PROC. 19th INT. CONF. HIGH ENERGY PHYSICS TOKYO, 1978
P5a Particle Spectroscopy
G. FLÛGGE
DESY
old hadrons, namely exotics. The main part §1. Introduction of my report will then be devoted to the new During the last years particle spectroscopy quarks charm and beauty. Being inspired by has evolved into a spectroscopy of leptons and Sosnowsky's talk,6 I will also try to offer you quarks. This era was initiated in 1974 by the a jet tour, starting with 2 jets in e+e~ reac discovery of the Jjcp mesons,1 quickly followed tions and leading eventually to a glimpse of by the new lepton2 r and finally the Ypsilon three gluon jets at the Ypsilon. meson.3 One is therefore tempted to outline this talk following the common prejudice that §11. Exotics high energy physics can be described by lep The possible existence of exotic particles has tons, quarks and their mutual interactions mainly been discussed in the context of two (Fig. 1). hypothetical quark compounds, dibaryons7 and baryonium.8 Dibaryons are constructed from the old baryons by doubling the quark content of the particle from 3 to 6 quarks. Similarly baryonium evolves from the concept of mesons qq by doubling the quark content giving qqqq states.
Fig. 1. Common belief on quarks, leptons and II. 1 Baryonium—broad states their mutual interactions. Experimentally baryonium is readily defined as mesonic states with strong coupling to an Let me first say a few words about the antibaryon-baryon (BB) system. First ob subjects I am not going to cover. I will not servations of this kind of phenomena were made talk about leptons. You just heard a beauti in the famous S, T and U states, which reveal ful review of the new lepton r by Gary themselves as a resonance in the elastic, total Feldman in the previous talk.4 I will be brief and annihilation cross sections of nucléon- also on the old quarks u, d and s since the antinucleon {NN) systems with a large elasti old hadron spectroscopy will be covered in city.8'9 the next talk by Cashmore.5 In my talk I Table I summarizes the situation encounter will just concentrate on one specific aspect of ed in 1977. The'S, J7 and U states were seen
Table I. Broad baryonium candidates. Masses and widths are given in MeV, the latter in brackets. Quantum numbers are indicated as far as they are known. 794 G. FLUGGE in many experiments on NN cross sections. usual framework of meson spectroscopy. In particular an analysis of the reaction pp-+ They could indeed be viewed as good candidates T:+7Z~ by Carter et ai gave clear evidence for for baryonium states. the existence of JPC=3~- (1=1), Jpc=4+ + The existence of such qqqq compounds was (7=0), and JPC = 5~- (7-1) states.9 In 1978 first predicted by Rosner from duality argu Carter et al. extended their analysis to the ments.16 For such meson states a strong reaction pp ->K+K~ and established the pre coupling to BB and the apparent reluctance to sence of 7=0 components with JPC=3~~ and decay into usual mesons can be explained 5"" as well.10 Dulude et ai analysed the reac by an OZI rule analogue17 (Fig. 2). tion pp-*7c0n0 and found a state with JPC = 2++ (1=0) at 2.1 GeV.11 Further data be came available from a measurement of iz~p-+ ppn by the Bari-Bonn-CERN-Daresbury- Glasgow-Liverpool-Milano- Vienna-Collabo ration at the Omega spectrometer at CERN.12 They found evidence for at least three broad resonances at 1950, 2100 and 2300 MeV with Fig. 2. Baryonium decay. Jp = l~, 3~ and 4+, respectively and may be an additional 2+ state at 2000 MeV.12 The elaboration of these ideas does explain In summary there is good and increasing both narrow and broad states in the BB system 18 evidence for the existence of broad NN states at least qualitatively. The three best candi and in the S, T and U range. However, the dates for narrow BB states are compiled in situation seems to be rather complex since 7=0 Table II together with the S state. and 1 Regge recurrences with J=\ 4 and 5, The first one, a narrow state at 2.95 GeV and maybe also J=l and 2 are encountered. with a width of less than 15 MeV, was first The best established state is certainly the S seen in 1977 at the CERN-Omega spectrom resonance13 (new evidence became available eter by the Bari-Bonn-CERN-Daresbury- from the Tokyo-Massachusetts Collaboration Glasgo w - Liverpool - Milano - Purdue - Vienna at this Conference14). However, there is no Collaboration in the reaction iz~p-*pp TT~ + Jp determination of this state so far, and it something. It showed up as a spike in the 19 may even have two Jp components.15 ppn" mass distribution. Since then this experiment has been repeated with 10 times 77.2 Baryonium—narrow states more statistics. As we heard on this Con We have good evidence for the existence of ference there are no definite new results yet. broad states coupled to the BB system. Of An analysis of part of the data did not con course, it is by no means clear that this has firm the effect.20 Consequently, the existence something to do with exotics. A possible of this resonance seems to be questionable. description of these states would for instance The other two candidates were seen in the be to view them as BB bound states. The reaction %~p-*%~ppp. Imposing the con narrow width of the S state could be explained dition that the forward proton and the TT~ due to its vicinity to the BB threshold. How form an N* or a J, the remaining pp system ever, in 1977 narrow high mass states coupled exhibits two spikes at 2.02 and 2.2 GeV (Fig. to BB were discovered. There seemed to be 3). They can be viewed as resonances in the no possible explanation for these states in the off shell pp scattering of the baryon exchange
Table II. Narrow baryonium candidates. Particle Spectroscopy 795
candidates and refer for all details to the parallel session. The first candidate is a pp resonance at 2.26 GeV first seen in the Argonne total cross section experiment with polarized targets and 26 3 beams. The resonance known as the F3 has a width of 200 MeV and the quantum numbers Jp= 3~(I=l).* The possible ex istence of further pp states was discussed on this Conference.27 The second candidate comes from the Tokyo- KEK measurements on the photodisintegration of deuterons.28 The analysis of these data Fig. 3. CERN-College de France-Ecole Polytechni reveals the possible existence of a z/J-resonance que-Orsay Collaboration: Backward {pp) mass at the mass of 2.38 GeV with a width of 200 distribution in the reaction iz~p->%~ppp exhibiting MeV, and /p-3+ (7=0). Jp = \+ cannot be two narrow peaks at 2.02 and 2.20 GeV, 9 and ruled out. 12 GeV data with selection on â and cos 6*(p,p)< The third candidate, a strange dibaryon 0 are shown. state, has been seen in many experiments.29'30 reaction. The experiment was repeated by the A recent analysis was carried out by the 30 Toronto-York-Purdue Collaboration with CERN-Heidelberg-Miinchen Collaboration. positive pions.22 They find some indication In the reaction K~&~^Apn~ they find a narrow for a 2.2 GeV state with a statistical significance bound state in the Ap system with a mass of of 2 standard deviations. The experiment 2.129 GeV, a width of less than 10 MeV, and does not confirm the 2.95 state. The Pitts S=-l. burgh-Massachusetts Collaboration has looked IIA Exotic quantum numbers into the reaction /?/?-*7r+7r~K+K~.23 They see Although there are several candidates for an indication for the existence of the 2.2 GeV + baryonium and dibaryon states, the only con state in the 7r~K K~ system wtith a statistical vincing argument in favour of exotic states significance of 4 to 5 standard deviations. If would be the discovery of states with exotic this were confirmed it would mean that the quantum numbers. 2.2 state is an isovector state. As we have Two searches for such states have been heard in P. Soding's talk there is also evidence reported at this Conference. The first one by for the 2.02 state being seen in the virtual the Indiana-Purdue-SLAC-Vanderbild Col photon production reaction TvP-^pPP at 31 24 laboration, does not show any evidence. The Cornell. The statistical significance of this second one, however, from the CERN-Omega effect is 3 standard deviations. To sum spectrometer by the Glasgow-DESY Collabo marize : There seems to be evidence confirming ration, does indeed show an effect in the reac the existence of two narrow states at 2.02 and tion K+p->Jp7i:+n.32 Applying a fit to this 2.20 GeV from several different experiments. reaction and constraining the Jpx+ system to II. 3 Dibaryons Let us see whether even higher combinations do exist, for example a 6 quark combination like the dibaryon states mentioned above. We all know at least one candidate for dibaryons, the deuteron. We know also that this is a nuclear force bound state and not Fig. 4. Glasgow-DESY Collaboration: (Apn+) mass the type of exotics we are looking for. Real distribution in the reaction K+p-^Apx+n with exotic dibaryon states were for instance pre an indication of a peak at 2.46 GeV. dicted in the MIT bag model by Jaffe in * Details were also given in V. A. Tsarev's talk at 25 1977. I will only summarize the three best this Conference. 796 G. FLÛGGE
+ either ÂJ++ or 2LP they see a spike in the e e~+hadrons with the two electrons tagged mass distribution of the Àpic+ system (Fig. 4). in the forward direction. The reaction was It has a statistical significance of 3 to 5 standard monitored by the two photon QED reaction deviations and certainly needs experimental e+e"-»e+e"e+e~ which was found to be in confirmation. good agreement with predictions. From the fact that no final states of the type e+e_ II.5 Summary +hadrons were found they could infer an upper To conclude this part there is no firm evi limit r(y^77)<11.5KeV (95% confindence dence for exotic quantum numbers so far. level). Previous results had been obtained by There have been many sightings of baryonium, ADONE (T<33 KeV) and Imperial College 35 narrow and wide, and dibaryon candidates. (rtot<.8MeV) groups. Qualitative arguments favour the exotic nature The other test quantity is the width F(J/ of the baryonium states. However, the high Again the coupling depends on the mass narrow states still need experimental quark charge and the predictions are 2.6 eV confirmation. Convincing evidence for the for fractional charge quarks and 13 eV for inte exotic nature of thesse states is certainly yet ger charge quarks.36 DASP has measured an missing. Consequently both experiments and upper limit of this Jj(p decay width giving theory have to be improved. r<5.\ eV (95% confidence level).37 Thus an integer charge of the quarks is ruled out by §111. New Quark Spectroscopy both experiments. The rest of my talk will be devoted to the discussion of the two new flavours of quark, III.B Charm charm and beauty. Since new results on the III. B. 1 Charmonium D meson were already discussed in the pre The cc system exhibits a series of bound vious talk by Feldman4 I will concentrate on states known as charmonium. The situa charmonium and the F meson in the context tion we faced one year ago is summarized in 38 of charm. Concerning beauty I will show Fig. 5. We have a rather firm knowledge the experimental evidence for Ypsilon and Ypsilon Prime in e+e" reactions; I shall also talk about the event topology in the Ypsilon region discussing evidence for a 2 jet structure outside the resonance and the search for 3 gluon jets.
Ill A Quark charge Before I go into a detailed discussion of the two new quarks let me briefly ask whether there is any experimental evidence supporting our common belief that quarks are fractionally charged. Two quantities might be used as a Fig. 5. The experimental knowledge on the char test for the quark charge. monium states (1977). The first one is the radiative width r(r]r-^jj). Since the rj' is dominated by the SU(3) singlet of the existence and even the spin assignment amplitude there is a strong dependence of this of the 3P states. The situation is much worse quantity on the charge of the quarks. For on the 1S states. Although the X(2820) was fractional charge quarks (Gell-Mann quarks) firmly established by the DASP collaboration37 a width of 7^=6.0 keV is calculated whereas and the existence of this state was confirmed for integer charge quarks (Han-Nambu quarks) in the reaction np-^yjn by the IHEP-CERN- the width is r=25.6 keV.33 Experimental Karlsruhe-Pisa-Vienna Collaboration,39 noth results on this quantity have become available ing—except its even C parity—is known about now from the Bonanza group at DES Y.34 its quantum numbers. In particular its iden + They look for the two photon process e e"-> tity as r]c is certainly still questionable. The Particle Spectroscopy 797 situation is even worse on the other state, the #(3455) which was only seen in the 77 cascade decay of Fig. 7. DESY-Heidelberg Collaboration: High vs low mass solution of the (J J/ creased mass resolution but of course less statistics the group got the result displayed in Fig. 7a which indicates a clearly separated excess of events at 3.6 GeV. From these 5 events above no background the group con Fig. 6. DESY-Heidelberg Collaboration : High mass cludes the possible existence of a state at solution of the (Ï J/ Figure 6 shows the results of this group on The scatter plot (Fig. 7) of the low mass the reaction with the directions against the high mass solution of their data shows however, that the high mass solution measured for both photons and the muons. is not unique. A low mass state at 3.18 GeV Constraining the two charged particles to could be equally possible. Table III sum the J\ Table III. Branching ratios of PJx states. BR( III.B.2 Charm particles The experimental problem is of course the You all know the exciting story of the dis high combinatorial background of photons covery of the D meson at SLAC38'45 and you from 2 to 3 TT0,S produced per event at these just heard a review of the situation by Feldman energies. The DASP group could, however, in the previous talk. Let me therefore only overcome this problem with a relatively good add some information on the particle which detection efficiency for photons (95% above was still missing in the multiplet of pseudoscalar an energy of 140 MeV) and an angular and mesons of SU(4) shown in Fig. 8. Evidence energy resolutions which combine to give a for the existence of this pseudoscalar meson mass resolution of about 80 MeV at the rj. F and its vector counterpart F* came from They select events with two charged particles the DASP detector at DESY.46 I am going and least 2 photons with an energy of more to describe their new data in some detail.47 than 140 MeV, an opening angle of more than 11.5° and a momentum vector sum of more than 300 MeV. With these cuts the TT° efficien cy is relatively low. Figure 9 shows the result of these measure ments for 6 different energy intervals. The full curves indicate fits to the iz° and 7] mass peaks on a background obtained by combin ing fs from different events. The dashed curves indicate the background below the rj signal. Note that there is no rj signal at 4.03 GeV whereas there is a clear signal at 4.16 GeV, a Fig. 8. The multiplet of pseudoscalar mesons in very strong signal at 4.42 GeV and maybe an SU(4). indication of rj production in the other energy regions. Figure 9(b) summarizes the data in IILB.3 F Meson terms of the inclusive cross section for rj pro If we assume that the mass of the F meson duction over the whole energy range from 4 is smaller than the sum of the masses of the to 5 GeV. For comparison the trend of the D and the K meson, the particle can only decay total cross section is indicated below the figure. weakly into an ss system in the final state. The figure shows the presence of rj production Consequently we expect KK, Fig. 9. DASP Collaboration: Inclusive rj production in e+e~" annihilation in the 4 to 5 GeV energy range. (a) Two photon mass distribution in different energy intervals. The full curve is the sum of the com binatorial background (photons taken from different events) and a fit to the n° and The dashed curve represents the background without rj production. (b) Inclusive cross section for rj production as a function of energy. The trend of the total cross section is given for comparison. Let me draw your attention again to the The next point to be checked is whether the fact that no production is present at 4.03 rj's really originate from a weakly decaying MeV. This is a crucial point in the whole particle. To check this all events were scanned argument since the spike at 4.03 GeV is known for the presence of electrons. Figure 10 for abundant D production.45 Consequently shows e.g., the result for the 4.4 GeV region, the lack of an rj signal at this point indicates where the rj signal was strongest. Electron that the yj production can not be explained by events are plotted against the y y mass. The any known source including D production and background due to misidentified electrons is decay. indicated by a full line. The figure shows that a strong signal above background is present in the region of the n0 and the rj mass. This indicates in particular that rj production is correlated with the emission of electrons indicating the presence of a weak decay. If Fig. 10. DASP Collaboration: Two photon mass distribution of events including electrons in the Fig. 11. DASP Collaboration: Two photon mass 4.42 GeV energy region. The full curve indicates distribution of events including an additional low the background from misidentified electrons. energy photon in three different energy intervals. 800 G. FLÙGGE we assume now that the rj production at 4.16 GeV rj production is strongly correlated with and 4.42 GeV is due to F production one might low energy photons. One is therefore urged suspect that at 4.42 GeV at least F* produc to look for direct evidence for FF* or F*F* tion is also involved. For the further argu production at 4.42 GeV. One assumes again ment let us therefore consider possible signa that the F* is cascading down to the F by tures for an F*. Assume F and F* are emitting a soft photon and that one of the F both ci, ~sc states, the F* being a spin exitation. particles decays into rj and TT. Therefore the In this case both F and F* are (1=0) states DASP group looked into the reaction and the F* can only decay into the F emitting e+e~-^+7r+a soft photon+X. an (1=0) system. We further assume that 43 events of this type were found. The the mass difference in the FF* system is about events were fitted to the hypothesis of F*F* equal to the mass difference in the DD* system or FF* production. Figure 12 shows the namely less than two times the pion mass. result for the case of FF* production. The The only possible decay mode for the F* will mass of the rjjt system is plotted against the then be the decay recoil mass. A clustering of 6 events can be seen at an rjiz mass value of 2.03±.06 GeV. These considerations led the DASP group The background is of the order of less than to look for the associated production of rj 0.5 events. with a soft photon possibly originating from the decay of F* into Fy. Figure 11 shows the result of this search displaying again the mass of the ?7 system at 3 different energy intervals. It shows again a strong rj signal at 4.42 GeV. No such signal is present above and below this energy range. This proves that at 4.42 Fig. 13. Schematic summary of the experimental situation on charm. Since the rjiz system cannot unabiguously be associated to the F or the F* and since also no clear distinction can be made between the FF* and the F*F* hypothesis the recoil mass is not very suitable to determine the mass of Fig. 12. DASP Collaboration: Events from the the F*. One can however infer the mass dif reaction e+e"-^ rs0ft+X at 4.42 GeV and ex cluding 4.42 GêV. A fit assuming e+e"->FF*; ference between F* and F from the energy F*-»rF, one F-^yn is applied. The figure shows distribution of soft photons. The result is biplots of the fitted mass distribution against the MF*-MF= 110±46 MeV. recoil mass and rjjc distributions for events with 2 Taking into account all efficiencies the DASP X <8 and a TZYJ mass difference |Mfit— Mmeas| <250 MeV. group determined a relative branching ratio Particle Spectroscopy 801 X(2.82), the existence of the states %(3.45) and The SLAG-LB L group also looked for possible ^(3.59) is not established. The quatnum num F production in the reaction bers of all three states are unknown, except for their even C-parity. The upper part of Fig. 13 indicates, how the production of D, D*, F and F* mesons Constraining these data to the hypothesis comes in with increasing energy: DD at the that they originated from the process e+e~-> FF with a subsequent decay of one of the F to one of the above (KKJ$>;T) systems they got a The evidence for FF production at the Columbia-Fermilab-Stony Brook Collabora corrected. The results of two other experi tion, the Y was found at DORIS by the ments, the DASP2 group61 and the DESY- PLUTO58 and DASP259 Collaborations simul Heidelberg 2 detector, which replaced the taneously. The original data of this search are shown in Fig. 14 which displays the visible cross section in both detectors as a function of energy. A clear signal at 9.46 GeV is seen in both experiments. From these original data both groups agreed on a mass value of Mr=9.46±-01 GeV, an electronic wdith of ree=1.3±.4 kcV and a total width of the resonance 7\ot <18 MeV. Note that the error in the mass is due to the DORIS calibration uncertainty and the width corresponds to the DORIS energy spread. These values already strongly favoured an interpretation of the Y being a bound state of Fig. 15. PLUTO Collaboration: Absolutely nor malized hadronic cross section in the T region. a new quark-antiquark pair with a charge of 1/3.58 III. C. 1 Ypsilon parameters The immediate issue of e+e" physics of the Y is of course a determination of the leptonic and the total width of the resoancnce. The leptonic width Fee can be inferred directly by integrating the hadronic cross section of the resonance according to the formula The integral extends to infinitely high energies which in practice means that radiative cor rections have to be applied properly. The absolutely normalized results of the PLUTO group are shown in Fig. 15. Outside the reso nance the cross section ratio is R=atot/aflfl= 5.2±1.0 in good agreement with the value of 4.7±1.0 measured at 5 GeV. Note that both Fig. 16. DASP 2 and DESY-Hamburg-Heidelberg- values include contributions from the heavy Munchen Collaborations: Visible cross section foi lepton r. The 9.4 GeV value is not radiatively e+e~->hadrons in the T region. Table IV. Results on Y (9.46). Particle Spectroscopy 803 PLUTO detector after its removal to PETRA, to be governed by the production of quark jets are shown in Fig. 16. (The latter detector with a characterstic angular distribution of was operated by a DESY-Hamburg-Heidel- l+cos2# due to the 1/2 spin of the quarks. berg-Munchen Collaboration.) Their values The resonance itself is expected to decay into are not absolutely normalized. For the deter gluons which then fragment into 3 jets in a 64 mination of the leptonic width ree both disc-like configuration. detectors used the PLUTO value of R. The To test these theoretical conjectures we have results of the three experiments are summarized analyzed our events in terms of sphericity. in Table IV. This quantity which was introduced by An attempt was made by the three groups Brodsky and Bjorken65 and later used success to determine the total width of the resonance. fully in the analysis of the S LAC-LB L data66 The procedure is to determine the JU pair is defined by branching ratio B^^ on the resonance. Assum ing fxt universality, the total width can then be obtained as r^^r^jB^. In all three experiments the determination of B^ suffers p±=momentum perpendicular to the à-axis. from very low statistics. For example the The limiting values of S are 0 in the limit of PLUTO group found 60 ju pairs off resonance two infinitely narrow jets and 1 in the limit of and 74 ju pairs on resonance.63 The angular an isotopic event. 64 distribution of these events is shown in Fig. 17. Also another quantity, thrust, which was Q7 The data are in good agreement with the first introduced by Brandt et al. will be used. expectation of l+cos2#. The values of B^ This quantity is defined as obtained from the three experiments are summarized again in Table IV. Pn=momentum parallel to the T-axis. T varies between the values of 1 for two line jets and 1/2 for isotropic events. Since it turns out that the features of the data in terms of thrust and sphericity are very similar681 will not discuss all aspects of both quantities. I will mostly concentrate on the sphericity, although sometimes the thrust axis will be used for convenience, because its definition is technically very simple. A word of caution Fig. 17. PLUTO Collaboration: Angular distribu should be said in this context: Although the tion of muon pairs produced in the Y region. mean angle between the jet axis defined by Data on and off resonance are combined. either S or T is zero, the distribution has a width of about 15°. This reflects the uncer Due to the large error on only lower tainty! nherent in defining the real jet axis.68 limits can be given on the total width of the III.C.3 Quark jets resonance. Even if all values are combined The existence of jets in e+e~ annihilation the error is still too large to obtain a two was first demonstrated by the SLAC-LBL standard-deviation upper limit of the total group.66 Their results are shown again in width. Again one can only obtain a lower Fig. 18. Their data are in good agreement limit of 25 keV at a 95 % confidence level. with the prediction of a jet model (full curve) If we take however 2*^=2.6% at face value whereas the phase space Monte-Carlo (dashed we find the 'best' value of curved) is completely ruled out at large ener gies. The PLUTO Collaboration has made a IILC.2 Event topology very similar analysis.68 The result is presented According to common prejudice the topology in Fig. 19. It shows the mean observed of events should change drastically in the re sphericity as a function of energy over the sonance region. The continuum is expected energy range from 3 to 10 GeV. The figure 804 G. FLÛGGE Fig. 18. SLAC-LBL Collaboration: First evidence for jets in e+e~ annihilation. Observed mean sphericity as a function of energy. The full and dashed lines show Monte-Carlo simulations of a jet and phase space model, respectively. Fig. 20. PLUTO Collaboration: Angular distribu tion of the jet axis as defined by thrust for two energies. Fig. 19. PLUTO Collaboration: Observed mean spericity of charged particles (>4 prongs) as a func tion of energy. The shaded region represents the phase-space prediction, the crossed one for two jets.69 shows again a dramatic fall over this energy range in good agreement with a two jet Monte-Carlo69 and in complete disagreement with phase space predictions. Note the small but significant change in sphericity at the charm threshold around 4 GeV. Fig. 21. PLUTO Collaboration: Angular distribu The angular distribution of the jet axis is tion 1/E dE/dÀ of neutral (data points) and charged (histogram) energy with respect to the thrust axis shown in Fig. 20. Data are in good agreement for three different thrust intervals. 1 is the angle with the theoretical expectation for spin 1/2 between the momentum vector and the thrust axis. Particle Spectroscopy 805 quark jets. A fit to the data with l+acos2# gives the values of a=0.76±0.3 at 7.7 GeV and 1.63±0.6 at 9.4 GeV. Two other interest ing properties of these jets can be read from Fig. 21. It shows the energy distribution of both charged and neutral energy with respect to the thrust axis for three different thrust intervals. The first observation is that the neutral energy flow follows almost exactly the energy flow of the charged particles. The Fig. 23. Off and on resonance contributions to the relative partition of neutral to charged energy annihilation cross section. can be determined from this figure to be about 0.8. Furthermore the half opening angle of the jets turns out to be of the order of 30°. A similar result is obtained, if one com pares the mean momenta perpendicular and parallel to the jet axis. Many observations on jets are best demons trated by looking at a typical event shown in Fig. 22. To summarize, there is clear (con firming) evidence for two jets in e+e~ annihila tion, the sphericity decreasing with increasing energy. The angular distribution of these jets is compatible with the quark spin being 1/2. Neutral and charged energy in these jets are strongly correlated and subtend a half opening angle of about 30°. Fig. 24. PLUTO Collaboration: Observed mean sphericity of charged particles (>4 prongs) includ ing the Y region. Values without (Y) and with (Y direct) subtraction of nondirect terms. now the T region. We notice a strong rise of the sphericity as we go across the resonance (inset of the figure). This increase gets even more pronounced if we extract the direct decay terms as indicated above. This value comes in fact very near to the Fig. 22. PLUTO Collaboration: A typical two jet event at 9.35 GeV CM. energy. value predicted by Hagiwara assuming a three gluon jet decay of the Ypsilon ('QCD' pre IILCA Change of topology at the Ypsilon diction).70 Note however that in terms of Whereas off resonace only quark pair pro sphericity there is only very little difference duction is at work, the on-resonance cross between the phase space and the QCD predic section is composed of three different processes, tion. as shown in Fig. 23. Since we are interested The features of these data change very little in the direct decay mechanism, the off resonance if we take thrust instead of sphericity. Figure and the vacuum polarization terms have to 25 shows a distribution of (1—the mean ob be subtracted in all distributions. The latter, served thrust) over the same energy range. which is proportional to RB^ represents Again there is a dramatic change of topology about 13% of the resonance cross section. in the T region and the direct term gets very Figure 24 shows again the mean observed close to the QCD prediction by Koller, Walsh sphericity over the full energy range including and Krasemann71 of not corrected for acceptance and one has to be cautious in comparing them directly with the prediction. Acceptance corrections are however not expected to be very large. The previous two figures showed a strong change of events topology in the charged energy flow. Figure 26 shows a comple mentary observation of the DESY-Heidelberg 2 group who have measured the distribution of the neutral sphericity and compared the differential sphericity off and on resonance.62 A striking difference is seen in the two dis tributions, the mean value changing from Fig, 26. DESY-Hamburg-Heidelberg-Munchen Collaboration: Observed sphericity distribution Fig. 27. DESY-Hamburg-Heidelberg-Munchen for events on and off resonance in the Y region. Collaboration : Obwerved charged multiplicity The sphericity is defined from the measured shower (including beam pipe conversion) on and off reson energies. ance in the Y region. Particle Spectroscopy 807 a considerable change of topology from a 2 jet structure outside the resonance to a more isotropic structure at the T9 a small increase of the charged multiplicity by about 1 unit as one goes from off to on resonance and no large change of the K$ content per event. A quantitative analysis of the change of topology in terms of thrust and sphericity shows that the change in the T region is about as expected from QCD (change from a 2 quark jet to a 3 gluon jet structure). However, the pro Fig. 28. PLUTO Collaboration: Visible cross sec ximity of phase space does not yet allow a tion for inclusive Ks° production in the Y region. firm conclusion on the existence of gluon jets. The trend of the total cross section is indicated by a III.C.7 Ypsilon prime dashed line. During the last weeks before the Conference the DASP 281'73 and the DESY-Heidelberg The last piece of information I want to 262,74 p proceeded into the region of mention is from the PLUTO group who gr0U S 10 GeV to search for the first excitation in the measured the inclusive K° production in the T family (V) suggested by the data of the 9.5 GeV energy region.63 Their result for the Columbia-Fermilab-Stony Brook Collabora- visible cross section is displayed in Fig. 28 as a function of energy. For comparison the total cross section is indicated by a dashed curve in the same figure with arbitrary normali zation. The comparison shows that the Kg production follows about the trend of the total cross sec tion. Quantitatively the comparison of on and off resonance cross sections yields a ratio of 4.0±1.7 for K° production, whereas it is about 2.5 for the total cross section. They conclude therefore that there is no significant change of Ks's produced per event if one goes through the resonance region. IIIC.6 Ypsilon summary In summary we have seen that the T is pro duced in e+e~ annihilation with a mass of 9.46±0.01 GeV, a leptonic width of ree= 1.2±.2keV, a branching ratio ^=2.6±1.4% and a total width of more than 25 keV (best value 50keV). These parameters strongly suggest that the Ypsilon is a quark-antiquark Fig. 29. DASP 2 and DESY-Hamburg-Heidelberg- bound state with a quark charge of —1/3. Fur Munchen Collaborations: Evidence for the Y' in ther observations in the resonance region are: e+e~ annihilation. Table V. Results on Yr (10.02) 808 G. FLÛGGE Fig. 30. Columbia-Fermilab-Stony Brook and DESY-Hamburg-Heidelberg-Mùnchen Collabo rations: The T family in hadronic and e+e~ reactions. tion. Figure 29 shows their result. There is ample evidence for the existence of a new heavy a resonance structure around 10.02 GeV with quark which is most probably of the 'beauty' a width compatible with the resolution of the type. To answer the question whether a 6th e+e" machine DORIS. In Table V the quark t would constitute perfect symmetry parameters of the Y' as found by the two between leptons and quarks again our answer groups are compiled together with the mean can now only be : values. The first surprising feature of these PETRA works and CESR and PEP will data is the relatively low mass difference follow soon! between Y and Y'. Figure 30 compares the FNAL and DESY data. The value is lower Acknowledgement than the one suggested by the Columbia- I am indebted to L. Montanet, V. Hepp, Fermilab-Stony Book Collaboration and in B. French, J. Six, B. Pietrzyk, D. Treille, Chan particular JM(r)=558±10 MeV is smaller Houg-Mo, H. Hogaasen, and V. Hungerbiihler than âM{ 2. M. L. Perl et al: Phys. Rev. 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K. Roller, H. Krasemann and T. F. Walsh: (1970) 1416. DESY 78/37 (1978) ; K. Roller : this Conference. 66. G. Hanson et al: Phys. Rev. Letters 35 (1975) 72. S. J. Brodsky, D. G. Coyne, T. A. de Grand and 1609; G. Hanson: XIII Rencontre de Moriond, R. R. Horgan: Phys. Letters 73B (1978) 203. Les Arcs (1978) and SLAC-PUB 2118 (1978). 73. C. W. Darden et al: DESY 78/44 (1978), sub 67. S. Brandt, Ch. Peyrou, R. Sosnowski and A. mitted to Phys. Letters. Wroblewski : Phys. Letters 12 (1964) 57 ; E. Fahri : 74. J.K. Bienîein et al: DESY 78/45 (1978; Phys. Rev. Letters 39 (1977) 1587. submitted to Phys. Letters. 68. PLUTO Collaboration, Ch. Berger et al : DESY 75. J. L. Rosner, C. Quigg and H. B. Thacker : Phys. 78/39 (1978) to be published in Phys. Letters; Letters 74B (1978) 350; J. L. Rosner: this Con G. Alexander: this Conference. ference. =0.19 to =0.37 with an error of 0.02 which is again very close to the QCD predic Fig. 25. PLUTO Collaboration: (1-observed mean 70 thrust) of charged particles (>4 prongs) including tion of =0.4. the Y region. Values without (Y) and with (Y IILC.5 Other properties of events in the direct) subtraction of nondirect terms. Ypsilon region A surprising observation72 which all three the value is again very close to the phase space groups agree on is the relatively small change prediction. in mean multiplicity as one passes from the The fact that the QCD and phase space continuum to the resonance. Figure 27 shows predictions are so similar may be suprising at the distribution of observed charged multiplici first sight, since one expects isotropic events in ty on and off resonance for the DESY- phase space and disc-like events in QCD. Heidelberg 2 detector. The mean charged mul However, at the low multiplicities encountered tiplicity changes from 6.4 off resonance to here phase space is not at all isotropic and the 7.3 on resonance (error 0.2) including the definition of sphericity and thrust always tends correction for non direct terms.62 A very to find a planar structure in the events. On similar increase of about one unit is also found the other hand we are dealing with 3 GeV by the PLUTO63 and the DASP 2 Collabo gluon jets in QCD which may be very broad rations.61 jets and hence the disc structure is smeared out. These features have been discussed in detail by G. Alexander in the parallel session.68 Note also that the sphericity and thrust values are