Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Calculation of $<p|\overline{u}u-\overline{d}d|p>$ from QCD sum rule and
  the neutron-proton mass difference   The proton matrix element of the isovector-scalar density,
$<p|\overline{u}u-\overline{d}d|p>/2M_p$, is calculated by evaluating the
nucleon current correlation function in an external isovector-scalar field
using the QCD sum-rule method. In addition to the usual chiral and gluon
condensates of the QCD vacuum, the response of the chiral condensates to the
external isovector field enters the calculation. The latter is determined by
two independent methods. One relates it to the difference between the up and
down quark chiral condensates and the other uses the chiral perturbation
theory. To first order in the quark mass difference $\delta m=m_d-m_u$, the
non-electromagnetic part of the neutron-proton mass difference is given by the
product $\delta m <p|\overline{u}u-\overline{d}d|p>/2M_p$; the resulting value
is in reasonable agreement with the experimental value.

14
The categories of this paper are: math-ph
4
The categories of this paper are: cond-mat
36
The categories of this paper are: cmp-lg
13
The categories of this paper are: quant-ph
9
The categories of this paper are: nlin
37
The categories of this paper are: supr-con
22
The categories of this paper are: chao-dyn
31
The categories of this paper are: ao-sci
2
The categories of this paper are: math
24
The categories of this paper are: dg-ga
16
The categories of this paper are: stat
8
The categories of this paper are: hep-ex
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The categories of this paper are: q-fin
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The categories of this paper are: adap-org
10
The categories of this paper are: q-bio
35
The categories of this paper are: mtrl-th