450 research outputs found
Model of charge and magnetic order formation in itinerant electron systems
We propose a simple model of charge and/or magnetic order formation in
systems containing both localized and itinerant electrons coupled by the
on-site, spin-dependent interaction that represents Coulomb repulsion and
Hund's rule (a generalized Falicov-Kimball model). Ground state properties of
the model are analyzed on the square lattice on a basis of the phase diagrams
that have been constructed rigorously, but in a restricted configurational
space. For intermediate values of the coupling constants there are considerable
ranges of itinerant electron densities where phases with complex charge and
magnetic structures of the localized electrons have lower energy than the
simplest antiferro- and ferromagnetic ones. A strong tendency towards the
antiferromagnetic coupling between spins of localized electrons has been
observed close to half-filling for any density of localized electrons,
including situations where the magnetic ions are diluted. For small band
fillings the ferromagnetic coupling between localized spins is predominant.Comment: 13 pages, 5 figure
Geosocial Networking: A Case from Ann Arbor, Michigan
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/91626/1/Geosocial.pd
Comment on "Hara's theorem in the constituent quark model"
It is pointed out that current conservation alone does not suffice to prove
Hara's theorem as it was claimed recently. By explicit calculation we show that
the additional implicit assumption made in such "proofs" is that of a
sufficiently localized current.Comment: 8 pages, Late
First observation of a narrow charm-strange meson DsJ(2632) -> Ds eta and D0 K+
We report the first observation of a charm-strange meson DsJ(2632) at a mass
of 2632.6+/-1.6 MeV/c^2 in data from SELEX, the charm hadro-production
experiment E781 at Fermilab. This state is seen in two decay modes, Ds eta and
D0 K+. In the Ds eta decay mode we observe an excess of 49.3 events with a
significance of 7.2sigma at a mass of 2635.9+/-2.9 MeV/c^2. There is a
corresponding peak of 14 events with a significance of 5.3sigma at 2631.5+/-1.9
MeV/c^2 in the decay mode D0 K+. The decay width of this state is <17 MeV/c^2
at 90% confidence level. The relative branching ratio Gamma(D0K+)/Gamma(Dseta)
is 0.16+/-0.06. The mechanism which keeps this state narrow is unclear. Its
decay pattern is also unusual, being dominated by the Ds eta decay mode.Comment: 5 pages, 3 included eps figures. v2 as accepted for publication by
PR
First Observation of the Doubly Charmed Baryon Xi_cc^+
We observe a signal for the doubly charmed baryon Xi_cc^+ in the charged
decay mode Xi_cc^+ --> Lambda_c^+ K- pi+ in data from SELEX, the charm
hadro-production experiment at Fermilab. We observe an excess of 15.9 events
over an expected background of 6.1 +/- 0.5 events, a statistical significance
of 6.3sigma. The observed mass of this state is (3519 +/- 1) MeV/c^2. The
Gaussian mass width of this state is 3MeV/c^2, consistent with resolution; its
lifetime is less than 33fsec at 90% confidence.Comment: 5 pages, 3 figures, accepted for publication in Physical Review
Letter
First Measurement of pi e -> pi e gamma Pion Virtual Compton Scattering
Pion Virtual Compton Scattering (VCS) via the reaction pi e --> pi e gamma
was observed in the Fermilab E781 SELEX experiment. SELEX used a 600 GeV/c pi-
beam incident on target atomic electrons, detecting the incident pi- and the
final state pi-, electron and gamma. Theoretical predictions based on chiral
perturbation theory are incorporated into a Monte Carlo simulation of the
experiment and are compared to the data. The number of reconstructed events (9)
and their distribution with respect to the kinematic variables (for the
kinematic region studied) are in reasonable accord with the predictions. The
corresponding pi- VCS experimental cross section is sigma=38.8+-13 nb, in
agreement with the theoretical expectation sigma=34.7 nb.Comment: 10 pages, 12 figures, 4 tables, 25 references, SELEX home page is
http://fn781a.fnal.gov/, revised July 21, 2002 in response to journal referee
Comment
Observation of the Cabibbo-suppressed decay Xi_c+ -> p K- pi+
We report the first observation of the Cabibbo-suppressed charm baryon decay
Xi_c+ -> p K- pi+. We observe 150 +- 22 events for the signal. The data were
accumulated using the SELEX spectrometer during the 1996-1997 fixed target run
at Fermilab, chiefly from a 600 GeV/c Sigma- beam. The branching fractions of
the decay relative to the Cabibbo-favored Xi_c+ -> Sigma+ K- pi+ and Xi_c+ ->
X- pi+ pi+ are measured to be B(Xi_c+ -> p K- pi+)/B(Xi_c+ -> Sigma+ K- pi+) =
0.22 +- 0.06 +- 0.03 and B(Xi_c+ -> p K- pi+)/B(Xi_c+ -> X- pi+ pi+) = 0.20 +-
0.04 +- 0.02, respectively.Comment: 5 pages, RevTeX, 3 figures (postscript), Submitted to Phys. Rev. Let
Block-Diagonalization and f-electron Effects in Tight-Binding Theory
We extend a tight-binding total energy method to include f-electrons, and
apply it to the study of the structural and elastic properties of a range of
elements from Be to U. We find that the tight-binding parameters are as
accurate and transferable for f-electron systems as they are for d-electron
systems. In both cases we have found it essential to take great care in
constraining the fitting procedure by using a block-diagonalization procedure,
which we describe in detail.Comment: 9 pages, 6 figure
Hara's Theorem and W-exchange in Hyperon Weak Radiative Decays
We reconsider Hara's theorem in its relation to the well-known properties of
beta-decay. All assumptions necessary for the theorem to be true are explicitly
formulated. Further, we study the W-exchange contribution to weak radiative
decays and show that it does not violate Hara's theorem. However, this
contribution reveals the essential role of particle mixing in symmetry
considerations and some peculiar features of gauge-invariant amplitudes under
perturbative expansion. Together they explain an effect, which was treated as
contradicting Hara's theorem, without any violation. The properties of
W-exchange we describe here may have more general importance and should be
taken into account in further detailed calculations of weak processes.Comment: 14 pages, LATEX, no figure
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