Observation of structure in the $J /\psi$ -pair mass spectrum
Aaij, Roel, Abellán Beteta, Carlos, Ackernley, Thomas, Adeva, Bernardo, Adinolfi, Marco, Afsharnia, Hossein, Aidala, Christine Angela, Aiola, Salvatore, Ajaltouni, Ziad, Akar, Simon, Albrecht, Johannes, Alessio, Federico, Alexander, Michael, Alfonso Albero, Alejandro, Aliouche, Zakariya, Alkhazov, Georgy, Alvarez Cartelle, Paula, Alves Jr, Antonio Augusto, Amato, Sandra, Amhis, Yasmine, Y. Amhis, L. An, Andreassi, Guido, Andreianov, Aleksei, Andreotti, Mirco, Archilli, Flavio, Artamonov, Alexander, Artuso, Marina, Arzymatov, Kenenbek, Aslanides, Elie, Atzeni, Michele, Audurier, Benjamin, Bachmann, Sebastian, Bachmayer, Marie, Back, John, Baker, Sophie, Baladron Rodriguez, Pablo, Balagura, Vladislav, Baldini, Wander, Baptista Leite, Juan, Barlow, Roger, Barsuk, Sergey, Barter, William, Bartolini, Matteo, Baryshnikov, Fedor, Basels, Jan-Marc, Bassi, Giovanni, Batozskaya, Varvara, Batsukh, Baasansuren, Battig, Alexander, Bay, Aurelio, Becker, Maik, Bedeschi, Franco, Bediaga, Ignacio, Beiter, Andrew, Belavin, Vladislav, Belin, Samuel, Bellee, Violaine, Belous, Konstantin, Belov, Ilia, I. Belyaev, G. Bencivenni, E. Ben-Haim, A. Berezhnoy, Roland Bernet, D. Berninghoff, H. C. Bernstein, C. Bertella, E. Bertholet, A. Bertolin, C. Betancourt, F. Betti, M. O. Bettler, Ia. Bezshyiko, S. Bhasin, J. Bhom, L. Bian, M. S. Bieker, S. Bifani, P. Billoir, Birch, Matthew, F. C. R. Bishop, A. Bizzeti, Mikkel Bjørn, M. P. Blago, Blake, Thomas, F. Blanc, S. Blusk, D. Bobulska, V. Bocci, J. A. Boelhauve, O. Boente García, T. Boettcher, A. Boldyrev, A. Bondar, N. Bondar, S. Borghi, M. Borisyak, M. Borsato, J. T. Borsuk
Abstract
Using proton-proton collision data at centre-of-mass energies of s=7,8 and 13TeV recorded by the LHCb experiment at the Large Hadron Collider, corresponding to an integrated luminosity of 9fb-1, the invariant mass spectrum of J/ψ pairs is studied. A narrow structure around 6.9GeV/c2 matching the lineshape of a resonance and a broad structure just above twice the J/ψ mass are observed. The deviation of the data from nonresonant J/ψ-pair production is above five standard deviations in the mass region between 6.2 and 7.4GeV/c2, covering predicted masses of states composed of four charm quarks. The mass and natural width of the narrow X(6900) structure are measured assuming a Breit-Wigner lineshape.