Nucleon Spin Structure Longitudinal Spin of the Proton. HUGS Summer School Jefferson National Laboratory June 2, 2011 Lecture 5

Nucleon Spin Structure Longitudinal Spin of the Proton HUGS Summer School Jefferson National Laboratory June 2, 2011 Lecture 5

RHIC as a Polarized Proton Collider Absolute Polarimeter (H jet) pc Polarimeters PHOBOS BRAHMS PHENIX Siberian Snakes Siberian Snakes STAR Spin Rotators (longitudinal polarizabon) Spin Rotators (longitudinal polarizabon) Spin flipper LINAC BOOSTER 5.9% Helical ParBal Siberian Snake Pol. H - Source AGS Internal Polarimeter 200 MeV Polarimeter pc Polarimeter 10-25% Helical ParBal Siberian Snake Without Siberian snakes: ν sp = Gγ = 1.79 E/m ~1000 depolarizing resonances With Siberian snakes (local 180 spin rotators): ν sp = ½ no first order resonances Two parbal Siberian snakes (11 and 27 spin rotators) in AGS 2

1 st step: truly global NLO QCD fit D. deflorian et al, PRL 101, 072001 (2008) Includes PHENIX Inclusive π 0 at 200 and 62 GeV and STAR inclusive Jets at 200 GeV along with world s polarized DIS data sets à Resulting ΔG is small, with large uncertainties 3

If the gluon s spin contribution is small ( remains small even after measuring it over the wide x-range.): What is the separation of q vs. qbar contribution to nucleon spin? Know how to do. Where is the remaining spin of the proton? Orbital motion of quarks and gluons? Learning how to do. 4

Anti-Quark Polarization measurement via W Large parity violating effect anticipated Measurement complimentary to SIDIS, but devoid of fragmentation function makes it cleaner! NLO analyses about to become available 5

Some insight in to what goes on. 6

W production @ RHIC 7

Single spin asymmetry in full detail. 8

First Observation of W s at RHIC Exciting few years ahead PHENIX 9

First Observation of W s at RHIC Exciting few years ahead STAR 10

Calibration of probes. W cross section PHENIX and STAR Data consistent with Standard model predicbons Of p- p cross secbon 11

A L for Wà e STAR and PHENIX central rapidity 12

Upgrades for W Physics: high η Both STAR and PHENIX can measure W->e near midrapidity Analysis of first 500 GeV data in 2009 is underway! What about larger rapidities? Both detectors are currently being upgrades e ± µ ± End Cap EMCal can tag electron. TPC does not give enough points for charge sign measurement à Forward GEM Tracker IdenBficaBon from MUID and charge sign/ momentum from MUTr. BUT, trigger is dominated by low momentum parbcles à ResisBve Plate Chambers à 5% of MUTr signal into trigger circuit Use bend in track to trigger high mom. muon 13

RHIC Luminosity Delivery Goal With ~70% beam polarization Experiments are getting ready for the program! (The only projecbon- plot in this talk!) 14

Transverse spin effects: Not the main charge given to me, but the spin spin puzzle can not be completely solved without some consideration of this. Reca# the Elephant in the Vi#age of Blind story? Understanding the subtleties of transverse spin effects is significantly more difficult, the framework for understanding them has evolved (and is evolving) over the past decade Details in Lectures by Accardi, Prokudin

Transverse spin introduction Kane, Pumplin, Repko 1978 Since people starved to measure effects at high p T to interpret them in pqcd frameworks, this was neglected as it was expected to be small.. However. Pion production in single transverse spin collisions showed us something different. 16

Early expectations Sing Spin Asymmetries For example: Azimuthal asymmetry in singly polarized pp collisions Left, forward Prediction: Kane, Pumplin and Repko PRL 41 1689 (1978) SSAs in hard scattering are expected to be very small: (leading twist effects). Measurements? Abhay Deshpande, Nucleon Spin Lectures 5 of 6 at HUGS 2011 17

Pion asymmetries: at most CM energies! ZGS/ANL s=4.9 GeV AGS/BNL s=6.6 GeV FNAL s=19.4 GeV RHIC s=62.4 GeV Suspect sok QCD effects at low scales, but they seem to remain relevant to perturbabve regimes as well 18

Other unexpected discoveries Large very forward neutron asymmetry found at RHIC. Center of Mass & p T dependence studied Not understood how it arises: a challenge to theorist 19

New at RHIC: pqcd Framework At 200 GeV Pion cross sections at both mid and forward rapidities described by NLO pqcd calculation. At 62.4 GeV pions are reasonably well described at both mid and forward rapidities NLL may be important 62.4 GeV 200 GeV 62.4 GeV 20

Transverse Single-Spin Asymmetry in η Meson Production Larger than the neutral pion! STAR 21

Transverse Single-Spin Asymmetry in η Meson Production Further evidence against a valence quark effect! Larger than the neutral pion! STAR 21

Sivers effect: due to transverse motion of quarks in the nucleon: initial state effect Phys Rev D41 (1990) 83; Phys Rev D43 (1991) 261 Top view x is longitudinal momentum fraction. Front view Quark transverse momentum in transversely polarized proton. INITIAL STATE EFFECT: Orbital angular momentum? Abhay Deshpande, Nucleon Spin Lectures 5 of 6 at HUGS 2011 22

Top view, Breit frame Red shift What does Sivers effect probe? Sivers function hep-ph/ 0703176 Hard probe (Parton, γ ) Blue shift Quarks orbital motion adds/ subtracts longitudinal momentum for negative/positive. Quark Orbital angular momentum PRD66 (2002) 114005 Parton Distribution Functions rapidly fall in longitudinal momentum fraction x. Final State Interaction between outgoing quark and target spectator. Generalized Parton Distribution Functions PRD59 (1999) 014013 23

Collins (Heppelmann) effect: Asymmetry in the fragmentation hadrons Example: Nucl Phys B396 (1993) 161, Nucl Phys B420 (1994) 565 Polarization of struck quark which fragments to hadrons. 24

π 200 GeV π,k,p 200 & 62.4 GeV π 62.4 GeV K 200 GeV Scales on plots different Kaon asymmetries not predicted K 62.4 GeV p Unfortunately no anb- proton measurement p 200 GeV 62.4 GeV 25 25

Measurements in Semi-Inclusive DIS Phenomenology: Goeke et al, Anselmino et al, Baccheqa et al, Vogelsang et al. 26

Although not expected, at any observable level, 400+ times the expected values of asymmetries have been routinely seen experimentally: both in ep and pp systems. Much work is now under progress to systematically study and understand them. Transverse motion/momentum of partons or Asymmetry in fragmentation process (final state) or Both. May be responsible. If it is the transvers momentum of quarks then it may have direct connection to orbital motion of partons, and hence connected to the total angular momentum contributing to the nucleon spin! Much more on this in lectures by A. Prokudin 27

Summary 28

Summary RHIC program will measure the anti-quark polarization in the next few years (same data sets would be used to improve the polarized gluon distribution) 28

Summary RHIC program will measure the anti-quark polarization in the next few years (same data sets would be used to improve the polarized gluon distribution) Transverse spin phenomena will be systematically studied as well, leading to better understanding of the richness of QCD However, will the proton spin puzzle be solved? What remains? ΔG at low x? Spin structure functions and its behavior at low x? If orbital angular motion plays a role, what is the orbital contribution from Gluons? Precision Measurements at a future facility! 28