Measurements • What can the large transverse single spin asymmetries tell us about the proton’s structure? • What is the gluon spin contribution to the proton? • Low-x behavior is unconstrained by experiment • What is the sea quark polarization?

13. Experimental Setup

14. Relativistic Heavy Ion Collider 2 counter-rotating packets of particles collide at 2 interactions points 108 ns between proton packets. Each packet has independent spin orientation (up or down).  Important for control of systematic effects in spin measurements. Typical pp collision rates: 2 MHz. DAQ bandwidth: 7 kHz Efficient triggering systems are essential for physics

15. Relativistic Heavy Ion Collider Performance • Accelerator performance improves every year of operation. • 2012 “Breakthrough” year for polarized proton performance. • ~135pb-1 delivered to experiments • 2013 PAC priority #1: “Running with polarized proton collisions at 500 GeV to provide an integrated luminosity of 750 pb-1 at an average polarization of 55%” 2012+2013 dataset will provide critical datasets for RHIC Spin Program

16. 2012 RHIC Running Review

17. PHENIX Experiment • Central Arms | η | < 0.35 • Identified charged hadrons • Neutral Pions • Direct Photon • J/Psi • Heavy Flavor • Muon Arms 1.2 < | η | < 2.4 • High momentum muons • J/Psi • Unidentified charged hadrons • Heavy Flavor • MPC 3.1 < | η | < 3.9 • Neutral Pion’s • Eta’s

18. Forward Calorimetry: Muon Piston Calorimeter • Design detector • PbWO4 crystals • Crystal wrap “party” • Detector shells • FNAL test beam • Drive to BNL • Prep for install • Install • Take data

19. MPC Performance Detector performance is excellent and behavior is well understood with both Monte-Carlo and data. Rare probes can be studied by using high-energy triggering system. Unpolarized pion cross-section agrees well with world-data.

20. MPC π0 and η meson Reconstruction Most interesting region: High Energy, High pT Where possible reconstruct meson’s invariant mass: Otherwise, measure high energy clusters & perform decomposition using Monte-Carlo Decay photonπ0Direct photon Fraction of clusters

21. p+p0+X at s=62.4 GeV/c2 0AN at High xF, s=62.4 GeV • xF>0 Non-zero and large asymmetries • Suggests effect originates from valence quark effect • Complementary to BRAHMS data

22. IsospinDependence, xF>0, s=62.4 GeV (Preliminary) + (ud) + (ud) 0 (uu+dd)/2 - (du) - (du) • Sign of AN seems consistent with sign of tranversity • However, transversity larger for u, but AN is larger for - • Pythiaclaims that originating quarks for mesons is:+: ~100%u -: 50/50% d/u 0: 25/75% d/u • u quark dominance in pion production over d’s.

23. s Dependence of 0 AN (Preliminary) • No strong dependence on s from 19.4 to 200 GeV • Varying experimental acceptance most likely causes spread in AN • Unexpected that AN does not vary over huge range of energy • pQCD does not reproduce low energy unpolarized cross-sections

24. η meson AN Results, s=200 GeV Conclusion:ANη meson consistent with π0arXiv:1206.1928 Preliminary Conclusion:ANη meson > with π0 Conclusion:ANη meson consistent with π0

25. Cluster AN, s=200 GeV • Suggestive drop in AN at high pTStatistical significance is not large enough • Recently acquired dataset will boost the significance. Data between preliminary and published

26. Cluster AN, s=200 GeV STAR 2γ method PHENIX inclusive cluster STAR data from:Phys. Rev. Lett. 101 (2008) 222001 • Hint that ANgamma is probably small. Direct photons are not sensitive to Collins effect  Suggests dominant mechanism not Sivers

27. HelicityMeasurements at RHIC • Inclusive Jet/hadron production Measured h c b Measured Spin sorted relative luminosities a d Fragmentation function from parton c to hadron h with momentum fraction z Hard scattering cross-section (calculable) Helicity distributions (to be extracted in global analysis)

28. Measuring ALL at RHIC h c b Requirements Longitudinal beam polarization(Dedicated effort needed to setup longitudinal beams) Luminosity monitorsNecessary to measure R, relative luminosity. Done using high-rate process and scalers. Detectors for measuring hadrons or jets. a d

29. RHIC ALL measurements • Hadron ALL precision reaches 10-3 but results are consistent with zero • Currently, measurement is systematics limited! • Dedicated studies performed in 2012 to address this PHENIX Mid-Rapidity| η | < 0.35 2005 2006 2009

30. RHIC ALL Measurements • Preliminary results from the STAR collaboration using Jets at mid-rapidity. • First non-zero ALL results from RHIC! • First look from DSSV collaboration: =0.13(global analysis needs to be redone)arXiv:1112.0904 • Like PHENIX ALL,measurement is performed at mid-rapidity xT=pT/ ( ½ √s )

31. Expected impact on ΔG Low-x region: Paucity of data. Large number of gluons make it possible for large spin contributions. High-x region: ΔG(x) at high x is an interesting measurement However, even if gluons are 100% polarized, the number of gluons dries up at high x  small possible contribution. 0 at ||<0.35: xg distribution vspT bin s=62 GeV s=200 GeV 2-2.5 GeV/c 4-5 GeV/c 9-12 GeV/c Phys.Rev.D80:034030,2009 Region probed by existing mid-rapidity measurements. Reminder: With existing probes: higher statistics and slightly lower range in x (√s=200  500 GeV) 2-2.5 GeV/c 4-5 GeV/c 9-12 GeV/c s=500 GeV

32. Measuring ΔG(x) at low-x • Strategy: Exploit forward kinematics to probe small-x • Expected asymmetries have been simulated (C. McKinney) • First measurements performed using MPC (S. Wolin) Simulation Measurement

33. Increasing precision on forward ALL Three essential components to forward ALL success: • High RHIC polarization and luminosityFrom 2012+2013 we expect a huge dataset. • Reduce systematic errors. • Dominant contribution from relative luminosity • Monte-Carlo + special accelerator studies in 2012 were performed with encouraging results. Followup studies planned for 2013. • Increase purity of MPC triggering system • Pre-2012: high fake rate from low-energy neutron backgrounds. • Post-2012: Electronics upgrade • Fully digital triggering system with “smart” trigger algorithm to reject isolated high energy towers.

34. Parton Helicity • W-production • W-boson AL Benefits: • “Clean” probe • High scale • u and d enter at same level • Simpler fragmentation from single hadron case Measured νe u Measured Spin sorted relative luminosities W+ d l+ Similar expression for W- Presented measurements measure leptons from decay of WKinematic smearing, nonetheless, at forward rapidity Suppressed at forward rapidity

35. Expected Lepton Asymmetries Mid-rapidity via We+/- Forward-rapidity Wμ+/- In both cases, experimental signature is high momentum lepton with small event rates

36. Wμ+/- Challenge Design Luminosity √s = 500 GeV σ=60mb L = 1.6 x1032/cm2/s Total X-sec rate＝9.6 MHz DAQ LIMIT=1-2 kHz Required Rejection 10,000 Default PHENIX Trigger: Rejection=200 ~ 500

37. Run11 Muon Trigger Hardware Absorber +Removes backgrounds Muon Tracker +Offline p measurement +Online trigger Muon ID +Low-p momentum threshold trigger RPC3 +Additional tracking +Timing information Absorber Muon Tracker RPC3 Muon ID

38. Run12 Muon Trigger Hardware FVTX Upgrade +Adds tracking RPC1 +Adds acceptance +Adds trigger rejection Additional Absorber +Shields detector from in-time backgrounds RPC1 FVTX Absorber Muon Tracker RPC3 Muon ID

39. Wμ: Trigger Commissioning Run13 Production Trigger Run12 Production Trigger Run11 Production Trigger Trigger Rejection Keep-out region where trigger will take too much DAQ bandwidth Collision Rate [MHz]

40. Wμ: Trigger Commissioning • Run12 Muon-like track turn on curve • Yield (Production Trigger) / Yield (Minimum Bias) South North Trigger maintains high rejection and selects high momentum tracks p (GeV/c)

41. Wμ: 2011 Results • First measurement at forward rapidities. • Results statistics limited. • In 2012+2013 dataset will be greatly expanded. μ+ μ-

42. Wμ: 2012+2013 Projected Statistical Errors Experimental Challenges: • Improving the existing S/BG ratio. • Finishing all shutdown activities • Bringing triggering system online quickly at the start of Run 13. • Operating PHENIX experiment efficiently to sample as much of delivered luminosity as possible.

43. Summary & Outlook: Transverse • Search for falloff of AN at high pTwill extend to higher pT with 2012 dataset • First hints that direct γ AN small  Hurts case for Siverseffect • In longer term: • With availability of high luminosity facilities: shift in hadron collisions towards “cleaner” probes. • Drell-Yan process is currently a hot topicExpected sign change in Sivers amplitude between DY and SIDIS • Possibilities at RHIC: • ANDY experiment recently shuttered • PHENIX: Spin running in near-term will be with longitudinal polarization. • Possibility at COMPASS: • Effort well underway to perform measurements.

44. Summary & Outlook • Gluon Helicity distributions • RHIC effort moving towards low-x ΔG measurements. • Forward region is essential for reaching lowest x possible. • Sea Quark Helicity Distributions • First AL Wμ results from PHENIX • Substantial dataset collected in 2012. • Decisive dataset coming in 2013.

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