Measurement of the parity-violating asymmetry in the 𝑁 → Δ transition at low 𝑄 2
Here, we report the measurement of the parity-violating asymmetry in the 𝑁 → Δ transition via the 𝑒 − + 𝑝 → 𝑒 − + Δ + reaction at two different kinematic points with low four-momentum transfer 𝑄 2 . Measurements were made with incident electron beam energies of 0.877 and 1.16 GeV, corresponding to 𝑄 2 values of 0.0111 and 0.0208 (GeV/c) 2 , respectively. These measurements put constraints on a low-energy constant in the weak Lagrangian, 𝑑 Δ , corresponding to a parity-violating electric-dipole transition matrix element. This matrix element has been shown to be large in the strangeness-changing channel, via weak hyperon decays such as Σ + → 𝑝𝛾. The measurements reported here constrain 𝑑 Δ in the strangeness-conserving channel. The final asymmetries were −0.65±1.00(stat.)±1.02(syst.) ppm (parts per million) for 0.877 GeV and −3.59±0.82(stat.)±1.33(syst.) ppm for 1.16 GeV. With these results we deduce a small value for 𝑑 Δ , consistent with zero, in the strangeness-conserving channel, in contrast to the large value for 𝑑 Δ previously reported in the strangeness-changing channel.