Abstract
We report measurements of charm-mixing parameters based on the decay-time-dependent ratio of $D^0\to K^+\pi^-$ to $D^0\to K^-\pi^+$ rates. The analysis uses a data sample of proton-proton collisions corresponding to an integrated luminosity of $5.0$ fb$^{-1}$ recorded by the LHCb experiment from 2011 through 2016. Assuming charge-parity (CP) symmetry, the mixing parameters are determined to be $x'^2=(3.9 \pm 2.7) \times10^{-5}$, $y'=(5.28 \pm 0.52) \times 10^{-3}$, and $R_D=(3.454 \pm 0.031)\times10^{-3}$. Without this assumption, the measurement is performed separately for $D^0$ and $\overline{D}{}^0$ mesons, yielding a direct CP-violating asymmetry $A_D =(-0.1\pm9.1)\times10^{-3}$, and magnitude of the ratio of mixing parameters $1.00< |q/p| <1.35$ at the $68.3\%$ confidence level. All results include statistical and systematic uncertainties and improve significantly upon previous single-measurement determinations. No evidence for CP violation in charm mixing is observed.
Highlights
The mass eigenstates of neutral charm mesons are linear combinations of the flavor eigenstates, jD1;2i 1⁄4 pjD0iÆ qjD 0i, where p and q are complex-valued coefficients
In the limit of charge-parity (CP) symmetry, oscillations are characterized by the dimensionless differences in mass, x ≡ Δm/Γ ≡ ðm2 − m1Þ/Γ, and decay width, y ≡ ΔΓ/2Γ ≡ ðΓ2 − Γ1Þ/2Γ, between the CP-even (D2) and CP-odd (D1) mass eigenstates, where Γ is the average decay width of neutral D mesons
The first fit allows for direct and indirect CP violation; the second allows only for indirect CP violation by imposing RþD 1⁄4 R−D; and the third is a fit under the CP-conservation hypothesis, in which all mixing parameters are common to the D0 and D 0 samples
Summary
The mass eigenstates of neutral charm mesons are linear combinations of the flavor eigenstates, jD1;2i 1⁄4 pjD0iÆ qjD 0i, where p and q are complex-valued coefficients This paper reports measurements of CP-averaged and CP-violating mixing parameters in D0–D 0 oscillations based on the comparison of the decay-time-dependent ratio of D0 → Kþπ− to D0 → K−πþ rates with the corresponding ratio for the charge-conjugate processes. CP violation and generate differences between ððx0þÞ2; y0þÞ and ððx0−Þ2; y0−Þ Experimental effects such as differing efficiencies for reconstructing WS and RS decays may bias the observed ratios of signal decays and, the mixing-parameter results. Candidate DÃþ mesons produced directly in the collision (primary DÃþ) are reconstructed while suppressing background contributions from charm mesons produced in the decay of bottom hadrons (secondary DÃþ) and misreconstructed decays Residual contaminations from such backgrounds are measured using control regions. The differences in the decay-time dependence of the WS D0 and D 0 samples are not examined until the analysis procedure is finalized
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