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1. Impact of recent updates to neutrino oscillation parameters on the effective Majorana neutrino mass in $0\nu \beta \beta$ decay

   https://doi.org/10.1103/PhysRevD.110.015026  

   Mei, Dongming; Dong, Kunming; Warren, Austin; Bhattarai, Sanjay (July 2024, Physical Review D)

   We investigate how recent updates to neutrino oscillation parameters and the sum of neutrino masses influence the sensitivity of neutrinoless double-beta ( $0\nu \beta \beta$) decay experiments. Incorporating the latest cosmological constraints on the sum of neutrino masses and laboratory measurements on oscillations, we determine the sum of neutrino masses for both the normal hierarchy (NH) and the inverted hierarchy (IH). Our analysis reveals a narrow range for the sum of neutrino masses, approximately $0.06\mathrm{eV}/{\mathrm{c}}^2$for NH and $0.102\mathrm{eV}/{\mathrm{c}}^2$for IH. Utilizing these constraints, we calculate the effective Majorana masses for both NH and IH scenarios, establishing the corresponding allowed regions. Importantly, we find that the minimum neutrino mass is nonzero, as constrained by the current oscillation parameters. Additionally, we estimate the half-life of $0\nu \beta \beta$decay using these effective Majorana masses for both NH and IH. Our results suggest that upcoming ton-scale experiments will comprehensively explore the IH scenario, while 100-ton-scale experiments will effectively probe the parameter space for the NH scenario, provided the background index can achieve 1 event/kton-year in the region of interest. Published by the American Physical Society2024 

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2. Spin-flavor precession of Dirac neutrinos in dense matter and its potential in core-collapse supernovae

   https://doi.org/10.1103/PhysRevD.108.103046  

   Sasaki, Hirokazu; Takiwaki, Tomoya; Balantekin, A. Baha (November 2023, Physical Review D)

   We calculate the spin-flavor precession (SFP) of Dirac neutrinos induced by strong magnetic fields and finite neutrino magnetic moments in dense matter. As found in the case of Majorana neutrinos, the SFP of Dirac neutrinos is enhanced by the large magnetic field potential and suppressed by large matter potentials composed of the baryon density and the electron fraction. The SFP is possible irrespective of the large baryon density when the electron fraction is close to 1/3. The diagonal neutrino magnetic moments that are prohibited for Majorana neutrinos enable the spin precession of Dirac neutrinos without any flavor mixing. With supernova hydrodynamics simulation data, we discuss the possibility of the SFP of both Dirac and Majorana neutrinos in core-collapse supernovae. The SFP of Dirac neutrinos occurs at a radius where the electron fraction is 1/3. The required magnetic field of the proto-neutron star for the SFP is a few 10^{14} G at any explosion time. For the Majorana neutrinos, the required magnetic field fluctuates from 10^{13} G to 10^{15} G. Such a fluctuation of the magnetic field is more sensitive to the numerical scheme of the neutrino transport in the supernova simulation. 

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3. Solar neutrinos and ${\nu }_2$ visible decays to ${\nu }_1$

   https://doi.org/10.1103/PhysRevD.109.013003  

   de_Gouvêa, André; Weill, Jean; Sen, Manibrata (January 2024, Physical Review D)

   Experimental bounds on the neutrino lifetime depend on the nature of the neutrinos and the details of the potentially new physics responsible for neutrino decay. In the case where the decays involve active neutrinos in the final state, the neutrino masses also qualitatively impact how these manifest themselves experimentally. In order to further understand the impact of nonzero neutrino masses, we explore how observations of solar neutrinos constrain a very simple toy model. We assume that neutrinos are Dirac fermions and there is a new massless scalar that couples to neutrinos such that a heavy neutrino— ${\nu }_2$with mass $m_2$—can decay into a lighter neutrino— ${\nu }_1$with mass $m_1$—and a massless scalar. We find that the constraints on the new physics coupling depend, sometimes significantly, on the ratio of the daughter-to-parent neutrino masses and that, for large-enough values of the new physics coupling, the “dark side” of the solar neutrino parameter space— ${\sin }^2{\theta }_{12}\sim 0.7$—provides a reasonable fit to solar neutrino data, if only ${}^8\mathrm{B}$or ${}^7\mathrm{Be}$neutrino data alone are considered, but no allowed region is found in the combined analysis. Our results generalize to other neutrino-decay scenarios, including those that mediate ${\nu }_2\to {\nu }_1{\overline{\nu }}_3{\nu }_3$when the neutrino mass ordering is inverted mass and $m_2>m_1\gg m_3$, the mass of ${\nu }_3$. Published by the American Physical Society2024 

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4. Search for heavy neutral leptons in final states with electrons, muons, and hadronically decaying tau leptons in proton-proton collisions at $$ \sqrt{s} $$ = 13 TeV

   https://doi.org/10.1007/JHEP06(2024)123  

   Hayrapetyan, A A; Tumasyan, A; Adam, W; Andrejkovic, J W; Bergauer, T; Chatterjee, S; Damanakis, K; Dragicevic, M; Hussain, P S; Jeitler, M; et al (June 2024, Journal of High Energy Physics)

   A<sc>bstract</sc> A search for heavy neutral leptons (HNLs) of Majorana or Dirac type using proton-proton collision data at$$ \sqrt{s} $$ $\sqrt{s}$= 13 TeV is presented. The data were collected by the CMS experiment at the CERN LHC and correspond to an integrated luminosity of 138 fb⁻¹. Events with three charged leptons (electrons, muons, and hadronically decaying tau leptons) are selected, corresponding to HNL production in association with a charged lepton and decay of the HNL to two charged leptons and a standard model (SM) neutrino. The search is performed for HNL masses between 10 GeV and 1.5 TeV. No evidence for an HNL signal is observed in data. Upper limits at 95% confidence level are found for the squared coupling strength of the HNL to SM neutrinos, considering exclusive coupling of the HNL to a single SM neutrino generation, for both Majorana and Dirac HNLs. The limits exceed previously achieved experimental constraints for a wide range of HNL masses, and the limits on tau neutrino coupling scenarios with HNL masses above the W boson mass are presented for the first time. 

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5. Improved cosmological constraints on the neutrino mass and lifetime

   https://doi.org/10.1007/JHEP08(2022)076  

   Abellán, Guillermo Franco; Chacko, Zackaria; Dev, Abhish; Du, Peizhi; Poulin, Vivian; Tsai, Yuhsin (August 2022, Journal of High Energy Physics)

   A bstract We present cosmological constraints on the sum of neutrino masses as a function of the neutrino lifetime, in a framework in which neutrinos decay into dark radiation after becoming non-relativistic. We find that in this regime the cosmic microwave background (CMB), baryonic acoustic oscillations (BAO) and (uncalibrated) luminosity distance to supernovae from the Pantheon catalog constrain the sum of neutrino masses ∑ m ν to obey ∑ m ν < 0 . 42 eV at (95% C.L.). While the bound has improved significantly as compared to the limits on the same scenario from Planck 2015, it still represents a significant relaxation of the constraints as compared to the stable neutrino case. We show that most of the improvement can be traced to the more precise measurements of low- ℓ polarization data in Planck 2018, which leads to tighter constraints on τ reio (and thereby on A s ), breaking the degeneracy arising from the effect of (large) neutrino masses on the amplitude of the CMB power spectrum. 

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