In this contribution I review recent experimental developments in the
spectroscopy of higher-mass mesons, especially candidate radial excitations
discussed at the WHS99 meeting in Frascati.Comment: Postscript, 14 pages, 1 fig, uses included frascatiphys.sty. Summary
  Report, Session on Radial Excitations, WHS99 (Frascati, 8-12 March 1999

Barnes, T.

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arXiv

In this contribution I discuss recent experimental developments in the Spectroscopy of higher-mass mesons, especially candidate radial excitations discussed at the WHS99 meeting in Frascati

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RADIAL EXCITATIONS*T. BarnesPhysics Division, Oak Ridge National Laboratory, Oak Ridge, TN 3783 l-6373Department of Physics, University of Tennessee, Knoxville, Tennessee 37996- 150 1to be nublished inProceedings ofWorkshop  on Hadron  SpectroscopyFrascati, ItalyMarch g-12,1999*Research supported by the U.S. Department of Energy under ContractDE-AC05960R22464 managed by Lockheed Martin Energy Research Corp.‘The submitted manuscript has been uuthored  by a contractor of the U.S. Government under contract No. DE-AC05960R22464. Accordingly. the U.S.Government retains u nonexclusive, royalty-free license to publish or reproduce the published form  of this contribution, or ullow others  to do so, for U.S.Government purposes.”Frascati Physics Series Vol. XXX (1997), pp. 000-000CONFERENCE  TITLE - Conference Town, Ott 3rd, 1996RADIAL EXCITATIONST. BarnesPhysics Division, Oak Ridge National LaboratoryOak Ridge, TN 37831-6373, USADepartment of Physics , University of TennesseeKnoxville, TN 37996-1501, USAABSTRACTIn this contribution I discuss recent experimental developments in the spectroscopyof higher-mass mesons, especially candidate radial excitations discussed at the WHS99meeting in Frascati.1 Introduction: Why radials?We now have strong evidence for a true Jpc = l-+ exotic at 1.6 GeV 1~ 2y 3, inp7r at BNL and VES, and q’;rr and blr at VES, and with a possible lighter state at1.4 GeV in r/r reported by BNL and Crystal Barrel ‘, 4y 5). Hadron spectroscopymay have finally found the hybrid mesons anticipated by theorists. Of course thereis an unresolved concern that these experimental masses are somewhat lighter thautheoretical expectations; both the flux-tube model 6, and recent LGT calculations 7)find that the lightest exotic should be a Jpc = l-+, albeit with a mass of M 1.9 -2.0 GeV.Since @g hybrids span flavor nonets,  there will be many more such statesif this is indeed a correct interpretation of the data. Specific models of hybrids suchas the flux-tube model and the bag model anticipate that there should be hybridTable 1: I=0 states reported in Crystal Barrel data in pfi + PsPs and rlr”?ro byBugg et al.F6++4++M(MeV) 1 l?(MeV)m2000(  40) 250(40)2365(  30) 300(  50)2240(40) 170(50)2210(40) 310(45)2065(  30) 225(30)1945(30) 220(40)2300(40) 270(40)2040(40) 190(40)I++ 2340(40)  340(40)0 + +  2335(25)  225(40)o + +  2095(10)  190(12)c o m m e n t s  1weak mrIr’vr”7To7,rpr”7rovery weak n-7rIdiscussed were rrr, vq, qr]‘, 37r”, 7x0, $r” and r/rr”ro. Some very interesting resultswere reported, which will allow us to quote some new estimates for the masses ofpreviously unknown higher-mass nfi quarkonium multiplets.The I=0 states reported by Bugg in rIT+7rT-,  LOX’,  77, qr]’ and ~rr”~o (takenfrom a recent preprint 14) are summarized in Table 1. Bugg also reported resultsfor I=1 states seen in 7r+r-,  37r”, r]7r” and qt7ro,  given in Table 2.If these results are confirmed, they represent a considerable contributionto the determination of the nR quarkonium spectrum in the mass region of 1.9-2.5GeV, which is especially relevant to searches for glueballs and excited hybrids.2.2 r hadronic decays at CLEO: the ai(1700)In addition to evidence for radially excited states in pp annihilation, we also heardresults from R.Baker 15) about the possible evidence for a radial excitation, theai (1700), in r hadronic decays. The process discussed was r- + v,7r-r”rro; this isdominated by pr, which originates primarily from the ai (1260). Since the ai (1260)appears clearly here, one might expect to see the radial excitation ai(1700) as well.This state is interesting as a benchmark for the 2P nfi multiplet, and in view of thereported exotic rri(1600) nearby in mass we must be especially careful in identifying2P qq states; nonexotic hybrids with l++ are predicted by the flux-tube model toTable 3: Possible radial excitations reported in other WHS99  sessions.lzzI=O:4++4++2+2++I2++2++2++2++2++0++I=l:3 - -3 - -2++E2++l - -1 - -0-+M(MeV) I’(MeV) mode2330(  30) ?90( 70)2330(  20) 240(40)2310(  30) 230( 80)2130(35) 270( 50)1980(50) 450(  100)1945(45) 130(70)1940(10) 150(  20)FZ 1645(20) x 200(30)1645(35) 230( 120)1980(30) 190(40)2300(  50) 240( 60) q7r+‘rr-2180(40) 260( 50) q?T+7r-1752(21)(4) 150(110)(34) yy + 7r+7r-7r”M 1670(20) M 280(70) r/r02150 [PDG] PGI P111450 [PDG] PGI P-l1400(40) 275( 50) PKcontributionqiT+Tr-wwq7h-K+K-rlrlrlrlwwToTo,  vlrlrlwore,  v-7Dorofeev (VES)Dorofeev (VES)Dorofeev (VES)Kirk (WA102)Peters (CBar)Kondashov (GAMS)Dorofeev (VES)Peters (CBar)Kondashov (GAMS)Kondashov (GAMS),Dorofeev (VES)Dorofeev (VES)Braccini (L3)Peters (CBar)Dorofeev (VES)Dorofeev (VES)Thoma (CBar)known, 17) and have been measured by E852, it should be straightforward to testthe strength of the possible ~~(1700) peak in fir at CLEO.3 Possible radial excitations reported in other WHS99 sessionsMany states were reported in talks in other WHS99 sessions which are plausible can-didates for radial excitations. Since these talks will be reviewed by the appropriatesession chairs I will not discuss them in general in any detail here, but instead simplyquote the quantum numbers, mass, width, author of the talk and the experiment(Table 3). Where these are especially interesting for the subject of,radial excitationsI will discuss the particular state subsequently.4 Theoretical aspects of identifying quarkonia and non-quarkonia4.1 MassesOne might wonder how any of these levels can be confidently identified as quarkonia,since many non@ states with the same quantum numbers are expected.4P3 D 2FFigure 1: Radially-excited.nA  multiplet levels suggested by recent data (see Table 4).numerical estimates of expected masses of radially excited levels one can of courseuse “mass systematics”  such as the radial Regge trajectories discussed by Bugg 13)and Peaslee  .24) Decay calculations and other matrix elements however requireexplicit meson wavefunctions, which are usually determined in a Godfrey-Isgur typemodel.4.2 Strong DecaysIt will be very interesting to see if these new, rather low-mass candidates for radiallyexcited qq levels can still be accommodated in a Coulomb plus linear potential model,or if there appears to be serious disagreement with this very widely used descriptionof meson spectroscopy.In addition to masses and quantum numbers, we can expect to have experi-mental data on some relative strong branching fractions. These can be very valuableindicators of the nature of a hadron; examples include the evidence that 4 = sS (aweak pn mode), f2( 1525) = sS (a weak 7r~ mode), and that $(3097) = cE (weak lighthadron modes generally). Similarly, discussions of the nature of the scalar statesfe(l500) and fe(1710) have centered on explanations of their strong branching frac-tions to 7rn, KI?, 77 and r/v’. (Here the situation is more complicated because theTable 7: Theoretical and observed partial widths of the ~(2040)~~(2040) rIr P pw blr f271. K K  K*K’t h y .  17) (qg) 1 2  MeV 3 3  MeV 5 4  MeV 2 0  MeV  1 0  MeV 8  MeV  9  MeVexpt. 2, - G 1 1.5(4) - 0.5(2) - -on branching fractions from well-established qQ states is badly needed to allow testsof the decay models. One of the few states discussed at WHS99 for which informationon relative mode strengths was reported was in the VES observation of the ~(2040).Although this is not a radial excitation, these data show how detailed comparisonswith theoretical branching fractions for radials may be possible in future. Therelative branching fractions of the ~(2040)  to f27r, ,OX and pw were reported, whichwe compare to the predictions of the 3Po model 17) in Table 7. (Only modes withtheoretical partial widths > 5 MeV are tabulated in Table 7; for the complete setsee Barnes et al. 17) .) Evidently there is good agreement at present accuracy, whichis a nontrivial test of the model since these modes represent different angular decayamplitudes.Unfortunately there have been few attempts to measure relative branchingfractions of higher-mass states, and none were reported for the candidate radial exci-tations discussed here. This is an extremely important topic for future experimentalstudies.5 The Future of RadialsFuture work on higher-mass qq spectroscopy will hopefully establish the masses ofmissing states in the known multiplets (especially those with masses and quantumnumbers expected for glueballs and hybrids), identify the higher-mass states to amass of at least N 2.5 GeV, and determine most branching fractions and decayamplitudes of a subset of these states in sufficient detail to be useful to theorists.Certain qq multiplets are especially interesting because their Jpc quantumnumbers and masses are similar to expectations to glueballs and hybrids; the q4levels either form a background and must be identified and eliminated as potentialexotica, or they may mix strongly with the glueball or hybrid states so that therelatively pure qq level does not exist in nature. Since we cannot say a priori whichpossibility is correct, it is especially important to clarify the experimental spectrumin these mass regions. Multiplets of special interest for this reason are:mode. These VV modes are quite interesting in that there are several subamplitudes,and the predicted amplitude ratios are nontrivial. As an example, the numerical 3Podecay amplitudes for f2 (1700) -+ ww are given in Table 10. If the S-wave and D-waveww amplitudes from this state could be separated and compared, we would have avery sensitive test of the decay model, and if there is agreement we could apply thesame model with more confidence to the decays of other candidate high-mass qqstates.As a final observation, the report by Braccini (L3 Collaboration) 28) of a2P candidate a2( 1750) in yy collisions is especially interesting because this is thefirst radial excitation to be reported in yy. Theoretically, radially excited qij statesshould appear with little suppression in yy collisions, 2g) but to date only this radialcandidate has been reported. There may actually be a problem here, as the massreported for an excited a2 state by Crystal Barrel, 30) M 1670(20)  MeV,  does notappear consistent with the L3 mass.l 3P and 4PThese multiplets, which experimentally lie at about 2.08 GeV and 2.34 GeV(for nfii; see Table 4), will be of interest because of the presence of the lightest tensorglueball. Of course there will be hybrids in this region as well, so we can expect acomplicated spectrum of overlapping resonances. At present there is no theoreticalguidance regarding decay modes of these qQ states; such a study would be difficultto motivate without evidence (for example from the 2P multiplet) that the decaymodels will give useful results for these high radial excitations.l 2s and 3sThe 2s and 3s multiplets are also interesting due to their proximity to thereported hybrid candidates ni(1405)  and 7ri(1600);  both the bag model and flux-tube model predict that the lightest l-+ hybrid has O-+ and l-- partners nearbyin mass. These should be observable as an overpopulation of states in the L,, = 0qq sectors.The 2s multiplet has historically been problematic because there are broadoverlapping states in the l-- sector; at least two states near 1.45 and 1.7 GeV areneeded to explain the data (notably e+e- -+ 7~ and w;rr). The observation of the~(1450)  in r decays at CLEO was reported here by Kravchenko “), who also notedthe absence of a K*(1410)  signal in Kr. The topic of vector meson spectroscopyin this mass region was recently reviewed by Donnachie and Kalashnikova, 31) whoconcluded that an additional vector was required in both I=0 and I=1 channels to fit4. A.Abele et al. (Crystal Barrel), Phys. Lett. B423, 175 (1998); C.Amsler, Rev.Mod. Phys. 70, 1293 (1998).5. U.Thoma, these proceedings.6. N.Isgur, R.Kokoski and J.Paton, Phys.  Rev. Lett. 54, 869 (1985); T.Barnes,F.E.Close and E.S.Swanson,  Phys. Rev. D52, 5242 (1995).7. C.Michael, Proceedings of HADRON97; P.Lacock et al. (UKQCD Collabora-tion), Phys. Lett. B401, 308 (1997); C.Bernard  et al. (MILC Collaboration),Phys. Rev. D56, 7039 (1997); CMorningstar,  Proceedings of HADRON97.8. For a recent review of LGT glueballs see M.Teper, hep-th/98121879. C.Amsler and F.E.Close,  Phys. Rev. D53, 295 (1996).10. J.Sexton, A.Vaccarino and D.Weingarten, Phys. Rev. Lett. 75, 4563 (1995).11. J.Weinstein and N.Isgur, Phys. Rev. D41, 2236 (1990).12. K.Dooley, E.S.Swanson  a n d  T.Barnes,  P h y s .  L e t t .  275B, 4 7 8  ( 1 9 9 2 ) ;N.Tornqvist,  Phys. Rev. Lett. 67, 556 (1991); T.E.O.Ericson  and G.Karl,  Phys.Lett. B309, 426 (1993).13. D.V.Bugg, these proceedings and private communication.14. A.V.Anisovich et al., “Amplitude Analysis of pip + ~+7r-,  7r07ro, ~7 and ~7’ fromThreshold to 2.5 GeV”.15. R.Baker (CLEO), contribution to this session.16. R.Kokoski and N.Isgur, Phys. Rev. D35, 907 (1987)17. T.Barnes,  F.E.Close,  P.R.Page and E.S.Swanson, Phys. Rev. D55, 4157 (1997).18. D.V.Amelin et al., Phys. Lett. B356, 595 (1995).19. I. Kravchenko (CLEO), these proceedings.20. R.McNulty (Delphi), these proceedings.21. T.Bergfeld et al. (CLEO Collaboration), Phys. Rev. Lett. 79, 2406 (1997).22. Particle Data Group, Eur. Phys. J. C3, 1 (1998).

Radial Excitations

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