A Study of Charged P-wave D Meson Production in Semileptonic B Decays. A search for the semileptonic decay of B mesons into final states involving charged D** as well as non resonant D0-pi is performed in a sample of approximately 3 million hadronic Z decays recorded with the ALEPH detector at LEP. Topological vertex criteria are used to separate the B -> D**+ l nu X signal from background as well as to search for the non-resonant component B -> D0 pi l nu X. Preliminary results for the branching fraction into resonant and non-resonant components are presented

Armstrong, S R

Bozzi, C

Calderini, G

Carpinelli, M

CERN Document Server

EPS95 Ref. eps0426Submitted to: Pa-2 Pa-14Pl-4 Pl-11EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHA Study of Charged P-wave D MesonProduction in Semileptonic B DecaysThe ALEPH CollaborationAbstractA search for the semileptonic decay of B mesons into nal states involvingcharged Das well as non resonant D is performed in a sample of approximately 3million hadronic Z decays recorded with the ALEPH detector at LEP. Topologicalvertex criteria are used to separate the B! D`X signal from background as wellas to search for the non-resonant component B ! D`X. Preliminary results forthe branching fraction into resonant and non-resonant components are presented.Contribution to the International Europhysics Conferenceon High Energy PhysicsEPS{HEP Brussels, Belgium, 27 Jul.{2 Aug. 1995OPEN-99-13327/05/951 IntroductionThe measured exclusive decays B!D`  and B!D`  only account for 616% [1]of the inclusive branching ratio, contrary to theoretical expectations [2]. Therefore, itis interesting to search for and to measure other exclusive semileptonic decays such asdirect four-body decays, B!D()`  or B!D` , where Ddenotes a generic L=1(P-wave) charm meson.Heavy Quark Eective Theory (HQET) predicts four neutral and four charged Dmesons [3], the properties of which are given in Table 1.Neutral states Charged statesMass Width Decay Modes Mass WidthJPJlq(MeV) (MeV) (MeV) (MeV)D11+1/2  2420>250 D  2420>250D00+1/2  2360>170 D  2360>170D11+3/2 24222 194 D 24273 298D22+3/2 24592 235 D, D 24593 216Table 1: Charm mesons with orbital excitations, their quantum numbers, masses, widths,and allowed strong decays into D and D in the innite heavy quark mass limit.Evidence for the narrow Jlq= 3=2 states has been established for both neutral [4] andcharged [5] mesons through their two body decays. The wide Jlq= 1=2 states are unob-served and cannot be distinguished from four-body decays with the available statistics.The term non-resonant is used in the following sections to label both four-body decaysand decays involving the wide Dresonances. Evidence for both neutral and chargednarrow Dstates in semileptonic B decays has been established by ARGUS [6], OPAL[7] and ALEPH [8].Given the displaced vertex topology of B decays produced at the Z resonance andthe capabilities of the ALEPH detector, a topological search for Din semileptonic Bdecays is possible. In a previous analysis [8], the ALEPH collaboration has examinedboth resonant B!D0` X and non-resonant B!D+ ` X semileptonic B decays.This paper presents a measurement of the decay modes1B!D+1` X, B!D+2` X,and their non-resonant counterparts. The data sample is 2.94  106hadronic decays ofthe Z collected with the ALEPH detector at LEP from 1991 to 1994.The ALEPH detector is described in detail elsewhere [9]. The relevant performancefor this analysis is the high precision obtained on the impact parameter resolution (25m+ 95m/p, where p is in GeV=c) with the tracking devices [10].1In this paper, charge conjugate reactions are always implied.12 Semileptonic B Decays into Narrow ResonancesSemileptonic B decays into P-wave charmed mesons have a distinctive 3-vertex topology(see Figure 1).-µ -pi **+D 0pi+e+ B K0-_eFigure 1: Vertex topology for the decay described in the text.For the narrow D+resonance search, the relevant process is the following:B0 ! D+` j !D0+j !D0+j !D0(0or )where +denotes the pion from D+decay. The neutral pion (or photon) from D0decay is not reconstructed.2The main background to the above signal processes is due to semileptonic B decaysinto D0where the +is a fragmentation track mistakenly associated to the B vertex. Thisis referred to as physics background. The other source of background is due to fake com-binations when reconstructing the D0, which is referred to as combinatorial background.Semileptonic B decays are identied in hadronic events containing a high momentumlepton and a fully reconstructed D0meson in the same hemisphere. Lepton identicationin ALEPH is described in detail elsewhere [11]. Electron candidates are required to havea momentum greater than 2 GeV/c, muon candidates are required to have a momentumgreater than 3 GeV/c.The D0is reconstructed in four decay channels: D0!K +, D0!K + +,D0!K0S+ , and D0!K +0. The mass of the D0candidates must lie within 2 ofthe known D0mass, where  is the standard deviation of the t to the D0mass distribu-tion. To eliminate contamination from D+decays, other charged tracks (denoted as \")are paired with the reconstructed D0: the event is rejected if any combination has a massdierence jM(D0\")-M(D0)j less than 5 MeV=c2from 145.5 MeV=c2. The momentumof the D0is required to be greater than 5 GeV=c for the K +and K + +decays,10 GeV=c for the K +0decay, and 7 GeV=c for the K0S+ decay. The specicionization measurement, when available, must be consistent with the expected rate forkaons for the K + +and K +0modes. For these modes, the kaon candidate musthave a momentum greater than 1.5 GeV=c. A cut is also made on the track probabilityto originate from the interaction point [13] in order to reject combinatorial backgroundfor the D0! K + +decay. The , K , and K0resonances are exploited for theD0!K +0decay mode. Neutral pions from this decay are identied by tting pairs ofECAL energy deposits using the constraint that the mass of the pair is consistent with the0mass. For D0!K0S+ we exploit the K resonance. K0Scandidates are rejected ifthe measured mass is more than 2 (10MeV=c2) from the nominal K0Smass. Pions fromthe K0Sdecay are required to be inconsistent with tracks originating from the interactionpoint.Reconstructed D0mesons and leptons are tted to a common B vertex using a topo-logical technique. Track candidates are required to have a 2vertex probability greaterthan 1% for both D0and D0` vertices. The D0` vertex is required to be separated by morethan three times the resolution of the decay length from the interaction point, and thedecay length resolution is required to be less than 500 m. The D0` vertex is required tobe upstream of the D0vertex. The invariant mass of the (D0`) candidates is required tobe within 2.7 and 5.5 GeV=c2.The D0candidate mass spectra for the four subsamples are shown in Figure 2. Table2 summarizes these subsample sizes and background rates from the ts to these distribu-tions.Pion candidates from Ddecays are selected using a momentum cut of 1 GeV/c. True+have a harder momentum spectrum than background fragmentation tracks. This cut3Figure 2: D0mass distribution for the decays described in the text.4D0resolutionChannel (MeV=c2) D0` bkg.D0! K +10 69632 20714D0! K + +7 39625 12911D0! K0S+ 9 18017 356D0! K +031 52533 23515Table 2: D mass resolutions with tted number of D` and background events within a2 window around the tted mass.also reduces uncertainties due to multiple scattering. A +candidate is rejected if itsimpact parameter with respect to the interaction point is less than twice its uncertainty.The distribution of the impact parameter with respect to the (D0`) vertex divided by itsuncertainty is used to build a function which gives the probabilityP that the +originatesfrom the D0` vertex. This procedure has been decribed in detail in [8] and [12]. Figure 3shows the P distribution for simulated signal events; Figure 4 refers to physics backgroundsimulated events. The P probability distribution for signal is expected to be at, whilethe P distribution for background peaks at zero allowing good background rejection. Pioncandidates are required to have P > 0:1, which eliminates 80% of the physics background.If more than one D0`+combination per event is found, the combination with the greatestprobability P is chosen. This happens for roughly 3%-5% of the events, depending on thechannel.2.1 ResultsThe parameter mis dened as the dierence between the measured masses of theD0+system and the D0and has a measurement error of 5 MeV=c2, which is much lessthan the natural widths of the D+1and D+2resonances. The mdistributions are shownin Figure 5 for the right sign sample, the wrong sign sample, and a sample obtained fromright sign combinations with a +candidate coming from the interaction point insteadof the D0` vertex, referred to as an anti-tagged vertex sample. This anti-tagged vertexsample is obtained by reversing the cuts on the impact parameter with respect to theinteraction point and on the probability P. There is a signicant excess in the right signsample corresponding to the D+1; no excess is found for the D+2resonance. The mdistribution is tted with 3 Breit-Wigner functions plus a background function. TwoBreit-Wigner account respectively for the D+1and D+2decays into D0+where the 0or from the D0decay is not reconstructed. The third Breit-Wigner accounts for the D+2decay into D0+. The masses and widths of the Breit-Wigner functions have been xedaccording to the D+1and D+2known values [1]. The tted number of events in the D+1signal peak is 32.49.4; for the D+2resonance, 0.27.4 events are tted. If the massesand widths of the resonances are allowed to vary in the t, values of 24314 MeV=c2and5Figure 3: The Probability distribution Pfor +to be associated with the D` vertexfor simulated signal events.Figure 4: The Probability distribution Pfor +to be associated with the D` vertexfor simulated physics background events.1910 MeV=c2are found for the mass and width of the D+1particle, which are consistentwithin errors with the known values. The product branching ratio for the D+1is:Br(b! B) Br(B! D+1` X) Br(D+1! D0+) == (2:06+0:58 0:64(stat)+0:30 0:53(syst)) 10 3The evaluation of systematic uncertainties is given in the next section.A limit can be set on the production of D+2. This is done by retting the distributionexcluding the region within 2 half-widths of the D+2Breit-Wigner. This t has beenused to estimate the background to any possible D+2contribution. At 95% condencelevel, we obtain:Br(b! B)Br(B! D+2` X) Br(D+2! D0+)  0:77 10 3The D+1product branching ratio agrees with its corresponding isospin conjugate coun-terpart, as measured in reference [8]. A suppression of D+2production relative to D+1production is observed. A similar suppression has been observed in a separate analysis[15].6Figure 5: mdistributions for the right-sign (top), wrong-sign (middle) and anti-tagged(bottom) samples. Bin width is 20 MeV=c2.7D0`Channel Signal EciencyD0! K +7.600.23%D0! K + +2.440.12%D0! K0S+ 1.520.12%D0! K +00.530.07%Table 3: Eciencies for B!D`  for the four decay mode. The errors are due to MonteCarlo statistics.2.2 Systematic UncertaintiesThe systematic error due to the uncertainty of Dmasses and widths is computed byretting the distribution varying the masses and widths of the resonances within theirpublished values [1]. The systematic error due to the background parametrization is ob-tained by tting with several dierent background shapes. There is a 10% uncertaintydue to the eciency of the 2requirement for the B vertex. The lepton identicationeciency has an uncertainty of 5%. There is also an uncertainty due to the B fragmen-tation process which aects the momentum distributions of the decay products. Thesystematic error due to the B mesons lifetime has been calculated by varying the lifetimewithin its published uncertainty. An uncertainty due to the shape of the distribution ofthe distance of the +from the D0` vertex is calculated varying the parameters of the tto this distribution by their uncertainties.Systematic uncertainties are summarized in Table 4.D0`Source syst(10 3)M,  of D+0:17 0:47 (Z!bb)/ (Z!had) 0.02Background Function 0.13B0Vertex Eciency 0.13Monte Carlo Statistics 0.06D+, D0Branching Ratios 0.06Lepton ID Eciency 0.07b Fragmentation 0.11B0Lifetime 0.05Probability Function P 0.09Total+0:30 0:53Table 4: Systematic uncertainties for D0` for the narrow resonance search.83 Semileptonic B Decays into Wide and Non-Resonant StatesAs in the previous analysis, the characteristic topology of the wide B !D+`  andnon-resonant B!D0`  decays permits unambigous identication. Using the D0` sam-ple selected in the previous section, more stringent requirements are applied to the +candidates. The error on the distance from the D0` vertex is required to be less than 150m, and the impact parameter with respect to the interaction point must be greater than3 times its uncertainty. If more than one track satises these criteria, the candidate withthe greatest momentum is chosen.3.1 ResultsThe probability distribution P, described in Section 2, is used to estimate the numberof signal events. For this search, the +candidates are required to have P > 0:2. Thecombinatorial background probability shape is taken fromD0` combinations which have aninvariant mass in the sidebands of the D0mass distributions. The corresponding number ofevents with P > 0:2 is subtracted after correcting for normalization. The number of eventsfrom physics background with P > 0:2 is estimated from the fraction of fragmentationpions that fall in the signal region, fP>0:2. From Monte-Carlo simulation fP>0:2= 0:1250:011. The probability distributions for the right-sign and wrong-sign samples are plottedin Figures 6 and 7. The excess of right-sign event combinations is evidence of wide- andnon-resonant structures. The corresponding mdistributions for the right-sign andwrong-sign samples are shown in Figure 8.The number of signal events in the P > 0:2 region is 70.117.0. The correspondingeciencies are listed in Table 5. The corresponding number of events in the wrong signcombinations is 35.D0`Channel Signal EciencyD0! K +5.000.19%D0! K + +1.760.11%D0! K0S+ 1.130.10%D0! K +00.4130.066%Table 5: Eciencies for B!D0`  for the four decay modes for the non-resonant search.The branching ratio for the sum of resonant Dand non-resonant contributions is:Br(b! B) Br(B! D0` X) = (6:46  1:57(stat) 0:74(syst)) 10 39Figure 6: Probability distribution P forright-sign sample for the non-resonantsearch. Contributions from combinato-rial and physics backgrounds are shown inhatched styles.Figure 7: Probability distribution P forwrong-sign sample for the non-resonantsearch. Contributions from combinato-rial and physics backgrounds are shown inhatched styles.where the evaluation of systematic uncertainties is given below.This result is consistent within errors with the corresponding isospin conjugate counter-part as measured in [8]3.2 Systematic UncertaintiesSystematic uncertainties for the non-resonant analysis are similar to those for the narrowresonant analysis discussed in Section 2.2. A systematic uncertainty of 10% on the e-ciency is added since it is not possible to distinguish the contribution due to wide resonantdecays from those due to non-resonant decays (the contribution from non-resonant decaysis not well known and could have a dierent eciency). The systematic uncertainties aregiven in Table 6.10Figure 8: mdistribution for right-sign (top) and wrong-sign (bottom) samples for thenon-resonant search. Bin width is 20 MeV=c2.11D0`Source syst(10 3)Wide vs. non.res. ec. 0.38 (Z!bb)/ (Z!had) 0.06Background Function 0.30B0Vertex Eciency 0.39Monte Carlo Statistics 0.23D+, D0Branching Ratios 0.19Lepton ID Eciency 0.20b Fragmentation 0.11B0Lifetime 0.05Probability Function P 0.12Total 0.74Table 6: Systematic uncertainties for D0` for the non-resonant search.4 ConclusionsEvidence for semileptonic B decays into both the D+1resonance and the non-resonantD0+(or into wide resonances) is observed by the ALEPH experiment at LEP. Theproduct branching ratio for the D+1production is:Br(b! B) Br(B! D+1` X) Br(D+1! D0+) == (2:06+0:58 0:64(stat)+0:30 0:53(syst)) 10 3From isospin arguments, Br(D+1!D0) must be less than 0.66. Using the branchingratio Br(b!B0)=0.370.03 from reference [16], we deduce Br(B0!D+1`X)(8.353.0)10 3.At the 95% condence level,Br(b! B) Br(B! D+2` X) Br(D+2! D0+)  0:77  10 3:The product branching ratio for the sum of the decays involving D0`X in the nalstate is:Br(b! B0) Br(B! D0` X) = (6:46  1:57(stat) 0:74(syst)) 10 3:12References[1] Particle Data Group: Phys. Rev. D50 Part I (August 1994).[2] M. Voloshin and M. Shifman, Sov. J. Nucl. Phys. 45 (1987) 292.M. Voloshin and M. Shifman, Sov. J. Nucl. Phys. 47 (1988) 511.N. Isgur, D. Scora, B. Grinstein, and M. B. Wise, Phys. Rev. D39 (1989) 799.P. Colangelo, G. Nardulli and N. Paver, Phys. Lett. B293 (1992) 207.[3] M. Neubert, Phys. Reports 245 (1994) 259.[4] H. Albrecht et al.(ARGUS Collaboration), Phys. Lett. B221 (1989) 422.H. Albrecht et al.(ARGUS Collaboration), Phys. Lett. B232 (1989) 398.J.C. Anjos et al.(E691 Collaboration), Phys. Rev. Lett. 62 (1989) 1717.P. Avery et al.(CLEO Collaboration), Phys. Rev. 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A Study of Charged P-wave D Meson Production in Semileptonic B Decays

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