Stars as Laboratories for Fundamental Physics - MPP Theory Group

Processes in a Nuclear Medium 157
Instead of an axion one may also emit a neutrino pair in Fig. 4.12.
The corresponding energy-loss rate was worked out by Muto and Tatsumi
(1988) who found
Q νν = 2π
945
˜C 2 A G 2 Fm 2 N A 2 κ2 0 T 6
|k π | . (4.83)
Here, ˜C A is the renormalized neutron neutral-current coupling constant;
in vacuum C A = −1.26/2. A somewhat different emission rate was
found by Senatorov and Voskresensky (1987).
A process related to that shown in Fig. 4.12 is pionic Compton
scattering πN → Na involving thermal pions. The equivalent of the
axion emission rate was calculated by Turner (1992) and by Raffelt and
Seckel (1995). This process could be of some importance in the hot
nondegenerate medium of a SN core. However, a thermal population of
pions yields a rate which is always less important than bremsstrahlung.
For a charged pion or kaon condensate, the equivalent of the modified
URCA process can occur by exchanging a meson with the condensate
rather than with a bystander nucleon. Recent calculations of
such processes include Senatorov and Voskresensky (1987), Muto and
Tatsumi (1988), and Thorsson et al. (1995).
4.9.2 Quark Matter
It has been speculated that a “neutron” star may actually undergo a
phase transition where the nucleons dissolve in favor of a quark-gluon
plasma. Notably, it is possible that the true ground state of nuclear
matter consists of “strange quark matter” with about equal numbers
of up, down, and strange quarks. Such a system can emit axions by
virtue of quark-quark bremsstrahlung (Fig. 4.13).
Fig. 4.13. Axion emission by quark-quark bremsstrahlung. There is a total
of eight amplitudes, four with the axion attached to each quark line, and an
exchange graph each with q 3 ↔ q 4 .