Acknowledgements

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)

CERN-PH-EP-2013-028 LHCb-PAPER-2012-054 6 March 2013

Observation of the decay

The LHCb collaborationAuthors are listed on the following pages.

The decay with is observed with a significance of using collision data corresponding to an integrated luminosity of collected by the LHCb experiment. The branching fraction of decays relative to that of the mode is measured to be

The last term is the uncertainty on the ratio .

Submitted to Physical Review Letters

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

 

LHCb collaboration

R. Aaij, C. Abellan Beteta, B. Adeva, M. Adinolfi, C. Adrover, A. Affolder, Z. Ajaltouni, J. Albrecht, F. Alessio, M. Alexander, S. Ali, G. Alkhazov, P. Alvarez Cartelle, A.A. Alves Jr, S. Amato, S. Amerio, Y. Amhis, L. Anderlini, J. Anderson, R. Andreassen, R.B. Appleby, O. Aquines Gutierrez, F. Archilli, A. Artamonov , M. Artuso, E. Aslanides, G. Auriemma, S. Bachmann, J.J. Back, C. Baesso, V. Balagura, W. Baldini, R.J. Barlow, C. Barschel, S. Barsuk, W. Barter, Th. Bauer, A. Bay, J. Beddow, F. Bedeschi, I. Bediaga, S. Belogurov, K. Belous, I. Belyaev, E. Ben-Haim, M. Benayoun, G. Bencivenni, S. Benson, J. Benton, A. Berezhnoy, R. Bernet, M.-O. Bettler, M. van Beuzekom, A. Bien, S. Bifani, T. Bird, A. Bizzeti, P.M. Bjørnstad, T. Blake, F. Blanc, J. Blouw, S. Blusk, V. Bocci, A. Bondar, N. Bondar, W. Bonivento, S. Borghi, A. Borgia, T.J.V. Bowcock, E. Bowen, C. Bozzi, T. Brambach, J. van den Brand, J. Bressieux, D. Brett, M. Britsch, T. Britton, N.H. Brook, H. Brown, I. Burducea, A. Bursche, G. Busetto, J. Buytaert, S. Cadeddu, O. Callot, M. Calvi, M. Calvo Gomez, A. Camboni, P. Campana, A. Carbone, G. Carboni, R. Cardinale, A. Cardini, H. Carranza-Mejia, L. Carson, K. Carvalho Akiba, G. Casse, M. Cattaneo, Ch. Cauet, M. Charles, Ph. Charpentier, P. Chen, N. Chiapolini, M. Chrzaszcz , K. Ciba, X. Cid Vidal, G. Ciezarek, P.E.L. Clarke, M. Clemencic, H.V. Cliff, J. Closier, C. Coca, V. Coco, J. Cogan, E. Cogneras, P. Collins, A. Comerma-Montells, A. Contu, A. Cook, M. Coombes, S. Coquereau, G. Corti, B. Couturier, G.A. Cowan, D. Craik, S. Cunliffe, R. Currie, C. D’Ambrosio, P. David, P.N.Y. David, I. De Bonis, K. De Bruyn, S. De Capua, M. De Cian, J.M. De Miranda, M. De Oyanguren Campos, L. De Paula, W. De Silva, P. De Simone, D. Decamp, M. Deckenhoff, L. Del Buono, D. Derkach, O. Deschamps, F. Dettori, A. Di Canto, H. Dijkstra, M. Dogaru, S. Donleavy, F. Dordei, A. Dosil Suárez, D. Dossett, A. Dovbnya, F. Dupertuis, R. Dzhelyadin, A. Dziurda, A. Dzyuba, S. Easo, U. Egede, V. Egorychev, S. Eidelman, D. van Eijk, S. Eisenhardt, U. Eitschberger, R. Ekelhof, L. Eklund, I. El Rifai, Ch. Elsasser, D. Elsby, A. Falabella, C. Färber, G. Fardell, C. Farinelli, S. Farry, V. Fave, D. Ferguson, V. Fernandez Albor, F. Ferreira Rodrigues, M. Ferro-Luzzi, S. Filippov, C. Fitzpatrick, M. Fontana, F. Fontanelli, R. Forty, O. Francisco, M. Frank, C. Frei, M. Frosini, S. Furcas, E. Furfaro, A. Gallas Torreira, D. Galli, M. Gandelman, P. Gandini, Y. Gao, J. Garofoli, P. Garosi, J. Garra Tico, L. Garrido, C. Gaspar, R. Gauld, E. Gersabeck, M. Gersabeck, T. Gershon, Ph. Ghez, V. Gibson, V.V. Gligorov, C. Göbel, D. Golubkov, A. Golutvin, A. Gomes, H. Gordon, M. Grabalosa Gándara, R. Graciani Diaz, L.A. Granado Cardoso, E. Graugés, G. Graziani, A. Grecu, E. Greening, S. Gregson, O. Grünberg, B. Gui, E. Gushchin, Yu. Guz, T. Gys, C. Hadjivasiliou, G. Haefeli, C. Haen, S.C. Haines, S. Hall, T. Hampson, S. Hansmann-Menzemer, N. Harnew, S.T. Harnew, J. Harrison, T. Hartmann, J. He, V. Heijne, K. Hennessy, P. Henrard, J.A. Hernando Morata, E. van Herwijnen, E. Hicks, D. Hill, M. Hoballah, C. Hombach, P. Hopchev, W. Hulsbergen, P. Hunt, T. Huse, N. Hussain, D. Hutchcroft, D. Hynds, V. Iakovenko, M. Idzik, P. Ilten, R. Jacobsson, A. Jaeger, E. Jans, P. Jaton, F. Jing, M. John, D. Johnson, C.R. Jones, B. Jost, M. Kaballo, S. Kandybei, M. Karacson, T.M. Karbach, I.R. Kenyon, U. Kerzel, T. Ketel, A. Keune, B. Khanji, O. Kochebina, I. Komarov, R.F. Koopman, P. Koppenburg, M. Korolev, A. Kozlinskiy, L. Kravchuk, K. Kreplin, M. Kreps, G. Krocker, P. Krokovny, F. Kruse, M. Kucharczyk, V. Kudryavtsev, T. Kvaratskheliya, V.N. La Thi, D. Lacarrere, G. Lafferty, A. Lai, D. Lambert, R.W. Lambert, E. Lanciotti, G. Lanfranchi, C. Langenbruch, T. Latham, C. Lazzeroni, R. Le Gac, J. van Leerdam, J.-P. Lees, R. Lefèvre, A. Leflat, J. Lefrançois, S. Leo, O. Leroy, B. Leverington, Y. Li, L. Li Gioi, M. Liles, R. Lindner, C. Linn, B. Liu, G. Liu, J. von Loeben, S. Lohn, J.H. Lopes, E. Lopez Asamar, N. Lopez-March, H. Lu, D. Lucchesi, J. Luisier, H. Luo, F. Machefert, I.V. Machikhiliyan, F. Maciuc, O. Maev, S. Malde, G. Manca, G. Mancinelli, U. Marconi, R. Märki, J. Marks, G. Martellotti, A. Martens, L. Martin, A. Martín Sánchez, M. Martinelli, D. Martinez Santos, D. Martins Tostes, A. Massafferri, R. Matev, Z. Mathe, C. Matteuzzi, E. Maurice, A. Mazurov, J. McCarthy, R. McNulty, A. Mcnab, B. Meadows, F. Meier, M. Meissner, M. Merk, D.A. Milanes, M.-N. Minard, J. Molina Rodriguez, S. Monteil, D. Moran, P. Morawski, M.J. Morello, R. Mountain, I. Mous, F. Muheim, K. Müller, R. Muresan, B. Muryn, B. Muster, P. Naik, T. Nakada, R. Nandakumar, I. Nasteva, M. Needham, N. Neufeld, A.D. Nguyen, T.D. Nguyen, C. Nguyen-Mau, M. Nicol, V. Niess, R. Niet, N. Nikitin, T. Nikodem, A. Nomerotski, A. Novoselov, A. Oblakowska-Mucha, V. Obraztsov, S. Oggero, S. Ogilvy, O. Okhrimenko, R. Oldeman, M. Orlandea, J.M. Otalora Goicochea, P. Owen, B.K. Pal, A. Palano, M. Palutan, J. Panman, A. Papanestis, M. Pappagallo, C. Parkes, C.J. Parkinson, G. Passaleva, G.D. Patel, M. Patel, G.N. Patrick, C. Patrignani, C. Pavel-Nicorescu, A. Pazos Alvarez, A. Pellegrino, G. Penso, M. Pepe Altarelli, S. Perazzini, D.L. Perego, E. Perez Trigo, A. Pérez-Calero Yzquierdo, P. Perret, M. Perrin-Terrin, G. Pessina, K. Petridis, A. Petrolini, A. Phan, E. Picatoste Olloqui, B. Pietrzyk, T. Pilař, D. Pinci, S. Playfer, M. Plo Casasus, F. Polci, G. Polok, A. Poluektov, E. Polycarpo, D. Popov, B. Popovici, C. Potterat, A. Powell, J. Prisciandaro, V. Pugatch, A. Puig Navarro, G. Punzi, W. Qian, J.H. Rademacker, B. Rakotomiaramanana, M.S. Rangel, I. Raniuk, N. Rauschmayr, G. Raven, S. Redford, M.M. Reid, A.C. dos Reis, S. Ricciardi, A. Richards, K. Rinnert, V. Rives Molina, D.A. Roa Romero, P. Robbe, E. Rodrigues, P. Rodriguez Perez, S. Roiser, V. Romanovsky, A. Romero Vidal, J. Rouvinet, T. Ruf, F. Ruffini, H. Ruiz, P. Ruiz Valls, G. Sabatino, J.J. Saborido Silva, N. Sagidova, P. Sail, B. Saitta, C. Salzmann, B. Sanmartin Sedes, M. Sannino, R. Santacesaria, C. Santamarina Rios, E. Santovetti, M. Sapunov, A. Sarti, C. Satriano, A. Satta, M. Savrie, D. Savrina, P. Schaack, M. Schiller, H. Schindler, M. Schlupp, M. Schmelling, B. Schmidt, O. Schneider, A. Schopper, M.-H. Schune, R. Schwemmer, B. Sciascia, A. Sciubba, M. Seco, A. Semennikov, K. Senderowska, I. Sepp, N. Serra, J. Serrano, P. Seyfert, M. Shapkin, I. Shapoval, P. Shatalov, Y. Shcheglov, T. Shears, L. Shekhtman, O. Shevchenko, V. Shevchenko, A. Shires, R. Silva Coutinho, T. Skwarnicki, N.A. Smith, E. Smith, M. Smith, M.D. Sokoloff, F.J.P. Soler, F. Soomro, D. Souza, B. Souza De Paula, B. Spaan, A. Sparkes, P. Spradlin, F. Stagni, S. Stahl, O. Steinkamp, S. Stoica, S. Stone, B. Storaci, M. Straticiuc, U. Straumann, V.K. Subbiah, S. Swientek, V. Syropoulos, M. Szczekowski, P. Szczypka, T. Szumlak, S. T’Jampens, M. Teklishyn, E. Teodorescu, F. Teubert, C. Thomas, E. Thomas, J. van Tilburg, V. Tisserand, M. Tobin, S. Tolk, D. Tonelli, S. Topp-Joergensen, N. Torr, E. Tournefier, S. Tourneur, M.T. Tran, M. Tresch, A. Tsaregorodtsev, P. Tsopelas, N. Tuning, M. Ubeda Garcia, A. Ukleja, D. Urner, U. Uwer, V. Vagnoni, G. Valenti, R. Vazquez Gomez, P. Vazquez Regueiro, S. Vecchi, J.J. Velthuis, M. Veltri, G. Veneziano, M. Vesterinen, B. Viaud, D. Vieira, X. Vilasis-Cardona, A. Vollhardt, D. Volyanskyy, D. Voong, A. Vorobyev, V. Vorobyev, C. Voß, H. Voss, R. Waldi, R. Wallace, S. Wandernoth, J. Wang, D.R. Ward, N.K. Watson, A.D. Webber, D. Websdale, M. Whitehead, J. Wicht, J. Wiechczynski, D. Wiedner, L. Wiggers, G. Wilkinson, M.P. Williams, M. Williams, F.F. Wilson, J. Wishahi, M. Witek, S.A. Wotton, S. Wright, S. Wu, K. Wyllie, Y. Xie, F. Xing, Z. Xing, Z. Yang, R. Young, X. Yuan, O. Yushchenko, M. Zangoli, M. Zavertyaev, F. Zhang, L. Zhang, W.C. Zhang, Y. Zhang, A. Zhelezov, A. Zhokhov, L. Zhong, A. Zvyagin.

Centro Brasileiro de Pesquisas Físicas (CBPF), Rio de Janeiro, Brazil

Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil

Center for High Energy Physics, Tsinghua University, Beijing, China

LAPP, Université de Savoie, CNRS/IN2P3, Annecy-Le-Vieux, France

Clermont Université, Université Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France

CPPM, Aix-Marseille Université, CNRS/IN2P3, Marseille, France

LAL, Université Paris-Sud, CNRS/IN2P3, Orsay, France

LPNHE, Université Pierre et Marie Curie, Université Paris Diderot, CNRS/IN2P3, Paris, France

Fakultät Physik, Technische Universität Dortmund, Dortmund, Germany

Max-Planck-Institut für Kernphysik (MPIK), Heidelberg, Germany

Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany

School of Physics, University College Dublin, Dublin, Ireland

Sezione INFN di Bari, Bari, Italy

Sezione INFN di Bologna, Bologna, Italy

Sezione INFN di Cagliari, Cagliari, Italy

Sezione INFN di Ferrara, Ferrara, Italy

Sezione INFN di Firenze, Firenze, Italy

Laboratori Nazionali dell’INFN di Frascati, Frascati, Italy

Sezione INFN di Genova, Genova, Italy

Sezione INFN di Milano Bicocca, Milano, Italy

Sezione INFN di Padova, Padova, Italy

Sezione INFN di Pisa, Pisa, Italy

Sezione INFN di Roma Tor Vergata, Roma, Italy

Sezione INFN di Roma La Sapienza, Roma, Italy

Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Kraków, Poland

AGH University of Science and Technology, Kraków, Poland

National Center for Nuclear Research (NCBJ), Warsaw, Poland

Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania

Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia

Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia

Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow, Russia

Institute for Nuclear Research of the Russian Academy of Sciences (INR RAN), Moscow, Russia

Budker Institute of Nuclear Physics (SB RAS) and Novosibirsk State University, Novosibirsk, Russia

Institute for High Energy Physics (IHEP), Protvino, Russia

Universitat de Barcelona, Barcelona, Spain

Universidad de Santiago de Compostela, Santiago de Compostela, Spain

European Organization for Nuclear Research (CERN), Geneva, Switzerland

Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland

Physik-Institut, Universität Zürich, Zürich, Switzerland

Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands

Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, The Netherlands

NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine

Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine

University of Birmingham, Birmingham, United Kingdom

H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom

Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom

Department of Physics, University of Warwick, Coventry, United Kingdom

STFC Rutherford Appleton Laboratory, Didcot, United Kingdom

School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom

School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom

Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom

Imperial College London, London, United Kingdom

School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom

Department of Physics, University of Oxford, Oxford, United Kingdom

Massachusetts Institute of Technology, Cambridge, MA, United States

Syracuse University, Syracuse, NY, United States

Pontifícia Universidade Católica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil, associated to

Institut für Physik, Universität Rostock, Rostock, Germany, associated to

University of Cincinnati, Cincinnati, OH, United States, associated to

P.N. Lebedev Physical Institute, Russian Academy of Science (LPI RAS), Moscow, Russia

Università di Bari, Bari, Italy

Università di Bologna, Bologna, Italy

Università di Cagliari, Cagliari, Italy

Università di Ferrara, Ferrara, Italy

Università di Firenze, Firenze, Italy

Università di Urbino, Urbino, Italy

Università di Modena e Reggio Emilia, Modena, Italy

Università di Genova, Genova, Italy

Università di Milano Bicocca, Milano, Italy

Università di Roma Tor Vergata, Roma, Italy

Università di Roma La Sapienza, Roma, Italy

Università della Basilicata, Potenza, Italy

LIFAELS, La Salle, Universitat Ramon Llull, Barcelona, Spain

IFIC, Universitat de Valencia-CSIC, Valencia, Spain

Hanoi University of Science, Hanoi, Viet Nam

Università di Padova, Padova, Italy

Università di Pisa, Pisa, Italy

Scuola Normale Superiore, Pisa, Italy

The meson, discovered by CDF at the Tevatron [1], is the only known meson composed of two flavours of heavy quarks, charm and beauty. Both quarks can decay via the weak interaction with the other quark being considered as a spectator, therefore a wide range of decay channels are possible. However, only a few of these channels have been experimentally observed [1, 2, 3, 4]. The LHC opens a new era for physics, with an expected production cross-section of at centre-of-mass energy TeV for the meson [5, 6]. The LHCb experiment has observed the decay  [7], and new channels such as  [8] have started to emerge.

We report here the first observation of the decay with and the measurement of the ratio of branching fractions . The inclusion of charge conjugate modes is implied throughout the paper. The relativistic quark model [9] and several other models [10, 11, 12, 13] make various theoretical predictions for this ratio of branching fractions. As a two-body decay, is under better control theoretically than , and therefore this measurement is particularly useful to test the models of decays. The decay mode is chosen as the normalisation channel because of its identical final state and similar event topology. Both channels take advantage of the large trigger efficiency due to the two muons in the final state.

The analysis is based on collision data corresponding to an integrated luminosity of at collected with the LHCb detector in 2011. The detector [14] is a single-arm forward spectrometer covering the pseudorapidity range , designed for the study of particles containing or quarks. The detector includes a high precision tracking system consisting of a silicon-strip vertex detector surrounding the interaction region, a large-area silicon-strip detector located upstream of a dipole magnet with a bending power of about , and three stations of silicon-strip detectors and straw drift tubes placed downstream. The combined tracking system has momentum resolution that varies from 0.4% at 5  to 0.6% at 100 , and impact parameter (IP) resolution of 20  for tracks with high transverse momentum (). Charged hadrons are identified using two ring-imaging Cherenkov detectors and good kaon-pion separation is achieved for tracks with momentum between 5  and 100 . Photon, electron and hadron candidates are identified by a calorimeter system consisting of scintillating-pad and preshower detectors, an electromagnetic calorimeter and a hadronic calorimeter. Muons are identified by a system composed of alternating layers of iron and multiwire proportional chambers. The trigger system [15] consists of a hardware stage, based on information from the calorimeter and muon systems, followed by a software trigger that applies a full event reconstruction and reduces the event rate from to around .

Candidate decays with , where denotes or , are selected by requiring a single muon or dimuon with high in the hardware trigger. In the software trigger, a charged particle is required to have , or if identified as a muon; alternatively a dimuon trigger requires two oppositely charged muons with , the invariant mass of the muon pair , and that the muon track pair has a decay length significance with respect to the primary vertex greater than 5.

Further offline selections require both muons to have , and a track fit per degree of freedom () of less than 5. The mass of the candidate is required to be within a window of centred around the known mass ( for and for [16]. The vertex fit is required to be less than 20, and the decay length significance larger than 5.

The candidate is reconstructed from the and a bachelor pion. The pion is required to have , a track fit and IP with respect to the primary interaction great than 4. The IP is defined as the difference between the of the primary vertex reconstructed with and without the considered track. The candidate is required to have mass within around the world average value [16] and a vertex fit .

A boosted decision tree (BDT) [17], trained on data and simulation, is used to perform further background suppression. The collisions are simulated using Pythia 6.4 [18] with a specific LHCb configuration [19]. The mesons are generated through the dominant hard subprocess with the dedicated generator Bcvegpy [20, 21]. Decays of hadronic particles are described by EvtGen [22] in which final state radiation is generated using Photos [23]. The interaction of the generated particles with the detector and its response are implemented using Geant4 [24, *Agostinelli:2002hh] as described in Ref. [26].

The choice of the variables used to train the BDT is based on two considerations: their power to separate signal and background, and the similarity of the distributions for the and candidates that causes the systematic uncertainties in the selections to cancel when the ratio of branching fractions is determined. The BDT input variables are: the IP ; the vertex fit ; the IP ; the of the distance between the vertex and the associated primary vertex; the of the candidate; and the from a refit of the decay vertex [27] using a or mass constraint and a constraint that the candidate points to the primary vertex.

The BDT is trained using a simulation sample for the signal and sidebands from the mass spectrum ( or ) for the background. The trained BDT is then applied to the data, and a signal estimator is calculated for each candidate; a large value indicates a signal-like candidate. The cut on the estimator is optimised to maximise the signal significance. The BDT selection efficiencies, estimated from simulation, for and candidates are 35.8% and 37.2% respectively, and the fraction of accepted background is as estimated from the sideband data.

After the BDT selection, it is further required that the unconstrained dimuon invariant mass is in the range for and for . Information on particle identification for pions and kaons is also used to suppress the reflection background due to decays. Figure 1 shows the invariant mass distributions of the and candidates.

Figure 1: Invariant mass distributions of candidates reconstructed as (a) and (b) . Points with error bars (black) show the data, the thick solid line (blue) represents the fit of the data, the dashed line (red) the signal distribution, the dotted line (green) the combinatorial background, the dot-dashed line (purple) the partially reconstructed background, and the thin solid line (light blue) the background from the channel.

The relative branching fraction is calculated using

(1)

where is the number of selected signal events and is the total efficiency.

The signal yields are obtained by performing an extended maximum likehood fit to the mass spectra in Fig. 1. The signal is modelled with a double-sided Crystal Ball function [28] with the tail parameters on both sides determined from simulation. The main background component for both channels is combinatorial and is modelled using an exponential function. At the lower end of the mass spectrum, the contribution from the partially reconstructed background is modelled by an ARGUS function [29] convolved with a Gaussian distribution. For the decay, the Cabibbo suppressed channel also contributes, and is fitted with a double-sided Crystal Ball function with all parameters fixed to values obtained from simulation. The observed signal yields are for and for . Therefore the ratio of yields is

The total efficiency is the product of the detector acceptance, and the trigger, reconstruction and selection efficiencies. Each contribution has been determined using simulated events for the two channels, and the ratio of the total efficiencies has been evaluated to be

where the uncertainty is due to the limited size of the simulated sample.

Several sources of systematic uncertainty have been considered. The measured ratio of signal yields is expected to be independent of the BDT selection, given that the distributions of training variables are very similar for the two channels. The ratio of signal yields is measured for different cuts on the BDT response, and is constant within the statistical uncertainties. The average of these ratios differs from the nominal value by 4.5%, which is taken as the systematic uncertainty due to the BDT selection.

The signal is fitted with a double-sided Crystal Ball function. Alternatively we determine the signal shape directly from the simulation using kernel estimation [30], and convolve it with a Gaussian function to take into account the detector resolution while allowing the mean of the mass to vary. This results in a difference with respect to the nominal ratio, which is taken as the uncertainty due to the signal shape.

To consider the contribution from partially reconstructed background, the background is fitted with an exponential function within a narrower range (). This results in a 2.9% change with respect to the nominal fit, and is assigned as a systematic uncertainty.

The statistical uncertainty on the simulation when estimating the ratio of efficiencies leads to an uncertainty of 0.9% on the ratio of branching fractions. The difference between data and simulation introduces a systematic uncertainty, especially from variables used as input for the BDT. The distributions of these variables in simulation and data are compared, after the background is subtracted from the data using the sPlot technique [31]. The difference is found to be negligible compared to the statistical fluctuation.

A summary of systematic uncertainties is given in Table 1. The total systematic uncertainty is 5.7%, with the most significant contribution coming from the BDT selection. Taking the systematic uncertainty into account and using the likelihood ratio test  [32], the significance of the decay is estimated to be a , where and represent the likelihood of the background-only hypothesis and the signal-plus-background hypothesis respectively.

Component Value  (%)
BDT selection 4.5
Signal shape 1.7
Background shape 2.9
Simulation sample size 0.9
Total 5.7
Table 1: Relative systematic uncertainties.

In summary, a search for the decay has been performed using a data sample corresponding to an integrated luminosity of at collected by LHCb in 2011. The signal yield is candidates, making the first observation of this decay channel. Using as normalisation channel, the ratio of branching fractions is measured to be

Furthermore, taking and  [16] and assuming universality of the electroweak interaction, we obtain

where the last term accounts for the uncertainty on . This result favours the prediction made by the relativistic quark model [9] in comparison with the other models.

Acknowledgements

We thank Prof. Chao-Hsi Chang for valuable discussions. We express our gratitude to our colleagues in the CERN accelerator departments for the excellent performance of the LHC. We thank the technical and administrative staff at the LHCb institutes. We acknowledge support from CERN and from the national agencies: CAPES, CNPq, FAPERJ and FINEP (Brazil); NSFC (China); CNRS/IN2P3 and Region Auvergne (France); BMBF, DFG, HGF and MPG (Germany); SFI (Ireland); INFN (Italy); FOM and NWO (The Netherlands); SCSR (Poland); ANCS/IFA (Romania); MinES, Rosatom, RFBR and NRC “Kurchatov Institute” (Russia); MinECo, XuntaGal and GENCAT (Spain); SNSF and SER (Switzerland); NAS Ukraine (Ukraine); STFC (United Kingdom); NSF (USA). We also acknowledge the support received from the ERC under FP7. 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 thankful for the computing resources put at our disposal by Yandex LLC (Russia), as well as to the communities behind the multiple open source software packages that we depend on.

{mcitethebibliography}

10 \mciteSetBstSublistModen \mciteSetBstMaxWidthFormsubitem) \mciteSetBstSublistLabelBeginEnd\mcitemaxwidthsubitemform

References

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