1. 1 Forward Physics with Polarized proton-proton Collisions at the experiment. John Koster RIKEN 2012/07/25

2. 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 Q² = negative momentum transfer squared p, proton e, electron , photon

3. 3 The three leading order, collinear PDFs Parton Distribution Functions q(x)  q(x)  T q(x) unpolarized PDF quark with momentum x=p quark /p proton in a nucleon helicity PDF quark with spin parallel to the nucleon spin in a longitudinally polarized nucleon transversity PDF quark with spin parallel to the nucleon spin in a transversely polarized nucleon

4. 4 Deep inelastic scattering (DIS) and Semi-inclusive DIS (SIDIS) pp collisions Probes to Study Polarized Proton Structure + Kinematics are “simple” (x,Q 2 ) + 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. -Most probes integrate over x and Q 2 +/- 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) Figures from DSSV: Prog.Part.Nucl.Phys. 67 (2012) 251-259

5. 5 Current Status of Distribution Functions MSTW 2008 NLO PDFs Eur.Phys.J.C63:189-285,2009 Selected experimental inputs: F 2 from Zeus D0: Phys.Rev.Lett.101:062001,2008E866: Phys. Rev. D 64 (2001) 052002 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.

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

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

8. 8 Possible A N Explanations: Transverse Momentum Dep. Distributions SPSP k T,p p p SPSP p p SqSq k T, π Sivers Effect: Introduce transverse momentum of parton relative to proton. Collins Effect: Introduce transverse momentum of fragmenting hadron relative to parton. Graphics from L. Nogach (2006 RHIC AGS Users Meeting) Correlation between Proton spin (S p ) and quark spin (S q ) + spin dep. frag. function Correlation between Proton spin (S p ) and parton transverse momentum k T,p

9. 9 Possible A N Explanations: Higher Twist Correlation Functions  No k T (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/p T PBPB PA↑PA↑ Graphic from Zhongbo Kang p T =0  A N =0 What is expected A N dependence on p T ? p T large, A N ~ 1/p T Low p T (TMD regime) So far, 1/p T has not been observed in proton-proton collisions

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

11. Connection to Partons in pp Collisions 11 Proton 1Proton 2 Detected hadron Mid-rapidityForward-rapidity Large contribution from gg scattering Symmetric x 1,x 2 distribution Forward rapidity: Selects large-x 1, small x 2 Dominant process: quark- gluon scattering.

12. Forward Rapidity Measurements 1.What can the large transverse single spin asymmetries tell us about the proton’s structure? 2.What is the gluon spin contribution to the proton? Low-x behavior is unconstrained by experiment 3.What is the sea quark polarization?

13. Experimental Setup 13

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 16

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

18. 18  Design detector  PbWO 4 crystals  Crystal wrap “party”  Detector shells  FNAL test beam  Drive to BNL  Prep for install  Install  Take data Forward Calorimetry: Muon Piston Calorimeter

19. MPC Performance 19 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 π 0 and η meson Reconstruction 20 Most interesting region: High Energy, High p T Where possible reconstruct meson’s invariant mass: Otherwise, measure high energy clusters & perform decomposition using Monte-Carlo Decay photon π 0 Direct photon Fraction of clusters

21.  0 A N at High x F,  s=62.4 GeV 21 p  +p  0 +X at  s=62.4 GeV/c 2 x F >0 Non-zero and large asymmetries Suggests effect originates from valence quark effect Complementary to BRAHMS data

22. Isospin Dependence, x F >0,  s=62.4 GeV  + (ud)  - (du) Sign of A N seems consistent with sign of tranversity However, transversity larger for u, but A N is larger for  - Pythia claims 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.  + (ud)  - (du)  0 (uu+dd)/  2 (Preliminary)

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

24. η meson A N Results,  s=200 GeV 24 Preliminary Conclusion: A N η meson > with π 0 Conclusion: A N η meson consistent with π 0 Conclusion: A N η meson consistent with π 0 arXiv:1206.1928

25.  Suggestive drop in A N at high p T Statistical significance is not large enough  Recently acquired dataset will boost the significance. 25 Data between preliminary and published Cluster A N,  s=200 GeV

26.  Hint that A N gamma is probably small. Direct photons are not sensitive to Collins effect  Suggests dominant mechanism not Sivers STAR data from: Phys. Rev. Lett. 101 (2008) 222001 STAR 2γ method PHENIX inclusive cluster

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

28. Measuring A LL at RHIC 28 a b d c h Requirements 1.Longitudinal beam polarization (Dedicated effort needed to setup longitudinal beams) 2.Luminosity monitors Necessary to measure R, relative luminosity. Done using high-rate process and scalers. 3.Detectors for measuring hadrons or jets.

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

30. RHIC A LL Measurements 30 x T =p T / ( ½ √s )

31. Expected impact on ΔG 31 Region probed by existing mid-rapidity measurements. Reminder: With existing probes: higher statistics and slightly lower range in x (√s=200  500 GeV) 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. Low-x region: Paucity of data. Large number of gluons make it possible for large spin contributions. Phys.Rev.D80:034030,2009 2-2.5 GeV/c 4-5 GeV/c 9-12 GeV/c 2-2.5 GeV/c 4-5 GeV/c 9-12 GeV/c  0 at |  |<0.35: x g distribution vs p T bin  s=500 GeV  s=62 GeV  s=200 GeV

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

33. Increasing precision on forward A LL Three essential components to forward A LL success: 1. High RHIC polarization and luminosity From 2012+2013 we expect a huge dataset. 2. 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. 3. 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. 33

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

35. Expected Lepton Asymmetries 35 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 36 Design Luminosity √s = 500 GeV σ=60mb L = 1.6 x10 32 /cm 2 /s Total X-sec rate ＝ 9.6 MHz Default PHENIX Trigger: Rejection=200 ~ 500 DAQ LIMIT=1-2 kHz Required Rejection 10,000

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

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

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

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

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

42. W  μ: 2012+2013 Projected Statistical Errors Experimental Challenges: 1. Improving the existing S/BG ratio. 2. Finishing all shutdown activities 3. Bringing triggering system online quickly at the start of Run 13. 4. Operating PHENIX experiment efficiently to sample as much of delivered luminosity as possible. 42

43. Summary & Outlook: Transverse  Search for falloff of A N at high p T will extend to higher p T with 2012 dataset  First hints that direct γ A N small  Hurts case for Sivers effect  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: A N DY experiment recently shuttered PHENIX: Spin running in near-term will be with longitudinal polarization. –Possibility at COMPASS: Effort well underway to perform measurements. 43

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 A L W  μ results from PHENIX –Substantial dataset collected in 2012.  Decisive dataset coming in 2013. 44

45.  Backup 45