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E+ E~ ANNIHILATION Hinrich Meyer, Fachbereich Physik, Universitдt

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E+ E~ ANNIHILATION Hinrich Meyer, Fachbereich Physik, Universitдt - 155 - e+e~ ANNIHILATION Hinrich Meyer, Fachbereich Physik, Universität Wuppertal, Fed. Rep. Germany CONTENT INTRODUCTION 1. e+e~ STORAGE RINGS .1 Energy and Size .2 Luminosity .3 Energy range of one Ring .4 Energie resolution .5 Polarization 2. EXPERIMENTAL PROCEDURES .1 Storage Ring Detectors .2 Background .3 Luminosity Measurement .4 Radiative Corrections 3. QED-REACTIONS + . 1 e+e~ T T~ .2 e+e"+y+y~ . 3 e+e~ -»• e+e~ . 4 e+e_ •+y y 4. HADRON PRODUCTION IN e+e~ANNTHILATION .1 Total Cross Section .2 Average Event Properties .3 Two Jet Structure .4 Three Jet Structure .5 Gluon Properties - 156 - 5. SEARCH FOR NEW PARTICLES .1 Experimental Methods .2 Search for the sixth Flavor (t) .3 Properties of Q Q Resonances .4 New Leptons 6. YY ~ REACTIONS .1 General Properties .2 Resonance Production .3 £ for e,Y Scattering .4 Hard Scattering Processes .5 Photon Structure Function INTRODUCTION e+e storage rings have been proven to be an extremely fruitful techno• logical invention for particle physics. They were originally designed to provide stringent experimental tests of the theory of leptons and photons QED (Quantum-Electro-Dynamics). QED has passed these tests beautifully up to the highest energies (PETRA) so far achieved. The great successes of the e+e storage rings however are in the field of strong interaction physics. The list below gives a very brief overview of the historical development by quoting the highlights of physics results from e+e storage rings with the completion of storage rings of higher and higher energy (see Fig.1). YEAR RING MAIN RESULT ON 1966 STANFORD, ADA e~e~ QED 1967 ACO, VEPP p,w,<t> vector mesons 1970 ADONE R M 2 at 2 GeV 1973 CEA R i 4 at 4-5 GeV 1974 SPEAR (ADONE DORIS) J/ii, \|/' cc resonance 1975 SPEAR T -lepton DORIS, SPEAR P , Y -states of cc 1975 c 1976 SPEAR 2 jet structure 1976 SPEAR, DORIS charm particles 1978 DORIS Y,Y' bB resonances 1979 PETRA 3 jet structure 1980 CE SR Y" Y'" bb resonances - 157 - Planned storage rings with energies larger than 40 GeV are bound to give completely new physics results this time probably mostly in the field of weak interactions, provided the basic machine physics problem for higher energy storage rings - low luminosity - can adequately be resolved. The first lecture will give information on properties and limitations of storage ring machines and about the basics of experimentation. Then the results of QED tests in the purely leptonic channels are presented. In the second lecture I try to cover information on hadron production at higher energies where jet-phenomena are important with emphasis on three jet events and their relations to some basic properties of QCD. In the third lecture the recent searches for new particle thresholds are reviewed and an account is given of some of the decay properties of heavy quark antiquark resonances. Finally(fourth lecture) the new field of photon-photon collisions where very little was known until very recently is briefly covered with discussions of C=+ resonance production and hard scattering phenomena. In recent years a large number of excellent reviews of e+e annihilation physics have appeared which the reader should consult for those parts of the field that I have been unable to even touch upon and also for the de• tails of the earlier important developments^. Since most of the very recent new results do come from the four experiments at PETRA (JADE,MARKJ,PLUTO, TASSO) I will frequently refer to those without trying to carefully follow up the historical development of a particular field of e+e physics. For some parts of the written version of the lectures updated information is given as it has appeared during the time just after the summer school took place. 1. e+e~ STORAGE RINGS The two most important parameters of an e e collision ring are the total collision energy E^M and the luminosity L, they will be briefly dis• cussed below. 1.1 Energy and Size The total energy is given by ( for head on collisions) Z Fcl = (Z£) = S (ßtV)*- where E (GeV) is the energy of the e- beams. Starting with the construction of the first storage rings in the early 1960th machines of ever increasing collision energy have been build and this trend is continuing with future projects presently very actively pur• sued2. The main goal of the new projects is to get to the mass of the Z° expected near E^ = 100 GeV. Fig. 1 shows the maximum energy E^ (max) reached in the various colliding beam machines vs. time indicating also the anti- E+E- COLLIDERS "i i i i i i i i i i 1 1 r T-.—IT—i—i i i 111 1 1—j—i—i M 11 1 1—r i I I I I I 1 1 1 1 1 1 1 1—7 E (GEV) 1 1 V CM / |_ p'(NRI) » 1 J - .../ í *. (LEP)^'" / (LEP)O 100 P (SPC) / (HERA) O / / / / / PETRAO/ / / (HERA)p / / / / / A. PETRA 10LR / O DORIS CESR O/ O SPEAR / /o / / / PEP O VEPP-4 CEA O/ / / / SPEAR / P ADONE /^DORIS / ; ADONE / / _ /O ACO : 1 y DCI / / YEAR VEPP -/2M E (GeV) H ADA, STANFORD , . ^ i ' i i i i 1111 i i i ' i 1111 i t i 1960 1970 1980 1990 10 100 + Fig. 1: Emax vs. time for e e~ storage Fig. 2: The bending radius p vs. E — —+ _ max rings. Also shown (dotted circles) are for e e storage rings. The line is the new projects under discussion. E2 max - 159 - cipated completion dates for some of the new projects. Surprisingly enough an exponential dependence emerges with Y measured in years and (A = 0,5 GeV in 1960). To keep energy loss by synchrotron radiation low enough the machine radius goes approximately like Erfiax (see Fig. 2). By simple minded extrapolation to the year 2000 a required diameter of 1.500 km would not fit into europe. Clearly this is a rather unsafe and unrealistic extrapolation. The relative sizes and geometries of e- storage rings can be easily illustrated with a site plan of DESY (Fig.3). Electrons and positrons are produced in the 400 MeV linac (1) stored and rebunched in PIA (2) transfered to DESY (3) accelerated and then transfered either to DORIS (4) or PETRA (5) the two machines used for physics programms. DORIS is a double ring structure now used in a one ring mode with two interaction points and 1x1 bunch ope• ration. PETRA has eightfold symmetry, with maximum 4x4 bunch operation now however used only in the 2x2 bunch mode with 4 interaction points. As a further illustration of the development of e+e storage rings the little ring PIA (2) is compared in size Fig. 4 to the first e e storage ring rea• lized in Stanford (1962) and in Fig. 5 PETRA is compared to LEP,most likely the largest ring ever to be realized. 1.2 Luminosity The colliding positrons and electrons are contained in rather short particles bunches (*>~(1,5 - 30) cm long the precise value depending on the accelerating Rf structure) circulating in opposite directions in the vacuum pipe of the storage ring. With n- the number of positrons and electrons in 2 the bunches of crosssection F (cm ) and a bunch revolution frequency _ -I f(sec ) the luminosity L is defined as i ~ sU+ M* -£/ F 0 + 11 2 Typical numbers are n- = 3.5 • 10 , F = 41T-0,1 • 0,01 (cm ) and f = 105 (sec-1) which leads to L~103° (cm-2 sec-1). As an illustration Fig.6 shows the maximum luminosity L , as function of E_„, for some of the e- 1 peak CM L storage rings. peak. is the luminosity just after the filling procedure is terminated and the bunches are colliding. The luminosity decays rapidly with time due to loss of particles from the bunches. This effect on the luminosity is in general somewhat balanced by a decreasing bunch crosssection F(see 1.3). A refill of the ring is therefore necessary every few hours as can be seen in Fig. 7 which shows the e- currents in PETRA over a twelf hour period. From general experience with all e- storage rings so far one obtains L for the integrated luminosity per day in terms of pea]t 160 RF RF Fig. 3; A site plan of DESY PIA •5m Fig. 4; A size comparison of Fig. 5: A possible site plan the storage ring PIA with the LEP with the PETRA ring for first e_e~ storage ring in parison. Note the factor of Stanford. ^ 1000 in scale to Fig. 4 - 161 - 4 Z r Lotir ~ LPJT/TK • M** ' /S- (c*~ ) 14 where the reduction by 1/5 takes account of the many factors that decrease the luminosity. It was always known and has recently become even clearer + (PETRA) that Lpeak is strongly limited for all e e~ storage rings due to 'beam beam effects'3. In practice this means that the bunch cross section F increases very rapidly with the bunch intensities n- and finally leading to complete disruption of the bunches in the moment of collision. This behaviour is not understood. At PETRA two modifications of the machine are prepared to still increase luminosity. The cross sections F of the bunches at the interaction point are decreased by stronger focussing of the beams (low ß-insertions) with an ex• pected gain of factor of 3. Secondly through the action of an 1 GHz cavity an effective bunch lengthening is achieved with the possibility of signifi• cantly increasing n* avoiding however a simultaneous increase of F (see 1.3).
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