Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On the Challenges of Cosmic-Ray Proton Shock Acceleration in the
  Intracluster Medium   Galaxy clusters host the largest particle accelerators in the Universe: Shock
waves in the intracluster medium (ICM), a hot and ionised plasma, that
accelerate particles to high energies. Radio observations pick up synchrotron
emission in the ICM, proving the existence of accelerated cosmic-ray electrons.
However, a sign of cosmic-ray protons, in form of $\gamma$-rays. remains
undetected. This is know as the missing $\gamma$-ray problem and it directly
challenges the shock acceleration mechanism at work in the ICM. Over the last
decade, theoretical and numerical studies focused on improving our knowledge on
the microphysics that govern the shock acceleration process in the ICM. These
new models are able to predict a $\gamma$-ray signal, produced by shock
accelerated cosmic-ray protons, below the detection limits set modern
$\gamma$-ray observatories. In this review, we summarise the latest advances in
solving the missing $\gamma$-ray problem.

11
The categories of this paper are: hep-lat
8
The categories of this paper are: hep-ex
29
The categories of this paper are: funct-an
24
The categories of this paper are: dg-ga
12
The categories of this paper are: nucl-th
16
The categories of this paper are: stat
7
The categories of this paper are: hep-th
1
The categories of this paper are: cs
27
The categories of this paper are: comp-gas
10
The categories of this paper are: q-bio
15
The categories of this paper are: nucl-ex
30
The categories of this paper are: q-alg
19
The categories of this paper are: eess
37
The categories of this paper are: supr-con
31
The categories of this paper are: ao-sci
13
The categories of this paper are: quant-ph