Accretion mediated spin-eccentricity correlations in LISA massive black hole binaries

We examine expected effective spin (χ_ eff,1 yr) and orbital eccentricity (e1 yr) correlations for a population of observable equal-mass massive black hole binaries (MBHBs) with total redshifted mass Mz~[104.5,107.5]~ M_ embedded in a circumbinary disc (CBD), one-year before merging in the LISA band. We find a strong correlation between measurable eccentricity and negative effective spin for MBHBs that are carried to merger by retrograde accretion. This is due to the well-established eccentricity pumping of retrograde accretion and the formation of retrograde CBD-aligned mini-discs, as observed in hydrodynamical simulations. Conversely, prograde accretion channels result in positive χ_ eff,1 yr and non-measurable e1 yr. This clear contrast between the two CBD orientations -- and particularly the unique signature of retrograde configurations -- provides a promising way to unlock the mysteries of MBHB formation channels in the LISA era.