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Register a new userMuon-to-Electron Conversion in Mirror Fermion Model with Electroweak Scale Non-Sterile Right-handed Neutrinos
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The muon-to-electron conversion in nuclei like aluminum, titanium and gold is studied in the context of a class of mirror fermion model with non-sterile right-handed neutrinos having mass at the electroweak scale. At the limit of zero momentum transfer and large mirror lepton masses, we derive a simple formula to relate the conversion rate with the on-shell radiative decay rate of muon into electron. Current experimental limits (SINDRUM II) and projected sensitivities (Mu2e, COMET and PRISM) for the muon-to-electron conversion rates in various nuclei and latest limit from MEG for the radiative decay rate of muon into electron are used to put constraints on the parameter space of the model. Depending on the nuclei targets used in different experiments, for the mirror lepton mass in the range of 100 to 800 GeV, the sensitivities of the new Yukawa couplings one can probe in the near future are in the range of one tenth to one hundred-thousandth, depending on the mixing scenarios in the model.
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The electric dipole moment of the electron is studied in detail in an extended mirror fermion model with the following unique features of (a) right-handed neutrinos are non-sterile and have masses at the electroweak scale, and (b) a horizontal symmetry of the tetrahedral group is used in the lepton and scalar sectors. We study the constraint on the parameter space of the model imposed by the latest ACME experimental limit on electron electric dipole moment. Other low energy experimental observables such as the anomalous magnetic dipole moment of the muon, charged lepton flavor violating processes like muon decays into electron plus photon and muon-to-electron conversion in titanium, gold and lead are also considered in our analysis for comparison. In addition to the well-known CP violating Dirac and Majorana phases in the neutrino mixing matrix, the dependence of additional phases of the new Yukawa couplings in the model is studied in detail for all these low energy observables.
The small neutrino mass observed in neutrino oscillations is nicely explained by the seesaw mechanism. Rich phenomenology is generally expected if the heavy neutrinos are not much heavier than the electroweak scale. A model with this feature built in has been suggested recently by Hung. The model keeps the standard gauge group but introduces chirality-flipped partners for the fermions. In particular, a right-handed neutrino forms a weak doublet with a charged heavy lepton, and is thus active. We analyze the lepton flavor structure in gauge interactions. The mixing matrices in charged currents (CC) are generally non-unitary, and their deviation from unitarity induces flavor changing neutral currents (FCNC). We calculate the branching ratios for the rare decays muto egamma and muto eebar e due to the gauge interactions. Although the former is generally smaller than the latter by three orders of magnitude, parameter regions exist in which muto egamma is reachable in the next generation of experiments even if the current stringent bound on muto eebar e is taken into account. If light neutrinos dominate for muto egamma, the latter cannot set a meaningful bound on unitarity violation in the mixing matrix of light leptons due to significant cancelation between CC and FCNC contributions. Instead, the role is taken over by the decay muto eebar e.
The KATRIN experiment aims to determine the absolute neutrino mass by measuring the endpoint region of the tritium $beta$ spectrum. As a large-scale experiment with a sharp energy resolution, high source luminosity and low background it may also be capable of testing certain theories of neutrino interactions beyond the standard model (SM). An example of a non-SM interaction are right-handed currents mediated by right-handed W bosons in the left-right symmetric model (LRSM). In this extension of the SM, an additional SU(2)$_mathrm R$ symmetry in the high-energy limit is introduced, which naturally includes sterile neutrinos and predicts the seesaw mechanism. In tritium $beta$ decay, this leads to an additional term from interference between left- and right-handed interactions, which enhances or suppresses certain regions near the endpoint of the beta spectrum. In this work, the sensitivity of KATRIN to right-handed currents is estimated for the scenario of a light sterile neutrino with a mass of some eV. This has been performed with a Bayesian analysis using Markov Chain Monte Carlo (MCMC). The simulations show that in principle KATRIN is able to set sterile neutrino mass-dependent limits on the interference strength. Thereby, the sensitivity is significantly increased if the $Q$ value of the $beta$ decay can be sufficiently constrained. However, the sensitivity is not high enough to improve current upper limits from right-handed W boson searches at the LHC.
We explain anomalies currently present in various data samples used for the measurement of the anomalous magnetic moment of electron ($a_e$) and muon ($a_mu$) in terms of an Aligned 2-Higgs Doublet Model with right-handed neutrinos. The explanation is driven by one and two-loop topologies wherein a very light CP-odd neutral Higgs state ($A$) contributes significantly to $a_mu$ but negligibly to $a_e$, so as to revert the sign of the new physics corrections in the former case with respect to the latter, wherein the dominant contribution is due to a charged Higgs boson ($H^pm$) and heavy neutrinos with mass at the electroweak scale. For the region of parameter space of our new physics model which explains the aforementioned anomalies we also predict an almost background-free smoking-gun signature of it, consisting of $H^pm A$ production followed by Higgs boson decays yielding multi-$tau$ final states, which can be pursued at the Large Hadron Collider.
In this article we consider the Standard Model extended by a number of (light) right-handed neutrinos, and assume the presence of some heavy physics that cannot be directly produced, but can be probed by its low-energy effective interactions. Within this scenario, we obtain all the gauge-invariant dimension-seven effective operators, and determine whether each of the operators can be generated at tree-level by the heavy physics, or whether it is necessarily loop generated. We then use the tree-generated operators, including those containing right-handed neutrinos, to put limits on the scale of new physics $ Lambda $ using low-energy measurements. We also study the production of same-sign dileptons at the Large Hadron Collider (LHC) and determine the constraints on the heavy physics that can be derived form existing data, as well as the reach in probing $ Lambda $ expected from future runs of this collider.
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