784,348 research outputs found
Light-by-light scattering in Double-Logarithmic Approximation
In the present paper we consider the elastic 2 -> 2 -scattering of virtual
photons at high energies in the forward kinematics at zero and non-zero values
of t. Accounting for both gluon and quark double-logarithmic (DL) contributions
to all orders in the QCD coupling, we obtain explicit expressions for
amplitudes of this process in Double-Logarithmic Approximation (DLA). First we
keep the QCD coupling fixed and then account for running coupling effects.
Applying the saddle-point method to the obtained expressions for the scattering
amplitude, we calculate the high-energy asymptotics of the amplitude, which
proved to be of the Regge form. The Reggeon bears the vacuum quantum numbers
and therefore it is a new, DL contribution to Pomeron. Comparison of the DL
Pomeron to the BFKL Pomeron shows that contribution of the DL Pomeron to the
high-energy asymptotics is of the same order as contribution of the BFKL
Pomeron, so the DL Pomeron should be taken into account together with the BFKL
Pomeron. We estimate the applicability region for the asymptotics of the
light-by-light scattering amplitude, where the the DL Pomeron can reliably
represent the parent amplitude.Comment: 16 pages, 4 figures. Multiple errors are corrected, 2 figs are added,
comparizon to BFKL is done in more detai
Two-loop low-energy effective action in Abelian supersymmetric Chern-Simons matter models
We compute two-loop low-energy effective actions in Abelian Chern-Simons
matter models with N=2 and N=3 supersymmetry up to four-derivative order.
Calculations are performed with a slowly-varying gauge superfield background.
Though the gauge superfield propagator depends on the gauge fixing parameter,
it is shown that the obtained results are independent of this parameter. In the
massless case the considered models are superconformal. We demonstrate that the
superconformal symmetry strongly restricts the form of two-loop quantum
corrections to the effective actions such that the obtained terms have simpler
structure than the analogous ones in the effective action of three-dimensional
supersymmetric electrodynamics (SQED) with vanishing topological mass.Comment: 1+30 pages; v2: references added, misprints corrected; v3: minor
changes, published versio
X-ray Brightening and UV Fading of Tidal Disruption Event ASASSN-15oi
We present late-time observations by Swift and XMM-Newton of the tidal
disruption event (TDE) ASASSN-15oi that reveal that the source brightened in
the X-rays by a factor of one year after its discovery, while it faded
in the UV/optical by a factor of . The XMM-Newton observations
measure a soft X-ray blackbody component with eV,
corresponding to radiation from several gravitational radii of a central black hole. The last Swift epoch taken almost 600 days after
discovery shows that the X-ray source has faded back to its levels during the
UV/optical peak. The timescale of the X-ray brightening suggests that the X-ray
emission could be coming from delayed accretion through a newly forming debris
disk, and that the prompt UV/optical emission is from the prior circularization
of the disk through stream-stream collisions. The lack of spectral evolution
during the X-ray brightening disfavors ionization breakout of a TDE "veiled" by
obscuring material. This is the first time a TDE has been shown to have a
delayed peak in soft X-rays relative to the UV/optical peak, which may be the
first clear signature of the real-time assembly of a nascent accretion disk,
and provides strong evidence for the origin of the UV/optical emission from
circularization, as opposed to reprocessed emission of accretion radiation.Comment: Accepted for publication in ApJ Letter
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