Measurement of Strange-Quark Contributions to the Nucleon's Form Factors atQ2=0.230   (GeV/c)2

Abstract

We report on a measurement of the parity-violating asymmetry in the scattering of longitudinally polarized electrons on unpolarized protons at a Q2 of 0.230 (GeV/c)(2) and a scattering angle of theta (e) = 30 degrees - 40 degrees. Using a large acceptance fast PbF2 calorimeter with a solid angle of delta omega = 0.62 sr, the A4 experiment is the first parity violation experiment to count individual scattering events. The measured asymmetry is A(phys)=(-5.44+/-0.54(stat)+/-0.26(sys))x10(-6). The standard model expectation assuming no strangeness contributions to the vector form factors is A(0) = (-6.30+/-0.43) x 10(-6). The difference is a direct measurement of the strangeness contribution to the vector form factors of the proton. The extracted value is G(s)(E) + 0.225G(s)(M) = 0.039+/-0.034 or F(s)(1) + 0.130F(s)(2) = 0.032+/-0.028.

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.…”

“…Conversion of photons into e + e − pairs formed a significant background, which was corrected for using a Monte Carlo simulation. Measurements of A P V from the proton were made using this apparatus in the forward angle at Q 2 points of 0.108 and 0.23 (GeV/c) 2 and in the backward angle at Q 2 = 0.22 (GeV/c) 2 [24][25][26].…”

A Measurement of the Weak Charge of the Proton through Parity Violating Electron Scattering using the Qweak Apparatus: A 21% Result

“…The weak charge of the proton measurement is only possible because of the successful PVES physics programs conducted by SAMPLE [24], A4 [25,26], HAPPEX [27,28], and G0 collaborations [29] whose goal were to study the electroweak neutral currents of the nucleons in order to measure the strange quark contribution to nucleon structure. The success of the Q weak experiment will provide the technical expertise for the next generation of the PV physics program starting with the proposed 12 GeV Møller experiment [30] whose goal is to measure the weak mixing angle three times more precisely than the Q weak experiment.…”

Demands on polarized electron sources by future experiments in parity violating electron scattering

“…The technique was originally used to verify the nature of the weak neutral-current [1,2], and more recently has been used to bound the size of the strange quark contribution to the nucleon form factors [3,4,5,6,7,8], verify the existence of a "neutron-skin" in 208 Pb [9,10], and to do precision tests of the Standard Model of particle physics [11,12]. Asymmetries measured in experiments so far have ranged from ∼200 parts-per-million (ppm) down to ∼0.2 ppm, depending on the physical process and kinematics.…”