1,800 research outputs found

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

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 $\nu^{}_{\rm
L}$ and right-handed antineutrinos $\overline{\nu}^{}_{\rm R}$ in future
experiments of neutrino capture on beta-decaying nuclei (e.g., $\nu^{}_e +
{^3{\rm H}} \to {^3}{\rm He} + e^-$ 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 $\nu^{}_{\rm R}$
and left-handed antineutrinos $\overline{\nu}^{}_{\rm L}$ can be efficiently
produced in the early Universe. It turns out that the capture rate can be
enhanced at most by $28\%$ due to the presence of relic $\nu^{}_{\rm R}$ and
$\overline{\nu}^{}_{\rm L}$ with a total number density of $95~{\rm cm}^{-3}$,
which should be compared to the number density $336~{\rm cm}^{-3}$ of cosmic
neutrino background. The enhancement has actually been limited by the latest
cosmological and astrophysical bounds on the effective number of neutrino
generations $N^{}_{\rm eff} = 3.14^{+0.44}_{-0.43}$ at the $95\%$ 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

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 $\tan \beta$, 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 $^{76}$Ge-based neutrinoless double-beta decay experiments

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 $^{76}\mathrm{Ge}$. Assuming no signals,
current and future experiments could place a $90\%$ lower limit on the half
life $T^{0\nu}_{1/2} \gtrsim 4\times 10^{26}~{\rm yr}$ and $T^{0\nu}_{1/2}
\gtrsim 7\times 10^{27}~{\rm yr}$, 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 $10^{4}~{\rm kg}\cdot{\rm yr}$
and a background rate of $10^{-4}~{\rm counts}/({\rm keV}\cdot{\rm kg}\cdot{\rm
yr})$, we find that a moderate evidence for normal neutrino mass ordering
(i.e., with a Bayes factor ${\cal B}$ given by $\ln({\cal B}) \simeq 2.5$ or a
probability about $92.3\%$ according to the Jeffreys scale) can be achieved if
the true value of effective neutrino mass $m^{}_{\beta\beta}$ turns out to be
below $0.01~{\rm eV}$.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

The Schechter-Valle theorem states that a positive observation of
neutrinoless double-beta ($0\nu \beta \beta$) 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 $0\nu \beta \beta$-decaying nuclei have placed a restrictive
upper bound on the Majorana neutrino mass $|\delta m^{ee}_\nu| < 7.43 \times
10^{-29}~{\rm eV}$ radiatively generated at the four-loop level. Furthermore,
we generalize this quantitative analysis of $0\nu \beta \beta$ decays to that
of the lepton-number-violating (LNV) meson decays $M^- \to {M^\prime}^+ +
\ell^-_\alpha + \ell^-_\beta$ (for $\alpha$, $\beta$ = $e$ or $\mu$). Given the
present upper limits on these rare LNV decays, we have derived the loop-induced
Majorana neutrino masses $|\delta m^{ee}_\nu| < 9.7 \times 10^{-18}~{\rm eV}$,
$|\delta m^{e\mu}_\nu| < 1.6 \times 10^{-15}~{\rm eV}$ and $|\delta m^{\mu
\mu}_\nu| < 1.0 \times 10^{-12}~{\rm eV}$ from $K^- \to \pi^+ + e^- + e^-$,
$K^- \to \pi^+ + e^- + \mu^-$ and $K^- \to \pi^+ + \mu^- + \mu^-$,
respectively. A partial list of radiative neutrino masses from the LNV decays
of $D$, $D_s^{}$ and $B$ mesons is also given.Comment: 10 pages, 1 figure, clarification added and references updated, Phys.
Lett. B in pres

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