23 research outputs found
TeV Astrophysics Constraints on Planck Scale Lorentz Violation
We analyze observational constraints from TeV astrophysics on Lorentz
violating nonlinear dispersion for photons and electrons without assuming any a
priori equality between the photon and electron parameters. The constraints
arise from thresholds for vacuum Cerenkov radiation, photon decay and
photo-production of electron-positron pairs. We show that the parameter plane
for cubic momentum terms in the dispersion relations is constrained to an order
unity region in Planck units. We find that the threshold configuration can
occur with an asymmetric distribution of momentum for pair creation, and with a
hard photon for vacuum Cerenkov radiation.Comment: 4 pages, RevTeX4, 1 figure. Some references and a footnote added,
improved discussion on the photon annihilation and GZK cutoff. Minor changes
of wording. Main results unchanged. Version to appear as a Rapid
Communication in PR
Observations of quasi-periodic solar X-ray emission as a result of MHD oscillations in a system of multiple flare loops
We investigate the solar flare of 20 October 2002. The flare was accompanied
by quasi-periodic pulsations (QPP) of both thermal and nonthermal hard X-ray
emissions (HXR) observed by RHESSI in the 3-50 keV energy range. Analysis of
the HXR time profiles in different energy channels made with the Lomb
periodogram indicates two statistically significant time periods of about 16
and 36 seconds. The 36-second QPP were observed only in the nonthermal HXR
emission in the impulsive phase of the flare. The 16-second QPP were more
pronounced in the thermal HXR emission and were observed both in the impulsive
and in the decay phases of the flare. Imaging analysis of the flare region, the
determined time periods of the QPP and the estimated physical parameters of
magnetic loops in the flare region allow us to interpret the observations as
follows. 1) In the impulsive phase energy was released and electrons were
accelerated by successive acts with the average time period of about 36 seconds
in different parts of two spatially separated, but interacting loop systems of
the flare region. 2) The 36-second periodicity of energy release could be
caused by the action of fast MHD oscillations in the loops connecting these
flaring sites. 3) During the first explosive acts of energy release the MHD
oscillations (most probably the sausage mode) with time period of 16 seconds
were excited in one system of the flare loops. 4) These oscillations were
maintained by the subsequent explosive acts of energy release in the impulsive
phase and were completely damped in the decay phase of the flare.Comment: 14 pages, 4 figure
Phenomenology of Particle Production and Propagation in String-Motivated Canonical Noncommutative Spacetime
We outline a phenomenological programme for the search of effects induced by
(string-motivated) canonical noncommutative spacetime. The tests we propose are
based, in analogy with a corresponding programme developed over the last few
years for the study of Lie-algebra noncommutative spacetimes, on the role of
the noncommutativity parameters in the dispersion relation. We focus on
the role of deformed dispersion relations in particle-production collision
processes, where the noncommutativity parameters would affect the threshold
equation, and in the dispersion of gamma rays observed from distant
astrophysical sources. We emphasize that the studies here proposed have the
advantage of involving particles of relatively high energies, and may therefore
be less sensitive to "contamination" (through IR/UV mixing) from the UV sector
of the theory. We also explore the possibility that the relevant deformation of
the dispersion relations could be responsible for the experimentally-observed
violations of the GZK cutoff for cosmic rays and could have a role in the
observation of hard photons from distant astrophysical sources.Comment: With respect to the experimental information available at the time of
writing version 1 of this manuscript (hep-th/0109191v1) the situation has
evolved significantly. Our remarks on the benefits of high-energy
observations found additional encouragement from the results reported in
hep-th/020925
New insights into the genetic etiology of Alzheimer's disease and related dementias
Characterization of the genetic landscape of Alzheimer's disease (AD) and related dementias (ADD) provides a unique opportunity for a better understanding of the associated pathophysiological processes. We performed a two-stage genome-wide association study totaling 111,326 clinically diagnosed/'proxy' AD cases and 677,663 controls. We found 75 risk loci, of which 42 were new at the time of analysis. Pathway enrichment analyses confirmed the involvement of amyloid/tau pathways and highlighted microglia implication. Gene prioritization in the new loci identified 31 genes that were suggestive of new genetically associated processes, including the tumor necrosis factor alpha pathway through the linear ubiquitin chain assembly complex. We also built a new genetic risk score associated with the risk of future AD/dementia or progression from mild cognitive impairment to AD/dementia. The improvement in prediction led to a 1.6- to 1.9-fold increase in AD risk from the lowest to the highest decile, in addition to effects of age and the APOE ε4 allele