36 research outputs found
Threshold effects and Planck scale Lorentz violation: combined constraints from high energy astrophysics
Recent work has shown that dispersion relations with Planck scale Lorentz
violation can produce observable effects at energies many orders of magnitude
below the Planck energy M. This opens a window on physics that may reveal
quantum gravity phenomena. It has already constrained the possibility of Planck
scale Lorentz violation, which is suggested by some approaches to quantum
gravity. In this work we carry out a systematic analysis of reaction
thresholds, allowing unequal deformation parameters for different particle
dispersion relations. The thresholds are found to have some unusual properties
compared with standard ones, such as asymmetric momenta for pair creation and
upper thresholds. The results are used together with high energy observational
data to determine combined constraints. We focus on the case of photons and
electrons, using vacuum Cerenkov, photon decay, and photon annihilation
processes to determine order unity constraints on the parameters controlling
O(E/M) Lorentz violation. Interesting constraints for protons (with photons or
pions) are obtained even at O((E/M)^2), using the absence of vacuum Cerenkov
and the observed GZK cutoff for ultra high energy cosmic rays. A strong
Cerenkov limit using atmospheric PeV neutrinos is possible for O(E/M)
deformations provided the rate is high enough. If detected, ultra high energy
cosmological neutrinos might yield limits at or even beyond O((E/M)^2).Comment: 35 pages, 13 Figures, RevTex4. Version published in PRD. Expanded
introduction, updated discussion of possible constraint if GZK cutoff is
confirmed. Corrected typos. Added and updated reference
High energy constraints on Lorentz symmetry violations
Lorentz violation at high energies might lead to non linear dispersion
relations for the fundamental particles. We analyze observational constraints
on these without assuming any a priori equality between the coefficients
determining the amount of Lorentz violation for different particle species. We
focus on constraints from three high energy processes involving photons and
electrons: photon decay, photo-production of electron-positron pairs, and
vacuum Cerenkov radiation. We find that cubic momentum terms in the dispersion
relations are strongly constrained.Comment: 7 pages, 1 figure, Talk presented at CPT01; the Second Meeting on CPT
and Lorentz Symmetry, Bloomington, Indiana, 15-18 Aug. 2001. Minor numerical
error corrected, gamma-decay constraint update
Modelling Planck-scale Lorentz violation via analogue models
Astrophysical tests of Planck-suppressed Lorentz violations had been
extensively studied in recent years and very stringent constraints have been
obtained within the framework of effective field theory. There are however
still some unresolved theoretical issues, in particular regarding the so called
"naturalness problem" - which arises when postulating that Planck-suppressed
Lorentz violations arise only from operators with mass dimension greater than
four in the Lagrangian. In the work presented here we shall try to address this
problem by looking at a condensed-matter analogue of the Lorentz violations
considered in quantum gravity phenomenology. Specifically, we investigate the
class of two-component BECs subject to laser-induced transitions between the
two components, and we show that this model is an example for Lorentz
invariance violation due to ultraviolet physics. We shall show that such a
model can be considered to be an explicit example high-energy Lorentz
violations where the ``naturalness problem'' does not arise.Comment: Talk given at the Fourth Meeting on Constrained Dynamics and Quantum
Gravity (QG05), Cala Gonone (Sardinia, Italy) September 12-16, 200
Lorentz Invariance and the semiclassical approximation of loop quantum gravity
It is shown that the field equations derived from an effective interaction
hamiltonian for Maxwell and gravitational fields in the semiclassical
approximation of loop quantum gravity using rotational invariant states (such
as weave states) are Lorentz invariant. To derive this result, which is in
agreement with the observational evidence, we use the geometrical properties of
the electromagnetic field.Comment: 6 page
Threshold configurations in the presence of Lorentz violating dispersion relations
A general characterization of lower and upper threshold configurations for
two particle reactions is determined under the assumptions that the single
particle dispersion relations E(p) are rotationally invariant and monotonic in
p, and that energy and momentum are conserved and additive for multiple
particles. It is found that at a threshold the final particle momenta are
always parallel and the initial momenta are always anti-parallel. The
occurrence of new phenomena not occurring in a Lorentz invariant setting, such
as upper thresholds and asymmetric pair production thresholds, is explained,
and an illustrative example is given.Comment: 5 pages, 3 figure
Modified Dispersion Relations from the Renormalization Group of Gravity
We show that the running of gravitational couplings, together with a suitable
identification of the renormalization group scale can give rise to modified
dispersion relations for massive particles. This result seems to be compatible
with both the frameworks of effective field theory with Lorentz invariance
violation and deformed special relativity. The phenomenological consequences
depend on which of the frameworks is assumed. We discuss the nature and
strength of the available constraints for both cases and show that in the case
of Lorentz invariance violation, the theory would be strongly constrained.Comment: revtex4, 9 pages, updated to match published versio
Lorentz violating kinematics: Threshold theorems
Recent tentative experimental indications, and the subsequent theoretical
speculations, regarding possible violations of Lorentz invariance have
attracted a vast amount of attention. An important technical issue that
considerably complicates detailed calculations in any such scenario, is that
once one violates Lorentz invariance the analysis of thresholds in both
scattering and decay processes becomes extremely subtle, with many new and
naively unexpected effects. In the current article we develop several extremely
general threshold theorems that depend only on the existence of some energy
momentum relation E(p), eschewing even assumptions of isotropy or monotonicity.
We shall argue that there are physically interesting situations where such a
level of generality is called for, and that existing (partial) results in the
literature make unnecessary technical assumptions. Even in this most general of
settings, we show that at threshold all final state particles move with the
same 3-velocity, while initial state particles must have 3-velocities
parallel/anti-parallel to the final state particles. In contrast the various
3-momenta can behave in a complicated and counter-intuitive manner.Comment: V1: 32 pages, 6 figures, 3 tables. V2: 5 references adde
Testing Lorentz invariance of dark matter
We study the possibility to constrain deviations from Lorentz invariance in
dark matter (DM) with cosmological observations. Breaking of Lorentz invariance
generically introduces new light gravitational degrees of freedom, which we
represent through a dynamical timelike vector field. If DM does not obey
Lorentz invariance, it couples to this vector field. We find that this coupling
affects the inertial mass of small DM halos which no longer satisfy the
equivalence principle. For large enough lumps of DM we identify a (chameleon)
mechanism that restores the inertial mass to its standard value. As a
consequence, the dynamics of gravitational clustering are modified. Two
prominent effects are a scale dependent enhancement in the growth of large
scale structure and a scale dependent bias between DM and baryon density
perturbations. The comparison with the measured linear matter power spectrum in
principle allows to bound the departure from Lorentz invariance of DM at the
per cent level.Comment: 42 pages, 9 figure
Einstein-aether as a quantum effective field theory
The possibility that Lorentz symmetry is violated in gravitational processes
is relatively unconstrained by experiment, in stark contrast with the level of
accuracy to which Lorentz symmetry has been confirmed in the matter sector. One
model of Lorentz violation in the gravitational sector is Einstein-aether
theory, in which Lorentz symmetry is broken by giving a vacuum expectation
value to a dynamical vector field. In this paper we analyse the effective
theory for quantised gravitational and aether perturbations. We show that this
theory possesses a controlled effective expansion within dimensional
regularisation, that is, for any process there are a finite number of Feynman
diagrams which will contribute to a given order of accuracy. We find that there
is no log-running of the two-derivative phenomenological parameters, justifying
the use of experimental constraints for these parameters obtained over many
orders of magnitude in energy scale. Given the stringent experimental bounds on
two-derivative Lorentz-violating operators, we estimate the size of matter
Lorentz-violation which arises due to loop effects. This amounts to an
estimation of the natural size of coefficients for Lorentz-violating
dimension-six matter operators, which in turn can be used to obtain a new bound
on the two-derivative parameters of this theory.Comment: 21 page