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Register a new userUndetected Higgs decays and neutrino masses in gauge mediated, lepton number violating models
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We discuss SUSY models in which renormalizable lepton number violating couplings hide the decay of the Higgs through h -> chi_1^0 + chi_1^0 followed by chi_1^0 -> tau + 2 jets or chi_1^0 -> u_tau + 2 jets and also explain neutrino masses. This mechanism can be made compatible with gauge mediated SUSY breaking.
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Lepton-number violation (LNV), in general, implies nonzero Majorana masses for the Standard Model neutrinos. Since neutrino masses are very small, for generic candidate models of the physics responsible for LNV, the rates for almost all experimentally accessible LNV observables -- except for neutrinoless double-beta decay -- are expected to be exceedingly small. Guided by effective-operator considerations of LNV phenomena, we identify a complete family of models where lepton number is violated but the generated Majorana neutrino masses are tiny, even if the new-physics scale is below 1 TeV. We explore the phenomenology of these models, including charged-lepton flavor-violating phenomena and baryon-number-violating phenomena, identifying scenarios where the allowed rates for $mu^-to e^+$-conversion in nuclei are potentially accessible to next-generation experiments.
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 the quantitative impact of the Schechter-Valle theorem, and find that current experimental lower limits on the half-lives of $0 u beta beta$-decaying nuclei have placed a restrictive upper bound on the Majorana neutrino mass $|delta m^{ee}_ u| < 7.43 times 10^{-29}~{rm eV}$ radiatively generated at the four-loop level. Furthermore, we generalize this quantitative analysis of $0 u beta beta$ decays to that of the lepton-number-violating (LNV) meson decays $M^- to {M^prime}^+ + ell^-_alpha + ell^-_beta$ (for $alpha$, $beta$ = $e$ or $mu$). Given the present upper limits on these rare LNV decays, we have derived the loop-induced Majorana neutrino masses $|delta m^{ee}_ u| < 9.7 times 10^{-18}~{rm eV}$, $|delta m^{emu}_ u| < 1.6 times 10^{-15}~{rm eV}$ and $|delta m^{mu mu}_ u| < 1.0 times 10^{-12}~{rm eV}$ from $K^- to pi^+ + e^- + e^-$, $K^- to pi^+ + e^- + mu^-$ and $K^- to pi^+ + mu^- + mu^-$, respectively. A partial list of radiative neutrino masses from the LNV decays of $D$, $D_s^{}$ and $B$ mesons is also given.
We propose a new mechanism to explain neutrino masses with lepton number conservation, in which the Dirac neutrino masses are generated at the two-loop level involving a dark matter candidate. In this model, branching ratios of lepton flavor violating decays of the Higgs boson can be much larger than those of lepton flavor violating decays of charged leptons. If lepton flavor violating decays of the Higgs boson are observed at future collider experiments without detecting lepton flavor violating decays of charged leptons, most of the models previously proposed for tiny neutrino masses are excluded while our model can still survive. We show that the model can be viable under constraints from current data for neutrino experiments, searches for lepton flavor violating decays of charged leptons and dark matter experiments.
Exotic Higgs decays are promising channels to discover new physics in the near future. We present a simple model with a new light scalar that couples to the Standard Model through a charged lepton-flavor violating interaction. This can yield exciting new signatures, such as $h to e^+ e^+ mu^-mu^-$, that currently have no dedicated searches at the Large Hadron Collider. We discuss this model in detail, assess sensitivity from flavor constraints, explore current constraints from existing multi-lepton searches, and construct a new search strategy to optimally target these exotic, lepton-flavor violating Higgs decays.
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 ongoing experiments, such as, NA62 and LHCb at CERN, Belle II at SuperKEK, as well as at the proposed future experiments, SHiP, MATHUSLA and FCC-ee. The RH Majorana neutrino $N$ mediating these meson decays provides a resonant enhancement of the rates, if the mass of $N$ lies in the range $(100, text{MeV}-6, text{GeV})$. We consider the effect of parent mesons velocity, as well as, the effect of finite detector size. Using the expected upper limits on the number of events for the LNV decay modes, $M_{1}^{-} toell_1^{-}ell_2^{-}pi^{+}$~($M_{1}=B, B_c,D, D_{s},text{and},K$), we analyze the sensitivity reach of the mixing angles $|V_{e N}|^{2}$, $|V_{mu N}|^{2}$, $|V_{tau N}|^{2}$, $|V_{e N}V_{mu N}|$, $|V_{e N}V_{tau N}|$ and $|V_{mu N}V_{tau N}|$ as a function of heavy neutrino mass $M_{N}$. We show that, inclusion of parent meson velocity can account to a large difference for active-sterile mixing, specially for $D$, $D_s$ meson decay at SHiP and $K$ meson decay at NA62. Taking into account the velocity of the $D_s$ meson, the future beam dump experiment SHiP can probe $|V_{eN}|^2 sim 10^{-9}$. For RH neutrino mass in between 2 - 5 GeV, MATHUSLA can provide best sensitivity reach of active-sterile mixings.
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