1,909 research outputs found
Restriction on the energy and luminosity of e+e- storage rings due to beamstrahlung
The role of beamstrahlung in high-energy e+e- storage-ring colliders (SRCs)
is examined. Particle loss due to the emission of single energetic
beamstrahlung photons is shown to impose a fundamental limit on SRC
luminosities at energies 2E_0 >~ 140 GeV for head-on collisions and 2E_0 >~ 40
GeV for crab-waist collisions. With beamstrahlung taken into account, we
explore the viability of SRCs in the E_0=240-500 GeV range, which is of
interest in the precision study of the Higgs boson. At 2E_0=240 GeV, SRCs are
found to be competitive with linear colliders; however, at 2E_0=400-500 GeV,
the attainable SRC luminosity would be a factor 15-25 smaller than desired.Comment: Latex, 5 pages. v2 differs only by minor changes is abstract and
introduction, one reference is added. v3 corresponds to the paper published
in PR
Jet Tomography of Hot and Cold Nuclear Matter
Medium modification of parton fragmentation functions induced by multiple
scattering and gluon bremsstrahlung is shown to describe the recent HERMES data
in deeply inelastic scattering (DIS) very well, providing the first evidence of
-dependence of the modification. The energy loss is found to be
GeV/fm for a 10-GeV quark in a nucleus. Including
the effect of expansion, analysis of the spectra in central
collisions at GeV yields an averaged energy loss equivalent to
GeV/fm in a static medium. Predictions for central
collisions at GeV are also given.Comment: 4 pages in RevTex with 3 ps figures, final version published in Phys.
Rev. Letter
Interaction corrections: temperature and parallel field dependencies of the Lorentz number in two-dimensional disordered metals
The electron-electron interaction corrections to the transport coefficients
are calculated for a two-dimensional disordered metal in a parallel magnetic
field via the quantum kinetic equation approach. For the thermal transport,
three regimes (diffusive, quasiballistic and truly ballistic) can be identified
as the temperature increases. For the diffusive and quasiballistic regimes, the
Lorentz number dependence on the temperature and on the magnetic field is
studied. The electron-electron interactions induce deviations from the
Wiedemann-Franz law, whose sign depend on the temperature: at low temperatures
the long-range part of the Coulomb interaction gives a positive correction,
while at higher temperature the inelastic collisions dominate the negative
correction. By applying a parallel field, the Lorentz number becomes a
non-monotonic function of field and temperature for all values of the
Fermi-liquid interaction parameter in the diffusive regime, while in the
quasiballistic case this is true only sufficiently far from the Stoner
instability.Comment: 11 pages, 5 figures. Appendix A revised, notes adde
Infrared Observations During the Secondary Eclipse of HD 209458b: I. 3.6-Micron Occultation Spectroscopy Using the VLT
We search for an infrared signature of the transiting extrasolar planet HD
209458b during secondary eclipse. Our method, which we call `occultation
spectroscopy,' searches for the disappearance and reappearance of weak spectral
features due to the exoplanet as it passes behind the star and later reappears.
We argue that at the longest infrared wavelengths, this technique becomes
preferable to conventional `transit spectroscopy'. We observed the system in
the wing of the strong nu-3 band of methane near 3.6 microns during two
secondary eclipses, using the VLT/ISAAC spectrometer at a spectral resolution
of 3300. Our analysis, which utilizes a model template spectrum, achieves
sufficient precision to expect detection of the spectral structure predicted by
an irradiated, low-opacity (cloudless), low-albedo, thermochemical equilibrium
model for the exoplanet atmosphere. However, our observations show no evidence
for the presence of this spectrum from the exoplanet, with the statistical
significance of the non-detection depending on the timing of the secondary
eclipse, which depends on the assumed value for the orbital eccentricity. Our
results reject certain specific models of the atmosphere of HD 209458b as
inconsistent with our observations at the 3-sigma level, given assumptions
about the stellar and planetary parameters.Comment: 26 pages, 8 figures Accepted to Astrophysical Journa
Review of the Safety of LHC Collisions
The safety of collisions at the Large Hadron Collider (LHC) was studied in
2003 by the LHC Safety Study Group, who concluded that they presented no
danger. Here we review their 2003 analysis in light of additional experimental
results and theoretical understanding, which enable us to confirm, update and
extend the conclusions of the LHC Safety Study Group. The LHC reproduces in the
laboratory, under controlled conditions, collisions at centre-of-mass energies
less than those reached in the atmosphere by some of the cosmic rays that have
been bombarding the Earth for billions of years. We recall the rates for the
collisions of cosmic rays with the Earth, Sun, neutron stars, white dwarfs and
other astronomical bodies at energies higher than the LHC. The stability of
astronomical bodies indicates that such collisions cannot be dangerous.
Specifically, we study the possible production at the LHC of hypothetical
objects such as vacuum bubbles, magnetic monopoles, microscopic black holes and
strangelets, and find no associated risks. Any microscopic black holes produced
at the LHC are expected to decay by Hawking radiation before they reach the
detector walls. If some microscopic black holes were stable, those produced by
cosmic rays would be stopped inside the Earth or other astronomical bodies. The
stability of astronomical bodies constrains strongly the possible rate of
accretion by any such microscopic black holes, so that they present no
conceivable danger. In the case of strangelets, the good agreement of
measurements of particle production at RHIC with simple thermodynamic models
constrains severely the production of strangelets in heavy-ion collisions at
the LHC, which also present no danger.Comment: This revised version incorporates the "Addendum on strangelets" as
Appendix, and updates the bibliograph
Reciprocal space mapping of magnetic order in thick epitaxial MnSi films
We report grazing incidence small angle neutron scattering (GISANS) and
complementary off-specular neutron reflectometry (OSR) of the magnetic order in
a single-crystalline epitaxial MnSi film on Si(111) in the thick film limit.
Providing a means of direct reciprocal space mapping, GISANS and OSR reveal a
magnetic modulation perpendicular to the films under magnetic fields parallel
and perpendicular to the film, where additional polarized neutron reflectometry
(PNR) and magnetization measurements are in excellent agreement with the
literature. Regardless of field orientation, our data does not suggest the
presence of more complex spin textures, notably the formation of skyrmions.
This observation establishes a distinct difference with bulk samples of MnSi of
similar thickness under perpendicular field, in which a skyrmion lattice
dominates the phase diagram. Extended x-ray absorption fine structure
measurements suggest that small shifts of the Si positions within the
unstrained unit cell control the magnetic state, representing the main
difference between the films and thin bulk samples
Testing collinear factorization and nuclear parton distributions with pA collisions at the LHC
Global perturbative QCD analyses, based on large data sets from
electron-proton and hadron collider experiments, provide tight constraints on
the parton distribution function (PDF) in the proton. The extension of these
analyses to nuclear parton distributions (nPDF) has attracted much interest in
recent years. nPDFs are needed as benchmarks for the characterization of hot
QCD matter in nucleus-nucleus collisions, and attract further interest since
they may show novel signatures of non- linear density-dependent QCD evolution.
However, it is not known from first principles whether the factorization of
long-range phenomena into process-independent parton distribution, which
underlies global PDF extractions for the proton, extends to nuclear effects. As
a consequence, assessing the reliability of nPDFs for benchmark calculations
goes beyond testing the numerical accuracy of their extraction and requires
phenomenological tests of the factorization assumption. Here we argue that a
proton-nucleus collision program at the LHC would provide a set of measurements
allowing for unprecedented tests of the factorization assumption underlying
global nPDF fits.Comment: 4 pages, 5 figure
Jet Quenching via Jet Collimation
The ATLAS Collaboration recently reported strong modifications of dijet
properties in heavy ion collisions. In this work, we discuss to what extent
these first data constrain already the microscopic mechanism underlying jet
quenching. Simple kinematic arguments lead us to identify a frequency
collimation mechanism via which the medium efficiently trims away the soft
components of the jet parton shower. Through this mechanism, the observed dijet
asymmetry can be accomodated with values of that lie in the
expected order of magnitude.Comment: 6 pages, 4 figure
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