  Liquid argon time projection chambers (LArTPCs) are widely used in particle
detection for their tracking and calorimetric capabilities. The particle
physics community actively builds and improves high-quality simulators for such
detectors in order to develop physics analyses in a realistic setting. The
fidelity of these simulators relative to real, measured data is limited by the
modeling of the physical detectors used for data collection. This modeling can
be improved by performing dedicated calibration measurements. Conventional
approaches calibrate individual detector parameters or processes one at a time.
However, the impact of detector processes is entangled, making this a poor
description of the underlying physics. We introduce a differentiable simulator
that enables a gradient-based optimization, allowing for the first time a
simultaneous calibration of all detector parameters. We describe the procedure
of making a differentiable simulator, highlighting the challenges of retaining
the physics quality of the standard, non-differentiable version while providing
meaningful gradient information. We further discuss the advantages and
drawbacks of using our differentiable simulator for calibration. Finally, we
provide a starting point for extensions to our approach, including applications
of the differentiable simulator to physics analysis pipelines.
