Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Axial momentum for the relativistic Majorana particle   The Hilbert space of states of the relativistic Majorana particle consists of
normalizable bispinors with real components, and the usual momentum operator $-
i \nabla$ can not be defined in this space. For this reason, we introduce the
axial momentum operator, $ - i \gamma_5 \nabla$ as a new observable for this
particle. In the Heisenberg picture, the axial momentum contains a component
which oscillates with the amplitude proportional to $m/E$, where $E$ is the
energy and $m$ the mass of the particle. The presence of the oscillations
discriminates between the massive and massless Majorana particle. We show how
the eigenvectors of the axial momentum, called the axial plane waves, can be
used as a basis for obtaining the general solution of the evolution equation,
also in the case of free Majorana field. Here a novel feature is a coupling of
modes with the opposite momenta, again present only in the case of massive
particle or field.

32
The categories of this paper are: atom-ph
10
The categories of this paper are: q-bio
25
The categories of this paper are: patt-sol
24
The categories of this paper are: dg-ga
35
The categories of this paper are: mtrl-th
26
The categories of this paper are: bayes-an
18
The categories of this paper are: econ
11
The categories of this paper are: hep-lat
30
The categories of this paper are: q-alg
31
The categories of this paper are: ao-sci
4
The categories of this paper are: cond-mat
14
The categories of this paper are: math-ph
21
The categories of this paper are: adap-org
12
The categories of this paper are: nucl-th
8
The categories of this paper are: hep-ex
37
The categories of this paper are: supr-con