Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Heavy Ion Collisions: Achievements and Challenges   A decade ago brief summary of the field could be formulated as a discovery of
strongly-coupled Quark-Gluon-Plasma, sQGP, making a very good liquid with
surprisingly small viscosity. Since 2010 we have LHC program, which added a lot
to our understanding, and now there seems to be a need to consolidate what we
learned and formulate a list of issues to be studied next. Hydrodynamical
perturbations, leading to higher harmonics of angular correlations, are
identified as long-lived sound waves. Recently studied reactions involving
sounds include phonon decays into two ("loop viscosity"), phonon+magnetic field
into photons/dileptons (sono-magneto-luminescence), and two phonons into a
gravity wave, a penetrating probe of the Big Bang. The mainstream issues in the
field now include a quest to study transition between $pp,pA$ and heavy ion
$AA$ collisions, with an aim to locate "the smallest drops" of the sQGP
displaying collective/hydrodynamics behavior. The issues related to
out-of-equilibrium stage of the collisions, and mechanisms of the
equilibration, in weak and strong coupling, are also hotly debated.

27
The categories of this paper are: comp-gas
28
The categories of this paper are: alg-geom
30
The categories of this paper are: q-alg
22
The categories of this paper are: chao-dyn
9
The categories of this paper are: nlin
25
The categories of this paper are: patt-sol
5
The categories of this paper are: gr-qc
33
The categories of this paper are: plasm-ph
17
The categories of this paper are: q-fin
29
The categories of this paper are: funct-an
32
The categories of this paper are: atom-ph
37
The categories of this paper are: supr-con
2
The categories of this paper are: math
19
The categories of this paper are: eess
16
The categories of this paper are: stat
7
The categories of this paper are: hep-th