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We report measurements of the parity-conserving beam-normal single-spin elastic scattering asymmetries Bn on 12C and 27Al, obtained with an electron beam polarized transverse to its momentum direction. These measurements add an additional kinematic point to a series of previous measurements of Bn on 12C and provide a first measurement on 27Al. The experiment utilized the Qweak apparatus at Jefferson Lab with a beam energy of 1.158 GeV. The average laboratory scattering angle for both targets was 7.7∘, and the average Q2 for both targets was 0.024 37 GeV2 (Q=0.1561 GeV). The asymmetries are Bn=−10.68±0.90(stat)±0.57(syst) ppm for 12C and Bn=−12.16±0.58(stat)±0.62(syst) ppm for 27Al. The results are consistent with theoretical predictions, and are compared to existing data. When scaled by Z/A, the Q dependence of all the far-forward angle (θ<10∘) data from 1H to 27Al can be described by the same slope out to Q≈0.35 GeV. Larger-angle data from other experiments in the same Q range are consistent with a slope about twice as steep. 

A beam-normal single-spin asymmetry generated in the scattering of transversely polarized electrons from unpolarized nucleons is an observable related to the imaginary part of the two-photon exchange process. We report a 2% precision measurement of the beam-normal single-spin asymmetry in elastic electron-proton scattering with a mean scattering angle of θlab=7.9° and a mean energy of 1.149 GeV. The asymmetry result is Bn=−5.194±0.067(stat)±0.082 (syst) ppm. This is the most precise measurement of this quantity available to date and therefore provides a stringent test of two-photon exchange models at far-forward scattering angles (θlab→0) where they should be most reliable. 

The explicit breaking of the axial symmetry by quantum fluctuations gives rise to the so-called axial anomaly. This phenomenon is solely responsible for the decay of the neutral pion π⁰into two photons (γγ), leading to its unusually short lifetime. We precisely measured the decay width Γ of the${\pi }^0\to \gamma \gamma$process. The differential cross sections for π⁰photoproduction at forward angles were measured on two targets, carbon-12 and silicon-28, yielding$\mathrm{\Gamma }\left({\pi }^0\to \gamma \gamma \right)=7.798\pm 0.056\left(\mathrm{stat}\mathrm{.}\right)\pm 0.109\left(\mathrm{syst}\mathrm{.}\right)\mathrm{ eV}$, where stat. denotes the statistical uncertainty and syst. the systematic uncertainty. We combined the results of this and an earlier experiment to generate a weighted average of$\mathrm{\Gamma }\left({\pi }^0\to \gamma \gamma \right)=7.802\pm 0.052\left(\mathrm{stat}\mathrm{.}\right)\pm 0.105\left(\mathrm{syst}\mathrm{.}\right)\mathrm{ eV}$. Our final result has a total uncertainty of 1.50% and confirms the prediction based on the chiral anomaly in quantum chromodynamics.