Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Gauge Invariance and the Unstable Particle   It is shown how to construct exactly gauge-invariant S-matrix elements for
processes involving unstable gauge particles such as the $Z^0$ boson. The
results are applied to derive a physically meaningful expression for the
cross-section $\sigma(e^+e^-\to Z^0Z^0)$ and thereby provide a solution to the
long-standing problem of the unstable particle.

16
The categories of this paper are: stat
12
The categories of this paper are: nucl-th
29
The categories of this paper are: funct-an
36
The categories of this paper are: cmp-lg
2
The categories of this paper are: math
15
The categories of this paper are: nucl-ex
13
The categories of this paper are: quant-ph
31
The categories of this paper are: ao-sci
35
The categories of this paper are: mtrl-th
11
The categories of this paper are: hep-lat
6
The categories of this paper are: astro-ph
8
The categories of this paper are: hep-ex
22
The categories of this paper are: chao-dyn
5
The categories of this paper are: gr-qc
28
The categories of this paper are: alg-geom
14
The categories of this paper are: math-ph