190 research outputs found
Disappointing model for ultrahigh-energy cosmic rays
Data of Pierre Auger Observatory show a proton-dominated chemical composition
of ultrahigh-energy cosmic rays spectrum at (1 - 3) EeV and a steadily heavier
composition with energy increasing. In order to explain this feature we assume
that (1 - 3) EeV protons are extragalactic and derive their maximum
acceleration energy, E_p^{max} \simeq 4 EeV, compatible with both the spectrum
and the composition. We also assume the rigidity-dependent acceleration
mechanism of heavier nuclei, E_A^{max} = Z x E_p^{max}. The proposed model has
rather disappointing consequences: i) no pion photo-production on CMB photons
in extragalactic space and hence ii) no high-energy cosmogenic neutrino fluxes;
iii) no GZK-cutoff in the spectrum; iv) no correlation with nearby sources due
to nuclei deflection in the galactic magnetic fields up to highest energies.Comment: 4 pages, 7 figures, the talk presented by A. Gazizov at NPA5
Conference, April 3-8, 2011, Eilat, Israe
On the Discovery of the GZK Cut-off
The recent claim of the '5 sigma' observation of the Greisen and Zatzepin and
Kuzmin cut-off by the HiRes group based on their nine years data is a
significant step toward the eventual solution of the one of the most intriguing
questions which has been present in physics for more than forty years. However
the word 'significance' is used in the mentioned paper in the sense which is
not quite obvious. In the present paper we persuade that this claim is a little
premature.Comment: 10 page
Report of the Working Group on the Composition of Ultra High Energy Cosmic Rays
For the first time a proper comparison of the average depth of shower maximum
() published by the Pierre Auger and Telescope Array Observatories
is presented. The distributions measured by the Pierre Auger
Observatory were fit using simulated events initiated by four primaries
(proton, helium, nitrogen and iron). The primary abundances which best describe
the Auger data were simulated through the Telescope Array (TA) Middle Drum (MD)
fluorescence and surface detector array. The simulated events were analyzed by
the TA Collaboration using the same procedure as applied to their data. The
result is a simulated version of the Auger data as it would be observed by TA.
This analysis allows a direct comparison of the evolution of with energy of both data sets. The
measured by TA-MD is consistent with a preliminary simulation of the Auger data
through the TA detector and the average difference between the two data sets
was found to be .Comment: To appear in the Proceedings of the UHECR workshop, Springdale USA,
201
Program transformations using temporal logic side conditions
This paper describes an approach to program optimisation based on transformations, where temporal logic is used to specify side conditions, and strategies are created which expand the repertoire of transformations and provide a suitable level of abstraction. We demonstrate the power of this approach by developing a set of optimisations using our transformation language and showing how the transformations can be converted into a form which makes it easier to apply them, while maintaining trust in the resulting optimising steps. The approach is illustrated through a transformational case study where we apply several optimisations to a small program
Highest Energy Cosmic Rays and results from the HiRes Experiment
The status of the field of ultrahigh energy cosmic rays is summarized, from
the point of view of the latest results of the High Resolution Fly's Eye
(HiRes) Experiment. HiRes results are presented, and compared with those of the
Akeno Giant Air Shower Array (AGASA), plus the Telescope Array and Pierre Auger
experiments. The HiRes measurements of the cosmic ray spectrum, and the
observation of the GZK cutoff are presented. HiRes results on composition,
searches for anisotropy, measurement of the proton-air total cross section, and
shapes of shower profiles are presented.Comment: 31 pages, 18 figures, submitted to Journal of Physics
Diffusive propagation of cosmic rays from supernova remnants in the Galaxy. I: spectrum and chemical composition
In this paper we investigate the effect of stochasticity in the spatial and
temporal distribution of supernova remnants on the spectrum and chemical
composition of cosmic rays observed at Earth. The calculations are carried out
for different choices of the diffusion coefficient D(E) experienced by cosmic
rays during propagation in the Galaxy. In particular, at high energies we
assume that D(E)\sim E^{\delta}, with and being the
reference scenarios. The large scale distribution of supernova remnants in the
Galaxy is modeled following the distribution of pulsars, with and without
accounting for the spiral structure of the Galaxy. We find that the stochastic
fluctuations induced by the spatial and temporal distribution of supernovae,
together with the effect of spallation of nuclei, lead to mild but sensible
violations of the simple, leaky-box-inspired rule that the spectrum observed at
Earth is with , where
is the slope of the cosmic ray injection spectrum at the sources. Spallation of
nuclei, even with the small rates appropriate for He, may account for slight
differences in spectral slopes between different nuclei, providing a possible
explanation for the recent CREAM observations. For we find that
the slope of the proton and helium spectra are and
respectively at energies above 1 TeV (to be compared with the measured values
of and ). For the hardening of the He
spectra is not observed. We also comment on the effect of time dependence of
the escape of cosmic rays from supernova remnants, and of a possible clustering
of the sources in superbubbles. In a second paper we will discuss the
implications of these different scenarios for the anisotropy of cosmic rays.Comment: 28 pages, To appear in JCA
Limits to the energy resolution of a single Air Cherenkov Telescope at low energies
The photon density on the ground is a fundamental quantity in all experiments
based on Cherenkov light measurements, e.g. in the Imaging Air Cherenkov
Telescopes (IACT). IACT's are commonly and successfully used in order to search
and study Very High Energy (VHE) gamma-ray sources. Difficulties with
separating primary photons from primary hadrons (mostly protons) in Cherenkov
experiments become larger at lower energies. I have calculated longitudinal and
lateral density distributions and their fluctuations at low energies basing on
Monte Carlo simulations (for vertical gamma cascades and protonic showers) to
check the influence of the detector parameters on the possible measurement.
Relative density fluctuations are significantly higher in proton than in photon
induced showers. Taking into account the limited detector field of view (FOV)
implies the changes of these calculated distributions for both types of primary
particles and causes an enlargement in relative fluctuations. Absorption due to
Rayleigh and Mie scattering has an impact on mean values but does not change
relative fluctuations. The total number of Cherenkov photons is more sensitive
to the observation height in gamma cascades than in proton showers at low
primary energies. The relative fluctuations of the density do not depend on the
reflector size in the investigated size range (from 240 m^2 up to 960 m^2).
This implies that a single telescope with a mirror area larger than that of the
MAGIC telescope cannot achieve better energy resolution than estimated and
presented in this paper. The correlations between longitudinal and lateral
distributions are much more pronounced for primary gamma-ray than for primary
proton showers.Comment: 21 pages, 11 figures, accepted for publication in Journal of Physics
Upper Bounds on the Neutrino-Nucleon Inelastic Cross Section
Extraterrestrial neutrinos can initiate deeply developing air showers, and
those that traverse the atmosphere unscathed may produce cascades in the ice or
water. Up to now, no such events have been observed. This can be translated
into upper limits on the diffuse neutrino flux. On the other hand, the
observation of cosmic rays with primary energies > 10^{10} GeV suggests that
there is a guaranteed flux of cosmogenic neutrinos, arising from the decay of
charged pions (and their muon daughters) produced in proton interactions with
the cosmic microwave background. In this work, armed with these cosmogenic
neutrinos and the increased exposure of neutrino telescopes we bring up-to-date
model-independent upper bounds on the neutrino-nucleon inelastic cross section.
Uncertainties in the cosmogenic neutrino flux are discussed and taken into
account in our analysis. The prospects for improving these bounds with the
Pierre Auger Observatory are also estimated. The unprecedented statistics to be
collected by this experiment in 6 yr of operation will probe the
neutrino-nucleon inelastic cross section at the level of Standard Model
predictions.Comment: To be published in JCA
Atmospheric Muon Flux at Sea Level, Underground, and Underwater
The vertical sea-level muon spectrum at energies above 1 GeV and the
underground/underwater muon intensities at depths up to 18 km w.e. are
calculated. The results are particularly collated with a great body of the
ground-level, underground, and underwater muon data. In the hadron-cascade
calculations, the growth with energy of inelastic cross sections and pion,
kaon, and nucleon generation in pion-nucleus collisions are taken into account.
For evaluating the prompt muon contribution to the muon flux, we apply two
phenomenological approaches to the charm production problem: the recombination
quark-parton model and the quark-gluon string model. To solve the muon
transport equation at large depths of homogeneous medium, a semi-analytical
method is used. The simple fitting formulas describing our numerical results
are given. Our analysis shows that, at depths up to 6-7 km w. e., essentially
all underground data on the muon intensity correlate with each other and with
predicted depth-intensity relation for conventional muons to within 10%.
However, the high-energy sea-level data as well as the data at large depths are
contradictory and cannot be quantitatively decribed by a single nuclear-cascade
model.Comment: 47 pages, REVTeX, 15 EPS figures included; recent experimental data
and references added, typos correcte
Cosmic Rays: The Second Knee and Beyond
We conduct a review of experimental results on Ultra-High Energy Cosmic Rays
(UHECR's) including measurements of the features of the spectrum, the
composition of the primary particle flux and the search for anisotropy in event
arrival direction. We find that while there is a general consensus on the
features in the spectrum -- the Second Knee, the Ankle, and (to a lesser
extent) the GZK Cutoff -- there is little consensus on the composition of the
primaries that accompany these features. This lack of consensus on the
composition makes interpretation of the agreed upon features problematic. There
is also little direct evidence about potential sources of UHECRs, as early
reports of arrival direction anisotropies have not been confirmed in
independent measurements.Comment: 46 pages, 30 figures. Topical Review to appear in J. Physics
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