Using D$^{*+}$ mesons exclusively reconstructed in the DELPHI detector at
LEP, orbital and radial excitations of non strange charmed mesons are studied.
The multiplicities of the two narrow $D^0_1$ and $D^{*0}_2$ orbital excitations
are measured in $Z^0 \to c \bar c$ and $Z^0 \to b \bar b$ decays. Preliminary
results are obtained on the production of broad D$^{**}$ states, using B meson
semi-leptonic decays. A narrow signal of 66 $\pm$ 14 events is observed in the
$(D^{*+} \pi^+ \pi^-)$ final state, interpreted as the first evidence of the
predicted $D^{*'}$ radial excitation.Comment: 5 page

Bourdarios, C.

English

arXiv

Using D$^{*+}$ mesons exclusively reconstructed in the DELPHI detector at LEP, orbital and radial excitations of non strange charmed mesons are studied. The multiplicities of the two narrow $D^0_1$ and $D^{*0}_2$ orbital excitations are measured in $Z^0 \to c \bar c$ and $Z^0 \to b \bar b$ decays. Preliminary results are obtained on the production of broad D$^{**}$ states, using B meson semi-leptonic decays. A narrow signal of 66 $\pm$ 14 events is observed in the $(D^{*+} \pi^+ \pi^-)$ final state, interpreted as the first evidence of the predicted $D^{*'}$ radial excitation.Using D$^{*+}$ mesons exclusively reconstructed in the DELPHI detector at LEP, orbital and radial excitations of non strange charmed mesons are studied. The multiplicities of the two narrow $D^0_1$ and $D^{*0}_2$ orbital excitations are measured in $Z^0 \to c \bar c$ and $Z^0 \to b \bar b$ decays. Preliminary results are obtained on the production of broad D$^{**}$ states, using B meson semi-leptonic decays. A narrow signal of 66 $\pm$ 14 events is observed in the $(D^{*+} \pi^+ \pi^-)$ final state, interpreted as the first evidence of the predicted $D^{*'}$ radial excitation

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STUDY OF D AND D0PRODUCTION IN B AND C JETS, WITH THE DELPHI DETECTORC. BOURDARIOSUniversite´ de Paris Sud, Laboratoire de l’Acce´le´rateur Line´aire, Baˆt. 200, B.P. 34, FR-91898 ORSAY CEDEXE-mail: claire.bourdarios@cern.chUsing D+ mesons exclusively reconstructed in the DELPHI detector at LEP, orbital and radial excitations of non strange charmedmesons are studied. The multiplicities of the two narrow D01 and D02 orbital excitations are measured in Z0 → cc¯ and Z0 → bb¯decays. Preliminary results are obtained on the production of broad D states, using B meson semi-leptonic decays. A narrowsignal of 66 ± 14 events is observed in the (D+pi+pi−) final state, interpreted as the first evidence of the predicted D0 radialexcitation.1 IntroductionFor mesons containing heavy and light quarks (Qq), andin the limit where the heavy quark mass is much largerthan the typical QCD scale, the spin −!sQ of the heavyquark decouples from other degrees of freedom. Thus,for strong decays, the total (spin+orbital) angular mo-mentum −!jq = −!sq +−!L of the light component is conserved.This heavy quark symmetry, together with quark poten-tial models used for lower mass mesons, allows the massesand decay widths of heavy mesons to be predicted 1.Figure 1: Spectroscopy of non-strange D mesons. The shaded areasshow predicted widths for these states. For clarity the expected D1and D2 decays involving a ρ meson or pipi pairs are not shown.The present knowledge of charmed meson spec-troscopy is summarized in Figure 1. The well establishedD and D mesons2 correspond to the two degenerate lev-els of the (L=0, jq = 1/2) state. The two (L=1, jq = 3/2)states have been clearly observed 2, because they havenarrow decay widths of about 20 MeV/c2. The mea-sured masses of the D01(2420) and D02 (2460) agree within20 MeV/c2 with the prediction of the models. Section 3presents a measurement of their production rate in ccand bb jets.The (L=1, jq = 1/2) states decay through a S waveand are expected to have large decay widths. Up to now,they have not been observed directly, but their total pro-duction rate is measured using B meson semi-leptonicdecays (section 4).In addition to these orbital excitations, ra-dial excitations of heavy mesons are foreseen.The D0and D0are expected to have masses of2.58 GeV/c2 and 2.64 GeV/c2 respectively, with a10-25 MeV/c2 uncertainty on the mass predictions 3.They are expected to decay, in S wave, into D()pipi.Section 5 presents the rst evidence for the D0meson,observed in the decay mode (Dpipi).2 D and D0reconstructionDELPHI 4 is a multipurpose LEP detector, with spe-cial emphasis on precise vertex and charged tracks mo-mentum reconstruction, and particle identication. Themicro-vertex detector provides 3 Rφ and 2 Z hits pertrack, with intrinsic resolutions of 7.6 and 9 µm. Formuons of 45 GeV/c momentum, a resolution of σ(p)/p of 3% is obtained, and the precision of the track extrap-olation to the beam collision point is 26  2 µm. Kaonand pion identication is performed using a Ring Imag-ing CHerenkov detector, and the ionisation loss in theTPC, which is the main tracking device. A total of 3.4million hadronic events is obtained from the 1992-1995data, at center-of-mass energies close to the Z0 mass.2.1 D reconstructionAll the decay channels considered here involve theD+ ! D0pi+ decay, followed by D0 ! (K−pi+) or1MKpi (GeV/c2)0200400600800100012001.6 1.8 2Entries / 10 MeV/c2MK3pi (GeV/c2)010020030040050060070080090010001.6 1.8 2Entries / 10 MeV/c2DELPHIFigure 2: Kpi and K3pi invariant mass distributions in (a) theD+ → (K−pi+)pi+ and (b) the D+ → (K−pi+pi−pi+)pi+ decaychannels.D0 ! (K−pi+pi−pi+). aTo reconstruct the D0 decay nal state, all (K−pi+)and (K−pi+pi−pi+) combinations are tried to t a sec-ondary vertex in space. Kinematical and track selec-tion cuts are described in detail in 5. Kaon candidatesare considered if they have a momentum larger than1 GeV/c and, in the K3pi channel, a loose kaon iden-tication is required. The D0 momentum and invari-ant mass are computed from the momenta of the decayproducts. Then, all charged particles with momentumbetween 0.4 GeV/c and 4.5 GeV/c and charge oppositeto that of the kaon candidate are used as pion candidatesfor the D+ ! D0pi+ decay. In the Kpi (K3pi) channel,events are selected if the mass dierence (MKpipi−MKpi)(resp. (MK3pipi−MK3pi)) is within  2 ( 1 ) MeV/c2 ofthe nominal value (MD − MD0). The D candidatesmust have an energy fraction XE(D) = E(D)/Ebeamgreater than 0.25. Figure 2 shows the distribution of theM(Kpi) and M(K3pi) invariant masses for the selectedevents. The tted D0 masses and widths are 1868  1(1869  1) and 19  1 (12  2) MeV/c2 . The recon-structed D0 mass is required to lie within  40 ( 30)MeV/c2 of the nominal D0 mass: 4661  88 (2164  65)D candidates are selected in the Kpi ( K3pi) channels.The selection eciency is estimated, using the simula-tion, to be 21% (8%).2.2 D01, D02 and D0reconstuctionSimilar selection criteria and vertex reconstruction areused to reconstruct narrow orbitally and radially excitedstates.aThroughout this paper, charge-conjugate states are always im-plied, and the pi from D decay is denoted as pi.In the case of D01 and D02 decaying into D+pi−, apion with a charge opposite that of the D+ is added, andthe D0pi+ pi− vertex is tted. All combinations are tried,provided the pion candidate has a momentum larger than1.0 (1.5) GeV/c in the Kpi (K3pi) channel. The recon-struction eciency is 14% (6%) in the Kpi ( K3pi) chan-nels.In the case of D0decaying into D+pi+pi−, all pairs ofoppositely charged pions are used to t a D0pi+pi− vertex.The pion candidates are required to have a momentumlarger than 0.6(1.0) GeV/c, and those compatible witha kaon according to particle identication are rejected.For a signal of mass 2640 MeV/c2, the reconstructioneciency is 4% (2%) in the Kpi ( K3pi) channels.In both cases, the precision on the invariantmass reconstruction is improved by correcting for a4 MeV/c2 shift observed in the D0 mass, by using:M(Dpi) = M(D0pipi) −M(D0pi) + mDM(Dpipi) = M(D0pipipi) −M(D0pi) + mD (1)where mD is the nominal D+ mass. The simulationpredicts a resolution of about 6 MeV/c2 on the mass re-construction, for both radial and orbital excitations.2.3 Selection of bb and cc samplesDue to the relatively long lifetimes of charmed and bot-tomed particles, heavy flavour events are characterizedby the presence of secondary vertices. The probabil-ity P that all tracks detected in the event come fromthe primary vertex is small: for bb events, a purity of90% is archieved, with an eciency of 60%, by requiringP  10−2.Charmed mesons from Z0 ! bb events are distin-guished from those in cc events by considering both theirenergy and lifetime informations. Bottom quarks frag-ment into a B hadron, which subsequently decays intoa D+ meson, whereas in cc events charmed mesons aredirectly produced in the fragmentation process. This dif-ference in the hadronization leads to a smaller energyfraction of XE(D) for bb events. Also, due to the bquark lifetime, the apparent flight of the D0 meson isgreater than the true decay length. Its measured propertime distribution is larger than the mean B meson life-time, 1.6 ps, compared to a true D0 lifetime of 0.4 ps.By combining these variables, bb and cc samples areselected, with high purities: 92 % for bb , 89 % for cc.In the bb sample, the combinatorial background ishigher, but is reduced by 50% using the kaon identica-tion, and also by asking that the impact parameter ofthe additional pion is positive, i.e. that the intersectionof the pion and D directions is on the same side of theprimary vertex as the D vertex. As a consequence, the2ratio of eciencies (Dpi)/(D) is 52% for bb , com-pared to 62% for cc .3 Study of narrow orbital excitationsFigure 3 shows the M(Dpi) invariant mass distribu-tion obtained for the sum of the bb and cc samples 6.A clear excess of (D+pi−) pairs is observed between2.4 and 2.5 GeV/c2, corresponding to the two overlap-ping contributions of the D01 and D02 . They are ttedby two Breit-Wigner functions, whose widths are xedto the measured world average 2, convoluted with theexperimental resolution. A total signal of (361  58)D01 + D02 events is tted, out of which (65  10) %is assigned to D01 . The masses are left free in the t,and the result is MD01 = 2425  3(stat) MeV/c2 andMD02 = 2461 6(stat) MeV/c2, i.e. consistent with theworld averages2. The helicity distributions are consistentwith the production of a Jp = 1+ and Jp = 2+ states.Figure 4 shows the same mass distribution, but forthe bb and cc samples separately. The same t is per-formed, but both D01 and D02 masses and widths arexed to the world average. The result of the t is(97  26) D01 and (69  27) D02 in the bb sample,(141  26) D01 and (104  26) D02 in the cc sample.In order to measure the D01 and D02 production rates,these results are unfolded from the reconstruction e-ciencies, signal purities, D01 and D02 decay widths intoD+pi−. The errors quoted below are statistical only.Systematic errors are still under study, but smaller thanthe statistical errors.The cc sample provides direct information on thecharm fragmentation. Results are:f(c! D01) = 1.9 0.4 (stat) %f(c! D02 ) = 4.3 1.3 (stat) % (2)Both results are in agreement with previous LEP andCLEO measurements. For the D01, the result is alsoin agreement with theoretical calculations 7, which pre-dicted 1.7%. For the D02 , the result is high compared tothe expectation (2.4 %), but has large errors. A more pre-cise measurement would need to use the D02 ! D+pi−channel, which is forbidden for the D01.For the bb sample, results are:f(b! D01) = 2.2 0.6 (stat) %f(b! D02 ) = 4.8 2.0 (stat) % (3)This shows that the charm fragmentation properties aresimilar, although in a dierent environment.M(D*pi)  (GeV/c2)02550751001251501752002252502.2 2.3 2.4 2.5 2.6 2.7 2.8Entries / 10 MeV/c2DELPHIFigure 3: Invariant mass distributions (D+pi−) (dots) and(D+pi+) (hatched histogram). The mass computation and thefit are explained in the text.M(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 4: Invariant mass distributions (D+pi−) (dots) and(D+pi+) (hatched histogram) for the separate bb¯ and cc¯ samples.The mass computation and the fit are explained in the text.34 Study of broad orbital excitations in B mesonsemileptonic decaysIn B meson semileptonic decays, only 60% to 70% ofthe nal states are described by D`ν` and D`ν`. Theremaining contribution is attributed to D. The totalproduction rate, including broad orbital excitations, canbe measured using the impact parameter of the pion,denoted pi, emitted in the decay chain B ! DX !(Dpi)X 0.Events are selected if a lepton with momentum largerthan 3 GeV/c2 is identied, and if its transverse momen-tum relative to the D+ is larger than 0.5 GeV/c2. Thekaon candidates in the D0 decay must have the samecharge as the lepton. 459 25 (288  19) events are se-lected in the Kpi(K3pi) channel. All remaining tracks, ofcharge opposite to that of the D+, are pi candidates.Kinematical and selection cuts are described in 8. Thebackground due to fake D+ associated to a true lep-ton `− is subtracted by using events in the tail of theD invariant mass distribution. The contribution of trueD+ associated to a fake lepton is subtracted using D`pairs with the wrong sign combination. The remaining111  16 events are due to true b semileptonic decaysinto D+`−X nal state, associated with a pi candi-date either from D decay, or from jet fragmentation.The shape of the two contributions are shown in gure 5.They are used to t the pi impact parameter distribu-tion shown in gure 6. From the result of the t, thefollowing branching ratio is obtained:BR(B− ! (D+pi−)`−νX)= 1.15 0.17(stat) 0.14(syst) % (4)Impact parameter (cm)020406080100120140160180-0.02 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14Entries / 40 µmImpact parameter (cm)0255075100125150175200-0.02 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14Entries / 40 µma) MC pi**b) MC fragmentationFigure 5: Impact parameter relative to the primary interactionvertex in simulated B semileptonic decays for a) pi from Ddecay and b) charged particles from jet fragmentation.Impact 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 6: Impact parameter relative to the primary interactionvertex in real data for a) right charge pi− and b) wrong chargepi+ candidates. The hatched area is the contribution from jetfragmentation.This result signicantly improves a previousDELPHI measurement, and is in agreement with otherLEP measurements.The (D+pi) invariant mass is also reconstructedand used to t, in the way described in the previoussection, the D01 and D02 narrow resonances. A signal of26.7  8.2 D01 is tted, and the corresponding productionrate is:BR(B− ! D01`−νX)= 0.72 0.22(stat) 0.13(syst) % (5)For the D02 state, 14.8  7.7 events are tted, i.e. asignal signicance smaller than 2 σ. More data would benecessary to estimate the corresponding branching ratio.5 Evidence for a narrow radial excitationFigure 7 shows the invariant mass distribution obtainedwhen two pions of opposite charges are added to the Dcandidate, and using the sum of the two bb and cc sam-ples. An excess of 66  14 (stat) events is observed in the(D+pi+pi−) combination. The signal is tted by a Gaus-sian distribution of free parameters: the χ2 per degree offreedom is 60/59, and would be 91/62 if the Gaussianwas removed. About (57  10)% of the signal is selectedin the cc sample.The tted mass is 2637 2 (stat)  6 (syst) MeV/c2.It is thus consistent with the predictions for the D0ra-dial excitation 3: 2640 MeV/c2. Other L=2 states arepredicted, but with masses higher by at least 50 MeV/c2 .4The width of the tted Gaussian is 7  2 MeV/c2, i.e.compatible with the detector resolution. Therefore, onlyan upper limit is derived: the full width of the signal issmaller than 15 MeV/c2 at 95 % C.L. There is no naturalexplanation of such a small value, neither for the D0norfor higher orbital excitations 9.Various checks were performed. Varying the back-ground shape and the kinematical cuts has no eectwithin statistics. No peculiar double counting was no-ticed, and the signal is stable when the pi is added tothe D0pi+pi− tracks in the vertex t. As explained above,mass shifts are studied using D01 and D02 narrow states,and a conservative systematic error of 6 MeV/c2 is at-tached to the mass measurement.The production rate of this signal can be comparedwith that of the D01 and D02 narrow states:<ND0>Br(D0!Dpi+pi−)∑J=1,2<ND()J>Br(D()J!Dpi)= 0.49 0.18(stat) 0.10(syst)(6)Most of the systematic uncertainties cancel in thisratio. The quoted systematics is due to the Monte-Carlostatistics, and to the uncertainties on widths and on thekaon rejection. This result is compatible, within its largeerrors, with the value obtained using the thermodynam-ical models already mentioned for orbital states 5,7.M(D*pipi)  (GeV/c2)010203040506070802.4 2.5 2.6 2.7 2.8 2.9 3Entries / 10 MeV/c2DELPHIFigure 7: Invariant mass distributions (D+pi+pi−) (dots) and(D+pi−pi−) (hatched histogram). The mass computation and thefit are explained in the text.6 ConclusionUsing about 7000 exclusively reconstructed D mesons,the D01 and D02 multiplicities are measured in cc events,and found to be consistent with theoretical calculations.The measured multiplicities in bb events are consistentwith the ones in cc events, both for the D01 and D02 .The total D production rate, involving a D+ in thenal state, is measured in B meson semileptonic decays.A narrow signal is observed in the (D+pi+pi−) -nal state, at the mass M = 2637  2 (stat)  6(syst) MeV/c2, interpreted as the rst evidence of thepredicted D0meson.AcknowledgementsI am very grateful to D. Bloch and P. Roudeau for theirhelp while preparing this talk, and to the DELPHI col-laboration for choosing me to give it.References1. N. Isgur and M.B. Wise, Phys. Rev. Lett.66(1991)1130 and ref. therein.2. Particle Data Group, \Review of Particle Proper-ties", Euro. Phys. J. C3(1998)1.3. S. Godfrey and N. Isgur, Phys. Rev.D32(1985)189.D. Ebert, R.N. Faustov and V.O. Galkin, \Massspectrum of orbitally and radially excited heavy-light mesons in the relativistic quark model",preprint HUB-EP-97/90, hep-ph/9712318(1997).4. DELPHI collab., P. Aarnio et al., Nucl. Instr.Meth. A303(1991)233.DELPHI collab., P. Aarnio et al., Nucl. Instr.Meth. A378(1996)57.5. DELPHI collab., P. Abreu et al., Phys. Lett.B426(1998)231-242and contribution to ICHEP’98 number 483.6. Contribution to ICHEP’98 number 240, DELPHI98-128 CONF 189.7. F. Becattini, J. Phys. G23(1997)1933.8. Contribution to ICHEP’98 number 239, DELPHI98-119 CONF 180.9. D. Melikhov and O. Pene, HEP-PH/9809308.5

Study of $D^{**}$ and $D^{*'}$ Production in B and C Jets, with the DELPHI Detector

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Study of D^{**} and D^{*'} Production in B and C Jets, with the Delphi Detector

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