We perform a comprehensive analysis of neutrinoless double beta (0 ) decay and its interplay with low-energy flavor observables in a radiative neutrino mass model with scalar leptoquarks ( $$ \overline{3} $$ , 1, 1/3) and $$ {\overset{\sim }{R}}_2\left(3,2,1/6\right) $$ . We carve out the parameter region consistent with constraints from neutrino mass and mixing, collider searches, as well as measurements of several flavor observables, such as muon and electron anomalous magnetic moments, charged lepton flavor violation and rare (semi)leptonic kaon and -meson decays, including the recent anomalies in $$ {R}_{D^{\left(\ast \right)}} $$ and $$ B\to K\nu \overline{\nu} $$ observables. We perform a global analysis to all the existing constraints and show the (anti)correlations between all relevant Yukawa couplings satisfying these restrictions. We find that the most stringent constraint on the parameter space comes from conversion in nuclei and $$ {K}^{+}\to {\pi}^{+}\nu \overline{\nu} $$ decay. We also point out a tension between the muon and electron ( – 2) anomalies in this context. Taking benchmark values from the combined allowed regions, we study the implications for 0 decay including both the canonical light neutrino and the leptoquark contributions. We find that for normal ordering of neutrino masses, the leptoquark contribution removes the cancellation region that occurs for the canonical case. The effective mass in presence of leptoquarks can lie in the desert region between the standard normal and inverted ordering cases, and this can be probed in future ton-scale experiments like LEGEND-1000 and nEXO.

United States

P. Dev

India

Srubabati Goswami

United Kingdom

Chayan Majumdar

Debashis Pachhar

Neutrinoless double beta decay from scalar leptoquarks: interplay with neutrino mass and flavor physics

The framework of three-flavor neutrino oscillation is a well-established phenomenon, but results from the short-baseline experiments, such as the Liquid Scintillator Neutrino Detector (LSND) and MiniBooster Neutrino Experiment (MiniBooNE), hint at the potential existence of an additional light neutrino state characterized by a mass-squared difference of approximately <math><mrow><mn>1</mn><mtext> </mtext><mtext> </mtext><msup><mrow><mi>eV</mi></mrow><mrow><mn>2</mn></mrow></msup></mrow></math>. The new neutrino state is devoid of all Standard Model (SM) interactions, commonly referred to as a “sterile” state. In addition, a sterile neutrino with a mass-squared difference of <math><mrow><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>2</mn></mrow></msup><mtext> </mtext><mtext> </mtext><msup><mrow><mi>eV</mi></mrow><mrow><mn>2</mn></mrow></msup></mrow></math> has been proposed to improve the tension between the results obtained from the Tokai to Kamioka (T2K) and the NuMI Off-axis <math><msub><mi>ν</mi><mi>e</mi></msub></math> Appearance (<math><mi>NO</mi><mi>ν</mi><mi>A</mi></math>) experiments. Further, the nonobservation of the predicted upturn in the solar neutrino spectra below 8 MeV can be explained by postulating an extra light sterile neutrino state with a mass-squared difference around <math><mrow><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>5</mn></mrow></msup><mtext> </mtext><mtext> </mtext><msup><mrow><mi>eV</mi></mrow><mrow><mn>2</mn></mrow></msup></mrow></math>. The hypothesis of an additional light sterile neutrino state introduces four distinct mass spectra depending on the sign of the mass-squared difference. In this paper, we discuss the implications of the above scenarios on the observables that depend on the absolute mass of the neutrinos; namely, the sum of the light neutrino masses (<math><mi>Σ</mi></math>) from cosmology, the effective mass of the electron neutrino from beta decay <math><mo>(</mo><msub><mi>m</mi><mi>β</mi></msub><mo>)</mo></math>, and the effective Majorana mass <math><mo>(</mo><msub><mi>m</mi><mrow><mi>β</mi><mi>β</mi></mrow></msub><mo>)</mo></math> from neutrinoless double beta decay. We show that some scenarios can be disfavored by the current constraints of the above variables. The implications for projected sensitivity of Karlsruhe Tritium Neutrino Experiment (KATRIN) and future experiments like Project-8, next Enriched Xenon Observatory (nEXO) are discussed.

Supriya Pan

Constraining the mass-spectra in the presence of a light sterile neutrino from absolute mass-related observables

We consider a general, anomaly free U<math><msup><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow><mrow><mo>′</mo></mrow></msup></math> extension of the Standard Model (SM) where the neutrino mass is generated at the tree level via the inverse seesaw mechanism. The model contains three right handed neutrinos, three additional singlet fermions, one extra complex scalar and a neutral gauge boson <math><mo>(</mo><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup><mo>)</mo></math>. Instead of resorting to a specific <math><mi>U</mi><mo>(</mo><mn>1</mn><mo>)</mo></math> extension, we consider a class of models by taking the <math><mi>U</mi><msup><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow><mrow><mo>′</mo></mrow></msup></math> charges of the scalars to be free parameters. Here, we assign one pair of the pseudo-Dirac degenerate sterile neutrinos as Dark Matter (DM) candidates which are produced by the freeze-in mechanism. Considering different mass regimes of the DM, <math><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></math> and reheating temperature, we obtain constraints on the <math><mi>U</mi><msup><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow><mrow><mo>′</mo></mrow></msup></math> charges giving the correct relic abundance. We have also obtained constraints on <math><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></math> mass and coupling from consideration of relic density as well as high energy collider experiments like ATLAS in case of heavy <math><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></math> or in intensity and lifetime frontier experiments like DUNE, FASERs, and ILC beam dump which are looking for light <math><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></math>. Additionally, in this model, the decay of pseudo-Dirac DM into active neutrinos can explain the 511 keV line observed by the INTEGRAL satellite.

Japan

Arindam Das

Chile

K.N. Vishnudath

Italy

Tanmay Kumar Poddar

Freeze-in production of sterile neutrino dark matter in a gauged U(1)′ model with inverse seesaw

The investigation of conservation/violation of conjugation and parity (CP) symmetry in the leptonic sector is essential to understanding the evolution of the universe. Lorentz invariance and CPT are fundamental symmetries of nature. The violation of Lorentz invariance can also lead to CPT violations. The standard three-flavor neutrino oscillation framework presents a scenario for observing the signature of Lorentz invariance and CP violations. This work focuses on the effect of Lorentz invariance-violating parameters on the sensitivity to CP violation. We investigate the sensitivity in two proposed configurations of the upcoming T2HK experiment: (i) one detector each placed at 295 km and 1100 km, and (ii) two identical detectors at 295 km. This study probes the effect of CPT-violating parameters  $$a_{e\mu },a_{e\tau },a_{\mu \tau }$$  <math> <mrow> <msub> <mi>a</mi> <mrow> <mi>e</mi> <mi>μ</mi> </mrow> </msub> <mo>,</mo> <msub> <mi>a</mi> <mrow> <mi>e</mi> <mi>τ</mi> </mrow> </msub> <mo>,</mo> <msub> <mi>a</mi> <mrow> <mi>μ</mi> <mi>τ</mi> </mrow> </msub> </mrow> </math>  .

Kaustav Chakraborty

Sensitivity to CP discovery in the presence of Lorentz invariance-violating potential at T2HK/T2HKK

Results from the experiments like LSND, and MiniBooNE hint towards the possible presence of an extra eV-scale sterile neutrino. The addition of such a neutrino will significantly impact the standard three-flavor neutrino oscillations; in particular, it can give rise to additional degeneracies due to new sterile parameters. In our work, we investigate how the sensitivity to determine the octant of the neutrino mixing angle <math><msub><mi>θ</mi><mn>23</mn></msub></math> is affected by introducing a sterile neutrino to the standard three generation framework. We compute the oscillation probabilities analytically in the presence of a sterile neutrino, using the approximation that <math><msub><mi>Δ</mi><mn>21</mn></msub></math>, the smallest mass squared difference, is zero. We use these probabilities to understand the degeneracies analytically at different baselines. We present our results of the sensitivity to octant of <math><msub><mi>θ</mi><mn>23</mn></msub></math> for beam neutrinos using a liquid argon time projection chamber (LArTPC). We also obtain octant sensitivity using atmospheric neutrinos using the same LArTPC detector. For the latter, we present our results assuming (i) no charge identification capability, and (ii) partial charge identification capability using the charge tagging ability of muon capture in argon. In addition, we include the charge tagging capability of muon capture in argon which allows one to differentiate between muon neutrino and antineutrino events. The combined sensitivity of beam and atmospheric neutrinos in a similar experimental setup is also delineated. We observe that by combining simulated data from the beam and atmospheric neutrinos (including charge-id for muons), the sensitivity to the octant of <math><msub><mi>θ</mi><mn>23</mn></msub></math> for true values of <math><msub><mi>θ</mi><mn>23</mn></msub><mo>=</mo><mn>4</mn><mn>1</mn><mi>°</mi><mo>(</mo><mn>4</mn><mn>9</mn><mi>°</mi><mo>)</mo></math> exceeds <math><mn>4</mn><mi>σ</mi><mo>(</mo><mn>3</mn><mi>σ</mi><mo>)</mo></math> for more than 50% values of true <math><msub><mi>δ</mi><mn>13</mn></msub></math>.

CERN

Animesh Chatterjee

Matter effect in presence of a sterile neutrino and resolution of the octant degeneracy using a liquid argon detector

Results from experiments like LSND and MiniBooNE hint towards the possible presence of an extra eV scale sterile neutrino. The addition of such a neutrino will significantly impact the standard three flavor neutrino oscillations. In particular, it can give rise to additional degeneracies due to additional sterile parameters. For an eV scale sterile neutrino, the cosmological constraints dictate that the sterile state is heavier than the three active states. However, for lower masses of sterile neutrinos, the sterile state can be lighter than one and/or more of the three states. In such cases, the mass ordering of the sterile neutrinos also becomes unknown, along with the mass ordering of the active states. In this paper, we explore the mass ordering sensitivity in the presence of a sterile neutrino assuming the mass squared difference <math><mo>|</mo><msub><mrow><mi>Δ</mi></mrow><mrow><mn>41</mn></mrow></msub><mo>|</mo></math> to be in the range <math><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>4</mn></mrow></msup><mo>−</mo><mn>1</mn></math> eV$^{2}$. We study (i) how the ordering of the active states, i.e. the determination of the sign of <math><msub><mrow><mi>Δ</mi></mrow><mrow><mn>31</mn></mrow></msub></math> gets affected by the presence of a sterile neutrino in the above mass range, (ii) the possible determination of the sign of <math><msub><mrow><mi>Δ</mi></mrow><mrow><mn>41</mn></mrow></msub></math> for <math><msub><mrow><mi>Δ</mi></mrow><mrow><mn>41</mn></mrow></msub></math> in the range <math><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>4</mn></mrow></msup><mo>−</mo><mn>0.1</mn></math> eV$^{2}$. This analysis is done in the context of a liquid argon detector using beam neutrinos traveling a distance of 1300 km and atmospheric neutrinos that propagate through a distance ranging from 10 - 10000 km, allowing resonant matter effects. Apart from presenting separate results from these sources, we also do a combined study and probe the synergy between these two in giving an enhanced sensitivity.

Probing mass orderings in presence of a very light sterile neutrino in a liquid argon detector

The pair annihilation of neutrinos  $$(\nu {\overline{\nu }}\rightarrow e^+e^-)$$  <math> <mrow> <mo>(</mo> <mi>ν</mi> <mover> <mi>ν</mi> <mo>¯</mo> </mover> <mo>→</mo> <msup> <mi>e</mi> <mo>+</mo> </msup> <msup> <mi>e</mi> <mo>-</mo> </msup> <mo>)</mo> </mrow> </math>   can energize violent stellar explosions such as gamma ray bursts (GRBs). The energy in this neutrino heating mechanism can be further enhanced by modifying the background spacetime over that of Newtonian spacetime. However, one cannot attain the maximum GRB energy  $$(\sim 10^{52}~\text {erg})$$  <math> <mrow> <mo>(</mo> <mo>∼</mo> <msup> <mn>10</mn> <mn>52</mn> </msup> <mspace width="3.33333pt"></mspace> <mtext>erg</mtext> <mo>)</mo> </mrow> </math>   in either the Newtonian background or Schwarzschild and Hartle–Thorne background. On the other hand, using modified gravity theories or the Quintessence field as background geometries, the maximum GRB energy can be reached. In this paper, we consider extending the standard model by an extra  $$U(1)_{{\mathrm{B-L}}}$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mrow> <mi>B</mi> <mo>-</mo> <mi>L</mi> </mrow> </msub> </mrow> </math>   gauge group and augmenting the energy deposition by neutrino pair annihilation process including contributions mediated by the  $$Z^{\prime }$$  <math> <msup> <mi>Z</mi> <mo>′</mo> </msup> </math>   gauge boson belonging to this model. From the observed energy of GRB, we obtain constraints on  $$U(1)_{{\mathrm{B-L}}}$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mrow> <mi>B</mi> <mo>-</mo> <mi>L</mi> </mrow> </msub> </mrow> </math>   gauge coupling in different background spacetimes. We find that the bounds on gauge coupling in modified gravity theories and quintessence background are stronger than those coming from the neutrino-electron scattering experiments in the limit of small gauge boson masses. Future GRB observations with better accuracy can further strengthen these bounds.

Tanmay Poddar

Arvind Mishra

Energizing gamma ray bursts via  $$Z^{\prime }$$  <math> <msup> <mi>Z</mi> <mo>′</mo> </msup> </math>   mediated neutrino heating

In this paper, we consider the possibility of generating the observed baryon asymmetry of the Universe via leptogenesis in the context of a triplet fermion-mediated type-III seesaw model of neutrino mass. With a hierarchical spectrum of the additional fermions, the lower bound on the lightest triplet mass is <math><mo>∼</mo><msup><mn>10</mn><mn>10</mn></msup><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math> for successful leptogenesis, a couple of orders higher than that of the type-I case. We investigate the possibility of lowering this bound in the framework of two-Higgs-doublet models. We find that the bounds can be lowered down to <math><msup><mn>10</mn><mn>7</mn></msup><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math> for a hierarchical spectrum. If we include the flavor effects, then a further lowering by one order of magnitude is possible. We also discuss if such lowering can be compatible with the naturalness bounds on the triplet mass.

Spain

Drona Vatsyayan

Lowering the scale of fermion triplet leptogenesis with two Higgs doublets

We present compact analytic expressions for 3-flavor neutrino oscillation probabilities with invisible neutrino decay, where matter effects have been explicitly included. We take into account the possibility that the oscillation and decay components of the effective Hamiltonian do not commute. This is achieved by employing the techniques of inverse Baker-Campbell-Hausdorff (BCH) expansion and the Cayley-Hamilton theorem applied in the 3-flavor framework. If only the vacuum mass eigenstate ν 3 decays, we show that the treatment of neutrino propagation may be reduced to an effective 2-flavor analysis in the One Mass Scale Dominance (OMSD) approximation. The oscillation probabilities for P  μμ , P  ee , P  eμ  and P  μe  — relevant for reactor, long baseline and atmospheric neutrino experiments — are obtained as perturbative expansions for the case of only ν 3 decay, as well as for the more general scenario where all components of the decay matrix are non-zero. The analytic results thus obtained match the exact numerical results for constant density matter to a high precision and provide physical insights into possible effects of the decay of neutrinos as they propagate through Earth matter. We find that the effects of neutrino decay are most likely to be observable in P  μμ . We also point out that at any long baseline, the oscillation dips in P  μμ  can show higher survival probabilities in the case with decay than without decay, and explain this feature using our analytic approximations.

Dibya Chattopadhyay

China

Amol Dighe

Analytic treatment of 3-flavor neutrino oscillation and decay in matter

We point out that the Hermitian and anti-Hermitian components of the effective Hamiltonian for decaying neutrinos cannot be simultaneously diagonalized by unitary transformations for all matter densities. We develop a formalism for the two-flavor neutrino propagation through matter of uniform density, for neutrino decay to invisible states. Employing a resummation of the Zassenhaus expansion, we obtain compact analytic expressions for neutrino survival and conversion probabilities, to first and second order in the “mismatch parameter” <math><mover><mi>γ</mi><mo>¯</mo></mover></math>.