Observation of Two New Excited <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msubsup><mml:mi mathvariant="normal">Ξ</mml:mi><mml:mi>b</mml:mi><mml:mn>0</mml:mn></mml:msubsup></mml:math> States Decaying to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msubsup><mml:mi mathvariant="normal">Λ</mml:mi><mml:mi>b</mml:mi><mml:mn>0</mml:mn></mml:msubsup><mml:msup><mml:mi>K</mml:mi><mml:mo>−</mml:mo></mml:msup><mml:msup><mml:mi>π</mml:mi><mml:mo>+</mml:mo></mml:msup></mml:math>
R. Aaij, A. S. W. Abdelmotteleb, C. Abellán Beteta, T. Ackernley, B. Adeva, M. Adinolfi, H. Afsharnia, C. Agapopoulou, C. A. Aidala, Simone Aiola, Ziad Ajaltouni, S. Akar, J. Albrecht, F. Alessio, M. Alexander, A. Alfonso Albero, Z. Aliouche, G. Alkhazov, P. Álvarez Cartelle, S. Amato, J. L. Amey, Y. Amhis, Liupan An, L. Anderlini, A. Andreianov, M. Andreotti, F. Archilli, A. Artamonov, M. Artuso, K. Arzymatov, E. Aslanides, M. Atzeni, B. Audurier, S. Bachmann, M. Bachmayer, J. J. Back, P. Baladrón Rodríguez, V. Balagura, W. Baldini, J. Baptista de Souza Leite, M. Barbetti, R. J. Barlow, S. Barsuk, W. Barter, M. Bartolini, F. Baryshnikov, J. M. Basels, S. Bashir, G. Bassi, B. Batsukh, A. Battig, A. Bay, A. Beck, M. Becker, F. Bedeschi, I. Bediaga, A. Beiter, V. Belavin, S. Belin, V. Bellée, K. Belous, I. Belov, I. Belyaev, G. Bencivenni, E. Ben-Haim, A. Berezhnoy, R. Bernet, D. Berninghoff, H. C. Bernstein, C. Bertella, A. Bertolin, C. Betancourt, F. Betti, Ia. Bezshyiko, S. Bhasin, J. Bhom, L. Bian, M. S. Bieker, S. Bifani, P. Billoir, M. Birch, F. C. R. Bishop, A. Bitadze, A. Bizzeti, M. Bjørn, M. P. Blago, T. Blake, F. Blanc, S. Blusk, D. Bobulska, J. A. Boelhauve, O. Boente García, T. Boettcher, A. Boldyrev, A. Bondar, N. Bondar, S. Borghi, M. Borisyak, M. Borsato, J. T. Borsuk
Abstract
Two narrow resonant states are observed in the ${\mathrm{\ensuremath{\Lambda}}}_{b}^{0}{K}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ mass spectrum using a data sample of proton-proton collisions at a center-of-mass energy of 13 TeV, collected by the LHCb experiment and corresponding to an integrated luminosity of $6\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$. The minimal quark content of the ${\mathrm{\ensuremath{\Lambda}}}_{b}^{0}{K}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ system indicates that these are excited ${\mathrm{\ensuremath{\Xi}}}_{b}^{0}$ baryons. The masses of the ${\mathrm{\ensuremath{\Xi}}}_{b}(6327{)}^{0}$ and ${\mathrm{\ensuremath{\Xi}}}_{b}(6333{)}^{0}$ states are $m[{\mathrm{\ensuremath{\Xi}}}_{b}(6327{)}^{0}]=6327.2{8}_{\ensuremath{-}0.21}^{+0.23}\ifmmode\pm\else\textpm\fi{}0.12\ifmmode\pm\else\textpm\fi{}0.24$ and $m[{\mathrm{\ensuremath{\Xi}}}_{b}(6333{)}^{0}]=6332.6{9}_{\ensuremath{-}0.18}^{+0.17}\ifmmode\pm\else\textpm\fi{}0.03\ifmmode\pm\else\textpm\fi{}0.22\text{ }\text{ }\mathrm{MeV}$, respectively, with a mass splitting of $\mathrm{\ensuremath{\Delta}}m=5.4{1}_{\ensuremath{-}0.27}^{+0.26}\ifmmode\pm\else\textpm\fi{}0.12\text{ }\text{ }\mathrm{MeV}$, where the uncertainties are statistical, systematic, and due to the ${\mathrm{\ensuremath{\Lambda}}}_{b}^{0}$ mass measurement. The measured natural widths of these states are consistent with zero, with upper limits of $\mathrm{\ensuremath{\Gamma}}[{\mathrm{\ensuremath{\Xi}}}_{b}(6327{)}^{0}]<2.20(2.56)$ and $\mathrm{\ensuremath{\Gamma}}[{\mathrm{\ensuremath{\Xi}}}_{b}(6333{)}^{0}]<1.60(1.92)\text{ }\text{ }\mathrm{MeV}$ at a 90% (95%) credibility level. The significance of the two-peak hypothesis is larger than nine (five) Gaussian standard deviations compared to the no-peak (one-peak) hypothesis. The masses, widths, and resonant structure of the new states are in good agreement with the expectations for a doublet of $1D$ ${\mathrm{\ensuremath{\Xi}}}_{b}^{0}$ resonances.