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Register a new userA new strategy for probing the Majorana neutrino CP violating phases and masses
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We propose a new strategy for detecting the CP-violating phases and the effective mass of muon Majorana neutrinos by measuring observables associated with neutrino-antineutrino oscillations in $pi^{pm}$ decays. Within the generic framework of quantum field theory, we compute the non-factorizable probability for producing a pair of same-charged muons in $pi^{pm}$ decays as a distinctive signature of $ u_{mu}-bar{ u_{mu}}$ oscillations. We show that an intense neutrino beam through a long baseline experiment is favored for probing the Majorana phases. Using the neutrino-antineutrino oscillation probability reported by MINOS collaboration, a new stringent bound on the effective muon-neutrino mass is derived.
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We consider the MSSM with see-saw mechanism of neutrino mass generation and soft SUSY breaking with flavour-universal boundary conditions at the GUT scale, in which the lepton flavour violating (LFV) decays muto e + gamma, tauto mu + gamma, etc.,are predicted with rates that can be within the reach of present and planned experiments. These predictions depend critically on the matrix of neutrino Yukawa couplings bf{Y_{ u}} which can be expressed in terms of the light and heavy right-handed (RH) neutrino masses, neutrino mixing matrix U_{PMNS}, and an orthogonal matrix bf{R}. We investigate the effects of Majorana CP-violation phases in U_{PMNS}, and of the RG running of light neutrino masses and mixing angles from M_Z to the RH Majorana neutrino mass scale M_R, on the predictions for the rates of LFV decays muto e + gamma, tau to mu + gamma and tauto e + gamma. Results for neutrino mass spectrum with normal hierarchy, values of the lightest u-mass in the range 0 leq m_1 leq 0.30 eV, and quasi-degenerate heavy RH Majorana neutrinos in the cases of bf{R} = bf{1} and complex matrix bf{R} are presented. We find that the effects of the Majorana CP-violation phases and of the RG evolution of neutrino mixing parameters can change by few orders of magnitude the predicted rates of the LFV decays mu to e + gamma and tau to e + gamma. The impact of these effects on the tau to mu + gamma decay rate is typically smaller and only possible for m_1 > 0.10 eV. If the RG running effects are negligible, in a large region of soft SUSY breaking parameter space the ratio of the branching ratios of the mu to e + gamma and tau to e + gamma (tau to mu + gamma) decays is entirely determined in the case of bf{R} cong bf{1} by the values of the neutrino mixing parameters at M_Z.
In the near future, the neutrinoless double-beta ($0 ubetabeta$) decay experiments will hopefully reach the sensitivity of a few ${rm meV}$ to the effective neutrino mass $|m^{}_{betabeta}|$. In this paper, we tentatively examine the sensitivity of future $0 ubetabeta$-decay experiments to neutrino masses and Majorana CP phases by following the Bayesian statistical approach. Provided experimental setups corresponding to the sensitivity of $|m^{}_{betabeta}| simeq 1~{rm meV}$, the null observation of $0 ubetabeta$ decays in the case of normal neutrino mass ordering leads to a very competitive bound on the lightest neutrino mass $m^{}_1$. Namely, the $95%$ credible interval turns out to be $1.6~{rm meV} lesssim m^{}_1 lesssim 7.3~{rm meV}$ or $0.3~{rm meV} lesssim m^{}_1 lesssim 5.6~{rm meV}$ when the uniform prior on $m^{}_1/{rm eV}$ or on $log^{}_{10}(m^{}_1/{rm eV})$ is adopted. Moreover, one of two Majorana CP phases is strictly constrained, i.e., $140^circ lesssim rho lesssim 220^circ$ for both priors of $m^{}_1$. In contrast, if a relatively worse sensitivity of $|m^{}_{betabeta}| simeq 10~{rm meV}$ is assumed, the constraint becomes accordingly $0.6~{rm meV} lesssim m^{}_1 lesssim 26~{rm meV}$ or $0 lesssim m^{}_1 lesssim 6.1~{rm meV}$, while two Majorana CP phases will be essentially unconstrained. In the same statistical framework, the prospects for the determination of neutrino mass ordering and the discrimination between Majorana and Dirac nature of massive neutrinos in the $0 ubetabeta$-decay experiments are also discussed. Given the experimental sensitivity of $|m^{}_{betabeta}| simeq 10~{rm meV}$ (or $1~{rm meV}$), the strength of evidence to exclude the Majorana nature under the null observation of $0 ubetabeta$ decays is found to be inconclusive (or strong), no matter which of two priors on $m^{}_1$ is taken.
We analyse the dependence of the rates of the LFV charged lepton decays mu to e + gamma, tau to e + gamma, tau to mu + gamma (l_i to l_j + gamma) and their ratios, predicted in the class of SUSY theories with see-saw mechanism of nu-mass generation and soft SUSY breaking with universal boundary conditions at the GUT scale, on the Majorana CP-violation phases in the PMNS neutrino mixing matrix and the ``leptogenesis CP-violating (CPV) parameters. The case of quasi-degenerate in mass heavy Majorana neutrinos is considered. The analysis is performed for normal hierarchical (NH), inverted hierarchical (IH) and quasi-degenerate (QD) light neutrino mass spectra. We show, in particular, that for NH and IH nu-mass spectrum and negligible lightest neutrino mass, all three l_i to l_j + gamma decay branching ratios, BR(l_i to l_j + gamma), depend on one Majorana phase, one leptogenesis CPV parameter and on the 3-neutrino oscillation parameters; if the CHOOZ mixing angle theta_13 is sufficiently large, they depend on the Dirac CPV phase in the PMNS matrix. The ``double ratios R(21/31) sim BR(mu to e + gamma)/BR(tau to e + gamma) and R(21/32) sim BR(mu to e + gamma)/BR(tau to mu + gamma) are determined by these parameters. The same Majorana phase enters into the NH and IH expressions for the effective Majorana mass in neutrinoless double beta decay, <m>.
We investigate effects of non-zero Dirac and Majorana CP violating phases on neutrino-antineutrino oscillations in a magnetic field of astrophysical environments. It is shown that in the presence of strong magnetic fields and dense matter, non-zero CP phases can induce new resonances in the oscillations channels $ u_e leftrightarrow bar{ u}_e$, $ u_e leftrightarrow bar{ u}_mu$ and $ u_e leftrightarrow bar{ u}_{tau}$. We also consider all other possible oscillation channels with $ u_mu$ and $ u_tau$ in the initial state. The resonances can potentially lead to significant phenomena in neutrino oscillations accessible for observation in experiments. In particular, we show that neutrino-antineutrino oscillations combined with Majorana-type CP violation can affect the $bar{ u}_e$/$ u_e$ ratio for neutrinos coming from the supernovae explosion. This effect is more prominent for the normal neutrino mass ordering. The detection of supernovae neutrino fluxes in the future experiments, such as JUNO, DUNE and Hyper-Kamiokande, can give an insight into the nature of CP violation and, consequently, provides a tool for distinguishing the Dirac or Majorana nature of neutrinos.
The Schechter-Valle theorem states that a positive observation of neutrinoless double-beta ($0 u beta beta$) 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 $0 u beta beta$-decaying nuclei have placed a restrictive upper bound on the Majorana neutrino mass $|delta m^{ee}_ u| < 7.43 times 10^{-29}~{rm eV}$ radiatively generated at the four-loop level. Furthermore, we generalize this quantitative analysis of $0 u beta beta$ 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 $|delta m^{ee}_ u| < 9.7 times 10^{-18}~{rm eV}$, $|delta m^{emu}_ u| < 1.6 times 10^{-15}~{rm eV}$ and $|delta m^{mu mu}_ u| < 1.0 times 10^{-12}~{rm eV}$ 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.
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