2,274 research outputs found

### Interpretation of High Energy String Scattering in terms of String Configurations

High energy string scattering at fixed momentum transfer, known to be
dominated by Regge trajectory exchange, is interpreted by identifying families
of string states which induce each type of trajectory exchange. These include
the usual leading trajectory $\alpha(t)=\alpha^\prime t+1$ and its daughters as
well as the ``sister'' trajectories $\alpha_m(t)=\alpha(t)/m-(m-1)/2$ and their
daughters. The contribution of the sister $\alpha_m$ to high energy scattering
is dominated by string excitations in the $m^{th}$ mode. Thus, at large $-t$,
string scattering is dominated by wee partons, consistently with a picture of
string as an infinitely composite system of ``constituents'' which carry zero
energy and momentum.Comment: 14 pages, phyzzx, psfig required, Florida Preprint UFIFT-94-

### Classical Effective Field Theory for Weak Ultra Relativistic Scattering

Inspired by the problem of Planckian scattering we describe a classical
effective field theory for weak ultra relativistic scattering in which field
propagation is instantaneous and transverse and the particles' equations of
motion localize to the instant of passing. An analogy with the non-relativistic
(post-Newtonian) approximation is stressed. The small parameter is identified
and power counting rules are established. The theory is applied to reproduce
the leading scattering angle for either a scalar interaction field or
electro-magnetic or gravitational; to compute some subleading corrections,
including the interaction duration; and to allow for non-zero masses. For the
gravitational case we present an appropriate decomposition of the gravitational
field onto the transverse plane together with its whole non-linear action. On
the way we touch upon the relation with the eikonal approximation, some
evidence for censorship of quantum gravity, and an algebraic ring structure on
2d Minkowski spacetime.Comment: 29 pages, 2 figures. v4: Duration of interaction is determined in Sec
4 and detailed in App C. Version accepted for publication in JHE

### Electromagnetic fields of a massless particle and the eikonal

Electromagnetic fields of a massless charged particle are described by a
gauge potential that is almost everywhere pure gauge. Solution of quantum
mechanical wave equations in the presence of such fields is therefore immediate
and leads to a new derivation of the quantum electrodynamical eikonal
approximation. The elctromagnetic action in the eikonal limit is localised on a
contour in a two-dimensional Minkowski subspace of four-dimensional space-time.
The exact S-matrix of this reduced theory coincides with the eikonal
approximation, and represents the generalisatin to electrodynamics of the
approach of 't Hooft and the Verlinde's to Planckian scattering.Comment: The missing overdot -- signifying the $\tau$ differentiation
$\nabla^2 {\dot \Omega}^{+ -} and {\dot k}^{+-}$ in eqs. (23) and (24) -- is
inserted. Also, obsolete macro has been fixed. Plain TeX, 13 page

### Supersymmetry, the Cosmological Constant and a Theory of Quantum Gravity in Our Universe

There are many theories of quantum gravity, depending on asymptotic boundary
conditions, and the amount of supersymmetry. The cosmological constant is one
of the fundamental parameters that characterize different theories. If it is
positive, supersymmetry must be broken. A heuristic calculation shows that a
cosmological constant of the observed size predicts superpartners in the TeV
range. This mechanism for SUSY breaking also puts important constraints on low
energy particle physics models. This essay was submitted to the Gravity
Research Foundation Competition and is based on a longer article, which will be
submitted in the near future

### Classicalization of Gravitons and Goldstones

We establish a close parallel between classicalization of gravitons and
derivatively-coupled Nambu-Goldstone-type scalars. We show, that black hole
formation in high energy scattering process represents classicalization with
the classicalization radius given by Schwarzschild radius of center of mass
energy, and with the precursor of black hole entropy being given by number of
soft quanta composing this classical configuration. Such an entropy-equivalent
is defined for scalar classicalons also and is responsible for exponential
suppression of their decay into small number of final particles. This parallel
works in both ways. For optimists that are willing to hypothesize that gravity
may indeed self-unitarize at high energies via black hole formation, it
illustrates that the Goldstones may not be much different in this respect, and
they classicalize essentially by similar dynamics as gravitons. In the other
direction, it may serve as an useful de-mystifier of
via-black-hole-unitarization process and of the role of entropy in it, as it
illustrates, that much more prosaic scalar theories essentially do the same.
Finally, it illustrates that in both cases classicalization is the defining
property for unitarization, and that it sets-in before one can talk about
accompanying properties, such as entropy and thermality of static classicalons
(black holes). These properties are by-products of classicalization, and their
equivalents can be defined for non-gravitational cases of classicalization.Comment: 23 page

### UV-Completion by Classicalization

We suggest a novel approach to UV-completion of a class of non-renormalizable
theories, according to which the high-energy scattering amplitudes get
unitarized by production of extended classical objects (classicalons), playing
a role analogous to black holes, in the case of non-gravitational theories. The
key property of classicalization is the existence of a classicalizer field that
couples to energy-momentum sources. Such localized sources are excited in
high-energy scattering processes and lead to the formation of classicalons. Two
kinds of natural classicalizers are Nambu-Goldstone bosons (or, equivalently,
longitudinal polarizations of massive gauge fields) and scalars coupled to
energy-momentum type sources. Classicalization has interesting phenomenological
applications for the UV-completion of the Standard Model both with or without
the Higgs. In the Higgless Standard Model the high-energy scattering amplitudes
of longitudinal $W$-bosons self-unitarize via classicalization, without the
help of any new weakly-coupled physics. Alternatively, in the presence of a
Higgs boson, classicalization could explain the stabilization of the hierarchy.
In both scenarios the high-energy scatterings are dominated by the formation of
classicalons, which subsequently decay into many particle states. The
experimental signatures at the LHC are quite distinctive, with sharp
differences in the two cases.Comment: 37 page

### Fundamental Strings as Black Bodies

We show that the decay spectrum of massive excitations in perturbative string
theories is thermal when averaged over the (many) initial degenerate states. We
first compute the inclusive photon spectrum for open strings at the tree level
showing that a black body spectrum with the Hagedorn temperature emerges in the
averaging. A similar calculation for a massive closed string state with winding
and Kaluza-Klein charges shows that the emitted graviton spectrum is thermal
with a "grey-body" factor, which approaches one near extremality. These results
uncover a simple physical meaning of the Hagedorn temperature and provide an
explicit microscopic derivation of the black body spectrum from a unitary $S$
matrix.Comment: some changes in the Discussion section and in the reference list. 11
pages, Late

### Effects of the Generalized Uncertainty Principle on the Inflation Parameters

We investigate the effects of the generalized uncertainty principle on the
inflationary dynamics of the early universe in both standard and braneworld
viewpoint. We choose the Randall-Sundrum II model as our underlying braneworld
scenario. We find that the quantum gravitational effects lead to a spectral
index which is not scale invariant. Also, the amplitude of density fluctuations
is reduced by increasing the strength of quantum gravitational corrections.
However, the tensor-to-scalar ratio increases by incorporation of these quantum
gravity effects. We outline possible manifestations of these quantum gravity
effects in the recent and future observations.Comment: 11 pages, revised version with new references, Accepted for
publication in IJMP

### Effective Action for High-Energy Scattering in Gravity

The multi-Regge effective action is derived directly from the linearized
gravity action. After excluding the redundant field components we separate the
fields into momentum modes and integrate over modes which correspond neither to
the kinematics of scattering nor to the one of exchanged particles. The
effective vertices of scattering and of particle production are obtained as
sums of the contributions from the triple and quartic interaction terms and the
fields in the effective action are defined in terms of the two physical
components of the metric fluctuation.Comment: 15 pages, LATE

### Exact Gravitational Shockwaves and Planckian Scattering on Branes

We obtain a solution describing a gravitational shockwave propagating along a
Randall-Sundrum brane. The interest of such a solution is twofold: on the one
hand, it is the first exact solution for a localized source on a
Randall-Sundrum three-brane. On the other hand, one can use it to study forward
scattering at Planckian energies, including the effects of the continuum of
Kaluza-Klein modes. We map out the different regimes for the scattering
obtained by varying the center-of-mass energy and the impact parameter. We also
discuss exact shockwaves in ADD scenarios with compact extra dimensions.Comment: 19 pages, 3 figures. v2: references added, minor improvements and
small errors correcte

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