  We introduce a novel grooming procedure, which is an extension of the
modified MassDrop tagging algorithm, tailored to the needs of deep inelastic
scattering (DIS). The new algorithm, which grooms the event as a whole, takes
advantage of the natural separation of current and target fragmentation in the
Breit frame, in order to eliminate radiation in the beam and central rapidity
regions. We study the groomed invariant mass in DIS and within soft-collinear
effective theory we construct a factorization theorem for the cross-section in
the back-to-back limit. In this limit we show that, up to a normalization
factor, the cross-section does not depend on the incoming hadronic matrix
element and we propose this measurement at HERA and the future electron-ion
collider (EIC) as a probe to hadronization, precision QCD, and cold nuclear
matter effects. We also give an event based definition of the Winner-Take-All
axis and comment on possible applications.
