Article
Reference
Search for neutral B meson decays to two charged leptons
L3 Collaboration
AMBROSI, Giovanni (Collab.), et al.
L3 Collaboration, AMBROSI, Giovanni (Collab.), et al . Search for neutral B meson decays to two charged leptons. Physics Letters. B , 1997, vol. 391, p. 474-480
DOI : 10.1016/S0370-2693(96)01583-3
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Physics Letters B 391 (1997) 474-480
PHYSICS LETTERS B
Search for neutral B meson decays to two charged leptons
L3 Collaboration
M. Acciarriab, 0. Adrianiq, M. Aguilar-Benitez”, S. Ahlen k, B. Alpat ai, J. Alcaraz”, G. Alemanni w, J. Allaby r, A. Aloisio ad, G. Alverson ‘, M.G. Alviggi ad, G. Ambrosi t,
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Wang P,476 M. Acciarri et al./ Physics Letters B 391 (1997) 474-480
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Y. Zhou ‘, G.Y. Zhu g, R.Y. Zhu *, A. Zichichi i,r,s, F. Ziegler aw
’ I.
Physikalisches Institut, RWTH, D-52056 Aachen, FRG 1 III. Physikalisches Institut, RWTH, D-52056 Aachen, FRG ’’ National Institute for High Energy Physics, NIKHEE and University of Amsterdam, NL-1009 DB Amsterdam, The Netherlands c University of Michigan, Ann Arbor; MI 48109, USA
’ Laboratoire d’Annecy-le-Vteux de Physique des Particules, LAPPIN2P3-CNRS, BP 110, F-74941 Annecy-le-Vieux CEDEX, France e Johns Hopkins University, Baltimore, MD 21218, USA
f Institute of Physics, University of Basel, CH-4056 Basel, Switzerland E Institute of High Energy Physics, IHEP, 100039 Beijing, China6
h Humboldt University D-10099 Berlin, FRG I i INFN-Sezione di Bologna, I-40126 Bologna, Italy j Tata Institute of Fundamental Research, Bombay 400 005, India
k Boston University, Boston, MA 02215, USA
’ Northeastern University, Boston, MA 02115, USA
m Institute of Atomic Physics and University of Bucharest, R-76900 Bucharest, Romania
” Central Research Institute for Physics of the Hungarian Academy of Sciences, H-I525 Budapest 114, Hungary2 Q Harvard University, Cambridge, MA 02139, USA
Q Massachusetts Institute of Technology, Cambridge, MA 02139, USA 4 INFN Sezione di Firenze and University of Florence, I-50125 Florence, Italy t European Laboratory for Particle Physics, CERN, CH-121 I Geneva 23, Switzerland
s World Laboratory, FBLJA Project, CH-1211 Geneva 23, Switzerland
’ University of Geneva, CH-IZII Geneva 4, Switzerland
u Chinese University of Science and Technology, USTC, Hefei, Anhui 230 029, China 6 v SEFI: Research Institute for High Energy Physics, PO. Box 9, SF-00014 Helsinki, Finland
w University of Iausanne, CH-IO15 Iausanne, Switzerland
’ INFN-Sezione di Lecce and Universita Degli Studi di Lecce, I-73100 Lecce, Italy Y Los Alamos National Laboratory, Los Alamos, NM 87544, USA
’ Institut de Physique Nucleaire de Lyon, IN2P3-CNRSUniversite’ Claude Bernard, F-69622 Villeurbanne, France aa Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas, CIEMAT, E-28040 Madrid, Spain3
ab INFN-Sezione di Milano, I-20133 Milan, Italy
ac Institute of Theoretical and Experimental Physics, ITEP, Moscow, Russia ad INFN-Sezione di Napoli and University of Naples, I-8OI25 Naples, Italy ae Department of Natural Sciences, University of Cyprus, Nicosia, Cyprus af University of Nymegen and NIKHEF NL-6525 ED Nymegen, The Netherlands
‘8 Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA ah California Institute of Technology, Pasadena, CA 91125, USA
ai INFN-Sezione di Perugia and Universitd Degli Studi di Perugia, I-06100 Perugia, Italy
=j Carnegie Mellon University Pittsburgh, PA 15213, USA 3k Princeton University Princeton, NJ 08544, USA
ae INFN-Sezione di Roma and University of Rome, “La Sapienza”, I-00185 Rome, Italy am Nuclear Physics Institute, St. Petersburg, Russia
‘Q University and INFN, Salerno, I-84100 Salerno, Italy i10 University of California, San Diego, CA 92093, USA
aQ Dept. de Fisica de Particulas Elementales, Univ. de Santiago, E-15706 Santiago de Compostela, Spain aq Bulgarian Academy of Sciences, Central Laboratory of Mechatronics and Instrumentation, BU-III3 Sofia, Bulgaria
a= Center for High Energy Physics, Korea Advanced Inst. of Sciences and Technology, 305-701 Taejon, South Korea as University of Alabama, Tuscaloosa, AL 35486, USA
Bt Vtrecht University and NIKHEF NL-3584 CB Utrecht, The Netherlands au Purdue University, West Lafayette, IN 47907, USA av Paul Scherrer Institut, PSI, CH-5232 Villigen, Switzerland Zw DESY-Institut fiir Hochenergiephysik, D-15738 Zeuthen, FRG
ay Eidgeniissische Technische Hochschule, ETH Ziirich, CH-8093 Ziirich, Switzerland
M. Acciarri et al./Physics Letters B 391 (1997) 474-480
az University of Hamburg, D-22761 Hamburg, FRG ha High Energy Physics Group, Taiwan, ROC
Received 29 October 1996 Editor: K. Winter
477
Abstract
The decays B$ Bg -+ e+e-, ,u”+pu-, e*pF are searched for in 3.5 million hadronic Z events, which constitute the full LEP I data sample collected by the L3 detector. No signals are observed, therefore upper limits at the 90% (95%) confidence levels are set on the following branching fractions:
Br(Bi -+ e+e-) < 1.4(1.8) x lo-‘; Br(Bf + e+e-) < 5.4(7.0) x 10m5;
Br(Bi -+ ,u+,u-) < l.O( 1.4) x 10V5; Br(Bf -+ ,zfpu-) < 3.8(5.1) x 10m5;
Br(Bi -+ e*pF) < 1.6(2.0) x 10m5; Br(Bf --+ e*p?) < 4.1(5.3) x 10p5.
The results for Bf -+ e+e- and Bf -+ e’,uT are the first limits set on these decay modes.
1. Introduction
Measurements of rare B hadron decay rates provide clean tests of the Standard Model and are in general sensitive to new physics. B” --+ C? decays 7 are par- ticularly clean both theoretically and experimentally.
The flavour-changing neutral current decays, B” --+
e+e- and B” -+ p+,u-, are forbidden at tree level.
They occur in the Standard Model due to higher-order processes with branching fractions < 0( 10e9) [ 11, which is beyond the sensitivity of LEP, and are sen- sitive to the Cabibbo-Kobayashi-Maskawa matrix el- ements, the top quark mass, and the B meson decay constants. B” --+ e * p F decays violate lepton number conservation and are forbidden in the Standard Model.
Observation of B” --+ @?- decays at LEP would be a clear indication for physics beyond the Standard Model. For example, two Higgs doublet models pre-
’ Supported by the German Bundesministerium Wr Bildung, Wis- senschaft, Forschung und Technologie.
2 Supported by the Hungarian OTKA fund under contract number T14459.
s Supported also by the Comisidn Interministerial de Ciencia y Technologia.
4 Also supported by CONICET and Universidad National de La Plats, CC 67, 1900 La Plata, Argentina.
5 Also supported by Panjab University, Chandigarh-160014, India.
6 Supported by the National Natural Science Foundation of China.
7 Throughout this paper we use B” to denote either Bi or Bz, and i?!- to denote e+e- or ,&pL- or e*p’. The latter case is a sum of e+p- and e-p+.
diet significant enhancements to the B” --+ e+e- de- cay rates [ 2-51.
This analysis is performed on data recorded during 1991-1995 running at the Z, corresponding to a sam- ple of approximately 3.5 million hadronic Z decays.
The mixed sample of B hadrons produced in Z decays provides an opportunity to study Bz meson decays which are not accessible at the center-of-mass energy of the Y(4S).
2. The L3 detector
The L3 detector is described in detail else- where [6,7]. The central tracking chamber is a Time Expansion Chamber (TEC) consisting of two coaxial cylindrical drift chambers with 12 inner and 24 outer sectors. The Z-chamber surrounding the TEC consists of two coaxial proportional cham- bers with cathode strip readout. The electromag- netic calorimeter is composed of bismuth germanate (BGO) crystals. Hadronic energy depositions are measured by a uranium-proportional wire chamber sampling calorimeter surrounding the BGO. Scintil- lator time-of-flight counters are located between the electromagnetic and hadronic calorimeters. The muon spectrometer, located outside the hadron calorimeter, consists of three layers of drift chambers measuring the muon trajectory in both the bending and the non- bending planes. All subdetectors are installed inside a solenoidal magnet which provides a uniform field
478 M. Acciarri et al./ Physics Letters B 391 (1997) 474-480
of 0.5 T.
The invariant mass resolution of pairs of electrons measured in the BGO calorimeter is approximately 70MeV for the typical kinematics of B” --) e+e- decays. Similarly, the di-muon mass resolution is 180MeV and the e*pF mass resolution is 140MeV.
3. Simulation of B” -+ J?P decays and backgrounds
The JETSET Monte Carlo program [ 8,9] is used to simulate hadronic Z decays. To model b quark frag- mentation the Peterson function [lo] is used as a function of XE = 2Ehadron/fi, with a mean value of (XE) = 0.703. The masses of the Bi and Bt mesons are assumed to be 5279 MeV and 5373 MeV respectively, which are consistent with the most recent world aver- age values [ 111. The events produced by JETSET are passed through the GEANT-based L3 detector simula- tion program [ 121 which allows for the effects of en- ergy loss, multiple scattering, decays and interactions in the detector material, as well as time-dependent de- tector effects. These events are then reconstructed us- ing the same algorithms as for the data.
To study the sensitivity of the L3 detector to events containing B” -+ &‘Y- decays, we first simulate e+e- --f bb events. Bj s mesons are forced to decay via the chain B& -+ .&C- (! = e,p). If, however, there is more than one Bi s meson in the event, only one is required to decay in this way. For the back- ground studies a sample of hadronic events is used which does not include the B” --+ &?- decay modes.
4. Event selection
Hadronic events are selected by making use of their characteristic energy distributions and high multiplic- ity [ 131. A total of 3 453 780 events from the 1991- 1995 data samples are selected.
Candidate electrons are selected in the barrel and endcap BGO calorimeters within 1 cos 01 < 0.97, where 0 is the polar angle. An electron is character- ized by an isolated energy cluster in the BGO with a shower shape consistent with that of electromagnetic particles. To reject photons, the cluster is required to match with a charged track to within 5 mrad in the
plane transverse to the beam direction. Electron can- didates are required to have an energy of more than 2 GeV.
Muon candidate tracks in the muon spectrometer are required to be within ( cos 81 < 0.8 and to have hits in at least two of the three t-4 layers and at least one of the two z layers. Backgrounds from punchthrough hadrons, decays in flight, and cosmic rays are sup- pressed by requiring the muon chamber track to point towards the primary vertex. Residual contamination due to cosmic rays which coincide with a genuine hadronic event is reduced to a negligible level by scin- tillator timing cuts. Muon candidates are required to have a momentum of more than 2GeV.
Due to the hard fragmentation of the b quark, the energy carried by the two leptons from a B” decay is large. Pairs of oppositely charged candidate leptons are therefore required to have a combined energy of more than 20GeV. The opening angle of the candi- date lepton pair is required to be less than 90”, to en- sure that both lepton candidates originated from the decay chain of the same primary quark. After these cuts the background consists predominantly of fake leptons with a small irreducible contribution from the decay chain b -+ c&-l; c --+ sC+v.
After applying these selection criteria the ef- ficiencies, E( B” -+ W- ) , are determined from Monte Carlo studies to be E( B” --+ efe- ) =
(39.0 f 5.2)%, e(B” --) p+,u-) = (39.2 f 4.8)%, and E(B’ -+ e*,uF) = (39.4 f 4.1)%, where the errors are dominated by systematic effects. The sys- tematic errors are estimated by comparing the data and Monte Carlo distributions of the selection vari- ables with various less stringent values for the cuts applied. A small contribution is also included from the uncertainty in the b quark fragmentation function.
5. Determination of branching fractions
Fig. 1 shows the expected invariant mass distribu- tions after application of the selection procedure de- scribed above, for a) B” -+ e+e-, b) B” --+ P+,u-, and c) B” 4 e * p F. Each is normalised to the in- tegrated luminosity of the data and corresponds to a nominally assumed branching fraction of Br(B’ + N-) = 10v5 for each process.
Fig. 1 also shows the mass spectra obtained from
h4. Acciarri et al./ Physics Letters B 391 (1997) 474-480 479
M(ee) GeV W-w.) GeV M(ep) GeV
>
4
8 ? 3 z 2 Y F 1
* 0
4 5 6 7
M(ee) GeV M(m-0 GeV M(ep) C&V
Fig. 1. Predictions for the dilepton invariant mass distributions for a) B” -+ e+e-, b) B” ----f p+p-, and c) B” -+ e*pF decays, assuming the integrated luminosity and efficiencies of this analysis and a nominal branching fraction of Br(B” ---t I?!?-) = low5 for each process. Mass spectra obtained from the data (solid line) for d) efe-, e) pf,uL-, and f) e*,uF; the dashed lines show the background predicted by Monte Carlo.
the data (solid line) for d) e+e-, e) ,u+p-, and f) e*,uF. The dashed line represents the background contribution which is estimated from the Monte Carlo sample of hadronic Z decays for which the same se- lection used for the data is applied. No signal is seen for Bi s
decays:
--) e+e-, Bi,, -+ ,ucc+,uu-, or Bj,, -+ e*pF Upper limits on the branching fractions for these processes are obtained from binned maximum- likelihood fits to the P!? invariant mass distributions.
For each di-lepton sample, the signal comprises two components, corresponding to Bj and Bf decays. The likelihood function is given by:
C(pd, Pus) hi”.
rI
e-(,%i+PCLd.I+ki) (
= pd,i + ,h,i + pb,i) N
i=l Ni!
where Ni is the number of observed data events in mass bin i, and pd,i, pu,t, and /Ab,i denote the expected numbers of events for Bz, Bf, and background respec-
tively.
The mass-dependences of the Bt and Bf signal com- ponents, /&j and ,Uu,,i, respectively, are taken from Figs. la), 1 b) , and Ic) . The total signal components, pu4 (q = d, s) , are given by:
Wills
ps=C+i=Br(Bi+Pe-) XNh ~‘2
i=l
x Rb6 x fq x E(B’ -+ e+e-)
where Nh = 3 453 780 is the total number of hadronic Z decays and R,, E r&/rh& = 0.222 f 0.0076 is taken from the L3 measurement [ 131. The fractions fd = (37.8 f 2.2)% and f, = (11.2?:.,8,)% [ 141 are the probabilities that a b quark is dressed in the frag- mentation to become a Bi or Bf meson respectively.
The background component, pb,i, is determined using the Monte Carlo sample of e+e- -+ hadrons events. The expected numbers of background events within f3a are 0.8, 2.6, and 2.4 events respectively for the efe-, p”+pu-, and e&,uF final states. The
480 M. Acciarri et al./Physics Letters B 391 (1997) 474-480 systematic errors on the background of between 10%
and 13% include detector effects, which are estimated by relaxing the selection cuts and comparing the data and Monte Carlo distributions of the selection vari- ables. This procedure accounts for systematic detector effects, uncertainties on the b quark fragmentation function, the heavy quark semi-leptonic branching fractions, and R,,. The statistical error on the back- ground is parametrised by a Poisson distribution.
The likelihood is evaluated as a function of the branching fractions Br(Bi -+ !+!-) and Br(Bt -+ lf4?), taking into account the errors on Rbs, fd, f,, the efficiencies, and the background parameterization. By varying Br(Bi -+ t+e-) and Br(Bf -+ C+!-) simultaneously we obtain the two-dimensional likelihood distribution. The one- dimensional likelihood distributions for the Bj (Bf) branching fractions are obtained by integrating the likelihood over all the values of the Bf (Bz) branch- ing fractions. The 90% (95%) confidence level limits are
Br(Bi -+ e+e-) < 1.4(1.8) x lo-‘;
Br(Bt -+ efe-) < 5.4(7.0) x 10w5;
Br(Bz --+ ,u+,u-) < l.O( 1.4) x 10p5;
Br(Bf --+ /_I,+/A-) < 3.8(5.1) x 10p5;
Br(Bi -+ e*pF) < 1.6(2.0) x 10w5;
Br(Bf -+ e*,uF) < 4.1(5.3) x 10p5.
The results for Bi --+ efe- and Bz --) e*,!&F are the first limits set on these decay modes. The results for Bad -+ e+e-, Bz -+ ,x+P-, Bz -+ e*pF, and By + p+p- are consistent with the slightly more stringent limits from CLEO [ 151 and CDF [ 161.
Acknowledgments
We wish to express our gratitude to the CERN ac- celerator divisions for the excellent performance of the LEP machine. We acknowledge the contributions of all the engineers and technicians who have partici- pated in the construction and maintenance of this ex- periment. Those of us who are not from member states thank CERN for its hospitality and help.
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