The experimental status of light vector meson spectroscopy is discussed. The
last results of $e^+e^-$ experiments obtained at the VEPP-2M collider in
Novosibirsk are described and the comparison with the old data in the mass
region from 1 GeV to 2.5 GeV is performed.Comment: 6 pages, 13 figures, e^+e^- Physics at Intermediate Energies,
  Workshop - Contribution T0

Ivanchenko, V.

English

arXiv

The experimental status of light vector meson spectroscopy is discussed. The last results of $e^+e^-$ experiments obtained at the VEPP-2M collider in Novosibirsk are described and the comparison with the old data in the mass region from 1 GeV to 2.5 GeV is performed

Ivanchenko, V N

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PSN T02EXPERIMENTAL STATUS REPORT ON VECTOR MESONSPECTROSCOPY V.Ivanchenko, BINP, Novosibirsk, RussiaAbstractThe experimental status of light vector meson spectroscopyis discussed. The last results of e+e− experiments obtainedat the VEPP-2M collider in Novosibirsk are described andthe comparison with the old data in the mass region from1 GeV to 2:5 GeV is performed.1 INTRODUCTIONFor the first time the e+e− spectroscopy study was per-formed in Novosibirsk in 60th at the VEPP-2 collider. Theshape of the (770) resonance have been measured [1].Since a lot of different experiments for spectroscopy havebeen done [2] and as a rule the most precise data on vectormeson parameters were obtained in e+e− production. Thecurrent status of the vector meson spectroscopy is follow-ing: All main states of qq systems are established. Charmonium and bottomonium families are wellknown. Excitation states of qq system for u; d; s quarks are notwell established. There are evidences for existence of KK or 4-quarksstates in the vector meson decays [3, 4, 5, 6]. There are evidences for existence of NN or 6-quarksstates [7].The main problems of the light vector meson spectroscopyconnect with the fact that in the mass region 2E = 1:4 2:5 GeV total integrated luminosity ’ 2 pb−1 was col-lected at DCI and ADONE. This statistic is incompatiblewith that collected in the energy regions of the charmoniumand bottomonium families.At the contrary, in the low energy region from the hadronproduction threshold to 1:4 GeV the systematic studieshave been performed in Novosibirsk at the e+e− colliderVEPP-2M. It was in operation from 1974 to 2000 and thetotal integrated luminosity ’ 80 pb−1 was collected. Im-portant measurements were done by OLYA [8, 9] and ND Invited talk at the workshop “e+e− Physics at Intermediate Ener-gies”, SLAC, Stanford, April,30 - May,2, 2001. Author is grateful toSLAC for the support.[10] experiments, but the main part of integrated luminos-ity were taken by the CMD-2 [11] and SND [12] experi-ments. Now the experimental program is finished, and thefinal data analysis is in a progress.2 PRODUCTION OF LIGHT VECTORMESONS IN ELECTRON-POSITRONCOLLISIONSMain advantages of the experiments on vector mesons pro-duction in e+e− annihilation are following: clean ini-tial state with the well defined quantum numbers, highmass resolution, good conditions for an exclusive reactionsstudy. The main problem of the e+e− data analysis connectwith uncertainties in the interference between several res-onances that often introduces model dependence into finalresults (for example [13]). There are also model depen-dences of the data analysis [14, 15, 16, 17, 18], which canbe resolved only after significant increasing of experimen-tal statistic.2.1 e+e− ! +− cross sectionThe precise measurement of the two pion production crosssection have been performed for many years [2, 9, 19].The systematic uncertainty of 0.6 % is achieved in the lastCMD-2 experiment [20] in the energy range below 1 GeV .For higher energies the results are not so precise, but DM2data [21] strongly emphasise the signal of (1700) (Fig.1).There is some wide enhancement in the the cross sectionaround 1:25 GeV which may be taken as an evidence forthe (1250) resonance, but at the same time another modelsare discussed [15, 22].2.2 e+e− ! +−0 cross sectionThe main contributions to three pion production cross sec-tion at low energy come from the !(782) and (1020) reso-nances. It is well known that the interference between theseresonances are destructive [13]. For many years in the en-ergy region above 1 GeV the experimental data was not soprecise [10, 23]. The last SND measurement [24] showsthat there are visible peak in the cross section at 1:25 GeV(Fig.2). After applying the radiative corrections and the de-tection efficiencies the total cross section was obtained inwhich the clear resonance signal is seen. Taking into ac-count the data below 1 GeV and the DM2 data [23] the setCenter-of-Mass Energy, MeV|F pi|2CMD2DM2ADONE10-210-1110500 1000 1500 2000 2500 3000202530354045700 750 800Figure 1: The e+e− ! +− cross section.2E,MeVσvis., nb0.511100 1200 1300 1400Figure 2: Visible cross section of the reaction e+e− !+−0.of fits were performed [24]. The best fit (Fig.3) requirescontributions of !, , !(1200), and !(1650) with the rela-tive phases (+), (-), (-), (+).2.3 e+e− ! +−+− cross sectionThe four charged pion production was studied by manygroups [2]. The most detailed investigation have been re-ported by CMD-2 [25]. In this work the PWA analysishave been performed and it was shown that the a1(1260)intermediate state dominates in the energy region below1:4 GeV . The SND results [26] confirm the CMD-2 data(Fig.4).2E,MeVσ0, nbSNDDM20123451000 1500 2000Figure 3: The e+e− ! +−0 cross section.Energy, MeVCross-section, nb   SND (this work)   CMD-2   ND   CMD   OLYA   DM20510152025301000 1100 1200 1300 1400 1500Figure 4: The e+e− ! +−+− cross section with therecent VEPP-2M data [26].2.4 e+e− ! +−00 cross sectionUsing the PWA analysis of the reaction e+e− !+−00 CMD-2 obtained [25] that a1(1260) and !0intermediate states dominate in the reactions mechanism(Figs.5,6). The recent SND data [26] are in agreement withthe CMD-2 results within the systematic uncertainty of theexperiments (Fig.7).2.5 e+e− ! +−+−0 cross sectionThe five pions production cross section have been studiedby CMD-2 [27] and DM2 [23]. It was shown that treediagrams (Figs.8,9) dominate in these reactions. In the!+− cross section the clear peak of the !(1650) is seenand probably some contribution of the !(1200) exists. Inthe +− reaction the clear peak of (1450) determinesthe cross section shape but some contribution of (1700) isnot excluded.Mrecoil(pi0)0200400400 600 800 1000 1200Figure 5: The invariant mass of +−0 in the reactione+e− ! +−00 [25].Minv(pi±pi0) Mrecoil(pi±)Minv(pi+pi-) Minv(pi0pi0)0255075100500 10000204060500 10000102030500 10000102030500 1000Figure 6: The two pions invariant mass in the reactione+e− ! +−00 [25].2.6 e+e− ! !0 cross sectionThe main reaction channel e+e− ! !0 ! +−00is seen in the four pion final state but less systematic un-certainty in the cross section measurement was achievedby SND using the e+e− ! !0 ! 00γ reaction[28]. Combining SND data with the data of DM2 [29]and CLEOII [30] the fit of the cross section was performed(Fig.12). Note, that there is a systematic bias between theDM2 and CLEOII data, which can be connected with a nor-malisation problem or with the bias in the energy scale.Energy, MeVCross-section, nb   SND (this work)   CMD-2   ND   OLYA   DM2051015202530354045900 1000 1100 1200 1300 1400 1500 1600Figure 7: The e+e− ! +−00 cross section with therecent VEPP-2M data [26]. pipiee+-ω ρpipipi+−γ VFigure 8: The e+e− ! !+− main diagram.2.7 e+e− ! γ cross sectionFirst indication of a radiative decay of radial excitations oflight vector mesons was found out by CMD-2 [31]. Twoevents of the reaction e+e− ! γ were identified. Theestimated production cross section is in agreement withthe data of CMD-2 [27] and DM2 [32] for the reactione+e− ! +−.γ ηρpipipipipi0+ −+−0ee+-VFigure 9: The e+e− ! +− main diagram.00.511.522.51400 1600 1800 2000 2200 2400Ec.m. , MeVCross section, nbCMD-2DM200.20.40.61300 1400Figure 10: The e+e− ! !+− cross section.024681400 1600 1800 2000 2200 2400Ec.m. , MeVCross section,  nbCMD-2: η → pi+pi-pi0CMD-2: η → γγDM2ND: η → γγ01231300 1400Figure 11: The e+e− ! +− cross section.2.8 e+e− ! KSKL; K+K− cross sectionsThe preliminary SND results on the cross section e+e− !KSKL [13, 33] together with the DM1 data [34] can besuccessfully fitted if the contributions of the , !, , and(1680) resonances are taken into account (Fig.13). Thedata on the reaction e+e− ! K+K− [8, 35] are in agree-ment with a such model.2.9 e+e− ! KK cross sectionsThe PWA analysis of the e+e− ! KK reaction havebeen performed by DM2 [36]. It was shown that isoscalar2E0, MeVσ(e+e-→ωpi0 →pi0 pi0 γ), nbCMD2DM2CLEOSND00.20.40.60.811.21.41.61.81000 1200 1400 1600 1800 2000Figure 12: The e+e− ! !0 cross section.10-310-210-111010 210 31 1.2 1.4 1.6 1.8 2 2.2Center of mass energy, GeVCross section, nb l --- CMD-2n --- SNDs --- DM1Figure 13: The e+e− ! KSKL cross section.process (1680) ! KK ! KSK0 dominates. Thecross section e+e− ! K+K−0 is small. The 1:45 GeVvector state observed in the hadron production [37] is notconfirmed in the e+e− production at VEPP-2M [33, 38].3 THE LIGHT VECTOR MESONSPECTRUMThe classification (Table 1) of the light vector mesons pro-posed by PDG [2] cannot be accepted without a seriousdiscussion. Some of resonances included in the table areno well established. In the contrary, the data on (1250)and !(1200) are ignored. The difficulty of the existingdata analysis connect with the low statistical accuracy ofthe data above 1:4 GeV . Moreover the model uncertaintyof resonances mass and width may exceed 200 MeV [18].Table 2: The level of experimental significance of the vector mesons in selected reactions: + - well established states, * -not well established states.(1250) !(1200) (1450) !(1420) (1700) !(1650) (1680) (2150)e+e− ! +− * * + *e+e− ! +−0 + * * *e+e− ! 4 + +e+e− ! !0 * +e+e− ! !+− * * + *e+e− ! +− + *e+e− ! γ *e+e− ! KSKL * +e+e− ! K+K− * * * +e+e− ! KK * +e+e− ! K+K−0 *−p ! !0n +−p ! 0n *pp; pn ! hadrons * * +γp ! hadrons * +Table 1: The classification of vector mesons by PDG’00.N2S+1LJ (uu− dd)=p2 (uu + dd)=p2 ss13S1 (770) !(782) (1020)23S1 (1450) !(1420) (1680)13D1 (1700) !(1650) -33S1 (2150)The quality of the experimental data is demonstrated in theTable 2 and the following conclusions can be done afterreview of the current data: (1250) is ignored by PDG but as pointed out byD. Peaslee [39] there are several old and new exper-iments (OMEGA [40], LASS [41], OBELIX [42, 43])in which some evidences for the (1250) were ob-tained. !(1200) is identified by +−0 cross section [24]. (1450) is identified by +−+− and +− pro-duction in e+e− and in pp experiments. !(1420) has no solid ground. (1700) is seen in the e+e− production in +−,!0, and +−00 final states. It is identified inthe gamma production [2] and in the pp production[44, 45]. !(1650) is identified by !+− cross section [23]. (1680) is identified by the KK cross section [36]. (2150) is identified in the hadron production of !0by GAMS [46].4 PROSPECTS FOR PEP-NThere are a set of questions which must be answered toclear the situation with excited states of the light vectormesons: Do (1250) exist? What is the nature of this object?It is 23S1 qq state or it is lowest 4-quark vector state? Do !(1200) is 23S1 qq state or it is lowest 4-quarkvector state? Do !(1420) exist? (1700), !(1650), and (1680) have practically thesame mass. They have to have common decay chan-nels, so its real inputs are hidden in cross sectionshapes because of the interference. Are there threeresonances (1700), !(1650), and (1680) or thereare only two? Do other light quarks states exist?The adequate e+e− collider for a such study is PEP-N.The experiment at PEP-N is able to provide a good effi-ciency and particle identification for hadron and radiativetransitions between different states in the energy region1 − 3 GeV . The other methods using existing facilitiesare not able to solve all problems of the spectroscopy ofthe light vector mesons because of following problems: Below 3 GeV the luminosity of existing e+e− collid-ers fall down. The designed maximum energy of theVEPP-2000 [47] is 2 GeV . Using the hadronic  decays is not possible to estab-lish the spectrum of vector mesons above 1:3 GeVbecause of kinematics. The Initial State Radiation method [48, 49] is a veryeffective method to demonstrate the cross sectionshape and to tag the most interesting phenomena, butthe precision of this method is not known and sometheoretical and experimental limits can be foreseen. The previous experience shows us that experimentsfor the hadron production, γ-production, and pp pro-duction cannot substitute precise e+e− experimentsfor the vector meson spectroscopy.5 CONCLUSIONS The knowledge of the vector meson spectroscopy isincomplete. The heavy quarkonium spectra are known much betterthan the spectrum of the light vector mesons. It is required to measure a complete set of hadron pro-duction cross sections in the energy region 1−3 GeVwith the integrated luminosity about 200 pb−1. This luminosity investment will provide an opportu-nity to study as traditional and exotic states, hadronicand radiative transitions. The two new e+e− projects VEPP-2000 [47] atNovosibirsk and PEP-N at SLAC are intend to bringa light on the light vector meson spectroscopy. 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Experimental Status Report on Vector Meson Spectroscopy

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