A measurement of the atomic hydrogen Lamb shift and the proton charge radius.

The surprising discrepancy between results from different methods for measuring the proton charge radius is referred to as the proton radius puzzle. In particular, measurements using electrons seem to lead to a different radius compared with those using muons. Here, a direct measurement of the n = 2 Lamb shift of atomic hydrogen is presented. Our measurement determines the proton radius to be r p = 0.833 femtometers, with an uncertainty of ±0.010 femtometers. This electron-based measurement of r p agrees with that obtained from the analogous muon-based Lamb shift measurement but is not consistent with the larger radius that was obtained from the averaging of previous electron-based measurements.

Note: A compact external-cavity diode laser, using feedback from an optical fiber.

We demonstrate a simple and compact external cavity diode laser (ECDL), where light reflected from the tip of an optical fiber provides feedback, and an interference filter provides frequency selection. The ECDL combines ease of construction with robust operation and exhibits a linewidth of 50 kHz.

Erratum: Mercury Monohalides: Suitability for Electron Electric Dipole Moment Searches [Phys. Rev. Lett. 114, 183001 (2015)].

This corrects the article DOI: 10.1103/PhysRevLett.114.183001.

Mercury monohalides: suitability for electron electric dipole moment searches.

Heavy polar diatomic molecules are the primary tools for searching for the T-violating permanent electric dipole moment of the electron (eEDM). Valence electrons in some molecules experience extremely large effective electric fields due to relativistic interactions. These large effective electric fields are crucial to the success of polar-molecule-based eEDM search experiments. Here we report on the results of relativistic ab initio calculations of the effective electric fields in a series of molecules that are highly sensitive to an eEDM, the mercury monohalides (HgF, HgCl, HgBr, and HgI). We study the influence of the halide anions on E_{eff}, and identify HgBr and HgI as attractive candidates for future electric dipole moment search experiments.

Order of magnitude smaller limit on the electric dipole moment of the electron.

The Standard Model of particle physics is known to be incomplete. Extensions to the Standard Model, such as weak-scale supersymmetry, posit the existence of new particles and interactions that are asymmetric under time reversal (T) and nearly always predict a small yet potentially measurable electron electric dipole moment (EDM), d(e), in the range of 10(-27) to 10(-30) e·cm. The EDM is an asymmetric charge distribution along the electron spin (S(→)) that is also asymmetric under T. Using the polar molecule thorium monoxide, we measured d(e) = (-2.1 ± 3.7stat ± 2.5syst) × 10(-29) e·cm. This corresponds to an upper limit of |d(e)| < 8.7 × 10(-29) e·cm with 90% confidence, an order of magnitude improvement in sensitivity relative to the previous best limit. Our result constrains T-violating physics at the TeV energy scale.