Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On the mass difference between proton and neutron   The Cottingham formula expresses the electromagnetic part of the mass of a
particle in terms of the virtual Compton scattering amplitude. At large photon
momenta, this amplitude is dominated by short distance singularities associated
with operators of spin 0 and spin 2. In the difference between proton and
neutron, chiral symmetry suppresses the spin 0 term. Although the angular
integration removes the spin 2 singularities altogether, the various pieces
occurring in the standard decomposition of the Cottingham formula do pick up
such contributions. These approach asymptotics extremely slowly because the
relevant Wilson coefficients only fall off logarithmically. We rewrite the
formula in such a way that the leading spin 2 contributions are avoided ab
initio. Using a sum rule that follows from Reggeon dominance, the numerical
evaluation of the e.m. part of the mass difference between proton and neutron
yields $m_{QED}^{p-n}=0.58\pm 0.16\,$MeV. The result indicates that the
inelastic contributions are small compared to the elastic ones.

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The categories of this paper are: nucl-th
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The categories of this paper are: math
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The categories of this paper are: adap-org
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The categories of this paper are: ao-sci
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The categories of this paper are: acc-phys
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The categories of this paper are: bayes-an
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The categories of this paper are: nucl-ex
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The categories of this paper are: gr-qc
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The categories of this paper are: q-bio
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The categories of this paper are: cmp-lg
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The categories of this paper are: supr-con
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The categories of this paper are: cond-mat
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The categories of this paper are: mtrl-th
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The categories of this paper are: comp-gas
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The categories of this paper are: chao-dyn
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The categories of this paper are: nlin