Probing the nature of the χc1(3872) state using radiative decays
R. Aaij, A. S. W. Abdelmotteleb, Carlos Abellán Beteta, F. Abudinén, T. Ackernley, A. A. Adefisoye, B. Adeva, M. Adinolfi, P. Adlarson, C. Agapopoulou, C. A. Aidala, Z. Ajaltouni, S. Akar, K. Carvalho Akiba, P. Albicocco, J. Albrecht, F. Alessio, M. Alexander, Z. Aliouche, P. Álvarez Cartelle, R. Amalric, S. Amato, J. L. Amey, Y. Amhis, L. An, L. Anderlini, M. Andersson, A. Andreianov, P. Andreola, M. Andreotti, D. Andreou, Alessia Anelli, D. Ao, F. Archilli, Matteo Argenton, S. Arguedas Cuendis, A. Artamonov, M. Artuso, E. Aslanides, R. Ataíde Da Silva, M. Atzeni, B. Audurier, D. Bacher, Irene Bachiller Perea, S. Bachmann, M. Bachmayer, J. J. Back, P. Baladrón Rodríguez, V. Balagura, W. Baldini, Luca Balzani, H. Bao, J. Baptista de Souza Leite, C. Pretel, M. Barbetti, I. R. Barbosa, R. J. Barlow, Mikhail Barnyakov, S. Barsuk, W. Barter, M. Bartolini, Justin Bartz, J. M. Basels, Saliha Bashir, G. Bassi, B. Batsukh, Piera Battista, A. Bay, A. Beck, M. Becker, F. Bedeschi, I. Bediaga, N. B. Behling, S. Belin, V. Bellée, K. Belous, I. Belov, I. Belyaev, G. Benane, G. Bencivenni, E. Ben-Haim, A. Berezhnoy, R. Bernet, S. Bernet Andres, A. Bertolin, C. Betancourt, F. Betti, J. Bex, Ia. Bezshyiko, J. Bhom, M. S. Bieker, N. V. Biesuz, P. Billoir, A. Biolchini, M. Birch, F. C. R. Bishop, A. Bitadze, A. Bizzeti, T. Blake, F. Blanc
Abstract
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.