1,800 research outputs found

    Relic Right-handed Dirac Neutrinos and Implications for Detection of Cosmic Neutrino Background

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    It remains to be determined experimentally if massive neutrinos are Majorana or Dirac particles. In this connection, it has been recently suggested that the detection of cosmic neutrino background of left-handed neutrinos νL\nu^{}_{\rm L} and right-handed antineutrinos νR\overline{\nu}^{}_{\rm R} in future experiments of neutrino capture on beta-decaying nuclei (e.g., νe+3H3He+e\nu^{}_e + {^3{\rm H}} \to {^3}{\rm He} + e^- for the PTOLEMY experiment) is likely to distinguish between Majorana and Dirac neutrinos, since the capture rate is twice larger in the former case. In this paper, we investigate the possible impact of right-handed neutrinos on the capture rate, assuming that massive neutrinos are Dirac particles and both right-handed neutrinos νR\nu^{}_{\rm R} and left-handed antineutrinos νL\overline{\nu}^{}_{\rm L} can be efficiently produced in the early Universe. It turns out that the capture rate can be enhanced at most by 28%28\% due to the presence of relic νR\nu^{}_{\rm R} and νL\overline{\nu}^{}_{\rm L} with a total number density of 95 cm395~{\rm cm}^{-3}, which should be compared to the number density 336 cm3336~{\rm cm}^{-3} of cosmic neutrino background. The enhancement has actually been limited by the latest cosmological and astrophysical bounds on the effective number of neutrino generations Neff=3.140.43+0.44N^{}_{\rm eff} = 3.14^{+0.44}_{-0.43} at the 95%95\% confidence level. For illustration, two possible scenarios have been proposed for thermal production of right-handed neutrinos in the early Universe.Comment: 16 pages, 4 figure, more discussions added, references updated, to appear in Nucl. Phys.

    A Further Study of the Frampton-Glashow-Yanagida Model for Neutrino Masses, Flavor Mixing and Baryon Number Asymmetry

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    In light of the latest neutrino oscillation data, we revisit the minimal scenario of type-I seesaw model, in which only two heavy right-handed Majorana neutrinos are introduced to account for both tiny neutrino masses and the baryon number asymmetry in our Universe. In this framework, we carry out a systematic study of the Frampton-Glashow-Yanagida ansatz by taking into account the renormalization-group running of neutrino mixing parameters and the flavor effects in leptogenesis. We demonstrate that the normal neutrino mass ordering is disfavored even in the minimal supersymmetric standard model with a large value of tanβ\tan \beta, for which the running effects could be significant. Furthermore, it is pointed out that the original scenario with a hierarchical mass spectrum of heavy Majorana neutrinos contradicts with the upper bound derived from a naturalness criterion, and the resonant mechanism with nearly-degenerate heavy Majorana neutrinos can be a possible way out.Comment: 24 pages, 4 figures, 2 tables, more discussions added, to appear in JHE

    Determination of neutrino mass ordering in future 76^{76}Ge-based neutrinoless double-beta decay experiments

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    Motivated by recent intensive experimental efforts on searching for neutrinoless double-beta decays, we perform a detailed analysis of the physics potential of the experiments based on 76Ge^{76}\mathrm{Ge}. Assuming no signals, current and future experiments could place a 90%90\% lower limit on the half life T1/20ν4×1026 yrT^{0\nu}_{1/2} \gtrsim 4\times 10^{26}~{\rm yr} and T1/20ν7×1027 yrT^{0\nu}_{1/2} \gtrsim 7\times 10^{27}~{\rm yr}, respectively. Then, how to report an evidence for neutrinoless double-beta decays is addressed by following the Bayesian statistical approach. For the first time, we present a quantitative description of experimental power to distinguish between normal and inverted neutrino mass orderings. Taking an exposure of 104 kgyr10^{4}~{\rm kg}\cdot{\rm yr} and a background rate of 104 counts/(keVkgyr)10^{-4}~{\rm counts}/({\rm keV}\cdot{\rm kg}\cdot{\rm yr}), we find that a moderate evidence for normal neutrino mass ordering (i.e., with a Bayes factor B{\cal B} given by ln(B)2.5\ln({\cal B}) \simeq 2.5 or a probability about 92.3%92.3\% according to the Jeffreys scale) can be achieved if the true value of effective neutrino mass mββm^{}_{\beta\beta} turns out to be below 0.01 eV0.01~{\rm eV}.Comment: 16 pages, 7 figures, the Jeffreys scale used, more discussions added, to appear in Phys. Rev.

    Majorana Neutrino Masses from Neutrinoless Double-Beta Decays and Lepton-Number-Violating Meson Decays

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    The Schechter-Valle theorem states that a positive observation of neutrinoless double-beta (0νββ0\nu \beta \beta) decays implies a finite Majorana mass term for neutrinos when any unlikely fine-tuning or cancellation is absent. In this note, we reexamine the quantitative impact of the Schechter-Valle theorem, and find that current experimental lower limits on the half-lives of 0νββ0\nu \beta \beta-decaying nuclei have placed a restrictive upper bound on the Majorana neutrino mass δmνee<7.43×1029 eV|\delta m^{ee}_\nu| < 7.43 \times 10^{-29}~{\rm eV} radiatively generated at the four-loop level. Furthermore, we generalize this quantitative analysis of 0νββ0\nu \beta \beta decays to that of the lepton-number-violating (LNV) meson decays MM++α+βM^- \to {M^\prime}^+ + \ell^-_\alpha + \ell^-_\beta (for α\alpha, β\beta = ee or μ\mu). Given the present upper limits on these rare LNV decays, we have derived the loop-induced Majorana neutrino masses δmνee<9.7×1018 eV|\delta m^{ee}_\nu| < 9.7 \times 10^{-18}~{\rm eV}, δmνeμ<1.6×1015 eV|\delta m^{e\mu}_\nu| < 1.6 \times 10^{-15}~{\rm eV} and δmνμμ<1.0×1012 eV|\delta m^{\mu \mu}_\nu| < 1.0 \times 10^{-12}~{\rm eV} from Kπ++e+eK^- \to \pi^+ + e^- + e^-, Kπ++e+μK^- \to \pi^+ + e^- + \mu^- and Kπ++μ+μK^- \to \pi^+ + \mu^- + \mu^-, respectively. A partial list of radiative neutrino masses from the LNV decays of DD, DsD_s^{} and BB mesons is also given.Comment: 10 pages, 1 figure, clarification added and references updated, Phys. Lett. B in pres