29,759 research outputs found
Probing the Structure of the Pomeron
We suggest that the pseudo-rapidity cut dependence of diffractive
deep-inelastic scattering events at HERA may provide a sensitive test of models
of diffraction. A comparison with the experimental cross section shows that the
Donnachie-Landshoff model and a simple two-gluon exchange model of the pomeron
model are disfavoured. However a model with a direct coupling of the pomeron to
quarks is viable for a harder quark--pomeron form factor, as is a model based
on the leading-twist operator contribution. We also consider a direct-coupling
scalar pomeron model. We comment on the implications of these results for the
determination of the partonic structure of the pomeron.Comment: 33 pages, LaTeX2e, 14 figures, uses axodra
The Classification of the Simply Laced Berger Graphs from Calabi-Yau spaces
The algebraic approach to the construction of the reflexive polyhedra that
yield Calabi-Yau spaces in three or more complex dimensions with K3 fibres
reveals graphs that include and generalize the Dynkin diagrams associated with
gauge symmetries. In this work we continue to study the structure of graphs
obtained from reflexive polyhedra. The objective is to describe the
``simply laced'' cases, those graphs obtained from three dimensional spaces
with K3 fibers which lead to symmetric matrices. We study both the affine and,
derived from them, non-affine cases. We present root and weight structurea for
them. We study in particular those graphs leading to generalizations of the
exceptional simply laced cases and . We show how
these integral matrices can be assigned: they may be obtained by relaxing the
restrictions on the individual entries of the generalized Cartan matrices
associated with the Dynkin diagrams that characterize Cartan-Lie and affine
Kac-Moody algebras. These graphs keep, however, the affine structure present in
Kac-Moody Dynkin diagrams. We conjecture that these generalized simply laced
graphs and associated link matrices may characterize generalizations of
Cartan-Lie and affine Kac-Moody algebras
Neutrino Oscillations Induced by Gravitational Recoil Effects
Quantum gravitational fluctuations of the space-time background, described by
virtual D branes, may induce neutrino oscillations if a tiny violation of the
Lorentz invariance (or a violation of the equivalence principle) is imposed. In
this framework, the oscillation length of massless neutrinos turns out to be
proportional to M/E^2, where E is the neutrino energy and M is the mass scale
characterizing the topological fluctuations in the vacuum. Such a functional
dependence on the energy is the same obtained in the framework of loop quantum
gravity.Comment: 5 pages, LaTex fil
Implications of Anomalous U(1) Symmetry in Unified Models: the Flipped SU(5) x U(1) Paradigm
A generic feature of string-derived models is the appearance of an anomalous
Abelian U(1)_A symmetry which, among other properties, constrains the Yukawa
couplings and distinguishes the three families from each other. In this paper,
we discuss in a model-independent way the general constraints imposed by such a
U(1)_A symmetry on fermion masses, R-violating couplings and proton-decay
operators in a generic flipped SU(5) x U(1)' model. We construct all possible
viable fermion mass textures and give various examples of effective low-energy
models which are distinguished from each other by their different predictions
for B-, L- and R-violating effects. We pay particular attention to predictions
for neutrino masses, in the light of the recent Super-Kamiokande data.Comment: 28 pages, reference adde
Integrability of irrotational silent cosmological models
We revisit the issue of integrability conditions for the irrotational silent
cosmological models. We formulate the problem both in 1+3 covariant and 1+3
orthonormal frame notation, and show there exists a series of constraint
equations that need to be satisfied. These conditions hold identically for
FLRW-linearised silent models, but not in the general exact non-linear case.
Thus there is a linearisation instability, and it is highly unlikely that there
is a large class of silent models. We conjecture that there are no spatially
inhomogeneous solutions with Weyl curvature of Petrov type I, and indicate
further issues that await clarification.Comment: Minor corrections and improvements; 1 new reference; to appear Class.
Quantum Grav.; 16 pages Ioplpp
Dynamical Formation of Horizons in Recoiling D Branes
A toy calculation of string/D-particle interactions within a world-sheet
approach indicates that quantum recoil effects - reflecting the gravitational
back-reaction on space-time foam due to the propagation of energetic particles
- induces the appearance of a microscopic event horizon, or `bubble', inside
which stable matter can exist. The scattering event causes this horizon to
expand, but we expect quantum effects to cause it to contract again, in a
`bounce' solution. Within such `bubbles', massless matter propagates with an
effective velocity that is less than the velocity of light in vacuo, which may
lead to observable violations of Lorentz symmetry that may be tested
experimentally. The conformal invariance conditions in the interior geometry of
the bubbles select preferentially three for the number of the spatial
dimensions, corresponding to a consistent formulation of the interaction of D3
branes with recoiling D particles, which are allowed to fluctuate independently
only on the D3-brane hypersurface.Comment: 25 pages LaTeX, 4 eps figures include
Background Dependent Lorentz Violation: Natural Solutions to the Theoretical Challenges of the OPERA Experiment
To explain both the OPERA experiment and all the known phenomenological
constraints/observations on Lorentz violation, the Background Dependent Lorentz
Violation (BDLV) has been proposed. We study the BDLV in a model independent
way, and conjecture that there may exist a "Dream Special Relativity Theory",
where all the Standard Model (SM) particles can be subluminal due to the
background effects. Assuming that the Lorentz violation on the Earth is much
larger than those on the interstellar scale, we automatically escape all the
astrophysical constraints on Lorentz violation. For the BDLV from the effective
field theory, we present a simple model and discuss the possible solutions to
the theoretical challenges of the OPERA experiment such as the Bremsstrahlung
effects for muon neutrinos and the pion decays. Also, we address the Lorentz
violation constraints from the LEP and KamLAMD experiments. For the BDLV from
the Type IIB string theory with D3-branes and D7-branes, we point out that the
D3-branes are flavour blind, and all the SM particles are the conventional
particles as in the traditional SM when they do not interact with the
D3-branes. Thus, we not only can naturally avoid all the known phenomenological
constraints on Lorentz violation, but also can naturally explain all the
theoretical challenges. Interestingly, the energy dependent photon velocities
may be tested at the experiments.Comment: RevTex4, 14 pages, minor corrections, references adde
- …