The classical conservation of the lepton number is an accidental symmetry present in the Standard Model (SM). Thus, we consider here a scenario where the SM is extended with a U(1) gauge group, promoting the lepton number to a local symmetry. The gauge anomaly cancellations necessitate the extension of the particle spectrum with several beyond the SM (BSM) particle fields. The extended lepton gauge group breaks around the TeV scale via spontaneous symmetry breaking, and a $\textit{Z}$ symmetry remains, which ensures the stability of the light $\textit{Z}$ odd BSM particles. Interestingly, the particle spectrum of the model has two distinct dark sectors with one having a Dirac type DM and the other one containing a Majorana type DM, thus resulting in a multipartite dark matter scenario. We have explored the available parameter space consistent with the observed dark matter relic density and direct detection measurements for both of the DM particles. Having a Majorana dark matter, we have also studied for the gamma line signatures to constraint the parameter space from the indirect dark matter detection experiments like FermiLAT and CTA.

India

Utkarsh Patel

 Avnish

Sudhanwa Patra

Kirtiman Ghosh

Multipartite dark matter in a gauge theory of leptons

While most of the results of the neutrino oscillation experiments can be accommodated within the standard paradigm of three active flavors, there are tantalizing hints of an eV-scale sterile neutrino from anomalous results of a few short baseline experiments. This additional light sterile neutrino is expected to leave an imprint on the physics observables pertaining to standard unknowns such as determination of the Dirac-type leptonic phase, $$\delta _{13},$$ the question of neutrino mass hierarchy and the octant of $$\theta _{23}.$$ The upcoming long baseline neutrino experiments such as T2HK, DUNE and P2O will be sensitive to active – sterile mixing. In the present work, we examine and assess the capability of these long baseline experiments to probe the sterile neutrino at the level of probabilities and event rates. We perform a detailed study by taking into account the values of parameters that are presently allowed and (a) study the impact on violation by examining the role played by various appearance and disappearance channels, (b) address the question of disentangling the intrinsic effects from extrinsic effects in the standard paradigm as well as three active plus one light sterile neutrino, and finally (c) assess the ability of these long baseline experiments to distinguish between the two scenarios. Our results indicate that for the true values of sterile parameters and for all values of $$\delta _{13},$$ the sensitivity of P2O is the lowest while the sensitivity of T2HK is modest $$(<3\,\sigma )$$ and the sensitivity of DUNE is $$> 3\,\sigma .$$ For larger values of the sterile mixing angles, there is an improvement in the sensitivity for all the three considered experiments.

Sabila Parveen

Kiran Sharma

Poonam Mehta

Signals of eV-scale sterile neutrino at long baseline neutrino experiments

We are considering a minimal U(1) extension of the Standard Model (SM) by promoting the baryon number as a local gauge symmetry to accommodate a stable dark matter (DM) candidate. The gauge theory of baryons induces non-trivial triangle gauge anomalies, and we provide a simple anomaly-free solution by adding three exotic fermions. A scalar spontaneously breaks the U(1) symmetry, leaving behind a discrete symmetry that ensures the stability of the lightest exotic fermion originally introduced to cancel the triangle gauge anomalies. Scenarios with weakly interacting DM candidates having non-zero hypercharge usually face stringent constraints from experimental bounds on the DM spin-independent direct-detection (SIDD) cross-section. In this work, we consider a two-component singlet-doublet fermionic dark matter scenario, which significantly relaxes the constraints from bounds on the DM SIDD cross-section for suppressed single-doublet mixing. We show that the model offers a viable parameter space for a cosmologically consistent DM candidate that can be probed through direct detection searches, collider experiments, and gravitational wave (GW) experiments.

 Taramati

Rameswar Sahu

Singlet-doublet fermionic dark matter in gauge theory of baryons

We consider the time reversal (T) transformation in neutrino oscillations in a model-independent way by comparing the observed transition probabilities at two different baselines at the same neutrino energy. We show that, under modest model assumptions, if the transition probability   <math> <msub> <mi>P</mi> <mrow> <msub> <mi>ν</mi> <mi>μ</mi> </msub> <mo>→</mo> <msub> <mi>ν</mi> <mi>e</mi> </msub> </mrow> </msub> </math>  $$ {P}_{\nu_{\mu}\to {\nu}_e} $$  around E  ν  ≃ 0.86 GeV measured at DUNE is smaller than the one at T2HK the T symmetry has to be violated. Experimental requirements needed to achieve good sensitivity to this test for T violation are to obtain enough statistics at DUNE for E  ν  ≲ 1 GeV (around the 2nd oscillation maximum), good energy resolution (better than 10%), and near-detector measurements with a precision of order 1% or better.

Germany

Sabya Chatterjee

Thomas Schwetz

Model-independent search for T violation with T2HK and DUNE

We explore the connection between low-scale CP-violating Dirac phase (δ) and high-scale leptogenesis in a Left-Right Symmetric Model (LRSM) with scalar bidoublet and doublets. The fermion sector of the model is extended with one sterile neutrino (S  L ) per generation to implement a double seesaw mechanism in the neutral fermion mass matrix. The double seesaw is performed via the implementation of type-I seesaw twice. The first seesaw facilitates the generation of Majorana mass term for heavy right-handed (RH) neutrinos (N  R ), and the light neutrino mass becomes linearly dependent on S  L  mass in the second. In our framework, we have taken charge conjugation (C) as the discrete left-right (LR) symmetry. This choice assists in deriving the Dirac neutrino mass matrix (M  D ) in terms of the light and heavy RH neutrino masses and light neutrino mixing matrix U  PMNS  (containing δ). We illustrate the viability of unflavored thermal leptogenesis via the decay of RH neutrinos by using the obtained M  D  with the masses of RH neutrinos as input parameters. A complete analysis of the Boltzmann equations describing the asymmetry evolution is performed in the unflavored regime, and it is shown that with or without Majorana phases, the CP-violating Dirac phase is sufficient to produce the required asymmetry in the leptonic sector within this framework for a given choice of input parameters. Finally, we comment on the possibility of constraining our model with the current and near-future oscillation experiments, which are aimed at refining the value of δ.

Pratik Adarsh

Italy

Purushottam Sahu

Leptogenesis in a Left-Right Symmetric Model with double seesaw

We examine the scope of the MOMENT experiment in the context of CP violation searches with the presence of extra eV scale sterile neutrino. MOMENT is a proposed medium baseline neutrino oscillation experiment using muon beams for neutrinos production, making it advantageous over π 0 background and other technical difficulties. We work over the first oscillation maxima which matches the peak value of flux with a run time of 5 years for both neutrino and anti-neutrino modes. We perform the bi-probability studies for both 3 and 3+1 flavor mixing schemes. The CP violation sensitivities arising from the fundamental CP phase δ 13 and unknown CP phase δ 14 are explored at the firm footing. Slight deteriorations are observed in CP violations induced by δ 13 as the presence of sterile neutrino is considered. We also look at the reconstruction of CP violations phases δ 13 and δ 14 and the MOMENT experiment shows significant capabilities in the precise measurement of δ 13 phase.

Impact of CP violation searches at MOMENT experiment with sterile neutrino

We discuss a left-right (L-R) symmetric model with the double seesaw mechanism at the TeV scale generating Majorana masses for the active left-handed (LH) flavor neutrinos <math><msub><mi>ν</mi><mrow><mi>α</mi><mi>L</mi></mrow></msub></math> and the heavy right-handed (RH) neutrinos <math><mrow><msub><mrow><mi>N</mi></mrow><mrow><mi>β</mi><mi>R</mi></mrow></msub><mo>,</mo><mi>α</mi><mo>,</mo><mi>β</mi><mo>=</mo><mi>e</mi><mo>,</mo><mrow><mi>μ</mi></mrow><mo>,</mo><mrow><mi>τ</mi></mrow></mrow></math>, which in turn mediate lepton number violating processes, including neutrinoless double beta decay. The Higgs sector is composed of two Higgs doublets <math><msub><mi>H</mi><mi>L</mi></msub></math>, <math><msub><mi>H</mi><mi>R</mi></msub></math>, and a bidoublet <math><mi>Φ</mi></math>. The fermion sector has the usual for the L-R symmetric models quarks and leptons, along with three <math><mi>S</mi><mi>U</mi><mo>(</mo><mn>2</mn><mo>)</mo></math> singlet fermion <math><msub><mi>S</mi><mrow><mi>γ</mi><mi>L</mi></mrow></msub></math>. The choice of bare Majorana mass term for these sterile fermions induces large Majorana masses for the heavy RH neutrinos leading to two sets of heavy Majorana particles <math><msub><mi>N</mi><mi>j</mi></msub></math> and <math><mrow><msub><mrow><mi>S</mi></mrow><mrow><mi>k</mi></mrow></msub><mo>,</mo><mrow><mi>j</mi></mrow><mo>,</mo><mrow><mi>k</mi><mo>=</mo><mn>1</mn></mrow><mo>,</mo><mn>2</mn><mo>,</mo><mn>3</mn></mrow></math>, with masses <math><msub><mi>m</mi><msub><mi>N</mi><mi>j</mi></msub></msub><mo>≪</mo><msub><mi>m</mi><msub><mi>S</mi><mi>k</mi></msub></msub></math>. Working with a specific version of the model in which the <math><msub><mi>ν</mi><mrow><mi>α</mi><mi>L</mi></mrow></msub><mo>−</mo><msub><mi>N</mi><mrow><mi>β</mi><mi>R</mi></mrow></msub></math> and the <math><msub><mi>N</mi><mrow><mi>β</mi><mi>R</mi></mrow></msub><mo>−</mo><msub><mi>S</mi><mrow><mi>γ</mi><mi>L</mi></mrow></msub></math> Dirac mass terms are diagonal, and assuming that <math><mrow><msub><mrow><mi>m</mi></mrow><mrow><msub><mrow><mi>N</mi></mrow><mrow><mi>j</mi></mrow></msub></mrow></msub><mo>∼</mo><mo>(</mo><mn>1</mn><mi>–</mi><mn>1000</mn><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math> and <math><mrow><mi>max</mi><mo>(</mo><msub><mrow><mi>m</mi></mrow><mrow><msub><mrow><mi>S</mi></mrow><mrow><mi>k</mi></mrow></msub></mrow></msub><mo>)</mo><mo>∼</mo><mo>(</mo><mn>1</mn><mi>–</mi><mn>10</mn><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>TeV</mi></mrow></math>, <math><msub><mi>m</mi><msub><mi>N</mi><mi>j</mi></msub></msub><mo>≪</mo><msub><mi>m</mi><msub><mi>S</mi><mi>k</mi></msub></msub></math>, we study in detail the new “nonstandard” contributions to the <math><mn>0</mn><mi>ν</mi><mi>β</mi><mi>β</mi></math> decay amplitude and half-life arising due to the exchange of virtual <math><msub><mi>N</mi><mi>j</mi></msub></math> and <math><msub><mi>S</mi><mi>k</mi></msub></math>. We find that in both cases of NO and IO light neutrino mass spectra, these contributions are strongly enhanced and are dominant at relatively small values of the lightest neutrino mass <math><mrow><msub><mrow><mi>m</mi></mrow><mrow><mn>1</mn><mo>(</mo><mn>3</mn><mo>)</mo></mrow></msub><mo>∼</mo><mo>(</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>4</mn></mrow></msup><mi>–</mi><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>2</mn></mrow></msup><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>eV</mi></mrow></math> over the light Majorana neutrino exchange contribution. In large part of the parameter space, the predictions of the model for the <math><mn>0</mn><mi>ν</mi><mi>β</mi><mi>β</mi></math> decay generalized effective Majorana mass and half-life are within the sensitivity range of the planned next generation of neutrinoless double beta decay experiments LEGEND-200 (LEGEND-1000), nEXO, KamlAND-Zen-II, CUPID, and NEXT-HD.

Japan

S. T. Petcov

Prativa Pritimita

Neutrinoless double beta decay in a left-right symmetric model with a double seesaw mechanism

With the recent experimental hint of new physics from flavor physics anomalies, combined with the evidence from neutrino masses and dark matter, we consider a minimal extension of SM with a scalar leptoquark and a fermion triplet. The scalar leptoquark with couplings to leptons and quarks can explain lepton flavor non-universality observables R  K ,   <math> <msub> <mi>R</mi> <msup> <mi>K</mi> <mfenced> <mo>∗</mo> </mfenced> </msup> </msub> </math>  $$ {R}_{K^{\left(\ast \right)}} $$ ,   <math> <msub> <mi>R</mi> <msup> <mi>D</mi> <mfenced> <mo>∗</mo> </mfenced> </msup> </msub> </math>  $$ {R}_{D^{\left(\ast \right)}} $$  and R  J/ψ . Neutral component of fermion triplet provides current abundance of dark matter in the Universe. The interesting feature of the proposal is that the minimal addition of these phenomenologically rich particles (scalar leptoquark and fermion triplet) assist in realizing the unification of the gauge couplings associated with the strong and electroweak forces of standard model when embedded in the non-supersymmetric SO(10) grand unified theory. We discuss on unification mass scale and the corresponding proton decay constraints while taking into account the GUT threshold corrections.

Aishwarya Bhatta

Rukmani Mohanta

Shivaramakrishna Singirala

Flavour anomalies and dark matter assisted unification in SO(10) GUT

We derive the lower bound on absolute scale of lightest neutrino mass for normal hierarchy and inverted hierarchy pattern of light neutrinos by studying the new physics contributions to charged lepton flavour violations in the framework of a TeV scale left–right symmetric model. In the model the fermion sector comprises of the usual quarks and leptons plus a fermion singlet per generation and the scalar sector consists of isospin doublets, triplets and a bidoublet. The framework allows large light-heavy neutrino mixing where the light neutrino mass formula is governed by natural type-II seesaw mechanism unlike the generic type-II seesaw dominance which assumes suppressed light-heavy neutrino mixing. We demonstrate how sizeable loop-induced contribution to light neutrino mass is kept under control such that light neutrino mass formula is dominantly explained by type-II seesaw mechanism. We examine the heavy neutrino contributions with large light-heavy neutrino mixing to charged lepton flavour violating processes like  $$\mu \rightarrow e \gamma $$  <math> <mrow> <mi>μ</mi> <mo>→</mo> <mi>e</mi> <mi>γ</mi> </mrow> </math>  ,  $$\mu \rightarrow 3 e$$  <math> <mrow> <mi>μ</mi> <mo>→</mo> <mn>3</mn> <mi>e</mi> </mrow> </math>   and  $$\mu \rightarrow e$$  <math> <mrow> <mi>μ</mi> <mo>→</mo> <mi>e</mi> </mrow> </math>   conversion inside a nuclei. We present a complementary study between neutrinoless double beta decay and charged lepton flavour violation taking into account single beta decay bound, double beta decay bound and cosmology bounds on neutrino mass sum.

Nitali Dash

Urjit Yajnik

Effect of large light-heavy neutrino mixing and natural type-II seesaw dominance to lepton flavor violation and neutrinoless double beta decay

We discuss an inverse seesaw model based on right-handed fermion specific <math><mi>U</mi><mo>(</mo><mn>1</mn><mo>)</mo></math> gauge symmetry and <math><msub><mrow><mi>A</mi></mrow><mrow><mn>4</mn></mrow></msub></math>-modular symmetry. These symmetries forbid unnecessary terms and restrict structures of Yukawa interactions which are relevant to inverse seesaw mechanism. Then we can obtain some predictions in neutrino sector such as Dirac-CP phase and sum of neutrino mass, which are shown by our numerical analysis. Besides the relation among masses of heavy pseudo-Dirac neutrino can be obtained since it is also restricted by the modular symmetry. We also discuss implications to lepton flavor violation and collider physics in our model.