Zvyagin35 LHCb Collaboration 1Centro Brasileiro de Pesquisas Fı´sicas CBPF, Rio de Janeiro, Brazil 2Universidade Federal do Rio de Janeiro UFRJ, Rio de Janeiro, Brazil 3 Center for High
Trang 1Measurements of the branching fractions and CP asymmetries of B ! J= c
and B ! c ð2SÞdecays
R Aaij,38C Abellan Beteta,33,nB Adeva,34M Adinolfi,43C Adrover,6A Affolder,49Z Ajaltouni,5J Albrecht,35
F Alessio,35M Alexander,48S Ali,38G Alkhazov,27P Alvarez Cartelle,34A A Alves, Jr.,22S Amato,2Y Amhis,36
J Anderson,37R B Appleby,51O Aquines Gutierrez,10F Archilli,18,35A Artamonov,32M Artuso,53,35E Aslanides,6
G Auriemma,22,mS Bachmann,11J J Back,45V Balagura,28,35W Baldini,16R J Barlow,51C Barschel,35S Barsuk,7
W Barter,44A Bates,48C Bauer,10Th Bauer,38A Bay,36I Bediaga,1S Belogurov,28K Belous,32I Belyaev,28
E Ben-Haim,8M Benayoun,8G Bencivenni,18S Benson,47J Benton,43R Bernet,37M.-O Bettler,17
M van Beuzekom,38A Bien,11S Bifani,12T Bird,51A Bizzeti,17,hP M Bjørnstad,51T Blake,35F Blanc,36C Blanks,50
J Blouw,11S Blusk,53A Bobrov,31V Bocci,22A Bondar,31N Bondar,27W Bonivento,15S Borghi,48,51A Borgia,53
T J V Bowcock,49C Bozzi,16T Brambach,9J van den Brand,39J Bressieux,36D Brett,51M Britsch,10T Britton,53
N H Brook,43H Brown,49A Bu¨chler-Germann,37I Burducea,26A Bursche,37J Buytaert,35S Cadeddu,15O Callot,7
M Calvi,20,jM Calvo Gomez,33,nA Camboni,33P Campana,18,35A Carbone,14G Carboni,21,kR Cardinale,19,35,i
A Cardini,15L Carson,50K Carvalho Akiba,2G Casse,49M Cattaneo,35Ch Cauet,9M Charles,52Ph Charpentier,35
N Chiapolini,37K Ciba,35X Cid Vidal,34G Ciezarek,50P E L L Clarke,47,35M Clemencic,35H V Cliff,44J Closier,35
C Coca,26V Coco,38J Cogan,6P Collins,35A Comerma-Montells,33A Contu,52A Cook,43M Coombes,43G Corti,35
B Couturier,35G A Cowan,36R Currie,47C D’Ambrosio,35P David,8P N Y David,38I De Bonis,4K de Bruyn,38
S De Capua,21,kM De Cian,37J M De Miranda,1L De Paula,2P De Simone,18D Decamp,4M Deckenhoff,9
H Degaudenzi,36,35L Del Buono,8C Deplano,15D Derkach,14,35O Deschamps,5F Dettori,39J Dickens,44
H Dijkstra,35P Diniz Batista,1F Domingo Bonal,33,nS Donleavy,49F Dordei,11A Dosil Sua´rez,34D Dossett,45
A Dovbnya,40F Dupertuis,36R Dzhelyadin,32A Dziurda,23S Easo,46U Egede,50V Egorychev,28S Eidelman,31
D van Eijk,38F Eisele,11S Eisenhardt,47R Ekelhof,9L Eklund,48Ch Elsasser,37D Elsby,42D Esperante Pereira,34
A Falabella,16,14,eC Fa¨rber,11G Fardell,47C Farinelli,38S Farry,12V Fave,36V Fernandez Albor,34M Ferro-Luzzi,35
S Filippov,30C Fitzpatrick,47M Fontana,10F Fontanelli,19,iR Forty,35O Francisco,2M Frank,35C Frei,35
M Frosini,17,fS Furcas,20A Gallas Torreira,34D Galli,14,cM Gandelman,2P Gandini,52Y Gao,3J-C Garnier,35
J Garofoli,53J Garra Tico,44L Garrido,33D Gascon,33C Gaspar,35R Gauld,52N Gauvin,36M Gersabeck,35
T Gershon,45,35Ph Ghez,4V Gibson,44V V Gligorov,35C Go¨bel,54D Golubkov,28A Golutvin,50,28,35A Gomes,2
H Gordon,52M Grabalosa Ga´ndara,33R Graciani Diaz,33L A Granado Cardoso,35E Grauge´s,33G Graziani,17
A Grecu,26E Greening,52S Gregson,44B Gui,53E Gushchin,30Yu Guz,32T Gys,35C Hadjivasiliou,53G Haefeli,36
C Haen,35S C Haines,44T Hampson,43S Hansmann-Menzemer,11R Harji,50N Harnew,52J Harrison,51
P F Harrison,45T Hartmann,55J He,7V Heijne,38K Hennessy,49P Henrard,5J A Hernando Morata,34
E van Herwijnen,35E Hicks,49K Holubyev,11P Hopchev,4W Hulsbergen,38P Hunt,52T Huse,49R S Huston,12
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,9J Keaveney,12I R Kenyon,42U Kerzel,35T Ketel,39A Keune,36
B Khanji,6Y M Kim,47M Knecht,36R F Koopman,39P Koppenburg,38M Korolev,29A Kozlinskiy,38L Kravchuk,30
K Kreplin,11M Kreps,45G Krocker,11P Krokovny,11F Kruse,9K Kruzelecki,35M Kucharczyk,20,23,35,j
V Kudryavtsev,31T Kvaratskheliya,28,35V N La Thi,36D Lacarrere,35G Lafferty,51A Lai,15D Lambert,47
R W Lambert,39E Lanciotti,35G Lanfranchi,18C Langenbruch,11T Latham,45C Lazzeroni,42R Le Gac,6
J van Leerdam,38J.-P Lees,4R Lefe`vre,5A Leflat,29,35J Lefranc¸ois,7O Leroy,6T Lesiak,23L Li,3L Li Gioi,5
M Lieng,9M Liles,49R Lindner,35C Linn,11B Liu,3G Liu,35J von Loeben,20J H Lopes,2E Lopez Asamar,33
N Lopez-March,36H Lu,3J Luisier,36A Mac Raighne,48F Machefert,7I V Machikhiliyan,4,28F Maciuc,10
O Maev,27,35J Magnin,1S Malde,52R M D Mamunur,35G Manca,15,dG Mancinelli,6N Mangiafave,44U Marconi,14
R Ma¨rki,36J Marks,11G Martellotti,22A Martens,8L Martin,52A Martı´n Sa´nchez,7M Martinelli,38
D Martinez Santos,35A Massafferri,1Z Mathe,12C Matteuzzi,20M Matveev,27E Maurice,6B Maynard,53
A Mazurov,16,30,35G McGregor,51R McNulty,12M Meissner,11M Merk,38J Merkel,9S Miglioranzi,35
D A Milanes,13M.-N Minard,4J Molina Rodriguez,54S Monteil,5D Moran,12P Morawski,23R Mountain,53
I Mous,38F Muheim,47K Mu¨ller,37R Muresan,26B Muryn,24B Muster,36J Mylroie-Smith,49P Naik,43T Nakada,36
R Nandakumar,46I Nasteva,1M Needham,47N Neufeld,35A D Nguyen,36C Nguyen-Mau,36,oM Nicol,7V Niess,5
N Nikitin,29T Nikodem,11A Nomerotski,52,35A Novoselov,32A Oblakowska-Mucha,24V Obraztsov,32S Oggero,38
PHYSICAL REVIEW D 85, 091105(R) (2012)
Trang 2S Ogilvy,48O Okhrimenko,41R Oldeman,15,35,dM Orlandea,26J M Otalora Goicochea,2P Owen,50B K Pal,53
J Palacios,37A Palano,13,bM Palutan,18J Panman,35A Papanestis,46M Pappagallo,48C Parkes,51C J Parkinson,50
G Passaleva,17G D Patel,49M Patel,50S K Paterson,50G N Patrick,46C Patrignani,19,iC Pavel-Nicorescu,26
A Pazos Alvarez,34A Pellegrino,38G Penso,22,lM Pepe Altarelli,35S Perazzini,14,cD L Perego,20,jE Perez Trigo,34
A Pe´rez-Calero Yzquierdo,33P Perret,5M Perrin-Terrin,6G Pessina,20A Petrolini,19,iA Phan,53E Picatoste Olloqui,33
B Pie Valls,33B Pietrzyk,4T Pilarˇ,45D Pinci,22R Plackett,48S Playfer,47M Plo Casasus,34G Polok,23A Poluektov,45,31
E Polycarpo,2D Popov,10B Popovici,26C Potterat,33A Powell,52J Prisciandaro,36V Pugatch,41A Puig Navarro,33
W Qian,53J H Rademacker,43B Rakotomiaramanana,36M S Rangel,2I Raniuk,40G Raven,39S Redford,52
M M Reid,45A C dos Reis,1S Ricciardi,46A Richards,50K Rinnert,49D A Roa Romero,5P Robbe,7E Rodrigues,48,51
F Rodrigues,2P Rodriguez Perez,34G J Rogers,44S Roiser,35V Romanovsky,32M Rosello,33,nJ Rouvinet,36T Ruf,35
H Ruiz,33G Sabatino,21,kJ J Saborido Silva,34N Sagidova,27P Sail,48B Saitta,15,dC Salzmann,37M Sannino,19,i
R Santacesaria,22C Santamarina Rios,34R Santinelli,35E Santovetti,21,kM Sapunov,6A Sarti,18,lC Satriano,22,m
A Satta,21M Savrie,16,eD Savrina,28P Schaack,50M Schiller,39H Schindler,35S Schleich,9M Schlupp,9
M Schmelling,10B Schmidt,35O Schneider,36A Schopper,35M.-H Schune,7R Schwemmer,35B Sciascia,18
A Sciubba,18,lM Seco,34A Semennikov,28K Senderowska,24I Sepp,50N Serra,37J Serrano,6P Seyfert,11M Shapkin,32
I Shapoval,40,35P Shatalov,28Y Shcheglov,27T Shears,49L Shekhtman,31O Shevchenko,40V Shevchenko,28A Shires,50
R Silva Coutinho,45T Skwarnicki,53N A Smith,49E Smith,52,46K Sobczak,5F J P Soler,48A Solomin,43F Soomro,18,35
B Souza De Paula,2B Spaan,9A Sparkes,47P Spradlin,48F Stagni,35S Stahl,11O Steinkamp,37S Stoica,26S Stone,53,35
B Storaci,38M Straticiuc,26U Straumann,37V K Subbiah,35S Swientek,9M Szczekowski,25P Szczypka,36T Szumlak,24
S T’Jampens,4E Teodorescu,26F Teubert,35C Thomas,52E Thomas,35J van Tilburg,11V Tisserand,4M Tobin,37
S Tolk,39S Topp-Joergensen,52N Torr,52E Tournefier,4,50S Tourneur,36M T Tran,36A Tsaregorodtsev,6N Tuning,38
M Ubeda Garcia,35A Ukleja,25U Uwer,11V Vagnoni,14G Valenti,14R Vazquez Gomez,33P Vazquez Regueiro,34
S Vecchi,16J J Velthuis,43M Veltri,17,gB Viaud,7I Videau,7D Vieira,2X Vilasis-Cardona,33,nJ Visniakov,34
A Vollhardt,37D Volyanskyy,10D Voong,43A Vorobyev,27V Vorobyev,31H Voss,10R Waldi,55S Wandernoth,11
J Wang,53D R Ward,44N K Watson,42A D Webber,51D Websdale,50M Whitehead,45D Wiedner,11L Wiggers,38
G Wilkinson,52M P Williams,45,46M Williams,50F F Wilson,46J Wishahi,9M Witek,23W Witzeling,35S A Wotton,44
K Wyllie,35Y Xie,47F Xing,52Z Xing,53Z Yang,3R Young,47O Yushchenko,32M Zangoli,14M Zavertyaev,10,a
F Zhang,3L Zhang,53W C Zhang,12Y Zhang,3A Zhelezov,11L Zhong,3and A Zvyagin35
(LHCb Collaboration)
1Centro Brasileiro de Pesquisas Fı´sicas (CBPF), Rio de Janeiro, Brazil
2Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil
3
Center 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
8LPNHE, 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
19
Sezione 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
Trang 325Soltan Institute for Nuclear Studies, 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
29Institute 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
33
Universitat 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 Vrije Universiteit, 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
45Department 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
49
Oliver 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, United States, 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
55Physikalisches Institut, Universita¨t Rostock, Rostock, Germany, associated to Physikalisches Institut,
Ruprecht-Karls-Universita¨t Heidelberg, Heidelberg, Germany (Received 19 March 2012; published 7 May 2012)
A study of B! J=c and B!c ð2SÞ decays is performed with data corresponding to
0:37 fb1of proton-proton collisions at ffiffiffi
s
p
¼ 7 TeV Their branching fractions are found to be BðB! J=c Þ ¼ ð3:88 0:11 0:15Þ 105 and BðB! c ð2SÞÞ ¼ ð2:52 0:26 0:15Þ 105;
where the first uncertainty is related to the statistical size of the sample and the second quantifies
systematic effects The measured CP asymmetries in these modes are AJ=CPc¼ 0:005 0:027
0:011 and ACPcð2SÞ¼ 0:048 0:090 0:011 with no evidence of direct CP violation seen
aP.N Lebedev Physical Institute, Russian Academy of Science (LPI RAS), Moscow, Russia
bUniversita` di Bari, Bari, Italy
c
Universita` di Bologna, Bologna, Italy
dUniversita` di Cagliari, Cagliari, Italy
eUniversita` di Ferrara, Ferrara, Italy
fUniversita` di Firenze, Firenze, Italy
gUniversita` di Urbino, Urbino, Italy
hUniversita` di Modena e Reggio Emilia, Modena, Italy
iUniversita` di Genova, Genova, Italy
jUniversita` di Milano Bicocca, Milano, Italy
kUniversita` di Roma Tor Vergata, Roma, Italy
lUniversita` di Roma La Sapienza, Roma, Italy
mUniversita` della Basilicata, Potenza, Italy
nLIFAELS, La Salle, Universitat Ramon Llull, Barcelona, Spain
oHanoi University of Science, Hanoi, Viet Nam
Published by the American Physical Society under the terms of theCreative Commons Attribution 3.0 License Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI
MEASUREMENTS OF THE BRANCHING FRACTIONS AND PHYSICAL REVIEW D 85, 091105(R) (2012)
Trang 4The Cabibbo-suppressed decay Bþ !cþ, where c
represents either a J=c orcð2SÞ, proceeds via a b ! ccd
quark transition Its branching fraction is expected to be
about 5% of the favored b ! c cs mode, Bþ !cKþ
(charge conjugation is implied unless otherwise stated)
The standard model predicts that for b ! c cs decays the
tree and penguin contributions have the same weak phase
and thus no direct CP violation is expected in Bþ !cKþ
For Bþ!cþ, the tree and penguin contributions have
different phases and CP asymmetries at the per mille level
may occur [1] An additional asymmetry may be
gener-ated, at the percent level, from long-distance rescattering,
particularly from decays that have the same quark content
ðD0D; DD0; Þ [2] Any asymmetry larger than this
would be of significant interest
In this paper, the CP asymmetries
Ac¼ BðB!cÞ BðBþ!cþÞ
BðB!cÞ þ BðBþ!cþÞ (1)
and charge-averaged ratios of branching fractions
Rc ¼ BðB!cÞ
are measured with thec reconstructed in the þ final
state From the latter, BðB!cÞ may be deduced
using the established B !cK branching fractions
[3] The CP asymmetry for Bþ !cð2SÞKþ
is also re-ported Bþ! J=cKþacts as a control mode in the
asym-metry analysis because it is well measured and no CP
violation is observed [3] Previous measurements of the
Bþ! J=cþ branching fractions and CP asymmetries
[4,5] have an accuracy of about 10% The Bþ !
cð2SÞhþðh ¼ K; Þ system is less precisely known due
to a factor ten lower branching fraction to the h final
state The world average for Acð2SÞKis 0:025 0:024 [3]
and there has been one measurement of Acð2SÞ¼ 0:022
0:086 [6]
The LHCb experiment [7] takes advantage of the high
b b and c c cross sections at the Large Hadron Collider to
record unprecedented samples of heavy hadron decays It
instruments the pseudorapidity range 2 < < 5 of the
proton-proton (pp) collisions with a dipole magnet and a
tracking system which achieves a momentum resolution of
0.4–0.6% in the range 5–100 GeV=c The dipole magnet
can be operated in either polarity and this feature is used to
reduce systematic effects due to detector asymmetries In
the sample analyzed here, 55% of data was taken with one
polarity, 45% with the other
The pp collisions take place inside a silicon-strip vertex
detector which has active material 8 mm from the beam
line It provides measurements of track impact parameters
with respect to primary collision vertices (PV) and precise
reconstruction of secondary Bþ vertices Downstream
muon stations identify muons by their penetration through
layers of iron shielding Charged particle identification
(PID) is realized using ring-imaging Cherenkov detectors
with three radiators: aerogel, C4F10and CF4 Events with a high transverse energy cluster in calorimeters or a high transverse momentum (pT) muon activate a hardware trig-ger About 1 MHz of such events are passed to a software-implemented high level trigger, which retains about 3 kHz The analysis is performed using 0:37 fb1 of data re-corded by LHCb in the first half of 2011 The decay chain
Bþ!chþ,c ! þ
is reconstructed from good qual-ity tracks which have a track-fit 2per degree of freedom
<5 The muons are required to have momentum,
p > 3 GeV=c, and pT> 0:5 GeV=c Selected hadrons have p > 5 GeV=c and pT> 1 GeV=c The two muon candidates are used to form a c resonance with vertex-fit
2< 10 The dimuon invariant mass is required to be withinþ3040 MeV=c2 of the nominalc mass [3]; the asym-metric limits allow for a radiative tail
The reconstructed Bþcandidate vertex is required to be
of good quality with a vertex-fit 2< 10 It is ensured to originate from a PV by requiring 2IP< 25 where the 2 considers the uncertainty on track impact parameters and the PV position In addition, the angle between the Bþ momentum vector and its direction of flight from the
PV must be <32ð10Þ mrad for cð2SÞhþ (J=chþ) Furthermore, neither the muons nor the hadron track may point back to any primary vertex with 2
IP< 4 It is required that the hardware trigger accepted a muon from the Bþ candidate or by activity in the rest of the event Hardware-trigger decisions based on the hadron are neglected to remove dependence on the correct emulation
of the calorimeter’s response to pions and kaons
The Bþ candidates are refitted [8] requiring all three tracks to originate from the same point in space and the c candidates to have their nominal mass [3] Candidates for which one muon gives rise to two tracks in the reconstruc-tion, one of which is then assumed to be the hadron, form
an artificial peaking background in the cð2SÞhþ analysis These candidates peak in the invariant mass distribution
of the same-sign muon-pion combination at m
245 MeV=c2, i.e the sum of the muon and pion rest masses Requiring m> 300 MeV=c2removes this background
In 2% of events two Bþcandidates are found If they decay within 2 mm of each other the candidate with the poorest quality vertex is removed; otherwise both are kept
When selecting J=chþ candidates, a requirement is made on the decay angle of the charged hadron as mea-sured in the rest frame of the Bþ with respect to the Bþ trajectory in the laboratory frame, cosðhÞ < 0 This re-quires the hadron to have flown counter to the trajectory of the Bþ candidate, hence lowering its average momentum
in the laboratory frame At lower momentum, the pion-kaon mass difference provides sufficient separation in the
Bþ invariant mass distribution, as shown in Fig 1 In the Bþ !cð2SÞhþ analysis, the average momentum of the hadrons is lower, so such a cut is unnecessary to separate the two modes
Trang 5Particle identification information is quantified as
dif-ferences between the logarithm of likelihoods, lnLh, under
five mass hypotheses, h 2 f; K; p; e; g Separation of
cþ candidates from cKþ is ensured by requiring that
the hadron track satisfies lnLK lnL¼ DLLK< 6
This value is chosen to ensure that most ( 95%) Bþ !
cþdecays are reconstructed as such These events form
the ‘‘pionlike’’ sample, as opposed to the kaonlike events
satisfying DLLK> 6 that are reconstructed under the
cKþhypothesis
The selected data are partitioned by magnet polarity,
charge and DLLK of the hadron track By keeping the
two magnet polarity samples separate, residual detection
asymmetries between the left and right sides of the detec-tor can be evaluated and hence facdetec-tor out Event yields are extracted by performing an unbinned, maximum-likelihood fit simultaneously to the eight distributions of
B invariant mass in the range 5000 < mB< 5780 MeV=c2 [9] Figure 2shows this fit to the data for Bþ! J=chþ, summed over magnet polarity The Bþ!cð2SÞhþdata is shown in Fig.3
The probability density function (PDF) used to describe these distributions has several components The correctly reconstructed, Bþ!chþ events are modeled by the function,
fðxÞ / exp
ðx Þ2 22þ ðx Þ2L;R
which describes an asymmetric peak of mean and width
, and where Lðx < Þ and Rðx > Þ parameterize the tails The mean is required to be the same for cKþ and
cþthough it can vary across the four charge polarity subsamples to account for different misalignment effects TableIshows the fitted values of the common tail parame-ters and the widths of the Bþ!chþpeaks averaged over the subsamples
The misidentifiedcKþevents form a displaced peaking structure to the left of thecþsignal and tapers to lower mass This is modeled by a Crystal Ball function [10] which is found to be a suitable effective PDF Its yield is added to that of the correctly identified events to calculate the total number of cKþevents
) 2
c
(MeV/
)
±
5000 5100 5200 5300 5400 5500
* h
-0.5
0
0.5
1
LHCb
FIG 1 Distribution of cosðhÞ versus the invariant mass of
Bþ! J=c þ candidates The curved structure contains
mis-identified Bþ! J=c Kþ
decays which separate from the Bþ! J=c þ
vertical band for cosðhÞ < 0 The partially
recon-structed background, B ! J=c K enters top left
FIG 2 (color online) Distributions of B! J=c h invariant mass, overlain by the total fitted PDF (thin line) Pion-like events, with DLLK< 6 are reconstructed as J=c and enter in the top plots All other events are reconstructed as J=c Kand are shown in the bottom plots on a logarithmic scale B decays are shown on the left, Bþ on the right The dark [red] curve shows the B! J=c
component, the light [green] curve represents B! J=c K The partially reconstructed contributions are shaded In the lower plots these are visualized with a dark (light) shade for B0s (Bþ or B0) decays In the top plots the shaded component are contributions from B ! J=c K (dark) and B ! J=c
(light)
MEASUREMENTS OF THE BRANCHING FRACTIONS AND PHYSICAL REVIEW D 85, 091105(R) (2012)
Trang 6The PDF modelling the small component of cþ
de-cays with DLLK> 6 is fixed entirely from simulation It
contributes negligibly to the total likelihood so the yield
must be fixed with respect to that of correctly identified
cþ events The efficiency of the PID cut is estimated
using samples of pions and kaons from D0 ! Kþ
decays which are selected with high purity without using
PID information These calibration events are reweighted
in bins of momentum to match the momentum distribution
of the large J=cKþ and cð2SÞKþ
samples By this technique, the following efficiencies are deduced for
DLLK< 6: J=c¼ ð95:8 1:0Þ%; cð2SÞ¼ ð96:6
1:0Þ% The errors, estimated from simulation, account for
imperfections in the reweighting and the difference of the
signal Kþand þmomenta
Partially reconstructed decays populate the region below
the Bþ mass Bþ=0 !cKþ decays, where the pion is
missed, are modeled in the kaonlike sample by a flat PDF
with a Gaussian edge A small B0!cKþ component
is needed to achieve a stable fit It is modeled with the
same shape as the partially reconstructed Bþ=0 decays
except shifted in mass by the B0 B0 mass difference,
þ87 MeV=c2 In the pionlike sample, cþ
back-grounds are assumed to enter with the same PDF, and
same proportion relative to the signal, as thecKþ
back-ground in the kaonlike sample A component of misidenti-fied Bþ=0! J=cKþ is also included with a fixed shape estimated from the data Lastly, a linear polynomial with a negative gradient is used to approximate the combinatorial background The slope of this component of the pionlike and kaonlike backgrounds can differ
The stability of the fit is tested with a large sample of pseudoexperiments Pull distributions from these tests are consistent with being normally distributed, demonstrating that the fit is stable under statistical variations The yields obtained from the signal extraction fit are shown in TableII
The observables, defined in Eqs (3) and (4) are calcu-lated by the fit, then modified by a set of corrections taken from simulation The acceptances of cþ and cKþ events in the detector are computed using PYTHIA[11] to generate the primary collision andEVTGEN[12] to model the Bþ decay The efficiency of reconstructing and select-ing cþ and cKþ decays is estimated with a bespoke simulation of LHCb based onGEANT4[13] It models the
FIG 3 (color online) Distributions of B!c ð2SÞh invariant mass See the caption of Fig 2 for details The partially reconstructed background in the pionlike sample is present but negligible yields are found
TABLE I Signal shape parameters from the B!c hfits
c K (MeV=c2) 7:84 0:04 6:02 0:08
c (MeV=c2) 8:58 0:27 6:12 0:75
TABLE II Raw fitted yields The labels ‘‘D’’ and ‘‘U’’ refer to the two polarities of the LHCb dipole
U 10 666 148 11 120 155
Trang 7interaction of muons and the two hadron species with the
detector material The total correction cK=cis 0:985
0:012 and 1:007 0:021 for RJ=c and Rcð2SÞrespectively
CP asymmetries are extracted from the observed charge
asymmetries ðARawÞ by taking account of instrumentation
effects The interaction asymmetry of kaons, AK
Det is expected to be nonzero, especially for low-momentum
particles This asymmetry, measured at LHCb using a
sample of Dþ! D0þ, D0 ! Kþ
decays, is
0:010 0:002 if the pion asymmetry is zero [14] The
null-asymmetry assumption for pions has been verified at
LHCb to an accuracy of 0.25% [15] These results are used
with enlarged uncertainties (0.004, for both kaons and
pions) to account for the different momentum spectra of
this sample and those used in the previous analyses
In summary, the CP asymmetry is defined as
Ach¼ Ach
Raw AProd Ah
where the production asymmetry, AProd, describes the
dif-ferent rates with which Band Bþhadronize out of the pp
collisions The observed, raw charge asymmetry in Bþ !
J=cKþis 0:012 0:004 Using Eq (4) with the
estab-lished CP asymmetry, AJ= c K¼ 0:001 0:007 [3], AProdis
estimated to be 0:003 0:009 This is applied as a
correction to the other modes reported here
The different contributions to the systematic
uncertain-ties are summarized in Table III They are assessed by
modifying the final selection, or altering fixed parameters
and rerunning the signal yield fit The maximum variation
of each observable is taken as their systematic uncertainty
The largest uncertainty is due to the use of simulation
to estimate the acceptance and selection efficiencies It
accounts for any bias due to imperfect modelling of the
detector and its relative response to pions and kaons
Another important contribution arises from the loose
trig-ger criteria that are employed This uncertainty is
esti-mated from the shift in the central values after rerunning
the fit using only those events where the muons passed the
software trigger The use of the PID calibration to estimate
the efficiency for pions to the DLLK< 6 selection also
contributes a significant systematic uncertainty
The measurements of Acdepend on the estimation of
AProdfrom the Bþ! J=cKþchannel The uncertainty on
AProdis determined by the statistical error of AJ=RawcKin the fit, the uncertainty on the world average of AJ= c K and the estimation of Ah
Det These effects are kept separate in the table where it is seen that the uncertainty on the nominal value of AJ=cK dominates Finally, it is noted that the detector asymmetries cancel for Acð2SÞK and a lower systematic uncertainty can be reported
The measured ratios of branching fractions are
RJ=c ¼ ð3:83 0:11 0:07Þ 102
Rcð2SÞ¼ ð3:95 0:40 0:12Þ 102; where the first uncertainty is statistical and the second systematic Rcð2SÞ is compatible with the one existing measurement, ð3:99 0:36 0:17Þ 102[6] The mea-surement of RJ=c is 3:2 lower than the current world average, ð5:2 0:4Þ 102 [3] Using the established measurements of the Cabibbo-favored branching fractions [3], we deduce
BðB! J=cÞ ¼ ð3:88 0:11 0:15Þ 105 BðB!cð2SÞÞ ¼ ð2:52 0:26 0:15Þ 105; where the systematic uncertainties are summed in quad-rature The measured CP asymmetries,
AJ=CPc¼ 0:005 0:027 0:011
ACPcð2SÞ¼ 0:048 0:090 0:011
AcCPð2SÞK¼ 0:024 0:014 0:008;
have comparable or better precision than previous results, and no evidence of direct CP violation is seen
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 CERN and at the LHCb institutes, and acknowledge sup-port from the National Agencies: CAPES, CNPq, FAPERJ and FINEP (Brazil); CERN; NSFC (China); CNRS/IN2P3 (France); BMBF, DFG, HGF and MPG (Germany); SFI
TABLE III Summary of systematic uncertainties The statistical fit errors are included for comparison
RJ= cð102Þ AJ= c Rcð2SÞð102Þ Ac ð2SÞ Acð2SÞK
MEASUREMENTS OF THE BRANCHING FRACTIONS AND PHYSICAL REVIEW D 85, 091105(R) (2012)
Trang 8(Ireland); INFN (Italy); FOM and NWO (The
Netherlands); SCSR (Poland); ANCS (Romania); MinES
of Russia and Rosatom (Russia); MICINN, 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 and the Region Auvergne
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