2,605 research outputs found
Limits on the low-energy antinucleon annihilations from the Heisenberg principle
Here a short synthesis is presented of the work, developed in the last two
years by the Brescia Collaboration, on the phenomenology of antinucleon-nucleon
and antinucleon-nucleus annihilation at small momenta (below 300 MeV/c in the
laboratory), with special stress on the role of general principles.Comment: Invited contribution at LEAP 2000 Conference, Venice August 200
Resonant and crossover phenomena in a multiband superconductor tuning the chemical potential near a band edge
Resonances in the superconducting properties, in a regime of crossover from
BCS to mixed Bose-Fermi superconductivity, are investigated in a two-band
superconductor where the chemical potential is tuned near the band edge of the
second mini-band generated by quantum confinement effects. The shape resonances
at T=0 in the superconducting gaps (belonging to the class of Feshbach-like
resonances) is manifested by interference effects in the superconducting gap at
the first large Fermi surface when the chemical potential is in the proximity
of the band edge of the second mini-band. The case of a superlattice of quantum
wells is considered and the amplification of the superperconducting gaps at the
3D-2D Fermi surface topological transition is clearly shown. The results are
found to be in good agreement with available experimental data on a
superlattice of honeycomb boron layers intercalated by Al and Mg spacer layers.Comment: 13 pages, 9 image
Angular Distributions for Knockout and Scattering of Protons in the Eikonal Approximation
The advent of new electron accelerators with few-GeV beam energies makes the
(e,e'p) reaction a promising tool for investigating new aspects of the
electromagnetic interaction. To this purpose it is crucial to set the scale of
Final-State Interactions (FSI) at high ejectile energies. We have analyzed the
constraints which allow for a meaningful generalization to (e,e'p) of the
Glauber method used in elastic (p,p) scattering. The FSI dominance at large
deflection angles produces in the distributions a universal behaviour
resembling the coherent diffractive scattering between the ejected proton and
the (residual) nucleus. It is argued that the comparison between (e,e'p) and
(p,p), when a meaningful one, may represent a convenient tool to disentangle
(exotic) effects due to FSI from those related to the (hard) electromagnetic
vertex.Comment: LaTeX, 21 pages, 4 figures uuencoded in a separate file. Submitted to
Phys. Rev.
Effects of azimuth-symmetric acceptance cutoffs on the measured asymmetry in unpolarized Drell-Yan fixed target experiments
Fixed-target unpolarized Drell-Yan experiments often feature an acceptance
depending on the polar angle of the lepton tracks in the laboratory frame.
Typically leptons are detected in a defined angular range, with a dead zone in
the forward region. If the cutoffs imposed by the angular acceptance are
independent of the azimuth, at first sight they do not appear dangerous for a
measurement of the cos(2\phi)-asymmetry, relevant because of its association
with the violation of the Lam-Tung rule and with the Boer-Mulders function. On
the contrary, direct simulations show that up to 10 percent asymmetries are
produced by these cutoffs. These artificial asymmetries present qualitative
features that allow them to mimic the physical ones. They introduce some
model-dependence in the measurements of the cos(2\phi)-asymmetry, since a
precise reconstruction of the acceptance in the Collins-Soper frame requires a
Monte Carlo simulation, that in turn requires some detailed physical input to
generate event distributions. Although experiments in the eighties seem to have
been aware of this problem, the possibility of using the Boer-Mulders function
as an input parameter in the extraction of Transversity has much increased the
requirements of precision on this measurement. Our simulations show that the
safest approach to these measurements is a strong cutoff on the Collins-Soper
polar angle. This reduces statistics, but does not necessarily decrease the
precision in a measurement of the Boer-Mulders function.Comment: 13 pages, 14 figure
Helicity Asymmetry for Proton Emission from Polarized Electrons in the Eikonal Regime
The nuclear response to longitudinally polarized electrons, detected in
coincidence with out-of-plane high-energy protons, is discussed in a simple
model where the ejectile wave function is approximated as a plane wave with a
complex wave vector. This choice is equivalent to solve the problem of
Final-State Interactions (FSI) in homogeneous nuclear matter, as the residual
nucleus can be described to a first approximation when dealing with very fast
emitted protons. The main advantage of the present method is that in the
framework of the Distorted-Wave Impulse Approximation (DWIA) it allows for an
analytical derivation of all the components of the nuclear response. It emerges
that cancellations among the leading contributions determine the very small
absolute size of the socalled fifth structure function and produce a nontrivial
asymptotic scaling of the related helicity asymmetry for large values of the
momentum transfer.Comment: 22 pages, RevTeX, 5 postscript figures encoded in separate file,
submitted to Phys. Rev.
Growing Cayley trees described by Fermi distribution
We introduce a model for growing Cayley trees with thermal noise. The
evolution of these hierarchical networks reduces to the Eden model and the
Invasion Percolation model in the limit , respectively.
We show that the distribution of the bond strengths (energies) is described by
the Fermi statistics. We discuss the relation of the present results with the
scale-free networks described by Bose statistics
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