518 research outputs found
BL Lac Objects in the Synchrotron Proton Blazar Model
We calculate the spectral energy distribution (SED) of electromagnetic
radiation and the spectrum of high energy neutrinos from BL Lac objects in the
context of the Synchrotron Proton Blazar Model. In this model, the high energy
hump of the SED is due to accelerated protons, while most of the low energy
hump is due to synchrotron radiation by co-accelerated electrons. To accelerate
protons to sufficiently high energies to produce the high energy hump, rather
high magnetic fields are required. Assuming reasonable emission region volumes
and Doppler factors, we then find that in low-frequency peaked BL Lacs (LBLs),
which have higher luminosities than high-frequency peaked BL Lacs (HBLs), there
is a significant contribution to the high frequency hump of the SED from pion
photoproduction and subsequent cascading, including synchrotron radiation by
muons. In contrast, in HBLs we find that the high frequency hump of the SED is
dominated by proton synchrotron radiation. We are able to model the SED of
typical LBLs and HBLs, and to model the famous 1997 flare of Markarian 501. We
also calculate the expected neutrino output of typical BL Lac objects, and
estimate the diffuse neutrino intensity due to all BL Lacs. Because pion
photoproduction is inefficient in HBLs, as protons lose energy predominantly by
synchrotron radiation, the contribution of LBLs dominates the diffuse neutrino
intensity. We suggest that nearby LBLs may well be observable with future
high-sensitivity TeV gamma-ray telescopes.Comment: 33 pages, 20 Figures. Astropart. Phys., accepte
The size of the proton - closing in on the radius puzzle
We analyze the recent electron-proton scattering data from Mainz using a
dispersive framework that respects the constraints from analyticity and
unitarity on the nucleon structure. We also perform a continued fraction
analysis of these data. We find a small electric proton charge radius, r_E^p =
0.84_{-0.01}^{+0.01} fm, consistent with the recent determination from muonic
hydrogen measurements and earlier dispersive analyses. We also extract the
proton magnetic radius, r_M^p = 0.86_{-0.03}^{+0.02} fm, consistent with
earlier determinations based on dispersion relations.Comment: 4 pages, 2 figures, fit improved, small modifications, section on
continued fractions modified, conclusions on the proton charge radius
unchanged, version accepted for publication in European Physical Journal
Characterization of large area avalanche photodiodes in X-ray and VUV-light detection
The present manuscript summarizes novel studies on the application of large
area avalanche photodiodes (LAAPDs) to the detection of X-rays and vacuum
ultraviolet (VUV) light. The operational characteristics of four different
LAAPDs manufactured by Advanced Photonix Inc., with active areas of 80 and 200
mm^2 were investigated for X-ray detection at room temperature. The best energy
resolution was found to be in the 10-18% range for 5.9 keV X-rays. The LAAPD,
being compact, simple to operate and with high counting rate capability (up to
about 10^5/s), proved to be useful in several applications, such as low-energy
X-ray detection, where they can reach better performance than proportional
counters. Since X-rays are used as reference in light measurements, the gain
non-linearity between 5.9 keV X-rays and light pulses was investigated. The
gain ratio between X-rays and VUV light decreases with gain, reaching 10 and 6%
variations for VUV light produced in argon (~128 nm) and xenon (~172 nm),
respectively, for a gain 200, while for visible light (~635 nm) the variation
is lower than 1%. The effect of temperature on the LAAPD performance was
investigated. Relative gain variations of about -5% per Celsius degree were
observed for the highest gains. The excess noise factor was found to be
independent on temperature, being between 1.8 and 2.3 for gains from 50 to 300.
The energy resolution is better for decreasing temperatures due mainly to the
dark current. LAAPDs were tested under intense magnetic fields up to 5 T, being
insensitive when used in X-ray and visible-light detection, while for VUV light
a significant amplitude reduction was observed at 5 T.Comment: 25 pages, 40 figures, submitted to JINS
Possible origins of macroscopic left-right asymmetry in organisms
I consider the microscopic mechanisms by which a particular left-right (L/R)
asymmetry is generated at the organism level from the microscopic handedness of
cytoskeletal molecules. In light of a fundamental symmetry principle, the
typical pattern-formation mechanisms of diffusion plus regulation cannot
implement the "right-hand rule"; at the microscopic level, the cell's
cytoskeleton of chiral filaments seems always to be involved, usually in
collective states driven by polymerization forces or molecular motors. It seems
particularly easy for handedness to emerge in a shear or rotation in the
background of an effectively two-dimensional system, such as the cell membrane
or a layer of cells, as this requires no pre-existing axis apart from the layer
normal. I detail a scenario involving actin/myosin layers in snails and in C.
elegans, and also one about the microtubule layer in plant cells. I also survey
the other examples that I am aware of, such as the emergence of handedness such
as the emergence of handedness in neurons, in eukaryote cell motility, and in
non-flagellated bacteria.Comment: 42 pages, 6 figures, resubmitted to J. Stat. Phys. special issue.
Major rewrite, rearranged sections/subsections, new Fig 3 + 6, new physics in
Sec 2.4 and 3.4.1, added Sec 5 and subsections of Sec
Characterization of large area avalanche photodiodes in X-ray and VUV-light detection
The present manuscript summarizes novel studies on the application of large
area avalanche photodiodes (LAAPDs) to the detection of X-rays and vacuum
ultraviolet (VUV) light. The operational characteristics of four different
LAAPDs manufactured by Advanced Photonix Inc., with active areas of 80 and 200
mm^2 were investigated for X-ray detection at room temperature. The best energy
resolution was found to be in the 10-18% range for 5.9 keV X-rays. The LAAPD,
being compact, simple to operate and with high counting rate capability (up to
about 10^5/s), proved to be useful in several applications, such as low-energy
X-ray detection, where they can reach better performance than proportional
counters. Since X-rays are used as reference in light measurements, the gain
non-linearity between 5.9 keV X-rays and light pulses was investigated. The
gain ratio between X-rays and VUV light decreases with gain, reaching 10 and 6%
variations for VUV light produced in argon (~128 nm) and xenon (~172 nm),
respectively, for a gain 200, while for visible light (~635 nm) the variation
is lower than 1%. The effect of temperature on the LAAPD performance was
investigated. Relative gain variations of about -5% per Celsius degree were
observed for the highest gains. The excess noise factor was found to be
independent on temperature, being between 1.8 and 2.3 for gains from 50 to 300.
The energy resolution is better for decreasing temperatures due mainly to the
dark current. LAAPDs were tested under intense magnetic fields up to 5 T, being
insensitive when used in X-ray and visible-light detection, while for VUV light
a significant amplitude reduction was observed at 5 T.Comment: 25 pages, 40 figures, submitted to JINS
Characterization of large area avalanche photodiodes in X-ray and VUV-light detection
The present manuscript summarizes novel studies on the application of large
area avalanche photodiodes (LAAPDs) to the detection of X-rays and vacuum
ultraviolet (VUV) light. The operational characteristics of four different
LAAPDs manufactured by Advanced Photonix Inc., with active areas of 80 and 200
mm^2 were investigated for X-ray detection at room temperature. The best energy
resolution was found to be in the 10-18% range for 5.9 keV X-rays. The LAAPD,
being compact, simple to operate and with high counting rate capability (up to
about 10^5/s), proved to be useful in several applications, such as low-energy
X-ray detection, where they can reach better performance than proportional
counters. Since X-rays are used as reference in light measurements, the gain
non-linearity between 5.9 keV X-rays and light pulses was investigated. The
gain ratio between X-rays and VUV light decreases with gain, reaching 10 and 6%
variations for VUV light produced in argon (~128 nm) and xenon (~172 nm),
respectively, for a gain 200, while for visible light (~635 nm) the variation
is lower than 1%. The effect of temperature on the LAAPD performance was
investigated. Relative gain variations of about -5% per Celsius degree were
observed for the highest gains. The excess noise factor was found to be
independent on temperature, being between 1.8 and 2.3 for gains from 50 to 300.
The energy resolution is better for decreasing temperatures due mainly to the
dark current. LAAPDs were tested under intense magnetic fields up to 5 T, being
insensitive when used in X-ray and visible-light detection, while for VUV light
a significant amplitude reduction was observed at 5 T.Comment: 25 pages, 40 figures, submitted to JINS
The First VERITAS Telescope
The first atmospheric Cherenkov telescope of VERITAS (the Very Energetic
Radiation Imaging Telescope Array System) has been in operation since February
2005. We present here a technical description of the instrument and a summary
of its performance. The calibration methods are described, along with the
results of Monte Carlo simulations of the telescope and comparisons between
real and simulated data. The analysis of TeV -ray observations of the
Crab Nebula, including the reconstructed energy spectrum, is shown to give
results consistent with earlier measurements. The telescope is operating as
expected and has met or exceeded all design specifications.Comment: Accepted by Astroparticle Physic
Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in âs = 7 TeV pp collisions with the ATLAS detector
A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fbâ1 of protonâproton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results
Search for displaced vertices arising from decays of new heavy particles in 7 TeV pp collisions at ATLAS
We present the results of a search for new, heavy particles that decay at a
significant distance from their production point into a final state containing
charged hadrons in association with a high-momentum muon. The search is
conducted in a pp-collision data sample with a center-of-mass energy of 7 TeV
and an integrated luminosity of 33 pb^-1 collected in 2010 by the ATLAS
detector operating at the Large Hadron Collider. Production of such particles
is expected in various scenarios of physics beyond the standard model. We
observe no signal and place limits on the production cross-section of
supersymmetric particles in an R-parity-violating scenario as a function of the
neutralino lifetime. Limits are presented for different squark and neutralino
masses, enabling extension of the limits to a variety of other models.Comment: 8 pages plus author list (20 pages total), 8 figures, 1 table, final
version to appear in Physics Letters
Measurement of the polarisation of W bosons produced with large transverse momentum in pp collisions at sqrt(s) = 7 TeV with the ATLAS experiment
This paper describes an analysis of the angular distribution of W->enu and
W->munu decays, using data from pp collisions at sqrt(s) = 7 TeV recorded with
the ATLAS detector at the LHC in 2010, corresponding to an integrated
luminosity of about 35 pb^-1. Using the decay lepton transverse momentum and
the missing transverse energy, the W decay angular distribution projected onto
the transverse plane is obtained and analysed in terms of helicity fractions
f0, fL and fR over two ranges of W transverse momentum (ptw): 35 < ptw < 50 GeV
and ptw > 50 GeV. Good agreement is found with theoretical predictions. For ptw
> 50 GeV, the values of f0 and fL-fR, averaged over charge and lepton flavour,
are measured to be : f0 = 0.127 +/- 0.030 +/- 0.108 and fL-fR = 0.252 +/- 0.017
+/- 0.030, where the first uncertainties are statistical, and the second
include all systematic effects.Comment: 19 pages plus author list (34 pages total), 9 figures, 11 tables,
revised author list, matches European Journal of Physics C versio
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