# Measurement of the mass difference between neutral charm-meson eigenstates

## TL;DR

This paper presents a highly precise measurement of the mass difference between neutral charm-meson eigenstates using a large dataset from the LHCb experiment, providing evidence for their mass difference and testing CP symmetry.

## Contribution

It introduces a novel approach to measure the mass difference with unprecedented precision, confirming the mass eigenstates differ in mass.

## Key findings

- Measured the normalized mass difference x_CP with high precision.
- Found results consistent with CP symmetry.
- Provided the first evidence of mass difference between 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 $x_{CP} = \left[2.7\pm1.6\text{(stat)}\pm0.4\text{(syst)}\right]\times10^{-3}$ and the $CP$-violating parameter $\Delta x = \left[-0.53\pm0.70\text{(stat)}\pm0.22\text{(syst)}\right]\times10^{-3}$. 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.