Evidence for <em>B_s⁰→ϕϕ</em> Decay and Measurements of Branching Ratio and <em>A_CP</em> for <em>B⁺→ ϕK⁺</em>

Article

Reference

Evidence for B

→ϕϕ Decay and Measurements of Branching Ratio and A

for B

→ ϕK

CDF Collaboration

CAMPANELLI, Mario (Collab.), et al .

Abstract

We present the first evidence of charmless decays of the B0s meson, the decay B0s→ϕϕ, and a measurement of the branching ratio BR(B0s→ϕϕ) using 180  pb−1 of data collected by the CDF II experiment at the Fermilab Tevatron collider. In addition, the BR and direct CP

asymmetry for the B+→ϕK+ decay are measured. We obtain

BR(B0s→ϕϕ)=[14+6−5(stat)±6(syst)]×10−6, BR(B+→ϕK+)=[7.6±1.3(stat)±0.6(syst)]×10−6, and ACP(B+→ϕK+)=−0.07±0.17(stat)+0.03−0.02(syst). Both decays are governed in the standard model by second order (penguin) b→ss¯s amplitudes.

CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Evidence for B

→ϕϕ Decay and Measurements of Branching Ratio and A

for B

→ ϕK

. Physical Review Letters, 2005, vol.

95, no. 03, p. 031801

DOI : 10.1103/PhysRevLett.95.031801

Available at:

http://archive-ouverte.unige.ch/unige:38102

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

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Evidence for B

! Decay and Measurements of Branching Ratio and A

for B

! K

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(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

4Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy

5Brandeis University, Waltham, Massachusetts 02254, USA

6University of California at Davis, Davis, California 95616, USA

7University of California at Los Angeles, Los Angeles, California 90024, USA

8University of California at San Diego, La Jolla, California 92093, USA

9University of California at Santa Barbara, Santa Barbara, California 93106, USA

10Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain

11Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA

12Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA

13Joint Institute for Nuclear Research, RU-141980 Dubna, Russia

14Duke University, Durham, North Carolina 27708, USA

15Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA

16University of Florida, Gainesville, Florida 32611, USA

17Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy

18University of Geneva, CH-1211 Geneva 4, Switzerland

19Glasgow University, Glasgow G12 8QQ, United Kingdom

20Harvard University, Cambridge, Massachusetts 02138, USA

21Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00044, Helsinki, Finland

22Hiroshima University, Higashi-Hiroshima 724, Japan

23University of Illinois, Urbana, Illinois 61801, USA

031801-2

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; 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

31Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

32Institute of Particle Physics, McGill University, Montre´al H3A 2T8, Canada, and University of Toronto, Toronto M5S 1A7, Canada

33University of Michigan, Ann Arbor, Michigan 48109, USA

34Michigan State University, East Lansing, Michigan 48824, USA

35Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia

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, Sezione di Padova-Trento, University of Padova, I-35131 Padova, Italy

43University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

44Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa, Siena and Scuola Normale Superiore, I-56127 Pisa, Italy

45University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

46Purdue University, West Lafayette, Indiana 47907, USA

47University of Rochester, Rochester, New York 14627, USA

48The Rockefeller University, New York, New York 10021, USA

49Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University of Roma ‘‘La Sapienza,’’ I-00185 Roma, Italy

50Rutgers University, Piscataway, New Jersey 08855, USA

51Texas A&M University, College Station, Texas 77843, USA

52Texas Tech University, Lubbock, Texas 79409, 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

60Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University di Roma ‘‘La Sapienza,’’ I-00185 Roma, Italy (Received 25 February 2005; published 15 July 2005)

We present the first evidence of charmless decays of the B⁰_s meson, the decay B⁰_s !, and a measurement of the branching ratioBRB⁰_s ! using 180 pb¹ of data collected by the CDF II experiment at the Fermilab Tevatron collider. In addition, the BR and directCPasymmetry for theB! K decay are measured. We obtain BRB⁰_s ! 14⁶₅stat 6syst 10⁶, BRB! K 7:61:3stat 0:6syst 10⁶, and A_CPB!K 0:070:17stat^0:03_0:02syst.

Both decays are governed in the standard model by second order (penguin)b!sss amplitudes.

DOI:10.1103/PhysRevLett.95.031801 PACS numbers: 13.25.Hw, 11.30.Er, 14.40.Nd

Recent measurements [1,2] of CP asymmetries for B⁰ !K_S⁰ andB⁰ !⁰K⁰_Ssuggest deviations from stan- dard model (SM) predictions inBmeson decays involving b!sss transitions, which also generate both B⁰_s! andB!K decays. Charmless decays ofB⁰ andB mesons provide among the most stringent tests of the Cabibbo-Kobayashi-Maskawa (CKM) matrix. Charmless B⁰_s decays, not yet experimentally established, offer im- portant additional ways of testing the theory. Of particular interest for the current and next generation experiments at hadron colliders are charmlessB⁰_s to vector-vector decays

into self-conjugate final states, such as B⁰_s !. Exploiting angular correlations between the final state particles inB⁰_s!VV decays allows the statistical separa- tion of theCP-even andCP-odd components of the decay amplitude. With sufficient statistics, this decomposition allows the measurement of theB⁰_s decay width difference (_s), CKM studies, and tests of decay polarization pre- dictions [3]. In particular, the B⁰_s! decay, which proceeds through a pure b!sss, has been considered as a probe for new physics scenarios [4] (e.g., in b!s penguin dominated decays) recently advocated to explain

the observed b!sss asymmetries. The measurement of the branching ratio BRB⁰_s ! gives insight into the size of penguin amplitudes as well as tightens the con- straints on poorly known form factors. Recent calculations predict BRB⁰_s ! between 1010⁶ [5] and 37 10⁶[6]. Within the SM theB!KdirectCPasym- metry (A_CP) is predicted not to exceed a few percent [7].

No single experiment [8] has yet reached the sensitivity to detect this asymmetry. Further measurements of decay rates and A_CP may help identifying the origin of the observed deviations from SM predictions inb!ssspen- guin dominatedBdecays.

We report the first evidence of B⁰_s ! decays, and present the first measurements of theCP-averaged BR and A_CPforB!K[9] at hadron colliders. To cancel the uncertainty in theBhadron production cross section and to reduce systematic uncertainties on detector efficiencies, the branching fractions are extracted from ratios of the decay rates of interest normalized to the establishedB⁰_s ! J= andB!J= Kdecay modes, which are charac- terized by the same number of decay vertices and charged tracks in the final state as in the signal modes.

In this analysis, we use180 pb¹ofpp collision data at s

p 1:96 TeVcollected by the upgraded collider detec- tor (CDF II) at the Fermilab Tevatron. The components of the CDF II detector pertinent to this analysis are briefly described. A more complete description can be found else- where [10]. We use tracks in the pseudorapidity range

jj&1[11] reconstructed by a silicon microstrip vertex

detector (SVX II) [12] and the central outer tracker (COT) [13], which are immersed in a 1.4 T solenoidal magnetic field. The SVX II detector consists of double-sided sensors arranged in five cylindrical layers. Surrounding the SVX II is the COT, an open cell drift chamber with 96 sense wires.

The integrated charge collected by each wire provides a measurement of the specific ionization (dE=dx) for charged particles, allowing a separation equivalent to 1.4 G between and K for p_T >2 GeV=c. A set of planar drift chambers, located outside the calorimeters and additional steel absorbers, is used to detect muons within jj 1with high purity.

A sample enriched with heavy flavor particles is selected by the three-level displaced track trigger. At level 1, charged tracks are reconstructed in the COT by the ex- tremely fast tracker (XFT) [14]. The trigger requires two oppositely charged tracks with transverse momentap_T 2 GeV=cand the scalar sump_T1p_T2 5:5 GeV=c. At level 2, the silicon vertex tracker [15] associates SVX II r-position measurements with XFT tracks, providing a precise measurement of the track impact parameter (d₀), the distance of closest approach of the track trajectory to the beam axis in the transverse plane. Decays of heavy flavor particles are identified by requiring two tracks with 120md₀ 1:0 mm and an opening angle 2 jj 90. A requirement L_xy>200m is also ap-

plied, where the two-dimensional decay length L_xy is calculated as the transverse distance from the beam axis to the two track intersection projected onto the total trans- verse momentum of the track pair. A complete event reconstruction is performed at level 3, where the level 1 and level 2 trigger requirements are confirmed.

Bcandidates are reconstructed by detecting!KK andJ= !decays. In the latter case, at least one of the muons has to be identified in the muon detectors to suppress contamination from other two-bodyJ= decays.

At least one pair of tracks (‘‘trigger tracks’’) has to satisfy the trigger requirements.B(B⁰_s) candidates are formed by fitting three (four) tracks withp_T>0:4 GeV=cto a com- mon vertex. Requiring a good vertex fit ² reduces back- ground from mismeasured tracks. Combinatoric background is reduced by exploiting several variables sensitive to the long lifetime and relatively hardp_T spec- trum of B mesons and the isolation ofB hadrons inside b-quark jets. For this purpose we define the quantityI_Ras the ratio of theB candidate p_T over the total transverse momenta of all tracks within a cone of radius R

²²

p 1 around the B flight direction.

Requiring theBflight direction to extrapolate back to the beam axis decreases background from partially recon- structed decays. The cut values on the discriminating var- iables are optimized by maximizing S=

SB

p for the

already observed B!K signal and S=1:5 pB

for B⁰_s ! whose branching ratio is unknown. The latter choice is equivalent to maximizing the potential to reach a3observation of a new signal [16]. The signal (S) is derived from a Monte Carlo (MC) simulation [17] of the CDF II detector and trigger that uses theBmeson momen- tum and rapidity distributions from Ref. [18], which were matched to CDF data. The background (B) is represented by appropriately normalized data selected with the same requirements as the signal except for the two-kaon invari- ant mass lying in the sideband region: 1:04< m_KK<

1:06 GeV=c².

We discuss first the B!K analysis. The optimi- zation results in the following requirements: vertex²<8, Bdecay lengthL_xy>350m,Breconstructed impact parameter d^B₀ <100m, isolation I_R>0:5, nontrigger track transverse momentum p^soft_T >1:3 GeV=c, and im- pact parameterd^soft₀ >120m.

TheB !Ktotal yield andA_CP, defined as

A_CPNB!K NB!K NB!K NB!K;

are extracted simultaneously from an extended unbinned maximum likelihood fit on four variables to the combined BandBsample: the three-kaon invariant mass (m_KKK), the invariant mass of thecandidate (m_KK), the cosine of the meson helicity angle (defined as the angle between the K momentum in the parent rest frame and the 031801-4

momentum of thein theBrest frame), and the measured dE=dxdeviation from the expected value for pions for the lowest momentum trigger track. The likelihood function has seven components: signal, partially reconstructedb! X decays, combinatoric background, B ! K⁰892, with K⁰ !K, B!f₀980K, with f₀!KK, nonresonant B!KKK, and nonresonantB!K. The normalizations of the last three components are fixed to theB!K⁰892 yield, determined in the fit, through their relative decay rates and detection efficiencies. For each component the likelihood function is the product of four one-dimensional probability density functions (PDFs) of the fit variables, which are assumed to be uncorrelated. Them_KKKandm_KK distributions are shown in Fig. 1 with projections for the different components.

A combination of MC simulation and sideband data is used to derive the PDF in each variable for the various fit components. Form_KKK the fully reconstructed signals are modeled by Gaussian functions. The mass and width of the Bsignals are determined from the fit in the case ofKKK final states, while for K they are fixed to the values predicted by the simulation. We derive a parametrization from simulation for the partially reconstructed decays that populate the low mass side of the m_KKK distribution. An exponential plus a constant describe the combinatoric background. In the PDF for m_KK, the resonance is described by a Breit-Wigner convoluted with a Gaussian resolution function, while the combinatoric background is modeled by an empirical phase space function. Shapes for other backgrounds are derived from simulation. The helicity PDFs for B decays are derived from simulation, while the combinatoric background PDF is modeled using data from thesideband. ThedE=dxPDFs for kaons and

pions are derived from a high statisticsD⁰!Ksam- ple obtained fromDdecays. We findN_K 47:08:4, A_CP 0:070:17, andN_K⁰7:86:0(statistical errors only) from which we estimate a B !f₀980K contamination of 11% under the signal peak.

Candidates for the normalization mode,B !J= K, are selected with the same requirements as theB!K candidates except for the invariant mass of the two muons being within 100 MeV=c² of the J= mass [8]. Using an extended likelihood fit of them_Kandmdistributions, we obtain a total yield ofN _K 43922(statistical error only). The asymmetry A_CPB!J= K 0:046 0:050is consistent with zero, as expected for this mode.

The relativeB!K decay rate is calculated using

BRB!K

BRB !J= K N_K N _K

BRJ= ! BR!KK

' _K '_K' _K;

where ' _K='_K 0:7210:011is the ratio of the com- bined trigger and selection efficiencies derived from MC simulations with a correction of about 5% due to the different trigger efficiency for muons and kaons as mea- sured in unbiased samples. The efficiency for identifying at least one of the decay muons, ' _K0:810:02, is ob- tained by weighting the expectedp_T spectra in our signal with the single muon identification efficiency measured as a function of p_T in a sample of inclusiveJ= ! decays. The relativeBRB!KandA_CPresults are reported in Table I. Systematic uncertainties on signal yield (1:4events) and asymmetry (0:03,0:02) are eval- uated by varying the PDFs used in the likelihood fit, including a variation of the f₀980 width from 40 to 100 MeV=c². The uncertainties on the determination of efficiency and muon identification introduce a 5.6% rela- tive error in the measurement of the branching ratio and have no effect for A_CP. The uncertainties from BR! KKandBRJ= !contribute an additional 2% to the total systematic. We determine a 0:005uncertainty on theA_CP measurement arising from a possible tracking efficiency asymmetry for K and K as measured in minimum bias data.

A ‘‘blind’’ search forB⁰_s !decays was performed fixing the selection requirements and evaluating the com- binatoric background from independent samples before

TABLE I. Results for B!K and B⁰_s! decays.

World average [8] !KK and J= ! BR have been used to obtain the BR ratio BRB!K=BRB! J= KandBRB⁰_s !=BRB⁰_s!J= . The first uncer- tainty is statistical, the second is systematic.

B!K B⁰_s!

Yield 47:08:41:4 7:3^3:2_2:50:4

BR ratio 7:61:30:6 10³ 10⁵₄1 10³ A_CP 0:070:17^0:03_0:02 —

2] [GeV/c mKKK

5 5.1 5.2 5.3 5.4 5.5 5.6 2 Entries / 20 MeV/c

0 5 10 15 20 25 30

2] [GeV/c mKKK

5 5.1 5.2 5.3 5.4 5.5 5.6 2 Entries / 20 MeV/c

0 5 10 15 20 25

30 Data

Total PDF Comb. Bkgd

K+ φ +→ B

φX

→ b

K+ K− K+

→ B+

π+ π− K+ +→ B

2] [GeV/c mKK

1 1.01 1.02 1.03 1.04 1.05 1.06 2 Entries / 2 MeV/c

0 5 10 15 20 25 30

2] [GeV/c mKK

1 1.01 1.02 1.03 1.04 1.05 1.06 2 Entries / 2 MeV/c

0 5 10 15 20 25 30

FIG. 1. m_KKK (left) and m_KK (right) distributions for B! K with projections of the total likelihood function and its components. B!KKK is the sum of nonresonant and B!f₀980K contributions, while B!K in- cludes nonresonant andB!K⁰892.

examining the signal region in the data. The signal is selected requiring two pairs of kaons having an invariant mass within15 MeV=c²of the world averagemass [8].

The optimized selection criteria are the following: vertex ²<10,B⁰_sdecay lengthL_xy>350m,B⁰_sreconstructed impact parameterd^B₀ <80m, minimummeson trans- verse momentum p_T >2:5 GeV=c, and the minimum of the two kaons’ impact parameters from each meson candidate d_{0 min}¹ >40m, d_{0 min}² >110m, where ₁ is the lower momentumcandidate. TheB⁰_s!can- didate mass distribution is shown in Fig. 2. In a region of 72 MeV=c²around the world average [8]B⁰_s mass, cor- responding to a window 3 times the expected mass reso- lution, we observe 8 events.

Two sources of background are expected in theB⁰_ssignal region: combinatoric background andB⁰ !K⁰ decays with the pion from theK⁰ decay treated as a kaon. The combinatoric background generates a smooth distribution in the invariant mass region close tom_B⁰_s. Its contribution in the signal region is0:350:37events (combining statis- tical and systematic uncertainties), estimated using a back- ground enriched sample where both meson candidates have invariant mass lying in the mass sideband region.

The B⁰ !K⁰ background results in an approximately Gaussian distribution underneath theB⁰_s signal. Its contri- bution, derived from simulation, is 0:370:18 events, where the error includes both statistical and systematic uncertainties, resulting in a signal yield of7:3^3:2_2:5 events.

The probability of a Poisson fluctuation of the background to the observed or higher number of events is1:310⁶, corresponding to4:7one-sided Gaussian significance.

For the determination of BRB⁰_s !, a normaliza- tion sample of B⁰_s !J= decays is selected, requiring

one pair of kaons and one pair of muons within 15 and 50 MeV=c²of the world averageandJ= mass, respec- tively. The other kinematic selection criteria are similar to theB⁰_s !mode. To extract the number ofB⁰_s!J=

events, the candidate invariant mass distribution, shown in Fig. 2, is fit with a binned maximum likelihood function using a Gaussian for the signal and an exponential for the background. The fit returnsN 6910stat 5syst events, where the systematic error is evaluated using alter- native background models for the low mass region where partially reconstructedBdecays are expected. We subtract 3:71:7background events from B⁰ !J= K⁰ decays, with the pion treated as a kaon, as estimated from simulation.

TheB⁰_s !decay rate is derived from the relation BRB⁰_s!

BRB⁰_s!J= N N

BRJ= ! BR!KK

' '' ;

where '=' 0:8210:015is derived from simula- tion as in theB !K case and' 0:920:05is obtained using the p_T spectra of the B⁰_s!J= signal and the muon identification efficiency curve discussed above.

The uncertainty on the B⁰_s !J= yield and back- ground evaluation contribute 8% to the relative systematic error. The efficiencies significantly depend on both the polarization of the decay vector particles and the assumed value of_ssince they affect the impact parameter of the decay products on which the trigger operates. We vary the longitudinal polarization of theB⁰_s !decay from 20%

to 80%, the B⁰_s !J= polarization amplitudes within 1 of the CDF measurement [19], and _s in the range 0:06<_s=_s<0:18to assign a relative systematic un- FIG. 2. Invariant mass distribution forB⁰_s!(left) andB⁰_s !J= (right) candidates. The arrows indicate the signal region for theB⁰_s !search. The fit described in the text is superimposed on them_KK distribution.

031801-6

certainty of 4%. Summing in quadrature all contributions, we estimate a total relative systematic uncertainty on the ratio of BRB⁰_s ! to BRB⁰_s !J= of 11%. To convert the ratio BRB⁰_s !=BRB⁰_s!J= re- ported in Table I in a measurement ofBRB⁰_s!, we use BRB⁰_s !J= 1:380:49 10³, obtained from correcting the CDF measurement [20] for the current value off_s=f_d, theB⁰_stoB⁰production ratio [8]. We derive BRB⁰_s ! 14⁶₅stat 6syst 10⁶, where the systematic uncertainty includes a 36% contribution due to the uncertainty onBRB⁰_s!J= .

In summary, we have usedpp collision data collected with the displaced track trigger of the CDF II detector to study two fully reconstructedb!sss penguin dominated Bdecays. Using the world average [8]BRB!J= K, we measure BRB!K 7:61:3stat 0:6syst 10⁶ and A_CPB !K 0:07 0:17stat^0:03_0:02syst, which agree with previous measure- ments [8]. We find the first evidence of a charmless vector- vectorB⁰_sdecay and measureBRB⁰_s !, in agreement with the estimate of Ref. [5] and the recently amended calculation in [6].

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 Bundesministerium 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, UK; the Russian Foundation for Basic Research; the Comision Interministerial de Ciencia y Tecnologia, Spain; and in part by the European Community’s Human Potential Program under Contract No. HPRN-CT-2002-00292, Probe for New Physics.

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