A new scheme for bright hard x-ray emission from laser wakefield electron accelerator is reported, where pure nitrogen gas is adopted. Intense Betatron x-ray beams are generated from ionization injected K-shell electrons of nitrogen into the accelera
ting wave bucket. The x-ray radiation shows synchrotron-like spectrum with total photon yield 8$times$10$^8$/shot and $10^8$ over 110keV. In particular, the betatron hard x-ray photon yield is 10 times higher compared to the case of helium gas under the same laser parameters. Particle-in-cell simulation suggests that the enhancement of the x-ray yield results from ionization injection, which enables the electrons to be quickly accelerated to the driving laser region for subsequent betatron resonance. Employing the present scheme,the single stage nitrogen gas target could be used to generate stable high brightness betatron hard x-ray beams.
The extension of the minimal standard model by three right-handed sterile neutrinos with masses smaller than the electroweak scale (nuMSM) is discussed in a Q_6 flavor symmetry framework. The lightness of the keV sterile neutrino and the near mass de
generacy of two heavier sterile neutrinos are naturally explained by exploiting group properties of Q_6. A normal hierarchical mass spectrum and an approximately mu-tau symmetric mass matrix are predicted for three active neutrinos. Nonzero theta_{13} can be obtained together with a deviation of theta_{23} from the maximality, where both mixing angles are consistent with the latest global data including T2K and MINOS results. Furthermore, the tiny active-sterile mixing is related to the mass ratio between the lightest active and lightest sterile neutrinos.
