Some aspects of positronium physics

Parapositronium can decay into three photons

“…In the low energy limit the photon spectrum is of O(x 5 ) in agreement with Low's theorem [12]. (14). Here, x = E/me is the energy of a photon, E, divided by the electron mass.…”

“…When one photon has maximal energy (x = 1), the other two photons must move collinearly and in the opposite direction. However, this configuration does not conserve angular momentum and thus the photon spectrum (14) vanishes at x = 1. This is in contrast with the spectrum of the orthopositronium decay into 3γ, which reaches its maximum at x = 1.…”

Improved limit on invisible decays of positronium

“…The singlet (1 1 S 0 ) state (parapositronium, p-Ps) decays predominantly into two photons with a lifetime in vacuum of τ p-Ps = 125 ps [6,7]. The longer lifetime of o-Ps (due to the phase-space and additional α suppression factors) gives an enhancement factor ≃ 10 3 in the sensitivity to an admixture of potential new interactions not accommodated in the Standard Model (SM) [9]. This paper focuses on a new search for o − P s → invisible decays.…”

Higgs →μτ in Abelian and non-Abelian flavor symmetry models

“…This will also lead to the lepton-flavor-violating decays τ → 3μ and τ → μγ [57,58]. Since the Z of a gauged U(1) L μ −L τ does not couple to first generation fermions, the experimental limits are not as stringent as for other U(1) models, and it might even be possible to use (a light) Z to resolve the longstanding 3-4σ anomaly surrounding the muon's magnetic moment [58][59][60][61][62][63][64][65][66]. An even lighter Z may induce long-range forces modifying neutrino oscillations [67], although this is not the limit of interest here.…”