⚛️ high-energy experiments

Observation of $\Xi_{b}^{0} \to \Xi^{0} J/\psi$ and evidence for $\Xi_{b}^{0} \to \Xi^{0} \psi(2S)$ decays

Using LHCb data from 2016–2018, this paper reports the first observation of the $\Xi_{b}^{0} \to \Xi^{0} J/\psi$ decay and evidence for $\Xi_{b}^{0} \to \Xi^{0} \psi(2S)$ , marking the first full reconstruction of the $\Xi^{0}$ hyperon at an LHC experiment and providing a measurement of their branching fraction ratio.

Original authors: LHCb collaboration, R. Aaij, M. Abdelfatah, A. S. W. Abdelmotteleb, C. Abellan Beteta, F. Abudinén, T. Ackernley, A. A. Adefisoye, B. Adeva, M. Adinolfi, P. Adlarson, C. Agapopoulou, C. A. Aidala, S. LHCb collaboration, R. Aaij, M. Abdelfatah, A. S. W. Abdelmotteleb, C. Abellan Beteta, F. Abudinén, T. Ackernley, A. A. Adefisoye, B. Adeva, M. Adinolfi, P. Adlarson, C. Agapopoulou, C. A. Aidala, S. Akar, K. Akiba, H. Al Saleh, P. Albicocco, J. Albrecht, R. Aleksiejunas, F. Alessio, P. Alvarez Cartelle, S. Amato, J. L. Amey, Y. Amhis, Z. Amos, L. An, L. Anderlini, P. Andreola, M. Andreotti, S. Andres Estrada, A. Anelli, D. Ao, C. Arata, F. Archilli, Z. Areg, M. Argenton, S. Arguedas Cuendis, L. Arnone, M. Artuso, E. Aslanides, R. Ataíde Da Silva, M. Atzeni, B. Audurier, J. A. Authier, D. Bacher, I. Bachiller Perea, S. Bachmann, M. Bachmayer, J. J. Back, M. Bai, Z. B. Bai, V. Balagura, A. Balboni, W. Baldini, Z. Baldwin, L. Balzani, H. Bao, J. Baptista de Souza Leite, C. Barbero Pretel, M. Barbetti, I. R. Barbosa, W. Barker, R. J. Barlow, M. Barnyakov, S. Baron, S. Barsuk, W. Barter, J. Bartz, S. Bashir, B. Batsukh, P. B. Battista, A. Bavarchee, A. Bay, A. Beck, M. Becker, F. Bedeschi, I. B. Bediaga, N. A. Behling, S. Belin, A. Bellavista, I. Belyaev, G. Bencivenni, E. Ben-Haim, J. L. M. Berkey, R. Bernet, A. Bertolin, L. Bertsch, F. Betti, J. Bex, O. Bezshyyko, S. Bhattacharya, M. S. Bieker, N. V. Biesuz, A. Biolchini, M. Birch, F. C. R. Bishop, A. Bitadze, A. Bizzeti, T. Blake, F. Blanc, J. E. Blank, S. Blusk, J. A. Boelhauve, O. Boente Garcia, T. Boettcher, A. Bohare, C. Bolognani, R. B. Bonacci, A. Bordelius, F. Borgato, S. Borghi, M. Borsato, J. T. Borsuk, E. Bottalico, S. A. Bouchiba, M. Bovill, T. J. V. Bowcock, A. Boyer, C. Bozzi, J. D. Brandenburg, A. Brea Rodriguez, N. Breer, C. Breitfeld, J. Brodzicka, J. Brown, E. Buchanan, M. Burgos Marcos, C. Burr, E. Butera, C. Buti, J. S. Butter, J. Buytaert, W. Byczynski, S. Cadeddu, H. Cai, Y. Cai, Y. Cai, A. Caillet, R. Calabrese, L. Calefice, M. Calvi, M. Calvo Gomez, P. Camargo Magalhaes, J. I. Cambon Bouzas, P. Campana, A. Campomagnani, A. C. Campos, A. F. Campoverde Quezada, Y. Cao, S. Capelli, M. Caporale, L. Capriotti, R. Caravaca-Mora, A. Carbone, L. Carcedo Salgado, R. Cardinale, A. Cardini, P. Carniti, L. Carus, R. Caspary, G. Casse, M. Cattaneo, G. Cavallero, V. Cavallini, S. Celani, I. Celestino, S. Cesare, A. J. Chadwick, M. Charles, Ph. Charpentier, E. Chatzianagnostou, R. Cheaib, M. Chefdeville, C. Chen, J. Chen, S. Chen, Z. Chen, A. Chen Hu, M. Cherif, S. Chernyshenko, X. Chiotopoulos, G. Chizhik, V. Chobanova, A. Christakakis, M. Chrzaszcz, Y. Chu, V. Chulikov, P. Ciambrone, X. Cid Vidal, P. Cifra, P. E. L. Clarke, M. Clemencic, H. V. Cliff, J. Closier, C. Cocha Toapaxi, V. Coco, A. Codovini, C. Codovini, J. Cogan, E. Cogneras, L. Cojocariu, S. Collaviti, P. Collins, T. Colombo, M. Colonna, A. Comerma-Montells, L. Congedo, J. Connaughton, A. Contu, N. Cooke, A. Corallo, G. Cordova, C. Coronel, I. Corredoira, A. Correia, G. Corti, G. C. Costantino, C. Cotirlan, J. Cottee Meldrum, B. Couturier, D. C. Craik, N. Crepet, M. Cruz Torres, M. Cubero Campos, E. Curras Rivera, R. Currie, C. L. Da Silva, X. Dai, J. Dalseno, C. D'Ambrosio, G. Darze, A. Davidson, O. De Aguiar Francisco, C. De Angelis, F. De Benedetti, J. de Boer, K. De Bruyn, S. De Capua, M. De Cian, U. De Freitas Carneiro Da Graca, F. De Gregorio, E. De Lucia, J. M. De Miranda, L. De Paula, A. De Robertis, E. De Santis, M. De Serio, P. De Simone, F. De Vellis, J. A. de Vries, F. Debernardis, D. Decamp, S. Dekkers, L. Del Buono, B. Delaney, B. Demaire-Lepape, J. Deng, O. Deschamps, F. Dettori, B. Dey, P. Di Nezza, S. Ding, Y. Ding, L. Dittmann, J. F. Diverchy, A. D. Docheva, A. Doheny, C. Dong, F. Dordei, J. Dorta Moreno, A. C. dos Reis, J. Dos Santos Oliveira, A. D. Dowling, L. Dreyfus, W. Duan, P. Duda, L. Dufour, V. Duk, P. Durante, M. M. Duras, J. M. Durham, K. Duwe, A. Dziurda, S. Easo, E. Eckstein, U. Egede, S. Eisenhardt, E. Ejopu, L. Eklund, M. Elashri, D. Elizondo Blanco, J. Ellbracht, S. Ely, A. Ene, T. Evans, F. Fabiano, S. Faghih, L. N. Falcao, B. Fang, R. Fantechi, L. Fantini, M. Faria, K. Farmer, F. Fassin, D. Fazzini, L. Felkowski, C. Feng, M. Feng, A. Fernandez Casani, M. Fernandez Gomez, B. Fernandez Rodino, J. Fernandez-John, A. D. Fernez, F. Ferrari, F. Ferreira Rodrigues, R. A. Fini, R. Fiorenza, M. Fiorini, M. Firlej, D. S. Fitzgerald, C. Fitzpatrick, T. Fiutowski, F. Fleuret, A. Fomin, M. Fontana, M. Fontes Vaz, L. A. Foreman, R. Forty, D. Foulds-Holt, V. Franco Lima, M. Franco Sevilla, M. Frank, E. Franzoso, G. Frau, C. Frei, D. A. Friday, J. Fu, Y. Fu, Q. Führing, T. Fulghesu, G. Galati, M. D. Galati, A. Gallas Torreira, D. Galli, S. Gambetta, M. Gandelman, P. Gandini, B. Ganie, H. Gao, R. Gao, T. Q. Gao, Y. Gao, Y. Gao, Y. Gao, L. M. Garcia Martin, P. Garcia Moreno, J. García Pardiñas, P. Gardner, L. Garrido, C. Gaspar, A. Gavrikov, J. George, E. Gersabeck, M. Gersabeck, T. Gershon, S. Ghizzo, Z. Ghorbanimoghaddam, F. I. Giasemis, V. Gibson, H. K. Giemza, A. L. Gilman, M. Giovannetti, A. Gioventù, L. Girardey, M. A. Giza, F. C. Glaser, V. V. Gligorov, C. Göbel, L. Golinka-Bezshyyko, E. Golobardes, A. Golutvin, S. Gomez Fernandez, A. G. Gomez Mongui, W. Gomulka, F. Goncalves Abrantes, I. Gonçales Vaz, M. Goncerz, G. Gong, S. Gong, J. A. Gooding, C. Gotti, E. Govorkova, J. P. Grabowski, L. A. Granado Cardoso, R. Grande Quartieri, E. Graugés, E. Graverini, L. Grazette, G. Graziani, A. T. Grecu, N. A. Grieser, L. Grillo, C. Gu, M. Guarise, L. Guerry, A. -K. Guseinov, Y. Guz, T. Gys, K. Habermann, T. Hadavizadeh, C. Hadjivasiliou, G. Haefeli, C. Haen, S. Haken, G. Hallett, P. M. Hamilton, Q. Han, S. Han, X. Han, S. Hansmann-Menzemer, N. Harnew, T. J. Harris, L. Hartman, M. Hartmann, S. Hashmi, J. He, N. Heatley, A. Hedes, F. Hemmer, C. Henderson, R. Henderson, R. D. L. Henderson, A. M. Hennequin, K. Hennessy, A. Henrot, J. Herd, P. Herrero Gascon, J. Heuel, A. Heyn, A. Hicheur, G. Hijano Mendizabal, J. Horswill, R. Hou, Y. Hou, D. C. Houston, N. Howarth, W. Hu, X. Hu, W. Hulsbergen, R. J. Hunter, D. Hutchcroft, M. Idzik, P. Ilten, A. Iohner, S. Jacevicius, H. Jage, S. J. Jaimes Elles, S. Jakobsen, T. Jakoubek, E. Jans, A. Jawahery, C. Jayaweera, A. Jelavic, V. Jevtic, Z. Jia, E. Jiang, X. Jiang, Y. Jiang, Y. J. Jiang, E. Jimenez Moya, N. Jindal, M. John, A. John Rubesh Rajan, D. Johnson, C. R. Jones, S. Joshi, B. Jost, J. Juan Castella, N. Jurik, I. Juszczak, K. Kalecinska, D. Kaminaris, S. Kandybei, M. Kane, Y. Kang, C. Kar, A. Kauniskangas, J. W. Kautz, M. K. Kazanecki, F. Keizer, M. Kenzie, T. Ketel, B. Khanji, S. Kholodenko, V. Kholoimov, G. Khreich, F. Kiraz, T. Kirn, V. S. Kirsebom, N. Kleijne, A. Kleimenova, D. Klekots, K. Klimaszewski, M. R. Kmiec, T. Knospe, R. Kolb, S. Koliiev, L. Kolk, A. Konoplyannikov, P. Kopciewicz, P. Koppenburg, A. Korchin, I. Kostiuk, O. Kot, S. Kotriakhova, E. Kowalczyk, O. Kravcov, M. Kreps, W. Krupa, W. Krzemien, O. Kshyvanskyi, S. Kubis, M. Kucharczyk, A. Kupsc, A. Kurzina, V. Kushnir, B. Kutsenko, J. Kvapil, I. Kyryllin, D. Lacarrere, P. Laguarta Gonzalez, A. Lai, A. Lampis, D. Lancierini, C. Landesa Gomez, G. Lanfranchi, C. Langenbruch, T. Latham, F. Lazzari, C. Lazzeroni, R. Le Gac, H. Lee, R. Lefèvre, M. Lehuraux, E. Lemos Cid, O. Leroy, T. Lesiak, E. D. Lesser, B. Leverington, A. Li, C. Li, C. Li, H. Li, J. Li, K. Li, L. Li, L. Li, P. Li, P. -R. Li, Q. Li, T. Li, T. Li, W. Li, Y. Li, Y. Li, Y. Li, Z. Li, Z. Lian, Q. Liang, X. Liang, Z. Liang, S. Libralon, A. Lightbody, J. Lin, S. Lin, T. Lin, R. Lindner, H. Linton, V. Lisovskyi, R. Litvinov, D. Liu, F. L. Liu, G. Liu, K. Liu, S. Liu, W. Liu, X. Liu, Y. Liu, Y. Liu, Y. L. Liu, G. Loachamin Ordonez, I. Lobo, A. Lobo Salvia, A. Loi, T. Long, F. C. L. Lopes, J. H. Lopes, A. Lopez Huertas, C. Lopez Iribarnegaray, Q. Lu, C. Lucarelli, D. Lucchesi, M. Lucio Martinez, Y. Luo, A. Lupato, M. Lupberger, E. Luppi, K. Lynch, J. Lyu, S. Lyu, X. -R. Lyu, H. Ma, S. Maccolini, F. Machefert, F. Maciuc, B. Mack, I. Mackay, L. M. Mackey, L. R. Madhan Mohan, M. J. Madurai, D. Magdalinski, J. J. Malczewski, S. Malde, L. Malentacca, G. Manca, C. Mancuso, R. Manera Escalero, A. Mangalasseri, F. M. Manganella, R. Mangrulkar, D. Manuzzi, S. Mao, D. Marangotto, J. F. Marchand, R. Marchevski, U. Marconi, L. Mareso, E. Mariani, S. Mariani, C. Marin Benito, J. Marks, A. M. Marshall, L. Martel, G. Martelli, G. Martellotti, L. Martinazzoli, M. Martinelli, C. Martinez, A. Martinez Armas, D. Martinez Gomez, D. Martinez Santos, F. Martinez Vidal, A. Martorell i Granollers, A. Massafferri, R. Matev, A. Mathad, C. Matteuzzi, K. R. Mattioli, L. Matzner, A. Mauri, E. Maurice, J. Mauricio, P. Mayencourt, J. Mazorra de Cos, M. Mazurek, D. Mazzanti Tarancon, M. McCann, N. T. McHugh, A. McNab, R. McNulty, B. Meadows, S. E. R. Medaer, D. Melnychuk, D. Mendoza Granada, P. Menendez Valdes Perez, F. M. Meng, M. Merk, A. Merli, L. Meyer Garcia, D. Miao, H. Miao, S. Mico, M. Mikhasenko, D. A. Milanes, A. Minotti, E. Minucci, B. Mitreska, D. S. Mitzel, R. Mocanu, A. Modak, L. Moeser, R. D. Moise, E. F. Molina Cardenas, T. Mombächer, M. Monk, T. Monnard, S. Monteil, A. Morcillo Gomez, G. Morello, M. J. Morello, M. P. Morgenthaler, A. Moro, J. Moron, W. Morren, A. B. Morris, A. G. Morris, R. Mountain, Z. Mu, N. Muangkod, E. Muhammad, F. Muheim, M. Mulder, K. Müller, F. Muñoz-Rojas, V. Mytrochenko, P. Naik, T. Nakada, R. Nandakumar, G. Napoletano, I. Nasteva, M. Needham, N. Neri, S. Neubert, N. Neufeld, J. Nicolini, D. Nicotra, E. M. Niel, L. Nisi, Q. Niu, B. K. Njoki, P. Nogarolli, P. Nogga, J. Nombela Royo, C. Normand, A. Novo Cal, J. Novoa Fernandez, G. Nowak, H. N. Nur, A. Oblakowska-Mucha, T. Oeser, O. Okhrimenko, R. Oldeman, F. Oliva, E. Olivart Pino, M. Olocco, R. H. O'Neil, J. S. Ordonez Soto, D. Osthues, J. M. Otalora Goicochea, P. Owen, A. Oyanguren, O. Ozcelik, F. Paciolla, A. Padee, K. O. Padeken, B. Pagare, T. Pajero, A. Palano, L. Palini, L. Palombini, M. Palutan, C. Pan, X. Pan, S. Panebianco, S. Paniskaki, L. Paolucci, A. Papanestis, M. Pappagallo, L. L. Pappalardo, C. Pappenheimer, C. Parkes, D. Parmar, G. Passaleva, D. Passaro, A. Pastore, M. Patel, J. Patoc, C. Patrignani, A. Paul, C. J. Pawley, A. Pellegrino, J. Peng, X. Peng, M. Pepe Altarelli, S. Perazzini, H. Pereira Da Costa, M. Pereira Martinez, A. Pereiro Castro, C. Perez, A. Perez Casas, P. Perret, A. Perrevoort, A. Perro, M. J. Peters, A. Petkovic, K. Petridis, A. Petrolini, S. Pezzulo, J. P. Pfaller, H. Pham, L. Pica, E. Picatoste Olloqui, M. Piccini, L. Piccolo, B. Pietrzyk, R. N. Pilato, D. Pinci, F. Pisani, M. Pizzichemi, V. M. Placinta, M. Plo Casasus, T. Poeschl, F. Polci, M. Poli Lener, A. Poluektov, I. Polyakov, E. Polycarpo, S. Ponce, D. Popov, K. Popp, K. Prasanth, C. Prouve, D. Provenzano, V. Pugatch, A. Puicercus Gomez, G. Punzi, J. R. 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Published 2026-09-15

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: LHCb collaboration, R. Aaij, M. Abdelfatah, A. S. W. Abdelmotteleb, C. Abellan Beteta, F. Abudinén, T. Ackernley, A. A. Adefisoye, B. Adeva, M. Adinolfi, P. Adlarson, C. Agapopoulou, C. A. Aidala, S. Akar, K. Akiba, H. Al Saleh, P. Albicocco, J. Albrecht, R. Aleksiejunas, F. Alessio, P. Alvarez Cartelle, S. Amato, J. L. Amey, Y. Amhis, Z. Amos, L. An, L. Anderlini, P. Andreola, M. Andreotti, S. Andres Estrada, A. Anelli, D. Ao, C. Arata, F. Archilli, Z. Areg, M. Argenton, S. Arguedas Cuendis, L. Arnone, M. Artuso, E. Aslanides, R. Ataíde Da Silva, M. Atzeni, B. Audurier, J. A. Authier, D. Bacher, I. Bachiller Perea, S. Bachmann, M. Bachmayer, J. J. Back, M. Bai, Z. B. Bai, V. Balagura, A. Balboni, W. Baldini, Z. Baldwin, L. Balzani, H. Bao, J. Baptista de Souza Leite, C. Barbero Pretel, M. Barbetti, I. R. Barbosa, W. Barker, R. J. Barlow, M. Barnyakov, S. Baron, S. Barsuk, W. Barter, J. Bartz, S. Bashir, B. Batsukh, P. B. Battista, A. Bavarchee, A. Bay, A. Beck, M. Becker, F. Bedeschi, I. B. Bediaga, N. A. Behling, S. Belin, A. Bellavista, I. Belyaev, G. Bencivenni, E. Ben-Haim, J. L. M. Berkey, R. Bernet, A. Bertolin, L. Bertsch, F. Betti, J. Bex, O. Bezshyyko, S. Bhattacharya, M. S. Bieker, N. V. Biesuz, A. Biolchini, M. Birch, F. C. R. Bishop, A. Bitadze, A. Bizzeti, T. Blake, F. Blanc, J. E. Blank, S. Blusk, J. A. Boelhauve, O. Boente Garcia, T. Boettcher, A. Bohare, C. Bolognani, R. B. Bonacci, A. Bordelius, F. Borgato, S. Borghi, M. Borsato, J. T. Borsuk, E. Bottalico, S. A. Bouchiba, M. Bovill, T. J. V. Bowcock, A. Boyer, C. Bozzi, J. D. Brandenburg, A. Brea Rodriguez, N. Breer, C. Breitfeld, J. Brodzicka, J. Brown, E. Buchanan, M. Burgos Marcos, C. Burr, E. Butera, C. Buti, J. S. Butter, J. Buytaert, W. Byczynski, S. Cadeddu, H. Cai, Y. Cai, Y. Cai, A. Caillet, R. Calabrese, L. Calefice, M. Calvi, M. Calvo Gomez, P. Camargo Magalhaes, J. I. Cambon Bouzas, P. Campana, A. Campomagnani, A. C. Campos, A. F. Campoverde Quezada, Y. Cao, S. Capelli, M. Caporale, L. Capriotti, R. Caravaca-Mora, A. Carbone, L. Carcedo Salgado, R. Cardinale, A. Cardini, P. Carniti, L. Carus, R. Caspary, G. Casse, M. Cattaneo, G. Cavallero, V. Cavallini, S. Celani, I. Celestino, S. Cesare, A. J. Chadwick, M. Charles, Ph. Charpentier, E. Chatzianagnostou, R. Cheaib, M. Chefdeville, C. Chen, J. Chen, S. Chen, Z. Chen, A. Chen Hu, M. Cherif, S. Chernyshenko, X. Chiotopoulos, G. Chizhik, V. Chobanova, A. Christakakis, M. Chrzaszcz, Y. Chu, V. Chulikov, P. Ciambrone, X. Cid Vidal, P. Cifra, P. E. L. Clarke, M. Clemencic, H. V. Cliff, J. Closier, C. Cocha Toapaxi, V. Coco, A. Codovini, C. Codovini, J. Cogan, E. Cogneras, L. Cojocariu, S. Collaviti, P. Collins, T. Colombo, M. Colonna, A. Comerma-Montells, L. Congedo, J. Connaughton, A. Contu, N. Cooke, A. Corallo, G. Cordova, C. Coronel, I. Corredoira, A. Correia, G. Corti, G. C. Costantino, C. Cotirlan, J. Cottee Meldrum, B. Couturier, D. C. Craik, N. Crepet, M. Cruz Torres, M. Cubero Campos, E. Curras Rivera, R. Currie, C. L. Da Silva, X. Dai, J. Dalseno, C. D'Ambrosio, G. Darze, A. Davidson, O. De Aguiar Francisco, C. De Angelis, F. De Benedetti, J. de Boer, K. De Bruyn, S. De Capua, M. De Cian, U. De Freitas Carneiro Da Graca, F. De Gregorio, E. De Lucia, J. M. De Miranda, L. De Paula, A. De Robertis, E. De Santis, M. De Serio, P. De Simone, F. De Vellis, J. A. de Vries, F. Debernardis, D. Decamp, S. Dekkers, L. Del Buono, B. Delaney, B. Demaire-Lepape, J. Deng, O. Deschamps, F. Dettori, B. Dey, P. Di Nezza, S. Ding, Y. Ding, L. Dittmann, J. F. Diverchy, A. D. Docheva, A. Doheny, C. Dong, F. Dordei, J. Dorta Moreno, A. C. dos Reis, J. Dos Santos Oliveira, A. D. Dowling, L. Dreyfus, W. Duan, P. Duda, L. Dufour, V. Duk, P. Durante, M. M. Duras, J. M. Durham, K. Duwe, A. Dziurda, S. Easo, E. Eckstein, U. Egede, S. Eisenhardt, E. Ejopu, L. Eklund, M. Elashri, D. Elizondo Blanco, J. Ellbracht, S. Ely, A. Ene, T. Evans, F. Fabiano, S. Faghih, L. N. Falcao, B. Fang, R. Fantechi, L. Fantini, M. Faria, K. Farmer, F. Fassin, D. Fazzini, L. Felkowski, C. Feng, M. Feng, A. Fernandez Casani, M. Fernandez Gomez, B. Fernandez Rodino, J. Fernandez-John, A. D. Fernez, F. Ferrari, F. Ferreira Rodrigues, R. A. Fini, R. Fiorenza, M. Fiorini, M. Firlej, D. S. Fitzgerald, C. Fitzpatrick, T. Fiutowski, F. Fleuret, A. Fomin, M. Fontana, M. Fontes Vaz, L. A. Foreman, R. Forty, D. Foulds-Holt, V. Franco Lima, M. Franco Sevilla, M. Frank, E. Franzoso, G. Frau, C. Frei, D. A. Friday, J. Fu, Y. Fu, Q. Führing, T. Fulghesu, G. Galati, M. D. Galati, A. Gallas Torreira, D. Galli, S. Gambetta, M. Gandelman, P. Gandini, B. Ganie, H. Gao, R. Gao, T. Q. Gao, Y. Gao, Y. Gao, Y. Gao, L. M. Garcia Martin, P. Garcia Moreno, J. García Pardiñas, P. Gardner, L. Garrido, C. Gaspar, A. Gavrikov, J. George, E. Gersabeck, M. Gersabeck, T. Gershon, S. Ghizzo, Z. Ghorbanimoghaddam, F. I. Giasemis, V. Gibson, H. K. Giemza, A. L. Gilman, M. Giovannetti, A. Gioventù, L. Girardey, M. A. Giza, F. C. Glaser, V. V. Gligorov, C. Göbel, L. Golinka-Bezshyyko, E. Golobardes, A. Golutvin, S. Gomez Fernandez, A. G. Gomez Mongui, W. Gomulka, F. Goncalves Abrantes, I. Gonçales Vaz, M. Goncerz, G. Gong, S. Gong, J. A. Gooding, C. Gotti, E. Govorkova, J. P. Grabowski, L. A. Granado Cardoso, R. Grande Quartieri, E. Graugés, E. Graverini, L. Grazette, G. Graziani, A. T. Grecu, N. A. Grieser, L. Grillo, C. Gu, M. Guarise, L. Guerry, A. -K. Guseinov, Y. Guz, T. Gys, K. Habermann, T. Hadavizadeh, C. Hadjivasiliou, G. Haefeli, C. Haen, S. Haken, G. Hallett, P. M. Hamilton, Q. Han, S. Han, X. Han, S. Hansmann-Menzemer, N. Harnew, T. J. Harris, L. Hartman, M. Hartmann, S. Hashmi, J. He, N. Heatley, A. Hedes, F. Hemmer, C. Henderson, R. Henderson, R. D. L. Henderson, A. M. Hennequin, K. Hennessy, A. Henrot, J. Herd, P. Herrero Gascon, J. Heuel, A. Heyn, A. Hicheur, G. Hijano Mendizabal, J. Horswill, R. Hou, Y. Hou, D. C. Houston, N. Howarth, W. Hu, X. Hu, W. Hulsbergen, R. J. Hunter, D. Hutchcroft, M. Idzik, P. Ilten, A. Iohner, S. Jacevicius, H. Jage, S. J. Jaimes Elles, S. Jakobsen, T. Jakoubek, E. Jans, A. Jawahery, C. Jayaweera, A. Jelavic, V. Jevtic, Z. Jia, E. Jiang, X. Jiang, Y. Jiang, Y. J. Jiang, E. Jimenez Moya, N. Jindal, M. John, A. John Rubesh Rajan, D. Johnson, C. R. Jones, S. Joshi, B. Jost, J. Juan Castella, N. Jurik, I. Juszczak, K. Kalecinska, D. Kaminaris, S. Kandybei, M. Kane, Y. Kang, C. Kar, A. Kauniskangas, J. W. Kautz, M. K. Kazanecki, F. Keizer, M. Kenzie, T. Ketel, B. Khanji, S. Kholodenko, V. Kholoimov, G. Khreich, F. Kiraz, T. Kirn, V. S. Kirsebom, N. Kleijne, A. Kleimenova, D. Klekots, K. Klimaszewski, M. R. Kmiec, T. Knospe, R. Kolb, S. Koliiev, L. Kolk, A. Konoplyannikov, P. Kopciewicz, P. Koppenburg, A. Korchin, I. Kostiuk, O. Kot, S. Kotriakhova, E. Kowalczyk, O. Kravcov, M. Kreps, W. Krupa, W. Krzemien, O. Kshyvanskyi, S. Kubis, M. Kucharczyk, A. Kupsc, A. Kurzina, V. Kushnir, B. Kutsenko, J. Kvapil, I. Kyryllin, D. Lacarrere, P. 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Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The universe is built from a small set of fundamental building blocks, but the way these pieces fit together to form the matter we see is a complex puzzle. Among the most elusive pieces are heavy particles containing a "beauty" quark, a type of matter that does not exist naturally on Earth and must be created in high-energy collisions. These particles are unstable and decay, or break apart, almost instantly into lighter, more stable forms. By studying how these heavy particles transform, scientists can test the standard rules of physics to see if they hold up under extreme conditions or if they hint at new, undiscovered forces. While scientists have spent decades watching how heavy particles made of three quarks behave when they contain a specific type of quark called a "strange" quark, the version containing a "strange" quark and a "bottom" quark has remained largely invisible to detectors, leaving a gap in our understanding of how these heavy baryons behave.

A team of researchers using the Large Hadron Collider beauty experiment, known as LHCb, has finally filled this gap. By analyzing data from proton-proton collisions collected between 2016 and 2018, the team reported the first clear observation of a specific heavy particle, the $\Xi^0_b$ baryon, decaying into a $\Xi^0$ hyperon and a $J/\psi$ meson. This is a significant milestone because it marks the first time the $\Xi^0$ hyperon has been fully reconstructed at a Large Hadron Collider experiment. The $\Xi^0$ is a neutral particle that decays into a lambda baryon and a neutral pion, a process that is notoriously difficult to track because the neutral pion breaks down into two photons that leave no direct track in the detector. The researchers managed to piece together the entire decay chain, effectively tracing the path of a particle that vanishes almost immediately after it is born.

The study also provided strong evidence for a second, similar decay where the heavy $\Xi^0_b$ baryon transforms into a $\Xi^0$ hyperon and a heavier cousin of the $J/\psi$ called the $\psi(2S)$ . While the first decay was observed with overwhelming statistical certainty, the second was seen with a level of confidence that suggests it is real, though not yet confirmed as a definitive discovery. The researchers measured how often the heavy particle chooses the heavier $\psi(2S)$ path compared to the lighter $J/\psi$ path. They found that the heavier route happens about 59 percent as often as the lighter one, a ratio that aligns well with measurements taken for other similar heavy particles by different experiments. This consistency suggests that the rules governing these decays are uniform across different types of heavy baryons.

The path to this discovery was fraught with technical challenges. The $\Xi^0$ hyperon lives long enough to travel a measurable distance before decaying, but its neutral pion daughter particle does not leave a track, making it hard to pinpoint exactly where the decay happened. To solve this, the team used a sophisticated mathematical technique to reconstruct the particle's path by assuming it originated from the main collision point and fitting its properties to known physical values. This method improved the precision of their measurements by a factor of two, allowing them to separate the rare signal events from the overwhelming background noise of other particle collisions. The team also had to filter out false signals caused by random combinations of particles that looked like the target decay but were not.

The successful observation of these decays demonstrates that the LHCb detector is capable of reconstructing complex baryon decays involving multiple neutral particles, a feat that was previously thought to be extremely difficult. This achievement opens the door to studying even rarer decays of the $\Xi^0_b$ baryon, including those that might reveal new physics beyond the current standard model. By proving that these elusive particles can be tracked and measured with high precision, the researchers have provided a new tool for exploring the fundamental structure of matter. The results, published in a letter to the physics community, confirm that the behavior of these heavy baryons follows the patterns predicted by existing theories, while simultaneously proving that the experimental techniques are now advanced enough to tackle the most challenging decay chains in the subatomic world.

Technical Summary: Observation of $\Xi^0_b \to \Xi^0 J/\psi$ and Evidence for $\Xi^0_b \to \Xi^0 \psi(2S)$

Problem and Motivation
Weakly decaying $b$ baryons provide a critical testing ground for the Standard Model, offering a perspective complementary to meson decays. Among the four ground-state $b$ baryons containing a single $b$ quark, the $\Xi^0_b$ (usb) remains the least studied. While its mass has been precisely measured, its production cross-section remains unknown, hindering absolute branching fraction measurements. Furthermore, the $\Xi^0$ hyperon had never been fully reconstructed at an LHC experiment due to the experimental challenges associated with its decay chain, specifically the neutral $\pi^0$ meson which lacks a defined trajectory. This paper addresses the first search for the decays $\Xi^0_b \to \Xi^0 J/\psi$ and $\Xi^0_b \to \Xi^0 \psi(2S)$ , aiming to observe these modes and measure the ratio of their branching fractions, a quantity where uncertainties related to the unknown $\Xi^0_b$ production fraction cancel.

Methodology
The analysis utilizes proton-proton collision data collected by the LHCb experiment between 2016 and 2018 at a center-of-mass energy of $\sqrt{s} = 13$ TeV, corresponding to an integrated luminosity of $5.4 \text{ fb}^{-1}$ . The signal processes are reconstructed via the decay chains:

$\Xi^0_b \to \Xi^0 \psi$ , where $\psi$ is either $J/\psi$ or $\psi(2S)$ .
$\psi \to \mu^+\mu^-$ .
$\Xi^0 \to \Lambda \pi^0$ , with $\Lambda \to p \pi^-$ and $\pi^0 \to \gamma\gamma$ .

The reconstruction of the $\Xi^0$ hyperon is experimentally challenging because the $\pi^0$ momentum vector cannot be determined directly from its decay products without assuming an origin. To mitigate the resulting bias in momentum measurement and the large background from soft $\pi^0$ mesons, a kinematic decay chain fit is employed. This fit constrains the $\Xi^0_b$ to originate from the primary vertex (PV) and fixes the masses of the $\psi$ , $\Xi^0$ , $\Lambda$ , and $\pi^0$ to their known values. This approach improves the invariant mass resolution by a factor of two.

Event selection involves a hardware trigger requiring high transverse momentum ( $p_T$ ) muons, followed by a software trigger selecting $\mu^+\mu^-$ pairs consistent with $\psi$ masses and displaced from the PV. Offline selection utilizes a Boosted Decision Tree (BDT) classifier trained on simulated signal and data sidebands to suppress combinatorial background, which is dominant due to the high-multiplicity environment and $\pi^0$ reconstruction challenges. Peaking backgrounds from misreconstructed $\Xi^-_b$ decays or $\Lambda^0_b \to \Lambda \psi$ with random $\pi^0$ mesons are suppressed via mass vetoes and kinematic requirements.

Yields are determined using an extended unbinned maximum-likelihood fit to the reconstructed $\Xi^0\psi$ invariant-mass distributions in the range $5400 < m(\Xi^0\psi) < 6200 \text{ MeV}/c^2$ . Signal components are modeled by a linear combination of two one-sided Crystal Ball functions, while backgrounds are modeled using exponential and double-sided Crystal Ball functions. The branching fraction ratio $R_{\Xi^0_b}$ is computed simultaneously for both decay modes.

Key Contributions and Results

First Observation: The paper reports the first observation of the decay $\Xi^0_b \to \Xi^0 J/\psi$ with overwhelming statistical significance.
First Evidence: Evidence for the decay $\Xi^0_b \to \Xi^0 \psi(2S)$ is presented with a significance exceeding $4\sigma$ .
Full $\Xi^0$ Reconstruction: This work marks the first time the $\Xi^0$ hyperon has been fully reconstructed at an LHC experiment, utilizing the $\Xi^0 \to \Lambda \pi^0$ decay. This achievement required handling the softest photon and $\pi^0$ candidates used in an LHCb measurement to date.
Branching Fraction Ratio: The ratio of branching fractions is measured as:
$\frac{\mathcal{B}(\Xi^0_b \to \Xi^0 \psi(2S))}{\mathcal{B}(\Xi^0_b \to \Xi^0 J/\psi)} = 0.59 \pm 0.19 \text{ (stat)} \pm 0.04 \text{ (syst)}$
The systematic uncertainty includes contributions from simulation sample size, fit models, BDT selection, and external branching fractions.

Significance
The observation of $\Xi^0_b \to \Xi^0 J/\psi$ demonstrates that baryonic decays involving multiple neutral particles can be reconstructed with good sensitivity at LHCb. The measured ratio $R_{\Xi^0_b}$ is compatible with the corresponding ratio recently measured by the CMS experiment for the $\Xi^-_b$ baryon and with measurements for the $\Lambda^0_b$ baryon by LHCb and ATLAS. The authors note that this result implies the rare decay $\Xi^0_b \to \Xi^0 \mu^+\mu^-$ is within reach of LHCb with increased data samples, given the successful reconstruction of the complex final state. The paper does not claim to resolve discrepancies between theoretical models (such as the covariant confined quark model vs. perturbative QCD) but provides the first experimental data point for these specific decay modes to facilitate such comparisons.

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