Measurement of the B Hadron Energy Distribution in Z0 Decays

a r X i v :h e p -e x /9707011v 2 9 J u l 1997

SLAC-PUB-7489

June 1997

MEASUREMENT OF THE B HADRON ENERGY DISTRIBUTION IN Z 0DECAYS ?

The SLD Collaboration ??Stanford Linear Accelerator Center Stanford University,Stanford,CA 94309

ABSTRACT

We have measured the B hadron energy distribution in Z 0decays using a sample of semi-leptonic B decays recorded in the SLD experiment at SLAC.The energy of each tagged B hadron was reconstructed using information from the lepton and a partially-reconstructed charm-decay vertex.We compared the scaled energy distribution with several models of heavy quark fragmentation.The average scaled energy of primary B

hadrons was found to be =0.716±0.011(stat .)+0.021

?0.022(syst .).

Submitted to Physical Review D

?

Work supported by Department of Energy contracts:DE-FG02-91ER40676(BU),DE-FG03-91ER40618

(UCSB),DE-FG03-92ER40689(UCSC),DE-FG03-93ER40788(CSU),DE-FG02-91ER40672(Col-orado),DE-FG02-91ER40677(Illinois),DE-AC03-76SF00098(LBL),DE-FG02-92ER40715(Mas-sachusetts),DE-FC02-94ER40818(MIT),DE-FG03-96ER40969(Oregon),DE-AC03-76SF00515(SLAC),DE-FG05-91ER40627(Tennessee),DE-FG02-95ER40896(Wisconsin),DE-FG02-92ER40704(Yale);National Science Foundation grants:PHY-91-13428(UCSC),PHY-89-21320(Columbia),PHY-92-04239(Cincinnati),PHY-95-10439(Rutgers),PHY-88-19316(Vanderbilt),PHY-92-03212(Washing-ton);The UK Particle Physics and Astronomy Research Council (Brunel and RAL);The Istituto Nazionale di Fisica Nucleare of Italy (Bologna,Ferrara,Frascati,Pisa,Padova,Perugia);The Japan-US Cooperative Research Project on High Energy Physics (Nagoya,Tohoku);The Korea Science and Engineering Foundation (Soongsil).

1.Introduction

The production of heavy hadrons (H)in e +e ?annihilation provides a laboratory for the study of heavy-quark (Q)jet fragmentation.This is commonly characterised in

terms of the observable x E H ≡2E H /

√

s is the c.m.energy.In contrast to

light-quark jet fragmentation one expects [1]the distribution of x E H ,D (x E H ),to peak at an x E H -value signi?cantly above 0.Since the hadronisation process is intrinsically non-perturbative D (x E H )cannot be calculated directly using perturbative Quantum Chromodynamics (QCD).However,the distribution of the closely-related variable x E Q

≡2E

Q /

√

s =29

and 35GeV [11].In more recent analyses [12,13]the scaled energy distribution D (x E B )has been measured by reconstructing B hadrons via their B →D l X decay mode;we have applied a similar technique.We used the precise SLD tracking system to select jets containing a B →D l X decay,where the charmed hadron D was identi?ed semi-inclusively from a secondary decay vertex formed from charged tracks.Each hadronic

vertex was then associated with a lepton l(l=e orμ)with large momentum transverse to the jet direction.Neutral energy depositions measured in the hermetic calorimeter, as well as the energies of charged tracks,that were not associated with the D l system were subtracted from the jet energy to yield the reconstructed B hadron energy.This measurement technique may be useful to B-lifetime or B-mixing analyses[14]where

√

the proper time t=L/

sinθ)μm,whereθis the polar angle with respect to the beam-line.This results in a mean resolution on reconstructed2-prong vertices(Section 3)ofσV

=400(25)μm for the projection on an axis along(perpendicular to) (⊥)

the vertex?ight direction.The LAC electromagnetic energy scale was calibrated from the measuredπ0→γγsignal[21,22];the electromagnetic energy resolution

isσE/E≈0.15/

have corrected the simulation to account for https://www.wendangku.net/doc/8b3802498.html,ing an event weighting technique we produced a generator-level distribution of B hadron energies in which the energy E B of20.7%of all B hadrons was adjusted to be E B?Eπ,where the pion energy

Eπwas produced according to an isotropic2-body decay distribution for B??→Bπ±, assuming a B??mass of5.7GeV/c2.Uncertainties in this simulation of B??production were taken into account in the systematic errors(Section7).

3.B Hadron Selection

Hadronic events were required to contain a lepton candidate within the barrel tracking system with|cosθ|<0.7.We then applied the JADE jet-?nding algorithm[31]to the LAC clusters in each selected event to de?ne a jet topology.With a jet-resolution criterion of y c=0.07,82.9%of the events were classi?ed as2-jet-like and17.1% as3-jet-like.Kinematic information based on this topological classi?cation was used subsequently(Section4)in the calculation of the B hadron energy.Events in which the lepton had a transverse momentum w.r.t.its jet axis,p t,of at least1GeV/c were retained for further analysis.In jets containing more than one such lepton only the highest-p t lepton was labelled for association with a D vertex and any lower-momentum leptons were used in the D-vertex-?nding.

In each selected jet we then searched for a secondary D vertex among the non-lepton tracks.Tracks were required to comprise at least40CDC hits and one VXD hit,to be well contained within the CDC with|cosθ|≤0.70,to have momentum in the range 0.151.Tracks from K0s andΛ0decays andγconversions were suppressed by requiring the distance of closest approach to the IP in the planes both perpendicular to,and containing,the beamline to be less than1cm.Two-prong vertices were?rst formed from all pairs of tracks whose distance-of-closest-approach was less than0.012 cm and whose?t to a vertex satis?edχ2<5.A multi-prong D-vertex candidate was

then de?ned to comprise the tracks in all accepted two-prong vertices in the jet,and to be located at the position of the two-prong vertex containing the track with the largest normalised transverse impact parameter d/σd.

The tracks in each D vertex were each assigned the charged pion mass and were then combined by adding their four-vectors to obtain the vertex invariant mass,m D, and the vertex momentum vector.The vertex?ight distance from the IP was projected onto the jet axis to obtain the quantity r D.Events were retained if at least one jet contained a D vertex with0.30.05cm,r D normalised by its error larger than unity,and the distance-of-closest-approach between the lepton track and the extrapolated D-vertex momentum vector was less than0.012cm.The lepton and D-vertex tracks were then?tted to a common candidate B vertex.The combined D-vertex and lepton invariant mass,m B,and the projection of the vector between the B-and D-vertex positions onto the D-vertex momentum vector,r B,were calculated. Events were selected in which m B<4.5GeV/c2,r B>0.025cm,and r B normalized by its error was larger than unity.

For the selected events,distributions of the number of tracks per D vertex,N D,and of m D,r D,m B,and r B are shown in Fig.1.Also shown are the simulated distributions in which the contribution from selected true B→D l X decays is indicated.In Fig.2the distributions of lepton transverse momentum with respect to the jet axis,p t,are shown for candidates passing all cuts except the requirement that p t be above1GeV/c;the simulated distributions are also shown,and the contributions from di?erent processes are indicated.The?nal sample comprises597events,293in the muon,and304in the electron,https://www.wendangku.net/doc/8b3802498.html,ing the simulation we estimate that the purity of this sample, de?ned to be the fraction of the tagged events whose identi?ed leptons l are from true B→D l X decays,is69.2%;a further18%of the selected events contain B decays with a cascade,punch-through or mis-identi?ed lepton,and are still useful.The estimated composition of the bˉb events in terms of the B hadron species is shown in Table1. The remaining12.8%of the event sample comprises non-bˉb events.The e?ciency for

selecting B hadron decays in the selected hadronic event sample is shown,as a function of x E

,in Fig.3;the overall e?ciency is1.1%.

B

4.Measurement of the B Energies

In each selected event we?rst de?ned the jet energies by using kinematic information. The2-jet events were divided into two hemispheres by the plane normal to the thrust axis and the jet in each hemisphere was assigned the beam energy.For the3-jet events we corrected the jet energies according to the angles between the jet axes, assuming energy and momentum conservation and massless https://www.wendangku.net/doc/8b3802498.html,belling the jets arbitrarily1,2and3,and the corresponding inter-jet anglesθ23,θ13andθ12,the corrected energy of jet1is given by:

√

E1=

it had no charged track extrapolating to it to within an angle4σcl from its centroid, whereσcl=

the measured distribution of x rec E

B ,D data(x rec E

B

),in Fig.7.Also shown in this?gure is

the simulated distribution in which the background contribution from non-bˉb events is indicated.

https://www.wendangku.net/doc/8b3802498.html,parison with Model Predictions

It is interesting to compare our measured B hadron energy distribution with the the-oretical predictions.The event generator used in our simulation is based on a per-turbative QCD‘parton shower’for production of quarks and gluons,together with the phenomenological Peterson function[6](Table2)to account for the fragmenta-tion of b and c quarks into B and D hadrons,respectively,within the iterative Lund string hadronisation mechanism[25];this simulation yields a generator-level primary B-hadron energy distribution with

B

>=0.693?.It is apparent(Fig.7)that this simulation does not reproduce the data well;theχ2for the comparison is36.7for15 bins.

We have also considered alternative forms of the fragmentation function based on the phenomenological model of the Lund group[7],the perturbative QCD calcula-tions of Braaten et al.[4],(BCFY)and of Nason et al.[2](NCM),as well as ad hoc parametrisations based on a function used by the ALEPH Collaboration[12]and on a third-order polynomial.These functions are listed in Table2.

In order to make a consistent comparison of each function with the data we adopted the following procedure.Starting values of the arbitrary parameters were assigned and

the corresponding distribution of scaled primary B hadron energies,D MC(x true

E B

),was reproduced in our MC-generated bˉb event sample,before simulation of the detector, by weighting events accordingly.The resulting distribution,after simulation of the detector,application of the analysis cuts and background subtraction,of reconstructed

B hadron energies,D MC(x rec E

B

),was then compared with the background-subtracted

data distribution and theχ2value was calculated.This process was iterated to?nd

the minimum inχ2,yielding a parameter set that gives an optimal description of the reconstructed data by the input fragmentation function.This procedure was applied for each function listed in Table2.The?tted parameters and minimumχ2values are listed in Table3,and the corresponding D MC(x rec E

B

)are compared with the data in Fig.8.Each function reproduces the data.We conclude that,within our resolution and with our current data sample,we are unable to distinguish between these functions. It should be noted,however,that the optimal third-order polynomial function has a

small negative minimum point in the region around x true

E B

=0.2;since this behaviour is unphysical we did not consider this function further in the analysis.

6.Correction of the B Energy Distribution

In order to compare our results with those from other experiments it is necessary

to correct the reconstructed scaled B hadron energy distribution D data(x rec E

B

)for the e?ects of non-B backgrounds,detector acceptance,event selection and analysis bias, and initial-state radiation,as well as for bin-to-bin migration e?ects caused by the ?nite resolution of the detector and the analysis technique.We also corrected for the e?ects of B??decays(Section2)to derive the primary B hadron energy distribution.

We applied a15×15matrix unfolding procedure to D data(x rec E

B

)to obtain an estimate

of the true distribution D data(x true

E B

):

D data(x true

E B )=??1(x true

E B

)·E(x true

E B

,x rec E

B

)·(D data(x rec E

B

)?S(x rec E

B

))(4)

where S is a vector representing the background contribution,E is a matrix to correct for bin-to-bin migrations,and?is a vector representing the e?ciency for selecting true B hadron decays for the analysis.

The matrices S,E and?were calculated from our MC simulation;the elements of?are shown in Fig.3.The matrix E incorporates a convolution of the input frag-mentation function with the resolution of the detector.We used in turn the Peterson,

Lund,BCFY,NCM and ALEPH functions,with the optimised parameters listed in Table3,to produce both a generator-level input primary B hadron energy distribution

D MC(x true

E B ),and a reconstructed distribution D MC(x rec E

B

),as discussed in the previous

section.In each case E was evaluated by examining the population migrations of true

B hadrons between bins of the input scaled B energy,x true

E B

,and the reconstructed

scaled B energy,x rec E

B

.

The data were then unfolded according to Eq.(4)to yield D data(x true

E B

),which is shown for each input fragmentation function in Fig.9.It can be seen that the shapes

of D data(x true

E B

)di?er systematically among the assumed input fragmentation functions. These di?erence were used to assign systematic errors,as discussed in the next section.

7.Systematic Errors

We have considered sources of systematic uncertainty that potentially a?ect our mea-surement of the B-hadron energy distribution.These may be divided into uncertainties in modelling the detector and uncertainties on experimental measurements serving as input parameters to the underlying physics modelling.For these studies our standard simulation,employing the Peterson fragmentation function,was used.

The uncertainty on the correction of the non-B neutral jet energy component E neu

frag (Section4)was estimated by changing the LAC cluster-energy selection requirement from100to200MeV,and by varying the LAC electromagnetic energy scale within our estimated uncertainty of±2.2%of its nominal value[21].In each case the di?erence in results relative to our standard procedure was taken as the systematic uncertainty.

A large source of detector modelling uncertainty was found to relate to knowledge of the charged tracking e?ciency of the detector,which we varied by our estimated

uncertainty of±2.4%.In addition,in each bin of x rec E

B

,we varied the estimated con-tribution from fake leptons in the data sample(Fig.2)by±25%.These uncertainties were assumed to be uncorrelated and were added in quadrature to obtain the detector

modelling uncertainty in each bin of x E

.

B

As a cross-check we also varied the event selection requirements.The thrust-axis containment cut was varied in the range0.65<|cosθT|<0.70,the minimum number of charged tracks required was increased from7to8,and the total charged-track energy requirement was increased from20to22GeV.In each case results consistent with the standard selection were obtained.As a further cross-check on jet axis modelling we systematically varied y c in the range0.01≤y c≤0.15and repeated the analysis;results consistent with the standard analysis were obtained.

A large number of measured quantities relating to the production and decay of charm and bottom hadrons are used as input to our simulation.In bˉb events we have considered the uncertainties on:the branching fraction for Z0→bˉb;the rates of production of

B u,B d and B s mesons,and B baryons;the rate of production of B??mesons,and the B??mass;the branching ratios for B→D?and B→D??;the lifetimes of B mesons and baryons;and the average charged multiplicity of B hadron decays. In cˉc events we have considered the uncertainties on:the branching fraction for Z0→cˉc;the charmed hadron fragmentation function;the rates of production of D0,D+ and D s mesons,and charmed baryons;and the charged multiplicity of charmed hadron decays.We have also considered the rate of production of sˉs in the jet fragmentation process,and the production of secondary bˉb and cˉc from gluon splitting.The world-average values[9,32]of these quantities used in our simulation,as well as the respective uncertainties,are listed in Table4.

The variation of each quantity within its uncertainty was produced in turn in our simulated event sample using an event weighting technique[32].The matrices S and E(Section6)were then reevaluated using the simulated events,and the data were recorrected.In each case the deviation w.r.t.the standard corrected result was taken as a separate systematic error.These uncertainties were conservatively assumed to be uncorrelated and were added in quadrature to obtain a total physics modelling

.

uncertainty in each bin of x E

B

The model-dependence of the unfolding procedure was estimated by considering the envelope of the unfolded results illustrated in Fig.9.In each bin of x E

B

we calculated the average value of the?ve unfolded results,as well as the r.m.s.deviation.The average value was taken as our central value in each bin,and the r.m.s.value was assigned as the respective unfolding uncertainty.

8.Summary and Conclusions

We have used the precise SLD tracking system to reconstruct the energies of B hadrons in e+e?→Z0events via the B→D l X decay mode.We estimate our resolution on the B energy to be about10%for roughly65%of the reconstructed decays.The distribu-

tion of reconstructed scaled B hadron energy,D(x rec E

B

),was compared with perturbative QCD and phenomenological model predictions;the calculations of Braaten,Cheung and Yuan and of Nason,Colangelo and Mele are consistent with our data,as are the phenomenological models of Peterson et al.and of the Lund group.The distribution was then corrected for bin-to-bin migrations caused by the resolution of the method and for selection e?ciency,as well as for the e?ects of B??production,to derive the energy distribution of primary B hadrons produced by Z0decays.Systematic uncer-

tainties in the correction were considered.The?nal corrected x E

B distribution D(x E

B

)

is listed in Table5and shown in Fig.10;the statistical,experimental systematic,and

unfolding uncertainties are indicated separately.

It is conventional to evaluate the mean of this distribution,

B

>.For each of

the?ve functions used to correct the data we evaluated

B

>from the distribution that corresponds to the optimised parameters;these are listed in Table3.We took the

average of the?ve values of

B

>as our central result,and de?ned the unfolding

uncertainty to be the r.m.s.deviation.We list in Table4the errors on

B

> resulting from the study of detector and physics modelling described in Section7.We

obtained:

B >=0.716±0.011(stat.)+0.009

?0.011

(exp.syst.)±0.019(unfolding),

where the systematic error is the sum in quadrature of the individual contributions

listed in Table4.It can be seen that

B

>is relatively insensitive to the variety of

allowed forms of the shape of the fragmentation function D(x E

B

).

Our results are in agreement with a previous measurement of the shape of the primary B hadron energy distribution at the Z0resonance[12],as well as with mea-surements of the shape[13]and mean value[10]of the distribution for weakly-decaying

B hadrons,after taking account of our estimate that the latter

B

>value is about

https://www.wendangku.net/doc/8b3802498.html,bining all systematic errors in quadrature we obtain

B

>=

0.716±0.011(stat.)+0.021

?0.022

(syst.).

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??List of Authors

K.Abe,(19)K.Abe,(30)T.Akagi,(28)N.J.Allen,(4)W.W.Ash,(28)?D.Aston,(28) K.G.Baird,(24)C.Baltay,(34)H.R.Band,(33)M.B.Barakat,(34)G.Baranko,(9) O.Bardon,(15)T.L.Barklow,(28)G.L.Bashindzhagyan,(18)A.O.Bazarko,(10) R.Ben-David,(34)A.C.Benvenuti,(2)G.M.Bilei,(22)D.Bisello,(21)G.Blaylock,(16) J.R.Bogart,(28)B.Bolen,(17)T.Bolton,(10)G.R.Bower,(28)J.E.Brau,(20)

M.Breidenbach,(28)W.M.Bugg,(29)D.Burke,(28)T.H.Burnett,(32)P.N.Burrows,(15) W.Busza,(15)A.Calcaterra,(12)D.O.Caldwell,(5)D.Calloway,(28)B.Camanzi,(11) M.Carpinelli,(23)R.Cassell,(28)R.Castaldi,(23)(a)A.Castro,(21)M.Cavalli-Sforza,(6) A.Chou,(28)E.Church,(32)H.O.Cohn,(29)J.A.Coller,(3)V.Cook,(32)R.Cotton,(4) R.F.Cowan,(15)D.G.Coyne,(6)G.Crawford,(28)A.D’Oliveira,(7)C.J.S.Damerell,(25) M.Daoudi,(28)R.De Sangro,(12)R.Dell’Orso,(23)P.J.Dervan,(4)M.Dima,(8)

D.N.Dong,(15)P.Y.C.Du,(29)R.Dubois,(28)B.I.Eisenstein,(13)R.Elia,(28)

E.Etzion,(33)S.Fahey,(9)D.Falciai,(22)C.Fan,(9)J.P.Fernandez,(6)M.J.Fero,(15)

R.Frey,(20)T.Gillman,(25)G.Gladding,(13)S.Gonzalez,(15)E.L.Hart,(29)

J.L.Harton,(8)A.Hasan,(4)Y.Hasegawa,(30)K.Hasuko,(30)S.J.Hedges,(3) S.S.Hertzbach,(16)M.D.Hildreth,(28)J.Huber,(20)M.E.Hu?er,(28)E.W.Hughes,(28)

H.Hwang,(20)Y.Iwasaki,(30)D.J.Jackson,(25)P.Jacques,(24)J.A.Jaros,(28)

Z.Y.Jiang,(28)A.S.Johnson,(3)J.R.Johnson,(33)R.A.Johnson,(7)T.Junk,(28) R.Kajikawa,(19)M.Kalelkar,(24)H.J.Kang,(26)I.Karliner,(13)H.Kawahara,(28)

H.W.Kendall,(15)Y.D.Kim,(26)M.E.King,(28)R.King,(28)R.R.Ko?er,(16)

N.M.Krishna,(9)R.S.Kroeger,(17)https://www.wendangku.net/doc/8b3802498.html,bs,(28)https://www.wendangku.net/doc/8b3802498.html,ngston,(20)https://www.wendangku.net/doc/8b3802498.html,th,(15) https://www.wendangku.net/doc/8b3802498.html,uber,(9)D.W.G.S.Leith,(28)V.Lia,(15)M.X.Liu,(34)X.Liu,(6)M.Loreti,(21) A.Lu,(5)H.L.Lynch,(28)J.Ma,(32)G.Mancinelli,(24)S.Manly,(34)G.Mantovani,(22)

T.W.Markiewicz,(28)T.Maruyama,(28)H.Masuda,(28)E.Mazzucato,(11)

A.K.McKemey,(4)

B.T.Meadows,(7)R.Messner,(28)P.M.Mockett,(32)

K.C.Mo?eit,(28)T.B.Moore,(34)D.Muller,(28)T.Nagamine,(28)S.Narita,(30)

U.Nauenberg,(9)H.Neal,(28)M.Nussbaum,(7)?Y.Ohnishi,(19)N.Oishi,(19)

D.Onoprienko,(29)L.S.Osborne,(15)R.S.Panvini,(31)C.H.Park,(27)H.Park,(20)

T.J.Pavel,(28)I.Peruzzi,(12)(b)M.Piccolo,(12)L.Piemontese,(11)E.Pieroni,(23) K.T.Pitts,(20)R.J.Plano,(24)R.Prepost,(33)C.Y.Prescott,(28)G.D.Punkar,(28) J.Quigley,(15)B.N.Ratcli?,(28)T.W.Reeves,(31)J.Reidy,(17)P.L.Reinertsen,(6) P.E.Rensing,(28)L.S.Rochester,(28)P.C.Rowson,(10)J.J.Russell,(28)O.H.Saxton,(28) T.Schalk,(6)R.H.Schindler,(28)B.A.Schumm,(6)J.Schwiening,(28)S.Sen,(34) V.V.Serbo,(33)M.H.Shaevitz,(10)J.T.Shank,(3)G.Shapiro,(14)D.J.Sherden,(28) K.D.Shmakov,(29)C.Simopoulos,(28)N.B.Sinev,(20)S.R.Smith,(28)M.B.Smy,(8) J.A.Snyder,(34)H.Staengle,(8)P.Stamer,(24)H.Steiner,(14)R.Steiner,(1)

M.G.Strauss,(16)D.Su,(28)F.Suekane,(30)A.Sugiyama,(19)S.Suzuki,(19) M.Swartz,(28)A.Szumilo,(32)T.Takahashi,(28)F.E.Taylor,(15)E.Torrence,(15) A.I.Tranda?r,(16)J.D.Turk,(34)https://www.wendangku.net/doc/8b3802498.html,her,(28)J.Va’vra,(28)C.Vannini,(23)E.Vella,(28)

J.P.Venuti,(31)R.Verdier,(15)P.G.Verdini,(23)D.L.Wagner,(9)S.R.Wagner,(28) A.P.Waite,(28)S.J.Watts,(4)A.W.Weidemann,(29)E.R.Weiss,(32)J.S.Whitaker,(3) S.L.White,(29)F.J.Wickens,(25)D.C.Williams,(15)S.H.Williams,(28)S.Willocq,(28) R.J.Wilson,(8)W.J.Wisniewski,(28)M.Woods,(28)G.B.Word,(24)J.Wyss,(21) R.K.Yamamoto,(15)J.M.Yamartino,(15)X.Yang,(20)J.Yashima,(30)S.J.Yellin,(5)

C.C.Young,(28)H.Yuta,(30)G.Zapalac,(33)R.W.Zdarko,(28)and J.Zhou,(20)

(1)Adelphi University,Garden City,New York11530

(2)INFN Sezione di Bologna,I-40126Bologna,Italy

(3)Boston University,Boston,Massachusetts02215

(4)Brunel University,Uxbridge,Middlesex UB83PH,United Kingdom

(5)University of California at Santa Barbara,Santa Barbara,California93106

(6)University of California at Santa Cruz,Santa Cruz,California95064

(7)University of Cincinnati,Cincinnati,Ohio45221

(8)Colorado State University,Fort Collins,Colorado80523

(9)University of Colorado,Boulder,Colorado80309

(10)Columbia University,New York,New York10027

(11)INFN Sezione di Ferrara and Universit`a di Ferrara,I-44100Ferrara,Italy

(12)INFN Lab.Nazionali di Frascati,I-00044Frascati,Italy

(13)University of Illinois,Urbana,Illinois61801

(14)https://www.wendangku.net/doc/8b3802498.html,wrence Berkeley Laboratory,University of California,Berkeley,California

94720

(15)Massachusetts Institute of Technology,Cambridge,Massachusetts02139

(16)University of Massachusetts,Amherst,Massachusetts01003

(17)University of Mississippi,University,Mississippi38677

(18)Moscow State University,Institute of Nuclear Physics119899Moscow,Russia

(19)Nagoya University,Chikusa-ku,Nagoya464Japan

(20)University of Oregon,Eugene,Oregon97403

(21)INFN Sezione di Padova and Universit`a di Padova,I-35100Padova,Italy

(22)INFN Sezione di Perugia and Universit`a di Perugia,I-06100Perugia,Italy

(23)INFN Sezione di Pisa and Universit`a di Pisa,I-56100Pisa,Italy

(24)Rutgers University,Piscataway,New Jersey08855

(25)Rutherford Appleton Laboratory,Chilton,Didcot,Oxon OX110QX United

Kingdom

(26)Sogang University,Seoul,Korea

(27)Soongsil University,Seoul,Korea156-743

(28)Stanford Linear Accelerator Center,Stanford University,Stanford,California

94309

(29)University of Tennessee,Knoxville,Tennessee37996

(30)Tohoku University,Sendai980Japan

(31)Vanderbilt University,Nashville,Tennessee37235

(32)University of Washington,Seattle,Washington98195

(33)University of Wisconsin,Madison,Wisconsin53706

(34)Yale University,New Haven,Connecticut06511

?Deceased

(a)Also at the Universit`a di Genova

(b)Also at the Universit`a di Perugia