Running a simple xp.generate_matched_gaussian_bunch() example, particles are created correctly, particles.zeta exists but particles.tau doesn't.

Code from https://xsuite.readthedocs.io/en/latest/particlesmanip.html, section [Example: Gaussian bunch]

Xsuite tracking did not appear to respond to the 'chi', 'charge ratio', or 'mass ratio' keywords (i.e. particles assigned with different values of these inputs exhibited the same deflection when tracked through a dipole).

Editing build_particles.py to add chi as a keyword argument to the build_particles function and adding
part_dict['chi'] = chi
inside the function appeared to restore the missing functionality.