Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Observation of double radiative capture on pionic hydrogen   We report the first observation of double radiative capture on pionic
hydrogen. The experiment was conducted at the TRIUMF cyclotron using the RMC
spectrometer, and detected $\gamma$--ray coincidences following $\pi^-$ stops
in liquid hydrogen. We found the branching ratio for double radiative capture
to be $(3.05 \pm 0.27(stat.) \pm 0.31(syst.)) \times 10^{-5}$. The measured
branching ratio and angle-energy distributions support the theoretical
prediction of a dominant contribution from the $\pi \pi \to \gamma \gamma$
annihilation mechanism.

12
The categories of this paper are: nucl-th
14
The categories of this paper are: math-ph
4
The categories of this paper are: cond-mat
0
The categories of this paper are: hep-ph
17
The categories of this paper are: q-fin
5
The categories of this paper are: gr-qc
26
The categories of this paper are: bayes-an
33
The categories of this paper are: plasm-ph
21
The categories of this paper are: adap-org
3
The categories of this paper are: physics
1
The categories of this paper are: cs
7
The categories of this paper are: hep-th
10
The categories of this paper are: q-bio
13
The categories of this paper are: quant-ph
32
The categories of this paper are: atom-ph
11
The categories of this paper are: hep-lat