  We formulate PanScales parton showers for hadron collisions so as to achieve
next-to-leading logarithmic (NLL) accuracy across a broad set of observables.
We do so specifically for colour singlet production. Relative to the existing
PanScales final-state showers, the main new question is that of how to
redistribute momentum imbalances from initial-state branching across the
remainder of the event. We presents tests of the showers at fixed order,
including the treatment of full colour for soft-collinear emissions and of spin
correlations in both the soft and collinear domains. We also include
comparisons to a formulation of a standard dipole shower, the current
leading-logarithmic state of the art. A forthcoming companion paper will
explore all-order tests of the new showers.
