Measurement of the charm mixing parameter $y_{CP} - y_{CP}^{K\pi}$ using two-body $D^0$ meson decays

Information

LHCb-PAPER-2021-041
CERN-EP-2022-022
arXiv:2202.09106 [PDF]

(Submitted on 18 Feb 2022)
Phys. Rev. D 105, 092013
Inspire 2035063

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Abstract

A measurement of the ratios of the effective decay widths of $D^0 \to \pi^-\pi^+$ and $D^0 \to K^-K^+$ decays over that of $D^0 \to K^-\pi^+$ decays is performed with the LHCb experiment using proton-proton collisions at a centre-of-mass energy of $13 \mathrm{TeV}$, corresponding to an integrated luminosity of $6 \mathrm{fb^{-1}}$. These observables give access to the charm mixing parameters $y_{CP}^{\pi\pi} - y_{CP}^{K\pi}$ and $y_{CP}^{KK} - y_{CP}^{K\pi}$, and are measured as $y_{CP}^{\pi\pi} - y_{CP}^{K\pi} = (6.57 \pm 0.53 \pm 0.16) \times 10^{-3}$, $y_{CP}^{KK} - y_{CP}^{K\pi} = (7.08 \pm 0.30 \pm 0.14) \times 10^{-3}$, where the first uncertainties are statistical and the second systematic. The combination of the two measurements is $y_{CP} - y_{CP}^{K\pi} = (6.96 \pm 0.26 \pm 0.13) \times 10^{-3}$, which is four times more precise than the previous world average.

Figures and captions

Sketch of a $ D ^0 \rightarrow K ^- K ^+ $ to $ D ^0 \rightarrow K ^- \pi ^+ $ matching.	Fig1.pdf [34 KiB] HiDef png [96 KiB] Thumbnail [52 KiB] *.C file
Matched versus original transverse momenta for the matching of a $K^-$ to a $\pi^-$ particle, related to the $ y_{ C P } ^{CC}$ measurement. The red line represents the requirement applied to the data sample, where candidates below the line are rejected. The plot is obtained with the 2017 MagUp sample.	Fig2.pdf [40 KiB] HiDef png [576 KiB] Thumbnail [425 KiB] *.C file
Distributions of $\Delta m$ for the (top) $ D ^0 \rightarrow K ^- \pi ^+ $ , (centre) $ D ^0 \rightarrow K ^- K ^+ $ , and (bottom) $ D ^0 \rightarrow \pi ^- \pi ^+ $ decay channels for the combined data sample. The signal and sideband regions employed to subtract the combinatorial background are delimited by the dashed vertical lines. The sum of the fit projections are overlaid.	Fig3a.pdf [81 KiB] HiDef png [176 KiB] Thumbnail [144 KiB] *.C file
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(Left) fraction and (right) average true $ D ^0 $ decay time of secondary decays as a function of the reconstructed $ D ^0 $ decay time, in units of the average $ D ^0 $ lifetime.	Fig4a.pdf [14 KiB] HiDef png [67 KiB] Thumbnail [39 KiB] *.C file
	Fig4b.pdf [15 KiB] HiDef png [106 KiB] Thumbnail [96 KiB] *.C file
(Left) normalised distributions of the $D^0$ decay angle $\cos \theta^{\ast}(h^-)$ in the raw condition, and (right) following both kinematic matching and reweighting procedures. The distributions are shown for the (top) $ y_{ C P } ^{CC}$, (middle) $ y_{ C P } ^{\pi\pi} - y_{ C P } ^{K\pi}$ and (bottom) $ y_{ C P } ^{KK} - y_{ C P } ^{K\pi}$ measurements. The plots are obtained with the 2017 MagUp sample.	Fig5a.pdf [18 KiB] HiDef png [191 KiB] Thumbnail [147 KiB] *.C file
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Results for (top) $ y_{ C P } ^{CC}$, (centre) $ y_{ C P } ^{\pi\pi} - y_{ C P } ^{K\pi}$ and (bottom) $ y_{ C P } ^{KK} - y_{ C P } ^{K\pi}$. The measurements employing raw data, and following both matching and weighting conditions are shown in green, blue and red, respectively. The measurements in purple employ the fit model where the presence of secondary decays is considered. The dashed vertical lines correspond to $\chi^2$ fits, used to determine the average values other all subsamples. In the y-axis labels, the data-taking year is abbreviated with the last two digits only and the magnet polarity Mag Up ( MagDown ) is abbreviated as "Up" ("Dw").	Fig6a.pdf [15 KiB] HiDef png [259 KiB] Thumbnail [207 KiB] *.C file
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Distributions of (left) $R^{\pi\pi}(t)$ and (right) $R^{KK}(t)$ using the full LHCb Run 2 data set, with the results of $ y_{ C P } ^{\pi\pi} - y_{ C P } ^{K\pi}$ and $ y_{ C P } ^{KK} - y_{ C P } ^{K\pi}$ overlaid as the blue slopes.	Fig7a.pdf [15 KiB] HiDef png [111 KiB] Thumbnail [106 KiB] *.C file
	Fig7b.pdf [15 KiB] HiDef png [110 KiB] Thumbnail [104 KiB] *.C file
Animated gif made out of all figures.	PAPER-2021-041.gif Thumbnail

Tables and captions

Results of the $\chi^2$ fits of Fig. 6 for each correction procedure. The results are shown in units of $10^{-3}$, while the values in parenthesis correspond to the $\chi^2$ of the fits, where the number of degrees of freedom is 7 for all measurements.	Table_1.pdf [55 KiB] HiDef png [63 KiB] Thumbnail [29 KiB] tex code
Systematic uncertainties for the $ y_{ C P } ^{\pi\pi} - y_{ C P } ^{K\pi}$ and $ y_{ C P } ^{KK} - y_{ C P } ^{K\pi}$ measurements.	Table_2.pdf [71 KiB] HiDef png [78 KiB] Thumbnail [35 KiB] tex code

Supplementary Material [file]

Supplementary material full pdf	supple[..].pdf [588 KiB]
This ZIP file contains supplementary material for the publication LHCb-PAPER-2021-041. The files are: supplementary.pdf : An overview of the extra figures .pdf, .png, .eps, .C : The figures in various formats (in folder figs-supplementary)	lhcb-logo.pdf [4 KiB] HiDef png [269 KiB] Thumbnail [142 KiB] *C file
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