Hai-Tao Li

Particle collisions on stringy black hole background

The collision of two particles in the background of a Sen black hole is studied. With the equations of motion of the particles, the center-of-mass energy is investigated when the collision takes place at the horizon of a Sen black hole. For an extremal Sen black hole, we find that the center-of-mass energy will be arbitrarily high with two conditions: (1) spin a ≠ 0 and (2) one of the colliding particles has the critical angular momentum lc = 2. For a nonextremal Sen black hole, we show that, in order to obtain an unlimited center-of-mass energy, one of the colliding particles should have the critical angular momentum lc′ = 2r+/a (r+ is the radius of the outer horizon for a nonextremal black hole). However, a particle with the angular momentum l = lc′ could not approach the black hole from outside of the horizon through free fall, which implies that the collision with arbitrarily high center-of-mass energy could not take place. Thus, there is an upper bound of the center-of-mass energy for the nonextremal black hole. We also obtain the maximal center-of-mass energy for a near-extremal black hole and the result implies that the Planck-scale energy is hard to be approached. Furthermore, we also consider the back-reaction effects. The result shows that, neglecting the gravitational radiation, it has a weak effect on the center-of-mass energy. However, we argue that the maximum allowed center-of-mass energy will be greatly reduced to below the Planck-scale when the gravitational radiation is included.

Fermion Resonances on Multi-field Thick Branes

Motivated by the recent work on the fermion resonances on scalar-constructed thick branes (arXiv:0901.3543 and arXiv:0904.1785), we extend the idea to multi-scalar generated thick branes and complete previous work. The fermion localization and reso- nances on the three-field and two-field thick branes are investigated. With the Numerov method, our numerical results show that the resonance states also exist in the brane be- sides the single-field thick branes and the two-field thick branes in the former cases. This interesting phenomenon is related to the internal structure of the brane and the coupling of fermions and scalars. We find that the Kaluza-Klein chiral decomposition of massive fermion resonances is the parity-chiral decomposition. For the couplings ��� k ��� and ��� k �� for three-field and two-field models with odd positive k, respectively, the number of the resonant states decreases with k. This result is opposite to the one obtained in the single-field de Sitter thick brane (arXiv:0904.1785).

Domain wall brane in squared curvature gravity

We suggest a thick braneworld model in the squared curvature gravity theory. Despite the appearance of higher order derivatives, the localization of gravity and various bulk matter fields is shown to be possible. The existence of the normalizable gravitational zero mode indicates that our four-dimensional gravity is reproduced. In order to localize the chiral fermions on the brane, two types of coupling between the fermions and the brane forming scalar is introduced. The first coupling leads us to a Schrödinger equation with a volcano potential, and the other a Pöschl-Teller potential. In both cases, the zero mode exists only for the left-hand fermions. Several massive KK states of the fermions can be trapped on the brane, either as resonant states or as bound states.

Fermion localization on asymmetric two-field thick branes

In this paper we investigate the localization of fermions on asymmetric thick branes generated by two scalars and χ. In order to trap fermions on the asymmetric branes with kink-like warp factors, the couplings with the background scalars are introduced, where F(χ, ) is a function of and χ. We find that the coupling does not support the localization of four-dimensional fermions on the branes. While, for the case , which is the kink-fermion coupling corresponding to one-scalar-generated brane scenarios, the zero mode of left-handed fermions could be trapped on the branes under some conditions.

Fermion Resonances on a Thick Brane with a Piecewise Warp Factor

In this paper, we mainly investigate the problems of resonances of massive KK fermions on a single scalar constructed thick brane with a piecewise warp factor matching smoothly. The distance between two boundaries and the other parameters are determined by one free parameter through three junction conditions. For the generalized Yukawa coupling

Deformed brane with finite extra dimension

\eta\bar{\Psi}\phi^{k}\Psi

Effects of temperature on thick branes and the fermion (quasi-)localization

with odd

Geometric Curvatures of Plane Symmetry Black Hole

k=1,3,5,...

Fermions on Asymmetric Bloch Branes

, the mass eigenvalue

Localization of gravity and bulk matters on a thick anti–de Sitter brane

m