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

A bstract : The radiative decays χ c1 (3872) → ψ(2S) γ and χ c1 (3872) → J/ψγ are used to probe the nature of the χ c1 (3872) state using proton-proton collision data collected with the LHCb detector, corresponding to an integrated luminosity of 9 fb − 1 . Using the B + → χ c1 (3872)K + decay, the χ c1 (3872) → ψ(2S) γ process is observed for the first time and the ratio of its partial width to that of the χ c1 (3872) → J/ψγ decay is measured to be $$ \frac{\Gamma_{\chi_{c1}(3872)\to \uppsi \left(2\textrm{S}\right)\gamma }}{\Gamma_{\chi_{c1}(3872)\to \textrm{J}/\uppsi \gamma }}=1.67\pm 0.21\pm 0.12\pm 0.04, $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mfrac> <mml:msub> <mml:mi>Γ</mml:mi> <mml:mrow> <mml:msub> <mml:mi>χ</mml:mi> <mml:mrow> <mml:mi>c</mml:mi> <mml:mn>1</mml:mn> </mml:mrow> </mml:msub> <mml:mfenced> <mml:mn>3872</mml:mn> </mml:mfenced> <mml:mo>→</mml:mo> <mml:mi>ψ</mml:mi> <mml:mfenced> <mml:mrow> <mml:mn>2</mml:mn> <mml:mi>S</mml:mi> </mml:mrow> </mml:mfenced> <mml:mi>γ</mml:mi> </mml:mrow> </mml:msub> <mml:msub> <mml:mi>Γ</mml:mi> <mml:mrow> <mml:msub> <mml:mi>χ</mml:mi> <mml:mrow> <mml:mi>c</mml:mi> <mml:mn>1</mml:mn> </mml:mrow> </mml:msub> <mml:mfenced> <mml:mn>3872</mml:mn> </mml:mfenced> <mml:mo>→</mml:mo> <mml:mi>J</mml:mi> <mml:mo>/</mml:mo> <mml:mi>ψ</mml:mi> <mml:mi>γ</mml:mi> </mml:mrow> </mml:msub> </mml:mfrac> <mml:mo>=</mml:mo> <mml:mn>1.67</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.21</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.12</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.04</mml:mn> <mml:mo>,</mml:mo> </mml:math> 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 χ c1 (3872) state as a pure D 0 $$ {\overline{\textrm{D}}}^{\ast 0} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mover> <mml:mi>D</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> <mml:mrow> <mml:mo>∗</mml:mo> <mml:mn>0</mml:mn> </mml:mrow> </mml:msup> </mml:math> + $$ {\overline{\textrm{D}}}^0 $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mover> <mml:mi>D</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> <mml:mn>0</mml:mn> </mml:msup> </mml:math> D *0 molecule questionable and strongly indicates a sizeable compact charmonium or tetraquark component within the χ c1 (3872) state.