Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The non-gravitational interactions of dark matter in colliding galaxy
  clusters   Collisions between galaxy clusters provide a test of the non-gravitational
forces acting on dark matter. Dark matter's lack of deceleration in the `bullet
cluster collision' constrained its self-interaction cross-section \sigma_DM/m <
1.25cm2/g (68% confidence limit) for long-ranged forces. Using the Chandra and
Hubble Space Telescopes we have now observed 72 collisions, including both
`major' and `minor' mergers. Combining these measurements statistically, we
detect the existence of dark mass at 7.6\sigma significance. The position of
the dark mass has remained closely aligned within 5.8+/-8.2 kpc of associated
stars: implying a self-interaction cross-section \sigma_DM/m < 0.47 cm2/g (95%
CL) and disfavoring some proposed extensions to the standard model.

30
The categories of this paper are: q-alg
12
The categories of this paper are: nucl-th
37
The categories of this paper are: supr-con
7
The categories of this paper are: hep-th
29
The categories of this paper are: funct-an
27
The categories of this paper are: comp-gas
9
The categories of this paper are: nlin
5
The categories of this paper are: gr-qc
11
The categories of this paper are: hep-lat
4
The categories of this paper are: cond-mat
19
The categories of this paper are: eess
33
The categories of this paper are: plasm-ph
26
The categories of this paper are: bayes-an
2
The categories of this paper are: math
18
The categories of this paper are: econ
31
The categories of this paper are: ao-sci