505 research outputs found
Enhanced Bound State Formation in Two Dimensions via Stripe-Like Hopping Anisotropies
We have investigated two-electron bound state formation in a square
two-dimensional t-J-U model with hopping anisotropies for zero electron
density; these anisotropies are introduced to mimic the hopping energies
similar to those expected in stripe-like arrangements of holes and spins found
in various transition metal oxides. In this report we provide analytical
solutions to this problem, and thus demonstrate that bound-state formation
occurs at a critical exchange coupling, J_c, that decreases to zero in the
limit of extreme hopping anisotropy t_y/t_x -> 0. This result should be
contrasted with J_c/t = 2 for either a one-dimensional chain, or a
two-dimensional plane with isotropic hopping. Most importantly, this behaviour
is found to be qualitatively similar to that of two electrons on the two-leg
ladder problem in the limit of t_interchain/t_intrachain -> 0. Using the latter
result as guidance, we have evaluated the pair correlation function, thus
determining that the bound state corresponds to one electron moving along one
chain, with the second electron moving along the opposite chain, similar to two
electrons confined to move along parallel, neighbouring, metallic stripes. We
emphasize that the above results are not restricted to the zero density limit -
we have completed an exact diagonalization study of two holes in a 12 X 2
two-leg ladder described by the t-J model and have found that the
above-mentioned lowering of the binding energy with hopping anisotropy persists
near half filling.Comment: 6 pages, 3 eps figure
The rare decays B --> K(*) anti-K(*) and R-parity violating supersymmetry
We study the branching ratios, the direct CP asymmetries in decays and the polarization fractions of decays by employing the QCD factorization in the minimal
supersymmetric standard model with R-parity violation. We derive the new upper
bounds on the relevant R-parity violating couplings from the latest
experimental data of , and some of these constraints
are stronger than the existing bounds. Using the constrained parameter spaces,
we predict the R-parity violating effects on the other quantities in decays which have not been measured yet. We find that the
R-parity violating effects on the branching ratios and the direct
asymmetries could be large, nevertheless their effects on the longitudinal
polarizations of decays are small. Near future
experiments can test these predictions and shrink the parameter spaces.Comment: 31 pages with 10 figure
Spin and energy transfer in nanocrystals without transport of charge
We describe a mechanism of spin transfer between individual quantum dots that
does not require tunneling. Incident circularly-polarized photons create
inter-band excitons with non-zero electron spin in the first quantum dot. When
the quantum-dot pair is properly designed, this excitation can be transferred
to the neighboring dot via the Coulomb interaction with either {\it
conservation} or {\it flipping} of the electron spin. The second dot can
radiate circularly-polarized photons at lower energy. Selection rules for spin
transfer are determined by the resonant conditions and by the strong spin-orbit
interaction in the valence band of nanocrystals. Coulomb-induced energy and
spin transfer in pairs and chains of dots can become very efficient under
resonant conditions. The electron can preserve its spin orientation even in
randomly-oriented nanocrystals.Comment: 13 pages, 3 figure
Anti-Search for the Glueball Candidate f_J(2220) in Two-Photon Interactions
Using 13.3 fb^{-1} of e^+e^- data recorded with the CLEO II and CLEO II.V
detector configurations at CESR, we have searched for f_J(2220) decays to
K^0_{S} K^0_{S} in untagged two-photon interactions. We report an upper limit
on the product of the two-photon partial width and the branching fraction,
Gamma_gamma gamma cdot B (f_J(2220) to K^0_{S} K^0_{S}) of less than 1.1 eV at
the 95% C.L: systematic uncertainties are included. This dataset is four times
larger than that used in the previous CLEO publication.Comment: 10 pages postscript, also available through
http://w4.lns.cornell.edu/public/CLNS, Submitted to PRD (R
Study of the B^0 Semileptonic Decay Spectrum at the Upsilon(4S) Resonance
We have made a first measurement of the lepton momentum spectrum in a sample
of events enriched in neutral B's through a partial reconstruction of B0 -->
D*- l+ nu. This spectrum, measured with 2.38 fb**-1 of data collected at the
Upsilon(4S) resonance by the CLEO II detector, is compared directly to the
inclusive lepton spectrum from all Upsilon(4S) events in the same data set.
These two spectra are consistent with having the same shape above 1.5 GeV/c.
From the two spectra and two other CLEO measurements, we obtain the B0 and B+
semileptonic branching fractions, b0 and b+, their ratio, and the production
ratio f+-/f00 of B+ and B0 pairs at the Upsilon(4S). We report b+/b0=0.950
(+0.117-0.080) +- 0.091, b0 = (10.78 +- 0.60 +- 0.69)%, and b+ = (10.25 +- 0.57
+- 0.65)%. b+/b0 is equivalent to the ratio of charged to neutral B lifetimes,
tau+/tau0.Comment: 14 page, postscript file also available at
http://w4.lns.cornell.edu/public/CLN
Update of the Search for the Neutrinoless Decay
We present an update of the search for the lepton family number violating
decay using a complete CLEO II data sample of 12.6 million
pairs. No evidence of a signal has been found and the
corresponding upper limit is \BR(\tau \to \mu\gamma) < 1.0 \times 10^{-6}
at 90% CL, significantly smaller than previous limits. All quoted results are
preliminary.Comment: 9 pages postscript, also available through
http://w4.lns.cornell.edu/public/CLN
Radiative Decay Modes of the Meson
Using data recorded by the CLEO-II detector at CESR we have searched for four
radiative decay modes of the meson: ,
, , and . We
obtain 90% CL upper limits on the branching ratios of these modes of , , and
respectively.Comment: 15 page postscript file, postscript file also available through
http://w4.lns.cornell.edu/public/CLN
Measurement of the Mass Splittings between the States
We present new measurements of photon energies and branching fractions for
the radiative transitions: Upsilon(2S)->gamma+chi_b(J=0,1,2). The masses of the
chi_b states are determined from the measured radiative photon energies. The
ratio of mass splittings between the chi_b substates,
r==(M[J=2]-M[J=1])/(M[J=1]-M[J=0]) with M the chi_b mass, provides information
on the nature of the bbbar confining potential. We find
r(1P)=0.54+/-0.02+/-0.02. This value is in conflict with the previous world
average, but more consistent with the theoretical expectation that r(1P)<r(2P);
i.e., that this mass splittings ratio is smaller for the chi_b(1P) triplet than
for the chi_b(2P) triplet.Comment: 11 page postscript file, postscript file also available through
http://w4.lns.cornell.edu/public/CLN
Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in √s = 7 TeV pp collisions with the ATLAS detector
A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fb−1 of proton–proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results
Jet size dependence of single jet suppression in lead-lead collisions at sqrt(s(NN)) = 2.76 TeV with the ATLAS detector at the LHC
Measurements of inclusive jet suppression in heavy ion collisions at the LHC
provide direct sensitivity to the physics of jet quenching. In a sample of
lead-lead collisions at sqrt(s) = 2.76 TeV corresponding to an integrated
luminosity of approximately 7 inverse microbarns, ATLAS has measured jets with
a calorimeter over the pseudorapidity interval |eta| < 2.1 and over the
transverse momentum range 38 < pT < 210 GeV. Jets were reconstructed using the
anti-kt algorithm with values for the distance parameter that determines the
nominal jet radius of R = 0.2, 0.3, 0.4 and 0.5. The centrality dependence of
the jet yield is characterized by the jet "central-to-peripheral ratio," Rcp.
Jet production is found to be suppressed by approximately a factor of two in
the 10% most central collisions relative to peripheral collisions. Rcp varies
smoothly with centrality as characterized by the number of participating
nucleons. The observed suppression is only weakly dependent on jet radius and
transverse momentum. These results provide the first direct measurement of
inclusive jet suppression in heavy ion collisions and complement previous
measurements of dijet transverse energy imbalance at the LHC.Comment: 15 pages plus author list (30 pages total), 8 figures, 2 tables,
submitted to Physics Letters B. All figures including auxiliary figures are
available at
http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2011-02
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