= Quark mass
{title2=$\mathcal L_m=-\sum_f m_f\bar q_fq_f$}
A <quark mass> is the coefficient of its <Dirac mass> term. This term couples left- and right-handed <quarks>, so nonzero <quark masses> explicitly break independent rotations of the two <chiralities>. For two equal masses, the vector <flavor symmetry> survives, while the nonsinglet <axial current> obeys the <flavor-nonsinglet axial Ward identity with equal quark masses>. In the renormalized theory the <mass> parameter depends on the subtraction scale and scheme, while measurable hadron masses are physical observables.
Back to article page
