分类： 物理学 >> 核物理学 提交时间： 2016-09-20
摘要： 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.
分类： 物理学 >> 核物理学 提交时间： 2016-09-14
摘要： 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.
分类： 物理学 >> 核物理学 提交时间： 2016-09-14
摘要： 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.
分类： 物理学 >> 核物理学 提交时间： 2016-09-13
摘要： 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.
分类： 物理学 >> 核物理学 提交时间： 2016-09-06
摘要： 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′++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 |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.
分类： 物理学 >> 核物理学 提交时间： 2016-09-05
摘要： 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.
分类： 物理学 >> 核物理学 提交时间： 2016-09-01
摘要： 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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