Strangeness Contribution to the Vector and Axial Form Factors of the Nucleon Stephen Pate, Glen MacLachlan, David McKee, Vassili Papavassiliou New Mexico State University JPARC Workshop on Hadron Structure KEK, Tsukuba, 1-December-2005 A combined analysis of HAPPEx, G 0, and BNL E734 data

Outline Program of parity-violating electron-nucleon elastic scattering experiments will measure the strange vector (electromagnetic) form factors of the nucleon --- but these experiments are insensitive to the strange axial form factor Use of neutrino and anti-neutrino elastic scattering data brings in sensitivity to the strange axial form factor as well Combination of forward PV data with neutrino and anti-neutrino data allows extraction of vector and axial form factors over a broad Q 2 range With better neutrino data, a determination of  s from the strange axial form factor is possible

Elastic Form Factors in Electroweak Interactions Elastic: same initial and final state particles, but with some momentum transfer q between them Electroweak: photon-exchange or Z-exchange The photon exchange (electromagnetic) interaction involves two vector operators, and thus two vector form factors, called F 1 and F 2, appear in the hadronic electromagnetic current:

Elastic Form Factors in Electroweak Interactions The Z-exchange (neutral current weak) interaction involves those same vector operators, but since it does not conserve parity it also includes axial-vector and pseudo-scalar operators. So, there are two additional form factors, G A and G P, in the hadronic weak current: (The pseudo-scalar form factor G P does not contribute to either PVeN scattering or to neutral-current elastic scattering, so we will ignore it hence.)

Conversion to Sachs Form Factors The vector form factors F 1 and F 2 are called respectively the Dirac and Pauli form factors. It is customary in low-energy hadronic physics to use instead the Sachs electric and magnetic form factors, G E and G M :

Flavor Decomposition of Form Factors Because the interaction between electrons or neutrinos and the quark constituents of the nucleon is point-like, we can re-write these nucleon form factors in terms of individual quark contributions. For example, the proton electric form factors: Because of the way we defined the form factors, the underlying quark form factors are defined by the tensor properties of the current operator and not the specific interaction. The interaction is represented by the multiplying coupling constants (electric or weak charges).

Charge Symmetry We will assume charge symmetry: that is, we assume that the transformation between proton and neutron is a rotation of  in isospin space. We also assume that the strange form factors in each nucleon are the same. The individual quark form factors are then global properties of the nucleons.

Strange Form Factors Our charge-symmetric expressions for the form factors allow us to solve for the up and down contributions: This is useful because the electromagnetic form factors of the proton and neutron are well known at low Q 2. Then we may eliminate the up and down form factors from all other formulae and focus on the strange form factors.

A QCD Relation for the Axial Current Therefore a measurement of the strange axial form factor can lead to an understanding of a portion of the nucleon spin puzzle --- a measurement of  s.

Features of parity-violating forward-scattering ep data measures linear combination of form factors of interest axial terms are doubly suppressed  (1  4sin 2  W ) ~  kinematic factor  ” ' ~ 0 at forward angles significant radiative corrections exist, especially in the axial term à parity-violating data at forward angles are mostly sensitive to the strange electric and magnetic form factors

Full Expression for the PV ep Asymmetry Note suppression of axial terms by (  sin   W  and  ” ' 

Things known and unknown in the PV ep Asymmetry

Features of elastic p data measures quadratic combination of form factors of interest axial terms are dominant at low Q 2 radiative corrections are insignificant [Marciano and Sirlin, PRD 22 (1980) 2695] à neutrino data are mostly sensitive to the strange axial form factor

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

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 =1.25 GeV covered the range 0.45 < Q 2 < 1.05 GeV 2 large liquid-scintillator target-detector system still the only elastic neutrino-proton cross section data available

Q 2 (GeV) 2 d  /dQ 2 ( p) (fm/GeV) 2 correlation coefficient               Uncertainties shown are total (stat and sys). Correlation coefficient arises from systematic errors. E734 Results

Forward-Scattering Parity-Violating ep Data These data must be in the same range of Q 2 as the E734 experiment. The original HAPPEx measurement: Q 2 = GeV 2 [PLB 509 (2001) 211 and PRC 69 (2004) ] The recent G 0 data covering the range 0.1 < Q 2 < 1.0 GeV 2 [PRL 95 (2005) ] E734 Q 2 range

Combination of the ep and p data sets Since the neutrino data are quadratic in the form factors, then there will be in general two solutions when these data sets are combined. Fortunately, the two solutions are very distinct from each other, and other available data can select the correct physical solution.

General Features of the two Solutions There are three strong reasons to prefer Solution 1: G A s in Solution 2 is inconsistent with DIS estimates for  s G M s in Solution 2 is inconsistent with the combined SAMPLE/PVA4/HAPPEx/G0 result of G M s = ~+0.6 at Q 2 = 0.1 GeV 2 G E s in Solution 2 is inconsistent with the idea that G E s should be small, and conflicts with expectation from recent G 0 data that G E s may be negative near Q 2 = 0.3 GeV 2 GEsGEs Consistent with zero (with large uncertainty) Large and positive GMsGMs Consistent with zero (with large uncertainty) Large and negative GAsGAs Small and negativeLarge and positive Solution 1 Solution 2 I only present Solution 1 in what follows.

HAPPEx, SAMPLE & PVA4 combined (nucl-ex/ )

G0 Projected HAPPEx, SAMPLE & PVA4 combined (nucl-ex/ )

G0 Projected HAPPEx & E734 [Pate, PRL 92 (2004) ] HAPPEx, SAMPLE & PVA4 combined (nucl-ex/ )

HAPPEx & E734 [Pate, PRL 92 (2004) ] G0 & E734 [to be published] G0 Projected First determination of the strange axial form factor.

HAPPEx & E734 [Pate, PRL 92 (2004) ] G0 & E734 [to be published] Q 2 -dependence suggests  s < 0 !

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

Look for Riska and Zou paper in the archive in a few weeks. [This follows work already presented by An, Riska and Zou, hep-ph/ ] determine a unique uudss configuration, in which the uuds system is radially excited and the s is in the ground state.

DIS vs. Form Factors The HERMES [PRL 92 (2004) ] result indicates that the strange quark helicity distribution  s(x) ~ 0 for x > 0.023, and the integral over their measured kinematics is also zero: One explanation: If these two results are both true, then the average value of  s(x) in the range 0 < x < must be ~  5. That’s not impossible, as s(x) is ~ in the range x~10 -2 to (CTEQ6). Are these results in contradiction? Not necessarily. At the same time, we have an indication from the analysis of PV ep and elastic p data that the full integral  s is negative.

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/ ] Accessible in deep-inelastic measurements

A future experiment to determine the three strange form factors and  s The program I have described determines the strange axial form factor down to Q 2 = 0.45 GeV 2 successfully, but it does not determine the Q 2 -dependence sufficiently for an extrapolation down to Q 2 = 0. A new experiment has been proposed to measure elastic and quasi- elastic neutrino-nucleon scattering to sufficiently low Q 2 to measure  s directly. A better neutrino experiment is needed, with a focus on determining these form factors. The large uncertainties in the E734 data limit their usefulness beyond what I have shown here.

FINeSSE* Determination of  s * B. Fleming (Yale) and R. Tayloe (Indiana), spokespersons

FINeSSE Determination of  s However, these uncertainty estimates do not include any contributions from nuclear initial state and final state effects. The calculation and understanding of these effects is a critical component of the FINeSSE physics program.

Theoretical Effort Related to FINeSSE Meucci, Giusti, and Pacati at INFN-Pavia [1] van der Ventel and Piekarewicz [2,3] Maieron, Martinez, Caballero, and Udias [4,5] Martinez, Lava, Jachowicz, Ryckebusch, Vantournhout, and Udias [6] [1] Nucl. Phys. A 744 (2004) 307. [2] Phys. Rev. C 69 (2004) [3] nucl-th/ , submitted to Phys. Rev. C [4] Phys. Rev. C 68 (2003) [5] Nucl. Phys. Proc. Suppl. 139 (2005) 226 [6] nucl-th/ , submitted to Phys. Rev. C These groups generally employ a relativistic PWIA for the baseline calculation, and use a variety of models to explore initial and final state nuclear effects (relativistic optical model or relativistic Glauber approximation, for example). Preliminary indications from [1] and [6] are that nuclear effects cancel very nicely in the ratios to be measured in FINeSSE.

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

In conclusion… Recent data from parity-violating electron-nucleon scattering experiments has brought the discovery of the strange vector form factors from the future into the present. Additional data from these experiments in the next few years will add to this new information about the strangeness component of the nucleon. However, an even richer array of results, including also the strange axial form factor and the determination of  s, can be produced if we can bring neutrino-proton scattering data into the analysis. The E734 data have insufficient precision and too narrow a Q 2 range to achieve the full potential of this physics program. The FINeSSE project can provide the necessary data to make this physics program a success.