Manimala Mitra

Neutrinoless Double Beta Decay and Heavy Sterile Neutrinos

The experimental rate of neutrinoless double beta decay can be saturated by the exchange of virtual sterile neutrinos, that mix with the ordinary neutrinos and are heavier than 200 MeV. Interestingly, this hypothesis is subject only to marginal experimental constraints, because of the new nuclear matrix elements. This possibility is analyzed in the context of the Type I seesaw model, performing also exploratory investigations of the implications for heavy neutrino mass spectra, rare decays of mesons as well as neutrino-decay search, LHC, and lepton flavor violation. The heavy sterile neutrinos can saturate the rate only when their masses are below some 10 TeV, but in this case, the suppression of the light-neutrino masses has to be more than the ratio of the electroweak scale and the heavy-neutrino scale; i.e., more suppressed than the naive seesaw expectation. We classify the cases when this condition holds true in the minimal version of the seesaw model, showing its compatibility (1) with neutrinoless double beta rate being dominated by heavy neutrinos and (2) with any light neutrino mass spectra. The absence of excessive fine-tunings and the radiative stability of light neutrino mass matrices, together with a saturating sterile neutrino contribution, imply an upper bound on the heavy neutrino masses of about 10 GeV. We extend our analysis to the Extended seesaw scenario, where the light and the heavy sterile neutrino contributions are completely decoupled, allowing the sterile neutrinos to saturate the present experimental bound on neutrinoless double beta decay. In the models analyzed, the rate of this process is not strictly connected with the values of the light neutrino masses, and a fast transition rate is compatible with neutrinos lighter than 100 meV.

Constraining Neutrino Mass from Neutrinoless Double Beta Decay

We re-analyze the compatibility of the claimed observation of neutrinoless double beta decay (0 ) in 76 Ge with the new limits on the half-life of 136 Xe from EXO-200 and KamLAND-Zen. Including recent calculations of the nuclear matrix elements (NMEs), we show that while the claim in 76 Ge is still compatible with the individual limits from 136 Xe for a few NME calculations, it is inconsistent with the KamLAND-Zen+EXO-200 combined limit for all but one NME. After imposing the most stringent upper limit on the sum of light neutrino masses from Planck, we nd that the canonical light neutrino contribution cannot satisfy the claimed 0 signature or saturate the current limit, irrespective of the NME uncertainties. However, inclusion of the heavy neutrino contributions, arising naturally in TeV-scale Left-Right symmetric models, can saturate the current limit of 0 . In a type-II seesaw framework, this imposes a lower limit on the lightest neutrino mass. Depending on the mass hierarchy, we obtain this limit to be in the range of 0.07 - 4 meV for a typical choice of the right-handed (RH) gauge boson and RH neutrino masses relevant for their collider searches. Using the 0 bounds, we also derive correlated constraints in the RH sector, complimentary to those from the LHC.

Prospects of Heavy Neutrino Searches at Future Lepton Colliders

We discuss the future prospects of heavy neutrino searches at next generation lepton colliders. In particular, we focus on the planned electron-positron colliders, operating in two different beam modes, namely,

Neutrino jets from high-mass WR gauge bosons in TeV-scale left-right symmetric models

e^+e^-

Effect of cancellation in neutrinoless double beta decay

and

Lepton number and lepton flavor violation through color octet states

e^-e^-

Implications of the Diboson Excess for Neutrinoless Double Beta Decay and Lepton Flavor Violation in TeV Scale Left Right Symmetric Model

. In the

Explaining a CMS

e^+e^-

eejj

beam mode, we consider various production and decay modes of the heavy neutrino (

Excess With

N