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Stars as Laboratories for Fundamental Physics - MPP Theory Group

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Processes in a Nuclear Medium 159<br />

than nuclear matter (Burrows 1979; Anand, Goyal, and Iha 1990). This<br />

observation h<strong>as</strong> little effect on the neutrino cooling rate of a quark<br />

star because the URCA processes, which are b<strong>as</strong>ed on charged-current<br />

reactions, dominate <strong>for</strong> both nuclear or quark matter.<br />

For axions, however, it may seem that the emission rate is much<br />

suppressed relative to nuclear matter. This is certainly true if “axion”<br />

stands <strong>for</strong> any generic pseudoscalar Nambu-Goldstone boson. The<br />

QCD axion, however, which w<strong>as</strong> introduced to solve the CP problem<br />

of strong interactions (Chapter 14) necessarily h<strong>as</strong> a two-gluon coupling<br />

which allows <strong>for</strong> the gluonic Primakoff effect (Fig. 4.14) which is<br />

analogous to the photon Primakoff effect discussed in Sect. 5.2.<br />

Fig. 4.14. Axion emission by the gluon Primakoff effect.<br />

Altherr (1991) h<strong>as</strong> calculated the emissivity of a hot quark-gluon<br />

pl<strong>as</strong>ma and found that it w<strong>as</strong> similar to that of a nuclear medium<br />

at the same temperature and density. Ellis and Salati (1990) found<br />

a much smaller emission rate because they included only the gluonic<br />

pl<strong>as</strong>mon decay process g T → g L + γ, much in analogy to the corresponding<br />

photonic process discussed in Sect. 5.2.2. This decay process,<br />

however, is only part of the axion emissivity by the gluon field fluctuations.<br />

For highly degenerate quark matter corresponding to old neutron<br />

stars the emission rate h<strong>as</strong> not be calculated <strong>as</strong> far <strong>as</strong> I know.<br />

4.9.3 Bubble Ph<strong>as</strong>e<br />

In a hot lepton-rich neutron star the nuclear medium in the density<br />

regime 1 2 ρ 0 to ρ 0 (nuclear matter density ρ 0 = 3×10 14 g cm −3 ) may<br />

<strong>for</strong>m a “bubble ph<strong>as</strong>e” with regions of low density embedded in the<br />

medium. Leinson (1993) h<strong>as</strong> estimated the bremsstrahlung rate <strong>for</strong> the<br />

neutral-current reactions N + bubble → bubble + N + νν. Naturally,<br />

he found that only the axial-vector current contributes. Also, it is e<strong>as</strong>y<br />

to translate his results into an axion emission rate. Apparently, the<br />

bubble ph<strong>as</strong>e can be important <strong>for</strong> early neutron-star cooling.

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