Measurement of the Ratios of Branching Fractions B(B_s⁰ -> D_s^-π+π+π−)/B(B⁰ -> D−π+π+π−) and B(B_s⁰ -> D−sπ+)/B(B⁰ -> D−π+)

Article

Reference

Measurement of the Ratios of Branching Fractions B(B

-> D

π+π+π−)/B(B

-> D−π+π+π−) and B(B

-> D−sπ+)/B(B

-> D−π+)

CDF Collaboration

CAMPANELLI, Mario (Collab.), et al .

Abstract

Using 355  pb−1 of data collected by the CDF II detector in pp¯ collisions at s√=1.96  TeV at the Fermilab Tevatron, we study the fully reconstructed hadronic decays B0(s)→D−(s)π+ and B0(s)→D−(s)π+π+π−. We present the first measurement of the ratio of branching fractions B(B0s→D−sπ+π+π−)/B(B0→D−π+π+π−)=1.05±0.10(stat)±0.22(syst). We also update our measurement of B(B0s→D−sπ+)/B(B0→D−π+) to 1.13±0.08(stat)±0.23(syst), improving the statistical uncertainty by more than a factor of 2. We find

B(B0s→D−sπ+)=[3.8±0.3(stat)±1.3(syst)]×10−3 and

B(B0s→D−sπ+π+π−)=[8.4±0.8(stat)±3.2(syst)]×10−3.

CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Measurement of the Ratios of Branching Fractions B(B

-> D

π+π+π−)/B(B

-> D−π+π+π−) and B(B

-> D−sπ+)/B(B

->

D−π+). Physical Review Letters , 2007, vol. 98, no. 06, p. 061802

DOI : 10.1103/PhysRevLett.98.061802

Available at:

http://archive-ouverte.unige.ch/unige:38382

Disclaimer: layout of this document may differ from the published version.

1 / 1

Measurement of the Ratios of Branching Fractions B B

! D

= B B

! D

and B B

! D

= B B

! D

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J. C. Yun,¹⁶L. Zanello,⁵¹A. Zanetti,⁵⁴I. Zaw,²¹X. Zhang,²³J. Zhou,⁵²and S. Zucchelli⁵ (CDF Collaboration)

1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China

2Argonne National Laboratory, Argonne, Illinois 60439, USA

3Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain

4Baylor University, Waco, Texas 76798, USA

5Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy

6Brandeis University, Waltham, Massachusetts 02254, USA

7University of California, Davis, Davis, California 95616, USA

8University of California, Los Angeles, Los Angeles, California 90024, USA

9University of California, San Diego, La Jolla, California 92093, USA

10University of California, Santa Barbara, Santa Barbara, California 93106, USA

11Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain

12Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA

13Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA

14Joint Institute for Nuclear Research, RU-141980 Dubna, Russia

15Duke University, Durham, North Carolina 27708, USA

16Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA

17University of Florida, Gainesville, Florida 32611, USA

18Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy

19University of Geneva, CH-1211 Geneva 4, Switzerland

20Glasgow University, Glasgow G12 8QQ, United Kingdom

21Harvard University, Cambridge, Massachusetts 02138, USA

22Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland

23University of Illinois, Urbana, Illinois 61801, USA

061802-2

24The Johns Hopkins University, Baltimore, Maryland 21218, USA

25Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany

26High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan

27Center for High Energy Physics: Kyungpook National University, Taegu 702-701, Korea;

Seoul National University, Seoul 151-742, Korea;

and SungKyunKwan University, Suwon 440-746, Korea

28Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

29University of Liverpool, Liverpool L69 7ZE, United Kingdom

30University College London, London WC1E 6BT, United Kingdom

31Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain

32Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

33Institute of Particle Physics: McGill University, Montre´al, Canada H3A 2T8;

and University of Toronto, Toronto, Canada M5S 1A7

34University of Michigan, Ann Arbor, Michigan 48109, USA

35Michigan State University, East Lansing, Michigan 48824, USA

36Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia

37University of New Mexico, Albuquerque, New Mexico 87131, USA

38Northwestern University, Evanston, Illinois 60208, USA

39The Ohio State University, Columbus, Ohio 43210, USA

40Okayama University, Okayama 700-8530, Japan

41Osaka City University, Osaka 588, Japan

42University of Oxford, Oxford OX1 3RH, United Kingdom

43University of Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy

44LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France

45University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

46Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa, Siena and Scuola Normale Superiore, I-56127 Pisa, Italy

47University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

48Purdue University, West Lafayette, Indiana 47907, USA

49University of Rochester, Rochester, New York 14627, USA

50The Rockefeller University, New York, New York 10021, USA

51Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University of Rome ‘‘La Sapienza,’’ I-00185 Roma, Italy

52Rutgers University, Piscataway, New Jersey 08855, USA

53Texas A&M University, College Station, Texas 77843, USA

54Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy

55University of Tsukuba, Tsukuba, Ibaraki 305, Japan

56Tufts University, Medford, Massachusetts 02155, USA

57Waseda University, Tokyo 169, Japan

58Wayne State University, Detroit, Michigan 48201, USA

59University of Wisconsin, Madison, Wisconsin 53706, USA

60Yale University, New Haven, Connecticut 06520, USA (Received 16 October 2006; published 6 February 2007) Using355 pb¹ of data collected by the CDF II detector inpp collisions atps

1:96 TeVat the Fermilab Tevatron, we study the fully reconstructed hadronic decays B⁰_s!D_s and B⁰_s! D_s. We present the first measurement of the ratio of branching fractions BB⁰_s ! D_s=BB⁰!D 1:050:10stat 0:22syst. We also update our measure- ment ofBB⁰_s !Ds=BB⁰!D to 1:130:08stat 0:23syst, improving the statistical uncertainty by more than a factor of 2. We findBB⁰_s!D_s 3:80:3stat 1:3syst 10³ andBB⁰_s !Ds 8:40:8stat 3:2syst 10³.

DOI:10.1103/PhysRevLett.98.061802 PACS numbers: 13.25.Hw, 14.40.Nd

Hadronic B meson decays provide important informa- tion on both weak and hadronic interactions of heavy flavored mesons. The dominant hadronic decay modes of theBmeson involve tree-level diagrams where theb!c transition leads to a charmed meson and a virtualWboson, which often emerges as a charged,, ora₁1260meson [1]. The measurement of the ratios of branching fractions

BB⁰_s!D_s=BB⁰!D [2]

reveals information about B decay mechanisms. One can attempt to separate the contributions of various processes in B⁰!D decay and then predict the B⁰_s !D_s branching fraction usingSU3[3,4] and further estimate flavorSU3symmetry breaking effects [5], which can be sizable [6]. The ratios of branching fractions are expected

to be close to 1 if the flavorSU3is a valid approximation and the contribution of the subleading diagram in theB⁰ ! Ddecay is small.

In this Letter, we present the first measurement of the ratio of branching fractions RD3 BB⁰_s ! D_s=BB⁰!D using D_s decays to , K⁰K, and and D decays to K. We also update our previous [7] measurement of the ratioRD BB⁰s !D_s=BB⁰ !D. We measure the ratios of branching fractions because most of the systematic uncertainties cancel due to the similarity of final state kinematics. The measurement is performed using a sample of inclusive heavy flavor decays, corre- sponding to an integrated luminosity of 355 pb¹ of pp collisions atps

1:96 TeV.

The components of the CDF II detector relevant for this analysis are briefly described below. A more complete description can be found elsewhere [8]. Charged particle tracks are reconstructed in the pseudorapidity rangejj&

1:0, whereis defined aslntan=2, andrepresents the angle between the particle and the proton beam direc- tion [9]. Tracks are reconstructed from hits in the silicon microstrip detector (SVX) and the central outer tracker (COT). Both detectors are inside a 1.4 T solenoidal mag- netic field. The SVX detector is composed of L00 (a single layer of silicon placed close to the beam pipe), SVX II (five cylindrical layers of double-sided sensors), and ISL (out- ermost layer of silicon), providing up to 8 coordinate measurements in the r- view [10]. Surrounding the SVX is the COT, an open cell drift chamber with 96 layers of sense wires [11]. A sample rich in charm and beauty hadrons is selected by a three-level displaced track trigger.

At level 1, tracks are reconstructed in the COT by the track trigger processor (XFT) [12]. The trigger requires two tracks with transverse momenta p_T>2 GeV=c and the scalar sum p_T1p_T2>4:0 GeV=c. The level 2 silicon vertex tracker [13] associates SVX IIr- position mea- surements with XFT tracks, providing a precise measure- ment of the track impact parameter (d₀), i.e., the distance of closest approach of the track helix to the beam axis in the transverse plane. Decays of heavy flavor particles are identified by requiring two tracks with 0:12 mm< d₀<

1 mmand an opening angle in the transverse plane2<

jj<90. A requirementL_xy>0:2 mmis also applied, whereL_xyis defined as the distance in the transverse plane from the beam line to the two-track vertex projected onto the two-track momentum vector. The level 3 trigger per- forms a full event reconstruction applying selection similar to levels 1 and 2 on offline quality quantities.

B candidate reconstruction starts with a collection of tracks. No particle identification is explicitly used, and tracks are assumed to be either a pion or a kaon to match the reconstruction hypothesis. A set of unique track com- binations making the,K⁰,D,D_s, andBcandidates is formed. The track combinations reconstructed in 3 dimen-

sions must be consistent with forming a vertex, and com- binations that fall outside a wide mass window around the mass of the respective meson are rejected.

The Monte Carlo simulation is an essential part of this analysis. It is used to optimize the selection cuts, model signal, and background and to study the trigger and recon- struction efficiency. We generate singleBhadrons with the program BGENERATOR [14]. The B-hadron decays are si- mulated withEVTGEN[15]. This package has been exten- sively tuned by experiments at the 4S resonance and reflects the measured properties ofBandDmeson decays.

The selection requirements used to reject combinatorial background are optimized by maximizingS=

SB p for each mode individually. The number of signal events (S) is derived from a Monte Carlo simulation of the CDF II detector and trigger. The number of background events (B) is estimated using data in the high-mass sideband interval mB 10B to mB 16B, where mB is the fitted mass andB 15 MeV=c² is the width of the signal peak. This sideband represents the combina- toric background underneath the signal peak. Selection requirements include cuts on the impact parameter of the B meson, the ²_r [16] of the B vertex fit in the transverse plane, the p_T of the pion from the B decay in B⁰_s!D_s, and a minimum p_T requirement of the tracks for decays with 6 tracks in the final state.

We exploit the narrow !KK resonance and K⁰!K resonance to suppress background by re- quiring 1010 MeV=c²< m<1029 MeV=c² and 840 MeV=c²< mK⁰<940 MeV=c². There are also re- quirements on L_xy=L_xy—the significance of the mea- surement ofL_xyforBandDvertices.

The assumptions on the relative contributions of reso- nant a₁ and and nonresonant in the B⁰! D signal affect RD3 because Monte Carlo simulation shows that their reconstruction efficiencies dif- fer by as much as 5%. We find that the contributions of the and nonresonant decays are small. The mass distributions were compared in data be- tweenB⁰andB⁰_s mesons and are compatible within statis- tics, as shown in Fig. 1. The resonant fractions in B⁰_s!D_s and B⁰!D decays are assumed to be identical.

To extractRD3[or, equivalently,RD], we use the following formula:

RD3 f_d f_s

B⁰ B⁰_s

BD BD_s

NB⁰_s

NB⁰; (1) where NB⁰_s and NB⁰ are the measured signal yields, B⁰=B⁰_sis the ratio of trigger and reconstruction effi- ciencies extracted from Monte Carlo simulation, f_d=f_s is the ratio ofbquark fragmentation fractions intoB⁰andB⁰_s mesons, and BD=BD_s is the ratio of the world average values for branching fractions of D and D_s mesons into the reconstructed final states [17].

061802-4

The yieldsNB⁰_sandNB⁰are extracted from the mass spectra in Fig. 2 using a binned likelihood fit and are summarized in TableI. In the normalization modesB⁰ ! D and B⁰ !D, we observe 8098 114stat and 328876stat signal candidates, respec- tively. The signal peaks are modeled with a sum of two Gaussians with the same mean values but different widths.

The combinatorial background is modeled with a sum of an

exponential function and a constant. The shapes of other physics backgrounds are modeled using Monte Carlo simulation, and their parametrization is fixed in the fits to the mass spectra.

There are several backgrounds whose mass distributions peak near the signal region and must be subtracted. They are the Cabibbo-suppressed decays B⁰_s!D_sK and B⁰_s!D_sKand misreconstructed baryon decays ⁰_b!_c and ⁰_b!_c. The ratio of Cabibbo-suppressedB⁰_s!D_sKbackground to the cor- responding signal was fixed to the world average ratio of branching fractions [17]. The ratio of Cabibbo-suppressed B⁰_s!D_sK decay to the signal is fixed to jV_usj²=jV_udj² 0:05. The fraction of_bbackground was fixed using the recent CDF measurement of B⁰_b! _c=BB⁰!D [18]. In all of the cases, the ratios are corrected for the relative trigger and reconstruc- tion efficiencies.

In the case of theB⁰_s!D_s decay, there is a reflection from B⁰!D. If one of the pions from a D!K decay is reconstructed as a kaon, a peak is produced under the B⁰_s signal region.

These events contribute in theB⁰_s!D_s,D_s ! K⁰K decay because the K⁰ resonance is broad. The fraction of B⁰ !D events under B⁰_s! D_s peaks is calculated from the observed num- ber of B⁰ mesons. The number of B⁰!D events under theB⁰_s !D_sK⁰K mass distri- bution is estimated to be 1416stat. The systematic uncertainty assigned due to theB⁰ !Dback-

2Number of Entries / 10 MeV/c

0 20 40 60 80 100

120 (a) combinatorics

ρ , Ds π s D*

→ Bs

π

→ D , Bd Λb s K,

→ D Bs other physics

0 10 20 30 40 50 60 70 80 90

(b)

combinatorics s X D*

→ Bs

0π sf πK, D s 2

→ D Bs other physics

5 5.5 6

2Number of Entries / 10 MeV/c

0 10 20 30 40 50 60 70 80 90

(c)

2] candidate Mass [GeV/c Bs

combinatorics s X D*

→ Bs

πK s 2

→ D , Bs π

→ D 3 Bd other physics

5 5.5 6

0 10 20 30 40 50 60 70

(d)

2] candidate Mass [GeV/c Bs

combinatorics

* X Ds

→ Bs

Λb πK, s 2

→ D Bs other physics

200 400 600 800 1000 1200 1400 1600 1800 2000 2200

(e) combinatorics

+ρ π, D D* 0→ B

π Ds

→ , Bs Λb D K, 0→ B other physics

5 5.5 6

200 400 600 800

1000 (f)

2] candidate Mass [GeV/c Bd

combinatorics

* X

→ D B0

π s3

→ D , Bs Λb πK,

→ D 2 B0 other physics

FIG. 2 (color online). Mass spectra for (a) B⁰_s!D_s, (b) B⁰_s !D_s, (c) B⁰_s !D_sK⁰K, (d) B⁰_s !D_s, (e) B⁰!D, and (f ) B⁰!D. The ‘‘other physics’’ category corresponds to the inclusiveB!DsXandB!DXdecays.

2] ) [GeV/c π+

π-

π+

M(

1 2 3

2 Number of Entries / 60 MeV/c

-5 0 5 10 15 20

π

π

π

D

→ B

π

π

π

D

→

s

B

FIG. 1 (color online). Comparison of the sideband subtracted mass spectrum of fromB⁰ andB⁰_s decays. Only the D_s !channel is used forB⁰_s. TheB⁰histogram is normal- ized toB⁰s.

ground subtraction is dominant in this channel and is a part of the ‘‘B⁰_s fit model’’ (see Table II). For B⁰_s ! D_s with D_s !, the corresponding B⁰ background fraction is very small due to the narrow width of the . The contamination of the double charm B⁰ ! DD_s, withD_s !, is estimated to be1%of the B⁰!D signal and is subtracted from the measured yield. The contamination ofB⁰_s !D_sD_s in the B⁰_s signal is found to be negligible. Applying a cut on the mass of three pions in the B⁰_s !D_s decay around the mass of the D_s meson does not change the measured yields.

The systematic uncertainty on the ratio of efficiencies B⁰_s=B⁰ comes from various physics sources. In all cases, the systematics were estimated by observing the change in the ratio of efficiencies when the effect was considered. The effect of the choice ofBmeson spectrum used by Monte Carlo simulation was determined by re- weighting the Monte Carlo events withp_Tspectrum based on next-to-leading order calculations [14] to match thep_T spectrum measured at CDF [19]. To estimate the system- atic uncertainty due to B and D lifetimes, we varied the assumed lifetime ofBandDmeson in signal Monte Carlo simulation within world average values [17]. The compo- sition systematic applies to the decay B⁰_s ! D_s only and is due to the limited knowledge of the resonances in D_s ! decay.

We assign a systematic uncertainty due to the unknown resonance structure of the system in B⁰_s! D_s decay by varying the fraction of the a₁ component in bothB⁰ andB⁰_s signal Monte Carlo simula- tion in a range consistent with the observed shape.

Systematic uncertainties due to the fit model are estimated by comparing the fitted yields after changing the mass range in which the fit is performed and also by varying the parameters of functions describing the backgrounds.

The systematic uncertainties are summarized in TableII.

The results of the measurements forRD3are sum- marized in TableI. To average the results of the measure- ments by the expected yield, we use Eq. (1), whereNB⁰_s is a sum of the yields in threeB⁰_s channels andB⁰_sis a linear combination of Monte Carlo efficiencies multiplied by the ratio of the branching fraction of D_s decay in a given channel andBD_s !. In the averaging pro-

cedure, small interference in the Dalitz plot between D_s !andD_s !K⁰Kcontributions was ignored.

Usingf_s=f_d 0:2590:038[17], we obtain:

RD3 1:050:10stat 0:07syst 0:14br 0:15pr:

The (br) and (pr) uncertainties refer to the uncertainty on theDmeson branching fractions and the ratio of fragmen- tation fractionsf_s=f_d, respectively.

Using Eq. (1) and the input from TableI, we obtain:

RD 1:130:08stat 0:05syst 0:15br 0:17pr:

The measurement ofRDis consistent with the previous CDF measurement [7] and supersedes that result with a statistical uncertainty reduced by a factor of 2. This mea- surement is consistent within uncertainties with the theo- retical prediction of 1:050:24 [3]. The agreement indicates the smallness of the flavorSU3breaking terms and limits the amplitude and phase difference of the sub- leading diagram (W-exchange) with respect to the tree diagram in theB⁰!Ddecay.

In conclusion, we have presented the first measurement of the ratio of branching fractionsRD3. We also have measured the ratio of branching fractionsRD, improv- ing the statistical uncertainty by more than a factor of 2.

TABLE I. Summary of event yields, ratios of efficiencies, and individual branching ratio measurements. The uncertainties listed on the yield and the ratio of efficiencies are statistical only.BD=BD_sis the ratio of BD!Kto the corresponding branching fraction of theD_s meson [BD_s !; !KK,BD_s !K⁰K; K⁰!K, orBD_s !].

Decay Yield B⁰_s=B⁰ BD=BD_s f_s=f_d BRB⁰_s=BRB⁰

D_s 49428 0:9130:004 4:400:59 0:2920:020stat 0:012syst

D_s 16017 0:8140:010 4:400:59 0:2630:029stat 0:018syst

D_sKK 9017 0:3520:009 3:800:77 0:2740:053stat 0:030syst D_s 4911 0:3970:009 7:801:50 0:2930:067stat 0:021syst

TABLE II. Summary of the relative systematics uncertainties on RD andRD3. The range of values appearing in the second column reflects the differences in the fit systematic of the three B⁰s decays.

Syst. uncertainty[%]

Effect B!D B!D3

B p_T spectrum 3:0 3:0

B⁰_s lifetime 2:1 2:1

3resonance structure 2:5

D_s !3composition 3:0

Trigger simulation 1:2 1:1

B⁰fit model 0:5 1:3

B⁰_s fit model 1:5 3:6–9:3

Total 4:2 6:7–10:9

061802-6

Using the world average values for BB⁰ !D and BB⁰ !D [17], we find BB⁰_s !D_s 3:80:3stat 1:3syst 10³ and BB⁰_s ! D_s 8:40:8stat 3:2syst 10³.

We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions.

This work was supported by the U.S. Department of Energy and National Science Foundation; the Italian Istituto Nazionale di Fisica Nucleare; the Ministry of Education, Culture, Sports, Science and Technology of Japan; the Natural Sciences and Engineering Research Council of Canada; the National Science Council of the Republic of China; the Swiss National Science Foundation; the A. P. Sloan Foundation; the Bundes- ministerium fu¨r Bildung und Forschung, Germany; the Korean Science and Engineering Foundation and the Korean Research Foundation; the Particle Physics and Astronomy Research Council and the Royal Society, United Kingdom; the Russian Foundation for Basic Research; the Comisio´n Interministerial de Ciencia y Tecnologı´a, Spain; in part by the European Community’s Human Potential Programme under Contract No. HPRN- CT-2002-00292; and the Academy of Finland.

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