All Results

Electroweak Vacuum Stability and Diphoton Excess at 750 GeV

Jue Zhang; Shun ZhouSubjects: Physics >> Nuclear Physics

Recently, both ATLAS and CMS collaborations at the CERN Large Hadron Collider (LHC) have announced their observations of an excess of diphoton events around the invariant mass of?750?GeV?with a local significance of?3.6σ?and?2.6σ, respectively. In this paper, we interpret the diphoton excess as the on-shell production of a real singlet scalar in the?pp→S→γγ?channel. To accommodate the observed production rate, we further introduce a vector-like fermion?F, which is carrying 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. |

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

Jue Zhang; Shun ZhouSubjects: Physics >> Nuclear Physics

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?νL?and right-handed antineutrinos?ν???R?in future experiments of neutrino capture on beta-decaying nuclei (e.g.,?νe+3H→3He+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?νR?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?νR?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?Neff=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. |

Subjects: Physics >> Nuclear Physics

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?76Ge. Assuming no signals, current and future experiments could place a?90%?lower limit on the half life?T0ν1/2?4×1026?yr?and?T0ν1/2?7×1027?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?104?kg?yr?and a background rate of?10?4?counts/(keV?kg?yr), we find that a moderate evidence for normal neutrino mass ordering (i.e., with a Bayes factor?B?given by?ln(B)?2.5?or a probability about?92.3%?according to the Jeffreys scale) can be achieved if the true value of effective neutrino mass?mββ?turns out to be below?0.01?eV. |

Subjects: Physics >> Nuclear Physics

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?tanβ, 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. |

Subjects: Physics >> Nuclear Physics

The Schechter-Valle theorem states that a positive observation of neutrinoless double-beta (0νββ) 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?0νββ-decaying nuclei have placed a restrictive upper bound on the Majorana neutrino mass?|δmeeν|<7.43×10?29?eV?radiatively generated at the four-loop level. Furthermore, we generalize this quantitative analysis of?0νββ?decays to that of the lepton-number-violating (LNV) meson decays?M?→M′++??α+??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?|δmeeν|<9.7×10?18?eV,?|δmeμν|<1.6×10?15?eV?and?|δmμμν|<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,?Ds?and?B?mesons is also given. |

Neutrino Mass Ordering in Future Neutrinoless Double Beta Decay Experiments

Jue ZhangSubjects: Physics >> Nuclear Physics

Motivated by recent intensive experimental efforts on searching for neutrinoless double beta decays, we present a detailed quantitative analysis on the prospect of resolving neutrino mass ordering in the next generation?76Ge-type experiments. |

Distinguishing Dirac/Majorana Sterile Neutrinos at the LHC

Claudio O. Dib; C. S. Kim; Kechen Wang; Jue Zhang?Subjects: Physics >> Nuclear Physics

We study the purely leptonic decays of W±→e±e±μ?ν and μ±μ±e?ν produced at the LHC, induced by sterile neutrinos with mass mN below MW in the intermediate state. Since the final state neutrino escapes detection, one cannot tell whether this process violates lepton number, what would indicate a Majorana character for the intermediate sterile neutrino. Our study shows that when the sterile neutrino mixings with electrons and muons are different enough, one can still discriminate between the Dirac and Majorana character of this intermediate neutrino by simply counting and comparing the above decay rates. After performing collider simulations and statistical analysis, we find that at the 14 TeV LHC with an integrated luminosity of 3000 fb?1, for two benchmark scenarios mN = 20 GeV and 50 GeV, at least a 3σ level of exclusion on the Dirac case can be achieved for disparities as mild as e.g. |UNe|2<0.7 |UNμ|2 or |UNμ|2<0.7 |UNe|2, provided that |UNe|2, |UNμ|2 are both above ?2×10?6. |

Radiative Corrections to the Sum Rule of Lepton Flavor Mixing

Jue Zhang; Shun ZhouSubjects: Physics >> Nuclear Physics

The simple correlation among three lepton flavor mixing angles?(θ12,θ13,θ23)?and the leptonic Dirac CP-violating phase?δ?is conventionally called a sum rule of lepton flavor mixing, which may be derived from a class of neutrino mass models with flavor symmetries. In this paper, we consider the sum rule?θ12≈θν12+θ13cosδ, where?θν12?stems from a constant mixing pattern in the neutrino sector and takes the value of?θν12=45??for the bi-maximal mixing (BM),?θν12=35.3??for the tri-bimaximal mixing (TBM) or?θν12=tan?1[2/(5√+1)]≈31.7??for the golden-ratio mixing (GR), and investigate the renormalization-group (RG) running effects on lepton flavor mixing parameters when this sum rule is assumed at a superhigh-energy scale. Quantitatively, we implement the Bayesian approach to explore the posterior distribution of?δ?at the low-energy scale, which becomes quite broad when the RG running effects are significant, as in the minimal supersymmetric standard model (MSSM) with a sizeable value of?tanβ. Moreover, we also discuss the compatibility of the above three mixing scenarios with current neutrino oscillation data, and observe that radiative corrections can increase such a compatibility for the BM scenario, resulting in a weaker preference for the TBM and GR ones. |

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