EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)

CERN-PH-EP-2014-115LHCb-PAPER-2014-020

3 June 2014

Observation of the Λ0b→ J/ψpπ−

decay

The LHCb collaboration†

Abstract

The first observation of the Cabibbo-suppressed decay Λ0b→ J/ψpπ− is reported

using a data sample of proton-proton collisions at 7 and 8 TeV, corresponding to anintegrated luminosity of 3 fb−1. A prominent signal is observed and the branchingfraction relative to the decay mode Λ0

b→ J/ψpK− is determined to be

B(Λ0b→ J/ψpπ−)

B(Λ0b→ J/ψpK−)

= 0.0824± 0.0025 (stat)± 0.0042 (syst).

A search for direct CP violation is performed. The difference in the CP asymmetriesbetween these two decays is found to be

ACP (Λ0b→ J/ψpπ−)−ACP (Λ0

b→ J/ψpK−) = (+5.7± 2.3 (stat)± 1.2 (syst))%,

which is compatible with CP symmetry at the 2.2σ level.

Submitted to JHEP

c© CERN on behalf of the LHCb collaboration, license CC-BY-3.0.

†Authors are listed on the following pages.

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LHCb collaboration

R. Aaij41, B. Adeva37, M. Adinolfi46, A. Affolder52, Z. Ajaltouni5, S. Akar6, J. Albrecht9,F. Alessio38, M. Alexander51, S. Ali41, G. Alkhazov30, P. Alvarez Cartelle37, A.A. Alves Jr25,38,S. Amato2, S. Amerio22, Y. Amhis7, L. An3, L. Anderlini17,g, J. Anderson40, R. Andreassen57,M. Andreotti16,f , J.E. Andrews58, R.B. Appleby54, O. Aquines Gutierrez10, F. Archilli38,A. Artamonov35, M. Artuso59, E. Aslanides6, G. Auriemma25,n, M. Baalouch5, A. Babeanu38,q,S. Bachmann11, J.J. Back48, A. Badalov36, V. Balagura31, W. Baldini16, R.J. Barlow54,C. Barschel38, S. Barsuk7, W. Barter47, V. Batozskaya28, V. Battista39, A. Bay39, L. Beaucourt4,J. Beddow51, F. Bedeschi23, I. Bediaga1, S. Belogurov31, K. Belous35, I. Belyaev31,E. Ben-Haim8, G. Bencivenni18, S. Benson38, J. Benton46, A. Berezhnoy32, R. Bernet40,M.-O. Bettler47, M. van Beuzekom41, A. Bien11, S. Bifani45, T. Bird54, A. Bizzeti17,i,P.M. Bjørnstad54, T. Blake48, F. Blanc39, J. Blouw10, S. Blusk59, V. Bocci25, A. Bondar34,N. Bondar30,38, W. Bonivento15,38, S. Borghi54, A. Borgia59, M. Borsato7, T.J.V. Bowcock52,E. Bowen40, C. Bozzi16, T. Brambach9, J. van den Brand42, J. Bressieux39, D. Brett54,M. Britsch10, T. Britton59, J. Brodzicka54, N.H. Brook46, H. Brown52, A. Bursche40,G. Busetto22,s, J. Buytaert38, S. Cadeddu15, R. Calabrese16,f , M. Calvi20,k, M. Calvo Gomez36,p,A. Camboni36, P. Campana18,38, D. Campora Perez38, A. Carbone14,d, G. Carboni24,l,R. Cardinale19,38,j , A. Cardini15, H. Carranza-Mejia50, L. Carson50, K. Carvalho Akiba2,G. Casse52, L. Cassina20, L. Castillo Garcia38, M. Cattaneo38, Ch. Cauet9, R. Cenci58,M. Charles8, Ph. Charpentier38, S. Chen54, S.-F. Cheung55, N. Chiapolini40, M. Chrzaszcz40,26,K. Ciba38, X. Cid Vidal38, G. Ciezarek53, P.E.L. Clarke50, M. Clemencic38, H.V. Cliff47,J. Closier38, V. Coco38, J. Cogan6, E. Cogneras5, P. Collins38, A. Comerma-Montells11,A. Contu15, A. Cook46, M. Coombes46, S. Coquereau8, G. Corti38, M. Corvo16,f , I. Counts56,B. Couturier38, G.A. Cowan50, D.C. Craik48, M. Cruz Torres60, S. Cunliffe53, R. Currie50,C. D’Ambrosio38, J. Dalseno46, P. David8, P.N.Y. David41, A. Davis57, K. De Bruyn41,S. De Capua54, M. De Cian11, J.M. De Miranda1, L. De Paula2, W. De Silva57, P. De Simone18,D. Decamp4, M. Deckenhoff9, L. Del Buono8, N. Deleage4, D. Derkach55, O. Deschamps5,F. Dettori38, A. Di Canto38, H. Dijkstra38, S. Donleavy52, F. Dordei11, M. Dorigo39,A. Dosil Suarez37, D. Dossett48, A. Dovbnya43, K. Dreimanis52, G. Dujany54, F. Dupertuis39,P. Durante38, R. Dzhelyadin35, A. Dziurda26, A. Dzyuba30, S. Easo49,38, U. Egede53,V. Egorychev31, S. Eidelman34, S. Eisenhardt50, U. Eitschberger9, R. Ekelhof9, L. Eklund51,38,I. El Rifai5, Ch. Elsasser40, S. Ely59, S. Esen11, H.-M. Evans47, T. Evans55, A. Falabella16,f ,C. Farber11, C. Farinelli41, N. Farley45, S. Farry52, RF Fay52, D. Ferguson50,V. Fernandez Albor37, F. Ferreira Rodrigues1, M. Ferro-Luzzi38, S. Filippov33, M. Fiore16,f ,M. Fiorini16,f , M. Firlej27, C. Fitzpatrick38, T. Fiutowski27, M. Fontana10, F. Fontanelli19,j ,R. Forty38, O. Francisco2, M. Frank38, C. Frei38, M. Frosini17,38,g, J. Fu21,38, E. Furfaro24,l,A. Gallas Torreira37, D. Galli14,d, S. Gallorini22, S. Gambetta19,j , M. Gandelman2, P. Gandini59,Y. Gao3, J. Garofoli59, J. Garra Tico47, L. Garrido36, C. Gaspar38, R. Gauld55, L. Gavardi9,G. Gavrilov30, E. Gersabeck11, M. Gersabeck54, T. Gershon48, Ph. Ghez4, A. Gianelle22,S. Giani’39, V. Gibson47, L. Giubega29, V.V. Gligorov38, C. Gobel60, D. Golubkov31,A. Golutvin53,31,38, A. Gomes1,a, H. Gordon38, C. Gotti20, M. Grabalosa Gandara5,R. Graciani Diaz36, L.A. Granado Cardoso38, E. Grauges36, G. Graziani17, A. Grecu29,E. Greening55, S. Gregson47, P. Griffith45, L. Grillo11, O. Grunberg62, B. Gui59, E. Gushchin33,Yu. Guz35,38, T. Gys38, C. Hadjivasiliou59, G. Haefeli39, C. Haen38, S.C. Haines47, S. Hall53,B. Hamilton58, T. Hampson46, X. Han11, S. Hansmann-Menzemer11, N. Harnew55,

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S.T. Harnew46, J. Harrison54, T. Hartmann62, J. He38, T. Head38, V. Heijne41, K. Hennessy52,P. Henrard5, L. Henry8, J.A. Hernando Morata37, E. van Herwijnen38, M. Heß62, A. Hicheur1,D. Hill55, M. Hoballah5, C. Hombach54, W. Hulsbergen41, P. Hunt55, N. Hussain55,D. Hutchcroft52, D. Hynds51, M. Idzik27, P. Ilten56, R. Jacobsson38, A. Jaeger11, J. Jalocha55,E. Jans41, P. Jaton39, A. Jawahery58, F. Jing3, M. John55, D. Johnson55, C.R. Jones47,C. Joram38, B. Jost38, N. Jurik59, M. Kaballo9, S. Kandybei43, W. Kanso6, M. Karacson38,T.M. Karbach38, S. Karodia51, M. Kelsey59, I.R. Kenyon45, T. Ketel42, B. Khanji20,C. Khurewathanakul39, S. Klaver54, O. Kochebina7, M. Kolpin11, I. Komarov39,R.F. Koopman42, P. Koppenburg41,38, M. Korolev32, A. Kozlinskiy41, L. Kravchuk33,K. Kreplin11, M. Kreps48, G. Krocker11, P. Krokovny34, F. Kruse9, W. Kucewicz26,o,M. Kucharczyk20,26,38,k, V. Kudryavtsev34, K. Kurek28, T. Kvaratskheliya31, V.N. La Thi39,D. Lacarrere38, G. Lafferty54, A. Lai15, D. Lambert50, R.W. Lambert42, E. Lanciotti38,G. Lanfranchi18, C. Langenbruch38, B. Langhans38, T. Latham48, C. Lazzeroni45, R. Le Gac6,J. van Leerdam41, J.-P. Lees4, R. Lefevre5, A. Leflat32, J. Lefrancois7, S. Leo23, O. Leroy6,T. Lesiak26, B. Leverington11, Y. Li3, M. Liles52, R. Lindner38, C. Linn38, F. Lionetto40,B. Liu15, G. Liu38, S. Lohn38, I. Longstaff51, J.H. Lopes2, N. Lopez-March39, P. Lowdon40,H. Lu3, D. Lucchesi22,s, H. Luo50, A. Lupato22, E. Luppi16,f , O. Lupton55, F. Machefert7,I.V. Machikhiliyan31, F. Maciuc29, O. Maev30, S. Malde55, G. Manca15,e, G. Mancinelli6,J. Maratas5, J.F. Marchand4, U. Marconi14, C. Marin Benito36, P. Marino23,u, R. Marki39,J. Marks11, G. Martellotti25, A. Martens8, A. Martın Sanchez7, M. Martinelli41,D. Martinez Santos42, F. Martinez Vidal64, D. Martins Tostes2, A. Massafferri1, R. Matev38,Z. Mathe38, C. Matteuzzi20, A. Mazurov16,f , M. McCann53, J. McCarthy45, A. McNab54,R. McNulty12, B. McSkelly52, B. Meadows57, F. Meier9, M. Meissner11, M. Merk41,D.A. Milanes8, M.-N. Minard4, N. Moggi14, J. Molina Rodriguez60, S. Monteil5, M. Morandin22,P. Morawski27, A. Morda6, M.J. Morello23,u, J. Moron27, A.-B. Morris50, R. Mountain59,F. Muheim50, K. Muller40, R. Muresan29, M. Mussini14, B. Muster39, P. Naik46, T. Nakada39,R. Nandakumar49, I. Nasteva2, M. Needham50, N. Neri21, S. Neubert38, N. Neufeld38,M. Neuner11, A.D. Nguyen39, T.D. Nguyen39, C. Nguyen-Mau39,r, M. Nicol7, V. Niess5,R. Niet9, N. Nikitin32, T. Nikodem11, A. Novoselov35, D.P. O’Hanlon48,A. Oblakowska-Mucha27, V. Obraztsov35, S. Oggero41, S. Ogilvy51, O. Okhrimenko44,R. Oldeman15,e, G. Onderwater65, M. Orlandea29, J.M. Otalora Goicochea2, P. Owen53,A. Oyanguren64, B.K. Pal59, A. Palano13,c, F. Palombo21,v, M. Palutan18, J. Panman38,A. Papanestis49,38, M. Pappagallo51, C. Parkes54, C.J. Parkinson9,45, G. Passaleva17,G.D. Patel52, M. Patel53, C. Patrignani19,j , A. Pazos Alvarez37, A. Pearce54, A. Pellegrino41,M. Pepe Altarelli38, S. Perazzini14,d, E. Perez Trigo37, P. Perret5, M. Perrin-Terrin6,L. Pescatore45, E. Pesen66, K. Petridis53, A. Petrolini19,j , E. Picatoste Olloqui36, B. Pietrzyk4,T. Pilar48, D. Pinci25, A. Pistone19, S. Playfer50, M. Plo Casasus37, F. Polci8, A. Poluektov48,34,E. Polycarpo2, A. Popov35, D. Popov10, B. Popovici29, C. Potterat2, E. Price46,J. Prisciandaro39, A. Pritchard52, C. Prouve46, V. Pugatch44, A. Puig Navarro39, G. Punzi23,t,W. Qian4, B. Rachwal26, J.H. Rademacker46, B. Rakotomiaramanana39, M. Rama18,M.S. Rangel2, I. Raniuk43, N. Rauschmayr38, G. Raven42, S. Reichert54, M.M. Reid48,A.C. dos Reis1, S. Ricciardi49, S. Richards46, M. Rihl38, K. Rinnert52, V. Rives Molina36,D.A. Roa Romero5, P. Robbe7, A.B. Rodrigues1, E. Rodrigues54, P. Rodriguez Perez54,S. Roiser38, V. Romanovsky35, A. Romero Vidal37, M. Rotondo22, J. Rouvinet39, T. Ruf38,F. Ruffini23, H. Ruiz36, P. Ruiz Valls64, G. Sabatino25,l, J.J. Saborido Silva37, N. Sagidova30,P. Sail51, B. Saitta15,e, V. Salustino Guimaraes2, C. Sanchez Mayordomo64,

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B. Sanmartin Sedes37, R. Santacesaria25, C. Santamarina Rios37, E. Santovetti24,l, M. Sapunov6,A. Sarti18,m, C. Satriano25,n, A. Satta24, D.M. Saunders46, M. Savrie16,f , D. Savrina31,32,M. Schiller42, H. Schindler38, M. Schlupp9, M. Schmelling10, B. Schmidt38, O. Schneider39,A. Schopper38, M.-H. Schune7, R. Schwemmer38, B. Sciascia18, A. Sciubba25, M. Seco37,A. Semennikov31, I. Sepp53, N. Serra40, J. Serrano6, L. Sestini22, P. Seyfert11, M. Shapkin35,I. Shapoval16,43,f , Y. Shcheglov30, T. Shears52, L. Shekhtman34, V. Shevchenko63, A. Shires9,R. Silva Coutinho48, G. Simi22, M. Sirendi47, N. Skidmore46, T. Skwarnicki59, N.A. Smith52,E. Smith55,49, E. Smith53, J. Smith47, M. Smith54, H. Snoek41, M.D. Sokoloff57, F.J.P. Soler51,F. Soomro39, D. Souza46, B. Souza De Paula2, B. Spaan9, A. Sparkes50, P. Spradlin51,F. Stagni38, M. Stahl11, S. Stahl11, O. Steinkamp40, O. Stenyakin35, S. Stevenson55, S. Stoica29,S. Stone59, B. Storaci40, S. Stracka23,38, M. Straticiuc29, U. Straumann40, R. Stroili22,V.K. Subbiah38, L. Sun57, W. Sutcliffe53, K. Swientek27, S. Swientek9, V. Syropoulos42,M. Szczekowski28, P. Szczypka39,38, D. Szilard2, T. Szumlak27, S. T’Jampens4, M. Teklishyn7,G. Tellarini16,f , F. Teubert38, C. Thomas55, E. Thomas38, J. van Tilburg41, V. Tisserand4,M. Tobin39, S. Tolk42, L. Tomassetti16,f , D. Tonelli38, S. Topp-Joergensen55, N. Torr55,E. Tournefier4, S. Tourneur39, M.T. Tran39, M. Tresch40, A. Tsaregorodtsev6, P. Tsopelas41,N. Tuning41, M. Ubeda Garcia38, A. Ukleja28, A. Ustyuzhanin63, U. Uwer11, V. Vagnoni14,G. Valenti14, A. Vallier7, R. Vazquez Gomez18, P. Vazquez Regueiro37, C. Vazquez Sierra37,S. Vecchi16, J.J. Velthuis46, M. Veltri17,h, G. Veneziano39, M. Vesterinen11, B. Viaud7,D. Vieira2, M. Vieites Diaz37, X. Vilasis-Cardona36,p, A. Vollhardt40, D. Volyanskyy10,D. Voong46, A. Vorobyev30, V. Vorobyev34, C. Voß62, H. Voss10, J.A. de Vries41, R. Waldi62,C. Wallace48, R. Wallace12, J. Walsh23, S. Wandernoth11, J. Wang59, D.R. Ward47,N.K. Watson45, D. Websdale53, M. Whitehead48, J. Wicht38, D. Wiedner11, G. Wilkinson55,M.P. Williams45, M. Williams56, F.F. Wilson49, J. Wimberley58, J. Wishahi9, W. Wislicki28,M. Witek26, G. Wormser7, S.A. Wotton47, S. Wright47, S. Wu3, K. Wyllie38, Y. Xie61, Z. Xing59,Z. Xu39, Z. Yang3, X. Yuan3, O. Yushchenko35, M. Zangoli14, M. Zavertyaev10,b, L. Zhang59,W.C. Zhang12, Y. Zhang3, A. Zhelezov11, A. Zhokhov31, L. Zhong3, A. Zvyagin38.

1Centro Brasileiro de Pesquisas Fısicas (CBPF), Rio de Janeiro, Brazil2Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil3Center for High Energy Physics, Tsinghua University, Beijing, China4LAPP, Universite de Savoie, CNRS/IN2P3, Annecy-Le-Vieux, France5Clermont Universite, Universite Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France6CPPM, Aix-Marseille Universite, CNRS/IN2P3, Marseille, France7LAL, Universite Paris-Sud, CNRS/IN2P3, Orsay, France8LPNHE, Universite Pierre et Marie Curie, Universite Paris Diderot, CNRS/IN2P3, Paris, France9Fakultat Physik, Technische Universitat Dortmund, Dortmund, Germany10Max-Planck-Institut fur Kernphysik (MPIK), Heidelberg, Germany11Physikalisches Institut, Ruprecht-Karls-Universitat Heidelberg, Heidelberg, Germany12School of Physics, University College Dublin, Dublin, Ireland13Sezione INFN di Bari, Bari, Italy14Sezione INFN di Bologna, Bologna, Italy15Sezione INFN di Cagliari, Cagliari, Italy16Sezione INFN di Ferrara, Ferrara, Italy17Sezione INFN di Firenze, Firenze, Italy18Laboratori Nazionali dell’INFN di Frascati, Frascati, Italy19Sezione INFN di Genova, Genova, Italy20Sezione INFN di Milano Bicocca, Milano, Italy21Sezione INFN di Milano, Milano, Italy

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22Sezione INFN di Padova, Padova, Italy23Sezione INFN di Pisa, Pisa, Italy24Sezione INFN di Roma Tor Vergata, Roma, Italy25Sezione INFN di Roma La Sapienza, Roma, Italy26Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Krakow, Poland27AGH - University of Science and Technology, Faculty of Physics and Applied Computer Science,Krakow, Poland28National Center for Nuclear Research (NCBJ), Warsaw, Poland29Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania30Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia31Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia32Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow, Russia33Institute for Nuclear Research of the Russian Academy of Sciences (INR RAN), Moscow, Russia34Budker Institute of Nuclear Physics (SB RAS) and Novosibirsk State University, Novosibirsk, Russia35Institute for High Energy Physics (IHEP), Protvino, Russia36Universitat de Barcelona, Barcelona, Spain37Universidad de Santiago de Compostela, Santiago de Compostela, Spain38European Organization for Nuclear Research (CERN), Geneva, Switzerland39Ecole Polytechnique Federale de Lausanne (EPFL), Lausanne, Switzerland40Physik-Institut, Universitat Zurich, Zurich, Switzerland41Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands42Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, TheNetherlands43NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine44Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine45University of Birmingham, Birmingham, United Kingdom46H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom47Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom48Department of Physics, University of Warwick, Coventry, United Kingdom49STFC Rutherford Appleton Laboratory, Didcot, United Kingdom50School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom51School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom52Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom53Imperial College London, London, United Kingdom54School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom55Department of Physics, University of Oxford, Oxford, United Kingdom56Massachusetts Institute of Technology, Cambridge, MA, United States57University of Cincinnati, Cincinnati, OH, United States58University of Maryland, College Park, MD, United States59Syracuse University, Syracuse, NY, United States60Pontifıcia Universidade Catolica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil, associated to 2

61Institute of Particle Physics, Central China Normal University, Wuhan, Hubei, China, associated to 3

62Institut fur Physik, Universitat Rostock, Rostock, Germany, associated to 11

63National Research Centre Kurchatov Institute, Moscow, Russia, associated to 31

64Instituto de Fisica Corpuscular (IFIC), Universitat de Valencia-CSIC, Valencia, Spain, associated to 36

65KVI - University of Groningen, Groningen, The Netherlands, associated to 41

66Celal Bayar University, Manisa, Turkey, associated to 38

aUniversidade Federal do Triangulo Mineiro (UFTM), Uberaba-MG, BrazilbP.N. Lebedev Physical Institute, Russian Academy of Science (LPI RAS), Moscow, RussiacUniversita di Bari, Bari, ItalydUniversita di Bologna, Bologna, Italy

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eUniversita di Cagliari, Cagliari, ItalyfUniversita di Ferrara, Ferrara, ItalygUniversita di Firenze, Firenze, ItalyhUniversita di Urbino, Urbino, ItalyiUniversita di Modena e Reggio Emilia, Modena, ItalyjUniversita di Genova, Genova, ItalykUniversita di Milano Bicocca, Milano, ItalylUniversita di Roma Tor Vergata, Roma, ItalymUniversita di Roma La Sapienza, Roma, ItalynUniversita della Basilicata, Potenza, ItalyoAGH - University of Science and Technology, Faculty of Computer Science, Electronics andTelecommunications, Krakow, PolandpLIFAELS, La Salle, Universitat Ramon Llull, Barcelona, SpainqUniversity of Utrecht, Utrecht, The NetherlandsrHanoi University of Science, Hanoi, Viet NamsUniversita di Padova, Padova, ItalytUniversita di Pisa, Pisa, ItalyuScuola Normale Superiore, Pisa, ItalyvUniversita degli Studi di Milano, Milano, Italy

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1 Introduction

The study of b-baryon decays is of considerable interest both to probe the dynamics ofheavy flavour decay processes and to search for the effects of physics beyond the StandardModel. Owing to their non-zero spin, b baryons provide the potential to improve thelimited understanding of the helicity structure of the underlying Hamiltonian [1].

Beauty baryons are copiously produced at the LHC, where the Λ0b baryon cross-section

is about half of the size of the B0 meson production in the forward region [2]. The ATLAS,CMS and LHCb collaborations measured the Λ0

b lifetime [3–5], and the masses of theground [5, 6] and first excited states [7]. The Λ0

b polarisation has been measured andfound to be compatible with zero [8]. The LHCb collaboration has studied Λ0

b decays tocharmonium [8,9], open charm [10,11], charmless states [12] and final states induced byelectroweak penguins [13]. No evidence for CP violation has been reported in decays ofbaryons. Searches with b-baryon decays have been performed with the decay channelsΛ0b → pπ−, pK− [14] and K0

Spπ− [12]. The corresponding theoretical literature is still

limited compared to that on B meson decays.The study of b → ccq decays can be used to constrain penguin pollution in the

determination of the CP -violating phases in B0 and B0s mixing [15]. While decays

originating from the b → ccs transitions, such as Λ0b → J/ψΛ or Λ0

b → J/ψpK−, arelargely dominated by the tree amplitudes, penguins amplitudes are enhanced in Cabibbo-suppressed b→ ccd transitions, such as the Λ0

b→ J/ψpπ− decay.This article reports the first observation of the Λ0

b→ J/ψpπ− decay and the determina-tion of its branching fraction relative to the Cabibbo-favoured mode Λ0

b→ J/ψpK−. Ameasurement of the CP asymmetry difference between the Λ0

b→ J/ψpπ− and Λ0b→ J/ψpK−

decays is also reported. The analysis is based on a data sample of proton-proton collisions,corresponding to an integrated luminosity of 1 fb−1 at a centre-of-mass energy of 7 TeVand 2 fb−1 at 8 TeV, collected with the LHCb detector.

2 Detector and software

The LHCb detector [16] is a single-arm forward spectrometer covering the pseudorapidityrange 2 < η < 5, designed for the study of particles containing b or c quarks. Thedetector includes a high-precision tracking system consisting of a silicon-strip vertexdetector surrounding the proton-proton interaction region, a large-area silicon-strip detectorlocated upstream of a dipole magnet with a bending power of about 4 Tm, and threestations of silicon-strip detectors and straw drift tubes [17] placed downstream of themagnet. The combined tracking system provides a momentum measurement with arelative uncertainty that varies from 0.4% at low momentum to 0.6% at 100 GeV/c, and animpact parameter measurement with a resolution of 20µm for charged particles with largetransverse momentum, pT. Different types of charged hadrons are distinguished using

1

information from two ring-imaging Cherenkov (RICH) detectors [18]. Photon, electron andhadron candidates are identified by a calorimeter system consisting of scintillating-pad andpreshower detectors, an electromagnetic calorimeter and a hadronic calorimeter. Muonsare identified by a system composed of alternating layers of iron and multiwire proportionalchambers [19]. The trigger [20] consists of a hardware stage, based on information fromthe calorimeter and muon systems, followed by a software stage, which applies a full eventreconstruction.

Candidate events are first required to pass the hardware trigger, which selects muonswith pT > 1.48 GeV/c. In the subsequent software trigger, at least one of the candidatemuons is required to be inconsistent with originating from any primary interaction. Finally,the muon pair is required to form a vertex that is significantly displaced from all primaryvertices (PV) and to have a mass within 120 MeV/c2 of the known J/ψ mass.

In the simulation, proton-proton collisions are generated using Pythia [21] with a spe-cific LHCb configuration [22]. Decays of hadronic particles are described by EvtGen [23],in which final state radiation is generated using Photos [24]. The interaction of thegenerated particles with the detector and its response are implemented using the Geant4toolkit [25] as described in Ref. [26].

3 Event Selection

The Λ0b→ J/ψpπ− and Λ0

b→ J/ψpK− decays are reconstructed with the J/ψ decaying totwo muons. Charge conjugation is implied throughout except in the definition of the CPasymmetry.

Candidate J/ψ → µ+µ− decays are reconstructed from oppositely charged particlespassing loose muon-identification requirements and with pT > 500 MeV/c. They arerequired to form a good quality vertex and have a mass in the range [3030, 3150] MeV/c2.This interval corresponds to about eight times the µ+µ− mass resolution at the J/ψ massand covers part of the J/ψ meson radiative tail.

Candidate Λ0b baryons are selected from combinations of J/ψ candidates and two

oppositely charged particles, one of which must be compatible with the proton hypothesis.The proton candidate is required to have a momentum, p, larger than 5 GeV/c, while thesecond charged particle must have p > 3 GeV/c. Both particles must have pT > 500 MeV/cand be inconsistent with coming from any PV. All four charged particles are required tobe consistent with coming from a common vertex.

The reconstructed mass and decay time of the Λ0b candidates are obtained from a

kinematic fit [27] that constrains the mass of the µ+µ− pairs to the known J/ψ mass andthe Λ0

b candidate to originate from the PV. If the event has multiple PVs, all combinationsare considered. Candidates are required to have a reconstructed decay time larger than0.2 ps.

To remove backgrounds from Λ0b→ J/ψΛ decays, candidates that have a pπ− mass

within 5 MeV/c2 of the Λ baryon mass are vetoed. To remove reflections from B0s→ J/ψφ

decays, candidates are also vetoed if the hadron-pair mass is less than 1035 MeV/c2 when

2

applying a K+ mass hypothesis to both particles.The remaining candidates are split into samples of Λ0

b→ J/ψpπ− and Λ0b→ J/ψpK−

according to the estimated probabilities that the charged meson candidate is a kaonor a pion. These probabilities are determined using a neural network (NN) exploitinginformation from the RICH detectors, calorimeter and muon systems, as well as trackquality. Particles with a larger pion probability are treated as Λ0

b→ J/ψpπ− candidates,otherwise they are treated as Λ0

b→ J/ψpK− candidates. In addition, the larger of thesetwo probabilities is required to be in excess of 5%. The Λ0

b candidates are required to bein the mass range [4900, 6100] MeV/c2. After this selection 4.3× 105 Λ0

b→ J/ψpπ− and1.9× 105 Λ0

b→ J/ψpK− candidates remain.The selection described above is not sufficient to isolate the small Λ0

b → J/ψpπ−

signal from the combinatorial background. The initial selection is therefore followed by amultivariate analysis, based on another NN [28]. The NN classifier’s output is used as thefinal selection variable.

The NN is trained entirely on data, using the Λ0b→ J/ψpK− signal as a proxy for the

Λ0b→ J/ψpπ− decay. The training is performed using half of the Λ0

b→ J/ψpK− candidateschosen at random. The other half is used to define the normalisation sample, allowingan unbiased measurement of the Λ0

b → J/ψpK− yield. The training uses signal andbackground weights determined using the sPlot technique [29] and obtained by performinga maximum likelihood fit to the unbinned mass distribution of the candidates meeting theloose selection criteria.

The fit probability density function (PDF) is defined as the sum of the Λ0b signal

component and the combinatorial background components. The parameterisation of theindividual components is described in the next section.

Several reflections from B0, B0s and Λ0

b decays, reconstructed using misidentifiedparticles, must be accounted for in the mass spectrum. In order to avoid the trainingbeing biased by these reflections, all candidates that have a mass compatible with theB0, B0

s or Λ0b mass after swapping the p and K assignments with any of π, K, or p are

removed from the training.The NN classifier uses information about the candidate kinematic distributions, vertex

and track quality, impact parameter and particle identification information on the proton.The most discriminating quantities are the proton particle identification probability, thekinematic fit quality and the kaon separation of the PV in this order. The variables thatare used in the NN are chosen to avoid correlations with the reconstructed Λ0

b mass andto have identical distributions in Λ0

b→ J/ψpπ− and Λ0b→ J/ψpK− simulated data.

Final selection requirements of the NN classifier output are chosen to optimise theexpected statistical precision on the Λ0

b→ J/ψpπ− signal yield. The expected signal andbackground yields entering the sensitivity estimation are obtained from the training sampleby scaling the number of surviving Λ0

b→ J/ψpK− candidates by the expected yield basedon an assumed branching fraction ratio of 0.1. The expected background is extrapolatedfrom the number of Λ0

b→ J/ψpπ− candidates in the mass range [5770, 6100] MeV/c2. Afterapplying the final requirement on the NN classifier output, the multivariate selectionrejects 99% of the background while keeping 75% of the Λ0

b→ J/ψpK− signal, relative to

3

the initial selection.After applying the full selection, about 0.1% of the selected events have more than one

candidate sharing at least one track, or more than one PV that can be used to determinethe kinematic properties of the candidate. In these cases one of the candidates or PVs isused at random.

4 Signal and background description

For the candidates passing the NN requirements, the yields of Λ0b→ J/ψpπ− and Λ0

b→J/ψpK− decays are determined from unbinned maximum likelihood fits to the massdistributions of reconstructed Λ0

b candidates. The PDF is defined as the sum of a Λ0b signal

component, a combinatorial background and the sum of several reflections.The signal shape is parametrised by a Gaussian distribution with power-law tails on

both sides, as indicated by simulation. The parameters describing the tails are taken fromsimulation, while the mean and width of the Gaussian are allowed to vary in the fit. Thecombinatorial background contribution is described by an exponential function, with yieldand slope parameter allowed to vary freely.

Several peaking backgrounds due to decays of b hadrons to J/ψ mesons and two chargedhadrons, where one or both hadrons are misidentified, survive the selection. In this fitthey are not vetoed, unlike in the training of the NN, except for candidates consistentwith the B0

s → J/ψφ hypothesis. Instead, their contribution is modelled by smoothednon-parametric functions determined from simulated data. The respective yields aredetermined by swapping the mass assignments of the p, π and K in turn and searching forpeaks at the B0, B0

s or Λ0b masses. In the Λ0

b→ J/ψpπ− fit, significant backgrounds arefound from the decays Λ0

b→ J/ψpK− (with K− identified as π−), B0→ J/ψK+π− (withK+ identified as p), and B0

s→ J/ψK+K− (with one K identified as p and the other as

π). In the Λ0b→ J/ψpK− fit, the main contributions are from the decays B

0→ J/ψπ+K−

(with π+ identified as p), B0s→ J/ψK+K− (with K+ identified as p), and Λ

0

b→ J/ψpK+

(with K− and p swapped). These yields are then used as Gaussian constraints on thenormalisation of the reflection background shapes. The results of the fits are shown inFig. 1. The contributions of the reflections are summed, except for the large Λ0

b→ J/ψpK−

reflection in the Λ0b→ J/ψpπ− fit, which is shown separately.

Low-mass contributions of partially reconstructed Λ0b → J/ψpπ−π0 and Λ0

b →J/ψpK−π0 decays, where the π0 is not considered in the combination, are investigated.Adding such a component to the fit, with a mass shape taken from simulation, results inyields compatible with zero and does not change the signal yields.

In total 11 179± 109 Λ0b→ J/ψpK− and 2102± 61 Λ0

b→ J/ψpπ− decays are obtained.The Λ0

b→ J/ψpK− yield, having been determined on the half of the data not used in thetraining, is multiplied by two, resulting in a ratio of Λ0

b→ J/ψpπ− to Λ0b→ J/ψpK− yields

of 0.0940± 0.0029.The shapes used in the mass fit are varied to determine a systematic uncertainty

related to the mass model. No significant differences are found when trying alternate

4

]2 [MeV/c-πpψJ/m5500 5600 5700

2C

andi

date

s pe

r 5

MeV

/c

0

200

400

600

800

1000

1200

1400

LHCb Data

-πpψJ/→bΛ -

pKψJ/→bΛ B reflections

Combinatorial

Total

]2 [MeV/c-pKψJ/m

5500 5600 5700

2C

andi

date

s pe

r 5

MeV

/c

10

210

310

Data-

pKψJ/→bΛ

B reflections

Combinatorial

Total

LHCb

Figure 1: Distribution of (left) J/ψpπ− and (right) J/ψpK− masses with fit projections overlaid.For Λ0

b→ J/ψpK− candidates, only the normalisation sample is shown.

signal parameterisation that still result in a good fit. Changing the peaking backgroundsPDF, or letting their yield free in the fit, change their relative fit fractions, as well as thatof the combinatorial background, but does not affect the signal yields. The combinatorialbackground model is changed from an exponential to a second-order polynomial, whichresults in a Λ0

b→ J/ψpπ− (Λ0b→ J/ψpK−) yield reduced by 2.1% (0.3%). These variations

are added in quadrature and used to estimate a systematic uncertainty on the ratio ofbranching fractions.

5 CP asymmetry

The same fit procedure is repeated separately for baryon (tagged by a positively chargedproton) and antibaryon candidates. All parameters of the fits are determined againseparately, except for the signal shape and the combinatorial background slope, whichare taken from the fit to all candidates. A total of 1131 ± 40 Λ0

b→ J/ψpπ−, 964 ± 38

Λ0

b→ J/ψpπ+, 5655± 77 Λ0b→ J/ψpK− and 5529± 76 Λ

0

b→ J/ψpK+ decays are found,corresponding to the raw asymmetries

Araw(Λ0b→ J/ψpπ−) ≡ N(Λ0

b→ J/ψpπ−)−N(Λ0

b→ J/ψpπ+)

N(Λ0b→ J/ψpπ−) +N(Λ

0

b→ J/ψpπ+)(1)

= (+7.9± 2.2)%,

Araw(Λ0b→ J/ψpK−) = (+1.1± 0.7)%.

The procedure to assess the systematic uncertainties related to the shape of the mass

5

distribution, described in Sec. 4, is repeated to determine the sensitivity of the rawasymmetries. A total variation of 0.7% is obtained, which is dominated by a change inAraw(Λ0

b→ J/ψpπ−) when using a second-order polynomial background model.The raw decay-rate asymmetry can be decomposed as

Araw(Λ0b→ J/ψph−) = ACP (Λ0

b→ J/ψph−) +Aprod(Λ0b)−Areco(h

+) +Areco(p), (2)

where the terms on the right hand side of the equation are the CP -violating, Λ0b production

and reconstruction asymmetries of the hadron h± = π±, K± and the proton, respectively.

Reconstruction asymmetries are defined following the convention Areco(h+) ≡ ε(h+)−ε(h−)

ε(h+)+ε(h−)

throughout, where ε is the reconstruction efficiency. The production asymmetry Aprod andthe proton reconstruction asymmetry cancel in the difference of the two asymmetries

∆ACP ≡ ACP (Λ0b→ J/ψpπ−)−ACP (Λ0

b→ J/ψpK−)

= Araw(Λ0b→ J/ψpπ−)−Araw(Λ0

b→ J/ψpK−) +Areco(π+)−Areco(K

+). (3)

The kaon and pion asymmetries can be determined from the raw asymmetry of the

B0 → J/ψK

∗(892)0 decay with K

∗(892)0→ K−π+. It has been measured [30] as

Araw(B0 → J/ψK

∗(892)0) ≡ N(B

0)−N(B0)

N(B0) +N(B0)

= (−1.10± 0.32± 0.06)%, (4)

where the first uncertainty is statistical and the second systematic. It can be decomposedas

Araw(B0 → J/ψK

∗(892)0) = ACP (B

0 → J/ψK∗(892)0)− κAprod(B0) (5)

+Areco(π+)−Areco(K

+)

≈ Areco(π+)−Areco(K

+), (6)

where κ is a dilution factor due to B0 mixing andAprod(B0) is the B0 production asymmetry,which is compatible with zero [31]. Under the assumption of no CP asymmetry in the

B0 → J/ψK

∗(892)0 decay and negligible production asymmetry, this value can thus be

taken as the combined kaon and pion reconstruction asymmetry, and is consistent withmeasurements in other decay modes to kaon and pions [31,32].

The difference of CP asymmetries in the Λ0b→ J/ψpπ− and Λ0

b→ J/ψpK− decays canthen be rewritten as

∆ACP = Araw(Λ0b→ J/ψpπ−)−Araw(Λ0

b→ J/ψpK−) +Araw(B0 → J/ψK

∗(892)0) (7)

= (+5.7± 2.3)%,

where the uncertainty is statistical only.The kaon and pion momenta in Λ0

b decays are not identical to those in

B0 → J/ψK

∗(892)0 decays, which could induce different detector asymmetries in the

Λ0b and B0 modes. This is investigated by weighting the Λ0

b→ J/ψpπ− and Λ0b→ J/ψpK−

6

data to match the pion and kaon momentum distributions observed in B0 → J/ψK

∗(892)0

decays. The value of ∆ACP changes by 0.8%, which is assigned as the systematic uncer-tainty related to reconstruction asymmetries.

Local CP asymmetries in the Dalitz plane are also searched for using the techniqueoutlined in Ref. [33]. No significant local asymmetries are found.

6 Efficiency corrections and systematic uncertainties

The raw quantities need to be corrected to determine the physics quantities. The efficiencyof the selection requirements is studied with simulation. Some quantities are known not tobe well reproduced in simulation, namely the Λ0

b transverse momentum and lifetime, theparticle multiplicity, and the Λ0

b→ J/ψpπ− and Λ0b→ J/ψpK− decay kinematic properties.

For all these quantities the simulated data are weighted to match the observed distributionsin data. They are obtained with the sPlot technique using the Λ0

b candidate mass as thecontrol variable.

For three-body b-hadron decays, both the signal decays and the dominant combinatorialbackgrounds populate regions close to the kinematic boundaries of the J/ψpπ− and J/ψpK−

Dalitz plot [34]. For more accurate modelling of these regions, it is convenient to transformthe conventional Dalitz space to a rectangular space (hereafter referred to as the squareDalitz plot [35]). We follow the procedure described in Ref. [12].

The Λ0b → J/ψpπ− and Λ0

b → J/ψpK− decays have different detector acceptance,reconstruction and selection efficiencies. They are determined from simulated data, whichare weighted to match the experimental data. The main differences are induced by

i. the detector acceptance, as the efficiency of Λ0b→ J/ψpK− is 6% larger than that for

the Λ0b→ J/ψpπ− decays due to the lower kinetic energy release in the former, which

causes smaller opening angles;

ii. the reconstruction and preselection efficiency, which is 4% larger in Λ0b→ J/ψpπ−

decays due to the average total and transverse momentum of the final state particlesbeing larger than in Λ0

b→ J/ψpK− decays;

iii. the particle identification requirements on the π− or K−, which are more efficient onΛ0b→ J/ψpπ− decays by 7%;

iv. the φ veto, which removes 7% (3%) of the Λ0b→ J/ψpK− (J/ψpπ−) signal.

Overall, these effects result in a further correction on the ratio of branching fractions ofΛ0b→ J/ψpπ− to Λ0

b→ J/ψpK− decays of 0.913± 0.040.The efficiency of particle identification is not perfectly modelled in simulation. The

kaon and pion identification efficiencies are further weighted after the kinematic weightingdescribed above, using a large sample of D∗-tagged D0 →K−π+ decays. The uncertaintiesare determined by varying the weights of the simulated data within their uncertainties,yielding a correction of 0.960 ± 0.010. The kinematic properties of the proton in the

7

Table 1: Corrections and related systematic uncertainties on the ratio of Λ0b → J/ψpπ− to

Λ0b → J/ψpK− branching fractions (BF) and on the difference between the CP asymmetries

∆ACP . The corrections are multiplicative on the branching fraction and additive for theasymmetry.

Source BF ∆ACPSimulation-based corrections 0.913± 0.040 -PID 0.960± 0.010 -Trigger 1.000± 0.010 -Λ0b lifetime 1.000± 0.001 -

Mass distribution model 1.000± 0.021 0.0± 0.7%

B0 → J/ψK

∗(892)0 − −1.1± 0.3%

Detection asymmetries − 0.0± 0.8%Total 0.876± 0.045 −1.1± 1.2%

Λ0b→ J/ψpπ− and Λ0

b→ J/ψpK− decays are found to be the same. The same applies tothe muons. The efficiencies of proton and muon identification therefore cancel in the ratioof branching fractions, as well as in the CP asymmetries.

The trigger efficiency is determined using simulation, which is validated using Λ0b→

J/ψpK− decays from data. Differences between the Λ0b → J/ψpπ− and Λ0

b → J/ψpK−

decays efficiencies are at the percent level and are assigned as systematic uncertainties.The value of the Λ0

b lifetime used in simulation is taken from Ref. [36], and is 3% lowerthan the current most precise measurement [3]. The simulated data is weighted to accountfor this and the difference is assigned as a systematic uncertainty.

The estimates of the systematic uncertainties described above are summarised in Table 1,along with the total obtained by summing them in quadrature. The uncertainty on theratio of branching fractions is dominated by the uncertainty on corrections obtained fromsimulation, mostly due to the unknown decay kinematic properties of the Λ0

b→ J/ψpπ−

decay. For ∆ACP , the mass model distribution and the detection asymmetries contributeabout equally.

7 Results and Conclusions

A signal of the Cabibbo-suppressed Λ0b→ J/ψpπ− decay is observed for the first time using

a data sample of proton-proton collisions at 7 and 8 TeV, corresponding to an integratedluminosity of 3 fb−1. Applying the appropriate corrections detailed in Table 1, the ratioof branching fractions is measured to be

B(Λ0b→ J/ψpπ−)

B(Λ0b→ J/ψpK−)

= 0.0824± 0.0025 (stat)± 0.0042 (syst).

Assuming these decays are dominated by tree b → ccd and b → ccs transitions,respectively, the ratio of Cabibbo-Kobayashi-Maskawa (CKM) matrix elements |Vcd|2/|Vcs|2

8

times ratio of phase space factors is approximately 0.08, compatible with the measured value.This ratio can also be compared to that of the decays Λ0

b→ Λ+c D

− and Λ0b→ Λ+

c D+s , which

involve the same quark lines as Λ0b→ J/ψpπ− and Λ0

b→ J/ψpK−, respectively. The ratio ofΛ0b→ Λ+

c D− and Λ0

b→ Λ+c D

+s has been measured as 0.042± 0.003 (stat)± 0.003 (syst) [11],

which is consistent with this measurement, when taking into account the D− and D+s

meson decay constants [36] and the different ratio of phase space factors.Background-subtracted and efficiency-corrected distributions of kinematic distributions

determined in the Λ0b→ J/ψpπ− decay are shown in Fig. 2. In this case the Λ0

b mass isfixed to its known value and the kinematic properties recomputed [27]. No attempt is madeto fit the decay rate on the Dalitz plane. The pπ− mass distribution shows a rich resonantstructure, as expected from fits to fixed-target experiment data [37,38], and suggests thepresence of the narrow N(1520) or N(1535), the N(1650), as well as the broad N(1440)resonances. No signs of exotic structures in the J/ψπ− or J/ψp mass distributions areseen.

The CP asymmetry difference between the Λ0b→ J/ψpπ− and Λ0

b→ J/ψpK− decays ismeasured to be

∆ACP = ACP (Λ0b→ J/ψpπ−)− ACP (Λ0

b→ J/ψpK−) = (+5.7± 2.3 (stat)± 1.2 (syst))%,

corresponding to a 2.2σ deviation from zero. No indications of large local CP asymmetriesin the Dalitz plane are observed. The precision of these measurements illustrate thepotential of Cabibbo-suppressed Λ0

b decays in studies of direct CP violation.

Acknowledgements

We express our gratitude to our colleagues in the CERN accelerator departments for theexcellent performance of the LHC. We thank the technical and administrative staff at theLHCb institutes. We acknowledge support from CERN and from the national agencies:CAPES, CNPq, FAPERJ and FINEP (Brazil); NSFC (China); CNRS/IN2P3 (France);BMBF, DFG, HGF and MPG (Germany); SFI (Ireland); INFN (Italy); FOM and NWO(The Netherlands); MNiSW and NCN (Poland); MEN/IFA (Romania); MinES and FANO(Russia); MinECo (Spain); SNSF and SER (Switzerland); NASU (Ukraine); STFC (UnitedKingdom); NSF (USA). The Tier1 computing centres are supported by IN2P3 (France),KIT and BMBF (Germany), INFN (Italy), NWO and SURF (The Netherlands), PIC(Spain), GridPP (United Kingdom). We are indebted to the communities behind themultiple open source software packages on which we depend. We are also thankful forthe computing resources and the access to software R&D tools provided by Yandex LLC(Russia). Individual groups or members have received support from EPLANET, MarieSk lodowska-Curie Actions and ERC (European Union), Conseil general de Haute-Savoie,Labex ENIGMASS and OCEVU, Region Auvergne (France), RFBR (Russia), XuntaGaland GENCAT (Spain), Royal Society and Royal Commission for the Exhibition of 1851(United Kingdom).

9

]4/c2 [GeV2 -πpm2 4 6

]4/c2

[G

eV2

- πψ

J/m

10

12

14

16

18

20

22

LHCb

]2 [GeV/c-πpm1 1.5 2 2.5

]2 c-1

[G

eV- πp

1/N

dN

/dm

0

0.5

1

1.5

2

2.5

3LHCb

]2 [GeV/c-πψJ/m3.5 4 4.5

]2 c-1

[G

eV- π

ψJ/

1/N

dN

/dm

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

LHCb

]2 [GeV/cpψJ/m4 4.5 5 5.5

]2 c-1

[G

eVp

ψJ/

1/N

dN

/dm

0

0.2

0.4

0.6

0.8

1

1.2 LHCb

Figure 2: Efficiency-corrected and background-subtracted Λ0b → J/ψpπ− Dalitz plane and

projections normalised to unit area on the pπ−, J/ψπ− and J/ψp axes.

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