  Ultrafast lasers are an increasingly important tool to control and stabilize
emergent phases in quantum materials. Among a variety of possible excitation
protocols, a particularly intriguing route is the direct light-engineering of
microscopic electronic parameters, such as the electron hopping and the local
Coulomb repulsion (Hubbard $U$). In this work, we use time-resolved x-ray
absorption spectroscopy to demonstrate the light-induced renormalization of the
Hubbard $U$ in a cuprate superconductor, La$_{1.905}$Ba$_{0.095}$CuO$_4$. We
show that intense femtosecond laser pulses induce a substantial redshift of the
upper Hubbard band, while leaving the Zhang-Rice singlet energy unaffected. By
comparing the experimental data to time-dependent spectra of single- and
three-band Hubbard models, we assign this effect to a $\sim140$ meV reduction
of the onsite Coulomb repulsion on the copper sites. Our demonstration of a
dynamical Hubbard $U$ renormalization in a copper oxide paves the way to a
novel strategy for the manipulation of superconductivity, magnetism, as well as
to the realization of other long-range-ordered phases in light-driven quantum
materials.
