Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Behavior of Matter under Extreme Conditions   The cores of neutron stars harbor the highest matter densities known to occur
in nature, up to several times the densities in atomic nuclei. Similarly,
magnetic field strengths can exceed the strongest fields generated in
terrestrial laboratories by ten orders of magnitude. Hyperon-dominated matter,
deconfined quark matter, superfluidity, even superconductivity are predicted in
neutron stars. Similarly, quantum electrodynamics predicts that in strong
magnetic fields the vacuum becomes birefringent. The properties of matter under
such conditions is governed by Quantum Chromodynamics (QCD) and Quantum
Electrodynamics (QED), and the close study of the properties of neutron stars
offers the unique opportunity to test and explore the richness of QCD and QED
in a regime that is utterly beyond the reach of terrestrial experiments.
Experimentally, this is almost virgin territory.

17
The categories of this paper are: q-fin
7
The categories of this paper are: hep-th
31
The categories of this paper are: ao-sci
36
The categories of this paper are: cmp-lg
35
The categories of this paper are: mtrl-th
30
The categories of this paper are: q-alg
25
The categories of this paper are: patt-sol
14
The categories of this paper are: math-ph
2
The categories of this paper are: math
11
The categories of this paper are: hep-lat
5
The categories of this paper are: gr-qc
29
The categories of this paper are: funct-an
32
The categories of this paper are: atom-ph
20
The categories of this paper are: acc-phys
3
The categories of this paper are: physics
18
The categories of this paper are: econ