The scalar triplet contribution to lepton flavour violation and neutrinoless double beta decay in Left-Right Symmetric Model

Abstract

We analyse in detail the scalar triplet contribution to the low-energy lepton flavour violating (LFV) and lepton number violating (LNV) processes within a TeV-scale left-right symmetric framework. We show that in both type-I and type-II seesaw dominance for the light neutrino masses, the triplet of mass comparable to or smaller than the largest right-handed neutrino mass scale can give sizeable contribution to the LFV processes, except in the quasi-degenerate limit of light neutrino masses, where a suppression can occur due to cancellations. In particular, a moderate value of the heaviest neutrino to scalar triplet mass ratio r O(1) is still experimentally allowed and can be explored in the future LFV experiments. Similarly, the contribution of a relatively light triplet to the LNV process of neutrinoless double beta decay could be significant, disfavouring a part of the model parameter space otherwise allowed by LFV constraints. Nevertheless, we find regions of parameter space consistent with both LFV and LNV searches, for which the values of the total effective neutrino mass can be accessible to the next generation tonscale experiments. Such light triplets can also be directly searched for at the LHC, thus providing a complementary probe of this scenario. Finally, we also study the implications of the triplet contribution for the left-right symmetric model interpretation of the recent diboson anomaly at the LHC.

Charged lepton flavour violcxmation and neutrinoless double beta decay in left-right symmetric models with type I+II seesaw

“…It can be seen from the plots shown in figure 6, 7 and 9 that this parameter can be larger than unity, implying that the doubly charged scalar masses can be as small as the heaviest right handed neutrino mass which can keep the scalar triplet masses well within the reach of LHC. This is in contrast to earlier results of [32] showing the scalar triplet to be at least ten times heavier than the heaviest right handed neutrino and the more recent work [57] where r was shown to be close to unity for a very small range of lightest neutrino mass. As can be seen from the plot in figure 7, we can have r ≥ 1 for almost all values of lightest neutrino mass in case of inverted hierarchy.…”

“…To be more specific, the authors of [57] showed that for heaviest right handed neutrino mass as low as 400 GeV, the triplet scalars are allowed to be as low as around 800 GeV for right handed charged gauge boson mass 3.5 TeV. Here we extend both these works [56,57] by considering more general type I and type II seesaw terms with comparable strength and study their implications in LFV processes µ → 3e, µ → eγ and LNV process like 0νββ. Instead of considering any specific mass matrix structure for either type I or type II seesaw mass matrix, we consider a very general mass matrix for one of the seesaw terms.…”

“…The authors showed that the current experimental bounds still allow light scalar triplet mass in MLRSM which was earlier thought to be around ten times heavier than the heaviest right handed neutrino mass [32]. To be more specific, the authors of [57] showed that for heaviest right handed neutrino mass as low as 400 GeV, the triplet scalars are allowed to be as low as around 800 GeV for right handed charged gauge boson mass 3.5 TeV. Here we extend both these works [56,57] by considering more general type I and type II seesaw terms with comparable strength and study their implications in LFV processes µ → 3e, µ → eγ and LNV process like 0νββ.…”

“…More recently, the authors of [32,52,53] studied the new physics contributions to 0νββ process for TeV scale MLRSM with dominant type II seesaw. There have also been several works [54][55][56][57] where type I seesaw limit was also included into the computation of 0νββ in MLRSM. Some more detailed analyses incorporating left-right gauge mixing were discussed in the works [58][59][60][61][62].…”

“…We then studied the new physics contributions to 0νββ amplitude by taking experimental constraints on LFV process µ → 3e, masses of triplet scalars, new gauge bosons and right handed neutrinos. In another recent work [57], scalar triplet contributions to LFV processes µ → 3e, µ → eγ as well as 0νββ were studied for either type I or type II dominant cases. The authors showed that the current experimental bounds still allow light scalar triplet mass in MLRSM which was earlier thought to be around ten times heavier than the heaviest right handed neutrino mass [32].…”