We prepare mixtures of ultracold CaF molecules and Rb atoms in a magnetic trap and study their
inelastic collisions. When the atoms are prepared in the spin-stretched state and the molecules in
the spin-stretched component of the first rotationally excited state, they collide inelastically with
a rate coefficient k2 = (6.6 ± 1.5) × 10−11 cm3
/s at temperatures near 100 µK. We attribute this
to rotation-changing collisions. When the molecules are in the ground rotational state we see no
inelastic loss and set an upper bound on the spin-relaxation rate coefficient of k2 < 5.8×10−12 cm3
/s
with 95% confidence. We compare these measurements to the results of a single-channel loss model
based on quantum defect theory. The comparison suggests a short-range loss parameter close to
unity for rotationally excited molecules, but below 0.04 for molecules in the rotational ground state.