DSpace at VNU: First observation of the decays B̄(s)0→Ds+K -π+π- and B̄s0→D s1(2536)+π-

DSpace at VNU: First observation of the decays B̄(s)0→Ds+K -π+π- and B̄s0→D s1(2536)+π- tài liệu, giáo án, bài giảng , l...

First observation of the decays
B0ðsÞ ! Dþ

s K

þ

and
B0s ! Ds1ð2536Þþ

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

PHYSICAL REVIEW D 86, 112005 (2012)

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

2Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil

3Center for High Energy Physics, Tsinghua University, Beijing, China

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

5Clermont Universite´, Universite´ Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France

6CPPM, Aix-Marseille Universite´, CNRS/IN2P3, Marseille, France

7LAL, Universite´ Paris-Sud, CNRS/IN2P3, Orsay, France

8

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

9Fakulta¨t Physik, Technische Universita¨t Dortmund, Dortmund, Germany

10Max-Planck-Institut fu¨r Kernphysik (MPIK), Heidelberg, Germany

11Physikalisches Institut, Ruprecht-Karls-Universita¨t Heidelberg, Heidelberg, Germany

12School of Physics, University College Dublin, Dublin, Ireland

13Sezione INFN di Bari, Bari, Italy

14Sezione INFN di Bologna, Bologna, Italy

15Sezione INFN di Cagliari, Cagliari, Italy

16Sezione INFN di Ferrara, Ferrara, Italy

17Sezione INFN di Firenze, Firenze, Italy

18Laboratori Nazionali dell’INFN di Frascati, Frascati, Italy

19Sezione INFN di Genova, Genova, Italy

20Sezione INFN di Milano Bicocca, Milano, Italy

21Sezione INFN di Roma Tor Vergata, Roma, Italy

22Sezione INFN di Roma La Sapienza, Roma, Italy

23Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Krako´w, Poland

24AGH University of Science and Technology, Krako´w, Poland

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

26Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania

27Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia

28Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia

29

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

30Institute for Nuclear Research of the Russian Academy of Sciences (INR RAN), Moscow, Russia

31Budker Institute of Nuclear Physics (SB RAS) and Novosibirsk State University, Novosibirsk, Russia

32Institute for High Energy Physics (IHEP), Protvino, Russia

33Universitat de Barcelona, Barcelona, Spain

34Universidad de Santiago de Compostela, Santiago de Compostela, Spain

35European Organization for Nuclear Research (CERN), Geneva, Switzerland

36Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), Lausanne, Switzerland

37Physik-Institut, Universita¨t Zu¨rich, Zu¨rich, Switzerland

38Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands

39Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, The Netherlands

40NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine

41Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine

42University of Birmingham, Birmingham, United Kingdom

43H.H Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom

44Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom

45

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

46STFC Rutherford Appleton Laboratory, Didcot, United Kingdom

47School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom

48School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom

49Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom

50Imperial College London, London, United Kingdom

51School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom

52Department of Physics, University of Oxford, Oxford, United Kingdom

53Syracuse University, Syracuse, New York, USA

54Pontifı´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

55Institut 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 decays
B0

s ! Dþ

sK

þ

and
B0! Dþ

sK

þ

are reported using an integrated luminosity of 1:0 fb
1recorded by the LHCb experiment The branching fractions, normalized

with respect to B
0

s! Dþ

s


þ

and B
0

s ! Dþ

sK

þ

, respectively, are measured to be

Bð
B 0

s !D þ

s K

þ

Þ

Bð
B 0

s !D þ

s


þ

Þ¼ ð5:2  0:5  0:3Þ  10
2 andBð
B 0 !D þ

s K

þ

Þ

Bð
B 0

s !D þ

s K

þ

Þ¼ 0:54  0:07  0:07, where the first

aFull author list given at end of the article

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

cUniversita` di Bari, Bari, Italy

dUniversita` di Bologna, Bologna, Italy

eUniversita` di Cagliari, Cagliari, Italy

fUniversita` di Ferrara, Ferrara, Italy

gUniversita` di Firenze, Firenze, Italy

hUniversita` di Urbino, Urbino, Italy

iUniversita` di Modena e Reggio Emilia, Modena, Italy

jUniversita` di Genova, Genova, Italy

kUniversita` di Milano Bicocca, Milano, Italy

lUniversita` di Roma Tor Vergata, Roma, Italy

mUniversita` di Roma La Sapienza, Roma, Italy

nUniversita` della Basilicata, Potenza, Italy

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

pHanoi University of Science, Hanoi, Viet Nam

qMassachusetts Institute of Technology, Cambridge, Massachusetts, USA

uncertainty is statistical and the second is systematic The
B0

s ! Dþ

sK

þ

decay is of particular interest as it can be used to measure the weak phase  First observation of the
B0

s! Ds1ð2536Þþ

,

Dþs1! Dþ

s


þ decay is also presented, and its branching fraction relative to
B0

s ! Dþ

s


þ

is found to beBð
B0s !D s1 ð2536Þ þ

;Dþs1!D þ

s


þÞ

Bð
B 0

s !D þ

s


þ

Þ ¼ ð4:0  1:0  0:4Þ  10
3.

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 and CP-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 phase   argð
Vub V ud

VcbV cdÞ, 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 as B0s ! D

sK [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
!

DK

þ

[12] for a time-independent measurement, or

B0

s ! Dþ

sK

þ

in the case of a time-dependent

analysis

The
B0! Dþ

sK

þ

decay, while having the same

final state as
B0

s ! Dþ

sK

þ

, receives contributions not only from the W-exchange process [Fig.1(d)], but also

from b ! c transitions in association with the production

of an extra s
s pair [Figs.1(e)and1(f )] The decay may also

proceed through mixing followed by a b ! u, W-exchange

process (not shown) However, this amplitude is Cabibbo-,

helicity- and color-suppressed and is therefore negligible

compared to the b ! c amplitude

This paper reports the first observation of B
0

s !

DþsK

þ

and
B0 ! Dþ

sK

þ

and measurements

of their branching fractions relative to
B0

s ! Dþ

s


þ

and
B0

s ! Dþ

sK

þ

, respectively The data sample is

based on an integrated luminosity of 1:0 fb
1of pp

colli-sions at ffiffiffi

s

p

¼ 7 TeV, collected by the LHCb experiment

The same data sample is also used to observe the

B0

s! Ds1ð2536Þþ

, Dþs1! Dþ

s


þ decay for the first time and measure its branching fraction relative to

B0

s! Dþ

s


þ

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 containing b or c quarks The detector includes a high precision tracking system consisting of a silicon-strip vertex detector surrounding the pp interaction 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% at 5 GeV=c to 0.6% at 100 GeV=c, and

an impact parameter (IP) resolution of 20 m 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

)

-π

+

π

( -K

(a)

b s

u s ub

V

c s

0 B

+ D )

-π

+

π

( -K

(b)

b

s

+ W

c

u

s

s D

)

-π

+

π

( -K s

0 B

cb V

us V

(c)

b

d

+ W

c

u

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

u g

(e)

b

d

c

d

s s

s D -K 0

ρ

0

d

g cb V

ud V (f)

FIG 1 (color online) Diagrams contributing to the B0

s,
B0

s !

DþsK

þ

(a–c) and
B0

s ! Dþ

sK

þ

(d–f ) decays, as described in the text In (a)–(d), the additional (
þ

) indicates that the K

þ

may be produced either through an excited strange kaon resonance decay, or through fragmentation

Published by the American Physical Society under the terms of

the Creative Commons Attribution 3.0 License Further

distri-bution of this work must maintain attridistri-bution to the author(s) and

the published article’s title, journal citation, and DOI

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 high pT sum of the

tracks and a significant displacement from the primary pp

interaction vertices (PVs) At least one track should have

pT> 1:7 GeV=c, an IP 2 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
B0ðsÞ decay candidates are formed by pairing

a Dþs ! KþK

þ candidate with either a


þ

(hereafter referred to as Xd) or a K

þ

combination

(hereafter referred to as Xs) Tracks used to form the Dþs and

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

using information from the ring-imaging Cherenkov

detec-tors, have pT in excess of 100 MeV=c and be significantly

detached from any reconstructed PV in the event

Signal Dþs candidates are required to have good vertex

fit quality, be significantly displaced from the nearest

PV and have invariant mass, MðKþK

þÞ, within

20 MeV=c2of the Dþs mass [21] To suppress

combinato-rial and charmless backgrounds, only those Dþs candidates

that are consistent with decaying through either the

 (MðKþK
Þ < 1040 MeV=c2) or
K0 (jMðK

þÞ

mK0j < 75 MeV=c2) resonances are used (here, mK0 is

the K0mass [21]) The remaining charmless background

yields are determined using the Dþs mass sidebands For

about 20% of candidates, when the Kþis assumed to be a

þ, the corresponding K

þ
þinvariant mass is

consis-tent with the Dþ mass To suppress cross feed from

B0 ! DþX decays, a tighter particle identification (PID)

requirement is applied to the Kþ in the Dþs ! KþK

þ

candidates when jMðK

þ
þÞ
mDþj < 20 MeV=c2

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

mass of the particles forming the Dþs candidate, after

replacing the Kþ mass with the proton mass, falls within

15 MeV=c2 of the þc 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

K

þ

combinations, where all invariant mass values

up to 3 GeV=c2 are accepted To reduce the level of combinatorial background, we demand that the Xd;svertex

is displaced from the nearest PV by more than 100 m in the direction transverse to the beam axis and that at least two of the daughter tracks have pT> 300 MeV=c Backgrounds to the
B0ðsÞ! Dþ

sK

þ

search from

B0

s! DðÞþs


þ

or
B0

s ! Dþ

sK
Kþ

decays are suppressed by applying more stringent PID requirements

to the K
and
þ 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! Dþ

sD
s decays, where Dþs !
þ


þ, Kþ


þ, it is required that MðXd;sÞ is more than 20 MeV=c2away from the Dþs mass Signal
B meson 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

sK

þ

, the vertex separation 2 between the Dþs and
B decay vertices is required to be greater than 9 Candidates passing all selection requirements are refit with both Dþs 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 simulated
B0s ! Dþ

sK

þ

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: IP 2, vertex separation 2, vertex fit

2, and pT; (ii) Dþs candidate: Flight distance significance from
B vertex;

(iii) Xd;scandidate: IP 2, maximum of the distances of closest approach between any pair of tracks in the decay;

(iv) Xd;s daughters: minðIP 2Þ, maxðIP 2Þ, minðpTÞ; and

(v) Dþs daughters: minðIP 2Þ, maxðIP 2Þ, 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 þ BðxcutÞ, 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 B
0

s ! Dþ

s


þ

and B
0

ðsÞ! Dþ

sK

þ

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 the
B0

ðsÞ! Dþ

sK

þ

mass fit, the difference between the mean
B0

sand
B0masses is fixed

to 87:35 MeV=c2 [21]

Several nonsignal b-hadron decays produce broad

peak-ing structures in the Dþs


þ

and DþsK

þ

invariant mass spectra For B
0

s! Dþ

s


þ

, the only significant source of peaking background is from

B0

s ! Dþ

s


þ

, where the photon or
0 from the

Dþs decay is not included in the reconstructed decay

Since the full decay amplitude for
B0s! Dþ

s


þ

is 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 B
0ðsÞ ! Dþ

sK

þ

, backgrounds from B
0ðsÞ!

Dþs K

þ

and from misidentified
B0

s!Dþ

s


þ

and B
0s ! Dþ

s


þ

decays are considered The

B0ðsÞ! Dþ

s K

þ

shape is fixed to be the same as

that obtained for the
B0

s! Dþ

s


þ

component in the
B0

s ! Dþ

s


þ

mass fit This same shape is

assumed for both
B0 and
B0

s, where for the former, a shift

by the
B0

B0

s mass difference is included For the

B0

s ! Dþ

s


þ

and
B0

s ! Dþ

s


þ

cross feed, simulated decays and kaon misidentification rates taken

from Dþcalibration data are used to obtain their expected

yields and invariant mass shapes The cross-feed

contribu-tion is about 3% of the
B0s! Dþ

s


þ

and
B0s !

Dþs


þ

yields; the corresponding cross-feed yields

are fixed in the
B0ðsÞ! Dþ

sK

þ

fit The shape is obtained by parametrizing the invariant mass spectrum

obtained from the simulation after replacing the

appropri-ate

mass in Xdwith 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! Dþ

s


þ

candidates passing all selection crite-ria The fitted number of
B0

s ! Dþ

s


þ

signal events

is 5683 83 While it is expected that most of the low mass background emanates from B
0

s ! Dþ

s


þ

decays, contributions from other sources such as

B0

s! Dþ

s


þ


0 are also possibly absorbed into this background component Figure3shows the invariant mass distribution for
B0

ðsÞ! Dþ

sK

þ

candidates The fitted signal yields are 402 33
B0 ! Dþ

sK

þ

and

216 21
B0

s! Dþ

sK

þ

events

]

2

c

mass [MeV/

-π

+

π

-π

+

D

2c

500 1000

Full PDF Signal PDF

X Bkg

+

D

→

0

B Comb Bkg

FIG 2 Invariant mass distribution for B
0

s ! Dþ

s


þ

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

+

D

Candidates / (10 MeV/ 50

100

150

LHCb Data Full PDF Signal PDFs

X Bkg + s D

→

0 B

X Bkg + D

→

0 B

Bkg

πππ

(*) s D

→

0 B Comb Bkg

FIG 3 Invariant mass distribution for
B0

ðsÞ! Dþ

sK

þ

candidates The fitted signal (dashed lines) and background shapes (shaded/hatched regions) are shown, as described in the text

The Dþs mass sidebands, defined to be from 35 to

55 MeV=c2 on either side of the nominal Dþs mass, are

used to estimate the residual charmless background that

may contribute to the observed signals The numbers of B0

s

decays in the Dþs sidebands are 61 16, 0þ5

0, and 9 5 for the
B0s ! Dþ

s


þ

,
B0s ! Dþ

sK

þ

and
B0 !

DþsK

þ

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 these
B0

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) the


þ

mass, (b) the smaller
þ

mass and (c) the larger
þ

mass in
B0s ! Dþ

s


þ

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

a1ð1260Þ
resonance 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 the pTspectra of the

] 2

c

Mass [MeV/

-π

+

π

-π

0

500

1000

1500

(a) LHCb Data Signal MC

] 2

c

Mass [MeV/

+

π

-π

Smaller

0 500 1000

1500

(b)

] 2

c

Mass [MeV/

+

π

-π

Larger

0 500 1000

1500

(c)

FIG 4 (color online) Invariant mass distributions for (a) Xd, (b) smaller
þ

mass in Xd and (c) the larger
þ

mass in Xd, from
B0

s ! Dþ

s


þ

decays using sWeights 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 the Dþs signal

and sidebands regions and the background corrected yield

The signal and sideband regions require Dþs candidates to

have invariant mass jMðKþK

þÞ
mDþ

sj < 20 MeV=c2 and

35 < jMðKþK

þÞ
mDþ

sj < 55 MeV=c2, respectively, where

mDþs is the Dþs mass [21]

Decay

Signal Region

Sideband Region Corrected Yield

B0

s ! Dþ

s


þ

5683 83 61 16 5622 85

B0

s ! Dþ

sK

þ

216 21 0þ5 216 22

B0! Dþ

sK

þ

402 33 9 5 393 33

] 2

c

Mass [MeV/

-π

+

π

-K

0

50

LHCb Data Signal MC

] 2

c

Mass [MeV/

+

π

-π

0 20 40

60

(b)

] 2

c

Mass [MeV/

+

π

-K

0 50

FIG 5 (color online) Invariant mass distributions for (a) Xs, (b)
þ

in Xsand (c) the K

þin Xs, from
B0

s ! Dþ

sK

þ

data using sWeights 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 ! Dþ

sK

þ

decay A peaked structure at low

K

þ

mass, consistent with contributions from

the lower-lying excited strange mesons, such as the

K1ð1270Þ
and K1ð1400Þ
, is observed As many of these

states decay through
K0 and 0 mesons, significant

con-tributions from these resonances are observed in the K

þ

and
þ

invariant mass spectra, respectively The

simu-lation provides a reasonable description of the distributions

in the data

Figure 6 shows the same distributions for B
0 !

DþsK

þ

The K

þ

invariant mass is quite

broad, with little indication of any narrow structures

There are indications of
K0 and 0 contributions in the

K

þand


þinvariant mass spectra, respectively, but

the contribution from resonances such as the K1ð1270Þ
or

K1ð1400Þ
appear to be small or absent In the K

þ

invariant mass spectrum, there may be an indication of a

K0ð1430Þ0 contribution The simulation, which models

the K

þ

final state as 10% K1ð1270Þ
, 10%

K1ð1400Þ
, 40% K
0

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

OF
B0s ! Ds1ð2536Þþ

A search for excited Dþs states, such as DþsJ !

Dþs


þ, contributing to the
B0

s! Dþ

s


þ

final state is performed Signal candidates within40 MeV=c2

of the nominal B0

s mass are selected, and from them the invariant mass difference, M ¼ MðDþs


þÞ
MðDþ

sÞ

is formed, where both
þ

combinations are included

The M distribution for candidates in the
B0

s signal window is shown in Fig 7 A peak corresponding to

the Ds1ð2536Þþ is observed, whereas no significant

struc-tures are observed in the upper B
0s 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 to 0:92 MeV=c2[21], and the Gaussian resolution is fixed to 3:8 MeV=c2 based on simulation

A signal yield of 20:0  5:1 signal events is observed at

a mass difference of 565:1  1:0 MeV=c2, which is con-sistent with the known Ds1ð2536Þþ
Dþ

s mass difference

of 566:63  0:35 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 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

2 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

0

50

LHCb Data Signal MC

] 2

c

Mass [MeV/

+

π

-π

0 20 40 60 80

(b)

] 2

c

Mass [MeV/

+

π

-K

0 50

100

(c)

FIG 6 (color online) Invariant mass distributions for (a) Xs, (b)
þ

in Xsand (c) the K

þin Xs, from
B0! Dþ

sK

þ

data using sWeights 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

B

2c

Candidates / (4 MeV/ 5 10 15

]

2

c

) [MeV/

+ s

)-M(D

+

π

-π

+ s

M(D

FIG 7 Distribution of the difference in invariant mass, MðDþs


þÞ
MðDþ

sÞ, using
B0

s ! Dþ

s


þ

candidates within 40 MeV=c2 of the known B0

s mass (points) and in the upper B0

s mass sidebands (filled histogram) The fit to the distribution is shown, as described in the text

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 ! Dþ

s


þ

), the signal function is integrated from 40 MeV=c2 below

to 40 MeV=c2 above the nominal B0

s mass A yield of

5505 85 events is found in this restricted mass interval

VII SELECTION EFFICIENCIES

The ratios of branching fractions can be written as

Bð
B0

s ! Dþ

sK

þ

Þ

Bð
B0

s ! Dþ

s


þ

Þ ¼

Yð
B0s ! Dþ

sK

þ

Þ

Yð
B0

s ! Dþ

s


þ

Þ srel

(1) and

Bð
B0 ! Dþ

sK

þ

Þ

Bð
B0

s ! Dþ

sK

þ

Þ

¼Yð
B0! DþsK

þ

Þ

Yð
B0

s! Dþ

sK

þ

Þ drel fs=fd; (2) where Y are the measured yields, s

rel¼ ð
B0

s !

Dþs


þ

Þ=ð
B0

s ! Dþ

sK

þ

Þ and d

rel¼ ð
B0

s !

DþsK

þ

ÞÞ=ð
B0! Dþ

sK

þ

Þ are the relative selection efficiencies (including trigger), and fs=fd ¼

0:267  0:021 [27] is the B0

s fragmentation fraction rela-tive to B0 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 ! Dþ

s


þ

decay is about 35% larger than

the values obtained in either the
B0s! Dþ

sK

þ

or

B0 ! Dþ

sK

þ

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
and
þ 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 the Dþs mass veto on MðXd;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 with MðXd;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 the MðXd;sÞ distributions (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!Dþ

s


þ and
B0

s! Dþ

s


þ

is estimated from simulation and is found to be 0:90  0:05

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 Bð
B0!Dþs K

þ

Þ

Bð
B 0

s !D þ

s K

þ

Þ, is from the b hadronization fraction, fs=fd¼ 0:267  0:021 [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 the Xd;sdecay The momentum spectra of the
B, Dþs,

Xd;sand the Xd;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;sÞ at 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% for
B0s!Dþ

sK

þ

,

B0s!Dþ

s


þ

and
B0!Dþ

sK

þ

, respectively, are the assigned uncertainties The systematic uncertainty

on the BDT efficiency is determined by fitting the
B0s!

Dþs


þ

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

s ! Dþ

s


þ

B
0

s ! Dþ

sK

þ

B
0! Dþ

sK

þ

Dþs veto corrected 1:013  0:003 1:013  0:003 1:017  0:005

M > 3 GeV=c2corrected 1:02  0:01 1:04  0:02 1:14  0:07

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 ! Dþ

sK

þ

and

B0 ! Dþ

sK

þ

PID efficiencies is obtained, which

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

s !

Dþs


þ

The trigger is fully simulated, and given

the identical number of tracks and the well-modeled pT

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 For
B0

s! Dþ

s


þ

, the difference in yield was 0.4% For
B0

s ! Dþ

sK

þ

, the maximum change was 4%, and for
B0 ! Dþ

sK

þ

, the maxi-mum shift was 1% In the
B0ðsÞ! Dþ

sK

þ

mass fit, the
B0

ðsÞ! Dþ

s K

þ

contribution was modeled using

the shape from the
B0s! Dþ

s


þ

mass fit To esti-mate an uncertainty from this assumption, the data were

fitted with the shape obtained from
B0

s ! Dþ

s K

þ

simulation A deviation of 5.5% in the fitted B
0 !

DþsK

þ

yield is found, with almost no change in

the
B0s ! Dþ

sK

þ

yield The larger sensitivity on

the
B0 yield than the
B0s yield arises because these

back-ground contributions have a rising edge in the vicinity of

the
B0 mass peak, which is far enough below the
B0

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 for
B0

s! Ds1ð2536Þþ

, Dþs1! Dþ

s


þ, 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 Bð
B0

s!

Ds1ð2536Þþ

;Dþs1!Dþ

s


þÞ=Bð
B0

s!Dþ

s


þ

Þ

IX RESULTS AND SUMMARY This paper reports the first observation of the

B0s! Dþ

sK

þ

, B
0 ! Dþ

sK

þ

and B
0s!

Ds1ð2536Þþ

, Dþs1! Dþ

s


þ decays The ratios of branching fractions are measured to be

Bð
B0

s ! Dþ

sK

þ

Þ

Bð
B0

s ! Dþ

s


þ

Þ¼ ð5:2  0:5  0:3Þ  10
2

Bð
B0 ! Dþ

sK

þ

Þ

Bð
B0

s ! Dþ

sK

þ

Þ¼ 0:54  0:07  0:07 and

Bð
B0

s! Ds1ð2536Þþ

; Dþs1 ! Dþ

s


þÞ

Bð
B0

s! Dþ

s


þ

Þ

¼ ð4:0  1:0  0:4Þ  10
3; where the uncertainties are statistical and systematic, respectively The
B0

s ! Dþ

sK

þ

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! Dþ

sK

þ

, can contribute about an additional 35% more signal events relative to the signal yield in B0

s! D

sK alone

The
B0! Dþ

sK

þ

branching fraction is about 50%

of that for
B0s ! Dþ

sK

þ

Compared to the
B0!

DþsK
decay that proceeds only via a W-exchange diagram, where Bð
B0 ! Dþ

sK
Þ=Bð
B0

s! Dþ

sK
Þ  0:1 [21], the ratio Bð
B0 ! Dþ

sK

þ

Þ=Bð
B0

s ! Dþ

sK

þ

Þ is about five times larger A consistent explanation of this larger B
0 ! Dþ

sK

þ

branching fraction is that only about 1=5 of the rate is from the W-exchange process [Fig 1(d)] and about 4=5 comes from the diagrams shown

in Figs 1(e) and 1(f) The observed MðXsÞ, MðK

þÞ and Mð
þ

Þ 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

0

s !D þ

s K

þ

Þ

Bð
B 0

s !D þ

s


þ

Þ Bð
B

0 !D þ

s K

þ

Þ

Bð
B 0

s !D þ

s K

þ

Þ