GAMMA-BAR

arxiv.org
[1506.08996] LHC Phenomenology of the Type II Seesaw: Observability of Neutral Scalars in the Nondegenerate Case

[ Authors ]
Zhi-Long Han, Ran Ding, Yi Liao
[ Abstract ]
This is a sequel to our previous work on LHC phenomenology of the type II seesaw model in the nondegenerate case ( arXiv:1502.05242 [hep-ph]). In this work, we further study the pair and associated production of the neutral scalars $H^0/A^0$. We restrict ourselves to the so-called negative scenario characterized by the mass order $M_{H^{\pm\pm}}>M_{H^\pm}>M_{H^0/A^0}$, in which the $H^0/A^0$ production receives significant enhancement from cascade decays of the charged scalars $H^{\pm\pm},~H^\pm$. We consider three important signal channels $b\bar{b}\gamma\gamma$, $b\bar{b}\tau^+\tau^-$, $b\bar{b}\ell^+\ell^-\cancel{E}_T$ and perform detailed simulations. We find that at 14 TeV LHC with an integrated luminosity of 3000/fb, a $5\sigma$ mass reach of 151 GeV, 150 GeV, and 180 GeV respectively is possible in the three channels from the pure Drell-Yan $H^0A^0$ production, while the cascade-decay enhanced $H^0/A^0$ production can push the mass limit further to 164 GeV, 177 GeV, and 200 GeV. The neutral scalars in the negative scenario are thus accessible in LHC run II.

arxiv.org
[1506.06870] Gapped Surface States in a Strong-Topological-Semimetal

[ Authors ]
A. P. Weber, Q. D. Gibson, Huiwen Ji, A. N. Caruso, A. V. Fedorov, R.J. Cava, T. Valla
[ Abstract ]
A three-dimensional strong-topological-insulator or -semimetal hosts topological surface states which are often said to be gapless so long as time-reversal symmetry is preserved. This narrative can be mistaken when surface state degeneracies occur away from time-reversal-invariant momenta. The mirror-invariance of the system then becomes essential in protecting the existence of a surface Fermi surface. Here we show that such a case exists in the strong-topological-semimetal Bi$_4$Se$_3$. Angle-resolved photoemission spectroscopy and \textit{ab initio} calculations reveal partial gapping of surface bands on the Bi$_2$Se$_3$-termination of Bi$_4$Se$_3$(111), where an 85 meV gap along $\bar{\Gamma}\bar{K}$ closes to zero toward the mirror-invariant $\bar{\Gamma}\bar{M}$ azimuth. The gap opening is attributed to an interband spin-orbit interaction that mixes states of opposite spin-helicity.