195 research outputs found
Computing the R^4 term at two Superstring Loops
We use a previously derived integral representation for the four graviton
amplitude at two loops in Superstring theory, whose leading term for vanishing
momenta gives the two-loop contribution to the R^4 term in the Effective
Action. We find by an explicit computation that this contribution is zero, in
agreement with a general argument implying the vanishing of the R^4 term beyond
one loop.Comment: 6 pages, Late
The decay of massive closed superstrings with maximum angular momentum
We study the decay of a very massive closed superstring (i.e. \alpha' M^2>>
1) in the unique state of maximum angular momentum. This is done in flat
ten-dimensional spacetime and in the regime of weak string coupling, where the
dominant decay channel is into two states of masses M_1, M_2. We find that the
lifetime surprisingly grows with the first power of the mass M: T =c \alpha' M.
We also compute the decay rate for each values of M_1, M_2. We find that, for
large M, the dynamics selects only special channels of decay: modulo processes
which are exponentially suppressed, for every decay into a state of given mass
M_1, the mass M_2 of the other state is uniquely determined.Comment: 22 pages, 4 figure
Anyon Mean Field as an Exact Limit of a Gauge Theory
We summarize a study of an Abelian gauge theory in 2+1 dimensions, the gauge
field being coupled to nonrelativistic Fermions. The Action for the gauge field
is a combination of the Maxwell term and a Chern-Simons (CS) term. We study the
limit of vanishing Fermions' charge, keeping fixed the gauge field mass induced
by the CS term. By considering a closed surface, in particular a torus to keep
translational invariance, we show that the Fermions do not decouple completely
from the gauge field, and in fact they behave as Anyons treated in the
-translationally invariant formulation of the- Mean Field approximation. We
describe the exact solution of this limiting case.Comment: 8 pages, LaTe
Thermodynamics of vortex lines in layered superconductors
We study the dissipative thermodynamics of vortex lines in layered
superconductors within a simple string model in the dilute limit of negligible
vortex interactions and compute the specific heat in presence of
arbitrary dissipation. The interplay of dissipation, inertia and elasticity is
shown to control the qualitative thermodynamical behavior and their relative
amount determines two very distinct regimes for the specific heat. In the
dissipation dominated case we find a behavior for a large
interval of temperature below .Comment: 10 pages, RevTe
Explicit Modular Invariant Two-Loop Superstring Amplitude Relevant for R^4
In this note we derive an explicit modular invariant formula for the two loop
4-point amplitude in superstring theory in terms of a multiple integral (7
complex integration variables) over the complex plane which is shown to be
convergent. We consider in particular the case of the leading term for
vanishing momenta of the four graviton amplitude, which would correspond to the
two-loop correction of the R^4 term in the effective Action. The resulting
expression is not positive definite and could be zero, although we cannot see
that it vanishes.Comment: 18 pages, the introduction (2.5 pages) is a comprehensive summary
including the main resul
Evidence for Nonvanishing Cosmological Constant in NonSusy Superstring Models
We reanalyse the computation of the cosmological constant at two
loops in recently proposed Superstring models without massless gravitini, both
in the theta-function based formalism and by a detailed computation in the more
explicit hyperelliptic description of the underlying genus two Riemann surface.
is expressed as the integral over the surface moduli of an amplitude
which is zero if susy is not completely broken, but we find it to be
nonvanishing in the susy breaking models which can be given an explicitly
workable fermionic formulation. Thus unfortunately the issue of getting
realistic and perturbatively viable models from Superstring Theory remains
open.Comment: 30 pages, Late
Semiclassical decay of strings with maximum angular momentum
We study the classical breaking of a highly excited (closed or open) string
state on the leading Regge trajectory, represented by a rotating soliton
solution, and we find the resulting solutions for the outgoing two pieces,
describing two specific excited string states. This classical picture
reproduces very accurately the precise analytical relation of the masses
and of the decay products found in a previous quantum computation. The
decay rate is naturally described in terms of a semiclassical formula. We also
point out some interesting features of the evolution after the splitting
process.Comment: 18 pages, latex, 7 figure
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