Search for the Decays B_(s)⁰ -> e⁺μ⁻ and B_(s)⁰ -> e⁺e⁻ in CDF Run II

Article

Reference

Search for the Decays B

-> e

μ

and B

-> e

e

in CDF Run II

CDF Collaboration

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

Abstract

We report results from a search for the lepton flavor violating decays B0s→e+μ− and B0→e+μ−, and the flavor-changing neutral-current decays B0s→e+e− and B0→e+e−. The analysis uses data corresponding to 2  fb−1 of integrated luminosity of pp collisions at s√=1.96   TeV collected with the upgraded Collider Detector (CDF II) at the Fermilab Tevatron. The observed number of B0 and B0s candidates is consistent with background expectations. The resulting Bayesian upper limits on the branching ratios at 90% credibility level are B(B0s→e+μ−)

CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Search for the Decays B

-> e

μ

and B

-> e

e

in CDF Run II. Physical Review Letters, 2009, vol. 102, no. 20, p. 201801

DOI : 10.1103/PhysRevLett.102.201801

Available at:

http://archive-ouverte.unige.ch/unige:38557

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

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Search for the Decays B

! e

and B

! e

e

in CDF Run II

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X. Zhang,²⁵Y. Zheng,^9,eand S. Zucchelli^6b,6a (CDF Collaboration)

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

2Argonne National Laboratory, Argonne, Illinois 60439, USA

3University of Athens, 157 71 Athens, Greece

4Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain

5Baylor University, Waco, Texas 76798, USA

6aIstituto Nazionale di Fisica Nucleare Bologna, I-40127 Bologna, Italy

6bUniversity of Bologna, I-40127 Bologna, Italy

7Brandeis University, Waltham, Massachusetts 02254, USA

8University of California, Davis, Davis, California 95616, USA

9University of California, Los Angeles, Los Angeles, California 90024, USA

10University of California, San Diego, La Jolla, California 92093, USA

11University of California, Santa Barbara, Santa Barbara, California 93106, USA

12Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain

13Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA

14Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA

15Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia

16Joint Institute for Nuclear Research, RU-141980 Dubna, Russia

17Duke University, Durham, North Carolina 27708, USA

18Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA

19University of Florida, Gainesville, Florida 32611, USA

20Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy

21University of Geneva, CH-1211 Geneva 4, Switzerland

22Glasgow University, Glasgow G12 8QQ, United Kingdom

201801-2

23Harvard University, Cambridge, Massachusetts 02138, USA

24Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland

25University of Illinois, Urbana, Illinois 61801, USA

26The Johns Hopkins University, Baltimore, Maryland 21218, USA

27Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany

28Center 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

29Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

30University of Liverpool, Liverpool L69 7ZE, United Kingdom

31University College London, London WC1E 6BT, United Kingdom

32Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain

33Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

34Institute of Particle Physics: McGill University, Montre´al, Que´bec, Canada H3A 2T8;

Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6;

University of Toronto, Toronto, Ontario, Canada M5S 1A7;

and TRIUMF, Vancouver, British Columbia, Canada V6T 2A3

35University of Michigan, Ann Arbor, Michigan 48109, USA

36Michigan State University, East Lansing, Michigan 48824, USA

37Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia

38University of New Mexico, Albuquerque, New Mexico 87131, USA

39Northwestern University, Evanston, Illinois 60208, USA

40The Ohio State University, Columbus, Ohio 43210, USA

41Okayama University, Okayama 700-8530, Japan

42Osaka City University, Osaka 588, Japan

43University of Oxford, Oxford OX1 3RH, United Kingdom

44aIstituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy

44bUniversity of Padova, I-35131 Padova, Italy

45LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France

46University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

47aIstituto Nazionale di Fisica Nucleare Pisa, I-56127 Pisa, Italy

47bUniversity of Pisa, I-56127 Pisa, Italy

47cUniversity of Siena, I-56127 Pisa, Italy

47dScuola Normale Superiore, I-56127 Pisa, Italy

48University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

49Purdue University, West Lafayette, Indiana 47907, USA

50University of Rochester, Rochester, New York 14627, USA

51The Rockefeller University, New York, New York 10021, USA

52aIstituto Nazionale di Fisica Nucleare, Sezione di Roma 1, I-00185 Roma, Italy

52bSapienza Universita` di Roma, I-00185 Roma, Italy

53Rutgers University, Piscataway, New Jersey 08855, USA

54Texas A&M University, College Station, Texas 77843, USA

55aIstituto Nazionale di Fisica Nucleare Trieste/Udine, I-34100 Trieste, Italy

55bUniversity of Trieste/Udine, I-33100 Udine, Italy

56University of Tsukuba, Tsukuba, Ibaraki 305, Japan

57Tufts University, Medford, Massachusetts 02155, USA

58Waseda University, Tokyo 169, Japan

59Wayne State University, Detroit, Michigan 48201, USA

60University of Wisconsin, Madison, Wisconsin 53706, USA

61Yale University, New Haven, Connecticut 06520, USA (Received 23 January 2009; published 21 May 2009)

We report results from a search for the lepton flavor violating decaysB⁰s!e^þandB⁰!e^þ, and the flavor-changing neutral-current decays B⁰s !e^þe and B⁰!e^þe. The analysis uses data corresponding to2 fb¹ of integrated luminosity ofpp collisions at ffiffiffi

ps

¼1:96 TeVcollected with the upgraded Collider Detector (CDF II) at the Fermilab Tevatron. The observed number ofB⁰andB⁰s can- didates is consistent with background expectations. The resulting Bayesian upper limits on the branching ratios at 90% credibility level are BðB⁰s!e^þÞ<2:010⁷, BðB⁰!e^þÞ<6:410⁸,

BðB⁰s !e^þeÞ<2:810⁷, andBðB⁰!e^þeÞ<8:310⁸. From the limits onBðB⁰_ðsÞ!e^þÞ, the following lower bounds on the Pati-Salam leptoquark masses are also derived:MLQðB⁰s!e^þÞ>

47:8 TeV=c², andMLQðB⁰!e^þÞ>59:3 TeV=c², at 90% credibility level.

DOI:10.1103/PhysRevLett.102.201801 PACS numbers: 13.20.He, 11.30.Hv, 12.15.Mm, 12.60.i

Rare particle decays that are either forbidden within the standard model (SM) of particle physics or are expected to have very small branching ratios provide excellent signa- tures with which to look for new physics and allow us to probe subatomic processes that are beyond the reach of direct searches. The decaysB⁰s !e^þandB⁰ !e^þ [1] are forbidden within the SM, in which lepton number and lepton flavor are conserved. However, the observation of neutrino oscillations indicates that lepton flavor is not conserved. To date, no lepton flavor violating (LFV) de- cays in the charged sector such asB⁰s!e^þ andB⁰ ! e^þ have been observed. These decays are allowed in models where the SM has been extended by heavy singlet Dirac neutrinos [2]. The LFV decays are also allowed in some physics scenarios beyond the SM, such as the Pati- Salam model [3] and supersymmetry models [4]. The grand-unification theory by Pati and Salam predicts a new interaction to mediate transitions between leptons and quarks via exchange of spin-1 gauge bosons, which are called Pati-Salam leptoquarks (LQ), that carry both color and lepton quantum numbers [3]. The lepton and quark components of the leptoquarks are not necessarily from the same generation [5,6], and the decays B⁰s ! e^þandB⁰!e^þcan be mediated by different types of leptoquarks. Processes involving flavor-changing neu- tral currents (FCNCs) can occur in the SM only through higher-order Feynman diagrams where new physics con- tributions can provide a significant enhancement. Com- pared to B⁰_ðsÞ!^þ [7], the FCNC decays of B⁰_ðsÞ! e^þe are further suppressed by the square of the ratio of the electron and muon massesðme=mÞ². The SM expec- tations for branching ratios ofB⁰_ðsÞ!e^þeare of the order of10¹⁵ [8].

In this Letter we report on a search for the LFV decays B⁰_s !e^þ and B⁰!e^þ and the FCNC decays B⁰_ðsÞ!e^þe, using a data sample corresponding to 2 fb¹ of integrated luminosity collected inpp collisions at ffiffiffi

ps

¼1:96 TeV. With no evidence for either the LFV or FCNC decays, we set upper limits on their branching ratios using the common reference decay B⁰ !K^þ, which has a precisely known branching ratio.

A detailed description of the CDF II detector can be found in Ref. [9]. Here we give a brief description of the detector elements most relevant to this analysis. Charged particle tracking is provided by a silicon microstrip detec- tor together with the surrounding open-cell wire drift chamber (COT), both immersed in a 1.4 T axial magnetic field. The tracking system provides precise vertex and momentum measurement for charged particles in the pseu-

dorapidity range jj<1:0[10]. Surrounding the tracking system are electromagnetic (CEM) and hadronic sampling calorimeters, arranged in a projective geometry. Drift chambers and scintillation counters are located behind the calorimeters to detect muons withinjj<0:6(CMU) and0:6<jj<1:0(CMX).

We use a data sample enriched in two-body B decays selected by a three-level trigger system using the extremely fast tracker [11] at level 1, and the silicon vertex trigger [12] at level 2. The trigger requires two oppositely charged tracks, each with a transverse momentump_T>2 GeV=c, and an impact parameter [13] 0:1< d0<1 mm. It also requires the scalar sum of the transverse momenta of the two tracks to be greater than5:5 GeV=c, the difference in the azimuthal angles of the tracks20<’ <135, and a transverse decay length [14]Lxy>200m. At the level-3 trigger stage, and in the off-line analysis, the trigger selec- tions are enforced with a more accurate determination of the same quantities. In the off-line analysis, additionally we require theB-meson isolationI >0:675[15], the point- ing angle <6:3[16], and a tighter selection ofLxy>

375m. These three thresholds were optimized in an unbiased way to obtain the best sensitivity for the searches using the procedure described in Ref. [17].

Electron and muon identification is applied in the selec- tion of B⁰sðB⁰Þ !e^þ and B⁰sðB⁰Þ !e^þe decay modes. The electron identification [18] requires that both the specific ionization (dE=dx) measured in the COT and the transverse and longitudinal shower shape as measured in the CEM be consistent with the hypothesis that the particle is an electron. The performance of electron iden- tification is optimized using pure electron samples recon- structed from !e^þe conversions and hadron and muon samples fromD⁰!K^þ,!p, andJ=c ! ^þ decays. We find the identification efficiency to be around 70% for electrons passing through the fiducial regions of the detectors used for electron identification.

The muon identification starts from tracks in the COT that are extrapolated into the muon detectors and are required to match hits in the muon systems. The muon selection efficiency for muons with pT>2 GeV=c in CMU or CMX has been measured to be greater than 99% using muons fromJ=c ^decays.

The mass resolutionm of fully reconstructedB-meson decays to two charged particles is about 28 MeV=c². Energy loss due to bremsstrahlung by electrons generates a tail on the low side of the mass distribution. This tail is more prominent for theB⁰s!e^þeandB⁰!e^þechan- nels, where two electrons are involved. We define search 201801-4

windows of ð5:262–5:477ÞGeV=c² for B⁰_s !e^þ and ð5:171–5:387Þ GeV=c²forB⁰ !e^þ. These correspond to a window around the nominal values of theB⁰_s andB⁰ masses [19] of approximately3_m. To recover some of the acceptance loss due to electron bremsstrahlung for the B⁰s !e^þe and B⁰!e^þe channels, we choose wider and asymmetric search windows ranging from6m below to3mabove the nominal values of theB⁰sandB⁰masses.

The search windows areð5:154–5:477ÞGeV=c²for theB⁰s

and ð5:064–5:387Þ GeV=c² for the B⁰. The sideband re- gions ð4:800–5:028Þ GeV=c² and ð5:549–5:800ÞGeV=c² are used to estimate the combinatorial backgrounds.

The background contributions considered include com- binations of random track pairs and partialB decays that accidentally meet the selection requirement (combinato- rial), and hadronic two-bodyBdecays in which both final particles are misidentified as leptons. The combinatorial background is evaluated by extrapolating the normalized number of events found in the sidebands to the signal region. The double-lepton misidentification rate is deter- mined by applying electron and muon misidentification probabilities to the number of two-body decays found in the search window.

Figure1shows the invariant mass distribution fore^þ candidates. We observe one event in theB⁰s mass window, and two events in theB⁰mass window, consistent with the estimated total background of0:80:6 events in the B⁰s

search window, and0:90:6in theB⁰window. The total background consists of two components: the combinatorial background in both channels is estimated to be0:70:6 events, the number of events where two tracks are mis- identified as electron and muon is estimated to be0:09 0:02for theB⁰_s case and0:220:04for theB⁰ case.

Figure2shows the invariant mass distributions fore^þe candidate pairs where both tracks were identified as elec- trons. We observe one event in theB⁰s mass window, and two events in theB⁰ mass window. We estimate the total background contributions to be2:71:8events in both the B⁰s and B⁰ mass windows. The dominant contribution comes from combinatorial background, 2:71:8, com- pared to the contribution where both tracks are misidenti- fied as electrons,0:0380:008, for bothB⁰sorB⁰.

We use the reference decayB⁰!K^þ to set a limit on BðB⁰_s!e^þ‘Þ (where ‘ is either e or ), using the following expression:

BðB⁰s !e^þ‘Þ

¼NðB⁰s !e^þ‘ÞBðB⁰ !K^þÞfd=fs

^rel_B0

s!e^þ‘NðB⁰ !K^þÞ :

The expression for theB⁰channels is identical, except that the ratio ofb-quark fragmentation probabilities,fd=fs, is not present. In the expression,NðB⁰s !e^þ‘Þis the calcu- lated upper limit on the number of B⁰s!e^þ‘ events, NðB⁰ !K^þÞ is the observed number of events from

the reference channel B⁰ !K^þ, BðB⁰!K^þÞ ¼ ð19:40:6Þ 10⁶ [19] is the branching ratio for the B⁰!K^þ decay, and ^rel_B0

s!e^þ‘ is the detector accep- tance and event selection efficiency for reconstructing B⁰s!e^þ‘ decays relative to that forB⁰!K^þ. The value offd=fsis3:860:59, where the (anti)correlation between the uncertainties has been accounted for [20]. To account for the differences in detector fiducial coverage and event selection efficiencies between the search and reference channel we use Monte Carlo events with a de- tailed simulation of the CDF detector response. Collision data are used to measure electron and muon identifica- tion efficiencies. We obtain ^rel_B0

s!e^þ¼0:2070:016, ^rel_B0!e^þ¼0:2100:012, ^rel_B0

s!e^þe¼0:1290:011, and ^rel_B0!e^þe ¼0:1280:011. These results of relative detector acceptance and efficiency also include effects of the different search windows for the e^þ and e^þe channels. The uncertainties listed above are the combined

) GeV/c2

e-

M(e+

4.8 4.9 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8

2Events/20MeV/c

0 1 2 3 4

- e+ e→0B - e+ e→s0B

Sideband Sideband

FIG. 2 (color online). Invariant mass distributions of e^þe pairs for events where both tracks passed the electron identifi- cation. TheB⁰s (B⁰) search window is indicated by the solid line (short dashed line). The sideband regions are indicated by dashed lines.

) GeV/c2

µ-

M(e+

4.8 4.9 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8

2Events/20MeV/c

0 1 2 3 4

-µ + e→ 0B -µ + e→ 0 sB

Sideband Sideband

FIG. 1 (color online). Invariant mass distribution of e^þ pairs for events where one track passed the electron identifica- tion and the other track the muon identification. The B⁰s (B⁰) search window is indicated by the solid line (short dashed line).

The sideband regions are indicated by the dashed lines.

statistical and systematic uncertainties. The latter include uncertainties from detector fiducial coverage, electron and muon identification efficiencies, detector material determi- nation,B⁰s andB⁰ pT spectrum, and B⁰s andB⁰ lifetimes.

The reference channel B⁰ !K^þ has been recon- structed using the same selection criteria except lepton identification. We find 6387214 B⁰!K^þ events, using a fitting procedure similar to that described in Ref. [21]. The uncertainty as returned by the fit is a combination of the mass fitting uncertainty and the sample composition uncertainties.

The upper limit on the branching ratio in each search window is obtained using the Bayesian approach [19], assuming a flat prior, and incorporating Gaussian uncer- tainties into the limit. The total systematic uncertainties, listed in TableI, are used as input for the limit calculation.

TableIIlists the upper limits we obtain on the branching ratios at 90% (95%) credibility level (C.L.).

Within the Pati-Salam leptoquark model, the following relationship between the BðB⁰_ðsÞ!e^þÞ and the lepto- quark mass (M_LQ) can be derived [5]:

BðB⁰_ðsÞ!e^þÞ ¼²sðMLQÞ 1 M_LQ⁴ F²_B0

ðsÞm³_B0 ðsÞR^{2 B}

0ðsÞ

@ ; where R¼^m_m^B0ðsÞ

b ð^sðMLQÞ

sðmtÞÞ⁴⁼⁷ð^sðmtÞ

sðmbÞÞ¹²⁼²³. The values and uncertainties of the quantities used in the calculation of MLQ are the following [19]: the top-quark mass mt

(171:22:1 GeV=c²), the bottom-quark massmb(4:20 0:17 GeV=c²), the charm quark mass mc (1:27 0:11 GeV=c²), the B⁰-meson mass m_B⁰ (5:279 53 0:000 33 GeV=c²), the B⁰_s-meson mass m_B⁰_s (5:3663

0:0006 GeV=c²), the B⁰-meson lifetime _B0 (1:530 0:009 ps), theB⁰_s-meson lifetime _B0

s (1:4700:027 ps), the coupling strength F_B⁰ (0:1780:014 GeV), andF_B⁰_s (0:2000:014 GeV) [22]. For the strong coupling con- stant we use sðM_Z⁰Þ ¼0:115, which is evolved to MLQ

using the Marciano approximation [23] assuming no col- ored particles exist with masses between mt and MLQ. Using the limits on the branching ratios listed in Table II, we calculate limits on the masses of the correspond- ing Pati-Salam leptoquarks of M_LQðB⁰_s !e^þÞ>

47:8ð44:9ÞTeV=c² and M_LQðB⁰ !e^þÞ>

59:3ð56:3ÞTeV=c² at 90% (95)% C.L. Figure 3 shows the limit and the relation between the leptoquark mass and the branching ratio for theB⁰s meson.

In summary, we report on a search for the lepton flavor violating decays B⁰s !e^þ and B⁰!e^þ and the flavor-changing neutral-current decays B⁰s!e^þe and B⁰!e^þe using data corresponding to 2 fb¹ of inte- grated luminosity collected in pp collisions at ffiffiffi

ps 1:96 TeV. This is the first search forB⁰_s !e^þeandB⁰!¼ e^þedecays at the Tevatron. We observe no evidence for

2 ] ) [ TeV/c µ

→ e

s

(B0

MLQ

30 40 50 60 70 80 90 100

)µ e → s

0B(B

10-8 10-7 10-6

Valencia and Willenbrock Ref. [5]

0.027 ps

± ) = 1.470 (Bs τ with:

0.0006 GeV/c2

± ) = 5.3663 m(Bs

0.17 GeV/c2

± = 4.20 mb

0.014 GeV

± ) = 0.200 F(Bs

@ 90% (95)% C.L.

) < 2.0 (2.6) x 10-7

µ

→ e

s

B(B0

@ 90% (95)% C.L.

) > 47.8(44.9) TeV/c2

µ

→ e

s

(B0

MLQ

90% C.L.

95% C.L.

FIG. 3 (color online). Leptoquark mass limit corresponding to the 90(95)% C.L. onBðB⁰s !e^þÞ. The error band is obtained by varying the values entering the theoretical calculation within their uncertainties. The uncertainties stemming from approxi- matings are not included.

TABLE I. Values used to calculate the limits onBðB⁰_ðsÞ!e^þÞandBðB⁰_ðsÞ!e^þeÞand their uncertainties.

Source Values BðB⁰_s !e^þÞ BðB⁰!e^þÞ BðB⁰_s !e^þeÞ BðB⁰!e^þeÞ

NðB⁰!K^þÞ 6387214 3.4% 3.4% 3.4% 3.4%

BðB⁰!K^þÞ ð19:40:6Þ 10⁶ 3.1% 3.1% 3.1% 3.1%

fd=fs 3:860:59 15.3% 15.3%

^rel_B0

s!e^þ 0:2070:016 7.6%

^rel_B0!e^þ 0:2100:012 5.9%

^rel_B0

s!e^þe 0:1290:011 8.9%

^rel_B0!e^þe 0:1280:011 8.9%

Total 17.7% 7.5% 18.3% 10.0%

TABLE II. Branching ratio limits at 90% (95)% C.L.

BðB⁰_s !e^þÞ<2:0ð2:6Þ 10⁷ BðB⁰!e^þÞ<6:4ð7:9Þ 10⁸ BðB⁰_s !e^þeÞ<2:8ð3:7Þ 10⁷ BðB⁰!e^þeÞ<8:3ð10:6Þ 10⁸

201801-6

these decays and set limits that are the most stringent to date. These results represent a significant improve- ment compared to the previous measurement [24] by CDF and the best results from B Factories [25–27] and LEP [28].

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 Founda- tion; 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 Science and Technology Facilities Council and the Royal Society, U.K.; the Institut National de Physique Nucleaire et Physique des Particules/CNRS; the Russian Foundation for Basic Research; the Ministerio de Ciencia e Innovacio´n, and Programa Consolider-Ingenio 2010, Spain; the Slovak R&D Agency; and the Academy of Finland.

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