2,168 research outputs found
Competitions of magnetism and superconductivity in FeAs-based materials
Using the numerical unrestricted Hartree-Fock approach, we study the ground
state of a two-orbital model describing newly discovered FeAs-based
superconductors. We observe the competition of a mode spin-density
wave and the superconductivity as the doping concentration changes. There might
be a small region in the electron-doping side where the magnetism and
superconductivity coexist. The superconducting pairing is found to be spin
singlet, orbital even, and mixed s + d wave (even
parity).Comment: 5 pages, 3 figure
Coexistence of Itinerant Electrons and Local Moments in Iron-Based Superconductors
In view of the recent experimental facts in the iron-pnictides, we make a
proposal that the itinerant electrons and local moments are simultaneously
present in such multiband materials. We study a minimal model composed of
coupled itinerant electrons and local moments to illustrate how a consistent
explanation of the experimental measurements can be obtained in the leading
order approximation. In this mean-field approach, the spin-density-wave (SDW)
order and superconducting pairing of the itinerant electrons are not directly
driven by the Fermi surface nesting, but are mainly induced by their coupling
to the local moments. The presence of the local moments as independent degrees
of freedom naturally provides strong pairing strength for superconductivity and
also explains the normal-state linear-temperature magnetic susceptibility above
the SDW transition temperature. We show that this simple model is supported by
various anomalous magnetic properties and isotope effect which are in
quantitative agreement with experiments.Comment: 7 pages, 4 figures; an expanded versio
Measurement of the branching fractions of psi(2S) -> 3(pi+pi-) and J/psi -> 2(pi+pi-)
Using data samples collected at sqrt(s) = 3.686GeV and 3.650GeV by the BESII
detector at the BEPC, the branching fraction of psi(2S) -> 3(pi+pi-) is
measured to be [4.83 +- 0.38(stat) +- 0.69(syst)] x 10^-4, and the relative
branching fraction of J/psi -> 2(pi+pi-) to that of J/psi -> mu+mu- is measured
to be [5.86 +- 0.19(stat) +- 0.39(syst)]% via psi(2S) -> (pi+pi-)J/psi, J/psi
-> 2(pi+pi-). The electromagnetic form factor of 3(pi+pi-) is determined to be
0.21 +- 0.02 and 0.20 +- 0.01 at sqrt(s) = 3.686GeV and 3.650GeV, respectively.Comment: 17pages, 7 figures, submitted to Phys. Rev.
Higher-order multipole amplitude measurement in
Using events collected with the BESIII detector at
the BEPCII storage ring, the higher-order multipole amplitudes in the radiative
transition are measured.
A fit to the production and decay angular distributions yields
and , where the first
errors are statistical and the second systematic. Here denotes the
normalized magnetic quadrupole amplitude and the normalized electric
octupole amplitude. This measurement shows evidence for the existence of the
signal with statistical significance and is consistent with
the charm quark having no anomalous magnetic moment.Comment: 14 pages, 4 figure
Observation of electron-antineutrino disappearance at Daya Bay
The Daya Bay Reactor Neutrino Experiment has measured a non-zero value for
the neutrino mixing angle with a significance of 5.2 standard
deviations. Antineutrinos from six 2.9 GW reactors were detected in
six antineutrino detectors deployed in two near (flux-weighted baseline 470 m
and 576 m) and one far (1648 m) underground experimental halls. With a 43,000
ton-GW_{\rm th}-day livetime exposure in 55 days, 10416 (80376) electron
antineutrino candidates were detected at the far hall (near halls). The ratio
of the observed to expected number of antineutrinos at the far hall is
. A rate-only analysis
finds in a
three-neutrino framework.Comment: 5 figures. Version to appear in Phys. Rev. Let
Search for psi(3770)\ra\rho\pi at the BESII detector at the Beijing Electron-Positron Collider
Non- decay \psppto \rhopi is searched for using a data sample of
taken at the center-of-mass energy of 3.773 GeV by the
BESII detector at the BEPC. No \rhopi signal is observed, and the upper limit
of the cross section is measured to be \sigma(\EETO \rhopi)<6.0 pb at 90% C.
L. Considering the interference between the continuum amplitude and the \pspp
resonance amplitude, the branching fraction of \pspp decays to is
determined to be \BR(\pspp\ra\rho\pi)\in(6.0\times10^{-6}, 2.4\times10^{-3})
at 90% C. L. This is in agreement with the prediction of the - and -wave
mixing scheme of the charmonium states for solving the ``\rhopi puzzle''
between \jpsi and \psp decays.Comment: 15 pages, 5 figure
Measurements of J/psi decays into phi pi^0, phi eta, and phi eta^prime
Based on 5.8x10^7 J/psi events detected in BESII, the branching fractions of
J/psi--> phi eta and phi eta^prime are measured for different eta and eta^prime
decay modes. The results are significantly higher than previous measurements.
An upper limit on B(J/psi--> phi pi^0) is also obtained.Comment: 9 pages, 10 figure
Observation and study of the decay
We report the observation and study of the decay
using events
collected with the BESIII detector. Its branching fraction, including all
possible intermediate states, is measured to be
. We also report evidence for a structure,
denoted as , in the mass spectrum in the GeV/
region. Using two decay modes of the meson ( and
), a simultaneous fit to the mass spectra is
performed. Assuming the quantum numbers of the to be , its
significance is found to be 4.4, with a mass and width of MeV/ and MeV, respectively, and a
product branching fraction
. Alternatively, assuming , the
significance is 3.8, with a mass and width of MeV/ and MeV, respectively, and a product
branching fraction
. The angular distribution of
is studied and the two assumptions of the
cannot be clearly distinguished due to the limited statistics. In all
measurements the first uncertainties are statistical and the second systematic.Comment: 10 pages, 6 figures and 4 table
Observation of and confirmation of its large branching fraction
The baryonic decay is observed, and the
corresponding branching fraction is measured to be
, where the first uncertainty is statistical
and second systematic. The data sample used in this analysis was collected with
the BESIII detector operating at the BEPCII double-ring collider with
a center-of-mass energy of 4.178~GeV and an integrated luminosity of
3.19~fb. The result confirms the previous measurement by the CLEO
Collaboration and is of greatly improved precision, which may deepen our
understanding of the dynamical enhancement of the W-annihilation topology in
the charmed meson decays
Observation of p pbar pi^0 and p pbar eta in psi' decays
The processes psi'-->p pbar pi^0 and psi'-->p pbar eta are studied using a
sample of 14 million psi' decays collected with the Beijing Spectrometer at the
Beijing Electron-Positron Collider. The branching fraction of psi'-->p pbar
pi^0 is measured with improved precision as (13.2\pm 1.0\pm 1.5)\times 10^{-5},
and psi'-->p pbar eta is observed for the first time with a branching fraction
of (5.8\pm 1.1\pm 0.7)\times 10^{-5}, where the first errors are statistical
and the second ones are systematic.Comment: 15 pages, 8 figures and 3 table
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