430 research outputs found
Do we need NLO Parton Distributions?
We discuss the uncertainty on processes computed using next-to-next-to
leading (NNLO) parton distributions (PDFs) due to the neglect of higher order
perturbative corrections in the PDF determination, in the specific case of
Higgs production in gluon fusion. By studying the behaviour of the perturbative
series for this process, we show that this uncertainty is negligible in
comparison to the theoretical uncertainty on the matrix element. We then take
this as a case study for the use of the Cacciari-Houdeau method for the
estimate of theoretical uncertainties, and show that the method provides an
effective way of treating theoretical uncertainties on the matrtix element and
the PDF on the same footing.Comment: 10 pages 5 figures. Final version, to be published in Phys. Lett. B.
Comparison with top production (figs 4-5) added. Several typos corrected and
references updated. Grant info adde
Solving the Altarelli-Parisi equations with truncated moments
The technique of truncated moments of parton distributions allows us to study
scaling violations without making any assumption on the shape of parton
distributions. The numerical implementation of the method is however difficult,
since the evolution equations for truncated moments are not diagonal. We
present a simple way to improve the efficiency of the numerical solution of the
evolution equations for truncated moments. As a result, the number of truncated
moments needed to achieve the required precision in the evolution is
significantly smaller than in the original formulation of the technique. The
method presented here can also be used to obtain the value of parton
distributions in terms of truncated moments, and therefore it can be viewed as
a technique for the solution of the Altarelli-Parisi equations.Comment: 10 pages Late
Evolution of truncated moments of singlet parton distributions
We define truncated Mellin moments of parton distributions by restricting the
integration range over the Bjorken variable to the experimentally accessible
subset x_0 < x < 1 of the allowed kinematic range 0 < x < 1. We derive the
evolution equations satisfied by truncated moments in the general (singlet)
case in terms of an infinite triangular matrix of anomalous dimensions which
couple each truncated moment to all higher moments with orders differing by
integers. We show that the evolution of any moment can be determined to
arbitrarily good accuracy by truncating the system of coupled moments to a
sufficiently large but finite size, and show how the equations can be solved in
a way suitable for numerical applications. We discuss in detail the accuracy of
the method in view of applications to precision phenomenology.Comment: 23 pages, 6 figures, LaTeX; factors of 2nf in Appendix C correcte
Are anonymity-seekers just like everybody else? An analysis of contributions to Wikipedia from Tor
User-generated content sites routinely block contributions from users of
privacy-enhancing proxies like Tor because of a perception that proxies are a
source of vandalism, spam, and abuse. Although these blocks might be effective,
collateral damage in the form of unrealized valuable contributions from
anonymity seekers is invisible. One of the largest and most important
user-generated content sites, Wikipedia, has attempted to block contributions
from Tor users since as early as 2005. We demonstrate that these blocks have
been imperfect and that thousands of attempts to edit on Wikipedia through Tor
have been successful. We draw upon several data sources and analytical
techniques to measure and describe the history of Tor editing on Wikipedia over
time and to compare contributions from Tor users to those from other groups of
Wikipedia users. Our analysis suggests that although Tor users who slip through
Wikipedia's ban contribute content that is more likely to be reverted and to
revert others, their contributions are otherwise similar in quality to those
from other unregistered participants and to the initial contributions of
registered users.Comment: To appear in the IEEE Symposium on Security & Privacy, May 202
Neural network approach to parton distributions fitting
We will show an application of neural networks to extract information on the
structure of hadrons. A Monte Carlo over experimental data is performed to
correctly reproduce data errors and correlations. A neural network is then
trained on each Monte Carlo replica via a genetic algorithm. Results on the
proton and deuteron structure functions, and on the nonsinglet parton
distribution will be shown.Comment: 4 pages, 5 eps figures. Talk given by Andrea Piccione at the "X
International Workshop on Advanced Computing and Analysis Techniques in
Physics Research", ACAT 2005, DESY-Zeuthen, Germany, 22-27 May 2005.
Corrected fig.
Determination of alpha_s from scaling violations of truncated moments of structure functions
We determine the strong coupling alpha_s(M_Z) from scaling violations of
truncated moments of the nonsinglet deep inelastic structure function F_2.
Truncated moments are determined from BCDMS and NMC data using a neural network
parametrization which retains the full experimental information on errors and
correlations. Our method minimizes all sources of theoretical uncertainty and
bias which characterize extractions of alpha_s from scaling violations. We
obtain alpha_s(M_Z) = 0.124 +0.004-0.007 (exp.) + 0.003- 0.004 (th.).Comment: 24 pages, 4 figures, latex with epsfig; neural network
parametrization available from http://sophia.ecm.ub.es/f2neura
Cooperation Between Stations in Wireless Networks
In a wireless network, mobile nodes (MNs) repeatedly perform tasks such as layer 2 (L2) handoff, layer 3 (L3) handoff and authentication. These tasks are critical, particularly for real-time applications such as VoIP. We propose a novel approach, namely Cooperative Roaming (CR), in which MNs can collaborate with each other and share useful information about the network in which they move. We show how we can achieve seamless L2 and L3 handoffs regardless of the authentication mechanism used and without any changes to either the infrastructure or the protocol. In particular, we provide a working implementation of CR and show how, with CR, MNs can achieve a total L2+L3 handoff time of less than 16 ms in an open network and of about 21 ms in an IEEE 802.11i network. We consider behaviors typical of IEEE 802.11 networks, although many of the concepts and problems addressed here apply to any kind of mobile network
Neural network determination of parton distributions: the nonsinglet case
We provide a determination of the isotriplet quark distribution from
available deep--inelastic data using neural networks. We give a general
introduction to the neural network approach to parton distributions, which
provides a solution to the problem of constructing a faithful and unbiased
probability distribution of parton densities based on available experimental
information. We discuss in detail the techniques which are necessary in order
to construct a Monte Carlo representation of the data, to construct and evolve
neural parton distributions, and to train them in such a way that the correct
statistical features of the data are reproduced. We present the results of the
application of this method to the determination of the nonsinglet quark
distribution up to next--to--next--to--leading order, and compare them with
those obtained using other approaches.Comment: 46 pages, 18 figures, LaTeX with JHEP3 clas
Recommended from our members
Seamless Layer-2 Handoff using Two Radios in IEEE 802.11 Wireless Networks
We reduces the false handoff probability significantly by introducing selective passive scanning
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Passive Duplicate Address Detection for Dynamic Host Configuration Protocol (DHCP)
During a layer-3 handoff, address acquisition via DHCP is often the dominant source of handoff delay, duplicate address detection (DAD) being responsible for most of the delay. We propose a new DAD algorithm, passive DAD (pDAD), which we show to be effective, yet introduce only a few milliseconds of delay. Unlike traditional DAD, pDAD also detects the unauthorized use of an IP address before it is assigned to a DHCP client
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