Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Measurement of the beam-helicity asymmetry $I^{\odot}$ in the
  photoproduction of $\pi^0$-pairs off the proton and off the neutron   Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz
for the photoproduction of neutral pion pairs in the reactions
$\vec{\gamma}p\rightarrow p\pi^0\pi^0$ and $\vec{\gamma}d\rightarrow
(n)p\pi^0\pi^0$, $\vec{\gamma}d\rightarrow (p)n\pi^0\pi^0$ off free protons and
off quasi-free nucleons bound in the deuteron for incident photon energies up
to 1.4 GeV. Circularly polarized photons were produced from bremsstrahlung of
longitudinally polarized electrons and tagged with the Glasgow magnetic
spectrometer. Decay photons from the $\pi^0$ mesons, recoil protons, and recoil
neutrons were detected in the 4$\pi$ covering electromagnetic calorimeter
composed of the Crystal Ball and TAPS detectors. After kinematic reconstruction
of the final state, excellent agreement was found between the results for free
and quasi-free protons. This demonstrates that the free-nucleon behavior of
such observables can be extracted from measurements with quasi-free nucleons,
which is the only possibility for the neutron. Contrary to expectations, the
measured asymmetries are very similar for reactions off protons and neutrons.
The results are compared to the predictions from the Two-Pion-MAID reaction
model and (for the proton) also to the Bonn-Gatchina coupled channel analysis.

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The categories of this paper are: cmp-lg
25
The categories of this paper are: patt-sol
20
The categories of this paper are: acc-phys
10
The categories of this paper are: q-bio
22
The categories of this paper are: chao-dyn
12
The categories of this paper are: nucl-th
29
The categories of this paper are: funct-an
9
The categories of this paper are: nlin
21
The categories of this paper are: adap-org
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The categories of this paper are: supr-con
2
The categories of this paper are: math
28
The categories of this paper are: alg-geom
34
The categories of this paper are: chem-ph
30
The categories of this paper are: q-alg
33
The categories of this paper are: plasm-ph
11
The categories of this paper are: hep-lat