Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Near-threshold $\eta$ production in $pp$ collisions   We study near-threshold $\eta$ meson production in $pp$ collisions within an
effective Lagrangian approach combined with the isobar model, by allowing for
the various intermediate nucleon resonances due to the $\pi$, $\eta$, and
$\rho$-meson exchanges. It is shown that the $\rho$-meson exchange is the
dominant excitation mechanism for these resonances, and the contribution from
the $N^*(1720)$ is dominant. The total cross section data can be reasonably
reproduced, and the anisotropic angular distributions of the emitted $\eta$
meson are consistent with experimental measurements. Besides, the invariant
mass spectra of $pp$ and $p\eta$ explain the data well at excess energy of 15
MeV, and are basically consistent with the data at excess energy of 40 MeV.
However, our model calculations cannot reasonably account for the two-peak
structure in the $p\eta$ distribution at excess energies of 57 and 72 MeV,
which suggests that a more complicated mechanism is needed at higher energy
region.

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The categories of this paper are: hep-th
31
The categories of this paper are: ao-sci
32
The categories of this paper are: atom-ph
5
The categories of this paper are: gr-qc
21
The categories of this paper are: adap-org
3
The categories of this paper are: physics
8
The categories of this paper are: hep-ex
10
The categories of this paper are: q-bio
28
The categories of this paper are: alg-geom
33
The categories of this paper are: plasm-ph
19
The categories of this paper are: eess
17
The categories of this paper are: q-fin
13
The categories of this paper are: quant-ph
25
The categories of this paper are: patt-sol
37
The categories of this paper are: supr-con
15
The categories of this paper are: nucl-ex