Atomic Spectroscopy Probes of New Physics

Abstract

Precision spectroscopy has long played a central role in testing the foundations of physics, from the early insights that led to the development of quantum mechanics to the validation of quantum electrodynamics and the determination of fundamental constants. Today, advances in atomic and molecular spectroscopy enable sensitive searches for physics beyond the Standard Model. A broad class of well-motivated extensions predicts new light degrees of freedom with feeble couplings to electrons, muons, and nucleons, giving rise to tiny spin-independent interactions that can be probed at low energies. In this review, we present a unified overview of spectroscopic searches for such interactions. We discuss the effective theoretical framework connecting fundamental interactions to atomic and nuclear observables, survey the key experimental and theoretical strategies, and review the atomic and molecular systems providing the strongest sensitivity. We conclude with updated spectroscopic constraints on representative benchmark models, highlighting the unique and complementary role of precision spectroscopy in exploring new fundamental interactions.

Figures (3)

• Figure 1: Schematic map of effective new-physics couplings to the proton, neutron, electron, and muon, highlighting the spectroscopic systems that connect them. Systems containing transitions essential for the determination of fundamental constants are shown in red, with the corresponding constant indicated in parentheses.
• Figure 2: Sensitivity reach of precision spectroscopic systems to new physics. The shaded sectors illustrate the sensitivity of various transitions in atomic, molecular, and exotic systems and to different combinations of effective new-physics couplings to electrons, muons, protons, and neutrons. The sectors are organized based on specific (products of) couplings probed by each systems, highlighting the complementarity between hydrogenic atoms, molecular ions, helium, positronium, muonium, and isotope-shift measurements. The radial axis encodes the strength of the probed coupling combinations, while the color shading indicates the characteristic interaction range.
• Figure 3: Bounds on spin-independent interactions obtained from a global fit to precision spectroscopic data, simultaneously wih a self-consistent determination of the relevant fundamental constants (see Section \ref{['sec:codata-np']}). The left panel shows the 99$\%$ CL constraints on $\alpha_{\varphi}\equiv g_{{\rm SI},e}g_{{\rm SI},p}/(4\pi)$ for four benchmark models, namely a dark photon (red), a $B-L$ gauge boson (black), a Higgs portal scalar (blue) and a featheron scalar (purple). The right panel shows the 99$\%$ CL constraints on individual spin-independent couplings to the electron (blue), muon (magenta) and nucleon assuming either isotriplet (red) or isosinglet (green) structure. Solid (dashed) curves correspond to a spin-0 mediator $\phi$ (spin-1 mediator $X_\mu$). Thicker bands indicate the mediator-mass ranges for which the fit prefers new physics over the SM at more than $4\sigma$.