HAPPEX: Strange Quarks in the Proton and ⁴He

HAPPEX results from our 2005 run for the parity‐violating asymmetry A _PV in elastic scattering of 3 GeV electrons off hydrogen and ⁴He targets at 〈θ_lab〉 ≈ 6.0° are presented. The ⁴He result is A_PV = (+6.40 ± 0.23 (stat) ± 0.12 (syst)) × 10⁻⁶. The hydrogen result is A_PV = (−1.58 ± 0.12 (stat) ± 0.04 (syst)) × 10⁻⁶. The asymmetry for hydrogen is a linear combination of $GEs$ and $GMs$⁠, the strange quark contributions to the electric and magnetic form factors of the nucleon, respectively, and that for ⁴He is a function of $GEs$ alone. The measurements imply $GEs + 0.09GMs = 0.007 ± 0.011 ± 0.006$ at 〈Q²〉 = 0.109 GeV² and $GEs = 0.002 ± 0.014 ± 0.007$ at 〈Q²〉 = 0.077 GeV².