Null tests from angular distributions in D→P1P2l+l−, l=e, μ decays on and off peak

We systematically analyze the full angular distribution in D→P1P2l+l− decays, where P1,2=π, K, l=e, μ. We identify several null tests of the standard model (SM). Notably, the angular coefficients I5,6,7, driven by the leptons’ axial-vector coupling C10(′), vanish by means of a superior Glashow-Iliopoulos-Maiani (GIM) cancellation and are protected by parity invariance below the weak scale. CP-odd observables related to the angular coefficients I5,6,8,9 allow us to measure CP asymmetries without D tagging. The corresponding observables A5,6,8,9 constitute null tests of the SM. Lepton universality in |Δc|=|Δu|=1 transitions can be tested by comparing D→P1P2μ+μ− to D→P1P2e+e− decays. Data for P1P2=π+π− and K+K− on muon modes are available from LHCb and on electron modes from BESIII. Corresponding ratios of dimuon to dielectron branching fractions are at least about an order of magnitude away from probing the SM. In the future electron and muon measurements should be made available for the same cuts as corresponding ratios RP1P2D provide null tests of e−μ universality. We work out beyond-SM signals model-independently and in SM extensions with leptoquarks.