Weinberg, Salam and Glashow on physics

Sheldon Glashow: making a patchwork quilt * In our next issue we will cover of theory into a tapestry. Here he is seen the award of the 1980 Nobel explaining how the long-awaited top quark may not really be necessary. Physics Prize to J.W. Cronin and V.L. Fitch. (Photo CERN 310.12.79)

Last year the Nobel Prize for physics was awarded to Steven Weinberg, Abdus Salam and Sheldon Glashow for the development of the theory which unifies electromagnetic and weak interactions*. The three reci­ pients each presented a lecture on the occasion of the presentation of the prize, and these lectures have been published in full (together with footnotes, appendices and refer­ ences) in the July edition of Reviews of Modern Physics (Vol. 52 No. 3). The lectures provide an illuminating insight into the challenges and rewards of modern physical theory and as such are well worth reading in their entirety. In addition, extracts illustrate several aspects of today's physics and physicists.

The objectives of physics and physicists

Weinberg: 'Our job in physics is to see things simply, to understand a great many it looks like ridges and valleys after work quilt. Electrodynamics, weak complicated phenomena in a unified all.' interactions, and strong interactions way, in terms of a few simple princi­ Salam also cites Einstein: were clearly separate disciplines, ples. At times, our efforts are illumi­ 'There is the apocryphal story separately taught and separate^ nated by a brilliant experiment, such about Einstein, who was asked what studied. There was no coherent the­ as the 1973 discovery of neutral he would have thought if experiment ory that described them all. Develop­ current neutrino reactions. But even had not confirmed the light deflec­ ments such as the observation of in the dark times between experi­ tion predicted by him. Einstein is parity violation, the successes of mental breakthroughs, there always supposed to have said, "Madam, I quantum electrodynamics, the dis­ continues a steady evolution of theo­ would have thought the Lord has covery of hadron resonances and the retical ideas, leading almost imper­ missed a most marvellous opportun­ appearance of strangeness were ceptibly to changes in previous ity." I believe, however, that the well-defined parts of the picture, but beliefs.' following quote from Einstein's Her­ they could not be easily fitted Salam quotes Feynman from an bert Spencer lecture of 1933 together. interview in the magazine 'Omni': expresses his, my colleagues', and Things have changed. Today we 'As long as it looks like the way my own views more accurately. have what has been called a "stand­ things are built [is] with wheels "Pure logical thinking cannot yield ard theory" of elementary particle within wheels, then you are looking us any knowledge of the empirical physics in which strong, weak, and for the innermost wheel — but it world; all knowledge of reality starts electromagnetic interactions all might not be that way, in which case from experience and ends in it." arise from a local symmetry princi­ you are looking for whatever the hell Glashow takes a more personal ple. It is, in a sense, a complete and it is you find !' In the same interview viewpoint: apparently correct theory, offering a he remarks 'a few years ago I was 'In 1956, when I began doing qualitative description of all particle very sceptical about the gauge theo­ theoretical physics, the study of ele­ phenomena and precise quantitative ries... I was expecting mist, and now mentary particles was like a patch- predictions in many instances. There Abdus Salam: after a brief foray into experimental physics as a student he soon turned his attention to theory. A wise move, it would appear.

(Photo CERN 371.10.79)

renormalizable theory of beta pro­ cesses without the possibility of a renormalizable electrodynamics. We should care to suggêstr'that a fully acceptable theory of these interac­ tions may only be achieved if they are treated together..." We used the original SU(2) gauge interaction of Yang and Mills. Things had to be arranged so that the charged cur­ rent, but not the neutral (electro­ magnetic) current, would violate parity and strangeness. Such a theory is technically possible to con­ struct, but it is both ugly and experi­ mentally false. We know now that neutral currents do exist and that the electroweak gauge group must be larger than SU(2).' Weinberg talks of his love affair' with broken symmetry: 'Sometime in 1 960 or early 1961, I learned of an idea which had origi­ nated earlier in solid state physics and had been brought into particle physics by those like Heisenberg, are no experimental data that con­ the sublime quality of patience — Nambu, and Goldstone, who had tradict the theory. In principle, if not patience in accumulating data, pa­ worked in both areas. It was the idea yet in practice, all experimental data tience with recalcitrant equipment of "broken symmetry," that a quan­ jean be expressed in terms of a small — which I sadly lacked. Reluctantly I tum theory could possess an exact number of "fundamental" masses turned my papers in, and started symmetry, and that the physical and coupling constants. The theory instead on quantum field theory with states might nevertheless not pro­ we now have is an integral work of Nicholas Kemmer in the exciting vide neat representations of the art: the patchwork quilt has become department of P.A.M. Dirac.' symmetry. In particular, a symmetry a tapestry. of the theory might turn out to be not Tapestries are made by many The search for electroweak a symmetry of the vacuum. „ artisans working together. The con­ unification As theorists sometimes do, I fell in tributions of separate workers can­ love with this idea. But as often not be discerned in the completed Glashow toyed with the idea of happens with love affairs, at first I work, and the loose and false threads electroweak unification at an early was rather confused about its impli­ have been covered over. So it is in stage in his physics career: cations. I thought (as it turned out, our picture of particle physics.' 'Schwinger, as early as 1956, wrongly) that the approximate sym­ Salam, the senior of the three believed that the weak and electro­ metries — parity, isospin, strange­ prizewinners, describes his early magnetic interactions should be ness, the eightfold way — might years in physics research: combined into a gauge theory. The really be exact symmetry principles, 'I started physics research thirty charged massive vector interme­ and that the observed violations of years ago as an experimental physi­ diary and the massless photon were these symmetries might somehow cist in the Cavendish, experimenting to be the gauge mesons. As his be brought about by spontaneous with tritium-deuterium scattering. student, I accepted this faith. In my symmetry breaking. It was therefore Soon I knew the craft of experimen­ 1 958 Harvard thesis, I wrote: "It is rather disturbing for me to hear of a tal physics was beyond me — it was of little value to have a potentially result of Goldstone, that in at least Steven Weinberg: a love affair' with broken symmetry.

(Photo CERN 349.12.79)

had been suspected to be the media­ tors of the weak interactions. The weak and electromagnetic interac­ tions could then be described in a unified way in terms of an exact but spontaneously broken gauge sym­ metry. And this theory would be renormalizable like quantum electro­ dynamics because it is gauge invar­ iant like quantum electrodynamics It was not difficult to develop c concrete model which embodied these ideas. I had little confidence then in my understanding of strong interactions, so I decided to concen­ trate on leptons.' Salam's account is more like a saga: 'For me, personally, the trek to gauge theories as candidates for fundamental physical theories started in earnest in September 1956 — the year I heard, at the Seattle Conference, Yang expound his and Lee's ideas on the possibility of the hitherto sacred principle of one simple case the spontaneous wrote our joint paper on the subject, I left-right symmetry being violated in breakdown of a continuous symme­ added an epigraph to the paper to the realm of the weak nuclear force. try like isospin would necessarily underscore the futility of supposing Lee and Yang had been led to con­ entail the existence of a massless that anything could be explained in sider abandoning left-right symme­ spin zero particle — what would terms of a noninvariant vacuum try for weak nuclear interactions as a today be called a "Goldstone state: it was Lear's retort to Cordelia, possible resolution of the kaon decay boson." It seemed obvious that "Nothing will come of nothing: puzzle. there could not exist any new type of speak again." Of course, The Physi­ I remember travelling back to Lon­ massless particle of this sort which cal Review protected the purity of don on an American Air Force trans­ would not already have been discov­ the physics literature, and removed port flight. Although I had been ered. the quote. Considering the future of granted, for that night, the status of a I had long discussions of this the noninvariant vacuum in theoreti­ Brigadierora Field Marshal — I don't problem with Goldstone at Madison cal physics it was just as well.' quite remember which — the plane in the summer of 1961, and then Later, Weinberg describes how he was very uncomfortable, full of cry­ with Salam while I was his guest at fell on the idea of electroweak unifi­ ing servicemen's children — that is, Imperial College in 1961-62. The cation: the children were crying, not the three of us soon were able to show 'At some point in the fall of 1 967,1 servicemen. I could not sleep. I kept that Goldstone bosons must in fact think while driving to my office at reflecting on why Nature should vio­ occur whenever a symmetry like MIT, it occurred to me that I had late left-right symmetry in weak isospin or strangeness is sponta­ been applying the right ideas to the interactions. neously broken, and that their wrong problem. It is not the rho Now the hallmark of most weak masses then remain zero to all meson that is massless: it is the interactions was Pauli's neutrino. orders of perturbation theory. I photon. And its partner is not the A1, While crossing over the Atlantic, a remember being so discouraged by but the massive intermediate bos­ deeply perceptive question about these zero masses that when we ons, which since the time of Yukawa the neutrino came back to me which Wolfgang Pauli, the father of the neutrino. His correspondence with Salam during the gestation of the new ideas on weak interactions makes interesting reading. One typical remark: if a theoretician says universal, it just means pure nonsense'.

Rudolf Peierls had asked when he 4%M was examining me for a Ph.D. a few years before. Peierls'question was: "The photon mass is zero because of Maxwell's principle of a gauge sym­ metry for electromagnetism; tell me, why is the neutrino mass zero?" I had then felt somewhat uncomforta­ ble at Peierls, asking for a Ph.D. viva, question of which he himself said fie did not know the answer. But during that comfortless night the answer came. The analogue for the neutrino of the gauge symmetry for the photon existed: it had to do with the masslessness of the neutrino, with symmetry under a particular transformation later christened "chi- ral symmetry". The existence of this symmetry for the massless neutrino must imply one of two possibilities for the neutrino interactions. Nature had the choice of an aesthetically satisfying but a left-right symmetry- violating theory, with a neutrino which travels exactly with the veloc­ ity of light; or alternatively a theory Zuleika Dobson, I wondered where I ideas similar to mine about chiral where left-right symmetry is pre­ could go next and the obvious place symmetry were expressed independ­ served, but the neutrino has a tiny was CERN in Geneva, with Pauli — ently by Landau and by Lee and nass — some ten thousand times the father of the neutrino — nearby Yang. smaller than the mass of the elec­ in Zurich. At that time CERN lived in I received Pauli's first, somewhat tron. a wooden hut just outside Geneva apologetic letter on 24 January It appeared at that time clear to airport. Besides my friends, Prentki 1957. Thinking that Pauli's spirit me what choice Nature must have and d'Espagnat, the hut contained a should by now be suitably crushed, I made. Surely, left-right symmetry gas ring on which was cooked the sent him two short notes I had writ­ must be sacrificed in all neutrino staple diet of CERN — Entrecôte à la ten in the meantime. These con­ interactions. I got off the plane the crème. The hut also contained Villars tained suggestions to extend chiral next morning, naturally very elated. I from MIT, who was visiting Pauli the symmetry to electrons and muons, rushed to the Cavendish, worked out same day in Zurich. I gave him my assuming that their masses were a the Michel parameter and a few paper. He returned the next day with consequence of what has come to other consequences of the symme­ a message from the Oracle: "Give be known as dynamical sponta­ try, rushed out again, got onto a train my regards to my friend Salam and neous symmetry breaking. With chi­ to Birmingham where Peierls lived. tell him to think of something bet­ ral symmetry for electrons, muons, To Peierls I presented my idea: he ter." and neutrinos, the only mesons that had asked the original question; This was discouraging, but I was could mediate weak decays of the could he approve of the answer? compensated by Pauli's excessive muons would have to carry spin one. Peierls' reply was kind but firm. He kindness a few months later, when Reviving thus the notion of charged said "I do not believe left-right sym­ Mrs. Wu's, Lederman's and intermediate spin-one bosons, one metry is violated in weak nuclear Telegdi's experiments were an­ could then postulate for these a type forces at all." nounced showing that left-right of gauge invariance which I called Thus rebuffed in Birmingham, like symmetry was indeed violated, and the "neutrino gauge." Pauli's reaction was swift and today call unification of basic forces Finally, in 1971 't Hooft showed in terrible. He wrote on 30 January — but I did not take this too serious­ a beautiful paper how the problem 1 957, then on 1 8 February and later ly. I felt this was a legacy of the could be solved: The proof was on 11, 12, and 13 March: "I am exasperation which Pauli had always subsequently completed by Lee and reading (along the shores of Lake felt at Einstein's somewhat formal- Zinn-Justin and by't Hooft and Velt- Zurich) in bright sunshine quietly istic attempts at unifying gravity with man. your paper... I am very much startled electromagnetism — forces which in I have to admit that when I first on the title of your paper 'Universal Pauli's phrase "cannot be joined — saw't Hooft's paper in 1 971, I was Fermi Interaction'... For quite a while for God hath rent them asunder." not convinced that he had found the I have for myself the rule if a theore­ But Pauli was absolutely right in way to prove renormalizability. The tician says universal it just means accusing me of darkness about the trouble was not with 't Hooft, bu^ pure nonsense. This holds particu­ problem of the masses of the Yang- with me: I was simply not familiar larly in connection with the Fermi Mills fields; one could not obtain a enough with the formalism on which interaction, but otherwise too, and mass without wantonly destroying 't Hooft's work was based.' now you too, Brutus, my son, come the gauge symmetry one had started Salam: with this word..." Earlier, on 30 Jan­ with. The problem was to be solved 'Both Weinberg and I suspected uary, he had written "There is a only seven years later with the that this theory was likely to be similarity between this type of gauge understanding of what is now renormalizable. Regarding sponta­ invariance and that which was pub­ known as the Higgs mechanism.' neously broken Yang-Mills-Shaw lished by Yang and Mills..." I quote theories in general this had earlier from his letter: "However, there are Renormalization been suggested by Englert, Brout, dark points in your paper regarding and Thiry. But this subject was not the vector field. If the rest mass is With the first formulations of the pursued seriously except at Velt- infinite (or very large), how can this electroweak theory, the spanner in man's school at Utrecht, where the be compatible with the gauge trans­ the works was their renormalization proof of renormalizability was given formation?" and he concluded his — there appeared to be no neat way by't Hooft in 1971. This was elabo­ letter with the remark: "Every reader of avoiding troublesome infinities in rated further by that remarkable phy­ will realize that you deliberately con­ the calculations. sicist, the late Benjamin Lee, work­ ceal here something and will ask you Weinberg: ing with Zinn-Justin, and by't Hooft the same questions." Although he The next question now was renor- and Veltman. In Coleman's eloquen- signed himself "With friendly re­ malizability. The Feynman rules for phrase "'t Hooft's work turned the gards," Pauli had forgotten his ear­ Yang-Mills theories with unbroken Weinberg-Salam frog into an en­ lier penitence. He was clearly and gauge symmetries had been worked chanted prince." rightly on the warpath. Now the fact out by deWitt, Faddeev, and Popov Glashow: that I was using gauge ideas similar and others, and it was known that 'Our labours were in vain. In the to the Yang-Mills gauge theory was such theories are renormalizable. spring of 1971, Veltman informed no news to me. This was because But in 1 967 I did not know how to us that his student Gerhart't Hooft the Yang-Mills theory (which mar­ prove that this renormalizability was had established the renormalizability ried gauge ideas of Maxwell with the not spoiled by the spontaneous sym­ of spontaneously broken gauge the­ internal symmetry SU(2) of which metry breaking. I worked on the ory. In pursuit of renormalizability, I the proton-neutron system consti­ problem on and off for several years, had worked diligently but I com­ tuted a doublet) had been inde­ partly in collaboration with students, pletely missed the boat' pendently invented by a Ph.D. pupil but I made little progress. With hind­ of mine, Ronald Shaw, at Cambridge sight, my main difficulty was that I The neutral current at the same time as Yang and Mills adopted a gauge now known as the had written. Shaw's work is rela­ unitarity gauge: this gauge has Electroweak unification implied tively unknown; it remains buried in several wonderful advantages, it the existence of a neutral current, his Cambridge thesis. exhibits the true particle spectrum of enabling weak interactions to hap­ I must admit I was taken aback by the theory, but it has the disadvan­ pen without altering the electric Pauli's fierce prejudice against uni- tage of making renormalizability charges of the participating parti­ versalism — against what we would totally obscure. cles. However the neutral current The heavy liquid bubble chamber Gargamelle in position at the CERN 28 GeVproton synchrotron where in 1973 it was the scene of the discovery of the neutral current (below). According to Weinberg, the electroweak theory predicted rates for the neutral current which were low enough to have escaped earlier detection, so there was every reason to look harder.

(Photo CERN 143.4.71)

was not discovered until the experi­ ments at CERN with the Gargamelle bubble chamber in 1973, six years after the publication of the electro­ weak ideas. Weinberg describes how he did not give up hope: 'Of course, the possibility of neu­ tral currents was nothing new. There -had been speculations about possi­ ble neutral currents as far back as 1937 by Gamow and Teller, Kem- mer, and Wentzel, and again in 1 958 by Bludman and Leite-Lopes. At­ tempts at a unified weak and elec­ tromagnetic theory had been made by Glashow, and Salam and Ward in the early 1 960s, and these had neu­ tral currents with many of the fea­ tures that Salam and I encountered in developing the 1967-68 theory. But since one of the predictions of our theory was a value for the mass of the intermediate boson, it made a definite prediction of the strength of the neutral currents. More important, now we had a comprehensive quantum field the­ ory of the weak and electromagnetic nteractions that was physically and mathematically satisfactory in the same sense as quantum electro­ dynamics — a theory that treated photons and intermediate vector bosons on the same footing, that was based on an exact symmetry principle, and that allowed one to carry calculations to any desired degree of accuracy. To test this the­ ory, it had now become urgent to found that in fact the 1967-68 construct a model of leptons alone settle the question of the existence theory predicted quite low rates, low seemed senseless: nuclear beta of the neutral currents. enough in fact to have escaped clear decay, after all, was the first and Late in 1 971, I carried out a study detection up to that time. So there foremost problem. One thing of the experimental possibilities. The was every reason to look a little seemed clear. The fact that the results were striking. Previous ex­ harder.' charged current violated strange­ periments had set upper bounds on Glashow's encounter with the ness would force the neutral current the rates of neutral current pro­ problem of the neutral current was to violate strangeness as well. It was cesses which were rather low, and somewhat more lengthy: already well known that strange­ many people had received the 'When I came upon the model in ness-changing neutral currents impression that neutral currents 1 960,1 had speculated on a possible were either strongly suppressed or were pretty well ruled out, but I extension to include hadrons. To absent. I concluded thatthe interme- Abdus Salam celebrates the award of his Nobel Prize with members of the Gar gamelle team. Salam learnt of the neutral current discovery from Paul Musset (on Salam's right in the photo) in the street in Aix-en-Provence, on his way to the 1973 European Conference on High Energy Physics.

(Photo CERN392.10.79)

diate boson of neutral currents had man and I devoted ourselves to the electroweak theory, we would have to be made very much heavier than exploitation of the unitary symmetry solved the problem of strangeness- its charged current counterparts. scheme. In the spring of 1964, I changing neutral currents. We did This was an arbitrary but permissible spent a short leave of absence in not. I had apparently quite forgotten act in those days: the symmetry Copenhagen. There, Bjorken and I my earlier ideas of electroweak syn­ breaking mechanism was unknown. suggested that the Gell-Mann- thesis. The problem which was expli­ I had "solved" the problem of Zweig system of three quarks should citly posed in 1961 was solved, in strangeness-changing neutral cur­ be extended to four. We called the principle, in 1 964. Noone, least of all rents by suppressing all neutral fourth quark the charmed quark. Part me, knew it. Perhaps we were all currents: the baby was lost with the of our motivation for introducing a befuddled by the chimera of relativ­ bath water. fourth quark was based on our mis­ ists SU(6), which arose at about this I returned briefly to the question of taken notions of hadron spectro­ time to cloud the minds of theo­ gauge theories of weak interactions scopy. But we also wished to rists. in a collaboration with Gell-Mann in enforce an analogy between the Five years later, John lliopoulos, 1961. We showed that a gauge weak leptonic current and the weak Luciano Maiani and I returned to the theory of weak interactions would hadronic current. Because there question of strangeness-changing inevitably run into the problem of were two weak doublets of leptons, neutral currents. \X seems incredible strangeness-changing neutral cur­ we believed there had to be two that the problem was totally ignored rents. We concluded that something weak doublets of quarks as well. for so long. We argued that un­ essential was missing. Indeed it was. The weak current Bjorken and I observed effects would be expected Only after quarks were invented introduced in 1964 was precisely to arise in any of the known weak could the idea of the fourth quark the GIM current. The associated interaction models and showed how and the GIM (Glashow-lliopoulos- neutral current, as we noted, con­ the unwanted effects would be elim­ Maiani) mechanism arise. served strangeness. Had we in­ inated with the conjectured exist­ From 1 961 to 1 964, Sidney Cole­ serted these currents into the earlier ence of a fourth quark. After lan- guishing for a decade, the problem of The future strong, and electromagnetic inter­ the selection rules of the neutral actions has been called a "grand current was finally solved. Of course, Weinberg is in poetic mood: unified theory", but a theory is not everyone believed in the pre­ 'I suppose that I tend to be optim­ neither grand nor unified unless it dicted existence of charmed ha- istic about the future of physics. And includes a description of gravita­ drons. nothing makes me more optimistic tional phenomena. We are still far This work was done fully three than the discovery of broken sym­ from Einstein's truly grand design.' years after the epochal work of metries. In the seventh book of The Salam expresses 'amazement': Weinberg and Salam and was pre­ Republic, Plato describes prisoners 'All I can say is that I am forever sented in seminars at Harvard and at who are chained in a cave and can and continually being amazed at the M.l.T. Neither I, nor my co-workers, see only shadows that things out­ depth revealed at each successive nor Weinberg, sensed the connec­ side cast on the cave wall. When level we explore. I would like to tion between the two endeavours. released from the cave at first their conclude with a prediction which We did not refer, nor were we asked eyes hurt, and for a while they think J. R. Oppenheimer made more than to refer, to the Weinberg-Salam that the shadows they saw in the twenty-five years ago and which has work in our paper. cave are more real than the objects been fulfilled today in a manner he The relevance became evident they now see. But eventually their did not live to see. More than any­ only a year later. Due to the work of't vision clears, and they can under­ thing else, it expresses the faith for Hooft, Veltman, Benjamin Lee, and stand how beautiful the real world is. the future with which this greatest of Zinn-Justin, it became clear that the We are in such a cave, imprisoned by decades in particle physics ends: Weinberg-Salam ansatz was in fact the limitations on the sorts of exper­ "Physics will change even more... If a renormalizable theory. With GIM, it iments we can do. In particular, we it is radical and unfamiliar... we think was trivially extended from a model can study matter only at relatively that the future will be only more of leptons to a theory of weak inter­ low temperatures, where symme­ radical and not less, only more actions. The ball was now squarely in tries are likely to be spontaneously strange and not more familiar, and the hands of the experimenters.' broken, so that nature does not that it will have its own new insights Especially amusing is Salam's appear very simple or unified. We for the inquiring human spirit." story of how he first came to hear of have not been able to get out of this the discovery of neutral currents: cave, but by looking long and hard at 'I still remember Paul Matthews the shadows on the cave wall, we and I getting off the train at Aix- can at least make out the shapes of en-Provence for the 1973 European symmetries, which though broken, Conference and foolishly deciding to are exact principles governing all walk with our rather heavy luggage phenomena, expressions of the to the student hostel where we were beauty of the world outside.' billeted. A car drove from behind us, Despite its successes, Glashow stopped, and the driver leaned out. does not hold out much hope for This was Musset whom I did not the present form of the electroweak know well personally then. He said: theory: "Are you Salam?" I said "Yes." He 'Let me stress that I do not believe said: "Get into the car. I have news that the standard theory will long for you. We have found neutral cur­ survive as a correct and complete rents." I will not say whether I was picture of physics. All interactions more relieved for being given a lift may be gauge interactions, but because of our heavy luggage or for surely they must lie within a unifying the discovery of neutral currents. At group. This would imply the exist­ the meeting that great and modest ence of a new and very weak inter­ man, Lagarrigue, was also present action which mediates the decay of and the atmosphere was that of a protons. All matter is thus inherently carnival — at least this is how it unstable, and can be observed to appeared to me.' decay. Such a synthesis of weak,