728 research outputs found
Features of Muon Arrival Time Distributions of High Energy EAS at Large Distances From the Shower Axis
In view of the current efforts to extend the KASCADE experiment
(KASCADE-Grande) for observations of Extensive Air Showers (EAS) of primary
energies up to 1 EeV, the features of muon arrival time distributions and their
correlations with other observable EAS quantities have been scrutinised on
basis of high-energy EAS, simulated with the Monte Carlo code CORSIKA and using
in general the QGSJET model as generator. Methodically various correlations of
adequately defined arrival time parameters with other EAS parameters have been
investigated by invoking non-parametric methods for the analysis of
multivariate distributions, studying the classification and misclassification
probabilities of various observable sets. It turns out that adding the arrival
time information and the multiplicity of muons spanning the observed time
distributions has distinct effects improving the mass discrimination. A further
outcome of the studies is the feature that for the considered ranges of primary
energies and of distances from the shower axis the discrimination power of
global arrival time distributions referring to the arrival time of the shower
core is only marginally enhanced as compared to local distributions referring
to the arrival of the locally first muon.Comment: 24 pages, Journal Physics G accepte
Ultra high energy neutrinos from gamma ray bursts
Protons accelerated to high energies in the relativistic shocks that generate
gamma ray bursts photoproduce pions, and then neutrinos in situ. I show that
ultra high energy neutrinos (> 10^19 eV) are produced during the burst and the
afterglow. A larger flux, also from bursts, is generated via photoproduction
off CMBR photons in flight but is not correlated with currently observable
bursts, appearing as a bright background. Adiabatic/synchrotron losses from
protons/pions/muons are negligible. Temporal and directional coincidences with
bursts detected by satellites can separate correlated neutrinos from the
background.Comment: Adiabatic/synchrotron losses from protons/pions/muons shown to be
negligible. Accepted for publication in Phys. Rev. Letters. RevTe
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
Search for Sub-TeV Gamma Rays Coincident with BATSE Gamma Ray Bursts
Project GRAND is a 100m x 100m air shower array of proportional wire chambers
(PWCs). There are 64 stations each with eight 1.29 m^2 PWC planes arranged in
four orthogonal pairs placed vertically above one another to geometrically
measure the angles of charged secondaries. A steel plate above the bottom pair
of PWCs differentiates muons (which pass undeflected through the steel) from
non-penetrating particles. FLUKA Monte Carlo studies show that a TeV gamma ray
striking the atmosphere at normal incidence produces 0.23 muons which reach
ground level where their angles and identities are measured. Thus,
paradoxically, secondary muons are used as a signature for gamma ray primaries.
The data are examined for possible angular and time coincidences with eight
gamma ray bursts (GRBs) detected by BATSE. Seven of the GRBs were selected
because of their good acceptance by GRAND and high BATSE Fluence. The eighth
GRB was added due to its possible coincident detection by Milagrito. For each
of the eight candidate GRBs, the number of excess counts during the BATSE T90
time interval and within plus or minus five degrees of BATSE's direction was
obtained. The highest statistical significance reported in this paper (2.7
sigma) is for the event that was predicted to be the most likely to be observed
(GRB 971110).Comment: To be presented at the XXVIII International Cosmic Ray Conference,
Tsukuba, Japa
Large Electric Dipole Moments of Heavy Neutrinos
In many models of CP violation, the electric dipole moment (EDM) of a heavy
charged or neutral lepton could be very large. We present an explicit model in
which a heavy neutrino EDM can be as large as e-cm, or even a factor
of ten larger if fine-tuning is allowed, and use an effective field theory
argument to show that this result is fairly robust. We then look at the
production cross section for these neutrinos, and by rederiving the Bethe-Block
formula, show that they could leave an ionization track. It is then noted that
the first signature of heavy neutrinos with a large EDM would come from
, leading to a very large rate for single photon plus
missing energy events, and the rate and angular distribution are found.
Finally, we look at some astrophysical consequences, including whether these
neutrinos could constitute the UHE cosmic rays and whether their decays in the
early universe could generate a net lepton asymmetry.Comment: 22 pages, 9 figure
Strangelets as Cosmic Rays beyond the Greisen-Zatsepin-Kuzmin Cutoff
Strangelets (stable lumps of quark matter) can have masses and charges much
higher than those of nuclei, but have very low charge-to-mass ratios. This is
confirmed in a relativistic Thomas-Fermi model. The high charge allows
astrophysical strangelet acceleration to energies orders of magnitude higher
than for protons. In addition, strangelets are much less susceptible to the
interactions with the cosmic microwave background that suppress the flux of
cosmic ray protons and nuclei above energies of -- eV (the
GZK-cutoff). This makes strangelets an interesting possibility for explaining
ultra-high energy cosmic rays.Comment: Physical Review Letters (in press
Consequences of parton's saturation and string's percolation on the developments of cosmic ray showers
At high gluon or string densities, gluons' saturation or the strong
interaction among strings, either forming colour ropes or giving rise to
string's percolation, induces a strong suppression in the particle
multiplicities produced at high energy. This suppression implies important
modifications on cosmic ray shower development. In particular, it is shown that
it affects the depth of maximum, the elongation rate, and the behaviour of the
number of muons at energies around 10^{17}-10^{18} eV. The existing cosmic ray
data point out in the same direction.Comment: Latex. 10 pages 2 figure
Induction of microRNAs, mir-155, mir-222, mir-424 and mir-503, promotes monocytic differentiation through combinatorial regulation
Acute myeloid leukemia (AML) involves a block in terminal differentiation of
the myeloid lineage and uncontrolled proliferation of a progenitor state. Using
phorbol myristate acetate (PMA), it is possible to overcome this block in THP-1
cells (an M5-AML containing the MLL-MLLT3 fusion), resulting in differentiation
to an adherent monocytic phenotype. As part of FANTOM4, we used microarrays to
identify 23 microRNAs that are regulated by PMA. We identify four PMA-induced
micro- RNAs (mir-155, mir-222, mir-424 and mir-503) that when overexpressed
cause cell-cycle arrest and partial differentiation and when used in
combination induce additional changes not seen by any individual microRNA. We
further characterize these prodifferentiative microRNAs and show that mir-155
and mir-222 induce G2 arrest and apoptosis, respectively. We find mir-424 and
mir-503 are derived from a polycistronic precursor mir-424-503 that is under
repression by the MLL-MLLT3 leukemogenic fusion. Both of these microRNAs
directly target cell-cycle regulators and induce G1 cell-cycle arrest when
overexpressed in THP-1. We also find that the pro-differentiative mir-424 and
mir-503 downregulate the anti-differentiative mir-9 by targeting a site in its
primary transcript. Our study highlights the combinatorial effects of multiple
microRNAs within cellular systems.Comment: 45 pages 5 figure
The Extremely High Energy Cosmic Rays
Experimental results from Haverah Park, Yakutsk, AGASA and Fly's Eye are
reviewed. All these experiments work in the energy range above 0.1 EeV. The
'dip' structure around 3 EeV in the energy spectrum is well established by all
the experiments, though the exact position differs slightly. Fly's Eye and
Yakutsk results on the chemical composition indicate that the cosmic rays are
getting lighter over the energy range from 0.1 EeV to 10 EeV, but the exact
fraction is hadronic interaction model dependent, as indicated by the AGASA
analysis. The arrival directions of cosmic rays are largely isotropic, but
interesting features may be starting to emerge. Most of the experimental
results can best be explained with the scenario that an extragalactic component
gradually takes over a galactic population as energy increases and cosmic rays
at the highest energies are dominated by particles coming from extragalactic
space. However, identification of the extragalactic sources has not yet been
successful because of limited statistics and the resolution of the data.Comment: The review paper including 21 figures. 39 pages: To be published in
Journal of Physics
- âŠ