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Strange Quark Contribution to the Proton Spin, from Elastic ep and n p Scattering

Strange Quark Contribution to the Proton Spin, from Elastic ep and n p Scattering. A combined analysis of HAPPEx, G 0 , and BNL E734 data. Stephen Pate New Mexico State University SPIN 2006 Kyoto, Japan, 3-October-2006. Outline.

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Strange Quark Contribution to the Proton Spin, from Elastic ep and n p Scattering

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  1. Strange Quark Contribution to the Proton Spin, from Elastic ep and np Scattering A combined analysis of HAPPEx, G0, and BNL E734 data Stephen Pate New Mexico State University SPIN 2006 Kyoto, Japan, 3-October-2006

  2. Outline • Initial indication of significant negative strange quark contribution to the proton spin came from SU(3)-based analysis of inclusive polarized DIS data from CERN (EMC); • Similar experiments at SLAC and CERN supported this finding; • An attempt to confirm this using neutrino-proton elastic scattering (BNL E734) was inconclusive; • HERMES made first significant measurement of strange quark polarized p.d.f., using semi-inclusive DIS, for 0.03 < x < 0.2; suggests strange quark contribution to the proton spin is essentially zero! • A combined analysis of BNL E734 np data with very recent HAPPEX and G0 forward-scattering ep data show the Q2-dependence of the strange axial form factor for 0.45 < Q2 < 1.0 GeV2; suggests the strange quark contribution is negative… • What to do now?

  3. Ds from polarized inclusive DIS Plan: Measure values of g1(x,Q2), extrapolated to x=0 and x=1, and combine with axial charges from hyperon beta-decay data to extract a flavor decomposition of the quark axial charges. [E.g. see B.W. Filippone and X. Ji, Adv. Nucl. Phys. 26 (2001) 1 and references therein…] Surprise #1: Quarks contribute only about 20% of the proton spin. Surprise #2: Strange quarks make a negative contribution: Ds ~ -0.15 Problems: Need SU(3) flavor symmetry to bring in the hyperon data. Extrapolation to x=0 problematic. => Unknown theoretical uncertainties.

  4. Ds from np elastic scattering Plan: Measure absolute cross section for np elastic scattering, and extract the strange contribution to the proton axial form factor. Extrapolate to Q2=0 to obtain the strange quark axial charge. Note: Elastic np scattering and inclusive DIS both measure the sum of strange quark and strange anti-quark contributions. BNL E734 measured this cross section for both neutrinos and anti-neutrinos, but result for Ds was inconclusive. Large uncertainty in cross sections. Lowest Q2 point (0.45 GeV2) not really low enough. Subsequent reanalyses (Garvey et al, Alberico et al.) came to similar non-conclusion.

  5. Ds from polarized semi-inclusive DIS Plan: Measure tagged structure function g1h(x,Q2) by observing leading hadrons in coincidence with scattered lepton. Use leading-order analysis to extract separated polarized quark distributions Dq(x). Integrate observed portion of Dq(x) to get estimates of axial charges. [See HERMES PRD 71 (2005) 012003and talk by Hal Jackson a few minutes ago…] Result: Advantages: No SU(3) assumption. No extrapolations to x=0. Can separate quark and anti-quark contributions. Disadvantages: Leading-order analysis. Some folks question validity of factorization at relatively low Q2. Result contradicts SU(3)-based analysis of inclusive DIS data.

  6. Ds from combination of ep and np elastic scattering data [finally we come to the main point of this talk…] Plan: Combine existing np data from BNL E734 with recent parity-violating ep scattering data from HAPPEx and G0. Extract the strangeness contribution to the proton electromagnetic and axial form factors point-by-point. Observe Q2-dependence of these form factors for the very first time. [See SP, PRL 92 (2004) 082002, and SP et al., hep-ex/0512032] Advantages: No SU(3) assumption. Get total integral over x by using elastic scattering. Neutrino scattering is very sensitive to the axial form factor.

  7. Features of parity-violating forward-scattering ep data • measures linear combination of form factors of interest • axial terms are doubly suppressed • (1 - 4sin2qW) ~ 0.075 • kinematic factor e”' ~ 0 at forward angles • significant radiative corrections exist, especially in the axial term • parity-violating data at forward angles are mostly sensitive to the strange electric and magnetic form factors

  8. Full Expression for the PV ep Asymmetry Note suppression of axial terms by (1 - 4sin2qW) ande”'.

  9. Things known and unknown in the PV ep Asymmetry

  10. Features of elastic np data • measures quadratic combination of form factors of interest • axial terms are dominant at low Q2 • radiative corrections are insignificant • [Marciano and Sirlin, PRD 22 (1980) 2695] • neutrino data are mostly sensitive to the strange axial form factor

  11. Elastic NC neutrino-proton cross sections Dependence on strange form factors is buried in the weak (Z) form factors.

  12. The BNL E734 Experiment • performed in mid-1980’s • measured neutrino- and antineutrino-proton elastic scattering • used wide band neutrino and anti-neutrino beams of <En>=1.25 GeV • covered the range 0.45 < Q2 < 1.05 GeV2 • large liquid-scintillator target-detector system • still the only elastic neutrino-proton cross section data available

  13. E734 Results Uncertainties shown are total (stat and sys). Correlation coefficient arises from systematic errors.

  14. Combination of the ep and np data sets E734 Q2 range • We use PV ep data in the same range of Q2 as the E734 experiment. • The original HAPPEx measurement: Q2 = 0.477 GeV2 [PLB 509 (2001) 211 and PRC 69 (2004) 065501] • The recent G0 data covering the range 0.1 < Q2 < 1.0 GeV2 [PRL 95 (2005) 092001]

  15. Combination of the ep and np data sets 1 2 Since the neutrino data are quadratic in the form factors, then there will be in general two solutions when these data sets are combined. Intersections between electron and neutrino data. Q2 = 0.5 GeV2 Fortunately, the two solutions are very distinct from each other, and other available data can select the correct physical solution.

  16. General Features of the two Solutions Solution 1 Solution 2 • There are three strong reasons to prefer Solution 1: • GAs in Solution 2 is inconsistent with all DIS estimates for Ds • GMs in Solution 2 is inconsistent with the recent HAPPEx result of GMs ~ 0 at Q2 = 0.1 GeV2 • GEs in Solution 2 is inconsistent with the idea that GEs should be small, and conflicts with expectation from recent G0 data that GEs may be negative near Q2 = 0.3 GeV2 I only present Solution 1 hereafter.

  17. First determination of the strange axial form factor. G0 & E734 [SP et al., hep-ex/0512032] HAPPEx & E734 [SP, PRL 92 (2004) 082002]

  18. Strange Axial Form Factor from ep and np Elastic Scattering Data G0 & E734 [SP et al., hep-ex/0512032] Q2-dependence suggests Ds < 0 ! HAPPEx & E734 [SP, PRL 92 (2004) 082002] E734 data quality and Q2 range again prevent a definitive conclusion, but the trend clearly suggests a negative strange quark contribution.

  19. What if….? If these two results are both true, then the average value of Ds(x) in the range x < 0.02 must be ~ -5. That’s not impossible, as s(x) is ~20-300 in the range x~10-2 to 10-3 (CTEQ6).But we would need a mechanism that would “turn on” the strange quark polarization suddenly at these low x values. Another more exotic possibility exists…

  20. A topological “x=0” contribution to the singlet axial charge? Accessible in a form factor measurement “subtraction at infinity” term from dispersion relation integration [Steven Bass, hep-ph/0411005] Accessible in deep-inelastic measurements

  21. What to do? We need to approach the problem from the point of view of both kinds of data. • The E734 data have insufficient precision and too narrow a Q2 range to determine the strange quark contribution to the proton spin. Better neutrino data is needed, with smaller uncertainties and points nearer Q2 = 0. A detailed understanding of the Q2-dependence of these form factors will not be possible until a more dense set of resolved data points are available. • The semi-inclusive DIS data need to be extended to lower x and higher Q2 to determine which of the two proposed explanations is correct, or perhaps to reveal some other explanation. A NLO analysis would also be a good idea.

  22. Better np elastic scattering data • FINeSSE (B. Fleming and R. Tayloe) at BNL or FNAL • A measurement of np elastic scattering is being considered for JPARC (NeuSpin, see talk by Y. Miyachi).

  23. Better semi-inclusive DIS data HERMES • Electron Ion Collider (EIC) • The problem of the strange quark contribution to the proton spin is an explicit part of the physics program at this new facility. • Improved version of HERMES semi-inclusive measurement will explore the quark polarizations to lower x and higher Q2. • [Figure is a simulation from Kinney and Stoesslein, AIP Conf. Proc. 588 (2001) 171.]

  24. Conclusion Strange quark contribution to the proton spin remains a mystery. HERMES semi-inclusive DIS data suggests zero contribution. Inclusive DIS data with SU(3)-based analysis, and independent data from form factor measurements, suggest a negative contribution. None of these measurements are definitive. Better neutrino data are needed to improve the form factor picture for a clean determination of Ds. => FINeSSE/NeuSpin Better semi-inclusive DIS data are needed to explore the low-x region to better understand the Ds(x) distribution. => EIC [This work supported by the US DOE.]

  25. More slides…

  26. FINeSSE (& G0) [exp. proposal: no nuclear initial or final state effects included in errors] G0 & E734 [to be published] HAPPEx & E734 [Pate, PRL 92 (2004) 082002] G0/HAPPEx/PVA4 Projected HAPPEx, SAMPLE & PVA4 combined (nucl-ex/0506011)

  27. determine a unique uudss configuration, in which the uuds system is radially excited and the s is in the ground state. An, Riska and Zou, hep-ph/0511223; Riska and Zou, nucl-th/0512102.

  28. G0 & E734 [to be published] Recent calculation bySilva, Kim, Urbano, and Goeke (hep-ph/0509281 and Phys. Rev. D 72 (2005) 094011)based on chiral quark-soliton model is in rough agreement with the data. HAPPEx & E734 [Pate, PRL 92 (2004) 082002]

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