Alfonso Albero, AlejandroCalvo Gómez, MíriamCamboni, AlessandroCoquereau, SamuelGarrido Beltrán, LluísGascón Fora, DavidGironella Gironell, PereGraciani Díaz, RicardoGraugés Pous, EugeniVilasis-Cardona, XavierLHCb Collaboration2020-10-052020-10-052019-06-140031-9007https://hdl.handle.net/2445/170962We 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.10 p.application/pdfengcc-by (c) Alfonso et. al. ,2019http://creativecommons.org/licenses/by/3.0/es/BosonsMesons (Física nuclear)BosonsMesons (Nuclear physics)info:eu-repo/semantics/article6944992020-10-05info:eu-repo/semantics/openAccess