Elastic electron-proton scattering (e−p) and the spectroscopy of hydrogen atoms are the two traditional methods used to determine the proton charge radius (r p). About a decade ago, a new method using muonic hydrogen (µH) atoms 1 found a significant discrepancy with the compilation of all previous results 2 , creating the "proton radius puzzle". Despite intensive worldwide experimental and theoretical efforts, the "puzzle" remains unresolved. In fact, a new discrepancy was reported between the two most recent spectroscopic measurements on ordinary hydrogen 3, 4. Here, we report on the PRad experiment, the first high-precision e − p experiment since the emergence of the "puzzle". For the first time, a magnetic-spectrometerfree method was employed along with a windowless hydrogen gas target, which overcame several limitations of previous e − p experiments and reached unprecedented small angles.

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 π0 into two photons (γγ), leading to its unusually short lifetime. We precisely measured the decay width Γ of the π0→ γγ process. The differential cross sections for π0 photoproduction at forward angles were measured on two targets, carbon-12 and silicon-28, yielding Γ(π0→ γγ)=7.798±0.056(stat.)±0.109(syst.) 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 Γ(π0→ γγ)=7.802±0.052(stat.)±0.105(syst.) eV. Our final result has a total uncertainty of 1.50% and confirms the prediction based on the chiral anomaly in quantum chromodynamics.

We have observed a linear pressure dependence of longitudinal relaxation time T 1 at 4.2 and 295 K in gaseous 3 He cells made of either bare Pyrex glass or Cs-or Rb-coated Pyrex due to paramagnetic sites in the cell wall. The paramagnetic wall relaxation is previously thought to be independent of 3 He pressure. We develop a model to interpret the observed wall relaxation by taking into account the diffusion process, and our model gives a good description of the data.