Topics in Superstring Perturbation Theory and Non-equilibrium Field Theory at Finite Temperatures

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With E. D'Hoker and D. P. Phong, we obtained analytically the two loop contribution to the cosmological constant in superstring theory, in some asymmetric orbifold vacua of physics interest. At the time, there were claims that this vacua had zero cosmological constant to all orders in perturbation theory, possibly leading to a hitherto unknown mechanism for preserving zero cosmological constant in spacetimes with no supersymmetry. This feature would be quite desirable from a phenomenological point of view. There was no clear statement on this issue, mostly due to the technical difficulties involved. Through explicit computation, we showed that in the relevant case, the cosmological constant is non-zero at the two loop level, contributing to the resolution of this issue. To date, there seems to be no systematic way of constructing asymmetric orbifolds. As a relevant related issue, we also systematically analyzed how to construct asymmetric orbifolds.Dynamics of non-equilibrium theories at finite temperatures is an important aspect of many fields in physics. However, there currently exist not that many quantitative results from first principles. With D. Kusnezov, H. Spohn and others, we have been computing the explicit behavior of physical quantities in non-equilibrium, from first principles. In particular, we have quantitatively shown how local equilibrium and linear response are broken under thermal gradients in some theories. Also, we elucidated the relationship between the microscopic aspects of the theory, such as Lyapunov exponents, and the macroscopic aspects of the theory, such as phase space dimensional loss and transport phenomena, under thermal gradients.The results mentioned above have been published in refereed journals and we are preparing some more publications on them. Furthermore, we believe that the results we have obtained will serve as the basis for further research.