Observation of $\bar{B}^0_{(s)}\rightarrow J/\psi f_1(1285)$ decays and measurement of the $f_1(1285)$ mixing angle

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Decays of Bs and B0 mesons into J/\psi \pi +\pi -\pi +\pi - final states, produced in pp collisions at the LHC, are investigated using data corresponding to an integrated luminosity of 3/fb collected with the LHCb detector. B^0_(s) -> J\psi f_1(1285) decays are seen for the first time, and the branching fractions are measured. Using these rates, the f_1(1285) mixing angle between strange and non-strange components of its wave function in the q-qbar structure model is determined to be \pm (24.0^{+3.1+0.6}_{-2.6-0.8}) degrees. Implications on the possible tetraquark nature of the f_1(1285) are discussed.

Figures and captions

Invariant mass distribution for $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+\pi^-\pi^+\pi^-$ combinations. The data are fit with Crystal Ball functions for $\overline{ B }{} ^0$ [(red) dashed curve] and $\overline{ B }{} ^0_ s $ [(purple) dot-dashed curve] signals, an exponential function for combinatoric background (black) dotted, and a Gaussian shape for lower mass background (blue) long-dashed. The total is shown with a (blue) solid curve.

fitdata.pdf [43 KiB]
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Background subtracted invariant mass distributions of the four pions in (a) $\overline{ B }{} ^0_ s $ and (b) $\overline{ B }{} ^0$ decays are shown in the histogram overlaid with the (black) filled points with the error bars indicating the uncertainties. The open (red) circles show the helicity $\pm$1 components of the signals.

m4pi_h[..].pdf [38 KiB]
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Fits to the four-pion invariant mass in (a) $\overline{ B }{} ^0_ s $ and (b) $\overline{ B }{} ^0$ decays. The data are shown as points, the signals components as (black) dashed curves, the combinatorial background by (black) dotted curves, and the higher mass resonance tail by (red) dot-dashed curves.

fitB-f[..].pdf [39 KiB]
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*.C file

Animated gif made out of all figures.


Tables and captions

Fit results for $\overline{ B }{} ^0_ s \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} f_1(1285)$ and $\overline{ B }{} ^0 \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} f_1(1285)$ decays.

Table_1.pdf [43 KiB]
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Systematic uncertainties of the branching fractions ${\cal{B}}(\overline{ B }{} \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} f_1(1285) , f_1(1285)\rightarrow \pi^+\pi^-\pi^+\pi^-)$ and the $\overline{ B }{} ^0 /\overline{ B }{} ^0_ s $ rate ratio. The "+" and "--" signs indicate the positive and negative uncertainties, respectively. All numbers are in (%).

Table_2.pdf [40 KiB]
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Branching fractions used for normalization.

Table_3.pdf [75 KiB]
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Created on 06 March 2021.