CP Violation In Neutrino Oscillations Research Articles

Predicting the baryon asymmetry with degenerate right-handed neutrinos

We consider the generation of a baryon asymmetry in an extension of the Standard Model with two singlet Majorana fermions that are degenerate above the electroweak phase transition. The model can explain neutrino masses as well as the observed matter-antimatter asymmetry, for masses of the heavy singlets below the electroweak scale. The only physical CP violating phases in the model are those in the PMNS mixing matrix, i.e. the Dirac phase and a Majorana phase that enter light neutrino observables. We present an accurate analytic approximation for the baryon asymmetry in terms of CP flavour invariants, and derive the correlations with neutrino observables. We demonstrate that the measurement of CP violation in neutrino oscillations as well as the mixings of the heavy neutral leptons with the electron, muon and tau flavours suffice to pin down the matter-antimatter asymmetry from laboratory measurements.

A Pythagoras-like theorem for CP violation in neutrino oscillations

The probabilities of νμ→νe and ν‾μ→ν‾e oscillations in vacuum are determined by the CP-conserving flavor mixing factors Rij≡Re(UμiUejUμj⁎Uei⁎) and the universal Jarlskog invariant of CP violation Jν≡(−1)i+jIm(UμiUejUμj⁎Uei⁎) (for i,j=1,2,3 and i<j), where U is the 3×3 Pontecorvo-Maki-Nakagawa-Sakata neutrino mixing matrix. We show that Jν2=R12R13+R12R23+R13R23 holds as a natural consequence of the unitarity of U. This Pythagoras-like relation may provide a novel cross-check of the result of Jν that will be directly measured in the next-generation long-baseline neutrino oscillation experiments. Indirect non-unitarity effects and terrestrial matter effects on Jν and Rij are also discussed.

CP violation in light neutrino oscillations and heavy neutrino decays: A general and explicit seesaw-bridged correlation

With the help of a block parametrization of the canonical seesaw flavor textures in terms of the Euler-like rotation angles and CP-violating phases, we derive a general and explicit expression for the Jarlskog invariant of CP violation in neutrino oscillations and compare it with the CP-violating asymmetries of heavy Majorana neutrino decays within the minimal seesaw framework which contains two right-handed neutrino fields. Two simplified scenarios are discussed to illustrate how direct or indirect the correlation between these two types of CP violation can be.

Photodetection and electronic system for the Hyper-Kamiokande Water Cherenkov detectors

Hyper-Kamiokande is a next-generation neutrino experiment built in Japan and is scheduled to become operational in 2027. It will be the world’s largest underground water Cherenkov detector instrumented with three kinds of photomultipliers (PMTs): 20- and 3- inch PMTs, supplemented with multi-PMTs. The physics program of the Hyper-Kamiokande experiment will address the most important, unsolved questions in physics, including CP violation in neutrino oscillations and nucleon decay. An overview of the experiment is presented, emphasizing photo-sensors and electronics.

Scintillator cubes for 3D neutrino detector SuperFGD

SuperFGD, a highly granular scintillator detector, is under construction to reduce systematic uncertainties in the T2K experiment in order to improve the sensitivity to CP-violation in neutrino oscillations. SuperFGD will be comprised of about 2×106 small (10×10×10 mm3) optically isolated polystyrene based plastic scintillator cubes with three orthogonal holes 1.5 mm in diameter. The readout of scintillating light from each cube is provided by three wavelength shifting fibers inserted into the three holes and coupled to MPPC micropixel photosensors. The cubes are covered with a white chemical reflector for optical isolation. The technology of making these cubes, their mechanical properties, their main characteristics obtained during tests with cosmic muons and at the CERN beamline, and the results of the temperature tests are presented in this paper.

Investigation of a kind of neutrino mass matrix

We carry out diagonalization of a kind of Majorana neutrino mass matrix of which the real part and imaginary part are commutative. For the matrix M, it is shown in a model-independent way that δ = ±π/2, which implies the maximal strength of CP violation in neutrino oscillations and atmospherical mixing angle would be in the ranges, π/4 < θ 23 < 3π/4, or −3π/4 < θ 23 < −π/4. It is shown that the Hermitian Majorana neutrino mass matrix M has only five real parameters and furthermore, only one free real parameter (A or D) if using the measured values of three mixing angles and mass squared differences as inputs.

Origin of neutrino masses and disappearance ofprimordial antimatter

<p indent="0mm">The standard model (SM) of particle physics has proved to be a huge success in describing the microscopic structures of matter and the behaviors of strong, weak and electromagnetic forces, but it is incomplete in some aspects. For example, the SM itself provides no answer about the origin of tiny neutrino masses, and it fails in accounting for the observed matter-antimatter asymmetry of the Universe. To go beyond the SM, one may wonder whether there exists an intrinsic connection between the solutions to these two fundamental issues in particle physics and cosmology. It is argued that the seesaw mechanism for neutrino mass generation and the associated leptogenesis mechanism for cosmological baryogenesis may pave a way for our deeper understanding of the origin of neutrino masses and mysterious disappearance of primordial antimatter of the Universe. Behind these two mechanisms is the existence of heavy and sterile Majorana neutrinos, which interact with the known (light and active) neutrinos via the Yukawa interactions. The SM contains no right-handed neutrino fields, and hence there is no way to write out a Dirac neutrino mass term. The simplest way to go beyond the SM is to introduce the right-handed neutrino fields and allow for lepton number violation. In this case the right-handed neutrino fields and their charge-conjugate counterparts may constitute a Majorana mass term which, together with the Yukawa interactions between left- and right-handed neutrino fields via the Higgs field, leads us to the origin of tiny Majorana masses for three active neutrinos. Such a seesaw mechanism attributes the smallness of active neutrino masses to the largeness of sterile neutrino masses as compared with the electroweak scale. The lepton-number-violating and CP-violating decays of those heavy degrees of freedom in the early Universe may produce a net lepton-antilepton asymmetry, and the latter may be partly converted into a net baryon-antibaryon asymmetry via the sphaleron process. This is just the leptogenesis mechanism, and it has attracted a lot of attention thanks to the observations of neutrino oscillations in the past decades. The seesaw and leptogenesis scenarios are correlated with each other, and they provide an elegant way of accounting for the cosmological baryon-antibaryon asymmetry and tiny neutrino masses. A pressing question is how to test such a “killing two birds with one stone” picture at low-energy experiments. It seems that a direct test of this picture is extremely difficult or even impossible, but one may always collect various “fossils” of heavy Majorana neutrinos which might have played a crucial role in the early Universe. In this connection it will be greatly helpful to verify the Majorana nature of massive neutrinos by observing the neutrinoless double-beta decays, to measure CP violation in neutrino oscillations and to test unitarity of the 3×3 flavor mixing matrix of three active neutrinos.

The impact of different parameterizations on the interpretation of CP violation in neutrino oscillations

CP violation in the lepton mass matrix will be probed with good precision in upcoming experiments. The amount of CP violation present in oscillations can be quantified in numerous ways and is typically parameterized by the complex phase δPDG in the standard PDG definition of the lepton mixing matrix. There are additional parameterizations of the lepton mixing matrix as well. Through various examples, we explore how, given the current data, different parameterizations can lead to different conclusions when working with parameterization dependent variables, such as δ. We demonstrate how the smallness of |Ue3| governs the scale of these results. We then demonstrate how δ can be misleading and argue that the Jarlskog is the cleanest means of presenting the amount of CP violation in the lepton sector. We also confirm that, among the different parameterizations considered, the standard PDG parameterization has a number of convenient features.

Significance of Broken μ − τ Symmetry in Correlating δ CP , θ 13 , Lightest Neutrino Mass, and Neutrinoless Double Beta Decay 0 ν β β

Leptonic CP violating phase δ CP in the light neutrino sector and leptogenesis via present matter-antimatter asymmetry of the Universe entails each other. Probing CP violation in light neutrino oscillation is one of the challenging tasks today. The reactor mixing angle θ 13 measured in reactor experiments, LBL, and DUNE with high precision in neutrino experiments indicates towards the vast dimensions of scope to detect δ CP . The correlation between leptonic Dirac CPV phase δ CP , reactor mixing angle θ 13 , lightest neutrino mass m 1 , and matter-antimatter asymmetry of the Universe within the framework of μ − τ symmetry breaking assuming the type I seesaw dominance is extensively studied here. Here, a SO(10) GUT model with flavor μ − τ symmetry is considered. In this work, the idea is to link baryogenesis through leptogenesis and the hint of CP violation in the neutrino oscillation data to a breaking of the mu-tau symmetry. Small tiny breaking of the μ − τ symmetry allows a large Dirac CP violating phase in neutrino oscillation which in turn is characterized by awareness of measured value of θ 13 and to provide a hint towards a better understanding of the experimentally observed near-maximal value of ν μ − ν τ mixing angle θ 23 ≃ π / 4 . Precise breaking of the μ − τ symmetry is achieved by adding a 120-plet Higgs to the 10 + 1 2 ¯ 6 -dimensional representation of Higgs. The estimated three-dimensional density parameter space of the lightest neutrino mass m 1 , δ CP , and reactor mixing angle θ 13 is constrained here for the requirement of producing the observed value of baryon asymmetry of the Universe through the mechanism of leptogenesis. Carrying out numerical analysis, the allowed parameter space of m 1 , δ CP , and θ 13 is found out which can produce the observed baryon to photon density ratio of the Universe.

CP violation in neutral lepton transition dipole moment

The CP violation in the neutrino transition electromagnetic dipole moment is discussed in the context of the Standard Model with an arbitrary number of right-handed singlet neutrinos. A full one-loop calculation of the neutrino electromagnetic form factors is performed in the Feynman gauge. A non-zero CP asymmetry is generated by a required threshold condition for the neutrino masses along with non-vanishing CP violating phases in the lepton flavour mixing matrix. We follow the paradiagm of CP violation in neutrino oscillations to parametrise the flavour mixing contribution into a series of Jarlskog-like parameters. This formalism is then applied to a minimal seesaw model with two heavy right-handed neutrinos denoted N1 and N2. We observe that the CP asymmetries for decays into light neutrinos N → νγ are extremely suppressed, maximally around 10−17. However the CP asymmetry for N2→ N1γ can reach of order unity. Even if the Dirac CP phase δ is the only source of CP violation, a large CP asymmetry around 10−5–10−3 is comfortably achieved.

Matter vs vacuum oscillations in atmospheric neutrinos

At present, both vacuum oscillations as well as matter modified oscillations can explain the data of atmospheric neutrino experiments and the long baseline accelerator neutrino experiments equally well. Given the important role the matter effects play in the determination of CP violation in neutrino oscillations, it is imperative to establish the signal for matter effects unambiguously. In this work, we study the ability of ICAL at INO to make a distinction between vacuum oscillations and matter modified oscillations. We find that it is possible to make a 3σ discrimination in ten years, whether the atmospheric mass-squared difference is positive or negative.

CP Violating Phase Originated from Right-handed Neutrino Mixing

We propose an idea that the observed CP violation in neutrino oscillation is originated from the phase in right-handed neutrino mixing by seesaw mechanism. We add small breaking terms $M_{ij}N_{R_i}N_{R_j} $ in model based on $A_4$ symmetry to generate non-diagonal right-handed neutrino mixing, which will give a small correction to TBM mixing via seesaw mechanism with nonzero reactor angle and CP violating phase. We estimate the CP violating phase by investigating the process of leptogenesis due to the decay of right-handed neutrino.

Disentangling genuine from matter-induced CP violation in neutrino oscillations

We prove [1] that, in any flavor transition, neutrino oscillation CP violating asymmetries in matter have two disentangled components: (a) a CPT-odd T-invariant term, non-vanishing iff there are interactions with matter; (b) a T-odd CPT-invariant term, non-vanishing iff there is genuine CP violation. As function of the baseline, these two terms are distinct L-even and L-odd observables, respectively. In the experimental region of terrestrial accelerator neutrinos, we calculate [2] their approximate expressions from which we prove that, at medium baselines, the CPT-odd component is small and nearly (δ-independent, so it can be subtracted from the experimental CP asymmetry as a theoretical background, provided the hierarchy is known. At long baselines, on the other hand, we find that (i) a Hierarchy-odd term in the CPT-odd component dominates the CP asymmetry for energies above the first oscillation node, and (ii) the CPT-odd term vanishes, independent of the CP phase δ, at E = 0.92 GeV(L/1300 km) near the second oscillation maximum, where the T-odd term is almost maximal and proportional to sin δ. A measurement of the CP asymmetry in these energy regions would thus provide separate information on (i) the neutrino mass ordering, and (ii) direct evidence of genuine CP violation in the lepton sector.

Hyper-Kamiokande

A next generation water Cherenkov detector Hyper-Kamiokande to be built in Japan is described. The main goals of this project include a sensitive measurement of CP violation in neutrino oscillations, a search for proton decay and study of solar, atmospherics and astrophysical neutrinos. Key features of the Hyper-Kamiokande detector are described. The main emphasis is put on large photosensors. The recent progress in development of near neutrino detectors is also presented.

On the beta -decay of the accelerated proton and neutrino oscillations: a three-flavor description with CP violation

The (inverse) beta -decay of uniformly accelerated protons (prightarrow n+ e^{+}+nu _e) has been recently analyzed in the context of two-flavor neutrino mixing and oscillations. It has been shown that the decay rates as measured by an inertial and comoving observer are in agreement, provided that: (i) the thermal nature of the accelerated vacuum (Unruh effect) is taken into account; (ii) the asymptotic behavior of neutrinos is described through flavor (rather than mass) eigenstates; (iii) the Unruh radiation is made up of oscillating neutrinos. Here we extend the above considerations to a more realistic scenario including three generations of Dirac neutrinos. By following the outlined recipe, we find that the equality between the two rates still holds true, confirming that mixing is perfectly consistent with the General Covariance of Quantum Field Theory. Notably, we prove that the analysis of CP violation in neutrino oscillations provides a further solid argument for flavor states as fundamental representation of asymptotic neutrino states. Our approach is finally discussed in comparison with the other treatments appeared in literature.

Latest oscillation results from T2K

T2K is a long-baseline neutrino oscillation experiment built to measure νμ disappearance and νe appearance using the J-PARC neutrino beam and the Super-Kamiokande detector. Presented here are the first results in the search for CP violation in neutrino oscillations with a combined analysis of appearance and disappearance channels using data from both neutrino- and antineutrino-mode beams. The data included in this analysis comprises 7.48×1020 protons on target in neutrino mode, giving 37 electron-like and 135 muon-like events at the far detector, and 7.47 × 1020 protons on target in antineutrino mode, giving 4 electron-like and 66 muon-like events. Including a constraint on sin2(2θ13) from reactor measurements, the 90% confidence interval for δCP spans the range [-2.95, -0.44] for the normal mass hierarchy.

Neutrino Phenomenology of a High Scale Supersymmetry Model**Supported by the National Natural Science Foundation of China under Grant Nos. 11375248 and 11875306

CP violation in the lepton sector, and other aspects of neutrino physics, are studied within a high scale supersymmetry model. In addition to the sneutrino vacuum expectation values (VEVs), the heavy vector-like triplet also contributes to neutrino masses. Phases of the VEVs of relevant fields, complex couplings, and Zino mass are considered. The approximate degeneracy of neutrino masses and can be naturally understood. The neutrino masses are then normal ordered, ∼0.020 eV, 0.022 eV, and 0.054 eV. Large CP violation in neutrino oscillations is favored. The effective Majorana mass of the electron neutrino is about 0.02 eV.

New results and perspectives in neutrino physics

A brief review of new results and perspectives in neutrino physics is presented. An emphasis on a search for CP violation in neutrino oscillations and a search for sterile neutrinos is given. Status of measurement of the direct neutrino mass measurement and searches for neutrinoless double beta decay are also discussed.

Resurrecting minimal Yukawa sector of SUSY SO(10)

Supersymmetric SO(10) models with Yukawa coupling matrices involving only a 10H and a {overline{126}}_H of Higgs fields can lead to a predictive and consistent scenario for fermion masses and mixings, including the neutrino sector. However, when coupled minimally to a symmetry breaking sector that includes a 210H and a 126H, these models lead either to an unacceptably small neutrino mass scale, or to non-perturbative values of the gauge couplings. Here we show that with the addition of a 54H to the symmetry breaking sector, the successful predictions of these models for fermion masses and mixings can be maintained. The 54H enables a reduction of the B − L symmetry breaking scale to an intermediate value of order 1012 GeV, consistent with the observed neutrino mass spectrum, while preserving perturbative gauge coupling unification. We obtain an excellent fit to all fermion masses and mixings in this framework. We analyze carefully the prediction of the model for CP violation in neutrino oscillations. Consistency with proton lifetime, however, requires a mini-split SUSY spectrum with the squarks and sleptons having masses of order 100 TeV, accompanied by TeV scale gauginos and Higgsinos. Such a spectrum may arise from pure gravity mediation, which would predict the partial lifetime for the decay pto overline{nu}{K}^{+} to be an order of magnitude above the current experimental limit.

Leptonic CP violation and proton decay in SUSY SO(10)

We study the correlation between proton lifetime and leptonic CP violation in a class of renormalizable supersymmetric SO(10) grand unified theories (GUTs) with 10, 126 and 120 Higgs fields, which provides a unified description of all fermion masses and a possible resolution for the strong CP problem. This specific model is unique in that it can so far be compatible with current proton lifetime limits for a supersymmetry (SUSY) breaking scale as low as 5 TeV due to the presence of a specific Yukawa texture. Our investigation reveals that a discovery of leptonic CP violation in neutrino oscillations would lead to substantial reduction of the parameter space of the model and the potential ruling out of type-II dominance in the neutrino mass seesaw. Furthermore, the previously predicted proton partial lifetimes are sufficiently long for only certain values of the leptonic CP phase.