7,227 research outputs found
Branching ratios, asymmetries and polarizations of decays
We analyzed the nonleptonic decays with by employing the perturbative QCD (PQCD) factorization
approach. Here the branching ratios, the asymmetries and the complete set
of polarization observables are investigated systematically. Besides the
traditional contributions from the factorizable and nonfactorizable diagrams at
the leading order, the next-to-leading order (NLO) vertex corrections could
also provide considerable contributions. The PQCD predictions for the branching
ratios of the decays are consistent
with the measured values within errors. As for decays, the branching ratios can reach the order of and could
be measured in the LHCb and Belle-II experiments. The numerical results show
that the direct asymmetries of the considered decays are very small. Thus
the observation of any large direct asymmetry for these decays will be a
signal for new physics. The mixing induced asymmetries in the neutral
modes are very close to , which suggests that these channels
can give a cross-check on the measurement of the Cabbibo-Kobayashi-Maskawa
(CKM) angle and . We found that the longitudinal polarization
fractions are suppressed to due to the large nonfactorizable
contributions. The magnitudes and phases of the two transverse amplitudes
and are roughly equal, which
is an indication for the approximate light quark helicity conservation in these
decays. The overall polarization observables of and
channels are also in good agreement with the
experimental measurements as reported by LHCb and BaBar. Other results can also
be tested by the LHCb and Belle-II experiments.Comment: 14 pages, 1 figure, 6 table
Semileptonic decays of meson to S-wave charmonium states in the perturbative QCD approach
Inspired by the recent measurement of the ratio of branching fractions
to and final states at the LHCb
detector, we study the semileptonic decays of meson to the S-wave ground
and radially excited 2S and 3S charmonium states with the perturbative QCD
approach. After evaluating the form factors for the transitions , where and denote pseudoscalar and vector S-wave charmonia,
respectively, we calculate the branching ratios for all these semileptonic
decays. The theoretical uncertainty of hadronic input parameters are reduced by
utilizing the light-cone wave function for meson. It is found that the
predicted branching ratios range from up to and could be
measured by the future LHCb experiment. Our prediction for the ratio of
branching fractions is in good
agreement with the data. For decays, the relative
contributions of the longitudinal and transverse polarization are discussed in
different momentum transfer squared regions. These predictions will be tested
on the ongoing and forthcoming experiments.Comment: 12 pages, 3 figures, 5 table
Quasi-two-body decays in the perturbative QCD approach
In this paper, we calculated the branching ratios of the quasi-two-body
decays
by employing the perturbative QCD (PQCD) approach. The contributions from the
-wave resonances , and were taken into
account. The two-pion distribution amplitude is
parameterized by the vector current time-like form factor to study
the considered decay modes. We found that (a) the PQCD predictions for the
branching ratios of the considered quasi-two-body decays are in the order of
, while the two-body decay rates are extracted from those for the
corresponding quasi-two-body decays; (b) the whole pattern of the pion form
factor-squared measured by the BABAR Collaboration could be
understood based on our theoretical results; (c) the general expectation based
on the similarity between and
decays are confirmed: is consistent with the measured
within errors; and (d) new ratios
and among the branching ratios of the
considered decay modes are defined and could be tested by future experiments.Comment: 10 pages, 3 figure
Improving thermoelectric properties of p-type Bi2Te3-based alloys by spark plasma sintering
AbstractHigh-performance (Bi2Te3)x(Sb2Te3)1βx bulk materials were prepared by combining fusion technique with spark plasma sintering, and their thermoelectric properties were investigated. The electrical resistivity and Seebeck coefficient increase greatly and the thermal conductivity decreases significantly with the increase of Bi2Te3 content, which leads to a great improvement in the thermoelectric figure of merit ZT. The maximum ZT value reaches 1.33 at 398 K for the composition of 20%Bi2Te3-80%Sb2Te3 with 3% (mass fraction) excess Te
Nematic crossover in BaFeAs under uniaxial stress
Raman scattering can detect spontaneous point-group symmetry breaking without
resorting to single-domain samples. Here we use this technique to study
, the parent compound of the "122" Fe-based
superconductors. We show that an applied compression along the Fe-Fe direction,
which is commonly used to produce untwinned orthorhombic samples, changes the
structural phase transition at temperature into a crossover
that spans a considerable temperature range above . Even in
crystals that are not subject to any applied force, a distribution of
substantial residual stress remains, which may explain phenomena that are
seemingly indicative of symmetry breaking above . Our results
are consistent with an onset of spontaneous nematicity only below
.Comment: 4 pages, 4 figure
- β¦