Search for CP violation in the lepton sector

One of the major open issues in neutrino physics is the possible existence of CP violation in the neutrino sector. Such an observation would have an important impact in various domains of physics, from high energy physics to cosmology. Its search requires future accelerator neutrino facilities producing intense and pure neutrino beams such as “beta-beams”. Here we review the different beta-beam scenarios proposed so far and discuss the present status, with a particular emphasis on the original baseline scenario and its feasibility. Alternative strategies for the CP violation search are to be pursued as well. A possibility is to search for CP violation effects in astrophysical environments. Here we present recent analytical and numerical results obtained in the context of core-collapse supernovae. In particular, we point out the conditions under which there can be CP violating effects in dense media and show numerical results on the supernova (anti-)neutrino fluxes and on the electron fraction, relevant for the r-process nucleosynthesis.     

Neutrino Mixing Matrix and Leptogenesis from Quantum Gravity

We consider non renormalization 1/M _x interaction term as a perturbation of the neutrino mass matrix. We find that for the degenerate neutrino mass spectrum. We assume that the neutrino masses and mixing arise through physics at a scale intermediate between Planck Scale and the electroweak scale. We also assume, above the electroweak breaking scale, neutrino masses are nearly degenerate and their mixing is bimaximal. The perturbation generates a non zero value of θ ₁₃, which is within reach of the high performance neutrino factory. In this paper, we find that the non zero value of θ ₁₃ due to Planck scale effects indicates the possibility of CP violation.     

CP Violation in Neutrino Oscillation Due to Planck Scale Effects

We consider non renormalization 1/M _x interaction term as a perturbation of the neutrino mass matrix. We find that for the degenerate neutrino mass spectrum. We assume that the neutrino masses and mixing arise through physics at a scale intermediate between Planck Scale and the electroweak scale. We also assume, above the electroweak breaking scale, neutrino masses are nearly degenerate and their mixing is bimaximal. The perturbation generates a non zero value of θ ₁₃, which is within reach of the high performance neutrino factory. In this paper, we find that the non zero value of θ ₁₃ due to Planck scale effects indicates the possibility of CP violation.     

Neutrino Phenomenology of a High Scale Supersymmetry Model

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 m_(ν1) and m_(ν2) 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.     

Non-unitary neutrino mixing and CP violation in the minimal inverse seesaw model

We propose a simplified version of the inverse seesaw model, in which only two pairs of the gauge-singlet neutrinos are introduced, to interpret the observed neutrino mass hierarchy and lepton flavor mixing at or below the TeV scale. This “minimal” inverse seesaw scenario (MISS) is technically natural and experimentally testable. In particular, we show that the effective parameters describing the non-unitary neutrino mixing matrix are strongly correlated in the MISS, and thus, their upper bounds can be constrained by current experimental data in a more restrictive way. The Jarlskog invariants of non-unitary CP violation are calculated, and the discovery potential of such new CP-violating effects in the near detector of a neutrino factory is discussed.     

A low energy neutrino factory with non-magnetic detectors

We show that a very precise neutrino/anti-neutrino event separation is not mandatory to cover the physics program of a low energy neutrino factory and thus non-magnetized detectors like water Cerenkov or liquid Argon detectors can be used. We point out, that oscillation itself strongly enhances the signal to noise ratio of a wrong sign muon search, provided there is sufficiently accurate neutrino energy reconstruction. Further, we argue that apart from a magnetic field, other means to distinguish neutrino from anti-neutrino events (at least statistically) can be explored. Combined with the fact that non-magnetic detectors potentially can be made very big, we show that modest neutrino/anti-neutrino separations at the level of 50% to 90% are sufficient to obtain good sensitivity to CP violation and the neutrino mass hierarchy for sin²2θ₁₃>¹⁰⁻³${\sin }^{2}2{\theta }_{13}>{10}^{\mathrm{-3}}$. These non-magnetized detectors have a rich physics program outside the context of a neutrino factory, including topics like supernova neutrinos and proton decay. Hence, our observation opens the possibility to use a multi-purpose detector also in a neutrino factory beam.     

电荷-宇称(CP)对称性破坏和夸克-轻子味物理--写在CP破坏发现和夸克理论提出40周年之际

简要地介绍了与电荷-宇称(CP)对称性破坏和夸克-轻子味物理有关的一些重要进展.从1964年发现CP破坏和提出夸克理论至今,这个领域就一直成为粒子物理研究的前沿领域,已研究和发展了整整40年,取得了许多辉煌的成就.在这篇文章中,着重评述了目前仍然热门的几个主要的研究方向:直接CP破坏的理论研究和实验验证,CP破坏机制和新的CP破坏源,中微子物理和新的味物理,夸克味物理和有效量子场理论,标准模型中味物理参数的预言和超对称大统一理论.同时对我国有关研究组在这些前沿方向做出的重要贡献作了重点简述.可以看出,在CP破坏和味物理这个重要前沿领域,仍然存在着许多未解之谜,使得粒子物理在21世纪既面临着巨大的挑战,又有着不断发展的机遇.     

在中微子振荡中的CP破坏

讨论了中微子味混合与中微子振荡的理论,定量地研究了在中微子振荡中的CP破坏效应.在一类超对称模型中,计算了真空中中微子振荡几率和 CP破坏效应.     

Future neutrino long baseline experiments

Very soon a new generation of reactor and accelerator neutrino oscillation experiments—Double Chooz, Daya Bay, Reno and T2K—will seek for oscillation signals generated by the mixing parameter θ₁₃. The knowledge of this angle is a fundamental milestone to optimize further experiments aimed at detecting CP violation in the neutrino sector. Leptonic CP violation is a key phenomenon that has profound implications in particle physics and cosmology but it is clearly out of reach for the aforementioned experiments. Since late 90’s, a worldwide activity is in progress to design facilities that can access CP violation in neutrino oscillation and perform high precision measurements of the lepton counterpart of the Cabibbo-Kobayashi-Maskawa matrix.     

Precision measurements and tau neutrino physics in a future accelerator neutrino experiment

We investigate prospects of building a future accelerator-based neutrino oscillation experiment in China, including site selection, beam optimization and tau neutrino physics aspects. CP violation, non-unitary mixing and non-standard neutrino interactions are discussed. We simulate neutrino beam setups based on muon and beta decay techniques and compare Chinese laboratory sites by their expected sensitivities. A case study on the Super Proton–Proton Collider and the China JinPing Laboratory is also presented. It is shown that the muon-decay-based beam setup can measure the Dirac CP phase by about 14.2° precision at 1σ CL, whereas non-unitarity can be probed down to ∣α_ij∣ ≲ 0.37 (i ≠ j = 1, 2, 3) and non-standard interactions to $| {\epsilon }_{{\ell }{\ell }^{\prime} }^{m}| \lesssim $ 0.11 (${\ell }\ne {\ell }^{\prime} =e$, μ, τ) at 90% CL, respectively.     

No-go for detecting CP violation via neutrinoless double beta decay

We present a necessary condition on the solar oscillation amplitude for CP violation to be detectable through neutrinoless double beta (0νββ) decay. It depends only on the fractional uncertainty in the ν_e–ν_e element of the neutrino mass matrix. We demonstrate that even under very optimistic assumptions about the sensitivity of future experiments to the absolute neutrino mass scale, and on the precision with which nuclear matrix elements that contribute to 0νββ decay are calculable, it will be impossible to detect neutrino CP violation arising from Majorana phases.     

Working group report: Neutrino and astroparticle physics

This is the report of neutrino and astroparticle physics working group at WHEPP-7. Discussions and work on CP violation in long baseline neutrino experiments, ultra high energy neutrinos, supernova neutrinos and water Cerenkov detectors are discussed.     

Neutrino Mass Hierarchy and CP Violation in Long Baseline Neutrino Experiment

We discuss the CP violation in long base line neutrino oscillation experiments. The direct measurement of CP violation is the difference of transitions probability between CP conjugate channels. The sign of Δ ₃₁ is not yet determined, we assume two mass hierarchy conditions, normal (Δ ₃₁>0) and inverted (Δ ₃₁<0). In this paper, we study the CP violation and neutrino mass hierarchy effect in vacuum and matter for long baseline BNL experiments. By an appropriate chose of experimental parameter, neutrino energy and traveled distance. We find that, in matter normal mass hierarchy en-chanced maximum CP violation over their invert mass hierarchy value by 12 %.     

Non-standard neutrino interactions in the Zee–Babu model

We investigate non-standard neutrino interactions (NSIs) in the Zee–Babu model. The size of NSIs predicted by this model is obtained from a full scan over the parameter space, taking into account constraints from low-energy experiments such as searches for lepton flavor violation (LFV) and the requirement to obtain a viable neutrino mass matrix. The dependence on the scale of new physics as well as on the type of the neutrino mass hierarchy is discussed. We find that NSIs at the source of a future neutrino factory may be at an observable level in the ν_e→ν_τ and/or ν_μ→ν_τ channels. In particular, if the doubly charged scalar of the model has a mass in reach of the LHC and if the neutrino mass hierarchy is inverted, a highly predictive scenario is obtained with observable signals at the LHC, in upcoming neutrino oscillation experiments, in LFV processes, and for NSIs at a neutrino factory.     

2540 km: bimagic baseline for neutrino oscillation parameters

We show that a source-to-detector distance of 2540 km, motivated recently [S. K. Raut, R. S. Singh, and S. U. Sankar, arXiv:0908.3741] for a narrow band superbeam, offers multiple advantages for a low energy neutrino factory with a detector that can identify muon charge. At this baseline, for any neutrino hierarchy, the wrong-sign muon signal is almost independent of CP violation and θ(13) in certain energy ranges. This allows the identification of the hierarchy in a clean way. In addition, part of the muon spectrum is also sensitive to the CP violating phase and θ(13), so that the same setup can be used to probe these parameters as well.     

About the Mixing and CP Violation in Neutrino System

Suppose that the geometrical explanation to the weak CP phase in quark sector is also valid for neutrinos, the mixing and CP violation in neutrino system are discussed. We find that a J_Cp larger than 3 × 10^-3 implies the large-mixing solution for solar neutrino problem. In the case of bi-maximal mixing, we predict relative large CP violation with J_Cp larger than 10^-3 in neutrino system, except the third mixing angle approaching to 0 or π/2 very closely.     

长基线中微子振荡几率和CP破坏效应

研究三味中微子在物质中的振荡,从理论上严格解出了绝热近似下在物质中三味中微子的质量平方矩阵本征值和物质中的中微子有效混合矩阵,并计算出三味中微子在长基线实验中的振荡几率和CP破坏效应.     

Are the New Physics Contributions from the Left-Right Symmetric Model Important for the Indirect CP Violation in the Neutral B Mesons?

No Heading Several works analyzing the new physics contributions from the Left-Right Symmetric Model to the CP violation phenomena in the neutral B mesons can be found in the literature. These works exhibit interesting and experimentally sensible deviations from the Standard Model predictions but at the expense of considering a low right scale υ_R around 1 TeV. However, when we stick to the more conservative estimates for υ_R, which say that it must be at least 10⁷ GeV, no experimentally sensible deviations from the Standard Model appear for indirect CP violation. This estimate for υ_R arises when the generation of neutrino masses is considered. In spite of the fact that this scenario is much less interesting and says nothing new about both the CP violation phenomenon and the structure of the Left-Right Symmetric Model, this possibility must be taken into account for the sake of completeness and when considering the see-saw mechanism that provides masses to the neutrino sector. ¹ Associate researcher of the Centro Internacional de Física, Ciudad Universitaria, Bogotá D.C., Colombia.     

θ_(13) and the Higgs Mass from High Scale Supersymmetry

In the framework in which supersymmetry is used for understanding fermion masses rather than stabilizing the electroweak scale, we elaborate on the phenomenological analysis for the neutrino physics. A relatively large sinθ13 0.13 is naturally obtained. The model further predicts vanishingly small CP violation in neutrino oscillations. While the high scale supersymmetry generically results in a Higgs mass of about 141 GeV, our model reduces this mass to 126 GeV via introducing SU(2)L triplet fields which make the electroweak vacuum metastable (with a safe lifetime) and also contribute to neutrino masses.     

Complex CKM from spontaneous CP violation without flavor changing neutral current

We analyse the general constraints on unified gauge models with spontaneous CP breaking that satisfy the conditions that (i) CP violation in the quark sector is described by a realistic complex CKM matrix, and (ii) there is no significant flavor changing neutral current effects in the quark sector. We show that the crucial requirement in order to conform to the above conditions is that spontaneous CP breaking occurs at a very high scale by complex vevs of standard model singlet Higgs fields. Two classes of models are found, one consisting of pure Higgs extensions and the other one involving fermionic extensions of the standard model. We give examples of each class and discuss their possible embeddings into higher unified theories. One of the models has the interesting property that spontaneous CP violation is triggered by spontaneous P violation, thereby linking the scale of CP violation to the seesaw scale for neutrino masses.