7,209 research outputs found
Anti-GZK effect in UHECR spectrum
In this paper we discuss the anti-GZK effect that arises in the framework of
the diffusive propagation of Ultra High Energy (UHE) protons. This effect
consists in a jump-like increase of the maximum distance from which UHE protons
can reach the observer. The position of the jump is independent of the
Intergalactic Magnetic Field (IMF) strength and depends only on the energy
losses of protons, namely on the transition energy from adiabatic and
pair-production energy losses. The Ultra High Energy Cosmic Rays (UHECR)
spectrum presents a low-energy steepening approximately at this energy, which
is very close to the position of the observed second knee. The dip, seen in the
universal spectrum as a signature of the proton interaction with the Cosmic
Microwave Background (CMB) radiation, is also present in the case of diffusive
propagation in magnetic fields.Comment: 4 pages, 4 eps figures, talk given at IFAE 2005: Incotri Fisica Alte
Energie, Catania, Italy, 30 March - 2 April 200
Super Heavy Dark Matter in light of BICEP2, Planck and Ultra High Energy Cosmic Rays Observations
The announcement by BICEP2 of the detection of B-mode polarization consistent
with primordial gravitational waves with a tensor-to-scalar ratio,
, challenged predictions from most inflationary models
of a lower value for . More recent results by Planck on polarized dust
emission show that the observed tensor modes signal is compatible with pure
foreground emission. A more significant constraint on was then obtained by
a joint analysis of Planck, BICEP2 and Keck Array data showing an upper limit
to the tensor to scalar ratio , excluding the case with low
statistical significance. Forthcoming measurements by BICEP3, the Keck Array,
and other CMB polarization experiments, open the possibility for making the
fundamental measurement of . Here we discuss how sets the scale for
models where the dark matter is created at the inflationary epoch, the
generically called super-heavy dark matter models. We also consider the
constraints on such scenarios given by recent data from ultrahigh energy cosmic
ray observatories which set the limit on super-heavy dark matter particles
lifetime. We discuss how super-heavy dark matter can be discovered by a precise
measurement of combined with future observations of ultra high energy
cosmic rays.Comment: 17 pages, 14 eps figures, accepted for publication in JCA
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
A dip in the UHECR spectrum and the transition from galactic to extragalactic cosmic rays
The dip is a feature in the diffuse spectrum of ultra-high energy (UHE)
protons caused by electron-positron pair production on the cosmic microwave
background (CMB) radiation. For a power-law generation spectrum , the
calculated position and shape of the dip is confirmed with high accuracy by the
spectra observed by the Akeno-AGASA, HiRes, Yakutsk and Fly's Eye detectors.
When the particle energies, measured in these detectors, are calibrated by the
dip, their fluxes agree with a remarkable accuracy. The predicted shape of the
dip is quite robust. The dip is only modified strongly when the fraction of
nuclei heavier than protons is high at injection, which imposes some
restrictions on the mechanisms of acceleration operating in UHECR sources. The
existence of the dip, confirmed by observations, implies that the transition
from galactic to extragalactic cosmic rays occurs at E \lsim 1\times 10^{18}
eV. We show that at energies lower than a characteristic value eV, the spectrum of extragalactic cosmic rays
flattens in all cases of interest, and it provides a natural transition to a
steeper galactic cosmic ray spectrum. This transition occurs at some energy
below , corresponding to the position of the so-called second knee.
We discuss extensively the constraints on this model imposed by current
knowledge of acceleration processes and sources of UHECR and compare it with
the traditional model of transition at the ankle.Comment: Version Accepted for Publication in Astroparticle Physics (minor
changes
Relativity in space-times with short-distance structure governed by an observer-independent (Planckian) length scale
I show that it is possible to formulate the Relativity postulates in a way
that does not lead to inconsistencies in the case of space-times whose
short-distance structure is governed by an observer-independent length scale.
The consistency of these postulates proves incorrect the expectation that
modifications of the rules of kinematics involving the Planck length would
necessarily require the introduction of a preferred class of inertial
observers. In particular, it is possible for every inertial observer to agree
on physical laws supporting deformed dispersion relations of the type , at least for certain types of .Comment: Same formulas and results as in 1st version, but a change of notation
is introduced in order to clarify that the studied illustrative example is
consistent with the R.P. for both choices of the overall sign. 1 ref added
and 2 refs upgraded. Some rewording of the text in Sec5, and addition of an
analogy with background fields in ordinary electromagnetism which I use to
illustrate difference between space-times with an observer-independent Lp,
and space-times in which Lp is introduced without modifications of Special
Relativit
Analytic calculations of the spectra of ultra high energy cosmic ray nuclei. II. The general case of background radiation
We discuss the problem of ultra high energy nuclei propagation in
extragalactic background radiations. The present paper is the continuation of
the accompanying paper I where we have presented three new analytic methods to
calculate the fluxes and spectra of ultra high energy cosmic ray nuclei, both
primary and secondary, and secondary protons. The computation scheme in this
paper is based on the analytic solution of coupled kinetic equations, which
takes into account the continuous energy losses due to the expansion of the
universe and pair-production, together with photo-disintegration of nuclei.
This method includes in the most natural way the production of secondary nuclei
in the process of photo-disintegration of the primary nuclei during their
propagation through extragalactic background radiations. In paper I, in order
to present the suggested analytical schemes of calculations, we have considered
only the case of the cosmic microwave background radiation, in the present
paper we generalize this computation to all relevant background radiations,
including infra-red and visible/ultra-violet radiations, collectively referred
to as extragalactic background light. The analytic solutions allow transparent
physical interpretation of the obtained spectra. Extragalactic background light
plays an important role at intermediate energies of ultra high energy cosmic
ray nuclei. The most noticeable effect of the extragalactic background light is
the low-energy tail in the spectrum of secondary nuclei.Comment: The paper is the second part of a two papers series, it is composed
by 25 pages and 16 eps figures, version accepted for publication on
Astroparticle Physic
Propagation of UHECRs in cosmological backgrounds: some results from SimProp
Ultra-High-Energy Cosmic Ray (UHECR) nuclei propagating in cosmological
radiation backgrounds produce secondary particles detectable at Earth. SimProp
is a one dimensional code for extragalactic propagation of UHECR nuclei,
inspired by the kinetic approach of Aloisio et al. As in this approach, only a
subset of nuclei and nuclear channels are used as representative. We discuss
the validation of the code and present applications to UHECR experimental
results. In particular we present the expected fluxes of neutrinos produced in
some astrophysical scenario.Comment: Poster presented by A. Di Matteo at the 33rd International Cosmic Ray
Conference, Rio De Janeiro (Brasil) July 2-9 201
Cosmogenic neutrinos and ultra-high energy cosmic ray models
We use an updated version of {\it SimProp}, a Monte Carlo simulation scheme
for the propagation of ultra-high energy cosmic rays, to compute cosmogenic
neutrino fluxes expected on Earth in various scenarios. These fluxes are
compared with the newly detected IceCube events at PeV energies and with recent
experimental limits at EeV energies of the Pierre Auger Observatory. This
comparison allows us to draw some interesting conclusions about the source
models for ultra-high energy cosmic rays. We will show how the available
experimental observations are almost at the level of constraining such models,
mainly in terms of the injected chemical composition and cosmological evolution
of sources. The results presented here will also be important in the evaluation
of the discovery capabilities of the future planned ultra-high energy cosmic
ray and neutrino observatories.Comment: 15 pages, 8 figures, some reference added, version accepted for
publication in JCA
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