Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Improved description of threshold pion electroproduction in chiral
  perturbation theory   We investigate neutral pion electroproduction off protons in the framework of
heavy baryon chiral perturbation theory. The chiral expansion of the S--wave
multipoles $E_{0+}$ and $L_{0+}$ is carried out to three orders. There appear
several undetermined low--energy constants. Three are taken from a recent study
of the new TAPS threshold $\pi^0$ photoproduction data, one is fixed from the
proton Dirac form factor and the novel S--wave constants appearing at orders
$q^4$ and $q^5$ are determined from a best fit (constrained by resonance
exchange) to the recent NIKHEF and MAMI data at photon momentum transfer
squared $k^2 = -0.1 \, \, {\rm GeV}^2$. The inclusion of a particular set of
dimension five operators is forced upon the fit by a soft--pion theorem which
severely constrains the momentum dependence of the longitudinal S--wave
multipole $L_{0+}$ at order $q^4$. We give predictions for lower photon
virtualities for the various multipoles and differential cross sections.
Further improvements are briefly touched upon.

2
The categories of this paper are: math
18
The categories of this paper are: econ
9
The categories of this paper are: nlin
21
The categories of this paper are: adap-org
37
The categories of this paper are: supr-con
5
The categories of this paper are: gr-qc
11
The categories of this paper are: hep-lat
36
The categories of this paper are: cmp-lg
24
The categories of this paper are: dg-ga
19
The categories of this paper are: eess
6
The categories of this paper are: astro-ph
10
The categories of this paper are: q-bio
16
The categories of this paper are: stat
30
The categories of this paper are: q-alg
29
The categories of this paper are: funct-an
20
The categories of this paper are: acc-phys