Strange Quark Contributions to Parity-Violating Asymmetries in the Backward Angle G0 Electron Scattering Experiment

Abstract

We have measured parity-violating asymmetries in elastic electron-proton and quasi-elastic electron-deuteron scattering at Q 2 = 0.22 and 0.63 GeV 2 . They are sensitive to strange quark contributions to currents in the nucleon, and to the nucleon axial current. The results indicate strange quark contributions of < ∼ 10% of the charge and magnetic nucleon form factors at these four-momentum transfers. We also present the first measurement of anapole moment effects in the axial current at these four-momentum transfers.PACS numbers: 11.30. Er, 14.20.Dh, 25.30.Bf At short distance scales, bound systems of quarks have relatively simple properties and QCD is successfully described by perturbation theory. However, on the size scale of the bound state, ∼ 1 fm, the QCD coupling constant is large and the effects of the color fields are a significant challenge, even in lattice QCD. In addition to valence quarks, e.g., uud for the proton, there is a sea of gluons and qq pairs that plays an important role. From a series of experiments measuring the parity-violating asymmetries of electrons scattered from protons and neutrons, we can extract the contributions of strange quarks to nucleon ground state charge and magnetic form factors. These strange quark contributions are exclusively part of the quark sea because there are no strange valence quarks in the nucleon. experiments have previously reported measurements of these parity-violating asymmetries. Using the combined forward angle asymmetries and the SAMPLE backward angle proton and deuteron measurements, a complete experimental determination of the strange quark vector currents and the axial current (see discussion below) has been made at a four-momentum transfer Q 2 = 0.1 GeV 2 [5]. In this paper, we report the first complete backward angle asymmetry measurements since the SAMPLE experiment, at the four-momentum transfers

First Determination of the Weak Charge of the Proton

“…The uncertainty of the asymmetry reported here is less than that of previous parity-violating electron scattering (PVES) experiments [10][11][12][13][14][15][16][17][18][19][20][21] directed at obtaining hadronic axial and strange form-factor information [22]. The theoretical interpretability of the Q weak measurement is very clean as it relies primarily on those previous PVES data instead of theoretical calculations to account for residual hadronic structure effects, which are significantly suppressed at the kinematics of this experiment.…”

“…Following the procedure outlined in [6,22], a global fit of asymmetries measured in PVES [10][11][12][13][14][15][16][17][18][19][20][21] on hydrogen, deuterium, and 4 He targets was used to extract Q p W from Eq. (4).…”

Measurement of the parity violating asymmetry in the quasielastic electron-deuteron scattering and improved determination of the magnetic strange form factor and the isovector anapole radiative correction

“…A third measurement of the parity-violating asymmetry in the quasielastic electron-deuteron scattering at backward angles will then isolate the effective isovector axial-vector form factor G [7]. The combination of the A4 results presented here with asymmetries measured previously in the A4 experiment [6,8], and with the asymmetries obtained in the G0 experiment at the Thomas Jefferson National Accelerator Facility [9,10], leads to a substantial reduction of the experimental uncertainty for the values of G e;ðT¼1Þ A and of G s M . The anapole radiative correction is extracted and compared to the theoretical calculations of Zhu et al [2].…”

“…A further reanalysis was performed for the published data of the G0 experiment [9,10]. The set of three asymmetries for each Q 2 have been used to determine are shown in Table II.…”

Parity violation in electron scattering

“…Four experimental programs in three laboratories, the Generation II PVES experiments, followed. The experiments were SAMPLE at the MIT Bates laboratory [16][17][18], the HAPPEX program at JLab [19][20][21], G0 at JLab [22,23], and the A4 program at Mainz [24][25][26]. By the time these programs were well underway, PVES had become accepted as a standard technique in electromagnetic facilities.…”