The production of charmed particles has been studied using 3.5 milllion hadronic Z decays collected by the DELPHI collaboration at LEP between 1992 and 1995. Large samples of D meson decays have been exclusively reconstructed, allowing to look for D$^*pi$ and D$^*pipi$ final states. The production fractions of the narrow D$_1^0(2420)$ and D$_2^{*0}(2460)$ orbital states are measured in c and b quark jets separately. Evidence for a radial state D$^{*'}(2637)$ is presented in the D$^{*+}pi^+pi^-$ decay mode. %DB Interesting perspectives to look for the wide orbital states in semileptonic B %DB decays are discussed

Bloch, D

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Charm Spectroscopy in DELPHI

Preprint - Server (24-June-1999)EXT-99-005IReS 99{14CHARM SPECTROSCOPY IN DELPHIDaniel BlochIReS, IN2P3-CNRS/Universite Louis Pasteur,BP 28, F-67037 Strasbourg Cedex2, FranceInvited talk given atWorkshop on Hadron SpectroscopyLaboratori Nazionali di Frascati - INFN, March 8-12, 1999Frascati (Rome) - ItalyInstitut de Recherches SubatomiquesUnite Mixte de Recherche CNRS-IN2P3 et Universite Louis PasteurInvited talk given at the Workshop on Hadron SpectroscopyLaboratori Nazionali di Frascati - INFN, March 8-12, 1999Frascati (Rome) - ItalyCHARM SPECTROSCOPY IN DELPHIDaniel BlochIReS, IN2P3-CNRS/Universite Louis PasteurBP 28, F-67037 Strasbourg Cedex2, Franceon behalf of the DELPHI collaborationABSTRACTThe production of charmed particles has been studied using 3.5 milllion hadronicZ decays collected by the DELPHI collaboration at LEP between 1992 and 1995.Large samples of D meson decays have been exclusively reconstructed, allowing tolook for D and D nal states. The production fractions of the narrow D01(2420)and D02(2460) orbital states are measured in c and b quark jets separately. Evidencefor a radial state D0(2637) is presented in the D++ decay mode.1 IntroductionIn the spectroscopy of charmmesons, the D and Dground states are well establishedwhile only the narrow orbital excitations D1and D2have been clearly observed sofar1). For mesons containing heavy and light quarks (Qq) and in the limitwhere theheavy quark mass is much larger than the typical QCD scale (mQ QCD), the spin~sQof the heavy quark and the total (spin+orbital) angular momentum~jq= ~sq+~Lof the light component are separately conserved by the strong interaction2). Thisheavy quark symmetry, together with quark potential models used for lower massmesons, allows the masses and decay widths of heavy mesons of total spin~J = ~sQ+~jqto be predicted3). The D and Dcorrespond to the two degenerate levels of theTable 1: Expected and observed orbital excitations of D0and D+mesons. The broad(L = 1, jq= 1=2) states are estimated. The narrow jq= 3=2 states are measured.Predicted decay modes not yet observed are indicated within parentheses.Mass   WidthJPjq(MeV/c2) (MeV/c2) Decay modesD00+1/2  2360  170 (D)D011+1/2  2430  250 (D)D+12427  5 28  8 D0+(D+0, D, D)D11+3/2D012422:2  1:8 18:9+4:6 3:5D+ (D00, D, D)D+22459  4 25+8 7D0+, D0+(D()+0, D, D)D22+3/2D022458:9  2:0 23  5 D+ , D+ (D()00, D, D)(L = 0, jq= 1=2) state. The L = 1 orbital excitations are grouped into twodegenerate levels with jq= 1=2 and jq= 3=2. The jq= 1=2 states decay through anS-wave and are expected to have a large decay width, whereas the jq= 3=2 statesdecay through a D-wave and are narrow. This scheme is summarized in Table 1 fornon-strange charmed mesons. The D1(2420) and D2(2460) have been observed withdecay widths of about 20 MeV/c2and are identied as the states with jq= 3=2 andJP= 1+and 2+, respectively1). Orbitally excited beauty mesons are expected topresent a similar scheme.In addition to these orbital excitations, radial excitations of heavy mesonsare also foreseen. Based on a QCD inspired relativistic quark model, the D0(JP=0 ) and D0(JP= 1 ) are expected with masses of 2.58 GeV/c2and 2.64 GeV/c2,respectively, with a 10-25 MeV/c2uncertainty on the mass predictions4). A morerecent relativistic quark model, which requires no expansion in function of the light-est quark mass, predicts masses of 2.58 GeV/c2and 2.63 GeV/c2, respectively, withan agreement of better than 20 MeV/c2for the observed charm orbital states5).The dominant decay modes of the D0and D0are expected to be into D and D,respectively, but decays into D() or through an intermediate orbital excitation arenot excluded. Decays with a single pion in the nal state would require a P-wavetransition and should be suppressed. Figure 1 shows the expected spectrum of thevarious D mesons with their decay modes involving S-, P- or D-wave transitions.The production of orbitally and radially excited charmed states are dis-cussed in the following. First, simple counting arguments, although model depen-dent6), of D0, D+and D+in cc and bb events show that charm states of heavierFigure 1: Spectroscopy of non-strange D mesons. The shaded areas show predictedwidths for these states. For clarity the expected D1and D2decays involving a meson or  pair are not shown.masses should contribute to the observed rates of ground states D mesons1. Thenthe narrow orbital D1(2420) and D2(2460) states, decaying into D+ are observedin c and b quark jets, separately. Looking for the D++ nal state a narrowsignal is observed, which is interpreted as a D0(2637) radial state.2 Tagging of c and b quark jetsThe DELPHI detector consists of several independent devices for tracking, calorime-try and particle identication7). A total of 3.5 million hadronic Z decays eventswas obtained from the 1992-1995 data at centre-of-mass energies close to the Z mass.This corresponds to 1.2 (1.5) million c (b) quark jets produced. Charmed mesonsfrom Z! bb events were distinguished from those in cc events by considering boththeir energy and lifetime informations.In bb events each bottom quark fragments into a B hadron which subse-quently decays to a D meson, whereas in cc events charmed mesons are directlyproduced in the fragmentation process. This dierence in the hadronisation leadsto a larger energy of D from primary charm quark (22 GeV in average) and a softerspectrum from bottom quark events (14 GeV in average).1Throughout the paper charge-conjugate states are implicitly included and the pion from theD+decay is denoted +.The apparent decay length L, or proper time t = L M(D)=pt(D), ofthe reconstructed D mesons are also ecient variables to select bb enriched samples.For a D0meson from charm quark fragmentation, t is the D0proper decay timeand is on average 0:4 ps1). For a D0from B decay, t is usually larger thanthe mean B lifetime of 1:6 ps1). From the simulation, selecting D0decays witht > 1 ps would retain about 10% from cc events and 60% from bb events.Due to the relatively long lifetimes of charmed and bottom particles, theheavy avour events are characterised by the presence of secondary vertices. Avariable PEwas dened as the probability that all charged particle tracks detectedin the event came from the primary vertex7). This variable was much smaller inbb events than in cc or light quark events. Typically a 90% purity was achieved for60% of bb events by selecting PEvalues less than 10 2.3 Charm counting in cc and bb eventsA combined t to the D meson energy and PEinformation8), lead to the productionrates quoted in Table 2. Isospin conservation suggests equal production rates ofcharged (cd) and neutral (cu) D mesons in the fragmentation of charm quarks incc events. A dierence in the observed D0and D+rates could arise from thedierence between the D0and the D+decay rates. The D+can decay into D0+,D+0or D+ while, due to their masses, the D0can only decay into D00or D01).Therefore the observed rate of D+!D0+could be responsible for the dierencebetween the D0and D+production rates.If fd(c) is dened as the probability for a charm quark to fragment into aprimary charged D or Dmeson (assumed equal to fu(c)), then the probabilities fora charm quark to fragment into the observed D+, D0and D+are expressed as:Pc!D+B= Y fd(c)Pc!D0= (1 + Y ) fd(c) (1)Pc!D+= (1  Y ) fd(c)where B= BR(D+! D0+) and Y = BVV+PwhereVV+Pis dened as the ratioof the vector meson rate to the total vector+pseudoscalar meson rate. The Y valuecan be obtained for cc events from a t to equation 1 using the result from Table 2:VV + P= 0:620  0:014(stat) 0:014(syst) 0:025(BR) ; (2)at four sigma below the naive spin counting expectation of 0.75, suggesting aneventual production of Dand D mesons from decays of higher D mass states.Table 2: Measured Rc(b)Pc(b)!DBR(D ! X) with stat. and syst. errors.Mode RcPc!DBR  103RbPb!DBR 103correlationD0! K +3.570  0.100  0.146 4.992  0.162  0.304 -0.46D+! K ++3.494  0.116  0.140 4.525  0.204  0.226 -0.38D+! (K +)+1.089  0.027  0.039 1.315  0.035  0.053 -0.34Table 3: Measured DJmultiplicities in b and c quark jets.(%) D01(2420) D02(2460)f(b! DJ) DELPHI 2:0  0:6 (stat) 4:8  2:0 (stat)OPAL 5:0  1:5 (stat+ syst) 4:7  2:7 (stat+ syst)ALEPH 2:3  0:7 (stat+ syst) < 2:0 (95% C:L:)f(c! DJ) DELPHI 1:9  0:4 (stat) 4:7  1:3 (stat)OPAL 2:1  0:8 (stat+ syst) 5:2  2:6 (stat+ syst)ALEPH 1:6  0:4 (stat+ syst) 4:7  1:0 (stat+ syst)CLEO 1:8  0:3 (stat+ syst) 1:9  0:3 (stat+ syst)theory 1.7 2.44 Study of D1and D2production in c and b quark jetsA direct search of the D01(2420) and D02(2460) orbital states decaying into D+ was thus performed9). The D+were exclusively reconstructed in the D+!D0+decay mode followed by D0! K +or D0! K ++ : a total of 7400D+was obtained. High purity (' 90%) bb and cc samples were selected separatelyusing both the energy and lifetime informations. Any additional pion of momentumlarger than 1  1:5 GeV/c and charge opposite to that of the D+was used to t aD0+ vertex. The pion selection eciency was estimated to be about 60%.To evaluate the M(D+ ) invariant mass, the following mass dierencewas computed: M(D+ ) = M(D0+ )  M(D0+) + mDwhere mDis thenominal D+mass1). This gives a resolution  of about 6 MeV/c2on M(D+ ),according to the simulation. TheM(D+ ) invariant mass distribution is presentedin Figure 2 for the selected bb and cc enriched samples (data points). An excess of(D+ ) pairs is observed between 2.4 and 2.5 GeV/c2, but not in the M(D++)distribution (hatched histogram). The (D+ ) mass spectrum was tted as thesum of three contributions: a background function of the form (M(D) mD m) exp( (M(D) mD m)) where mis the pion mass and ,  and  arefree parameters; for each D01and D02resonance, the convolution of a Breit-WignerM(D*pi)  (GeV/c2)01020304050607080901002.2 2.3 2.4 2.5 2.6 2.7 2.8Entries / 10 MeV/c2M(D*pi)  (GeV/c2)01020304050607080901002.2 2.3 2.4 2.5 2.6 2.7 2.8Entries / 10 MeV/c2DELPHIb samplec sampleFigure 2: Invariant mass (D+ ) (dots) and (D++) (hatched histogram).function and a Gaussian describing the experimental mass resolution. Fixing themass and width of both resonances to their world average value1), the number oftted D01and D02was 166  37 (245  37) in the bb (cc) enriched sample.Knowing from the simulation the eciencies and purities in both bb andcc samples, the production fractions of both DJstates are reported in Table 3. Theyare found in agreement with other CLEO and LEP measurements10), as well as atheoretical estimate of direct DJproduction in charm quark fragmentation11).5 Search for a radially excited D mesonRepeating a similar procedure as above, all remaining opposite charge + pairsproduced in the same direction as the D+candidates were used to t a D0+ vertex12). The additional pions were required to have a momentum larger than0:6   1:0 GeV/c and not to be identied as kaons. As previously, D++ com-binations belonging in bb or cc enriched samples were selected. The reconstructedinvariant mass was computed asM(D++ ) =M(D0++ ) M(D0+)+mD.This gave a resolution of about 6 MeV/c2and no bias, according to the simulation.The invariant mass distribution of all D++ candidates is presented in Figure 3.A narrow peak is observed whereas the wrong charge combinations D+  doM(D*pipi)  (GeV/c2)010203040506070802.4 2.5 2.6 2.7 2.8 2.9 3Entries / 10 MeV/c2DELPHIFigure 3: Invariant mass (D++ ) (dots) and (D+  ) (hatched histogram).not show any excess. A binned maximum likelihood t to the mass distributionwas performed by taking into account two contributions: a function of the form(M(D) mD  ) exp( (M(D) mD  )) for the background, with set to 340 MeV/c2, and a Gaussian function with free parameters to describe thenarrow peak. The tted number of events was 66  14(stat). The tted width was7  2(stat) MeV/c2, in agreement with the expected resolution, and the averagemass was found to be 2637  2(stat)  6(syst) MeV/c2, where the systematic er-ror was evaluated from a t to the narrow DJresonances of the previous section.About (57 10)% of these events were selected in the cc enriched sample. A Breit-Wigner form was also used to t the signal shape, giving a 95% C.L. upper limit of15 MeV/c2for the full decay width of the resonance.The observed signal is interpreted to come from a D0radial excitation(23S1) with an expected mass of about 2630   2640 MeV/c2 4; 5). It is howeverimportant to stress that recent theoretical computations of the expected width wouldpredict a much larger value for such a radial excitation13).The production rate of D0relative to that of D01and D02was evaluated:<ND0+> Br(D0+! D++ )PJ=1;2<ND()0J> Br(D()0J! D+ )= 0:49 0:18(stat) 0:10(syst): (3)This can be compared with models of primary hadron production which predict0:16  0:0111)or 0:25  0:0214)for cc events. The measured value in Z decays,where both cc and bb events contribute, is larger than these expectations but has alarge uncertainty.6 ConclusionThe study of charm meson excited states can still be improved at LEP. Other decaychannels (into D or DK) can also be used in DELPHI. The study of the productionrate of both narrow and broad DJstates can be achieved in B semileptonic decays,maybe providing a nice way to evaluate the mass and width of the broad states.7 AcknowledgementsI warmly acknowledge myDELPHI colleagues (especially J.-P. Gerber and R. Strub);as well as the local committee for its invitation and for the ecient organization ofthis fruitful workshop.References1. Review of Particle Physics, Eur. Phys. J. C3 (1998) 1.2. N. Isgur and M.B. Wise, Phys. Rev. Lett. 66 (1991) 1130 and ref. therein.3. J. Rosner, Comments on Nucl. and. Part. Phys. 16 (1986) 109 and ref. therein;S. Godfrey and R. Kokoski, Phys. Rev. D43 (1991) 1679;E.J. Eichten, C.T. Hill and C. Quigg, Phys. Rev. Lett. 71 (1993) 4116.4. S. Godfrey and N. Isgur, Phys. Rev. D32 (1985) 189.5. D. Ebert, R.N. Faustov and V.O. Galkin, Phys. Rev. D57 (1998) 5663.6. CLEO collab., Phys. Rev. D58 (1998) 052003 and ref. therein.7. DELPHI collab., P. Abreu et al., Nucl. Instr. & Meth. A378 (1996) 57.8. DELPHI collab., DELPHI Note 98-120 CONF 181 and ICHEP'98-122 (1998).9. DELPHI collab., DELPHI Note 98-128 CONF 189 and ICHEP'98-240 (1998).10. CLEO collab., P. Avery et al., Phys. Lett. B331 (1994) 236;OPAL collab., K. Ackersta et al., Zeit. Phys. C76 (1997) 425;ALEPH collab., D. Pallin et al., ALEPH 98-047 and ICHEP'98-021 (1998).11. F. Becattini, Zeit. Phys. C76 (1997) 269; J. Phys. G23 (1997) 1933.12. DELPHI collab., P. Abreu et al., Phys. Lett. B426 (1998) 231.13. D. Melikhov, O. Pene, Phys. Lett. B446 (1999) 336; P. Page, hep-ph/9809575.14. Yi-Jin Pei, Zeit. Phys. C72 (1996) 39.

Charm spectroscopy in DELPHI

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