Direct Limits on the Bs Oscillation Frequency

Abstract

We report results of a study of the Bs oscillation frequency using a large sample of Bs semileptonic decays corresponding to approximately 1 fb-1 of integrated luminosity collected by the D0 experiment at the Fermilab Tevatron Collider in 2002-2006. The amplitude method gives a lower limit on the Bs oscillation frequency at 14.8 ps-1 at the 95% C.L. At Δm_s = 19 ps-1, the amplitude deviates from the hypothesis A=0 (A=1) by 2.5 (1.6) standard deviations, corresponding to a two-sided C.L. of 1% (10%). A likelihood scan over the oscillation frequency, Δm_s, gives a most probable value of 19 ps-1 and a range of 17 < Δm_s < 21 ps-1 at the 90% C.L., assuming Gaussian uncertainties. This is the first direct two-sided bound measured by a single experiment. If Δm_s lies above 22 ps-1, then the probability that it would produce a likelihood minimum similar to the one observed in the interval 16 < Δm_s < 22 ps-1 is (5.0+-0.3)%.

Figures (3)

• Figure 1: $(K^+K^-)\pi^-$ invariant mass distribution for (a) the untagged $B^0_s$ sample, and (b) for candidates that have been flavor-tagged. The left and right peaks correspond to $\mu^+ D^{-}$ and $\mu^+ D^-_{s}$ candidates, respectively. The curve is a result of fitting a signal plus background model to the data.
• Figure 2: Value of $-\Delta\log\mathcal{L}$ as a function of $\Delta m_s$. Star symbols do not include systematic uncertainties, and the shaded band represents the envelope of all $\log\mathcal{L}$ scan curves due to different systematic uncertainties.
• Figure 3: $B^0_s$ oscillation amplitude as a function of oscillation frequency, $\Delta m_s$. The solid line shows the $\mathcal{A}=1$ axis for reference. The dashed line shows the expected limit including both statistical and systematic uncertainties.