40,582 research outputs found

    Hard X-ray and UV Observations of the 2005 January 15 Two-ribbon Flare

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    In this paper, we present comprehensive analysis of a two-ribbon flare observed in UV 1600{\AA} by Transition Region and Coronal Explorer and in HXRs by Reuven Ramaty High Energy Solar Spectroscopic Imager. HXR (25-100 keV) imaging observations show two kernels of size (FWHM) 15?? moving along the two UV ribbons. We find the following results. (1) UV brightening is substantially enhanced wherever and whenever the compact HXR kernel is passing, and during the HXR transit across a certain region, the UV count light curve in that region is temporally correlated with the HXR total flux light curve. After the passage of the HXR kernel, the UV light curve exhibits smooth monotonical decay. (2)We measure the apparent motion speed of the HXR sources and UV ribbon fronts, and decompose the motion into parallel and perpendicular motions with respect to the magnetic polarity inversion line (PIL). It is found that HXR kernels and UV fronts exhibit similar apparent motion patterns and speeds. The parallel motion dominates during the rise of the HXR emission, and the perpendicular motion starts and dominates at the HXR peak, the apparent motion speed being 10-40 km s-1. (3) We also find that UV emission is characterized by a rapid rise correlated with HXRs, followed by a long decay on timescales of 15-30 minutes. The above analysis provides evidence that UV brightening is primarily caused by beam heating, which also produces thick-target HXR emission. The thermal origin of UV emission cannot be excluded, but would produce weaker heating by one order of magnitude. The extended UV ribbons in this event are most likely a result of sequential reconnection along the PIL, which produces individual flux tubes (post-flare loops), subsequent non-thermal energy release and heating in these flux tubes, and then the very long cooling time of the transition region at the feet of these flux tubes.Comment: 8 figure

    Review of localization for 5d supersymmetric gauge theories

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    We give a pedagogical review of the localization of supersymmetric gauge theory on 5d toric Sasaki-Einstein manifolds. We construct the cohomological complex resulting from supersymmetry and consider its natural toric deformations with all equivariant parameters turned on. We also give detailed discussion on how the Sasaki-Einstein geometry permeates every aspect of the calculation, from Killing spinor, vanishing theorems to the index theorems.Comment: This is a contribution to the review volume `Localization techniques in quantum field theories' (eds. V. Pestun and M. Zabzine) which contains 17 Chapters. The complete volume is summarized in arXiv:1608.02952 and it can be downloaded at https://arxiv.org/src/1608.02952/anc/LocQFT.pdf or http://pestun.ihes.fr/pages/LocalizationReview/LocQFT.pd

    The exclusive Drell-Yan process and deeply virtual pion production

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    In this talk it is reported on analyses of l p -> l pi+ n and pi- p -> l+ l- n within the handbag approach. It is argued that recent measurements of hard pion production performed by HERMES and CLAS clearly indicate the occurrence of strong contributions from transversely polarized photons. The gamma*T -> pi transitions are described by the transversity GPDs accompanied by twist-3 pion wave functions. The experiments also require strong contributions from the pion pole which can be modeled as a classical one-pion exchange. With these extensions the handbag approach leads to results on cross sections and spin asymmetries in fair agreement with experiment. This approach is also used for an estimate of the partial cross sections for the exclusive Drell-Yan process.Comment: 8 pages, 6 figures, latex with style files jpsj-suppl.cls and cite.sty invited talk presented at MENU2016, Kyoto (Japan), July 2016. arXiv admin note: substantial text overlap with arXiv:1602.0380

    Rescattering effects in hadron-nucleus and heavy-ion collisions

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    We review the extension of the factorization formalism of perturbative QCD to {\it coherent} soft rescattering associated with hard scattering in high energy nuclear collisions. We emphasize the ability to quantify high order corrections and the predictive power of factorization approach in terms of universal nonperturbative matrix elements. Although coherent rescattering effects are power suppressed by hard scales of the scattering, they are enhanced by the nuclear size and could play an important role in understanding the novel nuclear dependence observed in high energy nuclear collisions.Comment: 8 pages, 13 figures, to be published in the Proceedings of 1st International Conference on Hard and Electromagnetic Probes of High Energy Nuclear Collisions (Hard Probe 2004), Ericeira, Portugal, Nov. 4-10, 200

    Single transverse-spin asymmetry in Drell-Yan lepton angular distribution

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    We calculate a single transverse-spin asymmetry for the Drell-Yan lepton-pair's angular distribution in perturbative QCD. At leading order in the strong coupling constant, the asymmetry is expressed in terms of a twist-3 quark-gluon correlation function T_F^{(V)}(x_1,x_2). In our calculation, the same result was obtained in both light-cone and covariant gauge in QCD, while keeping explicit electromagnetic current conservation for the virtual photon that decays into the lepton pair. We also present a numerical estimate of the asymmetry and compare the result to an existing other prediction.Comment: 15 pages, Revtex, 5 Postscript figures, uses aps.sty, epsfig.st

    Prediction of destabilizing blade tip forces for shrouded and unshrouded turbines

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    The effect of a nonuniform flow field on the Alford force calculation is investigated. The ideas used here are based on those developed by Horlock and Greitzer. It is shown that the nonuniformity of the flow field does contribute to the Alford force calculation. An attempt is also made to include the effect of whirl speed. The values predicted by the model are compared with those obtained experimentally by Urlicks and Wohlrab. The possibility of using existing turbine tip loss correlations to predict beta is also exploited. The nonuniform flow field induced by the tip clearnance variation tends to increase the resultant destabilizing force over and above what would be predicted on the basis of the local variation of efficiency. On the one hand, the pressure force due to the nonuniform inlet and exit pressure also plays a part even for unshrouded blades, and this counteracts the flow field effects, so that the simple Alford prediction remains a reasonable approximation. Once the efficiency variation with clearance is known, the presented model gives a slightly overpredicted, but reasonably accurate destabilizing force. In the absence of efficiency vs. clearance data, an empirical tip loss coefficient can be used to give a reasonable prediction of destabilizing force. To a first approximation, the whirl does have a damping effect, but only of small magnitude, and thus it can be ignored for some purposes

    QCD and Rescattering in Nuclear Targets

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    We review the extension of the factorization formalism of perturbative QCD to soft rescattering associated with hard processes in nuclei.Comment: Latex 16 pages, including 18 figures, to appear in the Proceedings of Hard Probe Collaboratio

    Solar flare hard X-ray spikes observed by RHESSI: a statistical study

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    Context. Hard X-ray (HXR) spikes refer to fine time structures on timescales of seconds to milliseconds in high-energy HXR emission profiles during solar flare eruptions. Aims. We present a preliminary statistical investigation of temporal and spectral properties of HXR spikes. Methods. Using a three-sigma spike selection rule, we detected 184 spikes in 94 out of 322 flares with significant counts at given photon energies, which were detected from demodulated HXR light curves obtained by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). About one fifth of these spikes are also detected at photon energies higher than 100 keV. Results. The statistical properties of the spikes are as follows. (1) HXR spikes are produced in both impulsive flares and long-duration flares with nearly the same occurrence rates. Ninety percent of the spikes occur during the rise phase of the flares, and about 70% occur around the peak times of the flares. (2) The time durations of the spikes vary from 0.2 to 2 s, with the mean being 1.0 s, which is not dependent on photon energies. The spikes exhibit symmetric time profiles with no significant difference between rise and decay times. (3) Among the most energetic spikes, nearly all of them have harder count spectra than their underlying slow-varying components. There is also a weak indication that spikes exhibiting time lags in high-energy emissions tend to have harder spectra than spikes with time lags in low-energy emissions.Comment: 16 pages, 13 figure

    Nuclear Modification to Parton Distribution Functions and Parton Saturation

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    We introduce a generalized definition of parton distribution functions (PDFs) for a more consistent all-order treatment of power corrections. We present a new set of modified DGLAP evolution equations for nuclear PDFs, and show that the resummed αsA1/3/Q2\alpha_s A^{1/3}/Q^2-type of leading nuclear size enhanced power corrections significantly slow down the growth of gluon density at small-xx. We discuss the relation between the calculated power corrections and the saturation phenomena.Comment: 4 pages, to appear in the proceedings of QM200
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