Observation of B_s⁰ -&gt; K⁺K⁻ and Measurements of Branching Fractions of Charmless Two-Body Decays of B⁰ and B_s⁰ Mesons in <em>pp</em> Collisions at s√=1.96  TeV

Article

Reference

Observation of B

-> K

K

and Measurements of Branching Fractions of Charmless Two-Body Decays of B

and B

Mesons in pp

Collisions at s√=1.96  TeV

CDF Collaboration

CAMPANELLI, Mario (Collab.), et al .

Abstract

We search for decays of the type B0(s)→h+h'− (where h,h′=K or π) in 180  pb−1 of pp collisions collected at the Tevatron by the upgraded Collider Detector at Fermilab. We report the first observation of the new mode B0s→K+K− with a yield of 236±32 events, corresponding to (fs/fd)×B(B0s→K+K−)/B(B0→K+π−)=0.46±0.08(stat)±0.07(syst), where fs/fd is the ratio of production fractions of B0s and B0. We find results in agreement with world averages for the B0 modes, and set the following new limits at 90% C.L.: B(B0s→K−π+)

CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Observation of B

-> K

K

and

Measurements of Branching Fractions of Charmless Two-Body Decays of B

and B

Mesons in pp Collisions at s√=1.96  TeV. Physical Review Letters , 2006, vol. 97, no. 21, p. 211802

DOI : 10.1103/PhysRevLett.97.211802

Available at:

http://archive-ouverte.unige.ch/unige:38361

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

1 / 1

Observation of B

! K

K

and Measurements of Branching Fractions of Charmless Two-Body Decays of B

and B

Mesons in pp Collisions at

p s

1:96 TeV

A. Abulencia,²³D. Acosta,¹⁷J. Adelman,¹³T. Affolder,¹⁰T. Akimoto,⁵⁵M. G. Albrow,¹⁶D. Ambrose,¹⁶S. Amerio,⁴³ D. Amidei,³⁴A. Anastassov,⁵²K. Anikeev,¹⁶A. Annovi,¹⁸J. Antos,¹M. Aoki,⁵⁵G. Apollinari,¹⁶J.-F. Arguin,³³ T. Arisawa,⁵⁷A. Artikov,¹⁴W. Ashmanskas,¹⁶A. Attal,⁸F. Azfar,⁴²P. Azzi-Bacchetta,⁴³P. Azzurri,⁴⁶N. Bacchetta,⁴³ H. Bachacou,²⁸W. Badgett,¹⁶A. Barbaro-Galtieri,²⁸V. E. Barnes,⁴⁸B. A. Barnett,²⁴S. Baroiant,⁷V. Bartsch,³⁰G. Bauer,³²

F. Bedeschi,⁴⁶S. Behari,²⁴S. Belforte,⁵⁴G. Bellettini,⁴⁶J. Bellinger,⁵⁹A. Belloni,³²E. Ben Haim,⁴⁴D. Benjamin,¹⁵ A. Beretvas,¹⁶J. Beringer,²⁸T. Berry,²⁹A. Bhatti,⁵⁰M. Binkley,¹⁶D. Bisello,⁴³R. E. Blair,²C. Blocker,⁶B. Blumenfeld,²⁴ A. Bocci,¹⁵A. Bodek,⁴⁹V. Boisvert,⁴⁹G. Bolla,⁴⁸A. Bolshov,³²D. Bortoletto,⁴⁸J. Boudreau,⁴⁷A. Boveia,¹⁰B. Brau,¹⁰

C. Bromberg,³⁵E. Brubaker,¹³J. Budagov,¹⁴H. S. Budd,⁴⁹S. Budd,²³K. Burkett,¹⁶G. Busetto,⁴³P. Bussey,²⁰ K. L. Byrum,²S. Cabrera,¹⁵M. Campanelli,¹⁹M. Campbell,³⁴F. Canelli,⁸A. Canepa,⁴⁸D. Carlsmith,⁵⁹R. Carosi,⁴⁶ S. Carron,¹⁵M. Casarsa,⁵⁴A. Castro,⁵P. Catastini,⁴⁶D. Cauz,⁵⁴M. Cavalli-Sforza,³A. Cerri,²⁸L. Cerrito,⁴²S. H. Chang,²⁷ J. Chapman,³⁴Y. C. Chen,¹M. Chertok,⁷G. Chiarelli,⁴⁶G. Chlachidze,¹⁴F. Chlebana,¹⁶I. Cho,²⁷K. Cho,²⁷D. Chokheli,¹⁴

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J. C. Yun,¹⁶L. Zanello,⁵¹A. Zanetti,⁵⁴I. Zaw,²¹F. Zetti,⁴⁶X. 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

211802-2

23University of Illinois, Urbana, Illinois 61801, USA

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

43Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, University of Padova, 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 7 July 2006; revised manuscript received 27 September 2006; published 22 November 2006) We search for decays of the typeB⁰_s!hh⁰(whereh; h⁰Kor) in180 pb¹ofpp collisions collected at the Tevatron by the upgraded Collider Detector at Fermilab. We report the first observation of the new mode B⁰_s!KK with a yield of 23632 events, corresponding to f_s=f_d BB⁰_s ! KK=BB⁰!K 0:460:08stat 0:07syst, wheref_s=f_d is the ratio of production frac- tions ofB⁰s andB⁰. We find results in agreement with world averages for the B⁰ modes, and set the following new limits at 90% C.L.:BB⁰_s !K<5:610⁶andBB⁰_s !<1:710⁶.

DOI:10.1103/PhysRevLett.97.211802 PACS numbers: 13.25.Hw, 14.40.Nd

The decay modes ofB mesons into pairs of charmless pseudoscalar mesons are effective probes of the quark- mixing (Cabibbo-Kobayashi-Maskawa, CKM) matrix and are sensitive to potential new physics effects. Their branch- ing fractions and CP asymmetries can be predicted with good accuracy and compared to rich experimental data available forB andB⁰ mesons, produced in large quan- tities in4Sdecays [1]. Measurements of similar modes predicted, but not yet observed, for the B⁰_s meson are important to complete our understanding ofBmeson de-

cays. The measurement of observables from both strange and nonstrangeBmesons allows a cancellation of hadronic uncertainties, thus enhancing the precision of the extrac- tion of physics parameters from experimental data [2–5].

The branching fraction of the B⁰_s!KK mode is a candidate for observing an unusually large breaking of U-spin symmetry, and is sensitive to anomalous electro- weak penguin contributions from new physics [4,6,7]. A combination of B⁰! and B⁰_s!KK observ- ables has been proposed as a way to directly determine

the phase of theV_ubelement of the CKM matrix (angle), or alternatively as a test of our understanding of dynamics of Bhadron decays, when compared with other determi- nations of[8]. TheB⁰_s !Kmode can also be used in measuring [3], and its CPasymmetry is a powerful model-independent test [9] of the source of the directCP asymmetry observed in theB⁰ !K mode [10]. The B⁰_s ! mode proceeds only through annihilation diagrams, which are currently poorly known and a source of significant uncertainty in many theoretical calculations [11]. Its features are similar to the as yet unobservedB⁰ ! KK mode, but it has a larger predicted branching frac- tion [11,12]; a measurement of both modes would allow a determination of the strength of penguin annihilation [4].

In this Letter we report the first observation of the de- cay B⁰_s !KK and perform the first measurement in hadron collisions of partial widths ofB⁰_s decays to pairs of charged pions and kaons. Throughout this Letter, C-conjugate modes are implied and branching fractions indicateCPaverages unless otherwise stated.

The measurements have been performed in a sample of 180 pb¹ ofpp collisions atps

1:96 TeV, recorded at the Tevatron collider by the upgraded Collider Detector at Fermilab (CDF II). CDF II is a multipurpose magnetic spectrometer surrounded by calorimeters and muon detec- tors [13]. The components of the detector pertinent to this analysis are described briefly below. A silicon microstrip detector (SVX II) [14] and a cylindrical drift chamber (COT) [15] immersed in a 1.4 T solenoidal magnetic field allow reconstruction of charged particles in the pseudor- apidity rangejj<1:0[16]. The SVX II consists of five concentric layers of double-sided silicon detectors with radii between 2.5 and 10.6 cm, each providing a measure- ment with15mresolution in thedirection. The COT has 96 measurement layers, between 40 and 137 cm in radius, organized into alternating axial and 2 stereo superlayers. The transverse momentum resolution is _pT=p_T ’0:15%p_T=GeV=c. The specific energy loss (dE=dx) of charged particles in the COT can be measured from the collected charge, which is encoded in the output pulse width of each wire.

Data were collected by a three-level trigger system, using a set of requirements dedicated toBhadron decays into charged particle pairs. At level 1, charged particle tracks are reconstructed in the COT transverse plane by a hardware processor (XFT) [17]. Two opposite-curvature tracks are required, with reconstructed transverse momenta p_T1; p_T2>2 GeV=c, the scalar sum p_T1p_T2>

5:5 GeV=c, and a transverse opening angle <135. At level 2, the Silicon Vertex Trigger (SVT) [18] combines XFT tracks with SVX II hits to measure the impact pa- rameterd(distance of closest approach to the beam line) of each valid track. The requirement of two tracks with 100m< d <1:0 mm reduces light-quark background by 2 orders of magnitude while preserving’50%of the

signal. A tighter opening-angle cut, 20< <135, selects two-body B decays from multibody with 97%

efficiency and reduces background further. Each track pair is then used to form a Bcandidate, which is required to have an impact parameter relative to the beam axisd_B<

140mand to have traveled a transverse distance L_xy>

200m. At level 3, a farm of computers confirms the selection with a full event reconstruction. The overall acceptance of the trigger selection is ’2%for Bmesons ofp_T>4 GeV=c.

In the offline analysis, combinatoric and light-quark backgrounds are effectively rejected by requiring the B candidate to be isolated. The isolation cut (I >0:5) [19]

has been chosen, together with tightened cuts on kinematic observables (L_xy>300m, d_B<80m, and d >

150m), by maximizing the quantityS=SB¹⁼² over all possible combinations of cuts. The background B is estimated from the data sidebands. The expected signal yield S is obtained from a detailed detector simulation assuming the momentum distribution of B mesons mea- sured by CDF [20], and normalized to the yield observed after the trigger selection. The overall efficiency of the chosen offline selection is’50%.

No more than oneBmeson candidate per event survives the selection, and a mass is assigned to each, assuming the pion mass for both decay products. The mass distribution, shown in Fig.1, exhibits an obvious peak in theB⁰_smass region. A binned fit of a Gaussian over an exponential background provides an estimate of 89347 signal events, with a width 382 MeV=c², compared to an expected mass resolution28 MeV=c² for an indi- vidualB⁰_s!hh⁰ mode. This indicates the presence of at least two distinct final states. Sizable signal contribu-

]² π c

π Invariant

5.0 5.2 5.4 5.6 5.8

2cEvents per 25 MeV/

0 50 100 150 200 250 300

]² c mass [GeV/

π π Invariant

5.0 5.2 5.4 5.6 5.8

2cEvents per 25 MeV/

0 50 100 150 200 250 300

π-

K+

→ B0

K-

K+

→

s

B0

π-

π+

→ B0

Background

FIG. 1. Invariant mass distribution ofB⁰_s!hh⁰candidates passing all selection requirements, using a pion mass assumption for both decay products. Cumulative projections of the like- lihood fit for each mode are overlaid.

211802-4

tions are expected from two known B⁰ modes, B⁰ ! and B⁰ !K, and two as yet unobserved B⁰_s modes, B⁰_s !KK and B⁰_s !K. Figure 1 shows that, as expected, the different modes are too closely spaced in mass to be clearly resolved and appear instead as a single peak somewhat broader than the mass resolu- tion. In addition to mass resolution, we use kinematic information along with particle identification to extract the different contributions. We incorporate all information in an unbinned likelihood fit, to statistically determine the contribution of each mode, and theCPasymmetry of the B⁰ !KmodeA_CP NB NB= NB NB.

For the kinematic portion, we use two loosely correlated observables to summarize the information carried by all possible values of invariant mass of the candidate B, re- sulting from different mass assignments to the two out- going particles [21]. They are the mass M calculated with the pion mass assignment to both particles, and the signed momentum imbalance 1p₁=p₂q₁, where p₁(p₂) is the lower (higher) of the particle momenta, and q₁is the sign of the charge of the particle of momentump₁. Using these two variables, the mass of any particular mode can be expressed, in the relativistic limit, as

M²_m1m2 M² 2 jjm²₂m²

1 jj 1¹m²₁m²; (1) wherem₁(m₂) is the mass of the lower (higher) momentum particle (Fig. 2, left panel). Particle identification (PID) information is provided by the measureddE=dxof the two tracks. In order to account for their dependence on particle momentum, we include in our fit the scalar sum p_tot p₁p₂as a fifth observable, which in conjunction with provides unique identification of the momenta of both particles.

With the chosen observables, the likelihood contribution of theith event is written as

Li 1bX

j

f_jL^kin_j L^PID_j bL^kin_bckL^PID_bck; (2) where the index ‘‘bck‘‘ labels background-related quanti- ties, the indexjruns over the eight distinguishableB⁰_s! hh⁰ modes (Fig. 2), and f_j are their fractions, to be determined by the fit together with the background fraction b. The L^kin_j is given by the product of the conditional probability density of M for given and the joint probability distribution P_j; p_tot. The mass distribution is a Gaussian centered at the value of M obtained from Eq. (1) by setting the appropriate particle masses for each decay modej. The Gaussian width_M283 MeV=c² was interpolated from the observed widths of other two- body decays (D⁰!K, J= !, and ! ), and the B⁰ andB⁰_s masses are set to the values measured by CDF [22] to cancel the common systematic uncertainty. The background mass distribution is fitted to an exponential function plus a constant. TheP_j; p_totis parameterized for each modejby a product of polynomial and exponential functions, fitted to Monte Carlo samples produced by a detailed detector simulation, while the corresponding distribution for the background is obtained from the mass sidebands of data.

The dE=dx response was calibrated over the tracking volume and time by means of a 97%-pure sample of 3 10⁵ D!D⁰! K decays, where the D⁰ decay products are identified by the charge of theDpion [23]. The observed response (Fig. 2, right panel) is well modeled by the convolution of a single-particle response function with a common baseline fluctuation, causing a 10% correlation between particles in the same event. Both effects are quasi-Gaussian with small tails and have been

q1

×

2)

1/p = (1 - p α

-1.0 -0.5 0.0 0.5 1.0

]2c-mass [GeV/ππInvariant 5.10

5.15 5.20 5.25 5.30 5.35

5.40 B⁰→π⁺π-

π-

K+

→ B0

π+

K- 0→

B -

+K

→K B0

dE/dx residual [ns]

-10 -8 -6 -4 -2 0 2 4 6 8 10

Frequency per 0.2 ns

0.00 0.01 0.02 0.03 0.04 0.05 0.06

K

π

FIG. 2 (color online). (Left panel) AverageMversusfor simulated samples ofB⁰events, whereKandKare treated separately. The solid curves are the corresponding first-order expressions from Eq. (1). The corresponding plots for theB⁰sare similar, but shifted for the mass difference. (Right panel) Distribution ofdE=dx(mean COT pulse width) around the average pion response, for calibration samples of kaons and pions (see text).

accurately modeled inL^PID. The separation between pions and kaons in the range 2< p_T<10 GeV=c is nearly constant at 1.4 standard deviations, corresponding to a resolution 1.7 times worse than a ‘‘perfect’’ PID, when measuring the relative fractions of the two particles in any given sample. TheL^PID_bck term allows for independent pion, kaon, proton, and electron components, which are free to vary independently in three mass regions (left, under, and right of the signal peak) to allow for possible variations due to the contribution of partially reconstructedBhadrons in the lower-mass region. Muons are indistinguishable from pions with the availabledE=dxresolution.

The fit of the data sample returns the yields listed in Table I. The observed resolutions are compatible with expectations from fitting Monte Carlo samples of the same size. Significant signals are seen for B⁰ !, B⁰ !K, and the previously unobservedB⁰_s !KK mode, while no evidence is obtained for B⁰_s !K, B⁰_s !, or B⁰!KK. As a check of our results, we performed an alternative fit based solely on kinematical information. Since theB⁰ !mode is indistinguish- able fromB⁰_s!KKin absence of PID information, we constrain its rate to its world-average value [24]. This fit confirms the main results, returning a yield of 19355 B⁰_s !KK events. To convert raw yields into relative branching fractions, we apply corrections for the different efficiencies of trigger and offline selection requirements for different decay modes; the relative efficiency correc- tions between modes do not exceed 19%. Most corrections are determined from the detailed detector simulation, with the following exceptions which are measured using data: A momentum-averaged relative isolation efficiency between B⁰_s andB⁰ of1:070:11has been determined from fully reconstructed samples of B⁰_s!J= , B⁰_s!D_s, B⁰ !J= K⁰, andB⁰ !D. The lower specific ion- ization of kaons with respect to pions in the COT is responsible for a ’5% lower efficiency to reconstruct a kaon by the XFT. This effect is measured in a sample of D!Kdecays triggered on two tracks, using the unbiased third track. The only correction needed byA_CPis

a 1:00:25% shift due to the different probability for K and K to interact with the tracker material. The accuracy of our control over instrumental charge asym- metries is confirmed by the smallness of the asymmetry (<0:5%) measured in the D⁰ !K mode [25]. The B⁰_s!KK and B⁰_s ! modes require a special treatment, since they contain a superposition of the flavor eigenstates of the B⁰_s. Their time evolution might differ from the flavor-specific modes if the width difference_s between theB⁰_smass eigenstates is significant. The current result is derived under the assumption that both modes are dominated by the short-livedB⁰_s component, that_sd, and_s=_s0:120:06[26]. The latter uncertainty is included in estimating the overall systematic uncertainty.

The dominant contributions to the systematic uncer- tainty are the following: the statistical uncertainty on iso- lation efficiency (B⁰_smodes), possible charge asymmetry of background (A_CP), and final state photon radiation (B⁰! ). The latter is conservatively estimated with the full effect predicted by QED calculations [27]. Smaller system- atic uncertainties are assigned for the following: mass scale and resolution; dE=dx response model; trigger efficien- cies; background shape and kinematics; Bmeson masses, lifetimes, and differences in momenta, allowed to vary by a factorm_B0

s m_B0=m_B0due to fragmentation effects [28].

The relative branching fractions obtained after applying all corrections are listed in Table I, where f_d and f_s indicate the production fractions, respectively, of B⁰ and B⁰_sfrom fragmentation of abquark inpp collisions. Upper limits are quoted for modes in which no significant signal is observed [29]. We also list absolute results obtained by normalizing our data to the world average of BB⁰! K and assuming for f_s=f_d the world average from

ppandee experiments [24].

The rate of the newly observed mode B⁰_s !KK favors the higher value367 10⁶ predicted by cal- culations based on QCD sum rules [4,6] implying large U-spin breaking in this process, although it is not statisti- cally incompatible with the expectation BB⁰_s! KK BB⁰ !Kfrom the assumption of exact TABLE I. Summary of results. The yields of the two annihilation modes (last two rows) were fixed to zero when fitting for the four main modes. Absolute branching fractions are normalized to the world-average valuesBB⁰!K 18:90:7 10⁶ and f_s=f_d0:260:039[24]. The first quoted uncertainty is statistical; the second is systematic.

Mode Yield Measured quantity DerivedB(10⁶)

B⁰!K 54230 A_CP 0:0130:0780:012

B⁰! 12127 ^BB_BB⁰0^!!K0:210:050:03 3:91:00:6

B⁰_s !KK 23632 _f^fs

d

BB⁰s!KK

BB⁰!K0:460:080:07 3367

B⁰s !K 325 _f^fs

d

BB⁰_s!K

BB⁰!K<0:08@90%C:L: <5:6@90%C:L:

B⁰_s! 1015 _BB^BB0^0s!

s!KK<0:05@90%C:L: <1:7@90%C:L:

B⁰!KK 1023 ^B_B^B_B⁰0^!K!K^K<0:10@90%C:L: <1:8@90%C:L:

211802-6

U-spin symmetry and negligible spectator contributions.

We also derive the ratio of U-spin – conjugate decays:

f_d=f_s BB⁰ ! =BB⁰_s ! KK 0:45 0:130:06, which can be related to theCPasymmetries in theB⁰!mode and to the CKM angle[8]. Our results for the B⁰ are in agreement with world-average values: BB⁰! 4:60:4 10⁶ and A_CPB⁰ !K 0:1130:020[30], although our A_CPmeasurement is also compatible with zero. The limit set on B⁰_s !K indicates a value at the lower end of current expectations [5,11]. The limit for the annihilation mode B⁰_s! is a large improvement over the pre- vious best limit [31], approaching the expectations from recent calculations [12,32].

In summary, we have measured relative branching frac- tions ofB⁰_s mesons into pairs of charmless charged me- sons. We find results in agreement with current world averages forB⁰ modes and observe for the first time the B⁰_s !KK mode. We set upper limits on unobserved modesB⁰!KK,B⁰_s!K, andB⁰_s !.

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 Bundesmini- sterium 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, U.K.; the Russian Foundation for Basic Research; the Comisio´n Interministerial de Ciencia y Tecnologı´a, Spain; the European Community’s Human Potential Programme under Contract No. HPRN-CT-2002-00292;

and the Academy of Finland.

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