Results from the four LEP experiments ALEPH, DELPHI, L3, and OPAL on the
spectroscopy of B and charmed mesons are presented. The predictions of Heavy
Quark Effective Theory (HQET) for the masses and the widths of excited L=1 B
mesons are supported by a new measurement from L3. A few B_c candidate eve nts
have masses consistent with the recent CDF observation and the predictions. New
results on Ddstar production and B -> Ddstar l nu are also presented. The
evidence for a Dstarpr meson reported recently by DELPHI is not supported by
OPAL and CLEO.Comment: 10 pages, 6 figure

Muheim, F.

English

arXiv

Franz Muheim

Crossref

B and D spectroscopy at LEP

Results from the four LEP experiments ALEPH, DELPHI, L3, and OPAL on the spectroscopy of B and charmed mesons are presented. The predictions of Heavy Quark Effective Theory (HQET) for the masses and the widths of excited L=1 B mesons are supported by a new measurement from L3. A few B_c candidate eve nts have masses consistent with the recent CDF observation and the predictions. New results on Ddstar production and B -> Ddstar l nu are also presented. The evidence for a Dstarpr meson reported recently by DELPHI is not supported by OPAL and CLEO.Results from the four LEP experiments ALEPH, DELPHI, L3, and OPAL on the spectroscopy of B and charmed mesons are presented. The predictions of Heavy Quark Effective Theory (HQET) for the masses and the widths of excited L=1 B mesons are supported by a new measurement from L3. A few B_c candidate eve nts have masses consistent with the recent CDF observation and the predictions. New results on Ddstar production and B -> Ddstar l nu are also presented. The evidence for a Dstarpr meson reported recently by DELPHI is not supported by OPAL and CLEO.Results from the four LEP experiments ALEPH, DELPHI, L3, and OPAL on the spectroscopy of B and charmed mesons are presented. The predictions of Heavy Quark Effective Theory (HQET) for the masses and the widths of excited L=1 B mesons are supported by a new measurement from L3. A few B_c candidate eve nts have masses consistent with the recent CDF observation and the predictions. New results on Ddstar production and B -> Ddstar l nu are also presented. The evidence for a Dstarpr meson reported recently by DELPHI is not supported by OPAL and CLEO.Results from the four LEP experiments ALEPH, DELPHI, L3, and OPAL on the spectroscopy of B and charmed mesons are presented. The predictions of Heavy Quark Effective Theory (HQET) for the masses and the widths of excited L=1 B mesons are supported by a new measurement from L3. A few B_c candidate eve nts have masses consistent with the recent CDF observation and the predictions. New results on Ddstar production and B -> Ddstar l nu are also presented. The evidence for a Dstarpr meson reported recently by DELPHI is not supported by OPAL and CLEO.Results from the four LEP experiments ALEPH, DELPHI, L3, and OPAL on the spectroscopy of B and charmed mesons are presented. The predictions of Heavy Quark Effective Theory (HQET) for the masses and the widths of excited L=1 B mesons are supported by a new measurement from L3. A few Bc+ candidate events have masses consistent with the recent CDF observation and the predictions. New results on D** production and B→D**lν are also presented. The evidence for a D*′ meson reported recently by DELPHI is not supported by OPAL and CLEO

Muheim, Franz

CERN Document Server

UGVA-DPNC 1998/11-180Nov. 1998B and D Spectroscopy at LEPFranz Muheim Universite´ de Gene`veDe´partement de physique nucle´aire et corpusculaire24, quai Ernest-Ansermet, 1211 Gene`ve 4, SwitzerlandEmail: Franz.Muheim@cern.chpresented at Heavy Quarks at Fixed Target HQ98 workshop, Fermilab, Oct. 1998.Abstract. Results from the four LEP experiments ALEPH, DELPHI, L3, and OPALon the spectroscopy of B and charmed mesons are presented. The predictions of HeavyQuark Effective Theory (HQET) for the masses and the widths of excited L = 1 Bmesons are supported by a new measurement from L3. A few B+c candidate eventshave masses consistent with the recent CDF observation and the predictions. Newresults on D production and B ! D`ν are also presented. The evidence for a D′meson reported recently by DELPHI is not supported by OPAL and CLEO.INTRODUCTIONDetailed understanding of the spectroscopy of orbitally excited heavy mesonscontaining a b or a c quark provides important information regarding the underlyingtheory. A flavor-spin symmetry arises from the fact that the mass of a heavy quarkQ is large relative to QCD. In this approximation, the spin ~sQ of the heavy quarkQ is conserved in the interactions, independently of the total angular momentum~jq = ~sq+~L of the light quark q. Corrections to this symmetry are a series expansionin 1=mQ; 1=m2Q, calculable in Heavy Quark Eective Theory (HQET) [1].TABLE 1. L = 1 B mesons containing a u or a d light quark with corresponding spin states,relative production rates, prediction for masses and widths, and two-body decay modes.Name jq JP Production Mass [MeV] Width [MeV] Decay ModeB0 1/2 0+ 1/12 MB1 − 12  150 Bpi S-waveB1 1/2 1+ 3/12 MB2  100  150 Bpi S-waveB1 3/2 1+ 3/12 5759 21 Bpi D-waveB2 3/2 2+ 5/12 5771 25 Bpi,Bpi D-waveThe L = 0 mesons, for which jq = 1=2, have two possible spin states: a pseudo-scalar P (JP = 0−) and a vector V (JP = 1−). If the spin of the heavy quarkis conserved independently, the relative production rate of these states is expectedto be V=(V + P ) = 0:75. Corrections due to the decay of higher excited statesare predicted to be small. Recent measurements of this rate for the B system [2,3]agree well with this ratio.In the case of L = 1 orbitally excited B mesons two sets of doublets are expected:the B0 and B1 (jq = 1=2) and the B1 and B2 (jq = 3=2) mesons (see Table 1). Theirrelative production rate follows from spin state counting (2J+1 states) [4]. For thedominant two-body decays, the jq = 1=2 states can decay via an S-wave transitionand their decay widths are expected to be broad in comparison to those of thejq = 3=2 states which must decay via a D-wave transition. Many measurementsexist for L = 1 orbitally excited charm mesons. All six narrow states, a doublet(D2 and D1) for each quark content (cu; c d and cs) are well established [17]. Thewide L = 1 states are hard to measure and have not been clearly identied.Several models based on HQET and on the charmed L = 1 meson data, havemade predictions for the masses and widths of orbitally excited B mesons [5{9](see Table 1). Some of these models place the average mass of the jq = 3=2 statesabove that of the jq = 1=2 states, while others predict the opposite (\spin-orbitinversion"). The mass splitting within each doublet is predicted to be 12 MeV.B SPECTROSCOPYAt LEP excited states of B mesons are produced. Each of the four experimentshas collected about 4 106 hadronic events out of which 0:9  106 events containB B pairs. In all B analyses, rst, the b-quark purity of the data sample isincreased by applying a lifetime based event tag. B mesons are reconstructedinclusively. Typically the two most energetic jets of the event are considered Bcandidates. The decay products of the B meson are separated from the backgrounddue to fragmentation particles using secondary vertex tagging (for charged decayparticles) or the rapidity of the decay products with respect to the B-jet axis. Analternative method is to fully reconstruct the B-meson decay chain which improvesthe resolution B-mass resolution but suers from low statistics.The decay of a B meson (B ! B()) is carried out via a strong interactionand thus the transition pion originates at the primary event vertex. In addition,the predicted masses for the L = 1 states correspond to relatively small Q values,so that the pion direction is forward with respect to the B-meson direction. Thetrack with the largest component of momentum in the direction of the B jet isselected.A rst measurement of L = 1 orbitally excited B mesons has been presentedby OPAL [10]. Using secondary vertex charge tagging to inclusively reconstructB mesons, the invariant mass distributions of B()+− and B()+K− combinationsshow enhancements consistent with the decay of B resonances as shown in Fig. 1.An excess of 1738  121 B()+− and 149  30 B()+K− candidates is observed inthe mass ranges 5.60 - 5.85 GeV and 5.80 - 6.00 GeV, respectively. The background-500501001502002503005.4 5.6 5.8 6 6.2 6.4(a)No. of entries / 25 MeVBpi mass (GeV) OPAL data-10010203040505.8 6 6.2 6.4 6.6(b)No. of entries / 25 MeVBK mass (GeV) OPAL dataFIGURE 1. The invariant mass distribution for (a) B()+pi− and (b) B()+K− combinationsafter subtraction of the background, respectively. The solid histograms shows the contributionfrom the B2 and the hatched histogram shows the contribution from the B1 state.is estimated with the wrong (like)-sign combinations. Fitting the excess with asingle Breit-Wigner function yields an average mass M(Bu,d) and a productionrate fB = B(b ! Bu,d)=B(b ! Bu,d). Throughout this paper, isospin symmetryis always employed to account for B decays via neutral pions. DELPHI andALEPH have made similar measurements using rapidity to inclusively reconstructB mesons [11,12]. The results are summarized in Table 2.TABLE 2. Inclusive B measurements.M(Bu,d) [MeV] fBOPAL 5681 11 0.270 0.056DELPHI 5734 5 17 0.32 0.018 0.06ALEPH 5703 4 10 0.279 0.016 0.059 +0.039−0.056A new measurement using an exclusive method is presented by ALEPH [13].Using many decay modes (B ! D()X, where X 2 [; ; a1] and B ! J= ( ′)K())238 charged and 166 neutral B meson candidates have been fully reconstructed.The sample has a B meson purity of 85 %. Each B candidate is then combinedwith a charged pion from the primary vertex. An excess of 45 13 events is seenin the right-sign sample compared to the wrong-sign sample. Fig. 2 shows a t tothe right-sign mass spectrum where the signal shape consists of ve Breit-Wignerpeaks. The relative masses, the widths, and the relative production rates of theindividual B mesons have been xed to the predictions from HQET. The mass ofthe B2 meson and the overall production rate are measured to be:MB2= 5739 +8−11+6−4 MeVfB = 0:31 0:09 +0.06−0.05 :05101520253035405.3 5.7 6.1 6.5 6.9 7.3BackgroundWide B**Narrow B**ALEPH  datamBpi (GeV/c2)Entries / 40 MeV/c2(a)00.511.522.533.545.4 5.5 5.6 5.7 5.8 5.9B0→B piB*1→B*piB1→B*piB*2→B*piB*2→B pimBpi (GeV/c2)Entries / 5 MeV/c2(b)FIGURE 2. (a) Bpi mass spectrum from data. The fit (histogram) includes the expected back-ground plus contributions from the narrow and wide B states. (b) An expanded view of thesignal region.A new analysis using an inclusive method is presented here by the L3 experiment[14]. Several techniques are used to both improve on the resolution of the B massspectrum and to unfold this resolution from the signal components. As a result,L3 is able to extract measurements for the masses and widths of both the D-waveB2 decays and the S-wave B1 decays.B meson candidates are reconstructed inclusively from all charged and neutralparticles with rapidity y > 1:6 relative to the original jet axis. The measurementof the direction of the B meson is determined by an error-weighted average ofthe direction of the measured secondary decay vertex and of the direction of theB candidate. The angular resolution obtained is 1 = 12 mrad for , and 1 =18 mrad for , respectively. The energy of the B meson candidate, EB, is estimatedby taking advantage of the known center-of-mass energy at LEP, Ecm, to beEB =E2cm −M2B +M2recoil2Ecm; (1)where Mrecoil is the mass of all particles in the event recoiling against the B candi-date. The dierence between reconstructed and generated values for the B-mesonenergy can be described by an asymmetric Gaussian with widths of 1:9 GeV and2:8 GeV.Fig. 3a) shows the resulting B invariant mass spectrum together with the ex-pected background from Monte Carlo. A clear signal due B ! B() decays isseen above the background which is well described by the simulation. Thus thebackground is parameterized by a threshold function, the shape of which is deter-mined from the Monte Carlo.Bpi Mass [GeV]L3Entries / 15 MeV020040060080010005.4 5.6 5.8 6 6.2 6.4 6.6 5.61 5.65 5.69 5.73 5.77Bpi Mass [GeV]0.000.020.040.06σ [GeV](b)L3FIGURE 3. (a) Mass spectrum for selected Bpi pairs. The dots are data and the shadedhistogram represents the expected background from Monte Carlo normalized to the sidebandregion 6.0− 6.6 GeV. (b) Linear fit to the extracted Bpi mass resolution for Monte Carlo signalcomponents generated at four different B mass values.To resolve the underlying structure of the signal, it is necessary to unfold eectsdue to detector resolution. The dominant sources of uncertainty for the massmeasurement are, with about equal magnitude, the angular and energy resolutionof the B meson. The dependence of the B mass resolution on the B mass isstudied by simulating signal events at four dierent values of B mass and Breit-Wigner width. Each signal B mass distribution is then t to a Breit-Wignerfunction convoluted with a Gaussian resolution. The Breit-Wigner width is xed toits generated value and the Gaussian resolution is extracted from the t and shownin Fig. 3b) as a function of the B mass together with a linear parameterization.The mass resolution is increasing with increasing B mass.The t function for the signal consists of ve Breit-Wigner mass peaks one forthe each of the ve decay modes allowed by spin-parity rules: B2 ! B;B,B1 ! B, B1 ! B, and B0 ! B. Each Breit-Wigner width is convoluted withthe mass dependent Gaussian resolution. No attempt is made to tag subsequentB ! Bγ decays, as the eciency for selecting the soft photon is low. The relativeproduction rate, and the mass splittings and the relative width within each doubletare constrained to the predictions from HQET (see Table 1).The B invariant mass distribution t with the signal and background functiondescribed above is shown in Fig. 4. The results of the t provide the rst mea-05001000Entries/15 MeVL35.4 5.6 5.8 6.0 6.2 6.4 6.6Bpi Mass [GeV]20100180260 backgroundsubtractedFIGURE 4. Fit to the data Bpi mass distribution with the five-peak signal function and thebackground function described in the text.surements of the masses and decay widths of the B2 (jq = 3=2) and B1 (jq = 1=2)mesons:MB2= (5770 6 (stat.) 4(syst)) MeVΓB2= (21 24 (stat.) 15(syst)) MeVMB1= (5675 12 (stat.) 4(syst)) MeVΓB1= (75 28 (stat.) 15(syst)) MeV :A total of 2652 232 events that occupy the signal region correspond to a relativeproduction rate fB for all L = 1 spin states offB = 0:39 0:06 (stat.) 0:06(syst) :Systematic errors are mainly due to the modelling of the background, the limitedknowledge of the signal function and the mass constraint within the doublets.These results disfavor recent theoretical models proposing spin-orbit inversion[8,9], but do agree well with several earlier models [5{7] and provide strong supportfor HQET.B+c STUDIESDELPHI, ALEPH, and OPAL have published searches for B+c mesons in Z decays[15]. No signals have been found. Table 3 shows the number of candidate eventsand the obtained upper limits on the production rates B(Z ! B+c X)  B(B+c !J= +; J= ‘+; J= +−+). The 3 B+c ! J= + candidates are consistent withTABLE 3. B+c studies at LEP.Decay mode Candidates Prod. Limit [10−5] at 90% CLDELPHI ALEPH OPAL DELPHI ALEPH OPALB+c ! J/ψpi+ 1 0 2 10.5 to 8.4 3.6 10.6B+c ! J/ψ`+ν 0 2 1 5.8 to 5.0 5.2 6.96B+c ! J/ψpi+pi−pi+ 1 - 0 17.5 - 5.53a background estimate of 2.3 expected events. A t to the mass yields the followingvalues: MJ/ψpi+ = 6:342  0:027 GeV (DELPHI) and MJ/ψpi+ = 6:32  0:06 GeV(OPAL average), respectively. Predictions for the B+c mass are in the range 6.24to 6.31 GeV. The CDF experiment at the Tevatron has recently reported theobservation of the B+c meson in the decay channel B+c ! J= ‘+ [16]. They nd20:4 +6.2−5.8 events and obtain a mass value of M(B+c ) = 6:40 0:39 0:13 GeV.D SPECTROSCOPYDuring the last year several new results on D production have been presentedby the LEP collaborations. D0 mesons are fully reconstructed in the decay chainTABLE 4. D production fractions B [%].Production mode OPAL DELPHI ALEPHb! D10 5.0 1.4 0.6 2.0 0.6 2.3 0.7b! D02 4.7 2.4 1.3 4.8 2.0 < 2.0c! D10 2.1 0.7 0.3 1.9 0.4 1.6 0.5c! D02 5.2 2.2 1.3 4.7 1.3 4.7 1.0c! D+s1 1.6 0.4 0.3 - 0.77 0.20 0.08c! D+s2 - - 1.3 0.5 0.2b! D+s1 - - 1.1 0.3 0.2b! D+s2 - - 2.2 0.8 0.5D0 ! D+−, D+ ! D0+, D0 ! K−+. High momentum D0 together withshort decay lengths are selected to obtain cc enriched samples whereas B and Dmeson vertexing is used to select bb enriched samples. Table 4 shows the results forthe D production fractions measured by OPAL, DELPHI, and ALEPH [18{20].M(D*+pi-)  (GeV/c2)02.557.51012.51517.52022.5252.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1Entries / 20 MeV/c2DELPHIImpact parameter (cm)020406080100120140-0.02 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14Entries / 75 µmDELPHIa) right sign pi**Impact parameter (cm)020406080100120140-0.02 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14Entries / 75 µmb) wrong sign pi**FIGURE 5. (a) Experimental D+pi− invariant mass distribution for D+pi−`− events. Thesolid line is a fit to the narrow D states plus background. The wrong sign D+pi−`+ candidatesare shown in the hatched histogram. (b) Impact parameter relative to the primary event vertexfor right charge and wrong charge pi candidates. The fit is described in the text.DELPHI has also presented a new B ! D‘ analysis [21]. In semileptonicevents, the decay chain D+ ! D0+, D0 ! K−+;K−+−+;K−+(0) is fullyreconstructed. The D+ candidates are then combined with opposite sign − andthe D+− mass, shown in Fig. 5a), is t to the narrow D states, resulting in thefollowing branching fraction: B(B− ! D10‘−) = 0:72 0:22 0:13%:A t to the impact parameter distribution of the bachelor pion  stemming fromthe D ! D transition for right (unlike)-sign and wrong (like)-sign combinations,as shown in Fig. 5b), allows to extract the following branching fraction B(B− !D+−‘−) = 1:15 0:17 0:14% where the signal comprises narrow and wide Dresonances plus non-resonant D+− combinations. These results are in agreementwith previous LEP and CLEO measurements.D0STUDIESDELPHI has recently reported an excess of events in the D+−+ mass spectrumas shown in Fig. 6a) [22]. The t yields N = 66  14 events, corresponding to aproduction rate fD′=fD = 0:490:180:10, a mass M = 263726 MeV anda width consistent with the experimental resolution. This mass value is consistentwith predictions of a radial excited D′meson.OPAL has performed a similar analysis [23]. The resulting D+−+ mass spec-trum is shown in Fig. 6b) for data and Monte Carlo events, where a DELPHI-likesignal has been added in the simulation. No excess is seen in the data (N < 32:8 at95 % CL) corresponding to a limit on the production rate of fD′=fD < 0:21 at95 % CL, thus not conrming the DELPHI result. CLEO also has examined theirD+−+ mass spectrum and does not conrm the DELPHI evidence [24].M(D*pipi)  (GeV/c2)010203040506070802.4 2.5 2.6 2.7 2.8 2.9 3Entries / 10 MeV/c2DELPHIOPAL Prelim.(a)m′ (GeV/c2)Entries / 10 MeV/c2Monte Carlo(b)m′ (GeV/c2)Entries / 10 MeV/c20204060801001202.3 2.4 2.5 2.6 2.7 2.8 2.9 30204060801001202.3 2.4 2.5 2.6 2.7 2.8 2.9 3FIGURE 6. (a) DELPHI invariant mass distributions D+pi+pi− (dots) and D+pi−pi− (hatchedhistogram). (b) OPAL D+pi+pi− mass distribution for data and Monte Carlo. A DELPHI-likesignal has been added in the simulation.ACKNOWLEDGEMENTSI wish to thank my colleagues from the other LEP collaborations for providingme with the results and the gure les. I also thank S. Goldfarb for his help whenpreparing this presentation.REFERENCES1. N. Isgur and M.B. Wise, Phys. Lett. B 232 (1989) 113; Phys. Lett. B 237 (1990)527; Their overview “Heavy Quark Symmetry” is included in B Decays, revised 2ndedition, edited by S. Stone (World Scientific, 1994) 231 and in Heavy Flavors, editedby A.J. Buras and M. Lindner (World Scientific, 1992) 234.2. L3 Collaboration, Phys. Lett. B 345 (1995) 589; Contribution to EPS Conference,Brussels EPS95 (1995).3. DELPHI Collaboration, Z. Phys. C 68 (1995) 353;ALEPH Collaboration, Z. Phys. C 69 (1996) 393;OPAL Collaboration, Z. Phys. C 74 (1997) 413.4. J.L. Rosner, Comments Nucl. Part. Phys. 16 (1986) 109;N. Isgur and M.B. Wise, Phys. Rev. Lett. 66 (1991) 1130.5. M. Gronau, A. Nippe and J.L. Rosner, Phys. Rev. D 47 (1993) 1988;M. 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B and D Spectroscopy at LEP

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