Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The p-bar p --> pi_0 pi_0 Puzzle   According to conventional theory, the annihilation reaction p-bar p --> pi_0
pi_0 cannot occur from a p-bar p atomic S state. However, this reaction occurs
so readily for antiprotons stopping in liquid hydrogen, that it would require
30% P-wave annihilations. Experimental results from other capture and p-bar p
annihilation channels show that the fraction of P-wave annihilations is less
than 6% in agreement with theoretical expectations. An experimental test to
determine whether this reaction can occur from an atomic S state is suggested.
If indeed this reaction is occurring from an atomic S state, then certain
neutral vector mesons should exhibit a pi_0 pi_0 decay mode, and this can also
be tested experimentally.

23
The categories of this paper are: solv-int
2
The categories of this paper are: math
19
The categories of this paper are: eess
4
The categories of this paper are: cond-mat
12
The categories of this paper are: nucl-th
20
The categories of this paper are: acc-phys
25
The categories of this paper are: patt-sol
26
The categories of this paper are: bayes-an
8
The categories of this paper are: hep-ex
32
The categories of this paper are: atom-ph
27
The categories of this paper are: comp-gas
9
The categories of this paper are: nlin
13
The categories of this paper are: quant-ph
1
The categories of this paper are: cs
3
The categories of this paper are: physics
28
The categories of this paper are: alg-geom