Neutrino-antineutrino synchrotron emission from magnetized dense quark matter

Using the Kadanoff-Baym formalism, we perform a detailed study of neutrino-antineutrino synchrotron emission from strongly magnetized, dense quark matter under conditions relevant to compact stars. Starting from an exact expression for the emission rate that fully accounts for Landau-level quantization of quarks, we derive an approximate formula applicable in the regime where quark chemical potentials are much larger than all other relevant energy scales. We demonstrate that the emission rate is largely controlled by a single dimensionless ratio between two low-energy scales: the Landau-level spacing at the Fermi surface, <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mo stretchy="false">|</a:mo><a:msub><a:mi>e</a:mi><a:mi>f</a:mi></a:msub><a:mi>B</a:mi><a:mo stretchy="false">|</a:mo><a:mo>/</a:mo><a:msub><a:mi>μ</a:mi><a:mi>f</a:mi></a:msub></a:math>, and the temperature of the quark matter, <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mi>T</e:mi></e:math>. When the ratio <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mo stretchy="false">|</g:mo><g:msub><g:mi>e</g:mi><g:mi>f</g:mi></g:msub><g:mi>B</g:mi><g:mo stretchy="false">|</g:mo><g:mo>/</g:mo><g:mo stretchy="false">(</g:mo><g:msub><g:mi>μ</g:mi><g:mi>f</g:mi></g:msub><g:mi>T</g:mi><g:mo stretchy="false">)</g:mo></g:math> approaches zero, many closely spaced Landau levels contribute to the emission, but the total rate vanishes as <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mi>B</m:mi><m:mo stretchy="false">→</m:mo><m:mn>0</m:mn></m:math>. In the opposite limit, where the ratio is large, the rate is dominated by transitions between adjacent levels and is exponentially suppressed due to Landau-level quantization, which limits the thermal activation of quarks near the Fermi surface. Our results show that, even in the presence of the strongest magnetic fields expected in compact stars, the synchrotron emission remains suppressed by more than 3 orders of magnitude compared to the direct Urca process. This implies that such emission is unlikely to play any substantial role in the cooling of magnetized quark stars, at least those made of unpaired quark matter phases.