Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Some Comments on Ghosts and Unitarity: The Pais-Uhlenbeck Oscillator
  Revisited   We give a simple discussion of ghosts, unitarity violation, negative norm
states and quantum vs classical behavior in the simplest model with four
derivative action - the Pais-Uhlenbeck oscillator. We also point out that the
normalizable "vacuum state" (in the sense defined below) of this model can be
understood as spontaneous breaking of the emergent conformal symmetry. We
provide an example of an interacting system that couples the "particle" and
"ghost" degrees of freedom and nevertheless remains unitary on both classical
and quantum level.

1
The categories of this paper are: cs
22
The categories of this paper are: chao-dyn
25
The categories of this paper are: patt-sol
33
The categories of this paper are: plasm-ph
35
The categories of this paper are: mtrl-th
31
The categories of this paper are: ao-sci
34
The categories of this paper are: chem-ph
21
The categories of this paper are: adap-org
30
The categories of this paper are: q-alg
18
The categories of this paper are: econ
28
The categories of this paper are: alg-geom
2
The categories of this paper are: math
4
The categories of this paper are: cond-mat
27
The categories of this paper are: comp-gas
29
The categories of this paper are: funct-an
36
The categories of this paper are: cmp-lg