Flavor Decomposition of Nucleon Spin via polarized SIDIS: JLab 12 GeV and EIC. Andrew Puckett Los Alamos National Laboratory INT 09/24/2010

Transcription

1 Flavor Decomposition of Nucleon Spin via polarized SIDIS: JLab 12 GeV and EIC Andrew Puckett Los Alamos National Laboratory INT 09/24/2010

2 Outline Nucleon Structure Nucleon spin structure Flavor decomposition of nucleon spin Inclusive DIS Semi-inclusive DIS pp collisions SIDIS A LL measurements JLab 12 GeV EIC

3 Preliminaries Nucleon Structure d 2 σ dxdy = 4πα 2 1 y x 2 y 2 M 2 xyq 2 Q 2 F 2 (x,q 2 ) + y 2 xf 1 (x,q 2 ) Inclusive DIS cross section in QED, dominant one-photon-exchange mechanism; unpolarized structure functions F 1, F 2. In the naive quarkparton model; we have: F 2 (x,q 2 ) = x q e q 2 (q(x) + q (x)) F 1 (x,q 2 ) = F 2 (x,q 2 ) /2x F 2p data with ZEUS NLO pqcd fit

4 Unpolarized PDFs MSTW2008 (arxiv: ) NNLO PDFs Unpolarized parton densities well constrained from a variety of different reaction channels; now selfconsistently up to NNLO accuracy

5 A = dσ dσ Polarized DIS and Spin Structure dσ + dσ = D(A 1 + ηa 2 ) A = dσ dσ dσ + dσ = d(a 2 ξa 1 ) A 1 = g 1 γ 2 g 2 F 1 A 2 = γ g 1 + g 2 F 1 [ ] γ 2 = 4M2 x 2 Q 2,R = 1+ γ 2 D = F 2 1 2xF 1 y[ ( 1+ γ 2 y /2)( 2 y) ] ( ) + 2( 1+ R) ( 1 y γ 2 y 2 /4) [ ] 1 y γ 2 y 2 /4 ( 1 y /2)( 1+ γ 2 y /2) y 2 1+ γ 2 d = D 1+ γ 2 y /2 1 y y 2 γ 2 /4 η = γ 1 y /2 ( ) ( ) 1+ γ 2 y /2 1 y /2 ξ = γ 1+ γ 2 y /2 Functions Form helicity asymmetries in the scattering of longitudinally polarized electrons on longitudinally and transversely (in the scattering plane) polarized nucleons Parallel and perp. asymmetries related to spin structure functions g 1 and g 2 Virtual photon-nucleon asymmetries A 1 and A 2 Approximation: A 1 A D ( 1+ γ 2 ) g 1 F 1 In the naive quark-parton model, g 1 is an incoherent sum over parton helicity distributions Δq, equal to the difference in number density of quarks polarized parallel and antiparallel to the nucleon spin: g 1 = 1 2 q e 2 q ( Δq + Δq )(x)

6 Experimental status of g 1 PDG 2010 approximate world proton, deuterium and neutron g 1 (from 3 He) data Virtually all data are obtained in inclusive DIS experiments on long. polarized targets Limitation of inclusive DIS only sensitive to Δq + Δq

7 Experimental Status of g 1 g 1 p data vs. Q 2 Binning in x Compared to NLO global analysis Blumlein and Bottcher, hep-ph/ At a given x, (up to) two orders of magnitude in Q 2 are covered by existing data Data in current x range at higher Q 2 and expanded x coverage highly desirable to test/calculate QCD evolution, and more precisely determine helicity PDFs

8 (Some of the) Physics of g 1 and the polarized PDFs (I) Proton spin crisis : flavor sum of first moments of helicity PDFs does not add up to the proton spin: ΔΣ = 1 0 Δ(u + u ) + Δ(d + d ) + Δ(s + s ) Experimental determinations of ΔΣ use the measured first moment of g 1 and external constraints from hyperon β-decay constants assuming flavor SU(3) symmetry The direct determination of ΔΣ from flavor-separated helicity PDF measurements would be highly desirable existing polarized DIS data don t provide sufficient independent observables, kinematic coverage, etc. On the other hand, SIDIS measurements come with significant caveats... If SU(3) is unbroken or not strongly broken in relating beta decays to nucleon spin structure, then where is the rest of the nucleon spin? ΔG, L q, L g,...

9 Experimental Status of Δq DSSV: PRD 80, (2009) Blumlein and Bottcher, hep-ph/ Recent Global Analyses: DSSV, includes SIDIS and pp data, asymmetric sea BB, DIS data only

10 DSSV: high x behavior of Δ(u+ubar)/(u+ubar) and Δ(d+dbar)/(d+dbar)

11 DSSV: relative impact of SIDIS vs. other data, individual SIDIS channels

12 Top left(right): Impact of all(sidis) data on strange quark polarization Bottom left: Impact of various types of data on ΔΣ

13 SIDIS and Flavor-Tagging Existing unpolarized SIDIS data included in NLO global analysis of fragmentation functions: DSS2007 (PRD 75, ) Considerable and growing evidence for QPM interpretation of SIDIS data Still need to verify for e.g. JLab 12 (deferred PR ) using ideally CLAS12 Based on the experimentally observed fact that the probability for quark q to fragment into hadron h is strongly, positively correlated with the valence quark content of that hadron By detecting and identifying specific hadrons in the final state carrying a significant fraction of the energy transferred to the struck quark, statistically map the flavor dependence of the measured observables (cross section, spin asymmetries,...) Collinear factorization for SIDIS implies (at LO...): dσ h e 2 q q(x)d h q (z) dxdydz q = dσ e 2 q q(x) dxdy q

14 Pioneering work by SMC, HERMES and COMPASS collaborations LO analysis to extract Δq, assume independent fragmentation: Δq in SIDIS: Progress so far

15 HERMES and COMPASS direct extraction of Δq from SIDIS A LL measurements, compared to various global analyses

16 Future SIDIS Δq measurements JLab 12 GeV: CLAS12; polarized proton, deuteron JLab Hall A: EIC: High lumi. polarized neutron ( 3 He), NH 3, 6 LiD BigBite + Super BigBite spectrometers polarized protons and light ions ( 3 He)

17 JLab 12 GeV PR : longitudinally polarized proton and deuteron targets CLAS12 spectrometer: Large acceptance Moderate resolution and PID capability Full pt coverage

18 JLab 12 GeV: Hall A 3 He as effective polarized neutron target High polarization P He = 65% High luminosity ~10 36 cm -2 s -1 (already achieved) large acceptance spectrometers: BigBite (already used successfully in several experiments; planned upgrades for 12 GeV) Super BigBite ( being developed for high Q 2 form factor program and other experiments requiring large acceptance and luminosity )

19 JLab Polarized 3 He Target ~90% ~1.5% ~8% Effective neutron polarizion in 3He is ~86% of nuclear polarization Beam current and polarization history of 3He targets in past experiments

20 Polarized Laser 795 nm 25 G Holding Field 230 o C 10 atm 3 He Some N 2, Rb, K Φ = 3 Pumping Chamber 40 cm Target Chamber Basic anatomy: hybrid Rb-K spin-exchange optical pumping 3 sets of Helmholtz coils provide holding field, orient target polarization in any direction R&D toward increasing luminosity by approximately 1 order of magnitude for GEn measurements at JLab 12

21 Expected 3He (Target) Performance Summary for JLab 12 GeV Experiments Polarization of 65% Luminosity: (up to) cm -2 s -1

22 Detector Layout Hall A Super BigBite Spectrometer Schematic illustration of experiment layout BigBite Spectrometer

23 BigBite Spectrometer BigBite performance in electron-mode well characterized in E (transversity) Planned upgrade: Replace MWDC with GEM trackers: Improved rate capability Improved resolution Add Gas Cerenkov with existing shower +preshower+scintillator BigBite momentum calibration in E for 1.2 GeV (1-pass) and 2.4 GeV (2-pass) beam using p(e,e )p

24 BigBite Performance Characteristics Acceptance: Angular: mrad (vertical) mrad (horizontal) Momentum: 200 MeV< p <...? Resolution: Angles: 3-10 mrad Momentum % Vertex: 3-10 mm

25 Super BigBite Spectrometer (SBS) Large-Acceptance dipole magnet, open geometry, measure momentum Hadron detection and PID: calorimeter (trigger and energy measurement, aid in track reconstruction) GEMs: high-rate, high resolution tracking RICH: Hadron PID Medium (angle-dependent) solid angle acceptance Large momentum acceptance (2-10 GeV/c)

26 SBS Magnet Illustration of SBS magnet in Hall A, with cut in yoke for beam path Table of resolution for 2.5 Tm Bdl

27 Super BigBite Spectrometer Schematic view of hadron detection and PID in SBS Previous experience with HERMES RICH counter Field orientation is horizontal (vertical bend) With detectors positioned vertically behind magnet, will detect upbending (π + /K + ) and downbending (π - /K - ) particles simultaneously with symmetric acceptance; reverse polarity to cancel any residual systematic acceptance effects.

28 Expected Physics Performance: SBS +BB Simple MC to estimate physics performance: LO cross section and A1 calculation using: MSTW2008 PDFs HKNS2007 Fragmentation Functions GRSV2000 polarized PDFs Reasonable assumptions on luminosity, target/ beam polarization, dilution Reasonable model of acceptance

29 SBS Kinematic Coverage

30 SBS Kinematic Coverage: Phase Space Q 2, x, and W, x strongly correlated Large z coverage independent of x: factorization check Significant p T coverage, also weakly correlated with x, check p T dependence

31 SBS Projections: A 1n (from 3 He) For a luminosity L = cm -2 s -1, the plots on the next slide are projected for an integrated luminosity Ldt of 2 weeks x L Statistical uncertainties on A 1h, in bins of x, integrated over z, p T, Q 2 Uncertainties for binning shown typically at the 10-3 level Multi(two)-dimensional (x,z) or three (x,z,p T ) binning possible with reasonable precision

32 ! A 1n! - A 1n + K A - 1n K A 1n x Bj Projected Errors for A1n for two weeks running on 3 He at L=1E37 Hz/cm 2 Approximate equality of A1n between pi+ and K+ reflects u-quark dominance of A1 and symmetry assumptions for the sea polarization in GRSV

33 SBS Projections: A 1p Using same detector setup with UVa polarized NH 3 target, we can measure A 1p for different SIDIS channels For solid targets, expect L~1e35 Hz/cm 2 P p (NH 3 ) = 80% Nitrogen dilution factor ~0.15 Assume 30 days beam time at nominal luminosity

34 x Bj +! A 1p! - A 1p + K A - 1p K A 1p With almost two orders of magnitude lower luminosity, more beam time is needed, and we cannot reach quite as high in x We still obtain good precision in the valence region for all four charged pion and kaon channels

35 Our projections compared to existing data

36 Example of Δq Accuracy: Δdv

37 Conclusions on JLab Hall A 12 GeV Large acceptance and high luminosity BigBite + SBS configuration in Hall A can provide substantial kinematic coverage and precision for flavor decomposition of nucleon spin in the JLab 12 GeV era. Compared to CLAS12, Hall A can take advantage of higher luminosity polarized 3He target in all possible spin configurations, provide complementary neutron data. For polarized proton (solid) targets, Hall A can be competitive with CLAS12, much easier to have transversely polarized proton target compared to CLAS12

38 Δq at an Electron-Ion Collider