6,511 research outputs found
Ferromagnetic barrier induced negative differential conductance on the surface of a topological insulator
We theoretically investigate the effect of the negative differential
conductance of a ferromagnetic barrier on the surface of a topological
insulator. Due to the changes of the shape and position of the Fermi surfaces
in the ferromagnetic barrier, the transport processes can be divided into three
kinds: the total, partial and blockade transmission mechanisms. The bias
voltage can give rise to the transition of the transport processes from partial
to blockade transmission mechanisms, which results in a giant effect of
negative differential conductance. With appropriate structural parameters, the
current-voltage characteristics show that the minimum value of the current can
reach to zero in a wide range of the bias voltage, and a large peak-to-valley
current ratio can be obtained.Comment: 4 figure
Radiative Decays of the Higgs Boson to a Pair of Fermions
We revisit the radiative decays of the Higgs boson to a fermion pair
where denotes a fermion in the Standard Model
(SM). We include the chirality-flipping diagrams via the Yukawa couplings at
the order , the chirality-conserving contributions
via the top-quark loops of the order , and the
electroweak loops at the order . The QED correction is
about and contributes to the running of fermion
masses at a similar level, which should be taken into account for future
precision Higgs physics. The chirality-conserving electroweak-loop processes
are interesting from the observational point of view. First, the branching
fraction of the radiative decay is about a half of
that of , and that of is more than
four orders of magnitude larger than that of , both of which
reach about . The branching fraction of is
of the order . All the leptonic radiative decays are potentially
observable at the LHC Run 2 or the HL-LHC. The kinematic distributions for the
photon energy or the fermion pair invariant mass provide non-ambiguous
discrimination for the underlying mechanisms of the Higgs radiative decay. We
also study the process and evaluate the observability at
the LHC. We find it comparable to the other related studies and better than the
channel in constraining the charm-Yukawa coupling.Comment: 23 pages, 6 figures, 6 tables; Minor corrections, references updated,
version published in JHE
The longitudinal and transverse distributions of the pion wavefunction from the present experimental data on the pion-photon transition form factor
It is noted that the low-energy behavior of the pion-photon transition form
factor is sensitive to the transverse distribution of the
pion wavefunction, and its high-energy behavior is sensitive to the
longitudinal one. Thus a careful study on can provide
helpful information on the pion wavefunction precisely. In this paper, we
present a combined analysis of the data on reported by the
CELLO, the CLEO, the BABAR and the BELLE collaborations. It is performed by
using the method of least squares. By using the combined measurements of BELLE
and CLEO Collaborations, the pion wavefunction longitudinal and transverse
behavior can be fixed to a certain degree, i.e. we obtain and for ,
where and are two parameters of a convenient pion wavefunction
model whose distribution amplitude can mimic the various longitudinal behavior
under proper choice of parameters. We observe that the CELLO, CLEO and BELLE
data are consistent with each other, all of which prefers the asymptotic-like
distribution amplitude; while the BABAR data prefers a more broad distribution
amplitude, such as the CZ-like one.Comment: 7 pages, 10 figure
Pion Electromagnetic Form Factor in the Factorization Formulae
Based on the light-cone (LC) framework and the factorization formalism,
the transverse momentum effects and the different helicity components'
contributions to the pion form factor are recalculated. In
particular, the contribution to the pion form factor from the higher helicity
components (), which come from the spin-space Wigner
rotation, are analyzed in the soft and hard energy regions respectively. Our
results show that the right power behavior of the hard contribution from the
higher helicity components can only be obtained by fully keeping the
dependence in the hard amplitude, and that the dependence in LC
wavefunction affects the hard and soft contributions substantially. A model for
the twist-3 wavefunction of the pion has been
constructed based on the moment calculation by applying the QCD sum rules,
whose distribution amplitude has a better end-point behavior than that of the
asymptotic one. With this model wavefunction, the twist-3 contributions
including both the usual helicity components () and the
higher helicity components () to the pion form
factor have been studied within the modified pQCD approach. Our results show
that the twist-3 contribution drops fast and it becomes less than the twist-2
contribution at . The higher helicity components in the
twist-3 wavefunction will give an extra suppression to the pion form factor.
When all the power contributions, which include higher order in ,
higher helicities, higher twists in DA and etc., have been taken into account,
it is expected that the hard contributions will fit the present experimental
data well at the energy region where pQCD is applicable.Comment: 4 pages, 2 figures, Prepared for International Conference on QCD and
Hadronic Physics, Beijing, China, 16-20 June 200
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