Observation of Bs-Bsbar Oscillations

TL;DR

The probability as a function of proper decay time that the Bs decays with the same, or opposite, flavor as the flavor at production, and a signal for Bs(0)-Bs(0) oscillations is found.

Abstract

We report the observation of Bs-Bsbar oscillations from a time-dependent measurement of the Bs-Bsbar oscillation frequency Delta ms. Using a data sample of 1 fb^-1 of p-pbar collisions at sqrt{s}=1.96 TeV collected with the CDF II detector at the Fermilab Tevatron, we find signals of 5600 fully reconstructed hadronic Bs decays, 3100 partially reconstructed hadronic Bs decays, and 61500 partially reconstructed semileptonic Bs decays. We measure the probability as a function of proper decay time that the Bs decays with the same, or opposite, flavor as the flavor at production, and we find a signal for Bs-Bsbar oscillations. The probability that random fluctuations could produce a comparable signal is 8 X 10^-8, which exceeds 5 sigma significance. We measure Delta ms = 17.77 +- 0.10 (stat) +- 0.07 (syst) ps^-1 and extract |Vtd/Vts| = 0.2060 +- 0.0007 (exp) + 0.0081 - 0.0060 (theor).

Figures (5)

• Figure 1: (Left panel) The invariant mass distributions for the $D^+_s(\phi\pi^+)$ candidates [inset] and the $\ell^-D^+_s(\phi\pi^+)$ pairs. The contribution labelled "false lepton & physics" refers to backgrounds from hadrons mimicking the lepton signature combined with real $D_s$ mesons and physics backgrounds such as $B^0\rightarrow D_s^+ D^-$, $D_s^+\rightarrow\phi\pi^+, D^-\rightarrow\ell^- X$. (Right panel) The invariant mass distribution for $\bar{B}^0_s\to D^+_s(\phi \pi^+) \pi^-$ decays including the contributions from $\bar{B}^0_s \rightarrow D_s^{*+} \pi^-$ and $\bar{B}^0_s \rightarrow D_s^+ \rho^-$. In this panel, signal contributions are drawn added on top of the combinatorial background.
• Figure 2: The invariant mass distributions for $\bar{B}^0_s\to D^+_s\pi^-$ (top panels) and $D^+_s\pi^-\pi^+\pi^-$ (bottom panels). Signal contributions are added on top of the combinatorial background. Contributions from partially reconstructed $B_s$ decays are taken into account in the fit and are not shown.
• Figure 3: (Left panel) The distribution of the correction factor $\kappa$ in semileptonic and partially reconstructed hadronic decays from Monte Carlo simulation. (Right panel) The average proper decay time resolution for $B_s$ decays as a function of proper decay time.
• Figure 4: The measured amplitude values and uncertainties versus the $B^0_s$-$\bar{B}^0_s$ oscillation frequency $\Delta m_s$. (Upper Left) Semileptonic decays only. (Lower Left) Hadronic decays only. (Upper Right) All decay modes combined. (Lower Right) The logarithm of the ratio of likelihoods for amplitude equal to one and amplitude equal to zero, $\Lambda = \log [ {\cal L}^{{\cal A}=0} / {\cal L}^{{\cal A}=1}(\Delta m_s) ]$, versus the oscillation frequency. The horizontal line indicates the value $\Lambda = -15$ that corresponds to a probability of $5.7\times 10^{-7}$ (5$\sigma$) in the case of randomly tagged data.
• Figure 5: The $B^0_s$-$\bar{B}^0_s$ oscillation signal measured in five bins of proper decay time modulo the measured oscillation period $2\pi/\Delta m_s$. The figure is described in the text.