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

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

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 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)

5 Current Status of Distribution Functions MSTW 2008 NLO PDFs Eur.Phys.J.C63: ,2009 Selected experimental inputs: F 2 from Zeus D0: Phys.Rev.Lett.101:062001,2008E866: Phys. Rev. D 64 (2001) 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 Current status of helicity distributions All plots from DSSV: PRD 80, (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 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, (2008)

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 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

Selected Extractions of Transverse Structure Sivers Collins Sivers Transversity Torino09

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.

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?

Experimental Setup 13

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

Relativistic Heavy Ion Collider Performance Accelerator performance improves every year of operation “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%” dataset will provide critical datasets for RHIC Spin Program

2012 RHIC Running Review 16

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  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

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.

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

 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

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)

 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)

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:

 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

 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) STAR 2γ method PHENIX inclusive cluster

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)

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.

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

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

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, GeV/c 4-5 GeV/c 9-12 GeV/c 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

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

Increasing precision on forward A LL Three essential components to forward A LL success: 1. High RHIC polarization and luminosity From 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 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

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

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

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

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

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

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

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

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

W  μ: 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 Operating PHENIX experiment efficiently to sample as much of delivered luminosity as possible. 42

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

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  Decisive dataset coming in

 Backup 45