Jue Zhang

Electroweak Vacuum Stability and Diphoton Excess at 750 GeV

Recently, both ATLAS and CMS collaborations at the CERN Large Hadron Collider (LHC) announced their observations of an excess of diphoton events around the invariant mass of 750 GeV with a local significance of 3.6 sigma and 2.6 sigma, respectively. In this paper, we interpret the diphoton excess as the on-shell production of a real singlet scalar in the pp -> S -> gamma gamma channel. To accommodate the observed production rate, we further introduce a vector-like fermion F, which carries both color and electric charges. The viable regions of model parameters are explored for this simple extension of the Standard Model (SM). Moreover, we revisit the problem of electroweak vacuum stability in the same scenario, and find that the requirement for the electroweak vacuum stability up to high energy scales imposes serious constraints on the Yukawa coupling of the vector-like fermion and the quartic couplings of the SM Higgs boson and the new singlet scalar. Consequently, a successful explanation for the diphoton excess and the absolute stability of electroweak vacuum cannot be achieved simultaneously in this economical setup.

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

A bstractIn 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.

Relic right-handed Dirac neutrinos and implications for detection of cosmic neutrino background

Abstract 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 L and right-handed antineutrinos ν ‾ R in future experiments of neutrino capture on beta-decaying nuclei (e.g., ν e + H 3 → He 3 + 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 R and left-handed antineutrinos ν ‾ 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 R and ν ‾ L with a total number density of 95 cm − 3 , which should be compared to the number density 336 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 eff = 3.14 − 0.43 + 0.44 at the 95 % confidence level. For illustration, two possible scenarios have been proposed for thermal production of right-handed neutrinos in the early Universe.

Renormalisation Group Corrections to the Littlest Seesaw Model and Maximal Atmospheric Mixing

A bstractThe Littlest Seesaw (LS) model involves two right-handed neutrinos and a very constrained Dirac neutrino mass matrix, involving one texture zero and two independent Dirac masses, leading to a highly predictive scheme in which all neutrino masses and the entire PMNS matrix is successfully predicted in terms of just two real parameters. We calculate the renormalisation group (RG) corrections to the LS predictions, with and without supersymmetry, including also the threshold effects induced by the decoupling of heavy Majorana neutrinos both analytically and numerically. We find that the predictions for neutrino mixing angles and mass ratios are rather stable under RG corrections. For example we find that the LS model with RG corrections predicts close to maximal atmospheric mixing, θ23 = 45° ± 1°, in most considered cases, in tension with the latest NOvA results. The techniques used here apply to other seesaw models with a strong normal mass hierarchy.

Majorana Neutrino Masses from Neutrinoless Double-Beta Decays and Lepton-Number-Violating Meson Decays

Abstract The Schechter–Valle theorem states that a positive observation of neutrinoless double-beta ( 0 ν β β ) decays implies a finite Majorana mass term for neutrinos when any unlikely fine-tuning or cancellation is absent. In this note, we reexamine the quantitative impact of the Schechter–Valle theorem, and find that current experimental lower limits on the half-lives of 0 ν β β -decaying nuclei have placed a restrictive upper bound on the Majorana neutrino mass | δ m ν e e | 7.43 × 10 − 29 eV radiatively generated at the four-loop level. Furthermore, we generalize this quantitative analysis of 0 ν β β decays to that of the lepton-number-violating (LNV) meson decays M − → M ′ + + l α − + l β − (for α , β = e or μ ). Given the present upper limits on these rare LNV decays, we have derived the loop-induced Majorana neutrino masses | δ m ν e e | 9.7 × 10 − 18 eV , | δ m ν e μ | 1.6 × 10 − 15 eV and | δ m ν μ μ | 1.0 × 10 − 12 eV from K − → π + + e − + e − , K − → π + + e − + μ − and K − → π + + μ − + μ − , respectively. A partial list of radiative neutrino masses from the LNV decays of D , D s and B mesons is also given.

Viability of exact tri-bimaximal, golden-ratio and bimaximal mixing patterns and renormalization-group running effects

A bstractIn light of the latest neutrino oscillation data, we examine whether the leptonic flavor mixing matrix can take on an exact form of tri-bimaximal (TBM), golden-ratio (GR) or bimaximal (BM) mixing pattern at a superhigh-energy scale, where such a mixing pattern could be realized by a flavor symmetry, and become compatible with experimental data at the low-energy scale. Within the framework of the Minimal Supersymmetric Standard Model (MSSM), the only hope for realizing such a possibility is to count on the corrections from the renomalization-group (RG) running. In this work we focus on these radiative corrections, and fully explore the allowed parameter space for each of these mixing patterns. We find that when the upper bound on the sum of neutrino masses Σν ≡ m1 + m2 + m3< 0.23 eV at the 95% confidence level from Planck 2015 is taken into account, none of these mixing patterns can be identified as the leptonic mixing matrix below the seesaw threshold. If this cosmological upper bound on the sum of neutrino masses were relaxed, the TBM and GR mixing patterns would still be compatible with the latest neutrino oscillation data at the 3σ level, but not at the 1σ level. Even in this case, no such a possibility exists for the BM mixing.

Looking into analytical approximations for three-flavor neutrino oscillation probabilities in matter

A bstractMotivated 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 η-gauge neutrino mass-squared difference Δ∗ ≡ ηΔ31 + (1 − η)Δ32 is introduced, where Δji≡mj2 − mi2 for ji = 21, 31, 32 are the ordinary neutrino mass-squared differences and 0 ≤ η ≤ 1 is a real and positive parameter. Expanding neutrino oscillation probabilities in terms of α ≡ Δ21/Δ∗, we demonstrate that the analytical formulas can be remarkably simplified for η = cos2θ12, with θ12 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 Oα2

Neutrinos and cosmological matter–antimatter asymmetry: A minimal seesaw with Frampton–Glashow–Yanagida ansatz

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