arXiv subjects

Shun Zhou

Publications and source records attributed to Shun Zhou.

At least 109 records · Page 6Linked to original sources

arXiv2015

Relic Right-handed Dirac Neutrinos and Implications for Detection of Cosmic Neutrino Background

It remains to be determined experimentally if massive neutrinos are Majorana or Dirac particles. In this connection, it has been recently suggested that the detection of cosmic neutrino background of left-handed neutrinos $ν^{}_{\rm L}$ and right-handed antineutrinos $\overlineν^{}_{\rm R}$ in future experiments of neutrino capture on beta-decaying nuclei (e.g., $ν^{}_e + {^3{\rm H}} \to {^3}{\rm He} + e^-$ for the PTOLEMY experiment) is likely to distinguish between Majorana and Dirac neutrinos, since the capture rate is twice larger in the former case. In this paper, we investigate the possible impact of right-handed neutrinos on the capture rate, assuming that massive neutrinos are Dirac particles and both right-handed neutrinos $ν^{}_{\rm R}$ and left-handed antineutrinos $\overlineν^{}_{\rm L}$ can be efficiently produced in the early Universe. It turns out that the capture rate can be enhanced at most by $28\%$ due to the presence of relic $ν^{}_{\rm R}$ and $\overlineν^{}_{\rm L}$ with a total number density of $95~{\rm cm}^{-3}$, which should be compared to the number density $336~{\rm cm}^{-3}$ of cosmic neutrino background. The enhancement has actually been limited by the latest cosmological and astrophysical bounds on the effective number of neutrino generations $N^{}_{\rm eff} = 3.14^{+0.44}_{-0.43}$ at the $95\%$ confidence level. For illustration, two possible scenarios have been proposed for thermal production of right-handed neutrinos in the early Universe.

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arXiv2015

Update on two-zero textures of the Majorana neutrino mass matrix in light of recent T2K, Super-Kamiokande and NO$ν$A results

The latest results from atmospheric and accelerator neutrino experiments indicate that the normal neutrino mass ordering $m^{}_1 < m^{}_2 < m^{}_3$, a maximal leptonic CP-violating phase $δ= 270^\circ$ and the second octant of neutrino mixing angle $θ^{}_{23} > 45^\circ$ are favored. In light of new experimental results, we update previous phenomenological studies on two-zero textures of the Majorana neutrino mass matrix $M^{}_ν$, in the flavor basis where the charged-lepton mass matrix $M^{}_l$ is diagonal. When the $1σ$ ranges of neutrino mixing parameters are taken into account, only four (i.e., ${\bf A}^{}_{1, 2}$ and ${\bf B}^{}_{2,4}$) among seven two-zero patterns of $M^{}_ν$ show the aforementioned features of neutrino mass spectrum, mixing angle $θ^{}_{23}$ and CP-violating phase $δ$, and thus are compatible with the latest neutrino oscillation data. The correlative relations among neutrino masses and mixing parameters have been derived analytically for these four patterns, and the allowed regions of neutrino mixing angles and the CP-violating phase are also given. Possible realizations of four viable two-zero textures via non-Abelian discrete flavor symmetries are discussed.

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arXiv2015

Determination of neutrino mass ordering in future $^{76}$Ge-based neutrinoless double-beta decay experiments

Motivated by recent intensive experimental efforts on searching for neutrinoless double-beta decays, we perform a detailed analysis of the physics potential of the experiments based on $^{76}\mathrm{Ge}$. Assuming no signals, current and future experiments could place a $90\%$ lower limit on the half life $T^{0ν}_{1/2} \gtrsim 4\times 10^{26}~{\rm yr}$ and $T^{0ν}_{1/2} \gtrsim 7\times 10^{27}~{\rm yr}$, respectively. Then, how to report an evidence for neutrinoless double-beta decays is addressed by following the Bayesian statistical approach. For the first time, we present a quantitative description of experimental power to distinguish between normal and inverted neutrino mass orderings. Taking an exposure of $10^{4}~{\rm kg}\cdot{\rm yr}$ and a background rate of $10^{-4}~{\rm counts}/({\rm keV}\cdot{\rm kg}\cdot{\rm yr})$, we find that a moderate evidence for normal neutrino mass ordering (i.e., with a Bayes factor ${\cal B}$ given by $\ln({\cal B}) \simeq 2.5$ or a probability about $92.3\%$ according to the Jeffreys scale) can be achieved if the true value of effective neutrino mass $m^{}_{ββ}$ turns out to be below $0.01~{\rm eV}$.

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arXiv2015

Theoretical Results on Neutrinos

In this talk, I first summarize our current knowledge about the fundamental properties of neutrinos and emphasize the remaining unsolved problems in neutrino physics. Then, recent theoretical results on neutrino mass models are introduced. Different approaches to understanding tiny neutrino masses, lepton flavor mixing and CP violation are presented. Finally, I report briefly some new progress in the studies of astrophysical neutrinos, including keV sterile neutrinos, supernova neutrinos and ultrahigh-energy cosmic neutrinos.

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arXiv2015

Electromagnetic neutrinos in laboratory experiments and astrophysics

An overview of neutrino electromagnetic properties, which open a door to the new physics beyond the Standard Model, is given. The effects of neutrino electromagnetic interactions both in terrestrial experiments and in astrophysical environments are discussed. The experimental bounds on neutrino electromagnetic characteristics are summarized. Future astrophysical probes of electromagnetic neutrinos are outlined.

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arXiv2015

A Further Study of the Frampton-Glashow-Yanagida Model for Neutrino Masses, Flavor Mixing and Baryon Number Asymmetry

In light of the latest neutrino oscillation data, we revisit the minimal scenario of type-I seesaw model, in which only two heavy right-handed Majorana neutrinos are introduced to account for both tiny neutrino masses and the baryon number asymmetry in our Universe. In this framework, we carry out a systematic study of the Frampton-Glashow-Yanagida ansatz by taking into account the renormalization-group running of neutrino mixing parameters and the flavor effects in leptogenesis. We demonstrate that the normal neutrino mass ordering is disfavored even in the minimal supersymmetric standard model with a large value of $\tan β$, for which the running effects could be significant. Furthermore, it is pointed out that the original scenario with a hierarchical mass spectrum of heavy Majorana neutrinos contradicts with the upper bound derived from a naturalness criterion, and the resonant mechanism with nearly-degenerate heavy Majorana neutrinos can be a possible way out.

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arXiv2015

Constraining Absolute Neutrino Masses via Detection of Galactic Supernova Neutrinos at JUNO

A high-statistics measurement of the neutrinos from a galactic core-collapse supernova is extremely important for understanding the explosion mechanism, and studying the intrinsic properties of neutrinos themselves. In this paper, we explore the possibility to constrain the absolute scale of neutrino masses $m^{}_ν$ via the detection of galactic supernova neutrinos at the Jiangmen Underground Neutrino Observatory (JUNO) with a 20 kiloton liquid-scintillator detector. In assumption of a nearly-degenerate neutrino mass spectrum and a normal mass ordering, the upper bound on the absolute neutrino mass is found to be $m^{}_ν< (0.83 \pm 0.24)~{\rm eV}$ at the 95% confidence level for a typical galactic supernova at a distance of 10 kpc, where the mean value and standard deviation are shown to account for statistical fluctuations. For comparison, we find that the bound in the Super-Kamiokande experiment is $m^{}_ν< (0.94 \pm 0.28)~{\rm eV}$ at the same confidence level. However, the upper bound will be relaxed when the model parameters characterizing the time structure of supernova neutrino fluxes are not exactly known, and when the neutrino mass ordering is inverted.

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arXiv2015

Renormalization group running of neutrino parameters

Neutrinos are the most elusive particles in our universe. They have masses at least one million times smaller than the electron mass, carry no electric charge, and very weakly interact with other particles, meaning they are rarely captured in terrestrial detectors. Tremendous efforts in the past two decades have revealed that neutrinos can transform from one type to another as a consequence of neutrino oscillations---a quantum mechanical effect over macroscopic distances---yet the origin of neutrino masses remains puzzling. The physical evolution of neutrino parameters with respect to energy scale may help elucidate the mechanism for their mass generation.

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arXiv2015

Supernova Bounds on keV-mass Sterile Neutrinos

Sterile neutrinos of keV masses are one of the most promising candidates for the warm dark matter, which could solve the small-scale problems encountered in the scenario of cold dark matter. We present a detailed study of the production of such sterile neutrinos in a supernova core, and derive stringent bounds on the active-sterile neutrino mixing angles and sterile neutrino masses based on the standard energy-loss argument.

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arXiv2015

Extrinsic and Intrinsic CPT Asymmetries in Neutrino Oscillations

We reconsider the extrinsic and possible intrinsic CPT violation in neutrino oscillations, and point out an identity, i.e., $A^{\rm CP}_{αβ} = A^{\rm CPT}_{βα} + A^{\rm T}_{αβ}$, among the CP, T, and CPT asymmetries in oscillations. For three-flavor oscillations in matter of constant density, the extrinsic CPT asymmetries $A^{\rm CPT}_{ee}$, $A^{\rm CPT}_{eμ}$, $A^{\rm CPT}_{μe}$, and $A^{\rm CPT}_{μμ}$ caused by Earth matter effects have been calculated in the plane of different neutrino energies and baseline lengths. It is found that two analytical conditions can be implemented to describe the main structure of the contours of vanishing extrinsic CPT asymmetries. Finally, without assuming intrinsic CPT symmetry in the neutrino sector, we investigate the possibility to constrain the difference of the neutrino CP-violating phase $δ$ and the antineutrino one $\overlineδ$ using a low-energy neutrino factory and the super-beam experiment ESS$ν$SB. We find that $|δ- \overlineδ| \lesssim 0.35π$ in the former case and $|δ- \overlineδ| \lesssim 0.7π$ in the latter case can be achieved at the $3σ$ confidence level if $δ= \overlineδ = π/2$ is assumed.

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arXiv2014

Nonstandard interaction effects on neutrino parameters at medium-baseline reactor antineutrino experiments

Precision measurements of leptonic mixing parameters and the determination of the neutrino mass hierarchy are the primary goals of the forthcoming medium-baseline reactor antineutrino experiments, such as JUNO and RENO-50. In this work, we investigate the impact of nonstandard neutrino interactions (NSIs) on the measurements of {sin^2 theta_{12}, Delta m^2_{21}} and {sin^2 theta_{13}, Delta m^2_{31}}, and on the sensitivity to the neutrino mass hierarchy, at the medium-baseline reactor experiments by assuming a typical experimental setup. It turns out that the true mixing parameter sin^2 theta_{12} can be excluded at a more than 3 sigma level if the NSI parameter varepsilon_{e mu} or varepsilon_{e tau} is as large as 2% in the most optimistic case. However, the discovery reach of NSI effects has been found to be small, and depends crucially on the CP-violating phases. Finally, we show that NSI effects could enhance or reduce the discrimination power of the JUNO and RENO-50 experiments between the normal and inverted neutrino mass hierarchies.

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arXiv2014

A partial $μ$-$τ$ symmetry and its prediction for leptonic CP violation

We find that the lepton flavor mixing matrix $U$ should possess a partial $μ$-$τ$ permutation symmetry $|U^{}_{μ1}| = |U^{}_{τ1}|$, and the latter predicts a novel correlation between the Dirac CP-violating phase $δ$ and three flavor mixing angles $θ^{}_{12}$, $θ^{}_{13}$ and $θ^{}_{23}$ in the standard parametrization. Inputting the best-fit values of these angles reported by Capozzi {\it et al}, we obtain the prediction $δ\simeq 255^\circ$ in the normal neutrino mass ordering, which is in good agreement with the best-fit result $δ\simeq 250^\circ$. In this connection the inverted neutrino mass ordering is slightly disfavored. If this partial $μ$-$τ$ symmetry is specified to be $|U^{}_{μ1}| = |U^{}_{τ1}| =1/\sqrt{6}~$, one can reproduce the phenomenologically-favored relation $\sin^2θ^{}_{12} = \left(1 - 2\tan^2θ^{}_{13}\right)/3$ and a viable two-parameter description of $U$ which were first uncovered in 2006. Moreover, we point out that the octant of $θ^{}_{23}$ and the quadrant of $δ$ can be resolved thanks to the slight violation of $|U^{}_{μ2}| = |U^{}_{τ2}|$ and $|U^{}_{μ3}| = |U^{}_{τ3}|$ either at the tree level or from radiative corrections.

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arXiv2014

Constraining Sterile Neutrinos Using Reactor Neutrino Experiments

Models of neutrino mixing involving one or more sterile neutrinos have resurrected their importance in the light of recent cosmological data. In this case, reactor antineutrino experiments offer an ideal place to look for signatures of sterile neutrinos due to their impact on neutrino flavor transitions. In this work, we show that the high-precision data of the Daya Bay experi\-ment constrain the 3+1 neutrino scenario imposing upper bounds on the relevant active-sterile mixing angle $\sin^2 2 θ_{14} \lesssim 0.06$ at 3$σ$ confidence level for the mass-squared difference $Δm^2_{41}$ in the range $(10^{-3},10^{-1}) \, {\rm eV^2}$. The latter bound can be improved by six years of running of the JUNO experiment, $\sin^22θ_{14} \lesssim 0.016$, although in the smaller mass range $ Δm^2_{41} \in (10^{-4} ,10^{-3}) \, {\rm eV}^2$. We have also investigated the impact of sterile neutrinos on precision measurements of the standard neutrino oscillation parameters $θ_{13}$ and $Δm^2_{31}$ (at Daya Bay and JUNO), $θ_{12}$ and $Δm^2_{21}$ (at JUNO), and most importantly, the neutrino mass hierarchy (at JUNO). We find that, except for the obvious situation where $Δm^2_{41}\sim Δm^2_{31}$, sterile states do not affect these measurements substantially.

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arXiv2013

Comment on "Is Dark Matter with Long-Range Interactions a Solution to All Small-Scale Problems of ΛCDM Cosmology?"

In Phys. Rev. Lett. 109 (2012) 231301, van den Aarssen et al. attempt to solve all the small-scale structure problems in the cold-dark-matter scenario by introducing interaction between the dark-matter particle and an MeV-mass vector boson. In addition, the latter is coupled to ordinary neutrinos, and the neutrino-vector boson coupling constant 10^{-5} < g_nu < 0.1 is required for that purpose. We show that this dark-matter scenario is subject to restrictive constraints from Big Bang Nucleosynthesis.

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arXiv2013

Effects of nonstandard neutrino interactions at PINGU

Neutrino oscillation experiments in the past decades have greatly improved our knowledge on neutrinos by measuring the fundamental neutrino parameters. The ongoing and upcoming neutrino oscillation experiments are intended to pin down the neutrino mass hierarchy and to discover the leptonic CP violation. By means of neutrino oscillograms, we analyze the impact of non-standard neutrino interactions on neutrino oscillations in the Earth matter. The standard neutrino oscillation probabilities may be significantly changed by non-standard interaction parameters, and in particular, the CP-violating effects in the energy range E = 1 ~ 20 GeV are greatly enhanced. In addition, the event rates of muon neutrinos in the proposed huge atmospheric neutrino experiment, PINGU at the South Pole, have been estimated in the presence of non-standard neutrino interactions. It has been found that the PINGU experiment has very good sensitivities to the non-standard neutrino interaction parameters.

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arXiv2013

Hybrid Textures of Majorana Neutrino Mass Matrix and Current Experimental Tests

Motivated by recent measurements of a relatively large θ_{13} in the Daya Bay and RENO reactor neutrino experiments, we carry out a systematic analysis of the hybrid textures of Majorana neutrino mass matrix $M_ν$, which contain one texture zero and two equal nonzero matrix elements. We show that three neutrino masses (m_1, m_2, m_3) and three leptonic CP-violating phases (δ, ρ, σ) can fully be determined from two neutrino mass-squared differences (δm^2, Δm^2) and three flavor mixing angles (θ_{12}, θ_{23}, θ_{13}). Out of sixty logically possible patterns of $M_ν$, thirty-nine are found to be compatible with current experimental data at the 3σlevel. We demonstrate that the texture zero of $M_ν$ is stable against one-loop quantum corrections, while the equality between two independent elements not. Phenomenological implications of $M_ν$ for the neutrinoless double-beta decay and leptonic CP violation are discussed, and a realization of the texture zero and equality by means of discrete flavor symmetries is illustrated.

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arXiv2013

Probing the leptonic Dirac CP-violating phase in neutrino oscillation experiments

The discovery of leptonic CP violation is one of the primary goals of next-generation neutrino oscillation experiments, which is feasible due to the recent measurement of a relatively large leptonic mixing angle θ_{13}. We suggest two new working observables ΔA^{\rm m}_{αβ} \equiv \max[A^{\rm CP}_{αβ}(δ)] - \min[A^{\rm CP}_{αβ}(δ)] and ΔA^{\rm CP}_{αβ}(δ) \equiv A^{\rm CP}_{αβ}(δ) - A^{\rm CP}_{αβ}(0) to describe the CP-violating effects in long-baseline and atmospheric neutrino oscillation experiments. The former signifies the experimental sensitivity to the leptonic Dirac CP-violating phase $δ$ and can be used to optimize the experimental setup, while the latter measures the intrinsic leptonic CP violation and can be used to extract $δ$ directly from the experimental observations. Both analytical and numerical analyses are carried out to illustrate their main features. It turns out that an intense neutrino beam with sub-GeV energies and a baseline of a few 100 km may serve as an optimal experimental setup for probing leptonic CP violation.

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arXiv2013

Texture Zeros for Dirac Neutrinos and Current Experimental Tests

The Daya Bay and RENO reactor neutrino experiments have revealed that the smallest neutrino mixing angle is in fact relatively large, i.e., theta_13 ~ 9 degrees. Motivated by this exciting progress, we perform a systematic study of the neutrino mass matrix M_nu with one or two texture zeros, in the assumption that neutrinos are Dirac particles. Among fifteen possible patterns with two texture zeros, only three turn out to be favored by current neutrino oscillation data at the 3 sigma level. Although all the six patterns with one texture zero are compatible with the experimental data at the 3 sigma level, the parameter space of each pattern is strictly constrained. Phenomenological implications of M_nu on the leptonic CP violation and neutrino mass spectrum are explored, and the stability of texture zeros against the radiative corrections is also discussed.

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