Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Pulsed light beams in vacuum with superluminal and negative group
  velocities   Gouy's phase of transversally limited pulses can create a strong anomalous
dispersion in vacuum leading to highly superluminal and negative group
velocities. As a consequence, a focusing pulse can diverge beyond the focus
before converging into it. A simple experiment is proposed.

25
The categories of this paper are: patt-sol
10
The categories of this paper are: q-bio
2
The categories of this paper are: math
4
The categories of this paper are: cond-mat
12
The categories of this paper are: nucl-th
15
The categories of this paper are: nucl-ex
19
The categories of this paper are: eess
32
The categories of this paper are: atom-ph
20
The categories of this paper are: acc-phys
0
The categories of this paper are: hep-ph
31
The categories of this paper are: ao-sci
1
The categories of this paper are: cs
16
The categories of this paper are: stat
17
The categories of this paper are: q-fin
36
The categories of this paper are: cmp-lg
37
The categories of this paper are: supr-con