Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A Break in the Highest Energy Cosmic Ray Spectrum: A Signature of New
  Physics?   Recent experimental data from the Fly's Eye and the Akeno array seem to
indicate significant structure in the ultrahigh energy cosmic ray spectrum
above $10^{18}\eV$. A statistically significant dip has been established at
about $5\times10^{18}\eV$. In addition, each experiment observed a different
superhigh energy event above $10^{20}\eV$ separated from the rest of the data
by about half a decade in energy. In this article we discuss what this implies
for the existence or non-existence of the ``Greisen-Zatsepin-Kuz'min cutoff'',
a long lasting and still open question in cosmic ray physics. This cutoff,
caused by energy losses in the cosmic microwave background, is predicted to
occur at a few times $10^{19}\eV$ if cosmic rays are produced by shock
acceleration of lower energy particles at extragalactic distances. We show that
from the spectral point of view, sources nearer than a few $\Mpc$ are still
consistent with the data at the $1\sigma$ level, provided these sources
accelerate particles beyond $3\times10^{20}\eV$. However, persistence of the
apparent gap in the existing data at the level of a 4 times higher total
exposure would rule out a wide range of acceleration models at $98\%$ C.L.,
whether they rely on nearby or extragalactic sources. This might hint to the
existence of a ``top down'' mechanism which produces an additional hard
component of ultrahigh energy particles directly, say, by decay from some
higher energy scale in contrast to bottom up acceleration of charged particles.
In this scenario a cutoff followed by a pronounced spectral flattening and
possibly even a gap could naturally be formed.

25
The categories of this paper are: patt-sol
16
The categories of this paper are: stat
9
The categories of this paper are: nlin
33
The categories of this paper are: plasm-ph
27
The categories of this paper are: comp-gas
18
The categories of this paper are: econ
4
The categories of this paper are: cond-mat
35
The categories of this paper are: mtrl-th
36
The categories of this paper are: cmp-lg
34
The categories of this paper are: chem-ph
8
The categories of this paper are: hep-ex
12
The categories of this paper are: nucl-th
22
The categories of this paper are: chao-dyn
13
The categories of this paper are: quant-ph
5
The categories of this paper are: gr-qc
20
The categories of this paper are: acc-phys