Probing the nature of the χc1(3872) state using radiative decays

Abstract

Abstract: The radiative decays χc1(3872) → ψ(2S) γ and χc1(3872) → J/ψγ are used to probe the nature of the χc1(3872) state using proton-proton collision data collected with the LHCb detector, corresponding to an integrated luminosity of 9 fb −1. Using the B+ → χc1(3872)K+ decay, the χc1(3872) → ψ(2S) γ process is observed for the first time and the ratio of its partial width to that of the χc1(3872) → J/ψγ decay is measured to be $$ \frac{\Gamma_{\chi_{c1}(3872)\to \uppsi \left(2\textrm{S}\right)\gamma }}{\Gamma_{\chi_{c1}(3872)\to \textrm{J}/\uppsi \gamma }}=1.67\pm 0.21\pm 0.12\pm 0.04, $$

Γ

χ

c 1

3872

→ ψ

2 S

γ

Γ

χ

c 1

3872

→ J / ψ γ

= 1.67 ± 0.21 ± 0.12 ± 0.04 ,

where the first uncertainty is statistical, the second systematic and the third is due to the uncertainties on the branching fractions of the ψ(2S) and J/ψ mesons. The measured ratio makes the interpretation of the χc1(3872) state as a pure D0

$$ {\overline{\textrm{D}}}^{\ast 0} $$

D ¯

∗ 0

+

$$ {\overline{\textrm{D}}}^0 $$

D ¯

0

D*0 molecule questionable and strongly indicates a sizeable compact charmonium or tetraquark component within the χc1(3872) state.