  Violation of scale symmetry, scale anomaly, being a radical concept in
quantum field theory, is of importance to comprehend the vacuum structure of
QCD, and should potentially contribute to the chiral phase transition in
thermal QCD, as well as the chiral and U(1) axial symmetry. Though it should be
essential, direct evidence of scale anomalies has never been observed in the
chiral phase transition. We propose a methodology to detect a scale anomaly in
the chiral phase transition, which is an electromagnetically induced scale
anomaly: apply a weak magnetic field background onto two-flavor massless QCD
with an extremely heavy strange quark, first observe the chiral crossover;
second, adjusting the strange quark mass to be smaller and smaller, observe the
second-order chiral phase transition, and then the first-order one in the
massless-three flavor limit. Thus, the second-order chiral phase transition,
observed as the evidence of the quantum scale anomaly, is a new critical
endpoint. It turns out that this electromagnetic scale anomaly gets most
operative in the weak magnetic field regime, rather than a strong field region.
We also briefly address accessibility of lattice QCD, a prospected application
to dense matter system, and implications to astrophysical observations, such as
gravitational wave productions provided from thermomagnetic QCD-like theories.
