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1 TWEVY-SECOND RCHTMYER MEMORAL LECTURE OF TKE AMZRCAN 7 L CF PHYSCS TEACHERS - January 24, 1963 The title of this talc, Photon and Electron High-Energy Physics: Present and Future
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1 TWEVY-SECOND RCHTMYER MEMORAL LECTURE OF TKE AMZRCAN 7 L CF PHYSCS TEACHERS - January 24, 1963 The title of this talc, Photon and Electron High-Energy Physics: Present and Future contains a contradiction: High-energy physics, usually defined as physics above the threshhold of production of unstable particles, cannot be properly separated into branches dealing with electrons and photons on the one hand, and other particles on the other. What wish to speak about is that area of high-energy physics to which electron and photon beams from accelerators have made important contributions and,are expected to do so in the future..~ Before discussing specific-physical problems, let me first discuss the question as to what is meant by high-energy eiectron or photon collisions in the relativistic sense. n general, when a photon (rest mass m. = 0) or an electron (rest mass m 0 = 0.31 Mev) participates in a collision it will transfer an energy A E and a momentum A ;;' in any given reference frame. n order to, describe the process in a manner independent of the motion of'the frame of reference, any physically meaningful results must depend on the covariant combination* s2 = DE2 - Ap2. (1) which is called the square of the four-momentum . f :..q2 0, the four-momentum is called time-like , if q2 C 0 the four-momentum is called space-like ; these names are chosen in analogy with the relativistic space-time interval i? = At2 - Ax2 (2) : ; where At and Ax are the time and space separation between two events. -ic V7e choose units in which c, the velocity of light, is unity., -l- 4 /..,, _._.._ ^_._...._.. _._.....A, : N-., f,i2 0 then there is a frame in which the two events are at the same Place, but separated in time; if T' 0, a frame can be found in which the two events are simultaneous but separaqd in space. n broad terms the importance of high-ener,v electron physics is based on the fact that, as far as is known,ele--- L,rons interact only through the electromagnetic interaction (and the app::oximate lolo --' bimes weaker Fermi interaction); at-'this time experiment and theory agzee exactly in all areas where this question has been put to the test. n contrast to proto% neutrons, pions, c :., electrons are not affected directly by the strong nuclear interaction. For this reason, again speaking generally, experiments in high-energy electron physics divide generally into three classes: 1) those processes which explore an unknown or poorly known structure, such as a nucleon or an artificially produced particle with the known action of electromagnetism (by known we mean the relativistic quantum description of the electromagnetic field, hereafter called QED for quantum electrodynamics), 2) the study of processes in which artificial unstable particles are created where the presence of a bombarding particle which possesses strong interaction (such as a nucleon or pion) would complicate analysis of the process, and 3) experiments which attempt to extend our range of q2,,the square of the four-momentum transfer, c,ver which Q,ED is known to be valid or to observe possible deviations. Let us examine the first of these applications. The best known of these are the now classical electron-scattering experiments in which the structure of nucleons and nuclei is examined by the angular and energy dependence of the elastic and inelastic electron-scattering cross-sections. n this case, the particle studied. is reay' i.e,, it has existed for a long time prior to the encounter.. As we shall see later a similar method can also be used on a virtual particle, i.e., a particle created and then destroyed during the brief interval permitted by quantum mechanics in which energy need not be conserved. Electron scattering is commonly visualized in the analogy to the classical -2- iheo:y of diffraction of vjaves of Jave A.ength h on an object of linear dimensions of order D. Scattering will then take place primarily into a forward cone of apex angle of order h/d. Analyzed in more detail, classical diffraction theory shows that the angular distribution expressed in terms of the scattering angle 8 is the ' Fourier transform of the density p (z) of scatterers in the object; more precisely, the amplitude A (6, h) of scattering is proportional to an integral over the distribution and over the volume of dimension D given by: (3) where the scattering wave vector z is the vector difference between the initial and final wave vectors of length h/2r( taken along the directions of the waves before and after scattering respectively; the magnitude of d is thus (2+) [2 sin (d/24 ; therefore, the scattering angular distribution effectively Fourier analyses the spatial distribution in terms of the wave number k . Altho-ugh there are, of course, many complicating factors, the relativistic generalization of this classical analysis relates the electron scattering amplitude to the Fourier analysis of the distribution in terms of the four-momentum transfer q2.i.e., the larger the.quantity q2, the finer is the spatial detail of the structure which can be examined. Let us examine this situation in the language of quantum mechanics. The actual scattering process (skei;ched in Fig. l).,.-._ ' _. _-.-.-_. i. :: * _- _._- --.-_ _ - _ in which; say, an electron scatters from the u.nkn~wn particle initially at rest, can also be analyzed by' stating that one or.more particles are exchanged which. transfer the four-momertum q, as shown in Fig _. _. ^._, . :. - _.-. - _- FlGURE 2. These diagrams, (which kill here not use in the completely formal Feynman graph ' sense) are interpreted as: follows: n Fig. 2, a proton (double line) approaches. an electron (single line) and they affect one another through the electromagnetic. -field. This diagram can be described by stating that the electron virtually emits a photon (wavy line) which is then absorbed by the proton. The four-momentum q is then carried by the virtual photon. The quantity which corresponds for high energy scattering to the diffraction amplitude A (e,,h) given by eq. (3) is called a form factorf (q'). n the physical interaction involving scattering of an electron from an unknown structure there is in general more than one form factor if the unknown structure has spin. and thus can change its spin state, or in case the unknown particle can be excited 'or disintegrated, i.e., if it can change its enera state _~-- --~-. ~- f scattering is elastic (i.e., if the energy state of the urknown particle is not changed) and if the rocess can be described by only a one-photon exchange corresponding to the diagram of Fig. 2a, then in general two form factors, called GE( q2 and GM s2 ) are required, the former corresponding to the case in which the spin of the particle to be explored is unchanged, and the latter to the case -4- in.jhich it changes by one unit of angular momentum along the axis of the momentum transfer. Figure 3 shows the values of GE (qa) and GM (q2) for two of the most important unknown particles: the proton and neutron. The data are those of the pioneering work of Hofstadter and collaborators combined with more modern data from Cornell, Orsay and Stanford. For reasons not discussed here the figure shows the g (isotopic scalar) and the difference (isotopic vector) between the Froton and neutron values of the form factors. -. us CORNELL! STANFORD jlh; T ; qz in f FG. 3a. sotopic vector form f'actorc. l?ig. jb. so.topic scalar form facto,3 How can we understand such data and what future growth of understanding can we expect? nitially the data were interpreted in analogy to the low energy diffraction picture discussed above ar.d thus one can construct models of charge and magnetic distributions which give some, intuitive picture. More fruitful.is the approach to relate the results to dynamic models of the nucleon arising from the recent discovery of Y+esonant or excited states of nucleons. To understand the subsequent discussion, the reader should remember the general shape of the curves of Fig. 3. Recent high energy experiments have shown that the nucleon can absorb various amounts of energy resulting in something closely akin to.a set of enera levels, in analogy to excited atomic states. Excess energy would be provided by. -5- absorption of a time like photon, i.e., a -photon gaining a lerger energy than momentum transfer. Among the excited states of the nucleons only some can contribute to the electromagnetic properties of the nucleon, due to certain selection rules involved. 'ihe most important state is the one in which the p-meson (spin 1; m. = 750 Mev) is!'osciuating about %he nucleon; this generates a resonance curve as a function of q* centering about a space like (i.e., excess energy) point at q2 equal to the square of the rest mass. On the other hand we note that for elastic scattering of electrons on stationery targets ia $i ae, i.e., q2 is space-like. What tight happen on the time-like side, i.e., the side on which excess energy can be transferred? A conjecture which corresponds to one of the currently explored resonances of the nucleon is shown in Fig. 4. -_- S-rORAGE RNG -- ELECTRON SCA-aTERNG - EN--S - EXPER TS,... / The elastic scattering thus measures the tail of the resonance curve. At present a single resonant nucleon staz 'e is not sufficient to fit the data; this msy be due to higher mass resonant contribution, to inadequacy in calculation, or due to inadequacies of the model of Fig. 2a in which only one photon is exchanged. This latter question can be analyzed by substituting positrons for electrons. in scattering experiments; if diagrams like Fig. 2b contribute, negative and positive electron scattering cross sections will differ; such experiments (instituted by J. Pine and collaborators) are in progress and there are indications of such differences for Large values of q*. have outlined how,in the past electric electron and positron scattering have illuminated the nucleon structure problem and formed a direct link to the resonances discovered in strongly interacting particle systems. What then is t'ne future? Clearly mu&l more work of the kind indicated needs to be done; in particular the situation in relation to neutron structure is still quite unsatis-. factory since, in the absence of free neutron targets, complications introduced by the deuteron structure seriously limits reliable analysis. Moreover, there are already indications -that analysis will become progressingly more difficult as q* increases in the space-like direction: first there is the contribution of more complex mechanisms, such as the one shown in Fig. 2b. Secondly, unless there will be new high mass resonances discovered which contribute to the scattering, the cross section will continue to decrease at high energies and thus the data rate of :,, experiments will go down. Thirdly, as we go to higher values of q* the validity of Q,ED is no longer established and such questions become intertwined into th,e study of particle structure. Finally, time-like values of q2 are inaccessible to electron scattering and hence, as seen in Fig. 4, the role of resonances has to be inferred by far away measurements on the q* axis. We will return later to experimental techniques involving collisions between electrons and positrons traveling in opposite directions; these have bearing on extending structure information along the time-like portion of the' q2 axis and also to examine the validity of QED. -7- \ HoV do we examine thiz structure of particles which are themselves unstable? k possible answer lies ill certain types of photo- and electro-production experiments involving such particles. Photo production of pions has been one of the earliest successful applications of electron accelerators; in fact the first information of the interaction between pions and nucleons have been inferred from the behavior of.photo-production cross sections. Phto-production of unstable particles can proceed, in broad terms, via the two alternate methods sketched in Fig. 5.. _ - --_._ PWOYON,7 P... n diagram 5a the photon interacts at A through its electromagnetic field with an electric or magnetic property (charge or magnetic movement) of the nucleon. The resultant excited nucleon then disintegrates at B into a nucleon and the particle P. The resultant production rate then depends on the forces acting at B between the particle. P and the nucleon, i.e., on information similar to that gained from scattering experiments of particles P generated as an external beam from an accelerator, scattered in a hydrogen target. -a- The absorption of a Fhoton of zex rest mass corresponds to a four-moxentun txnsfer q2 = 0, hence the absorption at A will give no new informetion. iiow-, ever, corresponding to e&ch photon - absorption process there is also an irxlastic clectron,scattering process, i.e., instead of absorbing energy from the fieid of a free photon, the process can be indilced by absorbing the field from a rapidly moving, electron. n this case Pig. a changes schematically to the fo,rm shown in Fig. 6. :,.-.-w _._._.. - _,--- w - _.... EO * FGURE, 6 -P- n this case q2 # 0; in fact the part of the process involving the inter- action at A and C are very similar to the ordinary electron scattering process of Fig. 2. Hence inelastic electron scattering can also give information on nucleon structure.. The second mechanism for photo-production of single particles is shown in Fig Here the photon is absorbed by the created unstable particle and thus the interaction is between the electromagnetic properties of this new particle and the field of the photon. f, as above, we substitute the field of an inelastically scattered electron for that of the photon, we find that the resultant scattering amplitude depends on the structure of the unstable particle. Hence, suc'h an electro-production process constitutes a virtual target of unstable particles for electron scattering. Successful exploitations of this scheme depend:. on isolating this process from other reaction channels; this should be possible in the future. We thus find that photo-production of unstable particles has taught us a great deal about the interaction of these particles with nuclei and about the structure of these particles themselves, and will continue to do so. Production of more than one particle opens up a new series of interesting phenomena. shall only discuss two: electromagnetic pair production and. peripheral production . Electromagnetic production of electron-positron pairs is the well-known process by which gamma rays of energy above 1 Mev can materialize ; it becomes the dominant absorption process for high energy electromagnetic radiation. A gamma ray by itself cannot convert into a pair of positive and negative particles and still conserve energy and momentum; a third, preferably heavy particle has to participate to absorb the recoil. Since this third particle (in general the nucleus of the nuclear species under gamma ray bombardment) can absorb the recoil just through its electrostatic field, the entire process is all electromagnetic , i.e., it can be totally analyzed in terms of well understood electromagnetic forces. Clearly the same type of theory adiusted only for the mass and spin of the charged particle pair to be produced can thus predict a production rate of any particle through this mechanism. *The process has been verified for muons (in addition to electrons) and thus lends itself well to a sys'tematic particle search. Such a search has recently been carried out at Stanford for particles intermediate to the electron and muon mass with (not unexpected) negative results; depending on observational techniques such a search can be continued in the future to higher masses. Of practical consequence is the production of pure high intensity muon beams; beams thus generated have a much smaller pion contamination than beams.?roduced by the decay of primary pions. Let us now consider a peripheral collision. We noted that as the energy of a photon becomes very large, a pion pair can be produced almost in vacua with only a small unbalance of momentum absorbed by a target nucleus (Fig. 7) _ We can thus consider a process in which only one of the pair fragments. escapes w'nile the other fl.agment interacts with the total cross section on the tarirret nucleus for the particle in question. u The resultant cross section is quite large in the fo,rward direction and in fact is likely to exceed the production cross section of corresponding tarticles bjr protons for secondary particles of energy near that of the b0mbardir.g particle; comparative cross sections are shown in 7 2 r;g. 8. As a result high energy electron accelerators will become highly competi- tive factories for secondary particles, including pions, k-particles, neutrinos, etc., in contrast to the s,ituation at lower energies where proton machines are superior in this respect. Finally, would like to discuss e;rperi., vents which aim to extend the range over which the correctness of QED has been established. Presently QED is known to be' correct through experiments covering the range from very large interaction distances.down to intervals as low as a small multiple of 10-i* cm. By the un- certainty principle examination of physical properties at small distances requires large transfers of momentum or, relativistically, four-momentum. n addition experiments designed to examine this question should, if possible only involve electrons, photons or muors because these are the only particles not interacting through nuclear forces; since in general nuciear forces are larger than electro- magnetic interactions, such forces wouid obscure observation of possible deviation from the laws of electromagnetic interaction. Actually, some.of the experiments involving protons can be interpreted as setting limits to Q,ZD; this can be done by comparing the results of different experiments in which the proton enters in the same manner, but in which different electromagnetic processes are involved. f proton targets are not involved then limits 'of electrodynamics can be explored either by a) comparing the electromagnetic properties of the free electron or muon with Q,ED, b) studying collision between electron or muon beams CA. c wit3 electrons at m, c! experiments between colliding electron beams. Of greatest importance in the first category is the CERN experiment on the g-factor OL c the muon; will not have the opportunity to discuss this beautiful result here beyond stating that no deviations from QZD were observed. Experiments of the second class suffer from the fact that the values of q2 which can be reached if a light particle, such as the electron, is struck at rest, will be very small even at a very high incident enera. Specifically, the value,' of the four-momentum transfer q2 if a particie of rest mass m 0 is struck by a very energetic incident particle of energy E 0 q2 = - 2moEo is given by which, in MeV becomes just q2 = - E. for an electron. Hence, a 10 GeV electron striking an electron at rest will produce a q value of only - (100 KeV)2. Hence, such electron knock-on experiments will extend our range of knowledge about Q,ZD only for very high incident energies indeed. Our greatest hope of examining the validity of QED further lies in colliding beam experiments. Such experiments are in progress at Stanford 'and by the Frascati, (taly) physicists using their storage ring at the French electron linear accelerator at Orsay. Figure 9 shows a diagram of the Stanford intersecting storage ring arrangement. Electrons are injected into eac n ring separately from a linear accelerator and stored in each ring.. Collisions occur in the common straight section.between the rings. Of possibly greatest interest are the future experiments in which electrons and positrons are stored in a single ring and are allowed to collide in specifie
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