Leptonic CP Violation Research Articles

Physics potential of the ESSnu SB

The ESSnu SB project proposes to base a neutrino “Super Beam” of unprecedented luminosity at the European Spallation Source. The original proposal identified the second peak of the oscillation probability as the optimal to maximize the discovery potential to leptonic CP violation. However this choice reduces the statistics at the detector and penalizes other complementary searches such as the determination of the atmospheric oscillation parameters, particularly the octant of theta _{23} as well as the neutrino mass ordering. We explore how these shortcomings can be alleviated by the combination of the beam data with the atmospheric neutrino sample that would also be collected at the detector. We find that the combination not only improves very significantly these drawbacks, but also enhances both the CP violation discovery potential and the precision in the measurement of the CP violating phase, for which the facility was originally optimized, by lifting parametric degeneracies. We then reassess the optimization of the ESSnu SB setup when the atmospheric neutrino sample is considered, with an emphasis in performing a measurement of the CP violating phase as precise as possible. We find that for the presently preferred value of delta sim -pi /2, shorter baselines and longer running time in neutrino mode would be optimal. In these conditions, a measurement better than 14^circ would be achievable for any value of the theta _{23} octant and the mass ordering. Conversely, if present and next generation facilities were not able to discover CP violation, longer baselines and more even splitting between neutrino and neutrino modes would be preferable. These choices would allow a 5 sigma discovery of CP violation for around a 60% of the possible values of delta and to determine its value with a precision around 6^circ if it is close to 0 or pi .

Distinguishing between the twin b-flavored unitarity triangles on a circular arc

With the help of the generalized Wolfenstein parametrization of quark flavor mixing and CP violation, we calculate fine differences between the twin b-flavored unitarity triangles defined by Vub⁎Vud+Vcb⁎Vcd+Vtb⁎Vtd=0 and Vud⁎Vtd+Vus⁎Vts+Vub⁎Vtb=0 in the complex plane. We find that apexes of the rescaled versions of these two triangles, described respectively by ρ‾+iη‾=−(Vub⁎Vud)/(Vcb⁎Vcd) and ρ˜+iη˜=−(Vub⁎Vtb)/(Vus⁎Vts), are located on a circular arc whose center and radius are given by O=(0.5,0.5cot⁡α) and R=0.5csc⁡α with α being their common inner angle. The small difference between (ρ‾,η‾) and (ρ˜,η˜) is characterized by ρ˜−ρ‾∼η˜−η‾∼O(λ2) with λ≃0.22 being the Wolfenstein expansion parameter, and these two apexes are insensitive to the two-loop renormalization-group running effects up to the accuracy of O(λ4). Some comments are also made on similar features of three pairs of the rescaled unitarity triangles of lepton flavor mixing and CP violation.

Sensitivity to leptonic CP violation at Hyper-Kamiokande

Hyper-Kamiokande is a next generation water Clierenkov detector, an order of magnitude larger than Super-Kamiokande. capable of large and diverse physics studies. Among these, it will study long baseline oscillation physics with great detail, thanks not only to a larger volume and higher detection efficiency, but also to an improved accelerator and near detector system at J-PARC. We study the effect of a new systematic error model 011 the overall sensitivity of the experiment to the neutrino oscillation parameters, with particular care for the CP violating phase. A variety of theoretical models will benefit from a precise measurement of δ CP, which Hyper-Kamiokande has the capability of constraining, with good precision. The accuracy of the measurement, however, is dictated by the systematic uncertainties and hence it is essential to assess the impact of the new error model 011 the sensitivity.

SU(5) grand unified theory with A4 modular symmetry

We present the first example of a grand unified theory (GUT) with a modular symmetry interpreted as a family symmetry. The theory is based on supersymmetric $SU(5)$ in 6d, where the two extra dimensions are compactified on a $T_2/\mathbb{Z}_2$ orbifold. We have shown that, if there is a finite modular symmetry, then it can only be $A_4$ with an (infinite) discrete choice of moduli, where we focus on $\tau = \omega=e^{i2\pi/3}$, the unique solution with $|\tau|=1$. The fields on the branes respect a generalised CP and flavour symmetry $A_4\ltimes \mathbb{Z}_2$ which is isomorphic to $S_4$ which leads to an effective $\mu-\tau$ reflection symmetry at low energies, implying maximal atmospheric mixing and maximal leptonic CP violation. We construct an explicit model along these lines with two triplet flavons in the bulk, whose vacuum alignments are determined by orbifold boundary conditions, analogous to those used for $SU(5)$ breaking with doublet-triplet splitting. There are two right-handed neutrinos on the branes whose Yukawa couplings are determined by modular weights. The charged lepton and down-type quarks have diagonal and hierarchical Yukawa matrices, with quark mixing due to a hierarchical up-quark Yukawa matrix.

The Hyper-K Near Detector Programme

The proposed Hyper-Kamiokande experiment (Hyper-K) is a next generation large water Cherenkov (WC) detector with a broad physics program consisting of neutrino beam measurements in search of leptonic CP violation, astrophysical measurements and a search for proton decay. Hyper-K will act as the far detector to measure the oscillated neutrino flux from the long-baseline beam of 0.6 GeV neutrinos/anti-neutrinos produced by a 1.3 MW proton beam at J-PARC in Japan. To minimise systematic uncertainties, particularly due to flux and cross-section uncertainties, detailed measurements of the unoscillated flux are required with a suite of near detectors. We review the challenges, and present the planned components of the near detector measurement suite, including a new intermediate Water Cherenkov Detector.

A comparative study between ESSnuSB and T2HK in determining the leptonic CP phase

In this paper, we perform a comparative analysis between the future proposed long-baseline experiments ESSnuSB and T2HK in measuring the leptonic CP phase [Formula: see text]. In particular, we study the effect of the neutrino mass ordering degeneracy and the leptonic mixing angle [Formula: see text] octant degeneracy in the measurement of leptonic CP violation and precision for both experiments. Since the ESSnuSB (T2HK) experiment probes the second (first) oscillation maximum to study neutrino oscillations, the effect of these degeneracies are significantly different in both experiments. Our main conclusion is that for the ESSnuSB experiment, the information on the neutrino mass ordering does not play a major role in the determination of [Formula: see text], which is not the case for the T2HK experiment. However, the information on the true octant compromises the CP sensitivity of the ESSnuSB experiment as compared to T2HK if [Formula: see text] lies in the lower octant. These conclusions are true for both the 540 km and 360 km baseline options for the ESSnuSB experiment. In addition, we investigate the effect of different running times in neutrino and antineutrino modes and the effect of [Formula: see text] precision in measuring [Formula: see text].

On the properties of the effective Jarlskog invariant for three-flavor neutrino oscillations in matter

In this paper, we show that the ratio of the effective Jarlskog invariant J˜ for leptonic CP violation in three-flavor neutrino oscillations in matter to its counterpart J in vacuum J˜/J≈1/(Cˆ12Cˆ13) holds as an excellent approximation, where Cˆ12≡1−2Aˆ⁎cos⁡2θ12+Aˆ⁎2 with Aˆ⁎≡acos2⁡θ13/Δ21 and Cˆ13≡1−2Accos⁡2θ13+Ac2 with Ac≡a/Δc. Here Δij≡mi2−mj2 (for ij=21,31,32) stand for the neutrino mass-squared differences in vacuum and θij (for ij=12,13,23) are the neutrino mixing angles in vacuum, while Δc≡Δ31cos2⁡θ12+Δ32sin2⁡θ12 and the matter parameter a≡22GFNeE are defined. This result has been explicitly derived by improving the previous analytical solutions to the renormalization-group equations of effective neutrino masses and mixing parameters in matter. Furthermore, as a practical application, such a simple analytical formula has been implemented to understand the existence and location of the extrema of J˜.

Fermion mass and mixing in a low-scale seesaw model based on the S4 flavor symmetry

Abstract We construct a low-scale seesaw model to generate the masses of active neutrinos based on $S_4$ flavor symmetry supplemented by the $Z_2 \times Z_3 \times Z_4 \times Z_{14}\times U(1)_L$ group, capable of reproducing the low-energy Standard Model (SM) fermion flavor data. The masses of the SM fermions and the fermionic mixing parameters are generated from a Froggatt–Nielsen mechanism after spontaneous breaking of the $S_4\times Z_2 \times Z_3 \times Z_4 \times Z_{14}\times U(1)_L$ group. The obtained values for the physical observables of the quark and lepton sectors are in good agreement with the most recent experimental data. The leptonic Dirac CP-violating phase $\delta _\mathrm{CP}$ is predicted to be $259.579^\circ$ and the predictions for the absolute neutrino masses in the model can also saturate the recent constraints.

A multi-PMT photodetector system for the Hyper-Kamiokande experiment

Hyper-Kamiokande (Hyper-K) is the next generation of large water Cherenkov neutrino detector with a fiducial volume which will be approximately 10 times larger than its precursor. Its broad physics program includes neutrinos from astronomical sources, nucleon decay, with the main focus the determination of leptonic CP violation. To detect the weak Cherenkov light generated by neutrino interactions or proton decay, the employment of the multi-PMT concept, first introduced in the KM3NeT detector, is considered as a possible solution. A multi-PMT Optical Module – a pressure vessel instrumented with multiple small diameter photosensors, read-out electronics and all required power supplies (incl. high voltage) – offers several advantages, such as weaker sensitivity to Earth’s magnetic field, increased granularity and directional information with an almost isotropic field of view. In this contribution the development of a multi-PMT module for Hyper-K is discussed.

Design study of pion and muon beam transport line for MOMENT

Abstract A MuOn-decay Medium-baseline NeuTrino beam facility (MOMENT) was proposed for the leptonic CP violation determination and other potential neutrino physics by offering extremely high flux neutrinos decayed from muons without cooling and acceleration. The huge transverse and large momentum acceptances are needed for the secondary beamline of MOMENT and the beam dynamics has been studied in detail. The whole beamline based on large-aperture superconducting solenoids is composed of several sections: charge selection section, pion decay channel, bending section, adiabatic taper and muon decay channel. To improve particles transmission and neutrino flux while maintaining low background at the far detector of 150 km, the curved solenoids with additional vertical dipole field are applied in the charge selection and bending sections. The adiabatic parameter changes between the sections are also adopted to enhance the neutrino flux. The end-to-end simulations have been carried out and the muon antineutrino flux at the far detector can reach 4.1 × 1011/m2/year with a low background of unwanted neutrino species.

Letter of interest for a neutrino beam from Protvino to KM3NeT/ORCA

The Protvino accelerator facility located in the Moscow region, Russia, is in a good position to offer a rich experimental research program in the field of neutrino physics. Of particular interest is the possibility to direct a neutrino beam from Protvino towards the KM3NeT/ORCA detector, which is currently under construction in the Mediterranean Sea 40 km offshore Toulon, France. This proposal is known as P2O. Thanks to its baseline of 2595 km, this experiment would yield an unparalleled sensitivity to matter effects in the Earth, allowing for the determination of the neutrino mass ordering with a high level of certainty after only a few years of running at a modest beam intensity of approx ~90~hbox {kW}. With a prolonged exposure (approx 1500hbox { kW},,hbox {year}), a 2sigma sensitivity to the leptonic CP-violating Dirac phase can be achieved. A second stage of the experiment, comprising a further intensity upgrade of the accelerator complex and a densified version of the ORCA detector (Super-ORCA), would allow for up to a 6sigma sensitivity to CP violation and a 10^circ {-}17^circ resolution on the CP phase after 10 years of running with a 450 kW beam, competitive with other planned experiments. The initial composition and energy spectrum of the neutrino beam would need to be monitored by a near detector, to be constructed several hundred meters downstream from the proton beam target. The same neutrino beam and near detector set-up would also allow for neutrino-nucleus cross section measurements to be performed. A short-baseline sterile neutrino search experiment would also be possible.

Correlations and degeneracies among the NSI parameters with tunable beams at DUNE

The Deep Underground Neutrino Experiment (DUNE) is a leading experiment in neutrino physics which is presently under construction. DUNE aims to measure the yet unknown parameters in the three flavour oscillation scenario which includes discovery of leptonic CP violation, determination of the mass hierarchy and determination of the octant of $\theta_{23}$. Additionally, the ancillary goals of DUNE include probing the sub-dominant effects induced by new physics. A widely studied new physics scenario is that of nonstandard neutrino interactions (NSI) in propagation which impacts the oscillations of neutrinos. We consider some of the essential NSI parameters impacting the oscillation signals at DUNE and explore the space of NSI parameters as well as study their correlations among themselves and with the yet unknown CP violating phase, $\delta$ appearing in the standard paradigm. The experiment utilizes a wide band beam and provides us with a unique opportunity to utilize different beam tunes at DUNE. We demonstrate that combining information from different beam tunes (low energy, LE and medium energy, ME) available at DUNE impacts the ability to probe some of these parameters and leads to altering the allowed regions in two-dimensional space of parameters considered.

On the determination of leptonic CP violation and neutrino mass ordering in presence of non-standard interactions: present status

We perform a global analysis of neutrino data in the framework of three massive neutrinos with non-standard neutrino interactions which affect their evolution in the matter background. We focus on the effect of NSI in the present observables sensitive to leptonic CP violation and to the mass ordering. We consider complex neutral current neutrino interactions with quarks whose lepton-flavor structure is independent of the quark type. We quantify the status of the “hints” for CP violation, the mass-ordering and non-maximality of θ23 in these scenarios. We also present a parametrization-invariant formalism for leptonic CP violation in presence of a generalized matter potential induced by NSI.

Fits to non-supersymmetric SO(10) models with type I and II seesaw mechanisms using renormalization group evolution

We consider numerical fits to non-supersymmetric SO(10)-based models in which neutrino mass is generated by the type-I or type-II seesaw mechanism or a combination of both. The fits are performed with a sophisticated top-down procedure, taking into account the renormalization group equations of the gauge and Yukawa couplings, integrating out relevant degrees of freedom at their corresponding mass scales, and using recent data for the Standard Model observables. We find acceptable fits for normal neutrino mass ordering only and with neutrino mass generated by either type-I seesaw only or a combination of types I and II seesaw in which type-I seesaw is dominant. Furthermore, we find predictions from the best fit regarding the small neutrino masses, the effective neutrinoless double beta decay mass, and the leptonic CP-violating phase. Finally, we show that the fits are rather insensitive to the chosen value of the unification scale.

Testing generalized CP symmetries with precision studies at DUNE

We examine the capabilities of the DUNE experiment in probing leptonic CP violation within the framework of theories with generalized CP symmetries characterized by the texture zeros of the corresponding CP transformation matrices. We investigate DUNE's potential to probe the two least known oscillation parameters, the atmospheric mixing angle $\theta_{23}$ and the Dirac CP-phase $\delta_{\rm CP}$. We fix theory-motivated benchmarks for ($ \sin^2\theta_{23}, \delta_{\rm CP} $) and take them as true values in our simulations. Assuming 3.5 years of neutrino running plus 3.5 years in the antineutrino mode, we show that in all cases DUNE can significantly constrain and in certain cases rule out the generalized CP texture zero patterns.

The role of leptons in electroweak baryogenesis

We investigate the role of leptons in electroweak baryogenesis by studying a relatively simple framework inspired by effective field theory that satisfies all Sakharov conditions. In particular, we study the effectiveness of CP-violating source terms induced by dimension-six Yukawa interactions for quarks and charged leptons. Despite the relatively small Yukawa coupling, CP-violating source terms involving taus are quite effective and can account for the observed matter-antimatter asymmetry. We obtain analytical and numerical expressions for the total baryon asymmetry, the former providing important insight into what makes lepton CP violation relatively effective compared to quark CP violation. Leptons also play an important role if the CP-violating source involves top quarks. While the tau Yukawa coupling in the Standard Model is small, it significantly enhances the baryon asymmetry by transferring the chiral asymmetry in quarks, which is washed out by strong sphalerons, to a chiral asymmetry in leptons. We conclude that leptons should not be ignored even if CP violation is limited to the quark sector. The role of leptons can be further increased in scenarios of new physics with additional chiral-symmetry-breaking interactions between quarks and leptons, as can happen in models with additional Higgs bosons or leptoquarks. Finally, we study CP-violating dimension-six Yukawa interactions for lighter quarks and leptons but conclude that these lead to too small baryon asymmetries.

Sign of CP violating phase in quarks and leptons

We discuss the relation between the CP violation of the quark mixing and that of the lepton mixing by investigating a CP violating observable, the Jarlskog invariant, as well as the CP violating Dirac phase. The down-type quark mass matrix with three zeros is given in terms of the minimal number of parameters, while the up-type quark mass matrix is diagonal. These quark mass matrices leading to the successful CKM mixing angles and CP violation are embedded in both the Pati-Salam and SU(5) models. The leptonic Jarlskog invariant JCPl (as well as CP violating Dirac phase) is examined for two cases: the neutrino mass matrix is diagonal or non-diagonal, where no additional CP violating phase is introduced apart from the Majorana phases. In the case of the diagonal neutrino mass matrix, the favorable sign of the leptonic CP violation is obtained, however, the magnitude of JCPl is at most mathcal{O} (10−4), which is too small compared with the expected value from the observation −0.02. In the case of the non-diagonal neutrino mass matrix where the tri-bimaximal mixing pattern is taken, we obtain the successful JCPl up to its sign.

Leptonic CP violation in flipped SU(5) GUT from Z12−I orbifold compactification

We obtain a phenomenologically acceptable PMNS matrix in a flipped SU(5) model, possessing the Z4R discrete symmetry, from the compactification of heterotic string E8×E8′. To analyze the Jarlskog determinant efficiently, we include the simple Kim-Seo form for the Pontecorbo-Maki-Nakagawa-Sakata matrix. We also noted that |δPMNS|≲63o for the normal hierarchy of neutrino masses with the PDG book parametrization.

CP transformed mixed μτ antisymmetry for neutrinos and its consequences

We propose a complex extension of mixed $\mu\tau$ antisymmetry in the neutrino Majorana mass matrix $M_\nu$. This can be implemented in the Lagrangian by a generalized CP transformation (labeled by a mixing parameter $\theta$) on the left-chiral flavor neutrino fields. We investigate its implications for leptonic CP violation and neutrino phenomenology in general. Interestingly, the $\mu\tau$ mixing parameter $\theta$ gets correlated with the Dirac CP phase $\delta$ and the atmospheric mixing angle $\theta_{23}$ through an analytical relation. In general, for arbitrary $\theta$, both $\theta_{23}$ and $\delta$ are nonmaximal. We discuss the corresponding results for the CP asymmetry parameter $A_{\mu e}$ in neutrino oscillation experiments. For a nonmaximal $\delta$, one of the two Majorana phases is different from $0$ or $\pi$, thereby leading to nonvanishing Majorana CP violation with observable consequences for the neutrinoless double beta ($\beta\beta0\nu$) decay process. We numerically work out in detail the predictions for that process in relation to various ongoing and forthcoming experiments. We also work out the predictions of our scheme on flavor flux ratios at neutrino telescopes. While exact CP transformed $\mu\tau$ interchange antisymmetry ($\theta=\pi/2$) leads to an exact equality among those ratios, taking a value $0.5$, a tiny deviation can cause a drastic change in them. Careful measurement of these flux ratios in future will further constrain the parameter $\theta$.

Highlight Talk from Super-Kamiokande

Super-Kamiokande (SK), a 50 kton water Cherenkov detector in Japan, is observing both atmospheric and solar neutrinos. It is also searching for supernova (relic) neutrinos, proton decays and dark matter-like particles. A three-flavor oscillation analysis was conducted with the atmospheric neutrino data to study the mass hierarchy, the leptonic CP violation term, and other oscillation parameters. In addition, the observation of solar neutrinos gives precise measurements of the energy spectrum and oscillation parameters. In this proceedings, we given an overview of the latest results from SK and the prospect toward the future project of SK-Gd.