[Paper Review] Charm Mixing and Lifetimes at Babar

A. Pompili|ArXiv.org|May 21, 2002

Historical Astronomy and Related Studies3 citations

TL;DR

This paper presents a preliminary measurement of the charm mixing parameter $ y = \Delta\Gamma/2\Gamma $ using 57.8 fb$^{-1}$ of data from the BaBar experiment at PEP-II. By comparing the $ D^0 $ lifetimes in Cabibbo-favored ($ K^-\pi^+ $) and Cabibbo-suppressed ($ K^-K^+ $, $ \pi^-\pi^+ $) decay modes under CP-conserving assumptions, it reports $ y = (1.4 \pm 1.0\text{ (stat.)} \pm^{0.6}_{-0.7}\text{ (syst.)})\% $, consistent with zero but suggesting a positive value not excluded by prior measurements.

ABSTRACT

Preliminary limits on the D^0 mixing parameter y = ΔΓ/ 2 Γare obtained using about 57.8 fb^-1 of data collected by BaBar in 2000 and 2001:y = (1.4 \pm 1.0 (stat.) +0.6 -0.7 (syst.))%. y is extracted, provided that CP is conserved, by measuring separately the D^0 lifetime for the Cabibbo-suppressed decay modes K- K+, π- π+ and the Cabibbo-favoured mode K- π+. Backgrounds are suppressed by D*-tag and particle identification requirements.

Motivation & Objective

• To measure the $ D^0 $ mixing parameter $ y = \Delta\Gamma/2\Gamma $ using time-dependent lifetime differences between Cabibbo-favored and suppressed decay modes.
• To test for physics beyond the Standard Model through enhanced $ y $, which could arise from final state interactions or new physics contributions.
• To reduce systematic uncertainties in $ y $ by exploiting lifetime ratio techniques that cancel common detector effects.
• To provide a precision measurement of $ y $ using a high-statistics $ c\bar{c} $ continuum sample from the BaBar experiment.

Proposed method

• Reconstruct $ D^0 \to h^+h^- $ candidates using $ D^{*+} \to D^0 \pi_s^+ $ tagging to determine proper time via flight length in the transverse plane.
• Apply stringent particle identification (DIRC, SVT, DCH) and $ \delta m = m(D^{*+}) - m(D^0) $ selection to suppress combinatorial backgrounds.
• Use unbinned maximum likelihood fits to proper time distributions to extract $ \tau(D^0) $ for each decay mode, with background modeled using sidebands.
• Compute the lifetime ratio $ \tau(K^-\pi^+)/\tau(h^-h^+) - 1 $ as a proxy for $ y_{\text{CP}} $, assuming CP conservation.
• Estimate systematic uncertainties via variations in tracking, PID, vertexing, alignment, and Monte Carlo statistics on large simulated samples.
• Apply beam spot constraint and refit $ D^* $ decay vertex using the beam spot to improve $ \delta m $ resolution and reduce background.

Experimental results

Research questions

• RQ1 What is the current experimental limit on the charm mixing parameter $ y = \Delta\Gamma/2\Gamma $, and how does it compare to SM predictions?
• RQ2 Can the lifetime difference between Cabibbo-favored and Cabibbo-suppressed $ D^0 $ decays be used to extract $ y $ with high precision?
• RQ3 To what extent do detector-related systematics affect the $ y $ measurement, and can they be mitigated via ratio techniques?
• RQ4 Is there evidence for non-zero $ y $, and does it suggest physics beyond the Standard Model?

Key findings

• The measured value of $ y $ for the $ K^-K^+ $ mode is $ 1.5 \pm 1.3\text{ (stat.)} \pm^{0.6}_{-0.7}\text{ (syst.)}\% $, consistent with zero within uncertainties.
• For the $ \pi^-\pi^+ $ mode, $ y = 1.0 \pm 1.7\text{ (stat.)} \pm^{1.2}_{-1.4}\text{ (syst.)}\% $, showing larger statistical uncertainty due to lower event yield.
• The average $ y $ value from both Cabibbo-suppressed modes is $ 1.4 \pm 1.0\text{ (stat.)} \pm^{0.6}_{-0.7}\text{ (syst.)}\% $, indicating a positive but non-significant deviation from zero.
• Systematic uncertainties are dominated by Monte Carlo statistics, with tracking and PID contributing significantly, especially for $ \pi\pi $.
• The results are consistent with previous measurements by BELLE and FOCUS, and no significant CP violation is observed in the $ y $ parameter.
• The analysis demonstrates that lifetime ratio techniques reduce systematic effects, making $ y $ a robust observable for future precision measurements.