- Email: [email protected]
Discussion on experimental developments and possibilities
PHYSICS REPORTS (Review Section of Physics Letters) 104, Nos. 2--4 (1984) 195--200. North-Holland, Amsterdam
Discussion on Experimental Developments and Possibilities
L. LEDERMAN: It seems to me and some of my colleagues at Fermilab that, whereas the mass region near 100 GeV is well covered by the ~p collider at CERN, by LEP and by the SLC, there are so many speculations in the mass range up to 1000 GeV and beyond, to 10 000 GeV that, in order to help you in what appears to be your extreme dilemma we must build a machine in the 20-40 TeV c.m. energy range as soon as possible. At Fermilab we have concentrated upon low cost and short R and D time and have been led to the venerable technology of iron magnets energized by a superconducting wire. Here we put the entire burden on engineering rather than physics-novel methods of mass production, no tunnels but reliable magnets enclosed in pipes buried in shallow trenches. Since the magnetic fields are small, the radii are large (15-20 km) and we need inexpensive, unpopulated, flat land, i.e. a "desert" and therefore the machine has been called a desertron. There are three stages in realizing any such project: (i) the technology, (ii) the strong consensus of the scientific (and especially theoretical) community and finally, (iii) the funding authority-hopefully these stages are not totally sequential. We are somewhere between (i) and (ii) and need very strong support from theorists. It is amusing to note the conflict of history and the present situation. History teaches us that every new domain of energy explored by new accelerators has given rise to unanticipated results. But the density of current speculations in the = T e V mass range is unprecedented and, if history is a valid guide, what is it that theoretical physics is missing?
R. HOFSTADTER: I support Leon Lederman's proposal enthusiastically. However, there is a limitation that occurs because all present accelerators, and Leon's also, operate with macroscopic electromagnetic cavities that produce the acceleration. I proposed, some time ago, using atoms in place of cavities and crystals containing atoms as the accelerators themselves. Exciting the atoms and then allowing particles, such as a proton to pass by such excited atoms in a crystal would allow the particle to be accelerated by the "dumping" of the excited atom's energy on the passing particle. However the energy gain must be greater than the energy loss-"friction" caused by the collision loss-in order for acceleration to occur. In this case the energy gradient is very much greater than in conventional accelerators, even Leon's proposed accelerator. This could get us into the higher energy regions, perhaps approaching GUTS energies. (Multiple scattering in the crystal accelerator is probably not important.) L. VAN HOVE: This is a question to C. Rubbia. How do you compare a very large, low magnetic field p~ collider (as Lederman's desertron) to a smaller, high magnetic field collider (as p~ in the LEP tunnel)? Could you state how they differ in luminosity? C. RUBBIA: In order to get luminosity two things are required: antiprotons and a given tune-shift. For a given tune-shift and a given number of antiprotons the luminosity is proportional to the frequency of rotation. So making small machines increases this factor right away by a factor of three or four. The second remark is that in order to collect a lot of antiprotons you must know how to do so. It takes a good many years to do so and since CERN has a long tradition now in developing such sources
196
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it is only reasonable to try to transmit these particles to yet another device. As far as feasibility is concerned, the work done by ISABELLE and Tevatron 1 and now also by H E R A and the Japanese and the Russians, who have developed a beautiful series of new magnets, makes clear that, at least from the point of view of construction, magnets of five or six Tesla are now well in hand. As far as the desert machine is concerned, it is again a prototype of a new system so I believe it will take a tremendous amount of research and development to produce a very simple and cheap superconducting magnet involving distances of hundreds of kilometers. So, although in order to reach 20-30 TeV such a machine is probably necessary, it will be on a different timescale than the smaller machines. H. FRITZSCH: I would like to ask Dr. Rubbia about his views on improving the detectors for colliding beam machines. Looking back at the history of the past ten years, the prospects do not look very encouraging in observing very small cross sections. For example, the ~ J/~0 production cross section at the ISR is about 10 .32 c m 2, however the J/~O has been missed during the first three years of ISR operation. Another example: until today one has not observed the b-flavored particles, although it is certain that pairs of these particles are produced at CESR, PETRA, PEP or the hadron colliders.
C. RUBBIA: Firstly I would like to agree substantially with what you are saying. For instance one of the most beautiful examples of colliding beam experiments is coming from SPEAR where a good machine and a good detector really went hand in hand. So detectors are absolutely essential for the realization of experiments. But nobody knows how to run with luminosity larger than 1031 and many people believe that this will remain true. Now with useful luminosity being limited by detectors at 10 31 it is better to improve the signal by putting the money into trying to produce higher energies than in developing new detectors. Therefore, build the highest energy machine you can, so that the cross section will go up. The second factor is that when the energy goes up things become simpler because the individual particles don't count so much anymore. The only thing which really counts is the jets. If the jets are really narrow at sufficiently high energy it should be possible to solve our problems. In particular the breakthrough may come in the field of calorimetry, in which individual particles don't matter anymore and you measure only the energy flow out of the interaction, ~ la Crystal Ball if you like. I am sure that in the year 2000 nobody will care if there's a pion at 35 °, they will just ask where the energy went. In doing this you go back to simple things again. Now we're going through a transition phase between difficult detection of individual particles and the general development of such calorimetry. What should be done is to concentrate on leptons, which are generally fine PT things, neutrinos, which are missing energies, muons, which will punch through, and quarks or gluons, which are the jets, and just don't look at any other particles. Then your detector will have little or no magnetic field, essentially just a calorimeter and lepton detector. R. HOFSTADTER: To solve the problem of new detectors for the very high multiplicities expected one should use conduction-type counters such as silver chloride or T1Br-TII which were first investigated in the late 1940's. One has to learn how to make the holes move in addition to the electrons. One, of course, needs crystals, or materials with high atomic number and high density to serve as these detector materials. The scintillator counter called "Crystal Ball" is the zeroth approximation to the new detectors. But a conduction counter (like a semiconductor detector) offers much higher angular resolution possibilities.
Discussion on experimental developments and possibilities
197
C. RUBBIA: I think what one is doing is extremely important but what you need is some sort of breakthrough. If the device you describe should ever materialize, it would be just the thing one needs. Y. NE'EMAN: The use of jets as indicators in very high energy collisions may be an arrow for future methods. In Astronomy, for instance, when dealing with very large distances, one utilizes the brightest galaxy in each cluster. This is similar in principle. C. RUBBIA: Yes, once you see jet events at the CERN p~ collider you know that this is the way to go. We have been greeted with a very important physics result: When you go above 100 GeV the jets are the dominant part of the cross section. The jets for us represent quarks. In a certain sense it seems to me that the best test of QCD will come from repeating the Rutherford experiment by scattering two quarks together and looking at what comes out. And this is in fact what is being done, with hadrons being the source for quark-gluon collisions. This is completely complementary to experiments with e+ewhere you create the matter through the annihilation graph. By the way, I have to tell you that we have very nice results from the collider where we can plot the jet-jet cross section and we come out in the end with essentially the Rutherford formula. Because the gluon is massless like the photon we get something over sine of the angle to the fourth power and we have results which actually show this. G. 't HOOFT: I would like to make a comment on the experimental search for monopoles. The reason for the existence of monopoles in unified theories is now well understood. Any theory with a compact covering group must have them. And it would be great if Cabrera's result were right! But which of all these unified models we should believe is another matter. I am not an expert on that and I am as puzzled and uncertain as most others here. Our predictions, or rather speculations, depend critically on such ideas. L.C. BIEDENHARN: I would like to put the monopole question more provocatively. It is, I believe, reasonably clear that, at a naive theoretical level, the two Stanford experiments (Fairbank's quark, and Cabrera's monopole) are in contradiction. But if we assume Cabrera's experiment is correct, and assume, as is reasonable, that electron-positron data show the quarks to have charge -~, 2, then the data are not in contradiction with the measured Dirac limit only if the monopole has colored flux as well. Other possibilities include a second photon. My question is to ask an expert to clarify this situation. G. 't HOOFT: You formulated the situation well. The complete Dirac quantization rule, if 1 and 2 are two differently charged particles, reads: E~ ( e T g ~ - e ' ~ g ? ) = 27rn, n integer (where e refers to electric charges and g to magnetic charges). One reads off that (1) monopoles themselves may be fractionally charged (dyons) and (2) contributions from various different kinds of Abelian or non-Abelian interactions must simply be added. So if Fairbank and Cabrera are both right, then an extra photon would be needed (color cannot be used because of color-electric confinement). Fractionally charged partons are compatible with monopoles if these partons indeed carry fundamental color charges and the monopoles are also color-magnetically charged. L. VAN HOVE: This is a slightly different version of the question of Prof. Biedenharn. Assume that there is strict confinement of QCD color for quarks and gluons, but that a monopole exists. What can be said about its color?
Higher Energy Physics
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G. 't HOOFT: The monopole must carry color-magnetic charge. Now we believe that confinement of color-electric charges is due to color-magnetic superconductivity of the vacuum. As a consequence then the color-magnetic charge of the monopole (even if it is "fractional") will be screened by the vacuum monopoles (and not "confined" to other color-magnetic monopoles). This screening takes place at hadronic distance scales. What we could see experimentally is a finite, hadronic cross section for this monopole in interactions with other hadrons. It participates in the strong interactions.
T. TAJIMA*: Method of accelerating particles to ultra-high energies by plasma wave created by laser beat Since the conventional accelerator has a slow wave structure by metal, the extremely large electric field required for acceleration necessarily breaks down the metal. A plasma is already discharged and causes no breakdown problem at any strength of field. Two considerations lead us to lasers for creating accelerating field in a plasma. One is their huge transverse field intensity (up to power density of 1016W/cm2). The other consideration is that of luminosity. As the cross-section decreases as 1/(energy)2, the luminosity of a high-energy accelerator has to increase as (energy)2. The luminosity is inversely proportional to the length of particle beam bunch, which again points to having shorter wavelength EM waves. Two intense lasers with frequencies Wo and o~1 and wave numbers ko and kl cause a beat wave of O) 0 - - 0 ) 1 and k o - k t . If we inject two beating lasers into a plasma and resonate with the plasma oscillation, i.e. ~Oo- wl = ~Op,where oJp is the plasma frequency, this excites intense l o n g i t u d i n a l electric fields (plasmon) through the process of Raman forward scattering. With intense lasers (eEo/mo~o ~ c, where E0 is laser electric field), it is possible to obtain intense longitudinal plasma wave field of EL ~-- mwpc/e. For a plasma of density 10 ~8c m -3, this electric field amounts to 1 GeV/cm (see ref. [1]). This intense longitudinal field structure (plasmon) is propagating along with the laser light with a phase velocity equal to c ( 1 - Wp~,o)',2~1/2 21 <_ ~." This wave can trap electrons and accelerate them to high energies. In one stage of the trapping process one obtains an energy of y m c 2 = (OJo/Wp)mC 2 in the frame of propagating plasmon, or an energy of 2 y E m c 2 = 2(Wo/COp)2mc: in the laboratory frame. If (OJo/~%) 30, this acceleration amounts to - 1 GeV over the distance la -~ c/o%(wo/OJp) 2 - 1 cm for the 1018c m -3 case. In order to achieve 100 TeV energy through this process, one would need l0 s stages. The laser beams have to be refocused and rephased in order to achieve this, which is quite a technical challenge. I am considering a process which I may call the relativistic Brillouin process instead of Raman process. This leads to a high phase velocity of plasma wave, Vph--C(1-- ,,,pl,O0) .2 / .251/2, where ~%i is ion plasma frequency, smaller by ( m / M ) ~/2 than the (electron) plasma frequency oJp. This way the number of stages of acceleration may be considerably reduced.
References l I] T. Tajima and J.M. Dawson, Phys. Rev. Lett. 43 (1979) 267. [2] T. Tajima, Proc. Oxford Conf. on Challenge of Ultra-high Energy, to be published (1982). [3] Experimental paper by C. Joshi, T. Tajima, JM. Dawson, H.A. Baldis, and N.A. Ebrahim, Phys. Rev. Lett. 47 (1981) 1285.
* Institute for Fusion Studies and Physics Department, University of Texas, Austin, Texas 78712, USA.
Discussion on experimental developments and possibilities
199
A. SALAM: Dr. Tajima was modest. In Oxford this scheme was the most promising one. I understand that there was an experiment which tested the principle you discussed. How well does the experiment test the theory. T. TAJIMA: The experiment initiated at UCLA by Joshi et al. [Phys. Rev. Lett. 47 (1981) 1285] is using a low power CO2 laser. Therefore, the nature of the experiment is merely a proof-of-principle type. An energy of electrons of - 1 M e V was obtained for oJp/Wo= 0.22. This was a single beam experiment in which the plasma wave grows as an instability. L. VAN HOVE: Do you have actual numbers for acceleration? T. TAJIMA: It is premature to give such numbers at this time. However, the electric field strength would be 1 GeV/cm for a plasma of 10lg cm -3 density, and if we can stage this 105 times successfully the energy would be 105 GeV = 100 TeV over a length of 105 cm = 1 km. D.H. HO: On the experiment related to the next energy scale or the breaking mass
In this conference, we have heard Prof. Weinberg speak about the next energy scale and the scale of symmetry breaking. Yesterday, Prof. Lederman talked about the concept of the "Desertron". Here I want to make two remarks about these topics. 1. It has been shown in 1978 by S. Miyake at the 19th Conference of High Energy Physics at Tokyo (who gave an excellent review about cosmic ray experiments) that there exists a lot of experimental phenomena which support the idea that the character of interactions changes for energies 1014-1015eV. Here I want to show that such a tendency is still supported by recent experiments. Some attractive phenomena in the super-high energy range (>100 TeV): (a) The mean multiplicity of secondary particles deviates distinctly from the s TM rule and possibly increases according to an s 1/3 or s 1/2 rule. Recently, new cosmic ray data for E -> 1015 eV reconfirmed that ( N ) ~ E 1/3 [M. Amennomori et al., Phys. Rev. D25 (1982) 2807]. (b) The average transverse momentum (pl) of the secondary particles is no longer a constant, but increases rapidly with s. (c) A "multi-core" appears in the spatial distribution of secondary particles produced by cosmic rays, i.e., on the emulsion film there are several "circular shadows" surrounded by some "discrete points". This picture has also been recently obtained by a Chinese cosmic ray group. An explanation of this is that there are iron nuclei in the primary cosmic rays. However, the amount of Fe present should be one order of magnitude larger than the amount of protons! (d) Some exotic events are well known, as the Centauro events (no photons). What I want to say is that these phenomena show an alternative way of supporting the idea of a Desertron. They are difficult to explain using ordinary theories (Fermi, Landau, Pomeranchuk, Hagedorn . . . . ) or by some newly opened channel, since these phenomena exhibit a general tendency for E_> 10 ~5eV. Possibly, these phenomena may be explained by the excitation of a new degree of freedom which is on a deeper level than, for example, the level of quarks. Recently, my colleague, Gao Chong-shou, and myself discussed these phenomena from this point of view (see preprints AS-ITP-82-003, 009, 013, 016, 019). I don't want to go into the details of these preprints at this conference. However, I would like to emphasize that these phenomena occurring in cosmic rays may be of further interest to both theorists and experimentalists.
Higher Energy Physics
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Owing to the great difficulty in doing cosmic ray experiments, their results are not very reliable, e.g., these pictures show that (N) :< s 1/2 and (p±) :~ s 1/2. However, we know, from the law of conservation of energy, that: <7"1" S1/2 (N) (Pi) - 4 since the transverse momentum can never be larger than the longitudinal momentum. However, these experiments still shouldn't be ignored, since they show new possibilities and can be improved on with further work. 2. The second remark I want to make is that it is very important to develop a method to measure the mass of G U T Magnetic Monopoles, provided the Cabrera monopole experiment is correct. As 't Hooft and Polyakov show, the mass of the GUMs is around: M~ ~ M~la
in which Mx is the grand unification mass. How big is this symmetry breaking mass? Its value is certainly of great theoretical importance! In order to measure the mass of a monopole, two quantities must be found; the momentum and the velocity. The velocity is easily determined by the "time of flight" method or estimated by ionization losses. The momentum determination is of greater difficulty. However, on closer examination this problem still offers some possibilities. In ordinary cosmic ray experiments, a cloud chamber with an area of 1 m × 1 m, a magnetic field of 7000 gauss, and space resolving power of about 1 mm will determine momenta of relativistic particles as big as 100-150 GeV/c. There exists such an apparatus in Yuan-Nan Province in the southwest part of China. Obviously, this value is far below the momenta of GUMs', which are generally around 1013-1017 GeV/c. However, there exist many factors which compensate to make monopoles possible to measure. Firstly, the charge will increase by a factor of g/e ~ 10 2. Secondly, a proton with momentum as big as 100 GeV/c is relativistic while such momentum corresponds to a non-relativistic GUM, so that again a factor of 100 appears. Thirdly, as many theoretical papers show, the monopole will have a velocity around 10-4 <- 13 -< 10-2; thus the time spent in the detector will increase by a factor of 1//3. This leads to the increase of the transverse displacement, s = ½at 2, by a factor of 1/'8 2. If one takes ,8 = 10-4, then the total amount of enhancement will be as big as a factor of 1012, as compared to Mm/Mp 10610 7 . If we further notice that if the acting length or the flying path in the magnetic field were to be increased by a factor of 10 (~ 10 m), and if the space resolution power were increased by a factor of 102, i.e., to around 10 ix, then finally we would get an enhancement factor of 1016. This value is quite close to the mass ratio of Mm/Mp! At least this kind of experiment would be sufficient to distinguish whether the monopole mass is of the order of 1016-107 GeV or of the order of 1012GeV. ~
L. VAN HOVE: In connection with your remarks on the multiplicity growth with energy, one should note that according to the first results of the P0 collider at CERN, the dispersion of the multiplicity gets large, but that the mean multiplicity only grows slowly with energy above the ISR range (roughly as In2 s). In cosmic ray collisions it is hard to get the true average multiplicity, and available estimates may be too high.
Discussion on Experimental Developments and Possibilities
L. LEDERMAN: It seems to me and some of my colleagues at Fermilab that, whereas the mass region near 100 GeV is well covered by the ~p collider at CERN, by LEP and by the SLC, there are so many speculations in the mass range up to 1000 GeV and beyond, to 10 000 GeV that, in order to help you in what appears to be your extreme dilemma we must build a machine in the 20-40 TeV c.m. energy range as soon as possible. At Fermilab we have concentrated upon low cost and short R and D time and have been led to the venerable technology of iron magnets energized by a superconducting wire. Here we put the entire burden on engineering rather than physics-novel methods of mass production, no tunnels but reliable magnets enclosed in pipes buried in shallow trenches. Since the magnetic fields are small, the radii are large (15-20 km) and we need inexpensive, unpopulated, flat land, i.e. a "desert" and therefore the machine has been called a desertron. There are three stages in realizing any such project: (i) the technology, (ii) the strong consensus of the scientific (and especially theoretical) community and finally, (iii) the funding authority-hopefully these stages are not totally sequential. We are somewhere between (i) and (ii) and need very strong support from theorists. It is amusing to note the conflict of history and the present situation. History teaches us that every new domain of energy explored by new accelerators has given rise to unanticipated results. But the density of current speculations in the = T e V mass range is unprecedented and, if history is a valid guide, what is it that theoretical physics is missing?
R. HOFSTADTER: I support Leon Lederman's proposal enthusiastically. However, there is a limitation that occurs because all present accelerators, and Leon's also, operate with macroscopic electromagnetic cavities that produce the acceleration. I proposed, some time ago, using atoms in place of cavities and crystals containing atoms as the accelerators themselves. Exciting the atoms and then allowing particles, such as a proton to pass by such excited atoms in a crystal would allow the particle to be accelerated by the "dumping" of the excited atom's energy on the passing particle. However the energy gain must be greater than the energy loss-"friction" caused by the collision loss-in order for acceleration to occur. In this case the energy gradient is very much greater than in conventional accelerators, even Leon's proposed accelerator. This could get us into the higher energy regions, perhaps approaching GUTS energies. (Multiple scattering in the crystal accelerator is probably not important.) L. VAN HOVE: This is a question to C. Rubbia. How do you compare a very large, low magnetic field p~ collider (as Lederman's desertron) to a smaller, high magnetic field collider (as p~ in the LEP tunnel)? Could you state how they differ in luminosity? C. RUBBIA: In order to get luminosity two things are required: antiprotons and a given tune-shift. For a given tune-shift and a given number of antiprotons the luminosity is proportional to the frequency of rotation. So making small machines increases this factor right away by a factor of three or four. The second remark is that in order to collect a lot of antiprotons you must know how to do so. It takes a good many years to do so and since CERN has a long tradition now in developing such sources
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Higher Energy Physics
it is only reasonable to try to transmit these particles to yet another device. As far as feasibility is concerned, the work done by ISABELLE and Tevatron 1 and now also by H E R A and the Japanese and the Russians, who have developed a beautiful series of new magnets, makes clear that, at least from the point of view of construction, magnets of five or six Tesla are now well in hand. As far as the desert machine is concerned, it is again a prototype of a new system so I believe it will take a tremendous amount of research and development to produce a very simple and cheap superconducting magnet involving distances of hundreds of kilometers. So, although in order to reach 20-30 TeV such a machine is probably necessary, it will be on a different timescale than the smaller machines. H. FRITZSCH: I would like to ask Dr. Rubbia about his views on improving the detectors for colliding beam machines. Looking back at the history of the past ten years, the prospects do not look very encouraging in observing very small cross sections. For example, the ~ J/~0 production cross section at the ISR is about 10 .32 c m 2, however the J/~O has been missed during the first three years of ISR operation. Another example: until today one has not observed the b-flavored particles, although it is certain that pairs of these particles are produced at CESR, PETRA, PEP or the hadron colliders.
C. RUBBIA: Firstly I would like to agree substantially with what you are saying. For instance one of the most beautiful examples of colliding beam experiments is coming from SPEAR where a good machine and a good detector really went hand in hand. So detectors are absolutely essential for the realization of experiments. But nobody knows how to run with luminosity larger than 1031 and many people believe that this will remain true. Now with useful luminosity being limited by detectors at 10 31 it is better to improve the signal by putting the money into trying to produce higher energies than in developing new detectors. Therefore, build the highest energy machine you can, so that the cross section will go up. The second factor is that when the energy goes up things become simpler because the individual particles don't count so much anymore. The only thing which really counts is the jets. If the jets are really narrow at sufficiently high energy it should be possible to solve our problems. In particular the breakthrough may come in the field of calorimetry, in which individual particles don't matter anymore and you measure only the energy flow out of the interaction, ~ la Crystal Ball if you like. I am sure that in the year 2000 nobody will care if there's a pion at 35 °, they will just ask where the energy went. In doing this you go back to simple things again. Now we're going through a transition phase between difficult detection of individual particles and the general development of such calorimetry. What should be done is to concentrate on leptons, which are generally fine PT things, neutrinos, which are missing energies, muons, which will punch through, and quarks or gluons, which are the jets, and just don't look at any other particles. Then your detector will have little or no magnetic field, essentially just a calorimeter and lepton detector. R. HOFSTADTER: To solve the problem of new detectors for the very high multiplicities expected one should use conduction-type counters such as silver chloride or T1Br-TII which were first investigated in the late 1940's. One has to learn how to make the holes move in addition to the electrons. One, of course, needs crystals, or materials with high atomic number and high density to serve as these detector materials. The scintillator counter called "Crystal Ball" is the zeroth approximation to the new detectors. But a conduction counter (like a semiconductor detector) offers much higher angular resolution possibilities.
Discussion on experimental developments and possibilities
197
C. RUBBIA: I think what one is doing is extremely important but what you need is some sort of breakthrough. If the device you describe should ever materialize, it would be just the thing one needs. Y. NE'EMAN: The use of jets as indicators in very high energy collisions may be an arrow for future methods. In Astronomy, for instance, when dealing with very large distances, one utilizes the brightest galaxy in each cluster. This is similar in principle. C. RUBBIA: Yes, once you see jet events at the CERN p~ collider you know that this is the way to go. We have been greeted with a very important physics result: When you go above 100 GeV the jets are the dominant part of the cross section. The jets for us represent quarks. In a certain sense it seems to me that the best test of QCD will come from repeating the Rutherford experiment by scattering two quarks together and looking at what comes out. And this is in fact what is being done, with hadrons being the source for quark-gluon collisions. This is completely complementary to experiments with e+ewhere you create the matter through the annihilation graph. By the way, I have to tell you that we have very nice results from the collider where we can plot the jet-jet cross section and we come out in the end with essentially the Rutherford formula. Because the gluon is massless like the photon we get something over sine of the angle to the fourth power and we have results which actually show this. G. 't HOOFT: I would like to make a comment on the experimental search for monopoles. The reason for the existence of monopoles in unified theories is now well understood. Any theory with a compact covering group must have them. And it would be great if Cabrera's result were right! But which of all these unified models we should believe is another matter. I am not an expert on that and I am as puzzled and uncertain as most others here. Our predictions, or rather speculations, depend critically on such ideas. L.C. BIEDENHARN: I would like to put the monopole question more provocatively. It is, I believe, reasonably clear that, at a naive theoretical level, the two Stanford experiments (Fairbank's quark, and Cabrera's monopole) are in contradiction. But if we assume Cabrera's experiment is correct, and assume, as is reasonable, that electron-positron data show the quarks to have charge -~, 2, then the data are not in contradiction with the measured Dirac limit only if the monopole has colored flux as well. Other possibilities include a second photon. My question is to ask an expert to clarify this situation. G. 't HOOFT: You formulated the situation well. The complete Dirac quantization rule, if 1 and 2 are two differently charged particles, reads: E~ ( e T g ~ - e ' ~ g ? ) = 27rn, n integer (where e refers to electric charges and g to magnetic charges). One reads off that (1) monopoles themselves may be fractionally charged (dyons) and (2) contributions from various different kinds of Abelian or non-Abelian interactions must simply be added. So if Fairbank and Cabrera are both right, then an extra photon would be needed (color cannot be used because of color-electric confinement). Fractionally charged partons are compatible with monopoles if these partons indeed carry fundamental color charges and the monopoles are also color-magnetically charged. L. VAN HOVE: This is a slightly different version of the question of Prof. Biedenharn. Assume that there is strict confinement of QCD color for quarks and gluons, but that a monopole exists. What can be said about its color?
Higher Energy Physics
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G. 't HOOFT: The monopole must carry color-magnetic charge. Now we believe that confinement of color-electric charges is due to color-magnetic superconductivity of the vacuum. As a consequence then the color-magnetic charge of the monopole (even if it is "fractional") will be screened by the vacuum monopoles (and not "confined" to other color-magnetic monopoles). This screening takes place at hadronic distance scales. What we could see experimentally is a finite, hadronic cross section for this monopole in interactions with other hadrons. It participates in the strong interactions.
T. TAJIMA*: Method of accelerating particles to ultra-high energies by plasma wave created by laser beat Since the conventional accelerator has a slow wave structure by metal, the extremely large electric field required for acceleration necessarily breaks down the metal. A plasma is already discharged and causes no breakdown problem at any strength of field. Two considerations lead us to lasers for creating accelerating field in a plasma. One is their huge transverse field intensity (up to power density of 1016W/cm2). The other consideration is that of luminosity. As the cross-section decreases as 1/(energy)2, the luminosity of a high-energy accelerator has to increase as (energy)2. The luminosity is inversely proportional to the length of particle beam bunch, which again points to having shorter wavelength EM waves. Two intense lasers with frequencies Wo and o~1 and wave numbers ko and kl cause a beat wave of O) 0 - - 0 ) 1 and k o - k t . If we inject two beating lasers into a plasma and resonate with the plasma oscillation, i.e. ~Oo- wl = ~Op,where oJp is the plasma frequency, this excites intense l o n g i t u d i n a l electric fields (plasmon) through the process of Raman forward scattering. With intense lasers (eEo/mo~o ~ c, where E0 is laser electric field), it is possible to obtain intense longitudinal plasma wave field of EL ~-- mwpc/e. For a plasma of density 10 ~8c m -3, this electric field amounts to 1 GeV/cm (see ref. [1]). This intense longitudinal field structure (plasmon) is propagating along with the laser light with a phase velocity equal to c ( 1 - Wp~,o)',2~1/2 21 <_ ~." This wave can trap electrons and accelerate them to high energies. In one stage of the trapping process one obtains an energy of y m c 2 = (OJo/Wp)mC 2 in the frame of propagating plasmon, or an energy of 2 y E m c 2 = 2(Wo/COp)2mc: in the laboratory frame. If (OJo/~%) 30, this acceleration amounts to - 1 GeV over the distance la -~ c/o%(wo/OJp) 2 - 1 cm for the 1018c m -3 case. In order to achieve 100 TeV energy through this process, one would need l0 s stages. The laser beams have to be refocused and rephased in order to achieve this, which is quite a technical challenge. I am considering a process which I may call the relativistic Brillouin process instead of Raman process. This leads to a high phase velocity of plasma wave, Vph--C(1-- ,,,pl,O0) .2 / .251/2, where ~%i is ion plasma frequency, smaller by ( m / M ) ~/2 than the (electron) plasma frequency oJp. This way the number of stages of acceleration may be considerably reduced.
References l I] T. Tajima and J.M. Dawson, Phys. Rev. Lett. 43 (1979) 267. [2] T. Tajima, Proc. Oxford Conf. on Challenge of Ultra-high Energy, to be published (1982). [3] Experimental paper by C. Joshi, T. Tajima, JM. Dawson, H.A. Baldis, and N.A. Ebrahim, Phys. Rev. Lett. 47 (1981) 1285.
* Institute for Fusion Studies and Physics Department, University of Texas, Austin, Texas 78712, USA.
Discussion on experimental developments and possibilities
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A. SALAM: Dr. Tajima was modest. In Oxford this scheme was the most promising one. I understand that there was an experiment which tested the principle you discussed. How well does the experiment test the theory. T. TAJIMA: The experiment initiated at UCLA by Joshi et al. [Phys. Rev. Lett. 47 (1981) 1285] is using a low power CO2 laser. Therefore, the nature of the experiment is merely a proof-of-principle type. An energy of electrons of - 1 M e V was obtained for oJp/Wo= 0.22. This was a single beam experiment in which the plasma wave grows as an instability. L. VAN HOVE: Do you have actual numbers for acceleration? T. TAJIMA: It is premature to give such numbers at this time. However, the electric field strength would be 1 GeV/cm for a plasma of 10lg cm -3 density, and if we can stage this 105 times successfully the energy would be 105 GeV = 100 TeV over a length of 105 cm = 1 km. D.H. HO: On the experiment related to the next energy scale or the breaking mass
In this conference, we have heard Prof. Weinberg speak about the next energy scale and the scale of symmetry breaking. Yesterday, Prof. Lederman talked about the concept of the "Desertron". Here I want to make two remarks about these topics. 1. It has been shown in 1978 by S. Miyake at the 19th Conference of High Energy Physics at Tokyo (who gave an excellent review about cosmic ray experiments) that there exists a lot of experimental phenomena which support the idea that the character of interactions changes for energies 1014-1015eV. Here I want to show that such a tendency is still supported by recent experiments. Some attractive phenomena in the super-high energy range (>100 TeV): (a) The mean multiplicity of secondary particles deviates distinctly from the s TM rule and possibly increases according to an s 1/3 or s 1/2 rule. Recently, new cosmic ray data for E -> 1015 eV reconfirmed that ( N ) ~ E 1/3 [M. Amennomori et al., Phys. Rev. D25 (1982) 2807]. (b) The average transverse momentum (pl) of the secondary particles is no longer a constant, but increases rapidly with s. (c) A "multi-core" appears in the spatial distribution of secondary particles produced by cosmic rays, i.e., on the emulsion film there are several "circular shadows" surrounded by some "discrete points". This picture has also been recently obtained by a Chinese cosmic ray group. An explanation of this is that there are iron nuclei in the primary cosmic rays. However, the amount of Fe present should be one order of magnitude larger than the amount of protons! (d) Some exotic events are well known, as the Centauro events (no photons). What I want to say is that these phenomena show an alternative way of supporting the idea of a Desertron. They are difficult to explain using ordinary theories (Fermi, Landau, Pomeranchuk, Hagedorn . . . . ) or by some newly opened channel, since these phenomena exhibit a general tendency for E_> 10 ~5eV. Possibly, these phenomena may be explained by the excitation of a new degree of freedom which is on a deeper level than, for example, the level of quarks. Recently, my colleague, Gao Chong-shou, and myself discussed these phenomena from this point of view (see preprints AS-ITP-82-003, 009, 013, 016, 019). I don't want to go into the details of these preprints at this conference. However, I would like to emphasize that these phenomena occurring in cosmic rays may be of further interest to both theorists and experimentalists.
Higher Energy Physics
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Owing to the great difficulty in doing cosmic ray experiments, their results are not very reliable, e.g., these pictures show that (N) :< s 1/2 and (p±) :~ s 1/2. However, we know, from the law of conservation of energy, that: <7"1" S1/2 (N) (Pi) - 4 since the transverse momentum can never be larger than the longitudinal momentum. However, these experiments still shouldn't be ignored, since they show new possibilities and can be improved on with further work. 2. The second remark I want to make is that it is very important to develop a method to measure the mass of G U T Magnetic Monopoles, provided the Cabrera monopole experiment is correct. As 't Hooft and Polyakov show, the mass of the GUMs is around: M~ ~ M~la
in which Mx is the grand unification mass. How big is this symmetry breaking mass? Its value is certainly of great theoretical importance! In order to measure the mass of a monopole, two quantities must be found; the momentum and the velocity. The velocity is easily determined by the "time of flight" method or estimated by ionization losses. The momentum determination is of greater difficulty. However, on closer examination this problem still offers some possibilities. In ordinary cosmic ray experiments, a cloud chamber with an area of 1 m × 1 m, a magnetic field of 7000 gauss, and space resolving power of about 1 mm will determine momenta of relativistic particles as big as 100-150 GeV/c. There exists such an apparatus in Yuan-Nan Province in the southwest part of China. Obviously, this value is far below the momenta of GUMs', which are generally around 1013-1017 GeV/c. However, there exist many factors which compensate to make monopoles possible to measure. Firstly, the charge will increase by a factor of g/e ~ 10 2. Secondly, a proton with momentum as big as 100 GeV/c is relativistic while such momentum corresponds to a non-relativistic GUM, so that again a factor of 100 appears. Thirdly, as many theoretical papers show, the monopole will have a velocity around 10-4 <- 13 -< 10-2; thus the time spent in the detector will increase by a factor of 1//3. This leads to the increase of the transverse displacement, s = ½at 2, by a factor of 1/'8 2. If one takes ,8 = 10-4, then the total amount of enhancement will be as big as a factor of 1012, as compared to Mm/Mp 10610 7 . If we further notice that if the acting length or the flying path in the magnetic field were to be increased by a factor of 10 (~ 10 m), and if the space resolution power were increased by a factor of 102, i.e., to around 10 ix, then finally we would get an enhancement factor of 1016. This value is quite close to the mass ratio of Mm/Mp! At least this kind of experiment would be sufficient to distinguish whether the monopole mass is of the order of 1016-107 GeV or of the order of 1012GeV. ~
L. VAN HOVE: In connection with your remarks on the multiplicity growth with energy, one should note that according to the first results of the P0 collider at CERN, the dispersion of the multiplicity gets large, but that the mean multiplicity only grows slowly with energy above the ISR range (roughly as In2 s). In cosmic ray collisions it is hard to get the true average multiplicity, and available estimates may be too high.