2,612 research outputs found
Electron-positron pair production in the external electromagnetic field of colliding relativistic heavy ions
The results concerning the production in peripheral highly
relativistic heavy-ion collisions presented in a recent paper by Baltz {\em{et
al.}} are rederived in a very straightforward manner. It is shown that the
solution of the Dirac equation directly leads to the multiplicity, i.e. to the
total number of electron-positron pairs produced by the electromagnetic field
of the ions, whereas the calculation of the single pair production probability
is much more involved. A critical observation concerns the unsolved problem of
seemingly absent Coulomb corrections (Bethe-Maximon corrections) in pair
production cross sections. It is shown that neither the inclusion of the
vacuum-vacuum amplitude nor the correct interpretation of the solution of the
Dirac equation concerning the pair multiplicity is able the explain (from a
fundamental point of view) the absence of Coulomb corrections. Therefore the
contradiction has to be accounted to the treatment of the high energy limit.Comment: 6 pages, 4 Postscript figures, uses svjour.cls/svepj.cl
Modification of surface energy in nuclear multifragmentation
Within the statistical multifragmentation model we study modifications of the
surface and symmetry energy of primary fragments in the freeze-out volume. The
ALADIN experimental data on multifragmentation obtained in reactions induced by
high-energy projectiles with different neutron richness are analyzed. We have
extracted the isospin dependence of the surface energy coefficient at different
degrees of fragmentation. We conclude that the surface energy of hot fragments
produced in multifragmentation reactions differs from the values extracted for
isolated nuclei at low excitation. At high fragment multiplicity, it becomes
nearly independent of the neutron content of the fragments.Comment: 11 pages with 13 figure
Observation of the phononic Lamb shift with a synthetic vacuum
The quantum vacuum fundamentally alters the properties of embedded particles.
In contrast to classical empty space, it allows for creation and annihilation
of excitations. For trapped particles this leads to a change in the energy
spectrum, known as Lamb shift. Here, we engineer a synthetic vacuum building on
the unique properties of ultracold atomic gas mixtures. This system makes it
possible to combine high-precision spectroscopy with the ability of switching
between empty space and quantum vacuum. We observe the phononic Lamb shift, an
intruiguing many-body effect orginally conjectured in the context of solid
state physics. Our study therefore opens up new avenues for high-precision
benchmarking of non-trivial theoretical predictions in the realm of the quantum
vacuum
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