Bounds on lepton flavor violating physics and decays of neutral mesons from τ(μ)→3ℓ , ℓγγ

Abstract

We study two-and three-body lepton flavor violating (LFV) decays involving leptons and neutral vector bosons V = ρ 0 , ω, φ, J/ψ, Υ, Z 0 , as well as pseudoscalar P = π 0 , η, η , ηc and scalar S = f0(500), f0(980), a0(980), χc0(1P ) mesons, without referring to a specific mechanism of LFV realization. In particular, we relate the rates of the three-body LFV decays τ (µ) → 3 , where = µ or e, to the two-body LFV decays (V, P ) → τ µ(τ e, µe), where V and P play the role of intermediate resonances in the decay process τ (µ) → 3 . From the experimental upper bounds for the branching ratios of τ (µ) → 3 decays, we derive upper limits for the branching ratios of (V, P ) → τ µ(τ e, µe). We compare our results to the available experimental data and known theoretical upper limits from previous studies of LFV processes and find that some of our limits are several orders of magnitude more stringent. Using the idea of quark-hadron duality, we extract limits on various quark-lepton dimension-six LFV operators from data on lepton decays. Some of these limits are either new or stronger than those existing in the literature.

Completeness and complementarity for μ → eγ, μ → $$ e\overline{e}e $$ and μA → eA

“…Constraints on LFV contact interactions have been tabulated in many previous publications. This manuscript differs from earlier works, such as [13][14][15][16][17], which mostly listed tree-level bounds, that is, constraints at the experimental scale on the coefficients of operators which contribute at tree level to the process. The sensitivities tabulated here are very similar to those of [18], which apply to coefficients at a short-distance scale ∼ 1/m W and include similar Standard Model loop corrections between m W and low energy; the results here complete those of [18] (where some operators are missing from the tables), extend them by a subset of τ ↔ operators, and also give the sensitivities in the SMEFT basis.…”

“…Black et al [54] give the sensitivity of various rare decays to a subset of SMEFT coefficients that contribute at tree level; a more complete study in the SMEFT was performed recently in [49]. The intermediate-state contribution of heavy quark mesons to leptonic LFV τ decays is considered in [17].…”

Lepton flavor violation and collider searches in a type I + II seesaw model

“…There are other sources of lepton flavor violation such as µ − e conversion but they are subdominant [100][101][102][103]. Other lepton flavor violating decays involving the τ lepton are less promising, unless one invokes a mechanism to significantly suppress µ → eγ [104][105][106][107][108][109][110][111][112][113]. Anyway, going back to the relevant muon decays, µ → eγ and µ → 3e, one can check that the current bounds read BR(µ → eγ) < 4.2×10 −13 , BR(µ → 3e) < 10 −12 , and future experiments aim BR(µ → eγ) < 4×10 −14 , BR(µ → 3e) < 10 −16 .…”

RGE effects on the LFV scale from meson decays

“…In this work, we study LFV in a model-independent way in the context of an effective LFV quark-lepton Lagrangian with the operators up to dimension-6, which can contribute to various LFV processes such as nuclear µ − e-conversion [17][18][19][20], deep inelastic 1 − 2 -conversion [21], decays of mesons M → 1 2 and leptons µ(τ ) → ee, γγ [20]. Previously, for the quark flavor-diagonal case, the Wilson Coefficients (WC) and the individual LFV scales for these operators have been constrained in the literature from the existing experimental limits on these processes (for a summary, see for example Ref.…”

“…Previously, for the quark flavor-diagonal case, the Wilson Coefficients (WC) and the individual LFV scales for these operators have been constrained in the literature from the existing experimental limits on these processes (for a summary, see for example Ref. [20]). We include in the analysis quark flavor non-diagonal operators, for which we derive new treelevel limits from current limits on the LFV rates of flavored mesons.…”