Search

We estimate the sensitivity of the DUNE experiment to new physics particles interacting with neutrinos, considering the dipole portal to heavy neutral leptons and a neutrinophilic scalar with lepton-number $2$ as examples. We demonstrate that neutrinos from the high-energy tail of the DUNE flux, with energies $E_{\nu}\gtrsim 5-10\text{ GeV}$, may significantly improve the sensitivity to these models, allowing to search for particles as heavy as $\simeq 10\text{ GeV}$. We also study the impact of the so-called tau-optimized neutrino beam configuration, which slightly improves sensitivity to the new physics models considered here. For both models, we consider new production channels (such as deep-inelastic scattering) and provide a detailed comparison of different signatures in the detector., Comment: 22 pages, 13 figures, consistent with published version

One-loop radiative corrections will lead to a small difference between the matter potentials developed by $\nu_\mu^{}$ and $\nu_{\tau}$ when they travel in a medium. By including such radiative corrections, we derive the exact expressions of the corresponding effective mass-squared differences and the moduli square of the lepton flavor mixing matrix elements $|\widetilde U_{\alpha i}^{}|^2$ (for $\alpha=e,\mu,\tau $ and $i=1,2,3$) in matter in the standard three-flavor mixing scheme and focus on their asymptotic behaviors when the matter density is very big (i.e., the matter effect parameter $A\equiv 2\sqrt{2} G_{\rm F}^{} N_e^{}E$ is very big). Different from the non-trivial fixed value of $|\widetilde U_{\alpha i}^{}|^2$ in the $A\to \infty$ limit in the case without radiative corrections, we get $|\widetilde U_{\alpha i}^{}|^2=0~{\rm or}~1$ under this extreme condition. The radiative corrections can significantly affect the lepton flavor mixing in dense matter, which are numerically and analytically discussed in detail. Furthermore, we also extend the discussion to the $(3+1)$ active-sterile neutrino mixing scheme., Comment: 33 pages, 12 figures, the version accepted for publication in JHEP

If finite but tiny masses of the three active neutrinos are generated via the canonical seesaw mechanism with three heavy sterile neutrinos, the 3\times 3 Pontecorvo-Maki-Nakagawa-Sakata neutrino mixing matrix V will not be exactly unitary. This kind of indirect unitarity violation can be probed in a precision reactor antineutrino oscillation experiment, but it may be entangled with terrestrial matter effects as both of them are very small. We calculate the probability of \overline{\nu}_e \to \overline{\nu}_e oscillations in a good analytical approximation, and find that, besides the zero-distance effect, the effect of unitarity violation is always smaller than matter effects, and their entanglement does not appear until the next-to-leading-order oscillating terms are taken into account. Given a 20-kiloton JUNO-like liquid scintillator detector, we reaffirm that terrestrial matter effects should not be neglected but indirect unitarity violation makes no difference, and demonstrate that the experimental sensitivities to the neutrino mass ordering and a precision measurement of \theta_{12} and \Delta_{21} \equiv m^2_2 - m^2_1 are robust., Comment: 21 pages, 6 figures, version to be published in PLB, more discussions added

Given the Dirac neutrino mass term, we explore the constraint conditions which allow the corresponding mass matrix to be invariant under the \mu-\tau reflection transformation, leading us to the phenomenologically favored predictions \theta_{23} = \pi/4 and \delta = 3\pi/2 in the standard parametrization of the 3\times 3 lepton flavor mixing matrix. If such a flavor symmetry is realized at a superhigh energy scale \Lambda_{\mu\tau}, we investigate how it is spontaneously broken via the one-loop renormalization-group equations (RGEs) running from \Lambda_{\mu\tau} down to the Fermi scale \Lambda_{\rm F}. Such quantum corrections to the neutrino masses and flavor mixing parameters are derived, and an analytical link is established between the Jarlskog invariants of CP violation at \Lambda_{\mu\tau} and \Lambda_{\rm F}. Some numerical examples are also presented in both the minimal supersymmetric standard model and the type-II two-Higgs-doublet model, to illustrate how the octant of \theta_{23}, the quadrant of \delta and the neutrino mass ordering are correlated with one another as a result of the RGE-induced \mu-\tau reflection symmetry breaking effects., Comment: 21 pages, 4 figures, some changes made, 2HDM considered, accepted for publication in JHEP

There are six leptonic unitarity triangles (LUTs) defined by six orthogonality conditions of the three-family lepton flavor mixing matrix in the complex plane. In the framework of the standard model or the minimal supersymmetric standard model, the evolutions of sides and inner angles of the six LUTs from a superhigh energy scale $\Lambda_{\rm H}^{}$ to the electroweak scale $\Lambda_{\rm EW}^{}$ due to the renormalization-group equation (RGE) running are derived in the integral form for both Dirac and Majorana neutrinos. Furthermore, the LUTs as an intuitively geometrical language are applied to the description of the RGE-induced $\mu$-$\tau$ reflection symmetry breaking analytically and numerically., Comment: 28 pages, 2 figures, version accepted for publication in Phys. Rev. D

It has proved convenient to define the effective lepton flavor mixing matrix $\widetilde{U}$ and neutrino mass-squared differences $\widetilde{\Delta}^{}_{ji} \equiv \widetilde{m}^2_j - \widetilde{m}^2_i$ (for $i,j =1,2,3$) to describe the phenomena of neutrino mixing and flavor oscillations in a medium, but the prerequisite is to establish direct and transparent relations between these effective quantities and their fundamental counterparts in vacuum. With the help of two sets of sum rules for $\widetilde{U}$ and $\widetilde{\Delta}^{}_{ji}$, we derive new and exact formulas for moduli of the nine elements of $\widetilde{U}$ and the sides of its three Dirac unitarity triangles in the complex plane. The asymptotic behaviors of $|\widetilde{U}^{}_{\alpha i}|^2$ and $\widetilde{\Delta}^{}_{ji}$ (for $\alpha = e, \mu, \tau$ and $i,j =1,2,3$) in very dense matter (namely, allowing the matter parameter $A = 2\sqrt{2} ~ G^{}_{\rm F} N^{}_e E$ to mathematically approach infinity) are analytically unraveled for the first time, and in this connection the confusion associated with the parameter redundancy of $\widetilde{\theta}^{}_{12}$, $\widetilde{\theta}^{}_{13}$, $\widetilde{\theta}^{}_{23}$ and $\widetilde{\delta}$ in the standard parametrization of $\widetilde{U}$ is clarified., Comment: 21 pages, 2 figures, version accepted by Nuclear Physics B

The preference of the normal neutrino mass ordering from the recent cosmological constraint and the global fit of neutrino oscillation experiments does not seem like a wise choice at first glance since it obscures the neutrinoless double beta decay and hence the Majorana nature of neutrinos. Contrary to this naive expectation, we point out that the actual situation is the opposite. The normal neutrino mass ordering opens the possibility of excluding the higher solar octant and simultaneously measuring the two Majorana CP phases in future $0\nu2\beta$ experiments. Especially, the funnel region will completely disappear if the solar mixing angle takes the higher octant. The combined precision measurement by the JUNO and Daya Bay experiments can significantly reduce the uncertainty in excluding the higher octant. With a typical $\mathcal O(\mbox{meV})$ sensitivity on the effective mass $|m_{ee}^{}|$, the neutrinoless double beta decay experiment can tell if the funnel region really exists and hence exclude the higher solar octant. With the sensitivity further improved to sub-meV, the two Majorana CP phases can be simultaneously determined. Thus, the normal neutrino mass ordering clearly shows phenomenological advantages over the inverted one., Comment: 8 pages, 5 figures, the version to be published in Chinese Physics C

If the neutrino mass spectrum turns out to be m^{}_3 < m^{}_1 < m^{}_2, one may choose to relabel it as m^{\prime}_1 < m^{\prime}_2 < m^{\prime}_3 such that all the masses of fundamental fermions with the same electrical charges are in order. In this case the columns of the 3\times 3 lepton flavor mixing matrix U should be reordered accordingly, and the resulting pattern U^\prime may involve one or two large mixing angles in the standard parametrization or its variations. Since the Majorana neutrino mass matrix keeps unchanged in such a mass relabeling, a possible \mu-\tau reflection symmetry is respected in this connection and its breaking effects are model-independently constrained at the 3\sigma level by using current experimental data., Comment: 17 pages, 5 figures, some changes made, new figures and discussions added, and accepted for publication in Chinese Phys. C

Motivated by tremendous progress in neutrino oscillation experiments, we derive a new set of simple and compact formulas for three-flavor neutrino oscillation probabilities in matter of a constant density. A useful definition of the $\eta$-gauge neutrino mass-squared difference $\Delta^{}_* \equiv \eta \Delta^{}_{31} + (1-\eta) \Delta^{}_{32}$ is introduced, where $\Delta^{}_{ji} \equiv m^2_j - m^2_i$ for $ji = 21, 31, 32$ are the ordinary neutrino mass-squared differences and $0 \leq \eta \leq 1$ is a real and positive parameter. Expanding neutrino oscillation probabilities in terms of $\alpha \equiv \Delta^{}_{21}/\Delta^{}_*$, we demonstrate that the analytical formulas can be remarkably simplified for $\eta = \cos^2 \theta^{}_{12}$, with $\theta_{12}^{}$ being the solar mixing angle. As a by-product, the mapping from neutrino oscillation parameters in vacuum to their counterparts in matter is obtained at the order of ${\cal O}(\alpha^2)$. Finally, we show that our approximate formulas are not only valid for an arbitrary neutrino energy and any baseline length, but also still maintaining a high level of accuracy., Comment: 34 pages, 5 figures; v2: reference added, to appear in JHEP

Given an accelerator-based neutrino experiment with the beam energy E \lesssim 1 GeV, we expand the probabilities of \nu_\mu \to \nu_e and \overline {\nu}_\mu \to \overline {\nu}_e oscillations in matter in terms of two small quantities \Delta_{21}/\Delta_{31} and A/\Delta_{31}, where \Delta_{21} \equiv m^2_2 - m^2_1 and \Delta_{31} \equiv m^2_3 - m^2_1 are the neutrino mass-squared differences, and A measures the strength of terrestrial matter effects. Our analytical approximations are numerically more accurate than those made by Freund in this energy region, and thus they are particularly applicable for the study of leptonic CP violation in the low-energy MOMENT, ESS\nuSM and T2K oscillation experiments. As a by-product, the new analytical approximations help us to easily understand why the matter-corrected Jarlskog parameter \widetilde{\cal J} peaks at the resonance energy E_* \simeq 0.14 GeV (or 0.12 GeV) for the normal (or inverted) neutrino mass hierarchy, and how the three Dirac unitarity triangles are deformed due to the terrestrial matter contamination. We also affirm that a medium-baseline neutrino oscillation experiment with the beam energy E lying in the E_* \lesssim E \lesssim 2 E_* range is capable of exploring leptonic CP violation with little matter-induced suppression., Comment: 36 pages, 11 figures. A crucial mistake removed, highlights changed, and a comparison with Freund's result made

Given the best-fit results of six neutrino oscillation parameters, we plot the Dirac and Majorana unitarity triangles (UTs) of the 3\times 3 lepton flavor mixing matrix to show their real shapes for the first time. The connections of the Majorana UTs with neutrino-antineutrino oscillations and neutrino decays are explored, and the possibilities of right or isosceles UTs are discussed. In the neutrino mass limit of m_1 \to 0 or m_3 \to 0, which is allowed by current experimental data, we show how the six triangles formed by the effective Majorana neutrino masses \langle m\rangle_{\alpha\beta} (for \alpha, \beta = e, \mu, \tau) and their corresponding component vectors look like in the complex plane. The relations of such triangles to the Majorana phases and to the lepton-number-violating decays H^{++} \to \alpha^+ \beta^+ in the type-II seesaw mechanism are also illustrated., Comment: 18 pages, 4 figures, more discussions added. Invited contribution to the Special Issue on Neutrino Oscillations for celebrating the Nobel Prize in Physics 2015 (Nucl. Phys. B in press)