Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: "Classical" instabilities and "quantum" speed-up in the evolution of
  neutrino clouds   We study some examples of collective behavior in neutrino clouds governed by
the neutral-current neutrino-neutrino interaction. The standard equations for
analyzing such systems are rederived in a two-step process: first, a
replacement of the full interaction Hamiltonian with a "forward" Hamiltonian
that contains only the momentum states that were initially occupied by a
neutrino of one flavor or another; second, a factorization assumption that
reduces the time evolution problem to the solution of coupled equations for the
expectations of various bilinear forms in the neutrino fields. We designate the
latter as the "classical" equations. We analyze some solutions of these
equations in cases in which the initial momentum and flavor distributions of
neutrinos are strongly anisotropic in space. In some cases we find an
instability that leads to rapid evolution of the flavor-angle distribution,
even when it is seeded by a very small initial flavor mixing (or alternatively
by a very small neutrino mass$^2$ term). Turning to the more complete case in
which we do not assume the classical factorization, but instead solve for the
evolution under the influence of the full "forward" Hamiltonian, we find the
possibility of rapid evolution, under our definition, even when there is no
seeding from conventional neutrino mixing.

27
The categories of this paper are: comp-gas
7
The categories of this paper are: hep-th
19
The categories of this paper are: eess
13
The categories of this paper are: quant-ph
32
The categories of this paper are: atom-ph
1
The categories of this paper are: cs
15
The categories of this paper are: nucl-ex
20
The categories of this paper are: acc-phys
16
The categories of this paper are: stat
12
The categories of this paper are: nucl-th
2
The categories of this paper are: math
22
The categories of this paper are: chao-dyn
26
The categories of this paper are: bayes-an
4
The categories of this paper are: cond-mat
5
The categories of this paper are: gr-qc
25
The categories of this paper are: patt-sol