Sanghyeon Chang

STABLE SUPERSTRING RELICS

We investigate the cosmological constraints on exotic stable matter states which arise in realistic free fermionic superstring models. These states appear in the superstring models due to a “Wilsonline” breaking of the unifying non-abelian gauge symmetry. In the models that we consider the unifying SO(10) gauge symmetry is broken at the string level to SO(6) x SO(4), SU(5) x U(1) or SU(3) x SU(2) x U(1)2. The exotic matter states are classified according to the patterns of the SO(10) symmetry breaking. In SO(6) x SO(4) and SU(5) x U(1) type models one obtains fractionally charged states with Qe.m. = ±12. In SU (3) x SU (2) x U(1)2 type models one also obtains states with the regular charges under the Standard Model gauge group but with “fractional” charges under the U(1)z′ symmetry. These states include down-like color triplets and electroweak doublets, as well as states which are Standard Model singlets. By analyzing the renormalizable and nonrenormalizable terms of the superpotential in a specific superstring model, we show that these exotic states can be stable. We investigate the cosmological constraints on the masses and relic density of the exotic states. We propose that, while the abundance and the masses of the fractionally charged states are highly constrained, the Standard Model-like states, and in particular the Standard Model singlet, are good dark matter candidates.

Studies of the motion and decay of axion walls bounded by strings

We discuss the appearance at the QCD phase transition, and the subsequent decay, of axion walls bounded by strings in

A Dark Matter Solution from the Supersymmetric Axion Model.

N=1

Spin-dependent Drell-Yan in QCD to O(αs2) (I). The non-singlet sector

axion models. We argue on intuitive grounds that the main decay mechanism is into barely relativistic axions. We present numerical simulations of the decay process. In these simulations, the decay happens immediately, in a time scale of order the light travel time, and the average energy of the radiated axions is

Axions from string decay

〈{\ensuremath{\omega}}_{a}〉\ensuremath{\simeq}{7m}_{a}

Drell-Yan non-singlet spin cross sections and spin asymmetry to O(αs2) (II)

for

Direct bound on the minimal Universal Extra Dimension model from the

{v}_{a}{/m}_{a}\ensuremath{\simeq}500.

resonance search at the Tevatron

〈{\ensuremath{\omega}}_{a}〉

Rapidity correlations and {Delta}G from prompt photon plus jet production in polarized pp collisions

is found to increase approximately linearly with