Leptonic Meson Decays into Invisible ALP

Abstract

The theoretical calculation of pseudo-scalar leptonic decay widths into an invisible ALP, M → ν a, is reviewed. Assuming generic flavour-conserving ALP couplings to SM fermions and a generic ALP mass, m a , the latest experimental results for pseudo-scalar leptonic decays are used to provide updated bounds on the ALPfermion Lagrangian sector. Constrains on the ALP-quark couplings obtained from these channels are not yet competitive with the ones derived from FCNC processes, like M → P a decays. These leptonic decays can, however, provide the most stringent model-independent upper bounds on ALP-leptons couplings for m a in the (sub)-GeV range.

ALP Production in Weak Mesonic Decays

“…For definiteness, m a = 0, f a = 1 TeV and c f = ±1 have been used. As noticed in Section 3.1 and 3.2, accidental cancellation can occurs in the tree-level amplitudes, depending on the relative sign between c Q (′) and c q (′) for all the processes but vector-to-vector decays, see Equations (20)(21)(22)(23)(24)(25)(26)(27) and Equations (41)(42)(43)(44)(45)(46). To make evident the impact of this accidental cancellation, the tree-level results in Table 1 has been shown with a (min − max) interval, obtained by setting c Q (′) ∕c q (′) = (+1, −1) respectively.…”

“…Flavor physics experiments have received more and more attention from the phenomenological community. [22,[24][25][26][47][48][49][50] Strong limits on the ALP couplings in Equation ( 1) can be derived, for example, through the study of the K → 𝜋a decay. In large part of the literature a flavor universal ALP-fermion coupling, often dubbed c aΦ , is assumed.…”

“…Note that the flavor conserving diagonal vector couplings are identically zero up to a shift in the electroweak anomalous coupling. Following most of the literature, [21][22][23][24][25][26] a low-energy CP and flavor conserving effective Lagrangian for ALP-fermion interactions can be introduced: 1 𝛿 a ferm = − 𝜕 𝜇 a 2f a c i fi 𝛾 𝜇 𝛾 5 f i ⊃ i a f a m i fi 𝛾 5 c i f i .…”

“…For completeness we report here, briefly, the semileptonic meson decays, M → 𝓁 𝜈 𝓁 a, derived in details in [26]. In Figure 4 the diagrams where the ALP is emitted from the meson are drawn.…”

Probing HNL‐ALP Couplings at Colliders

“…On the experimental side, the efforts pointed towards the construction of a consistent effective description of ALPs, [25][26][27][28][29][30][31][32] beyond the seminal study in Ref. [33], in order to investigate DOI: 10.1002/prop.202300027 possible signals at low-energy facilities, [34][35][36][37][38][39][40][41][42][43][44] colliders [25,[45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61] and in non-terrestrial environments. [62][63][64][65] Most of the analyses performed on collider data and from astrophysics deal with an on-shell ALP in the initial or final state of the considered processes, such as mono-Z, mono-W, and mono-Higgs searches or stellar cooling, and the associated bounds strongly depend on the ALP mass.…”