[Paper Review] Recent Charm Results From CLEO

A. J. S. Smith|arXiv (Cornell University)|May 31, 2002

Particle accelerators and beam dynamics1 references3 citations

TL;DR

This paper presents a Dalitz plot analysis of $D^0 \to K^0_S\pi^+\pi^-$ decays using 9 fb$^{-1}$ of $e^+e^-$ data from CLEO at CESR, observing the doubly Cabibbo-suppressed decay $D^0 \to K^{*+}(892)\pi^-$ with a branching fraction of $(0.6 \pm 0.3 \pm 0.2)\%$ and a strong phase shift of $(-3 \pm 11 \pm 8)^\circ$ relative to the Cabibbo-favored mode. It also sets a stringent upper limit of $R_{\rm mix} < 0.87\%$ at 95% CL on $D^0$-$\overline{D^0}$ mixing via $D^0 \to K^{*+}e^-\bar{\nu}_e$, providing key constraints on new physics beyond the Standard Model.

ABSTRACT

We describe two recent results using data collected with the CLEO detector at the CESR e+e- collider at energies near the Upsilon(4S). The first is a Dalitz plot analysis of the decay D0-&gt;K0spi+pi-. We observe a rich structure including the decay D0-&gt;K*+pi- which may be produced by D0-D0bar mixing or doubly Cabibbo-suppressed decays. We also search for D0-D0bar mixing in the decay D0-&gt;K*+e-nuebar. We observe no events and limit the mixing parameter R_mix to be less than 0.87% at the 95% confidence level.

Motivation & Objective

• To search for $D^0$-$\overline{D^0}$ mixing and doubly Cabibbo-suppressed decays in $D^0 \to K^0_S\pi^+\pi^-$ using a Dalitz plot analysis.
• To measure the spectroscopy of light mesons and glueball candidates via multi-body $D$ decays.
• To constrain $D^0$-$\overline{D^0}$ mixing by measuring the $CP$-violating parameter $R_{\rm mix}$ in the decay $D^0 \to K^{*+}e^-\bar{\nu}_e$.
• To extract the strong phase shift $\delta_s$ between Cabibbo-favored and doubly Cabibbo-suppressed amplitudes to resolve $x$ and $y$ mixing parameters.

Proposed method

• A maximum likelihood fit is performed on the Dalitz plot using variables $M^2(\pi^+\pi^-)$ and $M_{RS}$ to model resonant and non-resonant amplitudes with relativistic Breit-Wigner forms and Blatt-Weisskopf form factors.
• The $K^0_S$ is reconstructed from $\pi^+\pi^-$ pairs with a vertex fit requiring a confidence level > $10^{-4}$, and the $D^0$ candidate is reconstructed via $D^{*+} \to D^0\pi_s^+$ with the slow pion used for $D^0$/$\overline{D^0}$ tagging.
• The energy release $Q = M^* - M - m_\pi$ is used to define the signal region, with a core width of $220 \pm 4$ keV and a $D^*$ momentum cut of > 2.0 GeV/$c$.
• For the $D^0 \to K^{*+}e^-\bar{\nu}_e$ mode, the neutrino is pseudo-reconstructed using event thrust, slow pion, and $D^0$ daughter directions, with optimal weights derived from Monte Carlo simulations.
• The proper time distribution is fitted with a $t^2 e^{-t}$ dependence to extract signal yields, and $R_{\rm mix}$ is constrained via the ratio of wrong-signed to right-signed events.
• Systematic uncertainties are evaluated by varying fit parameters, resonance model choices, and background sideband regions.

Experimental results

Research questions

• RQ1 What is the branching fraction and strong phase of the doubly Cabibbo-suppressed decay $D^0 \to K^{*+}(892)\pi^-$?
• RQ2 Can $D^0$-$\overline{D^0}$ mixing be observed in the decay $D^0 \to K^{*+}e^-\bar{\nu}_e$?
• RQ3 What is the contribution of $D^0$-$\overline{D^0}$ mixing to the $K^0_S\pi^+\pi^-$ final state, and how does it affect the Dalitz plot structure?
• RQ4 What is the strong phase shift $\delta_s$ between the Cabibbo-favored and doubly Cabibbo-suppressed amplitudes in $D^0 \to K^0_S\pi^+\pi^-$ decays?
• RQ5 Can $CP$ violation be observed in charm decays at the few percent level in this analysis?

Key findings

• The decay $D^0 \to K^{*+}(892)\pi^-$ is observed with a branching fraction of $(0.6 \pm 0.3 \pm 0.2)\%$ relative to the Cabibbo-favored mode.
• The strong phase shift between the $K^{*+}(892)\pi^-$ and $K^{*+}(892)^{-}\pi^+$ amplitudes is measured to be $(-3 \pm 11 \pm 8)^\circ$.
• No $CP$-violating effects are observed in $D^0$ and $\overline{D^0}$ decays at the few percent level in the Dalitz plot analysis.
• The $D^0$-$\overline{D^0}$ mixing parameter $R_{\rm mix}$ is limited to less than $0.87\%$ at 95% confidence level using the $D^0 \to K^{*+}e^-\bar{\nu}_e$ decay mode.
• The Dalitz plot fit includes 18 resonant and non-resonant contributions, with the $K^{*}(892)^{-}$ mode dominating at $67.2\%$ of the total fit fraction.
• The $K_{0}(1430)^{-}$ and $K^{*}(1680)^{-}$ resonances contribute significantly, with fit fractions of $8.0\%$ and $2.3\%$, respectively.