Based on data corresponding to an integrated luminosity of 0.37 fb^1 collected by the LHCb experiment in 2011, the following ratios of branching fractions are measured: B(B0 > pi+ pi) / B(B0 > K+pi) = 0.262 +/ 0.009 +/ 0.017, (fs/fd) * B(Bs > K+K) / B(B^0 > K+pi) = 0.316 +/ 0.009 +/ 0.019, (fs/fd) * B(Bs >pi+ K) / B(B0 > K+pi) = 0.074 +/ 0.006 +/ 0.006, (fd/fs) * B(B0 > K+K) / B(Bs > K+K) = 0.018 {+ 0.008  0.007} +/ 0.009, (fs/fd) * B(Bs > pi+pi) / B(B0 > pi+pi) = 0.050 {+ 0.011  0.009} +/ 0.004, B(Lambda_b > p pi) / B(Lambda_b > p K) = 0.86 +/ 0.08 +/ 0.05, where the first uncertainties are statistical and the second systematic. Using the current world average of B(B0 > K+pi) and the ratio of the strange to light neutral B meson production fs/fd measured by LHCb, we obtain: B(B0 > pi+pi) = (5.08 +/ 0.17 +/ 0.37) x 10^6, B(Bs > K+K) = (23.0 +/ 0.7 +/ 2.3) x 10^6, B(Bs > pi+K) = (5.4 +/ 0.4 +/ 0.6) x 10^6, B(B0 > K+K) = (0.11 {+ 0.05  0.04} +/ 0.06) x 10^6, B(Bs > pi+pi) = (0.95 {+ 0.21  0.17} +/ 0.13) x 10^6. The measurements of B(Bs > K+K), B(Bs > pi+ K) and B(B0 > K+K) are the most precise to date. The decay mode Bs > pi+pi is observed for the first time with a significance of more than 5 sigma.
Distributions of (a) $\delta m=m_{K\pi\pi}m_{K\pi}$ for $D^{*+}\rightarrow D^0(K^\pi^+)\pi^+$ candidates and (b) invariant mass of $\Lambda \rightarrow p \pi^$ candidates, used for the PID calibration. The curves are the results of maximum likelihood fits. 
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Invariant $\pi^+\pi^$ mass for candidates passing the selection A of Table 1. The result of an unbinned maximum likelihood fit is overlaid. The main contributions to the fit model are also shown. 
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Momentum distributions of (a) pions and (b) kaons from $D^0$ decays in the PID calibration sample (histograms). For comparison, the points represent the inclusive momentum distribution of daughter particles in $H_b \rightarrow h^+h^{\prime}$ decays. The distributions are normalized to the same area. This example corresponds to selection A. 
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Invariant mass spectra corresponding to selection A for the mass hypotheses (a) $K^+\pi^$, (b) $\pi^+\pi^$, (c) $K^+K^$, (d) $pK^$ and (e) $p\pi^$, and to selection B for the mass hypothesis (f) $K^+\pi^$. The results of the unbinned maximum likelihood fits are overlaid. The main components contributing to the fit model are also shown. 
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Invariant mass spectra corresponding to selection C for the mass hypotheses (a, b) $K^+K^$ and (c, d) $\pi^+\pi^$. Plots (b) and (d) are the same as (a) and (c) respectively, but magnified to focus on the rare $B^0 \rightarrow K^+ K^$ and $B^0_s \rightarrow \pi^+ \pi^$ signals. The results of the unbinned maximum likelihood fits are overlaid. The main components contributing to the fit model are also shown. 
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Animated gif made out of all figures. 
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Summary of criteria adopted in the event selections A, B and C defined in the text. 
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Ratios of reconstruction efficiencies of the various channels, as determined from Monte Carlo simulation, corresponding to the three event selections of Table 1. PID efficiencies are not included here. The tight requirement on $t_{\pi\pi}$ used in selection C leads to a sizable difference from unity of the ratios in the last two rows, as the $B_s^0\rightarrow\pi^+\pi^$ and $B_s^0\rightarrow K^+K^$ decays proceed mainly via the short lifetime component of the $B^0_s$ meson. 
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PID efficiencies (in %), for the various mass hypotheses, corresponding to the event samples passing the selections A, B and C of Table 1. Different sets of PID requirements are applied in the three cases. 
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Ratios of PID efficiencies used to compute the relevant ratios of branching fractions, corresponding to selection A. 
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Signal yields determined by the unbinned maximum likelihood fits to the data samples surviving the event selections A, B and C of Table 1 with the associated PID criteria. Only statistical uncertainties are shown. 
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Ratios of signal yields needed for the measurement of the relative branching fractions. Only statistical uncertainties are shown. 
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Systematic uncertainties on the ratios of signal yields. The total systematic uncertainties are obtained by summing the individual contributions in quadrature. 
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Created on 20 February 2021.