First observation of the decays B0(s)→D+sK−π+π− and B0s→Ds1(2536)+π−

Original citation:

LHCb Collaboration (Including: Back, J. J., Dossett, D., Gershon, T. J., Harrison, P. F., Kreps, Michal, Latham, Thomas, Pilar, T., Poluektov, Anton, Reid, M. M., Silva Coutinho, R., Whitehead, M. and Williams, M. P.). (2012) First observation of the decays

B¯¯¯0(s)→D+sK−π+π− and B¯¯¯0s→Ds1(2536)+π−. Physical Review D (Particles, Fields, Gravitation and Cosmology), Volume 86 . Article Number 112005 .

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First observation of the decays

B

!

D

K

and

B

!

D

ð2536Þ

R. Aaij,38,aC. Abellan Beteta,33,pA. Adametz,11B. Adeva,34M. Adinolfi,43C. Adrover,6A. Affolder,49Z. Ajaltouni,5 J. Albrecht,35F. Alessio,35M. Alexander,48S. Ali,38G. Alkhazov,27P. Alvarez Cartelle,34A. A. Alves, Jr.,22S. Amato,2

Y. Amhis,36L. Anderlini,17,gJ. Anderson,37R. B. Appleby,51O. Aquines Gutierrez,10F. Archilli,18,35A. Artamonov,32 M. Artuso,53E. Aslanides,6G. Auriemma,22,nS. Bachmann,11J. J. Back,45C. Baesso,54W. Baldini,16R. J. Barlow,51

C. Barschel,35S. Barsuk,7W. Barter,44A. Bates,48Th. Bauer,38A. Bay,36J. Beddow,48I. Bediaga,1S. Belogurov,28 K. Belous,32I. Belyaev,28E. Ben-Haim,8M. Benayoun,8G. Bencivenni,18S. Benson,47J. Benton,43A. Berezhnoy,29

R. Bernet,37M.-O. Bettler,44M. van Beuzekom,38A. Bien,11S. Bifani,12T. Bird,51A. Bizzeti,17,iP. M. Bjørnstad,51 T. Blake,35F. Blanc,36C. Blanks,50J. Blouw,11S. Blusk,53A. Bobrov,31V. Bocci,22A. Bondar,31N. Bondar,27 W. Bonivento,15S. Borghi,48,51A. Borgia,53T. J. V. Bowcock,49C. Bozzi,16T. Brambach,9J. van den Brand,39 J. Bressieux,36D. Brett,51M. Britsch,10T. Britton,53N. H. Brook,43H. Brown,49A. Bu¨chler-Germann,37I. Burducea,26 A. Bursche,37J. Buytaert,35S. Cadeddu,15O. Callot,7M. Calvi,20,kM. Calvo Gomez,33,oA. Camboni,33P. Campana,18,35 A. Carbone,14,dG. Carboni,21,lR. Cardinale,19,jA. Cardini,15H. Carranza-Mejia,47L. Carson,50K. Carvalho Akiba,2 G. Casse,49M. Cattaneo,35Ch. Cauet,9M. Charles,52Ph. Charpentier,35P. Chen,3,36N. Chiapolini,37M. Chrzaszcz,23

K. Ciba,35X. Cid Vidal,34G. Ciezarek,50P. E. L. Clarke,47M. Clemencic,35H. V. Cliff,44J. Closier,35C. Coca,26 V. Coco,38J. Cogan,6E. Cogneras,5P. Collins,35A. Comerma-Montells,33A. Contu,52,15A. Cook,43M. Coombes,43

G. Corti,35B. Couturier,35G. A. Cowan,36D. Craik,45S. Cunliffe,50R. Currie,47C. D’Ambrosio,35P. David,8 P. N. Y. David,38I. De Bonis,4K. De Bruyn,38S. De Capua,51M. De Cian,37J. M. De Miranda,1L. De Paula,2 P. De Simone,18D. Decamp,4M. Deckenhoff,9H. Degaudenzi,36,35L. Del Buono,8C. Deplano,15D. Derkach,14

O. Deschamps,5F. Dettori,39A. Di Canto,11J. Dickens,44H. Dijkstra,35P. Diniz Batista,1M. Dogaru,26 F. Domingo Bonal,33,oS. Donleavy,49F. Dordei,11A. Dosil Sua´rez,34D. Dossett,45A. Dovbnya,40F. Dupertuis,36 R. Dzhelyadin,32A. Dziurda,23A. Dzyuba,27S. Easo,46,35U. Egede,50V. Egorychev,28S. Eidelman,31D. van Eijk,38 S. Eisenhardt,47R. Ekelhof,9L. Eklund,48I. El Rifai,5Ch. Elsasser,37D. Elsby,42A. Falabella,14,fC. Fa¨rber,11G. Fardell,47

C. Farinelli,38S. Farry,12V. Fave,36V. Fernandez Albor,34F. Ferreira Rodrigues,1M. Ferro-Luzzi,35S. Filippov,30 C. Fitzpatrick,35M. Fontana,10F. Fontanelli,19,jR. Forty,35O. Francisco,2M. Frank,35C. Frei,35M. Frosini,17,g S. Furcas,20A. Gallas Torreira,34D. Galli,14,dM. Gandelman,2P. Gandini,52Y. Gao,3J-C. Garnier,35J. Garofoli,53 P. Garosi,51J. Garra Tico,44L. Garrido,33C. Gaspar,35R. Gauld,52E. Gersabeck,11M. Gersabeck,35T. Gershon,45,35

Ph. Ghez,4V. Gibson,44V. V. Gligorov,35C. Go¨bel,54D. Golubkov,28A. Golutvin,50,28,35A. Gomes,2H. Gordon,52 M. Grabalosa Ga´ndara,33R. Graciani Diaz,33L. A. Granado Cardoso,35E. Grauge´s,33G. Graziani,17A. Grecu,26

E. Greening,52S. Gregson,44O. Gru¨nberg,55B. Gui,53E. Gushchin,30Yu. Guz,32T. Gys,35C. Hadjivasiliou,53 G. Haefeli,36C. Haen,35S. C. Haines,44S. Hall,50T. Hampson,43S. Hansmann-Menzemer,11N. Harnew,52S. T. Harnew,43 J. Harrison,51P. F. Harrison,45T. Hartmann,55J. He,7V. Heijne,38K. Hennessy,49P. Henrard,5J. A. Hernando Morata,34 E. van Herwijnen,35E. Hicks,49D. Hill,52M. Hoballah,5P. Hopchev,4W. Hulsbergen,38P. Hunt,52T. Huse,49N. Hussain,52 D. Hutchcroft,49D. Hynds,48V. Iakovenko,41P. Ilten,12J. Imong,43R. Jacobsson,35A. Jaeger,11M. Jahjah Hussein,5 E. Jans,38F. Jansen,38P. Jaton,36B. Jean-Marie,7F. Jing,3M. John,52D. Johnson,52C. R. Jones,44B. Jost,35M. Kaballo,9

S. Kandybei,40M. Karacson,35T. M. Karbach,35I. R. Kenyon,42U. Kerzel,35T. Ketel,39A. Keune,36B. Khanji,20 Y. M. Kim,47O. Kochebina,7V. Komarov,36,29R. F. Koopman,39P. Koppenburg,38M. Korolev,29A. Kozlinskiy,38 L. Kravchuk,30K. Kreplin,11M. Kreps,45G. Krocker,11P. Krokovny,31F. Kruse,9M. Kucharczyk,20,23,kV. Kudryavtsev,31

T. Kvaratskheliya,28,35V. N. La Thi,36D. Lacarrere,35G. Lafferty,51A. Lai,15D. Lambert,47R. W. Lambert,39 E. Lanciotti,35G. Lanfranchi,18,35C. Langenbruch,35T. Latham,45C. Lazzeroni,42R. Le Gac,6J. van Leerdam,38 J.-P. Lees,4R. Lefe`vre,5A. Leflat,29,35J. Lefranc¸ois,7O. Leroy,6T. Lesiak,23Y. Li,3L. Li Gioi,5M. Liles,49R. Lindner,35 C. Linn,11B. Liu,3G. Liu,35J. von Loeben,20J. H. Lopes,2E. Lopez Asamar,33N. Lopez-March,36H. Lu,3J. Luisier,36 H. Luo,47A. Mac Raighne,48F. Machefert,7I. V. Machikhiliyan,4,28F. Maciuc,26O. Maev,27,35J. Magnin,1M. Maino,20

S. Malde,52G. Manca,15,eG. Mancinelli,6N. Mangiafave,44U. Marconi,14R. Ma¨rki,36J. Marks,11G. Martellotti,22 A. Martens,8L. Martin,52A. Martı´n Sa´nchez,7M. Martinelli,38D. Martinez Santos,35D. Martins Tostes,2A. Massafferri,1 R. Matev,35Z. Mathe,35C. Matteuzzi,20M. Matveev,27E. Maurice,6A. Mazurov,16,30,35,fJ. McCarthy,42G. McGregor,51 R. McNulty,12M. Meissner,11M. Merk,38J. Merkel,9D. A. Milanes,13M.-N. Minard,4J. Molina Rodriguez,54S. Monteil,5 D. Moran,51P. Morawski,23R. Mountain,53I. Mous,38F. Muheim,47K. Mu¨ller,37R. Muresan,26B. Muryn,24B. Muster,36 J. Mylroie-Smith,49P. Naik,43T. Nakada,36R. Nandakumar,46I. Nasteva,1M. Needham,47N. Neufeld,35A. D. Nguyen,36

T. D. Nguyen,36C. Nguyen-Mau,36,pM. Nicol,7V. Niess,5N. Nikitin,29T. Nikodem,11A. Nomerotski,52,35

A. Novoselov,32A. Oblakowska-Mucha,24V. Obraztsov,32S. Oggero,38S. Ogilvy,48O. Okhrimenko,41R. Oldeman,15,35,e M. Orlandea,26J. M. Otalora Goicochea,2P. Owen,50B. K. Pal,53A. Palano,13,cM. Palutan,18J. Panman,35A. Papanestis,46 M. Pappagallo,48C. Parkes,51C. J. Parkinson,50G. Passaleva,17G. D. Patel,49M. Patel,50G. N. Patrick,46C. Patrignani,19,j

C. Pavel-Nicorescu,26A. Pazos Alvarez,34A. Pellegrino,38G. Penso,22,mM. Pepe Altarelli,35S. Perazzini,14,d D. L. Perego,20,kE. Perez Trigo,34A. Pe´rez-Calero Yzquierdo,33P. Perret,5M. Perrin-Terrin,6G. Pessina,20K. Petridis,50

A. Petrolini,19,jA. Phan,53E. Picatoste Olloqui,33B. Pie Valls,33B. Pietrzyk,4T. Pilarˇ,45D. Pinci,22S. Playfer,47 M. Plo Casasus,34F. Polci,8G. Polok,23A. Poluektov,45,31E. Polycarpo,2D. Popov,10B. Popovici,26C. Potterat,33 A. Powell,52J. Prisciandaro,36V. Pugatch,41A. Puig Navarro,36W. Qian,4J. H. Rademacker,43B. Rakotomiaramanana,36

M. S. Rangel,2I. Raniuk,40N. Rauschmayr,35G. Raven,39S. Redford,52M. M. Reid,45A. C. dos Reis,1S. Ricciardi,46 A. Richards,50K. Rinnert,49V. Rives Molina,33D. A. Roa Romero,5P. Robbe,7E. Rodrigues,48,51P. Rodriguez Perez,34

G. J. Rogers,44S. Roiser,35V. Romanovsky,32A. Romero Vidal,34J. Rouvinet,36T. Ruf,35H. Ruiz,33G. Sabatino,22,l J. J. Saborido Silva,34N. Sagidova,27P. Sail,48B. Saitta,15,eC. Salzmann,37B. Sanmartin Sedes,34M. Sannino,19,j R. Santacesaria,22C. Santamarina Rios,34R. Santinelli,35E. Santovetti,21,lM. Sapunov,6A. Sarti,18,mC. Satriano,22,n

A. Satta,21M. Savrie,16,fP. Schaack,50M. Schiller,39H. Schindler,35S. Schleich,9M. Schlupp,9M. Schmelling,10 B. Schmidt,35O. Schneider,36A. Schopper,35M.-H. Schune,7R. Schwemmer,35B. Sciascia,18A. Sciubba,18,mM. Seco,34

A. Semennikov,28K. Senderowska,24I. Sepp,50N. Serra,37J. Serrano,6P. Seyfert,11M. Shapkin,32I. Shapoval,40,35 P. Shatalov,28Y. Shcheglov,27T. Shears,49,35L. Shekhtman,31O. Shevchenko,40V. Shevchenko,28A. Shires,50

R. Silva Coutinho,45T. Skwarnicki,53N. A. Smith,49E. Smith,52,46M. Smith,51K. Sobczak,5F. J. P. Soler,48 F. Soomro,18,35D. Souza,43B. Souza De Paula,2B. Spaan,9A. Sparkes,47P. Spradlin,48F. Stagni,35S. Stahl,11 O. Steinkamp,37S. Stoica,26S. Stone,53B. Storaci,38M. Straticiuc,26U. Straumann,37V. K. Subbiah,35S. Swientek,9

M. Szczekowski,25P. Szczypka,36,35T. Szumlak,24S. T’Jampens,4M. Teklishyn,7E. Teodorescu,26F. Teubert,35 C. Thomas,52E. Thomas,35J. van Tilburg,11V. Tisserand,4M. Tobin,37S. Tolk,39D. Tonelli,35S. Topp-Joergensen,52

N. Torr,52E. Tournefier,4,50S. Tourneur,36M. T. Tran,36A. Tsaregorodtsev,6P. Tsopelas,38N. Tuning,38 M. Ubeda Garcia,35A. Ukleja,25D. Urner,51U. Uwer,11V. Vagnoni,14G. Valenti,14R. Vazquez Gomez,33 P. Vazquez Regueiro,34S. Vecchi,16J. J. Velthuis,43M. Veltri,17,hG. Veneziano,36M. Vesterinen,35B. Viaud,7I. Videau,7

D. Vieira,2X. Vilasis-Cardona,33,oJ. Visniakov,34A. Vollhardt,37D. Volyanskyy,10D. Voong,43A. Vorobyev,27 V. Vorobyev,31C. Voß,55H. Voss,10R. Waldi,55R. Wallace,12S. Wandernoth,11J. Wang,53D. R. Ward,44N. K. Watson,42

A. D. Webber,51D. Websdale,50M. Whitehead,45J. Wicht,35D. Wiedner,11L. Wiggers,38G. Wilkinson,52 M. P. Williams,45,46M. Williams,50,qF. F. Wilson,46J. Wishahi,9M. Witek,23W. Witzeling,35S. A. Wotton,44S. Wright,44 S. Wu,3K. Wyllie,35Y. Xie,47,35F. Xing,52Z. Xing,53Z. Yang,3R. Young,47X. Yuan,3O. Yushchenko,32M. Zangoli,14

M. Zavertyaev,10,bF. Zhang,3L. Zhang,53W. C. Zhang,12Y. Zhang,3A. Zhelezov,11L. Zhong,3and A. Zvyagin35

(The LHCb collaboration)

1

Centro Brasileiro de Pesquisas Fı´sicas (CBPF), Rio de Janeiro, Brazil

2_{Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil} 3_{Center for High Energy Physics, Tsinghua University, Beijing, China}

4_{LAPP, Universite´ de Savoie, CNRS/IN2P3, Annecy-Le-Vieux, France}

5_{Clermont Universite´, Universite´ Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France} 6_{CPPM, Aix-Marseille Universite´, CNRS/IN2P3, Marseille, France}

7_{LAL, Universite´ Paris-Sud, CNRS/IN2P3, Orsay, France} 8

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

9_{Fakulta¨t Physik, Technische Universita¨t Dortmund, Dortmund, Germany} 10_{Max-Planck-Institut fu¨r Kernphysik (MPIK), Heidelberg, Germany}

11_{Physikalisches Institut, Ruprecht-Karls-Universita¨t Heidelberg, Heidelberg, Germany} 12_{School of Physics, University College Dublin, Dublin, Ireland}

13_{Sezione INFN di Bari, Bari, Italy} 14_{Sezione INFN di Bologna, Bologna, Italy} 15_{Sezione INFN di Cagliari, Cagliari, Italy} 16_{Sezione INFN di Ferrara, Ferrara, Italy}

17_{Sezione INFN di Firenze, Firenze, Italy}

18_{Laboratori Nazionali dell’INFN di Frascati, Frascati, Italy} 19_{Sezione INFN di Genova, Genova, Italy}

21_{Sezione INFN di Roma Tor Vergata, Roma, Italy} 22_{Sezione INFN di Roma La Sapienza, Roma, Italy}

23_{Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Krako´w, Poland} 24_{AGH University of Science and Technology, Krako´w, Poland}

25_{National Center for Nuclear Research (NCBJ), Warsaw, Poland}

26_{Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania} 27_{Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia}

28_{Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia} 29

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

30_{Institute for Nuclear Research of the Russian Academy of Sciences (INR RAN), Moscow, Russia} 31_{Budker Institute of Nuclear Physics (SB RAS) and Novosibirsk State University, Novosibirsk, Russia}

32_{Institute for High Energy Physics (IHEP), Protvino, Russia} 33_{Universitat de Barcelona, Barcelona, Spain}

34_{Universidad de Santiago de Compostela, Santiago de Compostela, Spain} 35_{European Organization for Nuclear Research (CERN), Geneva, Switzerland}

36_{Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), Lausanne, Switzerland} 37_{Physik-Institut, Universita¨t Zu¨rich, Zu¨rich, Switzerland}

38_{Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands}

39_{Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, The Netherlands} 40_{NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine}

41_{Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine} 42_{University of Birmingham, Birmingham, United Kingdom}

43_{H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom} 44_{Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom}

45

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

46_{STFC Rutherford Appleton Laboratory, Didcot, United Kingdom}

47_{School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom} 48_{School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom}

49_{Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom} 50_{Imperial College London, London, United Kingdom}

51_{School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom} 52_{Department of Physics, University of Oxford, Oxford, United Kingdom}

53_{Syracuse University, Syracuse, New York, USA}

54_{Pontifı´cia Universidade Cato´lica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil,}

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

55_{Institut fu¨r Physik, Universita¨t Rostock, Rostock, Germany, associated to Physikalisches Institut,}

Ruprecht-Karls-Universita¨t Heidelberg, Heidelberg, Germany (Received 7 November 2012; published 20 December 2012)

The first observation of the decaysB0

s !DþsKþ andB0!DþsKþare reported using an integrated luminosity of1:0 fb1_{recorded by the LHCb experiment. The branching fractions, normalized}

with respect to B0

s!Dþsþ and B0s !DþsKþ, respectively, are measured to be

Bð_B0

s!DþsKþÞ

Bð_B0

s!DþsþÞ¼ ð5:20:50:3Þ 10

2 _andBðB0_!Dþ sKþÞ

Bð_B0

s!DþsKþÞ¼0:540:070:07, where the first

a_{Full author list given at end of the article.}

b_{P.N. Lebedev Physical Institute, Russian Academy of Science (LPI RAS), Moscow, Russia.} c_{Universita` di Bari, Bari, Italy.}

d_{Universita` di Bologna, Bologna, Italy.</sub> e<sub>Universita` di Cagliari, Cagliari, Italy.} f_{Universita` di Ferrara, Ferrara, Italy.</sub> g<sub>Universita` di Firenze, Firenze, Italy.} h_{Universita` di Urbino, Urbino, Italy.}

i_{Universita` di Modena e Reggio Emilia, Modena, Italy.</sub> j<sub>Universita` di Genova, Genova, Italy.}

k_{Universita` di Milano Bicocca, Milano, Italy.</sub> l<sub>Universita` di Roma Tor Vergata, Roma, Italy.} m_{Universita` di Roma La Sapienza, Roma, Italy.}

n_{Universita` della Basilicata, Potenza, Italy.}

o_{LIFAELS, La Salle, Universitat Ramon Llull, Barcelona, Spain.} p_{Hanoi University of Science, Hanoi, Viet Nam.}

q_{Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.}

uncertainty is statistical and the second is systematic. TheB0

s !DþsKþ decay is of particular interest as it can be used to measure the weak phase. First observation of theB0

s!Ds1ð2536Þþ,

Dþs1!Dþsþ decay is also presented, and its branching fraction relative toB0s !Dþsþ is found to beBðB0s!Ds1ð2536Þþ;Dþ_s1!DþsþÞ

Bð_B0

s!DþsþÞ ¼ ð4:01:00:4Þ 10

3_.

DOI:10.1103/PhysRevD.86.112005 PACS numbers: 13.25.Hw, 13.20.He

I. INTRODUCTION

In the Standard Model (SM), the amplitudes associated with flavor-changing processes depend on four Cabibbo-Kobayashi-Maskawa (CKM) [1,2] matrix parameters. Contributions from physics beyond the Standard Model (BSM) add coherently to these amplitudes, leading to potential deviations in rates andCP-violating asymmetries when compared to the SM contributions alone. Since the SM does not predict the CKM parameters, it is important to make precise measurements of their values in processes that are expected to be insensitive to BSM contributions. Their values then provide a benchmark to which BSM-sensitive measurements can be compared.

The least well-determined of the CKM parameters is the weak phaseargðVubVud

VcbVcdÞ, which, through direct mea-surements, is known to a precision of10o–12o [3,4]. It may be probed using time-independent rates of decays such as B!DK [5–7] or by analyzing the time-dependent decay rates of processes such asB0s !DsK

[8–11]. Sensitivity to the weak phase results from the interference between b!c and b!u transitions, as indicated in Figs. 1(a)–1(c). Such measurements may be extended to multibody decay modes, such as B ! DKþ [12] for a time-independent measurement, or

B0

s !DþsKþ in the case of a time-dependent

analysis.

The B0_!Dþ

sKþ decay, while having the same

final state as B0

s !DþsKþ, receives contributions

not only from theW-exchange process [Fig.1(d)], but also fromb!ctransitions in association with the production of an extrasspair [Figs.1(e)and1(f )]. The decay may also proceed through mixing followed by ab!u,W-exchange process (not shown). However, this amplitude is Cabibbo-, helicity- and color-suppressed and is therefore negligible compared to theb!camplitude.

This paper reports the first observation of B0

s !

DþsKþ andB0 !DþsKþ and measurements

of their branching fractions relative toB0

s !Dþsþ

andB0

s !DþsKþ, respectively. The data sample is

based on an integrated luminosity of1:0 fb1 _ofpp colli-sions atpffiffiffis¼7 TeV, collected by the LHCb experiment.

The same data sample is also used to observe the

B0

s!Ds1ð2536Þþ, Dþs1!Dþsþ decay for the

first time and measure its branching fraction relative to

B0

s!Dþsþ. The inclusion of charge-conjugated

modes is implied throughout this paper.

II. DETECTOR AND SIMULATION

The LHCb detector [13] is a single-arm forward spec-trometer covering the pseudorapidity range 2< <5, designed for the study of particles containingborcquarks. The detector includes a high precision tracking system consisting of a silicon-strip vertex detector surrounding theppinteraction region, a large-area silicon-strip detector located upstream of a dipole magnet with a bending power of about 4 Tm, and three stations of silicon-strip detectors and straw drift-tubes placed downstream. The combined tracking system has a momentum resolution (p=p) that varies from 0.4% at5 GeV=cto 0.6% at100 GeV=c, and an impact parameter (IP) resolution of 20m for tracks with high transverse momentum (pT). Charged hadrons are identified using two ring-imaging Cherenkov detectors. Photon, electron and hadron candidates are identified by a calorimeter system consisting of scintillating-pad and pre-shower detectors, an electromagnetic calorimeter and a hadronic calorimeter. Muons are identified by a system

b s c s cb V s u 0 s

B D+s

) -π + π ( -K (a) b s u s ub V c s 0 s B + s D ) -π + π ( -K (b) b s + W c u g _s s s + D ) -π + π ( -K s 0 B cb V us V (c) b d + W c u g s s s + D ) -π + π ( -K 0 B cb V ud V (d) b d c d s s s + D -π *0 K 0 B cb V ud V d u g (e) b d c d s s s + D -K 0 ρ 0 B _u d g cb V ud V (f)

FIG. 1 (color online). Diagrams contributing to theB0

s,B0s ! DþsKþ (a–c) and B0s !DþsKþ (d–f ) decays, as described in the text. In (a)–(d), the additional (þ) indicates that the Kþ may be produced either through an excited strange kaon resonance decay, or through fragmentation. Published by the American Physical Society under the terms of

composed of alternating layers of iron and multiwire proportional chambers.

The trigger consists of a hardware stage, based on information from the calorimeter and muon systems, followed by a software stage, which applies a full event reconstruction. The software trigger requires a two-, three-or four-track secondary vertex with a highpT sum of the tracks and a significant displacement from the primarypp interaction vertices (PVs). At least one track should have pT>1:7 GeV=c, an IP2 _{greater than 16 with respect to} all PVs, and a track fit 2_{=ndf<2, where ndf is the} number of degrees of freedom. The IP 2 _{is defined as} the difference between the 2 of the PV reconstructed with and without the considered particle. A multivariate algorithm is used for the identification of secondary vertices [14].

For the simulation, pp collisions are generated using

PYTHIA6.4 [15] with a specific LHCb configuration [16]. Decays of hadronic particles are described byEVTGEN[17] in which final state radiation is generated using PHOTOS

[18]. The interaction of the generated particles with the detector and its response are implemented using the

GEANT4toolkit [19] as described in Ref. [20].

III. SIGNAL SELECTION

Signal B0_ðsÞ decay candidates are formed by pairing a Dþs !KþKþ candidate with either a þ

(hereafter referred to as Xd) or a Kþ combination

(hereafter referred to asXs). Tracks used to form theDþs and

Xd;sare required to be identified as either a pion or a kaon

using information from the ring-imaging Cherenkov detec-tors, havepT in excess of100 MeV=cand be significantly detached from any reconstructed PV in the event.

SignalDþs candidates are required to have good vertex

fit quality, be significantly displaced from the nearest PV and have invariant mass, MðKþKþÞ, within

20 MeV=c2_{of theD}þ

s mass [21]. To suppress

combinato-rial and charmless backgrounds, only thoseDþs candidates

that are consistent with decaying through either the (MðKþKÞ<1040 MeV=c2_{) or K}0 _(jMðKþ_Þ mK0j<75 MeV=c2) resonances are used (here, m_K0 is theK0 _{mass [21]). The remaining charmless background} yields are determined using the Dþs mass sidebands. For

about 20% of candidates, when theKþis assumed to be a þ, the correspondingKþþinvariant mass is consis-tent with the Dþ mass. To suppress cross feed from

B0 _!Dþ_{X decays, a tighter particle identification (PID)} requirement is applied to theKþ in the Dþs !KþKþ

candidates when jMðKþþÞ mDþj<20 MeV=c2

(mDþ is the Dþ mass [21]). Similarly, if the invariant

mass of the particles forming the Dþs candidate, after

replacing theKþ mass with the proton mass, falls within

15 MeV=c2 _{of the} þ

c mass, tighter PID selection is

applied. The sizes of these mass windows are about

2.5 times the invariant mass resolution and are sufficient to render these cross-feed backgrounds negligible.

Candidates Xd and Xs are formed from þ or

Kþ combinations, where all invariant mass values up to 3 GeV=c2 are accepted. To reduce the level of combinatorial background, we demand that theXd;svertex

is displaced from the nearest PV by more than100min the direction transverse to the beam axis and that at least two of the daughter tracks have pT>300 MeV=c. Backgrounds to the B0_ðsÞ!DþsKþ search from

B0

s!DðÞþs þ or B0s !DþsKKþ decays are

suppressed by applying more stringent PID requirements to the K andþ in Xs. The PID requirements have an

efficiency of about 65% for selecting Xs, while rejecting

about 97% of the favored three-pion background. To sup-press peaking backgrounds from B0

s!DþsDs decays,

where Dþs !þþ, Kþþ, it is required that

MðXd;sÞis more than20 MeV=c2away from theDþs mass.

SignalBmeson candidates are then formed by combin-ing a Dþs with either an Xd or Xs. The reconstructed B

candidate is required to be well separated from the nearest PV with a decay time larger than 0.2 ps and to have a good quality vertex fit. To suppress remaining charmless back-grounds, which appear primarily in B0 _!Dþ

sKþ,

the vertex separation 2 _{between the D}þ

s and B decay

vertices is required to be greater than 9. Candidates passing all selection requirements are refit with bothDþs mass and

vertex constraints to improve the mass resolution [22]. To further suppress combinatorial background, a boosted decision tree (BDT) selection [23] with the AdaBoost algorithm[24] is employed. The BDT is trained using simulatedB0s !DþsKþ decays for the signal

distributions, and the high B mass sideband in data are used to model the backgrounds. The following 13 variables are used:

(i) B candidate: IP2_{, vertex separation}2_{, vertex fit} 2_{, andpT;}

(ii) Dþs candidate: Flight distance significance fromB

vertex;

(iii) Xd;scandidate: IP2, maximum of the distances of

closest approach between any pair of tracks in the decay;

(iv) Xd;s daughters: minðIP2Þ, maxðIP2Þ, minðpTÞ;

and

(v) Dþs daughters:minðIP2Þ,maxðIP2Þ,minðpTÞ,

where min and max denote the minimum and maximum of the indicated values amongst the daughter particles. The flight distance significance is the separation between the Dþs and B vertices, normalized by the uncertainty.

The training produces a single variable, x, that provides discrimination between signal decays and background contributions. The cut value is chosen by optimizing SðxcutÞ=pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiSðxcutÞ þBðxcutÞ, where SðxcutÞ and BðxcutÞ are the expected signal and background yields, respectively,

after requiring x > xcut. At the optimal point, a signal efficiency of 90% is expected while rejecting about 85% of the combinatorial background (after the previously discussed selections are applied). After all selections, about 3% of events have more than one signal candidate in both data and simulation. All candidates are kept for further analysis.

IV. FITS TO DATA

The B0

s !Dþsþ and B0_ðsÞ!DþsKþ

invariant mass spectra are each modeled by the sum of a signal and several background components. The signal shapes are obtained from simulation and are each described by the sum of a crystal ball (CB) [25] shape and a Gaussian function. The CB shape parameter that describes the tail toward low mass is fixed based on simu-lated decays. A common, freely varying scale factor multi-plies the width parameters in the CB and Gaussian functions to account for slightly larger resolution in data than in simulation. For theB0

ðsÞ!DþsKþ mass fit,

the difference between the meanB0

sandB0masses is fixed

to87:35 MeV=c2 _[21].

Several nonsignalb-hadron decays produce broad peak-ing structures in the Dþsþ and DþsKþ

invariant mass spectra. For B0

s!Dþsþ, the

only significant source of peaking background is from

B0

s !Dþs þ, where the photon or 0 from the

Dþs decay is not included in the reconstructed decay.

Since the full decay amplitude forB0s!Dþs þis

not known, the simulation may not adequately model the decay. Simulation is therefore used to provide an estimate for the shape, but the parameters are allowed to vary within one standard deviation about the fitted values.

For B0_ðsÞ!DþsKþ, backgrounds from B0ðsÞ!

Dþs Kþ and from misidentifiedB0s!Dþsþ

and B0s !Dþs þ decays are considered. The

B0_ðsÞ!Dþs Kþ shape is fixed to be the same as

that obtained for the B0

s!Dþs þ component in

the B0

s !Dþsþ mass fit. This same shape is

assumed for bothB0 _andB0

s, where for the former, a shift

by the B0_B0

s mass difference is included. For the

B0

s !Dþsþ and B0s !Dþs þ cross feed,

simulated decays and kaon misidentification rates taken fromDþcalibration data are used to obtain their expected yields and invariant mass shapes. The cross-feed contribu-tion is about 3% of the B0s!Dþsþ and B0s !

Dþs þ yields; the corresponding cross-feed yields

are fixed in the B0_ðsÞ!DþsKþ fit. The shape is

obtained by parametrizing the invariant mass spectrum obtained from the simulation after replacing the appropri-atemass inXdwith the kaon mass. The combinatorial

background is described by an exponential function whose slope is allowed to vary independently for both mass fits.

Figure 2 shows the invariant mass distribution for

B0s!Dþsþ candidates passing all selection

crite-ria. The fitted number ofB0

s !Dþsþsignal events

is 568383. While it is expected that most of the low mass background emanates from B0

s !Dþs þ

decays, contributions from other sources such as

B0

s!Dþsþ0 are also possibly absorbed into

this background component. Figure3shows the invariant mass distribution forB0

ðsÞ!DþsKþcandidates. The

fitted signal yields are 40233 B0 _!Dþ

sKþ and

21621 B0

s!DþsKþ events.

] 2 c mass [MeV/ -π + π -π + s D

5000 5200 5400 5600 5800

)

2_c

Candidates / (10 MeV/

500 1000

1500 LHCb Data

Full PDF Signal PDF X Bkg + s D → 0 s B Comb. Bkg

FIG. 2. Invariant mass distribution for B0

s !Dþsþ candidates. The fitted signal probability distribution function (PDF) is indicated by the dashed line and the background shapes are shown as shaded regions, as described in the text.

] 2 c mass [MeV/ -π + π -K + s D

5000 5200 5400 5600 5800

)

2

c

Candidates / (10 MeV/ 50 100 150 LHCb Data Full PDF Signal PDFs X Bkg + s D → 0 B X Bkg + s D → 0 s B Bkg πππ (*) s D → 0 s B Comb. Bkg

FIG. 3. Invariant mass distribution for B0

ðsÞ!DþsKþ

The Dþs mass sidebands, defined to be from 35 to

55 MeV=c2 _{on either side of the nominal D}þ

s mass, are

used to estimate the residual charmless background that may contribute to the observed signals. The numbers ofB0

s

decays in theDþs sidebands are6116,0þ50, and95for the B0s !Dþsþ, B0s !DþsKþ and B0 !

DþsKþ decays, respectively; they are subtracted

from the observed signal yields to obtain the corrected

number of signal decays. The yields in the signal and sideband regions are summarized in TableI.

V. MASS DISTRIBUTIONS OF

Xd;s AND TWO-BODY MASSES

In order to investigate the properties of theseB0

ðsÞdecays,

sWeights [26] obtained from the mass fits are used to determine the underlying Xd;s invariant mass spectra as

well as the two-body invariant masses amongst the three daughter particles. Figure4shows (a) theþmass, (b) the smaller þ mass and (c) the larger þ mass in B0s !Dþsþ data and simulated decays.

A prominent peak, consistent with the a1ð1260Þ!

þ, is observed, along with structures consistent with the 0 _{in the two-body masses. There appears to be} an offset in the peak position of the a1ð1260Þ between data and simulation. Since the mean and width of the a1ð1260Þresonance are not well known, and their values may even be process dependent, this level of agreement is reasonable. A number of other spectra have been compared between data and simulation, such as thepTspectra of the

] 2 c Mass [MeV/ -π + π -π

1000 2000 3000

)

2

c

Candidates / (200 MeV/

0 500 1000 1500 (a) LHCb Data Signal MC ] 2 c Mass [MeV/ + π -π Smaller

0 1000 2000 3000

)

2

c

Candidates / (100 MeV/

0 500 1000 1500 (b) ] 2 c Mass [MeV/ + π -π Larger

0 1000 2000 3000

)

2

c

Candidates / (100 MeV/

0 500 1000 1500

(c)

FIG. 4 (color online). Invariant mass distributions for (a)Xd, (b) smallerþmass inXd and (c) the largerþmass inXd, fromB0

s !Dþsþdecays usingsWeights. The points are the data and the solid line is the simulation. The simulated distribution is normalized to have the same yield as the data.

TABLE I. Summary of event yields from data in theDþs signal and sidebands regions and the background corrected yield. The signal and sideband regions require Dþs candidates to have invariant mass jMðKþKþÞ mDþsj<20 MeV=c

2 _and

35<jMðKþKþÞ mDþsj<55 MeV=c

2_{, respectively, where}

mDþs is theD

þ

s mass [21].

Decay

Signal Region

Sideband

Region Corrected Yield

B0

s !Dþsþ 568383 6116 562285

B0

s !DþsKþ 21621 0þ50 21622

B0_!Dþ

sKþ 40233 95 39333

] 2 c Mass [MeV/ -π + π -K

1000 2000 3000

)

2

c

Candidates / (200 MeV/

0 50 100 (a) LHCb Data Signal MC ] 2 c Mass [MeV/ + π -π

0 1000 2000 3000

)

2

c

Candidates / (100 MeV/

0 20 40 60 (b) ] 2 c Mass [MeV/ + π -K

0 1000 2000 3000

)

2

c

Candidates / (100 MeV/

0 50

100 _(c)

FIG. 5 (color online). Invariant mass distributions for (a)Xs, (b)þinXsand (c) theKþinXs, fromB0s !DþsKþdata usingsWeights. The points are data and the solid line is the simulation. The simulated distribution is normalized to have the same yield as the data.

Dþs,Xdand the daughter particles, and excellent agreement

is found.

Figure5 shows the corresponding distributions for the

B0

s !DþsKþ decay. A peaked structure at low

Kþ mass, consistent with contributions from the lower-lying excited strange mesons, such as the K1ð1270ÞandK1ð1400Þ, is observed. As many of these states decay throughK0 _and0 _{mesons, significant} con-tributions from these resonances are observed in theKþ andþinvariant mass spectra, respectively. The simu-lation provides a reasonable description of the distributions in the data.

Figure 6 shows the same distributions for B0 _! DþsKþ. The Kþ invariant mass is quite

broad, with little indication of any narrow structures. There are indications of K0 and 0 contributions in the Kþandþinvariant mass spectra, respectively, but the contribution from resonances such as theK1ð1270Þor K1ð1400Þ appear to be small or absent. In the Kþ invariant mass spectrum, there may be an indication of a

K0_ð1430Þ0 _{contribution. The simulation, which models} the Kþ final state as 10% K1ð1270Þ, 10% K1ð1400Þ, 40% K0 _{and 40% K}0_{, provides a} reasonable description of the data, which suggests that processes such as those in Figs.1(e)and1(f )constitute a large portion of the total width for this decay.

VI. FIRST OBSERVATION

OFB0s !Ds1ð2536Þþ

A search for excited Dþs states, such as DþsJ !

Dþsþ, contributing to the Bs0!Dþsþ final

state is performed. Signal candidates within40 MeV=c2 of the nominalB0

s mass are selected, and from them the

invariant mass difference,M¼MðDþsþÞ MðDþsÞ

is formed, where bothþ combinations are included. The M distribution for candidates in the B0

s signal

window is shown in Fig. 7. A peak corresponding to theDs1ð2536Þþ is observed, whereas no significant struc-tures are observed in the upper B0s mass sideband

(5450–5590 MeV=c2). The distribution is fitted to the sum of a signal Breit-Wigner shape convolved with a Gaussian resolution function, and a second order polyno-mial to describe the background contribution. The Breit-Wigner width is set to0:92 MeV=c2[21], and the Gaussian resolution is fixed to 3:8 MeV=c2 based on simulation. A signal yield of 20:05:1 signal events is observed at a mass difference of 565:11:0 MeV=c2, which is con-sistent with the knownDs1ð2536ÞþDþs mass difference

of 566:630:35 MeV=c2 [21]. The significance of the signal is 5.9, obtained by fitting the invariant mass distribution with the mean mass difference fixed to

566:63 MeV=c2 [21], and computing ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2 lnðL0=Lmax

p

Þ. Here, Lmax andL0 are the fit likelihoods with the signal yields left free and fixed to zero, respectively. Several

] 2

c

Mass [MeV/

-π

+

π

-K

1000 2000 3000

)

2

c

Candidates / (200 MeV/

0 50

100 _{LHCb Data}(a)

Signal MC

] 2

c

Mass [MeV/ +

π

-π

0 1000 2000 3000

)

2

c

Candidates / (100 MeV/

0 20 40 60 80

(b)

] 2

c

Mass [MeV/ +

π

-K

0 1000 2000 3000

)

2

c

Candidates / (100 MeV/

0 50 100

(c)

FIG. 6 (color online). Invariant mass distributions for (a)Xs, (b)þinXsand (c) theKþinXs, fromB0!DþsKþdata usingsWeights. The points are data and the solid line is the simulation. The simulated distribution is normalized to have the same yield as the data.

LHCb Data Total shape

Signal shape

Comb bkg shape

sideband

0 s

B

)

2_c

Candidates / (4 MeV/ 5

10 15

]

2

c

) [MeV/

+ s

)-M(D

+

π

-π

+ s

M(D

500 550 600 650 700

FIG. 7. Distribution of the difference in invariant mass, MðDþsþÞ MðDþsÞ, using B0s !Dþsþ candidates within 40 MeV=c2 _{of the known B}0

s mass (points) and in the upper B0

variations in the background shape were investigated, and in all cases the signal significance exceeded 5.5. This decay is therefore observed for the first time. To obtain the yield in the normalization mode (B0

s !Dþsþ),

the signal function is integrated from 40 MeV=c2 _below to 40 MeV=c2 _{above the nominal B}0

s mass. A yield of

550585events is found in this restricted mass interval.

VII. SELECTION EFFICIENCIES

The ratios of branching fractions can be written as

Bð_B0

s !DþsKþÞ

Bð_B0

s !DþsþÞ

¼YðB0s !DþsKþÞ

YðB0

s !DþsþÞ

s

rel

(1)

and

Bð_B0 _!Dþ

sKþÞ

Bð_B0

s !DþsKþÞ

¼YðB0!DþsKþÞ

YðB0

s!DþsKþÞ

d

relfs=fd; (2)

where Y are the measured yields, s

rel¼ðB0s !

DþsþÞ=ðB0s !DsþKþÞanddrel¼ðB0s !

DþsKþÞÞ=ðB0!DþsKþÞ are the relative

selection efficiencies (including trigger), and fs=fd ¼

0:2670:021[27] is theB0

s fragmentation fraction

rela-tive toB0_{. The ratios of selection efficiencies are obtained} from simulation, except for the PID requirements, which are obtained from a dedicated Dþ calibration sample, weighted to match the momentum spectrum of the particles that form Xd and Xs. The selection efficiencies for

each decay are given in Table II. The efficiency of the

B0s !Dþsþ decay is about 35% larger than

the values obtained in either the B0s!DþsKþ or

B0 !DþsKþ decay; the efficiencies of the latter

two are consistent with each other. The lower efficiency is due almost entirely to the tighter PID requirements on the K andþ in Xs. Two additional multiplicative

cor-rection factors, also shown in TableII, are applied to the measured ratio of branching fractions in Eqs. (1) and (2). The first is a correction for theDþs mass veto onMðXd;sÞ,

and the second is due to the requirement that MðXs;dÞ<

3 GeV=c2_{. The former, which represents a small} correc-tion, is estimated from the sWeight-ed distributions of MðXd;sÞ shown previously. For the latter, the fraction of

events withMðXd;sÞ>3 GeV=c2 is obtained from

simu-lation and scaled by the ratio of yields in data relative to simulation for the mass region 2:6< MðXs;dÞ<

3:0 GeV=c2_{. A 50% uncertainty is assigned to the} esti-mated correction. Based on the qualitative agreement between data and simulation in theMðXd;sÞdistributions

(see Sec.V) and the fact that the phase space approaches zero as MðXd;sÞ !3:5 GeV=c2, this uncertainty is

conservative. The relative efficiency between B0

s!

Ds1ð2536Þþ,Dþs1!Dþsþ and B0s!Dþsþ

is estimated from simulation and is found to be

0:900:05.

VIII. SYSTEMATIC UNCERTAINTIES

Several uncertainties contribute to the ratio of branching fractions. The sources and their values are listed in Table III. The largest uncertainty, which applies only to the ratio BðB0!DþsKþÞ

Bð_B0

s!DþsKþÞ, is from the b hadronization

fraction, fs=fd¼0:2670:021 [27], which is 7.9%.

Another large uncertainty results from the required correc-tion factor to account for the signal with MðXs;dÞ>

3 GeV=c2_{. Those corrections are described in Sec.VII.} The selection efficiency depends slightly on the model-ing of theXd;sdecay. The momentum spectra of theB,Dþs,

Xd;sand theXd;sdaughters have been compared to

simu-lation, and excellent agreement is found. The selection efficiency is consistent with being flat as a function of MðXd;sÞat the level of two standard deviations or less. To

assess a potential systematic uncertainty due to a possible MðXd;sÞ-dependent efficiency, the relative differences

between the nominal selection efficiencies and the ones obtained by reweighting the measured efficiencies by the Xd;s mass spectra in data are computed. The relative

deviations of 0.5%, 1.1%, and 1.2% forB0s!DþsKþ,

Bs0!Dþsþ and B0!DþsKþ, respectively,

are the assigned uncertainties. The systematic uncertainty on the BDT efficiency is determined by fitting the B0s!

Dþsþ mass distribution in data with and without

the BDT requirement. The efficiency is found to agree with simulation to better than the 1% uncertainty assigned to this source. In total, the simulated efficiencies have uncertainties of 1.6 and 1.9% in the two ratios of branch-ing fractions. The PID efficiency uncertainty is dominated by the usage of the Dþcalibration sample to determine

TABLE II. Selection efficiencies and correction factors for decay modes under study. The uncertainties on the selection efficiencies are statistical only, whereas the correction factors show the total uncertainty.

Quantity B0

s !Dþsþ B0s !DþsKþ B0!DþsKþ Total(104_{) 4:970:08 3:670:10 3:590:10}

Dþs veto corrected 1:0130:003 1:0130:003 1:0170:005 M >3 GeV=c2_{corrected 1:020:01 1:040:02 1:140:07}

the efficiencies of a given PID requirement [28]. This uncertainty is assessed by comparing the PID efficiencies obtained directly from simulated signal decays with the values obtained using a simulated Dþ calibration sample that is re-weighted to match the kinematics of the signal decay particles. Using this technique, an un-certainty of 2% each on the B0s !DþsKþ and

B0 _!Dþ

sKþ PID efficiencies is obtained, which

is 100% correlated, and a 1% uncertainty for B0

s !

Dþsþ. The trigger is fully simulated, and given

the identical number of tracks and the well-modeledpT spectra, the associated uncertainty cancels to first order. Based on previous studies [12], a 2% uncertainty is assigned.

The uncertainties in the signal yield determinations have contributions from both the background and signal model-ing. The signal shape uncertainty was estimated by varying all the fixed signal shape parameters one at a time by one standard deviation, and adding the changes in yield in quadrature (0.5%). A double Gaussian signal shape model was also tried, and the difference was negligible. For the combinatorial background, the shape was modified from a single exponential to either the sum of two exponentials, or a linear function. ForB0

s!Dþsþ, the difference in

yield was 0.4%. For B0

s !DþsKþ, the maximum

change was 4%, and for B0 !DþsKþ, the

maxi-mum shift was 1%. In theB0_ðsÞ!DþsKþ mass fit,

theB0

ðsÞ!Dþs Kþcontribution was modeled using

the shape from the B0s!Dþsþ mass fit. To

esti-mate an uncertainty from this assumption, the data were fitted with the shape obtained from B0

s !Dþs Kþ

simulation. A deviation of 5.5% in the fitted B0 _! DþsKþ yield is found, with almost no change in

the B0s !DþsKþ yield. The larger sensitivity on

the B0 yield than theB0s yield arises because these

back-ground contributions have a rising edge in the vicinity of theB0 _{mass peak, which is far enough below theB}0

smass

peak to have negligible impact. These yield uncertainties are added in quadrature to obtain the values shown in Table III. The uncertainties due to the finite simulation sample sizes are 3.0%.

The major source of systematic uncertainty on the branch-ing fraction forB0

s!Ds1ð2536Þþ,Dþs1 !Dþsþ, is

from the relative efficiency (5%), and on the fraction of events with M >3 GeV=c2 (10%). This 10% uncer-tainty is conservatively estimated by assuming a flat distribution in MðXdÞ up to 3 GeV=c2 and then a linear

decrease to zero at the phase space limit of

3:5 GeV=c2_{. Other systematic uncertainties related to} the fit model are negligible. Thus in total, a systematic uncertainty of 11% is assigned to the ratio BðB0

s!

Ds1ð2536Þþ;Dþs1!DþsþÞ=BðB0s!DþsþÞ.

IX. RESULTS AND SUMMARY

This paper reports the first observation of the

B0s!DþsKþ, B0 !DþsKþ and B0s!

Ds1ð2536Þþ, Dþs1!Dþsþ decays. The ratios of

branching fractions are measured to be

Bð_B0

s !DþsKþÞ

Bð_B0

s !DþsþÞ

¼ ð5:20:50:3Þ 102

Bð_B0 _!Dþ

sKþÞ

Bð_B0

s !DþsKþÞ

¼0:540:070:07

and

Bð_B0

s!Ds1ð2536Þþ; Dþ_s1 !DþsþÞ

Bð_B0

s!DþsþÞ

¼ ð4:01:00:4Þ 103_;

where the uncertainties are statistical and systematic, respectively. The B0

s !DþsKþ branching fraction

is consistent with expectations from Cabibbo suppression. This decay is particularly interesting because it can be used in a time-dependent analysis to measure the CKM phase. Additional studies indicate that this decay mode, with selections optimized for only B0

s!DþsKþ, can

contribute about an additional 35% more signal events relative to the signal yield inB0

s!DsK alone.

TheB0_!Dþ

sKþbranching fraction is about 50%

of that for B0s !DþsKþ. Compared to the B0!

DþsKdecay that proceeds only via aW-exchange diagram,

where BðB0 !DsþKÞ=BðB0s!DþsKÞ 0:1 [21], the

ratio BðB0 !DsþKþÞ=BðB0s !DsþKþÞ is

about five times larger. A consistent explanation of this larger B0 _!Dþ

sKþ branching fraction is that

only about1=5of the rate is from theW-exchange process [Fig. 1(d)] and about4=5comes from the diagrams shown in Figs. 1(e) and 1(f). The observed MðXsÞ, MðKþÞ

and MðþÞ distributions in Fig. 6 also support this explanation, as evidenced by the qualitative agreement with the simulation.

ACKNOWLEDGMENTS

We express our gratitude to our colleagues in the CERN accelerator departments for the excellent performance of

TABLE III. Summary of systematic uncertainties (in %) on the measurements of the ratios of branching fractions.

Source Bð

B0

s!DþsKþÞ

Bð_B0

s!DþsþÞ

Bð_B0_!

DþsKþÞ

Bð_B0

s!DþsKþÞ

fs=fd 7.9

MðXs;dÞ>3 GeV=c2 2.2 7.0

Efficiency 1.6 1.9

PID 2.2 0.0

Trigger 2.0 2.0

Signal yields 4.0 6.9

Simulated sample size 3.0 3.0

the LHC. We thank the technical and administrative staff at the LHCb institutes. We acknowledge support from CERN and from the following 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); and the NSF (USA). We also acknowledge the support received from the ERC under FP7. The Tier1 computing centers are supported by IN2P3 (France), KIT and BMBF (Germany), INFN (Italy), NWO and SURF (The Netherlands), CIEMAT, IFAE and UAB (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.

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