422 research outputs found
LPM effect as the origin of the jet fragmentation scaling in heavy ion collisions
We address a recent puzzling result from the LHC: the jet fragmentation
functions measured in and collisions appear very similar in spite
of a large medium-induced energy loss (we will call this "jet fragmentation
scaling", JFS). To model the real-time non-perturbative effects in the
propagation of a high energy jet through the strongly coupled QCD matter, we
adopt an effective dimensionally reduced description in terms of the
quasi-Abelian Schwinger theory. This theory is exactly soluble at any value of
the coupling and shares with QCD the properties of dynamical generation of
"mesons" with a finite mass and the screening of "quark" charge that are
crucial for describing the transition of the jet into hadrons. We find that
this approach describes quite well the vacuum jet fragmentation in
annihilation at at jet energies in the range of the LHC heavy ion
measurements ( is the ratio of hadron and jet momenta). In QCD medium, we
find that the JFS is reproduced if the mean free path of the jet is
short, fm, which is in accord with the small shear viscosity
inferred from the measurements of the collective flow. The JFS holds since at
short mean free path the quantum interference (analogous to the
Landau-Pomeranchuk-Migdal effect in QED) causes the produced mesons to have low
momenta , where GeV is the typical meson mass.
Meanwhile the induced jet energy loss at short mean free path is much larger
than naively expected in string models.Comment: 4 pages, 4 figure
Experimental Research of the Diffraction and Vavilov-Cherenkov Radiation Generation in a Teflon Target
Geometry of Vavilov-Cherekov (VChR) radiation when an electron moves close to
a dielectric target is in analogy to diffraction radiation (DR) geometry. In
this case we may expect DR generation from the upstream face of the target
besides that VChR. The joint observation of these booth types of radiation is
very interesting from the pseudo-photon viewpoint, which is applicable for
relativistic electrons. Unexpected results obtained in our experiment insist on
reflection about nature both DR and VChR. The experiment was performed on the
relativistic electron beam of the microtron of Tomsk Polytechnic University.Comment: This article will be published in Journal of Physic
Incorporating next-to-leading order matrix elements for hadronic diboson production in showering event generators
A method for incorporating information from next-to-leading order QCD matrix
elements for hadronic diboson production into showering event generators is
presented. In the hard central region (high jet transverse momentum) where
perturbative QCD is reliable, events are sampled according to the first order
tree level matrix element. In the soft and collinear regions next-to-leading
order corrections are approximated by calculating the differential cross
section across the phase space accessible to the parton shower using the first
order (virtual graphs included) matrix element. The parton shower then provides
an all-orders exclusive description of parton emissions. Events generated in
this way provide a physical result across the entire jet transverse momentum
spectrum, have next-to-leading order normalization everywhere, and have
positive definite event weights. The method is generalizable without
modification to any color singlet production process.Comment: 13 pages, 9 figure
Tensor polarization of deuterons passing through matter
It is shown that the magnitude of tensor polarization of the deuteron beam,
which arises owing to the spin dichroism effect, depends appreciably on the
angular width of the detector that registers the deuterons transmitted through
the target. Even when the angular width of the detector is much smaller than
the mean square angle of multiple Coulomb scattering, the beam's tensor
polarization depends noticeably on rescattering. When the angular width of the
detector is much larger than the mean square angle of multiple Coulomb
scattering (as well as than the characteristic angle of elastic nuclear
scattering), tensor polarization is determined only by the total reaction cross
sections for deuteron-nucleus interaction, and elastic scattering processes
make no contribution to tensor polarization.Comment: 18 pages, 3 figures, to be published in IO
Exotic radiation from a photonic crystal excited by an ultra-relativistic electron beam
We report the observation of an exotic radiation (unconventional
Smith-Purcell radiation) from a one-dimensional photonic crystal. The physical
origin of the exotic radiation is direct excitation of the photonic bands by an
ultra-relativistic electron beam. The spectrum of the exotic radiation follows
photonic bands of a certain parity, in striking contrast to the conventional
Smith-Purcell radiation, which shows solely a linear dispersion. Key
ingredients for the observation are the facts that the electron beam is in an
ultra-relativistic region and that the photonic crystal is finite. The origin
of the radiation was identified by comparison of experimental and theoretical
results.Comment: 4 pages, 5 figure
SUSY Magnetic Moments Sum Rules and Supersymmetry Breaking
It was recently shown that unbroken N=1 Susy relates, in a model independent
way, the magnetic transitions between states of different spin within a given
charged massive supermultiplet. We verify explicitly these sum rules for a
vector multiplet in the case of massless and massive fermions. The purpose of
this analysis is to provide the ground for the broken susy case. We study the
modifications of these results when an explicit soft Susy breaking realized
through a universal mass for all scalars is present. As a by-product we provide
a computation of the of the boson in the standard model which
corrects previous evaluations in the literature.Comment: 16+5 pages, Latex,(feynman.tex to print the figures), DFPD 94/TH/2
Experimental Observation of Energy Modulation in Electron Beams Passing Through Terahertz Dielectric Wakefield Structures
We report observation of a strong wakefield induced energy modulation in an
energy-chirped electron bunch passing through a dielectric-lined waveguide.
This modulation can be effectively converted into a spatial modulation forming
micro-bunches with a periodicity of 0.5 - 1 picosecond, hence capable of
driving coherent THz radiation. The experimental results agree well with
theoretical predictions.Comment: v3. Reviewers' suggestions incorporated. Accepted by PR
Reactor monitoring and safeguards using antineutrino detectors
Nuclear reactors have served as the antineutrino source for many fundamental
physics experiments. The techniques developed by these experiments make it
possible to use these very weakly interacting particles for a practical
purpose. The large flux of antineutrinos that leaves a reactor carries
information about two quantities of interest for safeguards: the reactor power
and fissile inventory. Measurements made with antineutrino detectors could
therefore offer an alternative means for verifying the power history and
fissile inventory of a reactors, as part of International Atomic Energy Agency
(IAEA) and other reactor safeguards regimes. Several efforts to develop this
monitoring technique are underway across the globe.Comment: 6 pages, 4 figures, Proceedings of XXIII International Conference on
Neutrino Physics and Astrophysics (Neutrino 2008); v2: minor additions to
reference
Energy loss in perturbative QCD
We review the properties of energetic parton propagation in hot or cold QCD
matter, as obtained in recent works. Advances in understanding the energy loss
- collisional and radiative - are summarized, with emphasis on the latter: it
features very interesting properties which may help to detect the quark-gluon
plasma produced in heavy ion collisions. We describe two different theoretical
approaches, which lead to the same radiated gluon energy spectrum. The case of
a longitudinally expanding QCD plasma is investigated. The energy lost by a jet
with given opening angle is calculated in view of making predictions for the
suppression (quenching) of hard jet production. Phenomenological implications
for the difference between hot and cold matter are discussed. Numerical
estimates of the loss suggest that it may be significantly enhanced in hot
compared to cold matter.Comment: 49 pages latex file with 11 embedded PS figures. Uses ar.sty
(included), one equation revised. submitted to Annual Review of Nuclear and
Particle Scienc
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