Current and future perspectives of positronium and muonium spectroscopy as dark sectors probe

Abstract

Positronium and Muonium are purely leptonic atoms and hence free of an internal sub-structure. This qualifies them as potentially well suited systems to probe the existence of physics beyond the Standard Model. We hence carry out a comprehensive study of the sensitivity of current Positronium and Muonium precision spectroscopy to several new physics scenarios. By taking properly into account existing experimental and astrophysical probes, we define clear experimental targets to probe new physics via precise spectroscopy. For Positronium we find that, in order for the spectroscopy bounds to reach a sensitivity comparable to the electron gyromagnetic factor, an improvement of roughly five orders of magnitude from state-of-the-art precision is required, which would be a challenge based on current technology. More promising is instead the potential reach of Muonium spectroscopy: in the next few years experiments like Mu-MASS at PSI will probe new regions of the parameter space testing the existence of medium/short range (MeV and above) spin-dependent and spin-independent dark forces between electrons and muons.

Intense beam of metastable Muonium

“…A determination of the Lamb shift in muonium with this accuracy will provide a stringent test of high-order recoil corrections in bound state QED [ 52 , 53 ], free of finite-size effects [ 1 ]. Moreover, it will be a sensitive probe for the existence of exotic dark-sector particles [ 54 ], new muonic forces [ 55 ], and hidden dimensions [ 56 , 57 ].…”

Observation of asymmetric line shapes in precision microwave spectroscopy of the positronium 2S13→2PJ3

“…Such forces could appear in two flavors, depending on the nature of the mediator of the interaction. Scalar and vector mediators lead to a spin-independent force, whereas the exchange of pseudoscalars or axion-like particles (ALPs) would give rise to spin-dependent interactions [73].…”

“…This interaction induces a shift of the atomic energy levels that depends on the mass of the mediator and the coupling strength. Therefore, for a particular shift, E , of Ps energy levels (in general the maximum between the experimental and theoretical uncertainties [73], although here it is relative to QED predictions), it is possible to constrain the coupling constant as a function of the mass of the mediator. In particular, for the 2 3 S1 → 2 3 P0 transition (2S → 2P, since the spin state is irrelevant for this interaction force), the energy shift is given by…”

Precision Microwave Spectroscopy of the Positronium n=2 Fine Structure

“…Trap-based positron beams [4] and new laser technology [38] can now be used to improve the precision of such measurements, which are potentially sensitive to new particles or forces (e.g., Refs. [39][40][41][42]).…”