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 $2.3\times 10^6$ $D^0\to K_S^0{\pi }^{+}{\pi }^{-}$ decays reconstructed in proton-proton collisions collected by the \lhcb experiment in 2011 and 2012. Allowing for $CP$ violation in mixing and in the interference between mixing and decay, we measure the $CP$-averaged normalized mass difference $x_{CP}=\left[2.7\pm 1.6\text{(stat)}\pm 0.4\text{(syst)}\right]\times 10^{-3}$ and the $CP$-violating parameter $\Delta x=\left[-0.53\pm 0.70\text{(stat)}\pm 0.22\text{(syst)}\right]\times 10^{-3}$. The results are consistent with $CP$ 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.

arXiv commentAll figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2019-001.html (LHCb public pages)

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