Hyperfine-interaction- and magnetic-field-induced Bose-Einstein-statistics suppressed two-photon transitions

TL;DR

This paper investigates how hyperfine interactions and magnetic fields can induce two-photon transitions in atoms that are normally forbidden by Bose-Einstein statistics, with implications for experimental tests of fundamental physics.

Contribution

It identifies and evaluates two mechanisms by which hyperfine interactions and magnetic fields relax the selection rule for two-photon transitions, expanding understanding of atomic transition behaviors.

Findings

Hyperfine interactions enable forbidden two-photon transitions in atoms with nuclear spin.

Magnetic fields relax the selection rule via Zeeman splitting and state mixing.

Theoretical results support experimental searches for Bose-Einstein-statistics violations.

Abstract

Two-photon transitions between atomic states of total electronic angular momentum $J_a=0$ and $J_b=1$ are forbidden when the photons are of the same energy. This selection rule is analogous to the Landau-Yang theorem in particle physics that forbids decays of vector particle into two photons. It arises because it is impossible to construct a total angular momentum $J_{2\gamma}=1$ quantum-mechanical state of two photons that is permutation symmetric, as required by Bose-Einstein statistics. In atoms with non-zero nuclear spin, the selection rule can be violated due to hyperfine interactions. Two distinct mechanisms responsible for the hyperfine-induced two-photon transitions are identified, and the hyperfine structure of the induced transitions is evaluated. The selection rule is also relaxed, even for zero-nuclear-spin atoms, by application of an external magnetic field. Once again, there are two similar mechanisms at play: Zeeman splitting of the intermediate-state sublevels, and off-diagonal mixing of states with different total electronic angular momentum in the final state. The present theoretical treatment is relevant to the ongoing experimental search for a possible Bose-Einstein-statistics violation using two-photon transitions in barium, where the hyperfine-induced transitions have been recently observed, and the magnetic-field-induced transitions are being considered both as a possible systematic effect, and as a way to calibrate the measurement.