  Primordial black holes can be produced by density fluctuations generated from
delayed vacuum decays of first-order phase transition. The primordial black
holes generated at the electroweak phase transition have masses of about
$10^{-5}$ solar mass. Such primordial black holes in the mass range can be
tested by current and future microlensing observations, such as Subaru HSC,
OGLE, PRIME and Roman telescope. Therefore, we may be able to explore new
physics models with strongly first-order electroweak phase transition via
primordial black holes. We examine this possibility by using models with
first-order electroweak phase transition in the standard model effective field
theory with dimension 6 and 8 operators. We find that depending on parameters
of the phase transition a sufficient number of primordial black holes can be
produced to be observed by above mentioned experiments. Our results would
suggest that primordial black holes can be used as a new probe of models with
strongly first-order electroweak phase transition, which has complementarity
with measurements of the triple Higgs boson coupling at future collider
experiments and observations of gravitational waves at future space-based
interferometers.
