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Precise measurement of the $\chi_{c1}$ and $\chi_{c2}$ resonance parameters with the decays $\chi_{c1,c2}\to J/\psi\mu^+\mu^-$

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Abstract

The decays $\chi_{c1} \rightarrow J/\psi \mu^+ \mu^-$ and $\chi_{c2} \rightarrow J/\psi \mu^+ \mu^-$ are observed and used to study the resonance parameters of the $\chi_{c1}$ and $\chi_{c2}$ mesons. The masses of these states are measured to be m(\chi_{c1}) = 3510.71 \pm 0.04(stat) \pm 0.09(syst)MeV , m(\chi_{c2}) = 3556.10 \pm 0.06(stat) \pm 0.11(syst)MeV , where the knowledge of the momentum scale for charged particles dominates the systematic uncertainty. The momentum-scale uncertainties largely cancel in the mass difference m(\chi_{c2}) - m(\chi_{c1}) = 45.39 \pm 0.07(stat) \pm 0.03(syst)MeV . The natural width of the $\chi_{c2}$ meson is measured to be $$\Gamma(\chi_{c2}) = 2.10 \pm 0.20(stat) \pm 0.02(syst)MeV .$$ These results are in good agreement with and have comparable precision to the current world averages.

Figures and captions

Mass distribution for selected $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \mu ^+\mu ^- $ candidates. The fit is shown in thick orange, the $\chi _{ c 1} $ and $\chi _{ c 2} $ signal components are shown by the thin red solid curve and the background component by the dashed blue curve.

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Background-subtracted $m(\mu ^+\mu ^- )$ distribution for $\chi _{ c 1} \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \mu ^+\mu ^- $ (solid red circles) and $\chi _{ c 2} \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \mu ^+\mu ^- $ (open blue squares) decays. The distributions are normalized to unit area. The curves show the expected distribution from the simulation, which uses the model described in Ref. [29].

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Animated gif made out of all figures.

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Tables and captions

Signal yields and resonance parameters from the nominal fit. No correction for final-state radiation is applied to the mass measurements at this stage.

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Systematic uncertainties (in $\mathrm{ ke V}$ ) on the mass and mass difference measurements.

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LHCb measurements, compared to both previous measurements from Ref. [40] and the current world averages from Ref. [20]. The quoted uncertainties includes statistical and systematic uncertainties.

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Supplementary Material [file]

Supplementary material full pdf

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Created on 05 March 2021.