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Measurement of the mass difference between neutral charm-meson eigenstates

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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.

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Distribution of (left) the difference between $ D ^{*+}$ and $ D ^0$ masses for $ D ^{*+} \rightarrow D ^0 (\rightarrow K ^0_{\mathrm{S}} \pi ^+ \pi ^- )\pi ^+ $ candidates and (right) $ D ^0$ mass for $\overline{ B }{} \rightarrow D ^0 (\rightarrow K ^0_{\mathrm{S}} \pi ^+ \pi ^- )\mu ^- X$ candidates.

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Smoothed efficiency as a function of $ m^{2}(\pi ^+ \pi ^- )$ and $t/\tau$ in $ D ^{*+} \rightarrow D ^0 (\rightarrow K ^0_{\mathrm{S}} \pi ^+ \pi ^- )\pi ^+ $ decays, as determined from the data with downstream $ K ^0_{\mathrm{S}}$ candidates.

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(Top) $ C P$ -averaged yield ratios and (bottom) differences of $ D ^0$ and $\overline{ D }{} {}^0$ yield ratios as functions of $t/\tau$ for each Dalitz bin. Prompt (closed points) and semileptonic (open points) data are shown separately. Fit projections over the prompt (solid line) and semileptonic (dashed line) data are overlaid.

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(Top) Dalitz-plot distribution of background-subtracted $ D ^0 \rightarrow K ^0_{\mathrm{S}} \pi ^+ \pi ^- $ candidates. (Bottom) Iso-$\Delta\delta$ binning of the $ D ^0 \rightarrow K ^0_{\mathrm{S}} \pi ^+ \pi ^- $ Dalitz plot, reproduced from \href{https://doi.org/10.1103/PhysRevD.82.112006}{Phys. Rev. {\bf D82} (2010) 112006}. The bins are symmetric with respect to the $m_+^2=m_-^2$ bisector; positive indices refer to bins in the (lower) $m_+^2 > m_-^2$ region; negative indices refer to those in the (upper) $m_+^2 < m_-^2$ region. Colors indicate the absolute value of the bin index $b$.

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Fit results. The first contribution to the uncertainty is statistical, and the second is systematic.

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Point estimates and 95.5% confidence-level (CL) intervals for the derived parameters. Uncertainties include statistical and systematic contributions.

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Results of the fit to the prompt data sample. The first contribution to the uncertainty is statistical, the second systematic.

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Results of the fit to the semileptonic data sample. The first contribution to the uncertainty is statistical, the second systematic.

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Supplementary Material [file]

This ZIP file contains supplemetary material for the publication LHCb-PAPER-2019-001. The files are: Supplementary-LHCb-PAPER-2019-001.pdf : An overview of the extra figures *.pdf, *.png, *.eps, *.C : The figures in various formats

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Created on 19 October 2019.