  Novel phases of matter with unique properties that emerge from quantum and
topological protection present an important thrust of modern research. Of
particular interest is to engineer these phases on demand using ultrafast
external stimuli, such as photoexcitation, which offers prospects of their
integration into future devices compatible with optical communication and
information technology. Here, we use MeV Ultrafast Electron Diffraction (UED)
to show how a transient three-dimensional (3D) Dirac semimetal state can be
induced by a femtosecond laser pulse in a topological insulator ZrTe$_5$. We
observe marked changes in Bragg diffraction, which are characteristic of bond
distortions in the photoinduced state. Using the atomic positions refined from
the UED, we perform density functional theory (DFT) analysis of the electronic
band structure. Our results reveal that the equilibrium state of ZrTe$_5$ is a
topological insulator with a small band gap of $\sim$25 meV, consistent with
angle-resolved photoemission (ARPES) experiments. However, the gap is closed in
the presence of strong spin-orbit coupling (SOC) in the photoinduced transient
state, where massless Dirac fermions emerge in the chiral band structure. The
time scale of the relaxation dynamics to the transient Dirac semimetal state is
remarkably long, $\tau \sim$160 ps, which is two orders of magnitude longer
than the conventional phonon-driven structural relaxation. The long relaxation
is consistent with the vanishing density of states in Dirac spectrum and slow
spin-repolarization of the SOC-controlled band structure accompanying the
emergence of Dirac fermions.
