# Precision measurement of the $B^0_s - \bar{B}^0_s$ oscillation frequency with the decay $B^0_s \to D^-_s \pi^+$

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## Abstract

A key ingredient to searches for physics beyond the Standard Model in $B^{0}_{s}$ mixing phenomena is the measurement of the $B^{0}_{s}$-$\bar{B}^{0}_{s}$ oscillation frequency, which is equivalent to the mass difference $\Delta m_{s}$ of the $B^{0}_{s}$ mass eigenstates. Using the world's largest $B^{0}_{s}$ meson sample accumulated in a dataset, corresponding to an integrated luminosity of 1.0 fb$^{-1}$, collected by the LHCb experiment at the CERN LHC in 2011, a measurement of $\Delta m_{s}$ is presented. A total of about 34,000 $B^{0}_{s}\rightarrowD^{-}_{s}\pi^{+}$ signal decays are reconstructed, with an average decay time resolution of 44 fs. The oscillation frequency is measured to be $\Delta m_{s}$ = 17.768 $\pm$ 0.023 (stat) $\pm$ 0.006 (syst) ps$^{-1}$, which is the most precise measurement to date.

## Figures and captions

 Invariant mass distributions for $B ^0_ s \!\rightarrow D ^-_ s \pi ^+$ candidates with the $D ^-_ s$ meson decaying as a) $D ^-_ s \!\rightarrow \phi( K ^+ K ^- )\pi ^-$ , b) $D ^-_ s \!\rightarrow K ^{*0} ( K ^+ \pi ^- ) K ^-$ , c) $D ^-_ s \!\rightarrow K ^+ K ^- \pi ^-$ nonresonant, d) $D ^-_ s \!\rightarrow K ^- \pi ^+ \pi ^-$ , and e) $D ^-_ s \!\rightarrow \pi ^- \pi ^+ \pi ^-$ . The fits and the various background components are described in the text. Misidentified backgrounds refer to background from $B ^0$ and $\Lambda ^0_ b$ decays with one misidentified daughter particle. m_phip[..].pdf [14 KiB] HiDef png [208 KiB] Thumbnail [70 KiB] m_ksta[..].pdf [14 KiB] HiDef png [178 KiB] Thumbnail [64 KiB] m_nonr[..].pdf [14 KiB] HiDef png [189 KiB] Thumbnail [71 KiB] m_kpip[..].pdf [15 KiB] HiDef png [183 KiB] Thumbnail [70 KiB] m_pipi[..].pdf [14 KiB] HiDef png [174 KiB] Thumbnail [64 KiB] Decay time distribution for the sum of the five decay modes for candidates tagged as mixed (different flavour at decay and production; red, continuous line) or unmixed (same flavour at decay and production; blue, dotted line). The data and the fit projections are plotted in a signal window around the reconstructed $B ^0_ s$ mass of 5.32 -- 5.55 ${\mathrm{\,Ge\kern -0.1em V\!/}c^2}$ . mixing[..].pdf [18 KiB] HiDef png [231 KiB] Thumbnail [58 KiB] Animated gif made out of all figures. PAPER-2013-006.gif Thumbnail

## Tables and captions

 Number of candidates and $B ^0_ s$ signal fractions in the mass range 5.32 -- 5.98 ${\mathrm{\,Ge\kern -0.1em V\!/}c^2}$ . Table_1.pdf [52 KiB] HiDef png [44 KiB] thumbnail [7 KiB] tex code Systematic uncertainties on the $\Delta m_{ s }$ measurement. The total systematic uncertainty is calculated as the quadratic sum of the individual contributions. Table_2.pdf [36 KiB] HiDef png [44 KiB] thumbnail [6 KiB] tex code

Created on 18 September 2018.Citation count from INSPIRE on 19 September 2018.