Observation of the Mass Difference Between Neutral Charm-Meson Eigenstates

Information

LHCb-PAPER-2021-009
CERN-EP-2021-099
arXiv:2106.03744 [PDF]

(Submitted on 07 Jun 2021)
Phys. Rev. Lett. 127, (2021) 111801
Inspire 1867376

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Abstract

A measurement of mixing and $C P$ violation in neutral charm mesons is performed using data reconstructed in proton-proton collisions collected by the LHCb experiment from 2016 to 2018, corresponding to an integrated luminosity of 5.4$\textrm{fb}^{-1}$. A total of $30.6$ million $D^{0}\rightarrow K^{0}_{\mathrm{S}}\pi^{+}\pi^{-}$ decays are analyzed using a method optimized for the measurement of the mass difference between neutral charm-meson eigenstates. Allowing for $C P$ violation in mixing and in the interference between mixing and decay, the mass and decay-width differences are measured to be $x_{C P} = \left[3.97\pm0.46\textrm{(stat)}\pm 0.29\textrm{(syst)}\right]\times10^{-3}$ and $y_{C P} = \left[4.59\pm1.20\textrm{(stat)}\pm0.85\textrm{(syst)}\right]\times10^{-3}$, respectively. The $C P$-violating parameters are measured as $\Delta x = \left[-0.27\pm0.18\textrm{(stat)}\pm0.01\textrm{(syst)}\right]\times10^{-3}$ and $\Delta y = \left[0.20\pm0.36\textrm{(stat)}\pm0.13\textrm{(syst)}\right]\times10^{-3}$. This is the first observation of a nonzero mass difference in the $D^0$ meson system, with a significance exceeding seven standard deviations. The data are consistent with $C P$ symmetry, and improve existing constraints on the associated parameters.

Figures and captions

Depiction of the interference of mixing and decay if a $ D ^0 $ and a $\overline{ D } {}^0 $ meson decay to a common final state $f$.	Fig1.pdf [3 KiB] HiDef png [27 KiB] Thumbnail [15 KiB] *.C file
"Binning" of the $ D ^0 \rightarrow K ^0_{\mathrm{S}} \pi ^+ \pi ^- $ Dalitz plot. Colors indicate the absolute value of the bin index $b$.	Fig2.pdf [405 KiB] HiDef png [379 KiB] Thumbnail [260 KiB] *.C file
Distribution of $\Delta m $ for the selected $ D ^{*+} \rightarrow D ^0 (\rightarrow K ^0_{\mathrm{S}} \pi ^+ \pi ^- )\pi ^+ $ candidates. The projection of the fit result is superimposed.	Fig3.pdf [41 KiB] HiDef png [184 KiB] Thumbnail [170 KiB] *.C file
(Top) $ C P$ -averaged yield ratios and (bottom) differences of $ D ^0$ and $\overline{ D } {}^0$ yield ratios as a function of $t/\tau$, shown for each Dalitz-plot bin with fit projections overlaid.	Fig4a.pdf [27 KiB] HiDef png [301 KiB] Thumbnail [281 KiB] *.C file
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Dalitz plot of background-subtracted $ D ^0 \rightarrow K ^0_{\mathrm{S}} \pi ^+ \pi ^- $ candidates.	Fig5.pdf [1 MiB] HiDef png [1 MiB] Thumbnail [539 KiB] *.C file
Animated gif made out of all figures.	PAPER-2021-009.gif Thumbnail

Tables and captions

Fit results of $ x_{ C P }$ , $ y_{ C P }$ , $\Delta x$ , and $\Delta y$ . The first contribution to the uncertainty is statistical, the second systematic.	Table_1.pdf [68 KiB] HiDef png [43 KiB] Thumbnail [19 KiB] tex code
Point estimates and 95.5% confidence-level (CL) intervals for $x$, $y$, $\|q/p\|$ and $\phi$. The uncertainties include statistical and systematic contributions.	Table_2.pdf [69 KiB] HiDef png [56 KiB] Thumbnail [27 KiB] tex code
Uncertainties in units of $10^{-3}$. The total systematic uncertainty is the sum in quadrature of the individual components. The uncertainties due to the external inputs and detection asymmetry calibration samples are included in the statistical uncertainty. These are also reported separately, along with the contributions due to the limited sample size, to ease comparison with other sources.	Table_3.pdf [49 KiB] HiDef png [125 KiB] Thumbnail [57 KiB] tex code

Supplementary Material [file]

This ZIP file contains supplementary material for the publication LHCb-PAPER-2021-099. The files are: Supplementary-LHCb-PAPER-2021-099.pdf : An overview of the extra figures .pdf, .png, .eps, .C : The figures in various formats	Fig10.pdf [1 MiB] HiDef png [369 KiB] Thumbnail [256 KiB] *C file
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