9,165 research outputs found
Search for a new Resonance decaying into Top-Antitop at Tevatron
In this report a new search for a narrow-width heavy resonance decaying into
top quark pairs (X -> ttbar) in ppbar collisions at sqrt(s)=1.96 TeV has been
performed using data collected by the D0 detector at the Fermilab Tevatron
collider. The analysis considers ttbar candidate events in the lepton+jets
channel using a lifetime tag to identify b-jets and the ttbar invariant mass
distribution to search for evidence of resonant production. The analyzed
dataset corresponds to an integrated luminosity of approximately 370 pb^-1.
Since no evidence for a ttbar resonance X is found, upper limits on sigma(X) x
B(X -> ttbar) for different hypothesized resonance masses using a Bayesian
approach are set. Within a topcolor-assisted technicolor model, the existence
of a leptophobic Z' boson with M(Z') < 680 GeV and width Gamma(Z') = 0.012
M(Z') can be excluded at 95% C.L..Comment: Talk given at the "International Europhysics Conference on High
Energy Physics", July 21st - 27th 2005, Lisboa, Portuga
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Chemical ageing and transformation of diffusivity in semi-solid multi-component organic aerosol particles
Recent experimental evidence underlines the importance of reduced diffusivity in amorphous semi-solid or glassy atmospheric aerosols. This paper investigates the impact of
diffusivity on the ageing of multi-component reactive organic particles representative of atmospheric cooking aerosols. We apply and extend the recently developed KM-SUB
model in a study of a 12-component mixture containing oleic and palmitoleic acids. We demonstrate that changes in the diffusivity may explain the evolution of chemical loss
rates in ageing semi-solid particles, and we resolve surface and bulk processes under transient reaction conditions considering diffusivities altered by oligomerisation. This new model treatment allows prediction of the ageing of mixed organic multi-component
aerosols over atmospherically relevant time scales and conditions. We illustrate the impact of changing diffusivity on the chemical half-life of reactive components in semisolid particles, and we demonstrate how solidification and crust formation at the particle
surface can affect the chemical transformation of organic aerosols
Non-Repudiation in Internet Telephony
We present a concept to achieve non-repudiation for natural language
conversations over the Internet. The method rests on chained electronic
signatures applied to pieces of packet-based, digital, voice communication. It
establishes the integrity and authenticity of the bidirectional data stream and
its temporal sequence and thus the security context of a conversation. The
concept is close to the protocols for Voice over the Internet (VoIP), provides
a high level of inherent security, and extends naturally to multilateral
non-repudiation, e.g., for conferences. Signatures over conversations can
become true declarations of will in analogy to electronically signed, digital
documents. This enables binding verbal contracts, in principle between
unacquainted speakers, and in particular without witnesses. A reference
implementation of a secure VoIP archive is exhibited.Comment: Accepted full research paper at IFIP sec2007, Sandton, South Africa,
14-16 May 200
A secure archive for Voice-over-IP conversations
An efficient archive securing the integrity of VoIP-based two-party
conversations is presented. The solution is based on chains of hashes and
continuously chained electronic signatures. Security is concentrated in a
single, efficient component, allowing for a detailed analysis.Comment: 9 pages, 2 figures. (C) ACM, (2006). This is the author's version of
the work. It is posted here by permission of ACM for your personal use. Not
for redistribution. The definitive version was published in Proceedings of
VSW06, June, 2006, Berlin, German
Variable Metric Random Pursuit
We consider unconstrained randomized optimization of smooth convex objective
functions in the gradient-free setting. We analyze Random Pursuit (RP)
algorithms with fixed (F-RP) and variable metric (V-RP). The algorithms only
use zeroth-order information about the objective function and compute an
approximate solution by repeated optimization over randomly chosen
one-dimensional subspaces. The distribution of search directions is dictated by
the chosen metric.
Variable Metric RP uses novel variants of a randomized zeroth-order Hessian
approximation scheme recently introduced by Leventhal and Lewis (D. Leventhal
and A. S. Lewis., Optimization 60(3), 329--245, 2011). We here present (i) a
refined analysis of the expected single step progress of RP algorithms and
their global convergence on (strictly) convex functions and (ii) novel
convergence bounds for V-RP on strongly convex functions. We also quantify how
well the employed metric needs to match the local geometry of the function in
order for the RP algorithms to converge with the best possible rate.
Our theoretical results are accompanied by numerical experiments, comparing
V-RP with the derivative-free schemes CMA-ES, Implicit Filtering, Nelder-Mead,
NEWUOA, Pattern-Search and Nesterov's gradient-free algorithms.Comment: 42 pages, 6 figures, 15 tables, submitted to journal, Version 3:
majorly revised second part, i.e. Section 5 and Appendi
Seismotectonic study of the Fergana region (Southern Kyrgyzstan): distribution and kinematics of local seismicity
We present new seismicity and focal-mechanism data for the Fergana basin and surrounding mountain belts in western Kyrgyzstan from a temporary local seismic network. A total of 210 crustal earthquakes with hypocentral depths shallower than 25 km were observed during a 12-month period in 2009/2010. The hypocenter distribution indicates a complex net of seismically active structures. The seismicity derived in this study is mainly concentrated at the edges of the Fergana basin, whereas the observed rate of seismicity within the basin is low. The seismicity at the dominant tectonic feature of the region, the Talas-Fergana fault, is likewise low, so the fault seems to be inactive or locked. To estimate the uncertainties of earthquake locations derived in this study, a strong explosion with known origin time and location is used as a ground truth calibration event which suggests a horizontal and vertical accuracy of about 1 km for our relocations. We derived 35 focal mechanisms using first motion polarities and retrieved a set of nine moment tensor solutions for earthquakes with moment magnitude (Mw) ranging from 3.3 to 4.9 by waveform inversion. The solutions reveal both thrust and strike-slip mechanisms compatible with a NW-SE direction of compression for the Fergana region. Two previously unknown tectonic structures in the Fergana region could be identified, both featuring strike-slip kinematics. The combined analysis of the results derived in this study allowed a detailed insight into the currently active tectonic structures and their kinematics where little information had previously been available
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