324 research outputs found
Classical ultrarelativistic bremsstrahlung in extra dimensions
The emitted energy and the cross-section of classical scalar bremsstrahlung
in massive particle collisions in D=4+d dimensional Minkowski space M_D as well
as in the brane world M_4 \times T^d is computed to leading ultra-relativistic
order. The particles are taken to interact in the first case via the exchange
of a bulk massless scalar field \Phi and in the second with an additional
massless scalar \phi confined together with the particles on the brane. Energy
is emitted as \Phi radiation in the bulk and/or \phi radiation on the brane. In
contrast to the quantum Born approximation, the classical result is unambiguous
and valid in a kinematical region which is also specified. For D=4 the results
are in agreement with corresponding expressions in classical electrodynamics.Comment: Preprint number adde
An electron jet pump: The Venturi effect of a Fermi liquid
A three-terminal device based on a two-dimensional electron system is
investigated in the regime of non-equilibrium transport. Excited electrons
scatter with the cold Fermi sea and transfer energy and momentum to other
electrons. A geometry analogous to a water jet pump is used to create a jet
pump for electrons. Because of its phenomenological similarity we name the
observed behavior "electronic Venturi effect".Comment: Journal of Applied Physics Special Topic: Plenary and Invited Papers
from the 30th International Conference on the Physics of Semiconductors,
Seoul, Korea, 2010; http://link.aip.org/link/?JAP/109/10241
Relaxation of hot electrons in a degenerate two-dimensional electron system: transition to one-dimensional scattering
The energy relaxation channels of hot electrons far from thermal equilibrium
in a degenerate two-dimensional electron system are investigated in transport
experiments in a mesoscopic three-terminal device. We observe a transition from
two dimensions at zero magnetic field to quasi--one-dimensional scattering of
the hot electrons in a strong magnetic field. In the two-dimensional case
electron-electron scattering is the dominant relaxation mechanism, while the
emission of optical phonons becomes more and more important as the magnetic
field is increased. The observation of up to 11 optical phonons emitted per hot
electron allows us to determine the onset energy of LO phonons in GaAs at
cryogenic temperatures with a high precision, \eph=36.0\pm0.1\,meV. Numerical
calculations of electron-electron scattering and the emission of optical
phonons underline our interpretation in terms of a transition to
one-dimensional dynamics.Comment: 15 pages, 9 figure
Small x divergences in the Similarity RG approach to LF QCD
We study small x divergences in boost invariant similarity renormalization
group approach to light-front QCD in a heavy quark-antiquark state. With the
boost invariance maintained, the infrared divergences do not cancel out in the
physical states, contrary to previous studies where boost invariance was
violated by a choice of a renormalization scale. This may be an indication that
the zero mode, or nontrivial light-cone vacuum structure, might be important
for recovering full Lorentz invariance.Comment: 23 pgs, 1 fig. Revised for publication: typos corrected, improved
discussion of regularizatio
Gauge Unification and Quark Masses in a Pati-Salam Model from Branes
We investigate the phase space of parameters in the Pati-Salam model derived
in the context of D-branes scenarios, requiring low energy string scale. We
find that a non-supersymmetric version complies with a string scale as low as
10 TeV, while in the supersymmetric version the string scale raises up to ~2 x
10^7 TeV. The limited energy region for RGE running demands a large tan(beta)
in order to have experimentally acceptable masses for the top and bottom
quarks.Comment: 11 pages, LaTeX, 7 figures include
Can Centauros or Chirons be the first observations of evaporating mini black holes?
We argue that the signals expected from the evaporation of mini black holes -
predicted in TeV-scale gravity models with large extra dimensions, and possibly
produced in ultra high energy collisions in the atmosphere - are quite similar
to the characteristics of the Centauro events, an old mystery of cosmic-ray
physics.Comment: 14 pages, LaTeX, one Table, uses rotate.sty and rotcapt.sty.
References added, typos corrected, footnotes moved into the tex
On the Need for Phenomenological Theory of P-Vortices or Does Spaghetti Confinement Pattern Admit Condensed-Matter Analogies?
Usually the intuition from condensed-matter physics is used to provide ideas
for possible confinement mechanisms in gauge theories. Today, with a clear but
puzzling ``spaghetti'' confinement pattern, arising after a decade of lattice
computer experiments, which implies formation of a fluctuating net of peculiar
magnetic vortices rather than condensation of the homogeneously distributed
magnetic monopoles, the time is coming to reverse the logic and search for
similar patterns in condensed matter systems. The main thing to look for in a
condensed matter setup is the simultaneous existence of narrow tubes
(-vortices or 1-branes) of direction-changing electric field and broader
tubes (Abrikosov lines) of magnetic field, a pattern dual to the one,
presumably underlying confinement in gluodynamics. As a possible place for this
search we suggest systems with coexisting charge-density waves and
superconductivity.Comment: 20 pages, 7 figures; to be published in ZhET
Back-Reaction In Lightcone QED
We consider the back-reaction of quantum electrodynamics upon an electric
field E(x_+) = - A'_-(x_+) which is parallel to x^3 and depends only on the
lightcone coordinate x_+ = (x^0 + x^3)/\sqrt{2}. Novel features are that the
mode functions have simple expressions for arbitrary A_-(x_+), and that one
cannot ignore the usual lightcone ambiguity at zero + momentum. Each mode of
definite canonical momenta k_+ experiences pair creation at the instant when
its kinetic momentum p_+=k_+ - e A_-(x_+) vanishes, at which point operators
from the surface at x_- =-\infty play a crucial role. Our formalism permits a
more explicit and complete derivation of the rate of particle production than
is usually given. We show that the system can be understood as the infinite
boost limit of the analogous problem of an electric field which is homogeneous
on surfaces of constant x^0.Comment: 37 pages, 2 figures, LaTeX 2 epsilo
Characterization of functional and phenotypic changes in anti-Gag vaccine-induced T cell responses and their role in protection after HIV-1 infection
Worldwide HIV-1 vaccine efforts are guided by the principle that HIV-specific T cell responses may provide protection from infection or delay overt disease. However, no clear correlates of T cell-mediated immune protection have been identified. Here, we examine in a HLA-B27(+) HIV seronegative vaccinee persistent HIV-specific vaccine-induced anti-Gag CD4(+) and CD8(+) T cell responses. Although these responses exhibited those characteristics (multifunctionality, appropriate memory phenotype, and targeting of epitopes associated with long-term nonprogression) predicted to correlate with protection from infection, the subject became HIV infected. After HIV infection, the vaccine-induced CD8(+) T cells expanded, but both CD4(+) and CD8(+) T cell responses acquired the functional and phenotypic patterns characteristic of chronic HIV infection. The virus quickly escaped the vaccine-induced T cell response, and the subject progressed more rapidly than expected for someone expressing the HLA-B27 allele. These data suggest that control of HIV by vaccine-elicited HIV-specific T cell responses may be difficult, even when the T cell response has those characteristics predicted to provide optimal protection
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