2,148 research outputs found
Renormalization group improved pQCD prediction for leptonic decay
The complete next-to-next-to-next-to-leading order short-distance and
bound-state QCD corrections to leptonic decay rate
has been finished by Beneke {\it et al.}
\cite{Beneke:2014qea}. Based on those improvements, we present a
renormalization group (RG) improved pQCD prediction for by applying the principle of maximum conformality (PMC). The PMC
is based on RG-invariance and is designed to solve the pQCD renormalization
scheme and scale ambiguities. After applying the PMC, all known-type of
-terms at all orders, which are controlled by the RG-equation, are
resummed to determine optimal renormalization scale for its strong running
coupling at each order. We then achieve a more convergent pQCD series, a
scheme- independent and more accurate pQCD prediction for
leptonic decay, i.e. keV, where the uncertainty is the squared average of
the mentioned pQCD errors. This RG-improved pQCD prediction agrees with the
experimental measurement within errors.Comment: 11 pages, 4 figures. Numerical results and discussions improved,
references updated, to be published in JHE
AT2019 avd: A tidal disruption event with a two-phase evolution
Tidal disruption events (TDEs) can uncover the quiescent super-massive black
holes (SMBHs) at the center of galaxies and also offer a promising method to
study them. After the disruption of a star by a SMBH, the highly elliptical
orbit of the debris stream will be gradually circularized due to the
self-crossing, and then the circularized debris will form an accretion disk.
The recent TDE candidate AT 2019avd has double peaks in its optical light
curve, and the X-ray emerges near the second peak. The durations of the peaks
are about 400 and 600 days, respectively, and the separation between them is ~
700 days. We fit and analyse its spectral energy distribution (SED) in
optical/UV, mid-infrared, and X-ray bands. We find that this source can be
interpreted as the circularization process in the first phase plus the delayed
accretion process in the second phase. Under this two-phase scenario, we use
the succession of self-crossing circularization model to fit the first peak,
and the delayed accretion model to fit the second peak. The fitting results are
consistent with the partial disruption of a 0.9 M_sun star by a 7 * 10^6 M_sun
SMBH with the penetration factor \beta ~ 0.6. Furthermore, we find the
large-amplitude (by factors up to ~ 5) X-ray variability in AT 2019avd can be
interpreted as the rigid-body precession of the misaligned disk due to the
Lense-Thirring effect of a spinning SMBH, with the disk precession period of 10
- 25 days.Comment: 14 pages, 11 figures, 4 tables, Submitted to Ap
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