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An observation of any lepton number violating process will undoubtedly point towards the existence of new physics and indirectly to the clear Majorana nature of the exchanged fermion. In this work, we explore the potential of a minimal extension of the Standard Model via heavy sterile fermions with masses in the $[ 0.1 - 10]$ GeV range concerning an extensive array of neutrinoless meson and tau decay processes. We assume that the Majorana neutrinos are produced on-shell, and focus on three-body decays. We conduct an update on the bounds on the active-sterile mixing elements, $|U_{ell_alpha 4} U_{ell_beta 4}|$, taking into account the most recent experimental bounds (and constraints) and new theoretical inputs, as well as the effects of a finite detector, imposing that the heavy neutrino decay within the detector. This allows to establish up-to-date comprehensive constraints on the sterile fermion parameter space. Our results suggest that the branching fractions of several decays are close to current sensitivities (likely within reach of future facilities), some being already in conflict with current data (as is the case of $K^+ to ell_alpha^+ ell_beta^+ pi^-$, and $tau^- to mu^+ pi^- pi^-$). We use these processes to extract constraints on all entries of an enlarged definition of a $3times 3$ effective Majorana neutrino mass matrix $m_{ u}^{alpha beta}$.
The Schechter-Valle theorem states that a positive observation of neutrinoless double-beta ($0 u beta beta$) decays implies a finite Majorana mass term for neutrinos when any unlikely fine-tuning or cancellation is absent. In this note, we reexamine
We study the discovery prospect of different three body lepton number violating~(LNV) meson decays $M_{1}^{-}toell_{1}^{-}ell_{2}^{-}M_{2}^{+}$ in the framework of right handed~(RH) neutrino extended Standard Model~(SM). We consider a number of ongoi
We search for lepton-number- and baryon-number-violating decays $tau^{-}tooverline{p}e^{+}e^{-}$, $pe^{-}e^{-}$, $overline{p}e^{+}mu^{-}$, $overline{p}e^{-}mu^{+}$, $overline{p}mu^{+}mu^{-}$, and $pmu^{-}mu^{-}$ using 921 fb$^{-1}$ of data, equivalen
We have calculated the decay $tau rightarrow u pi(K) gamma$. We present the photon energy spectrum, the meson-photon invariant mass spectrum and the integrated rate as a function of a photon energy cut or an invariant mass cut. Both the internal bre
We study lepton flavor violating (LFV) tau and B decays in models with heavy neutrinos to constrain the mixing matrix parameters U_{tau N}. We find that the best current constraints when the heavy neutrinos are purely left-handed come from LFV radiat