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

[to restricted-access page]

## 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. Figure1n.pdf [1 MiB] HiDef png [339 KiB] Thumbnail [97 KiB] *.C file 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]. Figure2.pdf [260 KiB] HiDef png [219 KiB] Thumbnail [68 KiB] *.C file Animated gif made out of all figures. PAPER-2017-036.gif Thumbnail

## 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. Table_1.pdf [44 KiB] HiDef png [108 KiB] Thumbnail [17 KiB] tex code Systematic uncertainties (in $\mathrm{ ke V}$ ) on the mass and mass difference measurements. Table_2.pdf [45 KiB] HiDef png [57 KiB] Thumbnail [8 KiB] tex code 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. Table_3.pdf [46 KiB] HiDef png [54 KiB] Thumbnail [8 KiB] tex code

## Supplementary Material [file]

 Supplementary material full pdf supple[..].pdf [193 KiB] Fig1_supp.pdf [10 KiB] HiDef png [164 KiB] Thumbnail [46 KiB] *C file Fig2_supp.pdf [10 KiB] HiDef png [159 KiB] Thumbnail [51 KiB] *C file Fig3_supp.pdf [9 KiB] HiDef png [173 KiB] Thumbnail [56 KiB] *C file

Created on 14 June 2019.Citation count from INSPIRE on 14 June 2019.