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Opposite-side flavour tagging of $B$ mesons at the LHCb experiment

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Abstract

The calibration and performance of the opposite-side flavour tagging algorithms used for the measurements of time-dependent asymmetries at the LHCb experiment are described. The algorithms have been developed using simulated events and optimized and calibrated with B+ -> J/psi K+, B0 -> J/psi K*0 and B0 -> D*- mu+ nu_mu decay modes with 0.37 fb^-1 of data collected in pp collisions at sqrt(s) = 7 TeV during the 2011 physics run. The opposite-side tagging power is determined in the B+ -> J/psi K+ channel to be (2.10 +- 0.08 +- 0.24) %, where the first uncertainty is statistical and the second is systematic.

Figures and captions

Mass distribution of OS tagged $ B ^+ \!\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^+ $ events. Black points are data, the solid blue line, red dotted line and green area are the overall fit, the signal and the background components, respectively.

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Distributions of (a) $K^+\pi^-$ invariant mass, (b) mass difference $m(K\pi\pi)$$-$$m(K\pi)$ and (c) decay time of the $ B ^0 \!\rightarrow D ^{*-} \mu ^+ \nu _\mu $ events. Black points with errors are data, the blue curve is the fit result. The other lines represent signal (red dot-dashed), $\overline{ D }{} ^0$ -from-$B$ decay background (gray dashed), $ B ^+$ background (green short dashed), $D^*$ prompt background (magenta solid). The combinatorial background is the magenta filled area.

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Raw mixing asymmetry of $ B ^0 \!\rightarrow D ^{*-} \mu ^+ \nu _\mu $ events in the signal mass region when using the combination of all OS taggers. Black points are data and the red solid line is the result of the fit. The lower plot shows the pulls of the residuals with respect to the fit.

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Mass distribution of OS tagged $ B ^0 \!\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^{*0} $ events. Black points are data, the solid blue line, red dotted line and green area are the overall fit, the signal and the background components, respectively.

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Raw mixing asymmetry of the $ B ^0 \!\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^{*0} $ events in the signal mass region, for all OS tagged events. Black points are data and the red solid line is the result of the fit. The lower plot shows the pulls of the residuals with respect to the fit.

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Distribution of the calibrated mistag probability for the single OS taggers and their combination for $ B ^+ \!\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^+ $ events selected in a $\pm24$ $ {\mathrm{\,MeV\!/}c^2}$ mass window around the $ B ^+$ mass.

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Raw mixing asymmetry as a function of $ B $ decay time in $ B ^0 \!\rightarrow D ^{*-} \mu ^+ \nu _\mu $ events, in the signal mass region, using the OS tagger. Events are split into seven samples of decreasing mistag probability $\eta_c$.

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Measured mistag fraction ( $\omega$ ) versus calculated mistag probability ($\eta_c$) calibrated on $ B ^+ \!\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^+ $ signal events for the OS tagger, in background subtracted events. Left and right plots correspond to $ B ^+ \!\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^+ $ and $ B ^0 \!\rightarrow D ^{*-} \mu ^+ \nu _\mu $ signal events. Points with errors are data, the red lines represent the result of the mistag calibration, corresponding to the parameters of Table ???.

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Animated gif made out of all figures.

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Tables and captions

Selection criteria for the OS muon, electron and kaon taggers.

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Tagging performance in the $ B ^+ \!\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^+ $ channel. Uncertainties are statistical only.

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Tagging performance in the $ B ^0 \!\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^{*0} $ channel. Uncertainties are statistical only.

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Tagging performance in the $B^0 \rightarrow D^{*-} \mu^+ \nu_\mu$ channel. Uncertainties are statistical only.

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Fit values and correlations of the OS combined mistag calibration parameters measured in the $ B ^+ \!\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^+ $ , $ B ^0 \!\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^{*0} $ and $ B ^0 \!\rightarrow D ^{*-} \mu ^+ \nu _\mu $ channels. The uncertainties are statistical only.

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Systematic uncertainties on the calibration parameters $p_0$ and $p_1$ obtained with $ B ^+ \!\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^+ $ events.

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Tagging efficiency, mistag probability and tagging power calculated from event-by-event probabilities for $ B ^+ \!\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^+ $ , $ B ^0 \!\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^{*0} $ , $ B ^0 \!\rightarrow D ^{*-} \mu ^+ \nu _\mu $ and $ B ^0_s \!\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \phi$ signal events. The quoted uncertainties are obtained propagating the statistical (first) and systematic (second) uncertainties on the calibration parameters determined from the $ B ^+ \!\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^+ $ events.

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Created on 18 December 2018.Citation count from INSPIRE on 18 December 2018.