Nonperturbative Case Research Articles

Back reaction of a conformal field on a three-dimensional black hole

The first order corrections to the geometry of the (2+1)-dimensional black hole due to back-reaction of a massless conformal scalar field are computed. The renormalized stress energy tensor used as the source of Einstein equations is computed with the Green function for the black-hole background with transparent boundary conditions. This tensor has the same functional form as the one found in the nonperturbative case which can be exactly solved. Thus, a static, circularly symmetric and asymptotically anti-de Sitter black hole solution of the semiclassical equations is found. The corrections to the thermodynamic quantities are also computed.

No future for the fourth generation?

We discuss bounds on and implications of a possible fourth generation of quarks and leptons. We derive lower bounds on the masses of fourth generation quarks from the recent experimental bound mτ' ⩾ 41 GeV, for both nonsupersymmetric and supersymmetric models. We also derive upper bounds on these masses from the requirement of a perturbative unification, and briefly discuss the non-perturbative case. We then show that a fourth generation renders the conventional mechanism of cosmological baryogenesis via Higgs-decay problematic, and tends to result in a large supersymmetry breaking scale in minimal supergravity models. We finally show that the incorporation of a fourth generation in superstring models can only work if the superpotential fulfills certain specific conditions, which appear to be in conflict with cosmological requirements.