THEORETICAL INTERPRETATION OF e+e- RESULTS Haim Harari Weizmann Institute of Science, Rehovot, Israel and Stanford Linear Accelerator Center, Stanford University Stanford, California, U.S.A. Summary First Generation Fermions: Are quarks and leptons Recent experimental results and theoretical ideas pointlike? Do quarks come in three colors? Do they related to e+e- scattering are reviewed. have the "usual" charges of i and - j-? Second Generation Fermions: Is the right-handed muon 1. INTRODUCTION: A Long List of Answerable Questions in an SU(2) x U(l) singlet? Is the cs current a V-A The study of electron-positron collisions has current? What is the cc potential? Where are the gradually become the most productive experimental pseudoscalar charmonium states? Do semileptonic D-decays method of uncovering new particles and new phenomena. proceed mainly to strange particles, as predicted by Beginning with the "November revolution" of 1974 and GIM? What are the features of nonleptonic D-decays? continuing at an incredibly rapid pace since that What are the properties of F+, A+ , L etc? time, the 1JJ family, the T-lepton, the D+ and DO, c c Third Generation Fermions: Does the top quark exist, various D*'s and possibly F+ were discovered in e+e­ and if so, what is its mass? Do B-mesons and T-mesons collisions, while the T-states, the charmed baryons as exist? Are the charged currents involving t and b, well as quark jets and now gluon jets have been studied. left-handed? What are the values of the generalized Our present theoretical picture of the fundamental Cabibbo angles 8 ,8 and the Kobayashi-Maskawa phase particles and their interactions is very compact. We 2 3 o? Can these parameters account for all weak transitions describe everything in terms of twelve gauge bosons among the three generations of quarks? What are the (w+,w-,ZO,y and eight gluons) and twelve quarks and properties of T? Is ~'t massless? leptons, neatly arranged in three "generations": Beyond: Are there additional generations, following (u,d,v ,e); (c,s,v ,lJ); (t,b,v ,'t). All interactions e lJ 't the same pattern? Are there "exotic" quarks and of these bosons and ferrnions are described by an leptons? SU(3)c xSU(2)x U(l) gauge theory, in which SU(3)c x U(l) is an exact symmetry while the remaining symmetries Our list of questions seems almost endless. However, are spontaneously broken, presumably by the Higgs as we shall see in this report, all of them are related mechanism. A very long list of theoretical questions to recent experimental e+e- results or to e+e- accompanies this "standard" picture. Some of these experiments which will be performed during the next few questions have been answered during the last five years. We will review the present theoretical situa­ years. Others are still open. However, every single tion in view of the new experimental data which were one of them relates to past, present or future presented at this conference, and will pose some experiments in electron-positron scattering. An in­ answerable questions to future experiments. complete list of these questions, arranged according A schematic "map" of present and future e+e­ to the various gauge fields or the relevant fermionic energies is shown in figure 1. Every new energy range "building blocks" includes: has, so far, yielded some entirely new features. We ~: Are electrons "pointlike"? Up to what energies is are almost guaranteed that this will continue to be the QED valid? case, at least up to energies of 200 GeV or so. I, Weak Interactions: Do W+,W- and ZO exist? Do they personally, believe that beyond that range, many have the predicted masses? Are there additional, surprises are awaiting us. heavier, Wand Z bosons? Is SU(2) x U(l) the full electrowe~k group? Do Higgs mesons exist and if so, are they composite? ~: Do gluons exist? Do they have J=l? What are the properties of gluon jets? Are gluons "flavor-blind"? What is the value of a ? Does it "run" as a function s of momentum? Is there a three-gluon coupling? -198- uds b t ----+ more quarks ? ~ more leptons ? Z WW+ - ~ ~ - more W,Z bosons. Strol7g weak interactions ? qqg ~~5,6 T Higgs mesons? Technic%r ? pw¢ G/uebol/s? 1GeV 10GeV 100GeV 1TeV 10TeV Fig. 1: A schematic "map" of e+e- scattering. An impressive array of major discoveries covers the left half of the map. Speculations and hopes for the future are on the right. 2. The Pattern of Fermion Generations E 'V.3D GeV indicate that,3,4,5,6 at most, an addition- c.m. 1 al Q= - '3 quark may exist below that energy, and that 2.1.. "Exotic" Quarks and Leptons. any "exotic" quarks and leptons are unlikely. It is Electron-positron collisions are particularly extremely important to pursue the search for such states. suitable for the discovery of new types of quarks and While we do not understand the generation pattern, we at leptons. One of the most remarkable experimental least have a well-defined puzzle. It would be useful to facts is actually the absence (up to E 'V 30 GeV) of c.m. know whether the puzzle is stated properly! any "exotic" quarks and leptons. All known quarks 2 I ~k? have charges "3 or -"3 and are, presumably, color 2.2. Where is the Top The PETRA experiments indicate that the rT threshold triplets. All known leptons have charges 0 or -1 and 3 has not been reached until E =31.6 GeV. ,4,5,6 This are colorless. All known quarks and leptons are c.m. means that the lowest lyin.g tt ("topponium") bound state probably J = 1- obj ects. The observed pattern of genera­ cannot be below, say, 29 G€~V and that the mass of the tions involves unexplained repetitions of these quantum t-quark must be above 14 or 15 GeV. Most (but not all) numbers, reminding us of the repeated discoveries of theoretical predictions place the t-quark between 11 and 8U(3) octets, decuplets and singlets of hadrons in the 7 15 GeV. However, none of these predictions are based 1960's. In analogy with the concept of exotic hadrons on any fundamental principle or serve as a serious test we may define "exotic quarks and leptons" to be any of important new ideas. One popular relation among such particles which do not repeat the existing quark masses which has bef~n derived from various pattern. Exotic leptons. might have Q = ±2 and/or spin 8 1. Exotic quarks might be color sextets and/or have sets of assumptions is: spin ~ and/or have Q etc. Fortunately, all m em -m m em em =i, -t u ct d S b such objects should be relatively easy to detect in (m -m +m )3 e+e- collisions, and should contribute to the R-value t c u more than their nonexotic counterparts. Various Using the "current masses" for the light quarks, this authors have suggested that the b-quark may be a color 'V gives mt 13 GeV. However, an important ingredient in sextet! or t hat t h e L- Iepton has 2spln .2 3 . Th ere 1S. all the derivations is the "technical" assumption that neither theoretical need nor experimental evidence for there are only two Higgs multiplets. With three, one these proposals, but some of them cannot yet be could get almost any value of fit. If m is well above t definitely excluded. The observed values of R at 14 or 15 GeV, it would simply mean that some of the technical assumptions arE~ not valid. -199 - It would be very nice if the tt state is found some­ respectively. Much more puzzling and difficult to where around 34 GeV, at the highest luminosity point of explain was the claim that X(2.85) had a substantial PETRA and PEP and at the highest available energy which branching fractions into y+y while X(3.4S) liked to still allows the production of TT pairs and, at the decay into Y+~. Theoretically, one would expect that same time, offers the best opportunity of studying the the hadronic widths of these states are of the order gluon jets in topponium decay. Let us hope that this of a few MeV and that their radiative decays are quite will be the case, and that the t-mass will turn out to rare. l4 have the "most profitable" value of 17 GeV. It is with great relief that we have now heard about the disappearance of these two states. The 2.3. How Many Generations? 14 beautiful results of the crystal ball experiment The present list of quark and lepton mass-values provided us with new upper limits for B(w+y+X)eB(X+yy) reads almost like a table of random numbers. We and for B(llJ' +Y+X(3.45))B(X(3.45) +Y+1JJ). These limits cannot make any sense out of it. The discovery of convince us that the simple theoretical notions are, additional generations might eventually give us some again, reasonable and that the X(2.8S) and X(3.45) clues concerning mass regularities. There are several puzzles were either unfortunate statistical fluctua­ arguments which place limitations on the number of tions or experimental errors (or both). quarks and leptons or on their maximal allowed mass values. Such arguments are based on asymptotic freedom,9 on calculations of helium abundance in cosmological models,lO on radiative correctionsll to the Weinberg mass relation Mw = M cOS8 and on the BACKGROUND SUBTRACTED Z 12 W properties of the Higgs potential.