218 research outputs found
Electron correlation effects in electron-hole recombination in organic light-emitting diodes
We develop a general theory of electron--hole recombination in organic light
emitting diodes that leads to formation of emissive singlet excitons and
nonemissive triplet excitons. We briefly review other existing theories and
show how our approach is substantively different from these theories. Using an
exact time-dependent approach to the interchain/intermolecular charge-transfer
within a long-range interacting model we find that, (i) the relative yield of
the singlet exciton in polymers is considerably larger than the 25% predicted
from statistical considerations, (ii) the singlet exciton yield increases with
chain length in oligomers, and, (iii) in small molecules containing nitrogen
heteroatoms, the relative yield of the singlet exciton is considerably smaller
and may be even close to 25%. The above results are independent of whether or
not the bond-charge repulsion, X_perp, is included in the interchain part of
the Hamiltonian for the two-chain system. The larger (smaller) yield of the
singlet (triplet) exciton in carbon-based long-chain polymers is a consequence
of both its ionic (covalent) nature and smaller (larger) binding energy. In
nitrogen containing monomers, wavefunctions are closer to the noninteracting
limit, and this decreases (increases) the relative yield of the singlet
(triplet) exciton. Our results are in qualitative agreement with
electroluminescence experiments involving both molecular and polymeric light
emitters. The time-dependent approach developed here for describing
intermolecular charge-transfer processes is completely general and may be
applied to many other such processes.Comment: 19 pages, 11 figure
Improved KL->pi e nu Form Factor and Phase Space Integral with Reduced Model Uncertainty
Using the published KTeV sample of 2 million KL-> pi e nu decays and a new
form factor expansion with a rigorous bound on higher order terms, we present a
new determination of the KL->pi e nu form factor and phase space integral.
Compared to the previous KTeV result, the uncertainty in the new form factor
expansion is negligible and results in an overall uncertainty in the phase
space integral (IKe) that is a factor of two smaller: IKe = 0.15392 +- 0.00048
\.Comment: 3 pages, 2 figures, submitted to PRD Rapid Communicatio
Measurements of the Decay
The E799-II (KTeV) experiment at Fermilab has collected 83262 events above a background of 79 events. We measure a decay width,
normalized to the (\pi^0 \to \gamma\gamma, \pi^0 to
\gamma\gamma, \pi^0_D \to e^+e^-\gamma) decay width, of K_L \to
e^+e^-\gamma. We also measure parameters of two form factor models. In the Bergstrom, Masso, and Singer
(BMS) parametrization, we find \caks = -0.517 \pm 0.030_{stat} \pm
0.022_{syst}. We separately fit for the first parameter of the D'Ambrosio,
Isidori, and Portoles (DIP) model and find \adip = -1.729 \pm 0.043_{stat} \pm
0.028_{syst}.Comment: 5 pages, 3 figures, submitted to PR
Electroproduction, photoproduction, and inverse electroproduction of pions in the first resonance region
Methods are set forth for determining the hadron electromagnetic structure in
the sub--threshold timelike region of the virtual-photon ``mass'' and
for investigating the nucleon weak structure in the spacelike region from
experimental data on the process at low energies. These
methods are formulated using the unified description of photoproduction,
electroproduction, and inverse electroproduction of pions in the first
resonance region in the framework of the dispersion-relation model and on the
basis of the model-independent properties of inverse electroproduction.
Applications of these methods are also shown.Comment: The revised published version; Revtex4, 18 pages, 6 figure
Search for the Rare Decay KL --> pi0 ee
The KTeV/E799 experiment at Fermilab has searched for the rare kaon decay
KL--> pi0ee. This mode is expected to have a significant CP violating
component. The measurement of its branching ratio could support the Standard
Model or could indicate the existence of new physics. This letter reports new
results from the 1999-2000 data set. One event is observed with an expected
background at 0.99 +/- 0.35 events. We set a limit on the branching ratio of
3.5 x 10^(-10) at the 90% confidence level. Combining the results with the
dataset taken in 1997 yields the final KTeV result: BR(KL --> pi0 ee) < 2.8 x
10^(-10) at 90% CL.Comment: 4 pages, three figure
Search for Light Gluinos via the Spontaneous Appearance of pi+pi- Pairs with an 800 GeV/c Proton Beam at Fermilab
We searched for the appearance of pi+pi- pairs with invariant mass greater
than 648 MeV in a neutral beam. Such an observation could signify the decay of
a long-lived light neutral particle. We find no evidence for this decay. Our
null result severely constrains the existence of an R0 hadron, which is the
lightest bound state of a gluon and a light gluino, and thereby also the
possibility of a light gluino. Depending on the photino mass, we exclude the R0
in the mass and lifetime ranges of 1.2 -- 4.6 GeV and 2E-10 -- 7E-4 seconds,
respectively. (To Appear in Phys. Rev. Lett.)Comment: Documentstyle aps,epsfig,prl (revtex), 6 pages, 7 figure
Radiative Decay Width Measurements of Neutral Kaon Excitations Using the Primakoff Effect
We produce a sample consisting of 147 candidate events, with minimal
backgrounds, of the mixed axial vector pair K1(1270)-K1(1400) by exciting Kl's
in the Coulomb field of lead and report the first measurements of the radiative
widths Gamma_r(K1(1400)) = 280.8+-23.2(stat)+-40.4(syst) keV and
Gamma_r(K1(1270)) = 73.2+- 6.1(stat)+-28.3(syst) keV. We also place 90% CL
upper limits Gamma_r(K*(1410)) < 52.9 keV for the vector state and
Gamma_r(K2*(1430)) < 5.4 keV for the tensor state. These measurements allow for
significant tests of quark-model predictions of radiative widths for the
low-lying vector mesons.Comment: PRL-size article, 4 figure
Search for the Decay K_L -> pi^0 nu nubar using pi^0 -> e^+ e^- gamma
We report on a search for the decay K_L -> pi^0 nu nubar, carried out as a
part of E799-II, a rare K_L decay experiment at Fermilab. Within the Standard
Model, the K_L -> pi^0 nu nubar decay is dominated by direct CP violating
processes, and thus an observation of the decay implies confirmation of direct
CP violation. Due to theoretically clean calculations, a measurement of B(K_L
-> pi^0 nu nubar) is one of the best ways to determine the CKM parameter eta.
No events were observed, and we set an upper limit B(K_L -> pi^0 nu nubar) <
5.9 times 10^-7 at the 90% confidence level.Comment: 5 pages, 4 figure
A Measurement of the Branching Ratio of
We report on a study of the decay carried out as
a part of the KTeV/E799 experiment at Fermilab. The 1997 data yielded a sample
of 1543 events, including an expected background of events. An
effective form factor was determined from the observed distribution of the
invariant mass. Using this form factor in the calculation of the
detector acceptance, the branching ratio was measured to be .Comment: 5 pages, 4 figure
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