5,726 research outputs found
Charged pion photoproduction with the Delta(1232) baryon beyond the resonance region
We investigate the charged pion photoproduction off the proton target with
the Delta(1232) baryon in the final state, i.e. gamma p -> pi^- Delta^++(1232)
and gamma p -> pi^+ Delta^0(1232), based on the effective Lagrangian method,
beyond the resonance region, E_cm >~ GeV. We employ the pi- and rho-meson Regge
trajectories in the t-channel, in addition to the proton- and Delta-pole, in
the s- and u-channels respectively, and the contact-interaction contributions.
A specific scheme for the form factor which satisfies the Ward-Takahashi
identity, crossing symmetry, and on-shell condition, is taken into account. To
discuss the validity of the Regge approach within the Effective Lagrangian
method, we also consider a smooth interpolation between the Regge- and
Feynman-propagators for the t-channel meson exchanges as a function of sqrt(s).
We present the numerical results for the energy and angular dependences of the
cross sections, and double-polarization observable. It turns out that the
present framework shows the significance of the pi-exchange and contact
interaction terms to reproduce the experimental data qualitatively well.
Especially, the interpolation between the two propagators plays a crucial role
to reproduce the high-energy experimental data. The pi^+ decay-angle
distribution is also studied using the Delta^++ decay frame, i.e. the
Gottfried-Jackson frame. The present results will be a useful guide for future
high-energy photon-beam experiments.Comment: 14 pages, 24 figure
Lambda(1520) photoprodcution with Regge contribution
In this talk, we report our recent progresses on the Lambda(1520)
photoproduction using the effective Lagrangian approach. In addition to the
tree-level Born diagrams, we take into account the Regge-trajectories for the
possible strange-meson exchanges in the t channel. We compute the angular and
energy dependences of the production process, including polarization
observables, such as the photon-beam asymmetry and the polarization-transfer
coefficients, resulting in good qualitative agreement with current experimental
data. We also compute the K^- angle distribution function in the
Gottfried-Jackson frame, using the polarization-transfer coefficients in the z
direction.Comment: 3 pages, 1 figure, Talk given at the international conference
BARYONS'10, 7~11 Dec 2010, Osaka, Japa
Beyond the Dark matter effective field theory and a simplified model approach at colliders
Direct detection of and LHC search for the singlet fermion dark matter (SFDM)
model with Higgs portal interaction are considered in a renormalizable model
where the full Standard Model (SM) gauge symmetry is imposed by introducing a
singlet scalar messenger. In this model, direct detection is described by an
effective operator m_q \bar{q} q \bar{\chi} \chi as usual, but the full
amplitude for monojet + \not E_T involves two intermediate scalar propagators,
which cannot be seen within the effective field theory (EFT) or in the
simplified model without the full SM gauge symmetry. We derive the collider
bounds from the ATLAS monojet + \not E_T as well as the CMS t\bar{t} + \not E_T
data, finding out that the bounds and the interpretation of the results are
completely different from those obtained within the EFT or simplified models.
It is pointed out that it is important to respect unitarity, renormalizability
and local gauge invariance of the SM.Comment: 7 pages, 3 figures, version published in Phys. Lett.
Competition between structural distortion and magnetic moment formation in fullerene C
We investigated the effect of on-site Coulomb interactions on the structural
and magnetic ground state of the fullerene C based on
density-functional-theory calculations within the local density approximation
plus on-site Coulomb corrections (LDA+). The total energies of the high
symmetry () and distorted () structures of C were
calculated for different spin configurations. The ground state configurations
were found to depend on the forms of exchange-correlation potentials and the
on-site Coulomb interaction parameter , reflecting the subtle nature of the
competition between Jahn-Teller distortion and magnetic instability in
fullerene C. While the non-magnetic state of the distorted
structure is robust for small , a magnetic ground state of the undistorted
structure emerges for larger than 4 eV when the LDA
exchange-correlation potential is employed.Comment: 4 figures, 1 tabl
Evidence for a preformed Cooper pair model in the pseudogap spectra of a Ca10(Pt4As8)(Fe2As2)5 single crystal with a nodal superconducting gap
For high-Tc superconductors, clarifying the role and origin of the pseudogap
is essential for understanding the pairing mechanism. Among the various models
describing the pseudogap, the preformed Cooper pair model is a potential
candidate. Therefore, we present experimental evidence for the preformed Cooper
pair model by studying the pseudogap spectrum observed in the optical
conductivity of a Ca10(Pt4As8)(Fe2As2)5 (Tc = 34.6 K) single crystal. We
observed a clear pseudogap structure in the optical conductivity and observed
its temperature dependence. In the superconducting (SC) state, one SC gap with
a gap size of {\Delta} = 26 cm-1, a scattering rate of 1/{\tau} = 360 cm-1 and
a low-frequency extra Drude component were observed. Spectral weight analysis
revealed that the SC gap and pseudogap are formed from the same Drude band.
This means that the pseudogap is a gap structure observed as a result of a
continuous temperature evolution of the SC gap observed below Tc. This provides
clear experimental evidence for the preformed Cooper pair model.Comment: 15 pages, 4 figure
Temperature-dependent evolutions of excitonic superfluid plasma frequency in a srong excitonic insulator candidate, TaNiSe
We investigate an interesting anisotropic van der Waals material,
TaNiSe, using optical spectroscopy. TaNiSe has been
known as one of the few excitonic insulators proposed over 50 years ago.
TaNiSe has quasi-one dimensional chains along the -axis. We have
obtained anisotropic optical properties of a single crystal TaNiSe
along the - and -axes. The measured - and -axis optical
conductivities exhibit large anisotropic electronic and phononic properties.
With regard to the -axis optical conductivity, a sharp peak near 3050
cm at 9 K, with a well-defined optical gap ( 1800
cm) and a strong temperature-dependence, is observed. With an increase
in temperature, this peak broadens and the optical energy gap closes around
325 K(). The spectral weight redistribution with respect to the
frequency and temperature indicates that the normalized optical energy gap
() is . The
temperature-dependent superfluid plasma frequency of the excitonic condensation
in TaNiSe has been determined from measured optical data. Our
findings may be useful for future research on excitonic insulators.Comment: 17 pages, 5 figure
Theory of Luminescent Emission in Nanocrystal ZnS:Mn with an Extra Electron
We consider the effect of an extra electron injected into a doped quantum dot
. The Coulomb interaction and the exchange interaction between the
extra electron and the states of the Mn ion will mix the wavefunctions, split
the impurity energy levels, break the previous selection rules and change the
transition probabilities. Using this model of an extra electron in the doped
quantum dot, we calculated the energy and the wavefunctions, the luminescence
probability and the transition lifetime and compare with the experiments. Our
calculation shows that two orders of magnitudes of lifetime shortening can
occur in the transition when an extra electron is present.Comment: 15 pages, 2 Figs No change in Fig
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