Dimension distortion by right coset projections in the Heisenberg group

We study the family of vertical projections whose fibers are right cosets of horizontal planes in the Heisenberg group, $\mathbb{H}^n$. We prove lower bounds for Hausdorff dimension distortion of sets under these mappings, with respect to the Euclidean metric and also the natural quotient metric. We show these bounds are sharp in a large part of the range of possible dimension, and give conjectured sharp lower bounds for the remaining part of the range. Our result also lets us improve the known almost sure lower bound for the standard family of vertical projections in $\mathbb{H}^n$.