1,594 research outputs found
Drell-Yan Lepton Angular Distribution at Small Transverse Momentum
We investigate the dependence of the Drell-Yan cross section on lepton polar
and azimuthal angles, as generated by the lowest-order QCD annihilation and
Compton processes. We focus in particular on the azimuthal-angular
distributions, which are of the form cos(phi) and cos(2phi). At small
transverse momentum q_T of the lepton pair, q_T << Q, with Q the pair mass,
these terms are known to be suppressed relative to the phi-independent part of
the Drell-Yan cross section by one or two powers of the transverse momentum.
Nonetheless, as we show, like the phi-independent part they are subject to
large logarithmic corrections, whose precise form however depends on the
reference frame chosen. These logarithmic contributions ultimately require
resummation to all orders in the strong coupling. We discuss the potential
effects of resummation on the various angular terms in the cross section and on
the Lam-Tung relation.Comment: 13 pages, 2 figures; reference added, minor other changes, matches
version to be published in PR
Flow ring valve is simple, quick-acting
Two porting rings, one within the other, control gas or liquid flow by using seal buttons as the sliding valve closers. Multiporting within the ring allows close control of the flow by the slight rotation of the outer porting ring
Synchrotron outbursts in Galactic and extragalactic jets, any difference?
We discuss differences and similarities between jets powered by super-massive black holes in quasars and by stellar-mass black holes in microquasars. The comparison is based on multi-wavelength radio-to-infrared observations of the two active galactic nuclei 3C 273 and 3C 279, as well as the two galactic binaries GRS 1915+105 and Cyg X-3. The physical properties of the jet are derived by fitting the parameters of a shock-in-jet model simultaneously to all available observations. We show that the variable jet emission of galactic sources is, at least during some epochs, very similar to that of extragalactic jets. As for quasars, their observed variability pattern can be well reproduced by the emission of a series of self-similar shock waves propagating down the jet and producing synchrotron outbursts. This suggests that the physical properties of relativistic jets is independent of the mass of the black hol
Synchrotron flaring behaviour of CygnusX-3 during the February-March 1994 and September 2001 outbursts
Aims: In this paper we study whether the shock-in-jet model, widely used to
explain the outbursting behaviour of quasars, can be used to explain the radio
flaring behaviour of the microquasar Cygnus X-3.
Method: We have used a method developed to model the synchrotron outbursts of
quasar jets, which decomposes multifrequency lightcurves into a series of
outbursts. The method is based on the Marscher & Gear (1985) shock model, but
we have implemented the modifications to the model suggested by Bjornsson &
Aslaksen (2000), which make the flux density increase in the initial phase less
abrupt. We study the average outburst evolution as well as specific
characteristics of individual outbursts and physical jet properties of Cyg X-3.
Results: We find that the lightcurves of the February-March 1994 and
September 2001 outbursts can be described with the modified shock model. The
average evolution shows that instead of the expected synchrotron plateau, the
flux density is still increasing during the synchrotron stage. We also find
that high frequency peaking outbursts are shorter in duration than the ones
peaking at lower frequencies. Finally, we show that the method can be used,
complementary to radio interferometric jet imaging, for deriving the physical
parameters such as the magnetic field strength and the energy density of
relativistic electrons in the jet of Cyg X-3.Comment: 8 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
Synchrotron Outbursts in Galactic and Extra-galactic Jets, Any Difference?
We discuss differences and similarities between jets powered by super-massive
black holes in quasars and by stellar-mass black holes in microquasars. The
comparison is based on multi-wavelength radio-to-infrared observations of the
two active galactic nuclei 3C 273 and 3C 279, as well as the two galactic
binaries GRS 1915+105 and Cyg X-3. The physical properties of the jet are
derived by fitting the parameters of a shock-in-jet model simultaneously to all
available observations. We show that the variable jet emission of galactic
sources is, at least during some epochs, very similar to that of extra-galactic
jets. As for quasars, their observed variability pattern can be well reproduced
by the emission of a series of self-similar shock waves propagating down the
jet and producing synchrotron outbursts. This suggests that the physical
properties of relativistic jets is independent of the mass of the black hole.Comment: 4 pages, 1 figure, Proceedings IAU Symposium No. 238. Black Holes:
from Stars to Galaxies - across the Range of Masse
Multiple roles for UV RESISTANCE LOCUS8 in regulating gene expression and metabolite accumulation in arabidopsis under solar ultraviolet radiation
Photomorphogenic responses triggered by low fluence rates of ultraviolet B radiation (UV-B; 280–315 nm) are mediated by the UV-B photoreceptor UV RESISTANCE LOCUS8 (UVR8). Beyond our understanding of the molecular mechanisms of UV-B perception by UVR8, there is still limited information on how the UVR8 pathway functions under natural sunlight. Here, wild-type Arabidopsis (Arabidopsis thaliana) and the uvr8-2 mutant were used in an experiment outdoors where UV-A (315–400 nm) and UV-B irradiances were attenuated using plastic films. Gene expression, PYRIDOXINE BIOSYNTHESIS1 (PDX1) accumulation, and leaf metabolite signatures were analyzed. The results show that UVR8 is required for transcript accumulation of genes involved in UV protection, oxidative stress, hormone signal transduction, and defense against herbivores under solar UV. Under natural UV-A irradiance, UVR8 is likely to interact with UV-A/blue light signaling pathways to moderate UV-B-driven transcript and PDX1 accumulation. UVR8 both positively and negatively affects UV-A-regulated gene expression and metabolite accumulation but is required for the UV-B induction of phenolics. Moreover, UVR8-dependent UV-B acclimation during the early stages of plant development may enhance normal growth under long-term exposure to solar UV
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