3,038 research outputs found
Screening of Coulomb Impurities in Graphene
We calculate exactly the vacuum polarization charge density in the field of a
subcritical Coulomb impurity, , in graphene. Our analysis is based on
the exact electron Green's function, obtained by using the operator method, and
leads to results that are exact in the parameter , where is
the "fine structure constant" of graphene. Taking into account also
electron-electron interactions in the Hartree approximation, we solve the
problem self-consistently in the subcritical regime, where the impurity has an
effective charge , determined by the localized induced charge. We find
that an impurity with bare charge Z=1 remains subcritical, , for any , while impurities with and higher can become
supercritical at certain values of .Comment: 4 pages, 2 figure
The early evolution of the earth, the beginning of its geological history: how and when the granitoid magmas appeared
The Earth has a number of differences from the planets of the Solar System and other star-planetary systems. These differences were acquired during its formation and geological history. In the early Chaotic eon occurred the accretion of the Earth, the separation of the primary substance of the Earth into a mantle and a nucleus, a satellite of the Earth - the Moon appeared. 4500 Ma ago in the Gadey aeon the geological history of the Earth began. At this time, the endogenous processes on the Earth were controlled to a great extent by meteorite-asteroid bombardments, which caused large-scale melting and differentiation of the upper shells of the Earth. In the magmatic chambers differentiation proceeded until the appearance of melts of granitoid composition. The continental crust of Gadey time was almost completely destroyed by meteoric bombardments, the last heavy bombardment occurred at the end of the Gadey aeon 4000-3900 Ma ago. The geological situation of the Gadey time can be judged only from the preserved zircons from the rocks of that epoch. In particular, their geochemical features indicate that the Earth has an atmosphere. The Gadey eon was replaced by the Archean one, from which the processes of self-organization began to predominate on the Earth. At this time, a crust composed of komatiite-basalt and tonalite-trondhjemite-granodiorite (TTG) series of rocks was formed. In its formation, the processes of sagduction (vertical growth of the crust) over the rising mantle plumes was played the leading role. At the same time the lower basaltic crust was bured in the mantle, eclogitized and melted, which led to the appearance of the sodium series of TTG rocks. At the end of the Archean 3.1-3.0 Ga tectonics of the cover (LID tectonics), which determined the style of the structure and development of the Archean crust, is replaced by the tectonics of small plates, which was later replaced by modern tectonics - the tectonics of plates combined with mantle plumes
Study of Gluon versus Quark Fragmentation in and Events at \sqrt{s}=10 GeV
Using data collected with the CLEO II detector at the Cornell Electron
Storage Ring, we determine the ratio R(chrg) for the mean charged multiplicity
observed in Upsilon(1S)->gggamma events, to the mean charged multiplicity
observed in e+e- -> qqbar gamma events. We find R(chrg)=1.04+/-0.02+/-0.05 for
jet-jet masses less than 7 GeV.Comment: 15 pages, postscript file also available through
http://w4.lns.cornell.edu/public/CLN
Resonant Structure of and Decays
The resonant structure of the four pion final state in the decay is analyzed using 4.27 million pairs
collected by the CLEO II experiment. We search for second class currents in the
decay using spin-parity analysis and establish an
upper limit on the non-vector current contribution. The mass and width of the
resonance are extracted from a fit to the
spectral function. A partial wave analysis of the resonant structure of the
decay is performed; the spectral decomposition of
the four pion system is dominated by the and final
states.Comment: 34 pages postscript, also available through
http://w4.lns.cornell.edu/public/CLN
Observation of Radiative Leptonic Decay of the Tau Lepton
Using 4.68 fb^{-1} of e^+e^- annihilation data collected with the CLEO II
detector at the Cornell Electron Storage Ring (CESR) we have studied tau
radiative decays tau -> mu nu nu gamma and tau -> e nu nu gamma. For a 10 MeV
minimum photon energy in the tau rest frame, the branching fraction of
radiative tau decay to a muon or electron is measured to be
(3.61+-0.16+-0.35)*10^{-3} or (1.75+-0.06+-0.17)*10^{-2}, respectively. The
branching fractions are in agreement with the Standard Model theoretical
predictions.Comment: 11 pages postscript, also available through
http://w4.lns.cornell.edu/public/CLN
Two-Body B Meson Decays to and -- Observation of {'}K$
In a sample of 6.6 million produced B mesons we have observed decays B ->
eta' K, with branching fractions BR(B+ -> eta' K+ = 6.5 +1.5 -1.4 +- 0.9) x
and BR(B0 -> eta' K0 = 4.7 +2.7 -2.0 +- 0.9) x . We have
searched with comparable sensitivity for 17 related decays to final states
containing an eta or eta' meson accompanied by a single particle or low-lying
resonance. Our upper limits for these constrain theoretical interpretations of
the B -> eta' K signal.Comment: 12 page postscript file, postscript file also available through
http://w4.lns.cornell.edu/public/CLN
Measurement of the Partila Width and Form Factor Parameters
We have studied the decay , where . From a fit to the differential decay rate we measure the
rate normalization and form factor slope
, and, using measured values of , find . The resulting branching
fractions are and .
The form factor parameters are in agreement with those measured in decays, as predicted by heavy quark effective theory.Comment: 11 pages, postscript file also available through
http://w4.lns.cornell.edu/public/CLN
Study of Semileptonic Decays of B Mesons to Charmed Baryons
Using data collected by the CLEO II detector at the center-of-mass energy on
or near the Upsilon(4S) resonance, we have determined the 90% confidence level
upper limit B(Bbar --> Lambda_c^+ e- X)/B(Bbar --> Lambda_c^+ X, Lambdabar_c^-
X) < 0.05 for electrons with momentum above 0.6 GeV/c. We have also derived the
ratio B(B^- --> Lambda_c^+ pbar e- nubar_e)/B(Bbar --> Lambda_c^+ pbar X) <
0.04 at the 90% confidence level and measured the ratio B(Bbar --> Lambda_c^+
pbar X)/B(Bbar --> Lambda_c^+ X, Lambdabar_c^- X) = 0.57 +- 0.05 +- 0.05.Comment: 9 page postscript file, postscript file also available through
http://w4.lns.cornell.edu/public/CLN
Further Search for the Two-Photon Production of the Glueball Candidate
The CLEOII detector at the Cornell e+ e- storage ring CESR has been used to
search for the two-photon production of the decaying into pi+ pi-.
No evidence for a signal is found in data corresponding to an integrated
luminosity of 4.77/fb and a 95% CL upper limit on of 2.5 eV is set. If this result is combined with the BES Collaboration's
measurement of in radiative decay, a 95% CL
lower limit on the stickiness of the of 73 is obtained. If the
recent CLEO result for \Gamma_{two-photon} * BR{\K_S K_S} is combined with
the present result, the stickiness of the is found to be larger
than 102 at the 95% CL. These results for the stickiness (the ratio of the
probabilities for two-gluon coupling and two-photon coupling) provide further
support for a substantial neutral parton content in the .Comment: 8 pages, postscript file also available through
http://w4.lns.cornell.edu/public/CLN
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