8,105 research outputs found

    Two-Photon-Exchange Effects and Ξ”(1232)\Delta(1232) Deformation

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    The two-photon-exchange (TPE) contribution in epβ†’epΟ€0ep\rightarrow ep\pi ^0 with W=MΞ”W=M_{\Delta} and small Q2Q^2 is calculated and its corrections to the ratios of electromagnetic transition form factors REM=E1+(3/2)/M1+(3/2)R_{EM} = E_{1+}^{(3/2)}/M_{1+}^{(3/2)} and RSM=S1+(3/2)/M1+(3/2)R_{SM} = S_{1+}^{(3/2)}/M_{1+}^{(3/2)}, are analysed. A simple hadronic model is used to estimate the TPE amplitude. Two phenomenological models, MAID2007 and SAID, are used to approximate the full epβ†’epΟ€0ep\rightarrow ep\pi ^0 cross sections which contain both the TPE and the one-photon-exchange (OPE) contributions. The genuine the OPE amplitude is then extracted from an integral equation by iteration. We find that the TPE contribution is not sensitive to whether MAID or SAID is used as input in the region with Q2<2Q^2<2 GeV2^2. It gives small correction to REMR_{EM} while for RSMR_{SM}, the correction is about -10\% at small Ο΅\epsilon and about 1%1\% at large Ο΅\epsilon for Q2β‰ˆ2.5Q^2\approx2.5 GeV2^2. The large correction from TPE at small Ο΅\epsilon must be included in the analysis to get a reliable extraction of RSMR_{SM}.Comment: Talk given at Conference:C16-07-2

    Faster Random Walks By Rewiring Online Social Networks On-The-Fly

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    Many online social networks feature restrictive web interfaces which only allow the query of a user's local neighborhood through the interface. To enable analytics over such an online social network through its restrictive web interface, many recent efforts reuse the existing Markov Chain Monte Carlo methods such as random walks to sample the social network and support analytics based on the samples. The problem with such an approach, however, is the large amount of queries often required (i.e., a long "mixing time") for a random walk to reach a desired (stationary) sampling distribution. In this paper, we consider a novel problem of enabling a faster random walk over online social networks by "rewiring" the social network on-the-fly. Specifically, we develop Modified TOpology (MTO)-Sampler which, by using only information exposed by the restrictive web interface, constructs a "virtual" overlay topology of the social network while performing a random walk, and ensures that the random walk follows the modified overlay topology rather than the original one. We show that MTO-Sampler not only provably enhances the efficiency of sampling, but also achieves significant savings on query cost over real-world online social networks such as Google Plus, Epinion etc.Comment: 15 pages, 14 figure, technical report for ICDE2013 paper. Appendix has all the theorems' proofs; ICDE'201
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