Using data collected with the LHCb detector in protonproton collisions at a centreofmass energy of 7 TeV, the hadronic decay Bs > J/psi f0(980) is observed. This CP eigenstate mode could be used to measure mixinginduced CP violation in the B_s system. Using a fit to the pi+ pi mass spectrum with interfering resonances gives R_{f0/phi} = [Gamma(Bs > J/psi f0, f0 > pi+ pi)]/[Gamma(Bs > J/psi phi, phi > K+K)] = 0.252^{+0.046+0.027}_{0.0320.033}, where the uncertainties are statistical and systematic, respectively.
Decay diagram for $B_s^0\to J/\psi (f_0{\rm or }\phi)$ decays. 
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The $\mu^+\mu^$ invariant mass for candidates satisfying the trigger and analysis requirements and having $t_z>0.5$ ps. The data points are shown as circles; the error bars are smaller than the circle radii. The dashed line shows the Crystal Ball signal function \cite{CB}, the dotted line the background and the solid line the sum. 
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The invariant mass of $J/\psi K^+K^$ combinations when the $K^+K^$ pair is required to be with $\pm$20 MeV of the $\phi$ mass. The data have been fit with a Gaussian signal function whose mass and width are allowed to float and linear background function shown as a dashed line. The solid curve shows the sum. 
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(a) The invariant mass of $J/\psi\pi^+\pi^$ combinations when the $\pi^+\pi^$ pair is required to be with $\pm$90 MeV of the $f_0(980)$ mass. The data have been fit with a signal Gaussian and several background functions. The thin (red) solid curve shows the signal, the longdashed (brown) curve the combinatorial background, the dashed (green) curve the $B^+\to J/\psi K^+(\pi^+) $ background, the dotted (blue) curve the ${B}^0\to J/\psi K^{*0}$ background, the dashdot curve (purple) the ${B}^0\to J/\psi \pi^+\pi^$ background, the barely visible dotted curve (black) the sum of $B_s^0\to J/\psi \eta'$ and $J/\psi\phi$ backgrounds, and the thicksolid (black) curve the total. (b) The same as above but for likesign dipion combinations. 
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Angular distributions of events within $\pm$30 MeV of the ${B}_s^0$ mass and $\pm$90 MeV of the $f_0$ mass after likesign background subtraction. (a) The cosine of the angle of the $\mu^+$ with respect to the ${B}_s^0$ direction in the $J/\psi$ rest frame for ${B}_s^0\to J/\psi \pi^+\pi^$ decays. The data are fit with a function $f(\cos\theta_{J/\psi})=1\alpha\cos^2\theta_{J/\psi}$. (b) The cosine of the angle of the $\pi^+$ with respect to the ${B}_s^0$ direction in the dipion rest frame for ${B}_s^0\to J/\psi\pi^+\pi^$ decays. The data are fit with a flat line. 
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The invariant mass of $\pi^+\pi^$ combinations when the $J/\psi\pi^+\pi^$ is required to be within $\pm$30 MeV of the ${B}_s^0$ mass. The dashed curve is the likesign background that is taken from the data both in shape and absolute normalization. The dotted curve is the result of the fit using Eq. 2 and the solid curve the total. 
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Animated gif made out of all figures. 
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Relative systematic uncertainties on $R_{f_0/\phi}$ (%). Both negative and positive changes resulting from the parameter variations are indicated in separate columns. 
Table_1.pdf [49 KiB] HiDef png [81 KiB] Thumbnail [36 KiB] tex code 
Created on 27 February 2021.