Global analysis of CP-violation in atoms, molecules and role of medium-heavy systems

TL;DR

This paper develops a qualitative model for analyzing P,T-violation in atoms and molecules, highlighting the importance of medium-heavy systems and providing computational results to guide future experiments in detecting physics beyond the Standard Model.

Contribution

A simple qualitative model is introduced for global analysis of P,T-odd parameters, emphasizing the value of medium-heavy molecules and confirming predictions with density functional theory calculations.

Findings

Medium-heavy molecules with Z ≤ 54 are valuable for constraining P,T-violating parameters.

The model guides the selection of atoms and molecules based on angular momentum and nuclear charge.

Density functional theory calculations support the model's predictions.

Abstract

Detection of parity (P) and time-reversal (T) symmetry-odd electric dipole moments (EDMs) within currently achievable resolution would evidence physics beyond the Standard Model of particle physics. Via the CPT-theorem, which includes charge conjugation (C), such low-energy searches complement high-energy physics experiments that probe CP-violation up to the TeV scale. Heavy-elemental atoms and molecules are considered to be among the most promising candidates for a first direct detection of P,T-violation due to enhancement effects that increase steeply with increasing nuclear charge number $Z$. However, different P,T-odd sources on the subatomic level can contribute to molecular or atomic EDMs, which are target of measurements, and this complicates obtaining rigorous bounds on P,T-violation on a fundamental level. Consequently, several experiments of complementary sensitivity to these individual P,T-odd sources are required for this purpose. Herein, a simply-applicable qualitative model is developed for global analysis of the P,T-odd parameter space from an electronic-structure theory perspective. Rules of thumb are derived for the choice of atoms and molecules in terms of their angular momenta and nuclear charge number. Contrary to naive expectations from $Z$-scaling laws, it is demonstrated that medium-heavy molecules with $Z\leq54$ can be of great value to tighten global bounds on P,T-violating parameters, in particular, if the number of complementary experiments increases. The model is confirmed by explicit density functional theory calculations of all relevant P,T-odd electronic structure parameters in systems that were used in past experiments or are of current interest for future experiments, respectively: the atoms Xe, Cs, Yb, Hg, Tl, Ra, Fr and the molecules CaOH, SrOH, YO, CdH, BaF, YbF, YbOH, HfF$^+$, WC, TlF, PbO, RaF, ThO, ThF$^+$ and PaF$^{3+}$.