Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Spin of the proton and orbital motion of quarks   Effect of the quark intrinsic motion on the proton spin structure functions
is demonstrated. It is shown, that the covariant version of the quark-parton
model taking into account the orbital motion gives the consistent picture of
the proton spin structure, which is based on the valence quarks. This picture
is supported by the recent data, which indicate, that the spin contributions
from the sea quarks and gluons are compatible with zero.

2
The categories of this paper are: math
31
The categories of this paper are: ao-sci
18
The categories of this paper are: econ
14
The categories of this paper are: math-ph
20
The categories of this paper are: acc-phys
11
The categories of this paper are: hep-lat
1
The categories of this paper are: cs
25
The categories of this paper are: patt-sol
17
The categories of this paper are: q-fin
8
The categories of this paper are: hep-ex
7
The categories of this paper are: hep-th
22
The categories of this paper are: chao-dyn
13
The categories of this paper are: quant-ph
12
The categories of this paper are: nucl-th
36
The categories of this paper are: cmp-lg
29
The categories of this paper are: funct-an