Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Domain Walls in QCD   QCD was shown to have a nontrivial vacuum structure due to the topology of
the theta = theta+2*pi*n parameter. As a result of this nontrivial topology, in
the large N_c limit, quasi-stable QCD domain walls appear, characterized by a
transition in the singlet eta' field. We discuss the physics of these QCD
domain walls as well as related axion domain walls and we present a new type of
axion wall which also contains an eta' transition. We argue that these domain
walls are topologically stable in the limit N_c->infinity and classically
stable for large but finite N_c, however, they can decay through a tunneling
process. We argue that the qualitative features of these QCD domain walls --
namely their classical stability -- persist to the realistic case of N_c=3 and
that it is at least possible that their lifetime could be macroscopically
large. If it is, then QCD domain walls could play an important role in the
evolution of early universe and may be detectable in energetic collisions such
as those at the Relativistic Heavy Ion Collider (RHIC).

21
The categories of this paper are: adap-org
34
The categories of this paper are: chem-ph
35
The categories of this paper are: mtrl-th
8
The categories of this paper are: hep-ex
15
The categories of this paper are: nucl-ex
18
The categories of this paper are: econ
13
The categories of this paper are: quant-ph
2
The categories of this paper are: math
19
The categories of this paper are: eess
28
The categories of this paper are: alg-geom
26
The categories of this paper are: bayes-an
20
The categories of this paper are: acc-phys
31
The categories of this paper are: ao-sci
33
The categories of this paper are: plasm-ph
16
The categories of this paper are: stat
14
The categories of this paper are: math-ph