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Register a new userRevealing neutrino nature and $CPT$ violation with decoherence effects
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We study decoherence effects on mixing among three generations of neutrinos. We show that in presence of a non--diagonal dissipation matrix, both Dirac and Majorana neutrinos can violate the $CPT$ symmetry and the oscillation formulae depend on the parametrization of the mixing matrix. We reveal the $CP$ violation in the transitions preserving the flavor, for a certain form of the dissipator. In particular, the $CP$ violation affects all the transitions in the case of Majorana neutrinos, unlike Dirac neutrinos which still preserve the $CP$ symmetry in one of the transitions flavor preserving. This theoretical result shows that decoherence effects, if exist for neutrinos, could allow to determine the neutrino nature and to test fundamental symmetries of physics. Next long baseline experiments could allow such an analysis. We relate our study with experiments by using the characteristic parameters and the constraints on the elements of the dissipation matrix of current experiments.
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We analyze many aspects of the phenomenon of the decoherence for neutrinos propagating in long baseline experiments. We show that, in the presence of an off-diagonal term in the dissipative matrix, the Majorana neutrino can violate the CP T symmetry, which, on the contrary, is preserved for Dirac neutrinos. We show that oscillation formulas for Majorana neutrinos depend on the choice of the mixing matrix U. Indeed, different choices of U lead to different oscillation formulas. Moreover, we study the possibility to reveal the differences between Dirac and Majorana neutrinos in the oscillations. We use the present values of the experimental parameters in order to relate our theoretical proposal with experiments.
In order to accommodate the neutrino oscillation signals from the solar, atmospheric, and LSND data, a sterile fourth neutrino is generally invoked, though the fits to the data are becoming more and more constrained. However, it has recently been shown that the data can be explained with only three neutrinos, if one invokes CPT violation to allow different masses and mixing angles for neutrinos and antineutrinos. We explore the nature of neutrinos in such CPT-violating scenarios. Majorana neutrino masses are allowed, but in general, there are no longer Majorana neutrinos in the conventional sense. However, CPT-violating models still have interesting consequences for neutrinoless double beta decay. Compared to the usual case, while the larger mass scale (from LSND) may appear, a greater degree of suppression can also occur.
If dark energy (DE) couples to neutrinos, then there may be apparent violations of Lorentz/CPT invariance in neutrino oscillations. The DE-induced Lorentz/CPT violation takes a specific form that introduces neutrino oscillations that are energy independent, differ for particles and antiparticles, and can lead to novel effects for neutrinos propagating through matter. We show that ultra-high-energy neutrinos may provide one avenue to seek this type of Lorentz/CPT violation in u_mu- u_tau oscillations, improving the current sensitivity to such effects by seven orders of magnitude. Lorentz/CPT violation in electron-neutrino oscillations may be probed with the zenith-angle dependence for high-energy atmospheric neutrinos. The ``smoking gun, for DE-neutrino coupling would, however, be a dependence of neutrino oscillations on the direction of the neutrino momentum relative to our peculiar velocity with respect to the CMB rest frame. While the amplitude of this directional dependence is expected to be small, it may nevertheless be worth seeking in current data and may be a target for future neutrino experiments.
The sidereal time dependence of MiniBooNE electron neutrino and anti-electron neutrino appearance data are analyzed to search for evidence of Lorentz and CPT violation. An unbinned Kolmogorov-Smirnov test shows both the electron neutrino and anti-electron neutrino appearance data are compatible with the null sidereal variation hypothesis to more than 5%. Using an unbinned likelihood fit with a Lorentz-violating oscillation model derived from the Standard Model Extension (SME) to describe any excess events over background, we find that the electron neutrino appearance data prefer a sidereal time-independent solution, and the anti-electron neutrino appearance data slightly prefer a sidereal time-dependent solution. Limits of order 10E-20 GeV are placed on combinations of SME coefficients. These limits give the best limits on certain SME coefficients for muon neutrino to electron neutrino and anti-muon neutrino to anti-electron neutrino oscillations. The fit values and limits of combinations of SME coefficients are provided.
The neutrino parameters determined from the solar neutrino data and the anti-neutrino parameters determined from KamLAND reactor experiment are in good agreement with each other. However, the best fit points of the two sets differ from each other by about $10^{-5}$ eV$^2$ in mass-square differenc and by about $2^circ$ in the mixing angle. Future solar neutrino and reactor anti-neutrino experiments are likely to reduce the uncertainties in these measurements. This, in turn, can lead to a signal for CPT violation in terms a non-zero difference between neutrino and anti-neutrino parameters. In this paper, we propose a CPT violating mass matrix which can give rise to the above differences in both mass-squared difference and mixing angle and study the constraints imposed by the data on the parameters of the mass matrix.
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