The existence of light sterile neutrinos, as predicted in several models, can help to explain a number of observations starting from dark mater to recent anomalies in short baseline experiments. In this paper we consider two models - Left-Right Symme
tric Zee model and Extended Seesaw model, that can naturally accommodate the presence of light sterile neutrinos in the eV to MeV mass scale. We perform a detailed study on the neutrinoless double beta decay process which receives major contributions from diagrams involving these light sterile neutrinos. Considering a number of theoretical and experimental constraints, including light neutrino masses and mixings, unitarity of the mixing matrix etc., we compare our predicted values of the half-life of neutrinoless double beta decay with the experimental limits. This can put significant constraints on the neutrino mass, active-sterile neutrino mixing and several other important parameters in these models.
In a simple extension of the standard model (SM), a pair of vector like lepton doublets ($L_1$ and $L_2$) and a $SU(2)_L$ scalar doublet ($eta$) have been introduced to help in accommodating the discrepancy in determination of the anomalous magnetic
moments of the light leptons, namely, $e$ and $mu$. Moreover, to make our scenario friendly to a Dirac like neutrino and also for a consistent dark matter phenomenology, we specifically add a singlet scalar ($S$) and a singlet fermion ($psi$) in the set-up. A discrete symmetry $mathcal{Z}_2timesmathcal{Z}_2^prime$ has been imposed under which all the SM particles are even while the new particles may be assumed to have odd charges. In a bottom-up approach, with a minimal particle content, we systematically explore the available parameter space in terms of couplings and masses of the new particles. Here a number of observables associated with the SM leptons have been considered, e.g., masses and mixings of neutrinos, $(g-2)$ anomalies of $e$, $mu$, charged lepton flavor violating (cLFV) observables and the dark matter (DM) phenomenology of a singlet-doublet dark matter. Neutrinos, promoted as the Dirac type states, acquire mass at one loop level after the discrete $mathcal{Z}_2^prime$ symmetry gets softly broken, while the unbroken $mathcal{Z}_2$ keeps the dark matter stable. The mixing between the singlet $psi$ and the doublet vector lepton can be constrained to satisfy the electroweak precision observables and the spin independent (SI) direct detection (DD) cross section of the dark matter. In this analysis, potentially important LHC bounds have also been discussed.
The present anomaly in muon anaomalous magnetic moment can be explained by the presence of a muon-philic $X$ boson which could be a scalar particle or a vector particle with mass less than twice the mass of muon. The muon-philic $X$ boson could inter
estingly not be a parity eigenstate as well. If there exists such a boson, irrespective of its parity, it can be directly observed in the decay $J/psi to mu^- mu^+ X$ where $X$ remains invisible. We show that by using the angular distribution or the distribution of events in the square Dalitz plot, along with two well defined dimensionless ratios, one can clearly distinguish among the various spin-parity possibilities. This would constitute an important probe of both the existence and the nature of this new physics possibility.
The various global analyses of available neutrino oscillation data indicate the presence of the standard $3+0$ neutrino oscillation picture. However, there are a few short baseline anomalies that point to the possible existence of a fourth neutrino (
with mass in the eV-scale), essentially sterile in nature. Should sterile neutrino exist in nature and its presence is not taken into consideration properly in the analyses of neutrino data, the interference terms arising due to the additional CP phases in presence of a sterile neutrino can severely impact the physics searches in long baseline (LBL) neutrino oscillation experiments. In the current work we consider one light (eV-scale) sterile neutrino and probe all the three CP phases ($delta_{13}$, $delta_{24}$, $delta_{34}$) in the context of the upcoming Deep Underground Neutrino Experiment (DUNE) and also estimate how the results improve when data from NOvA, T2K and T2HK are added in the analysis. We illustrate the $Delta chi^2$ correlations of the CP phases among each other, and also with the three active-sterile mixing angles. Finally, we briefly illustrate how the relevant parameter spaces in the context of neutrinoless double beta decay get modified in light of the bounds in presence of a light sterile neutrino.
We explore relativistic freeze-in production of scalar dark matter in gauged $B-L$ model, where we focus on the production of dark matter from the decay and annihilation of Standard Model (SM) and $B-L$ Higgs bosons. We consider the Bose-Einstein (BE
) and Fermi-Dirac (FD) statistics, along with the thermal mass correction of the SM Higgs boson in our analysis. We show that in addition to the SM Higgs boson, the annihilation and decay of the $B-L$ scalar can also contribute substantially to the dark matter relic density. Potential effects of electroweak symmetry breaking (EWSB) and thermal mass correction in BE framework enhance the dark matter relic substantially as it freezes-in near EWSB temperature via scalar annihilation. However, such effects are not so prominent when the dark matter freezes-in at a later epoch than EWSB, dominantly by decay of scalars. The results of this analysis are rather generic, and applicable to other similar scenarios.
We consider the extension of the Standard Model (SM) with an inert Higgs doublet that also contains two or three sets of $SU(2)_L$ triplet fermions with hypercharge zero and analyze the stability of electroweak vacuum for the scenarios. The model rep
resents a Type-III inverse seesaw mechanism for neutrino mass generation with a Dark matter candidate.An effective potential approach calculation with two-loop beta function have been carried out in deciding the fate of the electroweak vacuum. Weak gauge coupling $g_2$ shows a different behaviour as compared to the Standard Model. The modified running of $g_2$, along with the Higgs quartic coupling and Type-III Yukawa couplings become crucial in determining the stability of electroweak vacuum. The interplay between two and three generations of such triplet fermions reveals that extensions with two generations is favoured if we aspire for Planck scale stability. Bounds on the Higgs quartic couplings, Type-III Yukawa and number of triplet fermion generations are drawn for different mass scale of Type-III fermions. The phenomenologies of inert doublet and Type-III fermions at the LHC and other experiments are commented upon.
We explore the signatures of the $tilde{R}_2$ class of leptoquark (LQ) models at the proposed $e^- p$ and $e^+p$ colliders. We carry out an analysis for the proposed colliders LHeC and FCC-eh with center of mass (c.m.) energy 1.3 TeV and 3.46 TeV, re
spectively. For $tilde{R}_2$ class of LQ models, there are a number of final states that can arise from LQ production and its subsequent decay. In this report we do a detailed cut-based analysis for the $l^{pm}j$ final state. We also discuss the effect of polarized electron and positron beams on LQ production and in turn on $l^{pm}j$ production. At LHeC, the final state $l^+j$ has very good discovery prospect. We find that, only 100 $text{fb}^{-1}$ of data can probe LQ mass upto 1.2 TeV with $5sigma$ significance, even with a generic set of cuts. On the contrary, at FCC-eh, one can probe LQ masses upto 2.2 TeV (for $e^-$ beam) and 3 TeV (for $e^+$ beam), at more than $5sigma$ significance with luminosity $1000,text{fb}^{-1}$ and $500,text{fb}^{-1}$, respectively.
It is well known that for the pure standard model triplet fermionic WIMP-type dark matter (DM), the relic density is satisfied around 2 TeV. For such a heavy mass particle, the production cross-section at 13 TeV run of LHC will be very small. Extendi
ng the model further with a singlet fermion and a triplet scalar, DM relic density can be satisfied for even much lower masses. The lower mass DM can be copiously produced at LHC and hence the model can be tested at collider. For the present model we have studied the multi jet ($geq 2,j$) + missing energy ($cancel{E}_{T}$) signal and show that this can be detected in the near future of the LHC 13 TeV run. We also predict that the present model is testable by the earth based DM direct detection experiments like Xenon-1T and in future by Darwin.
We consider an extension of the Standard Model (SM) augmented by two neutral singlet fermions per generation and a leptoquark. In order to generate the light neutrino masses and mixing, we incorporate inverse seesaw mechanism. The right handed neutri
no production in this model is significantly larger than the conventional inverse seesaw scenario. We analyze the different collider signatures of this model and find that the final states associated with three or more leptons, multi jet and at least one b-tagged and (or) $tau$-tagged jet can probe larger RH neutrino mass scale. We have also proposed a same-sign dilepton signal region associated with multiple jets and missing energy that can be used to distinguish the the present scenario from the usual inverse seesaw extended SM.
In the light of the recent result from KamLAND-Zen (KLZ) and GERDA Phase-II, we update the bounds on the effective mass and the new physics parameters, relevant for neutrinoless double beta decay ($0 u beta beta$). In addition to the light Majorana n
eutrino exchange, we analyze beyond standard model contributions that arise in Left-Right symmetry and R-Parity violating supersymmetry. The improved limit from KLZ constrains the effective mass of light neutrino exchange down to sub-eV mass regime 0.06 eV. Using the correlation between the $^{136}rm{Xe}$ and $^{76}rm{Ge}$ half-lives, we show that the KLZ limit individually rules out the positive claim of observation of $0 ubetabeta$ for all nuclear matrix element compilation. For the Left-Right symmetry and R-parity violating supersymmetry, the KLZ bound implies a factor of 2 improvement of the effective mass and the new physics parameters. The future ton scale experiments such as, nEXO will further constrain these models, in particular, will rule out standard as well as Type-II dominating LRSM inverted hierarchy scenario.
