131 research outputs found
The Effective Field Theory of Codimension-two Branes
Distributional sources of matter on codimension-two and higher branes are
only well-defined as regularized objects. Nevertheless, intuition from
effective field theory suggests that the low-energy physics on such branes
should be independent of any high-energy regularization scheme. In this paper,
we address this issue in the context of a scalar field model where matter
fields (the standard model) living on such a brane interact with bulk fields
(gravity). The low-energy effective theory is shown to be consistent and
independent of the regularization scheme, provided the brane couplings are
renormalized appropriately at the classical level. We perform explicit
computations of the classical renormalization group flows at tree and one-loop
level, demonstrate that the theory is renormalizable against codimension-two
divergences, and extend the analysis to several physical applications such as
electrodynamics and brane localized kinetic terms.Comment: 30 pages + appendices, matches journal versio
Unitary null energy condition violation in cosmologies
A non-singular cosmological bounce in the Einstein frame can only take place
if the Null Energy Condition (NEC) is violated. We explore situations where a
single scalar field drives the NEC violation and derive the constraints imposed
by demanding tree level unitarity on a cosmological background. We then focus
on the explicit constraints that arise in P(X) theories and show that
constraints from perturbative unitarity make it impossible for the NEC
violation to occur within the region of validity of the effective field theory
without also involving irrelevant operators that arise at a higher scale that
would enter from integrating out more massive degrees of freedom. Within the
context of P(X) theories we show that including such operators allows for a
bounce that does not manifestly violate tree level unitarity, but at the price
of either imposing a shift symmetry or involving technically unnatural small
operator coefficients within the low-energy effective field theory.Comment: 35 pages, 1 figur
- …