Z lepton flavour violation as a probe for new physics at future $$e^+e^-$$ colliders

Abstract

In this work we assess the potential of discovering new physics by searching for lepton-flavour-violating (LFV) decays of the Z boson, $$Z\rightarrow \ell _i \ell _j$$ Z → ℓ i ℓ j , at the proposed circular $$e^+e^-$$ e + e - colliders CEPC and FCC-ee. Both projects plan to run at the Z-pole as a “Tera Z factory”, i.e., collecting $${\mathcal {O}}\left( 10^{12} \right) $$ O 10 12 Z decays. In order to discuss the discovery potential in a model-independent way, we revisit the LFV Z decays in the context of the Standard Model effective field theory and study the indirect constraints from LFV $$\mu $$ μ and $$\tau $$ τ decays on the operators that can induce $$Z\rightarrow \ell _i \ell _j$$ Z → ℓ i ℓ j . We find that, while the $$Z\rightarrow \mu e$$ Z → μ e rates are beyond the expected sensitivities, a Tera Z factory is promising for $$Z\rightarrow \tau \ell $$ Z → τ ℓ decays, probing New Physics at the same level of future low-energy LFV observables.

Renormalization group improved implications of semileptonic operators in SMEFT

“…While finalizing our paper we notice ref. [87] on arXiv which presented a study of the LFV decays through Z-flavour violation in the context of future colliders. The discussion on the LFV decays partly overlaps with the present work.…”

Leptonic CP violation from a vector-like lepton

“…The flavor changing Z decays for example currently constrain |X | 10 −3 . In the future Tera Z factories are expected to reach a sensitivity similar of low energy CLFV observables [61] in the channels Z → τ . Lepton flavor universality violation tests are already considered in the fit of ref.…”

Leptoflavorgenesis: baryon asymmetry of the Universe from lepton flavor violation

“…Here, we note that nonzero µ e implies some other nonzero µ i to satisfy the conservation of hypercharge, which is implicitly taken into account in Eqs. ( 14) to (16). In this case, the electron charge is converted to q ∆ f as described by the above transport equations.…”

“…Also, the discovery of neutrino oscillations revealed that the lepton flavor symmetries, U (1) L e −L µ × U (1) L µ −L τ , are violated [6-10], which further implies the violation of U (1) B −L if neutrinos are Majorana fermions [11]. However, such interactions are so feeble that they could be inefficient up to T B −L ∼ 10 13 GeV, depending on possible UV completions of the neutrino sector.Charged-lepton flavor violation (CLFV) is currently attracting a lot of attention [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27], since its discovery would undoubtedly imply physics beyond the SM (plus massive neutrinos), enabling us to probe new physics at extremely high energy scales, such as O 10 8 GeV by µ → eγ [13] and O 10 9 GeV by µ → ea, where a is a pseudo-Nambu-Goldstone boson like an axion or familon [22,28]. 1 This is because CLFV induced by the SM neutrino masses is extremely suppressed by the GIM mechanism [49].…”

“…Charged-lepton flavor violation (CLFV) is currently attracting a lot of attention [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27], since its discovery would undoubtedly imply physics beyond the SM (plus massive neutrinos), enabling us to probe new physics at extremely high energy scales, such as O 10 8 GeV by µ → eγ [13] and O 10 9 GeV by µ → ea, where a is a pseudo-Nambu-Goldstone boson like an axion or familon [22,28]. 1 This is because CLFV induced by the SM neutrino masses is extremely suppressed by the GIM mechanism [49].…”