First Observation of the Decay B_s⁰ -> D_s^-D_s⁺ and Measurement of Its Branching Ratio

Article

Reference

First Observation of the Decay B

-> D

D

and Measurement of Its Branching Ratio

CDF Collaboration

CLARK, Allan Geoffrey (Collab.), et al.

Abstract

We report the observation of the exclusive decay B0s→D−sD+s at the 7.5 standard deviation level 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 measure the relative branching ratio B(B0s→D−sD+s)/B(B0→D−D+s)=1.44+0.48−0.44. Using the world average value for B(B0→D−D+s), we find B(B0s→D−sD+s)=(9.4+4.4−4.2)×10−3. This provides a lower bound ΔΓCPs/Γs≥2B(B0s→D−sD+s)>1.2×10−2 at 95% C.L.

CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . First Observation of the Decay B

->

D

D

and Measurement of Its Branching Ratio. Physical Review Letters, 2008, vol. 100, no.

02, p. 021803

DOI : 10.1103/PhysRevLett.100.021803

Available at:

http://archive-ouverte.unige.ch/unige:38493

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

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First Observation of the Decay B

! D

D

and Measurement of Its Branching Ratio

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Y. Zheng,^8,cand S. Zucchelli⁵ (CDF Collaboration)^a

1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, People’s 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

14Comenius University, 842 48 Bratislava, Slovakia and Institute of Experimental Physics, 040 01 Kosice, Slovakia

15Joint Institute for Nuclear Research, RU-141980 Dubna, Russia

16Duke University, Durham, North Carolina 27708, USA

17Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA

18University of Florida, Gainesville, Florida 32611, USA

19Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy

20University of Geneva, CH-1211 Geneva 4, Switzerland

21Glasgow University, Glasgow G12 8QQ, United Kingdom

22Harvard University, Cambridge, Massachusetts 02138, USA

23Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland

021803-2

24University of Illinois, Urbana, Illinois 61801, USA

25The Johns Hopkins University, Baltimore, Maryland 21218, USA

26Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany

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

Seoul National University, Seoul 151-742, Korea;

Sungkyunkwan University, Suwon 440-746, Korea;

Korea Institute of Science and Technology Information, Daejeon, 305-806, Korea;

Chonnam National University, Gwangju, 500-757, 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

36University of New Mexico, Albuquerque, New Mexico 87131, USA

37Northwestern University, Evanston, Illinois 60208, USA

38The Ohio State University, Columbus, Ohio 43210, USA

39Okayama University, Okayama 700-8530, Japan

40Osaka City University, Osaka 588, Japan

41University of Oxford, Oxford OX1 3RH, United Kingdom

42Istituto Nazionale di Fisica Nucleare, University of Padova, Sezione di Padova-Trento, I-35131 Padova, Italy

43LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France

44University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

45Istituto Nazionale di Fisica Nucleare Pisa, University of Pisa, University of Siena, and Scuola Normale Superiore, I-56127 Pisa, Italy

46University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

47Purdue University, West Lafayette, Indiana 47907, USA

48University of Rochester, Rochester, New York 14627, USA

49The Rockefeller University, New York, New York 10021, USA

50Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University of Rome ‘‘La Sapienza,’’ I-00185 Roma, Italy

51Rutgers University, Piscataway, New Jersey 08855, USA

52Texas A&M University, College Station, Texas 77843, USA

53Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy

54University of Tsukuba, Tsukuba, Ibaraki 305, Japan

55Tufts University, Medford, Massachusetts 02155, USA

56Waseda University, Tokyo 169, Japan

57Wayne State University, Detroit, Michigan 48201, USA

58University of Wisconsin, Madison, Wisconsin 53706, USA

59Yale University, New Haven, Connecticut 06520, USA (Received 25 July 2007; published 16 January 2008)

We report the observation of the exclusive decayB⁰_s !D_sD_s at the 7.5 standard deviation level using 355 pb¹ of data collected by the CDF II detector inpp collisions atps

1:96 TeVat the Fermilab Tevatron. We measure the relative branching ratioBB⁰_s !DsDs=BB⁰!DDs 1:44^0:48_0:44. Using the world average value forBB⁰!DD_s, we findBB⁰_s !D_sD_s 9:4^4:4_4:2 10³. This provides a lower bound^CPs =s2BB⁰_s!DsDs>1:210²at 95% C.L.

DOI:10.1103/PhysRevLett.100.021803 PACS numbers: 13.25.Hw, 14.40.Nd

TheB⁰_sB⁰_s system exhibits mixing, with two distinct mass eigenstates B_H and B_L having a mass difference m_sm^H_s m^L_s, which has recently been measured [1].

In the standard model, these two states have decay widths ^L_s and^H_s, with difference_s^L_s ^H_s and average _s ^L_s ^H_s=2. To good approximation, the two mass eigenstates are expected to be eigenstates ofCP:B^even_s and B^odd_s , so that _s^CP_s , where ^CP_s B^even_s

B^odd_s . The Standard Model predicts_s=_s0:147 0:060[2] with a reasonably small uncertainty. A measure- ment of this quantity can therefore provide a sensitive test and in case of a discrepancy it would be a good indicator for new physics. Measuring the B⁰_s decay rate to Ds Ds , where Ds stands for either D_s or D_s , determines ^CP_s =_s, assuming the b!ccs transitions are dominated by these decays, and neglecting small

CP-odd components [3]:

^CP_s

_s 2BB⁰_s !Ds Ds : (1) The inclusive measurement of the B⁰_s !Ds Ds

decay rate has been reported previously [4] using B⁰_s ! X correlations. In this Letter we present the first ob- servation of the exclusive decayB⁰_s !D_sD_s [5], measure the ratio of its branching fraction with respect to that for B⁰ !DD_s, and set a lower bound on^CP_s =_s. We use CDF II detector data corresponding to355 pb¹ of inte- grated luminosity ofppcollisions at

ps

1:96 TeVat the Fermilab Tevatron [6].

This analysis depends primarily on the charged particle tracking systems. Charged particle tracks are reconstructed using the hits in the silicon microstrip detector system and the central outer tracker (COT) in the pseudorapidity range jj 1:0, where is defined as ln tan=2 and represents the angle between the particle and the proton beam direction [7]. Both detectors are inside a 1.4 T uni- form magnetic field. The silicon detector is composed of L00 (single layer of silicon microstrip sensors close to the beam pipe), the silicon vertex detector (SVX II) (five cylindrical layers of double-sided sensors), and intermedi- ate silicon layers, providing up to 8 coordinate measure- ments in the r- view [8]. Surrounding the SVX is the COT, a cylindrical drift chamber with 96 layers of sense wires [9].

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 (Extremely Fast Tracker, XFT) [10]. 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 sili- con vertex trigger [11] associates SVX II r- position measurements with Extremely Fast Tracker tracks and provides a precise measurement of the track impact pa- rameter (d₀), 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 with0:12 mmd₀ 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 applies the level 1 and level 2 selection requirements after a full event reconstruction.

We measure the branching fraction ratio BB⁰_s ! D_sD_s=BB⁰ !DD_s in which the D_s meson decay rates and part of the systematic uncertainties cancel. In searching for B⁰_s!D_sD_s we require the decay D_s ! . To enhance the search sensitivity we reconstructD_s meson candidates in the,K⁰K, ordecay channels for both the B⁰ and B⁰_s signals. The ratio is measured independently for three D_s decay modes and is calculated using

BB⁰_s!D_sD_s BB⁰!DD_s N_B⁰_s

N_B⁰ _B⁰ _B⁰_s

f_d f_s

BD!K BD_s ! ;

(2) whereN_B⁰_s andN_B⁰are the measured signal yields,_B⁰=_B⁰_s is the ratio of reconstruction and trigger efficiencies ex- tracted from Monte Carlo simulation,f_s=f_dis the ratio ofb quark fragmentation fractions intoB⁰_sandB⁰mesons, and BD !K=BDs ! is the ratio of branching fractions determined by other experiments.

All tracks used in reconstruction must have p_T>

350 MeV=cand are assumed to be either pions or kaons depending on the specific reconstruction hypothesis. The reconstruction of B⁰ !DKD_s, for ex- ample, begins by searching for D_s ! candidates.

We require two oppositely charged tracks to form ! KKand then add a third track to formD_s !. The reconstruction of D mesons uses the D!K mode. We reconstruct B⁰ !DD_s candidates by apply- ing a fit to six tracks with constraints on a primaryBmeson decay vertex, two secondaryDmeson decay vertices, and the masses of theDmesons.

Monte Carlo simulations are used to optimize the selec- tion requirements, to derive fitting functions for signal and background, and to determine the trigger and reconstruc- tion efficiencies. Single B hadrons are generated without fragmentation products of underlying event particles, and their decays are simulated usingEVTGEN[12]. The detector response, including the trigger, is modeled using the CDF simulation package [13]. The selection requirements are optimized by maximizing the significance of the Monte Carlo simulated signal, scaled to the expected yield, relative to the combinatorial background using a method valid for low statistics [14]. Combinatorial background is fitted in the interval 5:4;6:0GeV=c² and extrapolated into a60 MeV=c² wide signal region centered around the appropriate B meson mass. Selection requirements are made on the minimum p_T of the tracks, the impact pa- rameter of theBmeson, and the ² masses ofandK⁰ candidates. We also make requirements on the significance of theL_xy measurement,L_xy=L_xy, whereL_xyis the L_xy uncertainty, of B and D meson vertices, and of the displacement of the D meson vertices with respect to the Bmeson vertex. For decays involving resonant states, we require 1010 MeV=c²< mKK<1029 MeV=c² for candidates and 840 MeV=c²< mK<

940 MeV=c² for K⁰ candidates. The background from B⁰!DKD_s is removed from the B⁰_s!D_sD_sK⁰K signal by reconstructing D_s !K⁰K as D!K and removing events with theDcandidate mass in the range1845 MeV=c²<

mK<1893 MeV=c².

Figure1shows the reconstructed mass spectra forB⁰_s! D_sD_s and B⁰!DD_s decays. The signal yields N_B⁰_s andN_B⁰are extracted from a binned likelihood fit of these 021803-4

spectra. The fitting functions for all the modes have terms describing the signal, combinatorial background, partially reconstructedBhadrons, and contributions fromBdecays to different D_s decay modes. The combinatorial back- ground is represented by the sum of a constant plus an exponential. The signal and partially reconstructed modes are fitted with templates that have fixed shapes derived from simulation and floating normalizations. Each signal template is parametrized by two Gaussians with different widths and a common mean.

In fitting B⁰_s!D_sD_s distributions, we fix the signal masses to Particle Data Group values [15], and we fix the Gaussian signal widths, dominated by detector resolution, to the values obtained from Monte Carlo simulation. We also limit the mass range in the fit to value above 5:3 GeV=c² avoiding a detailed description of the well separated physics background.

We treat the background from the decay mode B⁰ ! D to the signal for B⁰!DK

D_sK⁰K and B⁰ !DKD_s modes by introducing templates normalized to the B⁰ yield. Similarly, we introduce the template normalized to the B⁰_s yield to model B⁰_s!D_s back- ground under the signal for B⁰_s!D_s D_s mode. These corrections lead to a less than 2% change in the signal.

Monte Carlo studies show that aBmeson signal, recon- structed in a specific D_s decay mode, have contributions from misreconstructed B meson candidates decaying through otherD_s channels. The decayB⁰ !DD_s, fol- lowed by D_s !f₀980KK, contributes to the reconstructed B⁰ !DKD_s. Similarly, B meson decays followed by a nonresonant D_s ! KK contribute to theBmeson signal reconstructed with D_s !K⁰K. The D_s !KK decay model takes into account the measured branching fractions of its resonant substructure [16]. The aforementioned effects are taken into account by introducing correction templates

2 Entries per 5.0 MeV/c 0

10 20 30 40 50

60 B⁰→ D^- ( K⁺π^-π^- ) D⁺_s ( φπ⁺ ) Total Fit Combinatorial

(*)+

Ds

D(*)-

→ B0

-)X π π-

(K+

D-

→ B0

+) π (f0 +

)Ds

π-

π-

(K+

D-

→ B0 2 Entries per 5.0 MeV/c 0

10 20 30 40 50 60

0 10 20 30 40 50

60 B⁰→ D^- ( K⁺π^-π^- ) D⁺s ( K^*0 K⁺ ) Total Fit Combinatorial

(*)+

Ds

D(*)-

→ B0

)X π-

π-

(K+

D-

→ B0

+) π K+

(K- +

)Ds

π-

π-

(K+

D-

→ B0

0 10 20 30 40 50 60

2] Mass [GeV/c

5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7

0 10 20 30 40 50

60 B⁰→ D^-(K⁺π^-π^-) D⁺s (π⁺π⁺π^-) Total Fit Combinatorial

(*)+

Ds

D(*)-

→ B0

-)X π π-

(K+

D-

→ B0

π-

π+

π+ -) π π-

(K+

D-

→ B0

2] Mass [GeV/c

5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7

0 10 20 30 40 50 60

2 Entries per 5.0 MeV/c₀

1 2 3 4 5 6

7 B⁰s→ D^-_s ( φπ^- ) D⁺_s ( φπ⁺ )

Total Fit

+) π K-

(K+ +

)Ds

π-

φ

-( Ds

→

0

Bs 2 Entries per 5.0 MeV/c₀

1 2 3 4 5 6 7

0 1 2 3 4 5 6

7 B⁰s→ D^-s ( φπ^- ) D⁺s ( K^*0 K⁺ )

Total Fit

+) π φ

+( Ds -

D(s)

→ B0

0 1 2 3 4 5 6 7

2] Mass [GeV/c

5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8

0 1 2 3 4 5 6

7 B⁰s→ D^-s ( φπ^- ) D⁺s ( π⁺π⁺π^- )

Total Fit

-) X π φ

-( Ds

→

0

Bs

2] Mass [GeV/c

5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8

0 1 2 3 4 5 6 7

FIG. 1 (color online). Mass spectra for B⁰!DDs (left) and B⁰s !DsDs (right) where Ds ! (top), Ds !K⁰K (middle),D_s !(bottom). The decomposition of the background into combinatorial background and several backgrounds fromB hadron decays is shown. ForB⁰ decays data are represented with dots and error bars, while for the small event yieldB⁰_s channels a histogram is shown.

TABLE I. Summary of event yields, efficiencies, measured ratios of branching fractions, and corresponding uncertainties for three D_s decay modes.

K⁰K

NB⁰!DD_s 183 15 128 13 84 13

NB⁰_s!DsDs 9:2^3:5_2:9 6:0^3:4_2:7 8:3^3:5_2:8 B⁰_s !D_sD_s=B⁰!DD_s 0:88 0:03 0:53 0:02 0:63 0:02 BB⁰_s !D_sD_s =BB⁰!DD_s 0:98^0:38_0:32 1:51^0:87_0:70 2:67^1:20_0:99

systematic uncertainty 0:06=0:08 0:15=0:25 0:27=0:29

(f_s=f_d) uncertainty 0:14 0:22 0:39

BD_s !=BD!Kuncertainty 0:13 0:20 0:36

with relative normalizations derived from Monte Carlo simulations and result in a 4% correction to the signal yield. Other b hadron backgrounds are described with templates derived from semigeneric simulations (B! D_sX), where one of theD_s mesons is forced to decay in the signal channel and the rest of the decay chain (X) follows the best available measurement of branching fractions.

Yields and ratios of reconstruction efficiencies extracted from signal simulations are summarized in TableI. Using Eq. (2) and the latest PDG [15] valuesf_s=f_d0:259 0:038, andBD_s ! B!KK 2:16 0:28 10², we calculate the ratio of branching fractions BB⁰_s !D_sD_s=BB⁰ !DD_s for the three D_s modes shown in TableIalong with corresponding statisti- cal uncertainties, systematic uncertainties discussed below, and the uncertainties from the measurements off_s=f_dand BD_s !=BD!K.

The systematic uncertainties, summarized in Table II, are evaluated from the change in the ratio of the branching fractions BB⁰_s!D_sD_s=BB⁰!DD_s for each ef- fect under consideration. Fit systematic uncertainties are estimated by varying the fit window, binning, and template parameters. The normalizations of background templates underneath the signal peaks are varied using the measured branching fractions [15] within their uncertainties and the effect is included in the fit systematics in Table II. The uncertainty due to theB mesonp_T spectrum is evaluated by comparing the efficiency determined from simulation based on next-to-leading-order calculations [17] and on the measured B hadron spectrum [18]. The effect of meson lifetimes is studied by varying the world averageB⁰_sandB⁰ lifetimes within their uncertainties in the simulations.

Trigger-related systematic uncertainties are estimated from simulations. The effects due to the limited knowledge of theD_s ! composition are studied by vary- ing the relative branching fractions of the components of the decay within their PDG [15] uncertainties. Finally, using the combinatorial background from data to optimize the selection introduces a bias. This effect has been esti-

mated using simulation based on the expected combinato- rial background distribution.

The significance of theB⁰_s !D_sD_s signal is given by the ratio of likelihoods of the mass fits, where we use the one of full fit model divided by the one of the same model but excluding the signal component. The individual sig- nificances of the signal reconstructed with B⁰_s! D_sD_s, B⁰_s !D_sD_sK⁰K, and B⁰_s!D_sD_s decay modes are 5:8, 3:4, and 4:4, respectively. From the product of three likelihoods we find the combined result consistent with an observation ofB⁰_s!D_sD_s at a7:5significance.

When combining the three results, the fit systematic uncertainties are weighed by the measured yields. The rest of the systematic uncertainties, except for the D_s ! composition uncertainty, are considered com- mon for all three modes. We find

BB⁰_s!D_sD_s

BB⁰!DD_s 1:44^0:38_0:31stat^0:08_0:12syst 0:21f_s=f_d

0:20

BD!K

BDs !

; (3) which we combine withBB⁰ !DD_s 6:5 2:1 10³ [15] and determine

BB⁰_s !D_sD_s 9:4^4:4_4:2 10³; (4) from which we derive a lower limit on^CP_s =:

^CP_s

_s 2BB⁰_s !D_s D_s 2BB⁰_s !D_sD_s

1:210² at 95% C:L: (5) In the derivation of the lower limit we take into account the Poisson statistical fluctuations of the signal yields and the Gaussian distribution for systematics uncertainties.

We have presented the first observation of the decay B⁰_s!D_sD_s and have measured its branching fraction with respect to B⁰ !DD_s. We set a lower bound on ^CP_s =_s, which at the 95% confidence level requires a nonzero decay rate difference and agrees with theoretical predictions [2] and other experimental data: 0:06<

_s=_s<0:28at the 95% confidence level [15].

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 Foun- dation; the A. P. Sloan Foundation; the Bundes- TABLE II. Systematic uncertainties in [%] for the B⁰s=B⁰

relative branching fraction measurements for three D_s decay modes.

K⁰K B⁰!DDs fit 2:3 4:2 8:4 B⁰_s !D_sD_s fit 4:4 8:2 4:6

Bmesonp_T spectrum 3:0 3:0 3:0

B⁰_s lifetime 2:0 2:0 2:0

trigger 1:0 1:0 1:0

Ds !composition - - 3:0

optimization bias 5:0 13:0 4:0

Total ^6:2_{8:0 16:4}^{9:9 10:3}_11:0

021803-6