Measurement of the Mass Difference Between Neutral Charm-Meson Eigenstates

Abstract

We report a measurement of the mass difference between neutral charm-meson eigenstates using a novel approach that enhances sensitivity to this parameter. We use 2.3 × 10 6 D 0 → K 0 S π + π − decays reconstructed in proton-proton collisions collected by the LHCb experiment in 2011 and 2012. Allowing for C P violation in mixing and in the interference between mixing and decay, we measure the C P -averaged normalized mass difference x C P = [ 2.7 ± 1.6 ( stat ) ± 0.4 ( syst ) ] × 10 − 3 and the C P -violating parameter Δ x = [ − 0.53 ± 0.70 ( stat ) ± 0.22 ( syst ) ] × 10 − 3 . The results are consistent with C P symmetry. These determinations are the most precise from a single experiment and, combined with current world-average results, yield the first evidence that the masses of the neutral charm-meson eigenstates differ.