Where Are We in Particle Physics? by S

Where are we in particle physics? by S. B. Treiman

Technologically on course — installation of superconducting magnets below the Fermilab main ring.

(Photo Fermilab)

The fourth in a series of HEPAP (High Energy Physics Advisory Panel) Sub- panels, formed from time to time to review the status of the US high ener gy physics programme, last year had to confront an unusually wide array of opportunities and problems. The scientific opportunities and chal lenges are well known. They spring from the prodigious experimental and theoretical strides of the past decade and lay out the case for a new round of accelerator and other facili ties — to pursue the critical leads opened up by the recent develop ments and, as always, to allow for the unexpected. A number of delicious possibili ties, in various stages of definition, were in view: the Stanford Linear Collider for electron-positron phy sics at 100 GeV centre-of-mass en ergy; a Cornell conception for an el ectron-positron collider at a similar energy, based on superconducting r.f. technology; possible electron- proton collider facilities, etc. Nev traditional place. tion that we may be almost 'there' is ertheless, the prevailing climate was The current difficulties seem to be of course risky, if not outrageous, such as to focus attention mainly on mainly fiscal and technological ; there and even the most forward of the the problems. The problems are also are also problems, shared with Eu proponents of this view qualify it in well known — the financial ones, rope, associated with the complex various ways. which increasingly constrain the pro sociology of the large collaborations Everyone acknowledges that there gramme and utilization of existing that are increasingly called for in high are crucial tests to be made and facilities and the technological ones, energy experimentation. But let me information to be found in the com associated with the large supercon set these things aside and turn in ing round of experimentation; so ducting accelerator projects at Fer stead to another kind of foreboding that, given only the resources milab and Brookhaven. that has surfaced in certain quarters needed to exploit the visible scien Now the Saver/Tevatron project and that generated a great deal of tific opportunities, we surely face at Fermilab seems technologically heated debate among HEPAP mem very exciting times. Moreover, the more surely on course, but there bers in the off hours. proponents acknowledge, even if have been magnet problems for ISA- The recent years of particle phy everything goes as expected, that BELLE at Brookhaven and the nation sics have witnessed enormous pro there will remain much more to be al financial picture is more constrain gress, culminating in what seem to known than can be revealed in the ing than ever. The situation has per be very far-reaching synthesis. It is next round of experimentation. In haps fostered a magnified picture this rising curve of advancement that deed, there are very stirring visions among US physicists of the vigour of testifies to the scientific vitality of the about what may lie out there beyond the European programme but one field and that makes the case for con the immediately foreseeable do also imagines — or hopes — that tinuing support. So great is the sense mains of direct, experimental at the hurdles will, in time, be sur of achievement, however, that one tack. mounted and that US high energy dares to ask whether perhaps we are The trouble, however, is this : they physics will be able to resume its almost 'there' already. The proposi conceive that these farther reaches

404 CERN Courier, November 1981 Assembly of the UA2 detector at the CERN proton-antiproton collider — an example of the vigour of the European high energy physics programme.

(Photo CERN 84.9.81)

much of its modern history, this trou ble-making has been especially easy. In the limited domain of electron- photon physics, the great quantita tive successes of quantum electro dynamics had convinced most peo ple already by the late 1940s of the correctness of the general notions of relativistic qQantum field theory. At about the same time, however, the explosive proliferation of new parti cle types, with all their complicated interaction chemistry, had set in. This explosion of new phenomena dominated, and theory, at best, could only limp along with partial and ever-shifting insights and rough pro visional models. Serious visions of a realistic and fundamental dynamics receded. There was no encompassing ortho doxy to shoot at. To be sure in corn ers there were some people toying with new dynamical ideas — like spontaneously broken symmetry and non-Abelian gauge theories. The may lie forever beyond direct exper fussy about consistency, in view of possibilities, to those who could fol imental investigation and that, for the possibility of experimental error low the developments, looked very what can be reached, we may alrea and/or theoretical refinement. interesting indeed, at least as amuse dy have the basic framework in When the foundations for some ments, but the connections with real hand. domain of science seem to be in ity were still obscure. The goal of our field is an under hand, what is there left to do within By the early 1960s the one thing standing of the fundamental struc that domain ? Well, plenty! There still that was clear to most people was ture of matter. In discussing our loca remain all the rich and varied phe that the different hadron species tion along the axis of understanding, nomena themselves and the work of were already far too numerous for for present purposes I have in mind relating the phenomena to the foun any of them to be regarded as funda our grasp of the fundamental laws, dations. It is not easy, for example, mental in any reasonable sense; and as distinct from a mastery of all the to get to superconductivity straight the idea of a more parsimonious sub implications of those laws for phe away from Schrodinger's equation, structure, the quarks, was born, nomena. In principle, until one can the Pauli principle, and Coulomb's though unaccompanied by a serious work out and test a sufficiently wide law. For some fields there also arises and detailed quark dynamics. One range of predictions, one can't be the possibility of exploiting the new could get away with only three quark sure that the right foundations are in understanding for practical applica flavours at the time. The number has hand. Nevertheless, it is a fact that tions. Above all, for those of a cer since grown ominously to five, with great syntheses are established and tain mentality, there remains the task reasonable expectation of at least accepted from time to time on the of making trouble for the established one more — the 'top' quark. basis of vastly more limited evi orthodoxy — by searching for indi Then, by the early part of the last dence, at least initially, provided that cations of its limitations and thereby decade, the whole outlook for the the conjectured theoretical struc opening up new fields to be con strong interactions began to change tures look both pretty and consist quered. dramatically. At SLAC, then sub ent. At the start one is not even so For particle physics, throughout sequently at CERN and Fermilab, the

CERN Courier, November 1981 405 The discovery of the neutral current at CERN — an example of a testable prediction.

experimental study of deep inelastic scattering of leptons by nucleons revealed a certain scaling behaviour which suggested that the scattering takes place off point-like consti tuents in the nucleon — the quarks we now think. The quarks, it ap peared, behave as if free in these high momentum transfer transac tions. This made for a nice physical picture, embodied in the parton mod el, but at a deeper field theoretic level, it was hard to understand what was going on. One has to explain rents were subsequently discovered ton distributions, are still needed. why the distorting strong interac and are among the great experimen Nevertheless, there have been suc tions among quarks become effec tal triumphs of the past decade. Best cesses and there are as yet no ob tively unimportant at large momen of all, the new picture unified two vious contradictions. One can still be tum transfers, or equivalently, at hitherto separate classes of interac sceptical — the whole thing could be short distances. tions, the electromagnetic and the wrong and the successes an acci This was an important clue to weak. dent ; or the theory could be general strong interaction dynamics. It led to In QCD, for the strong interactions, ly on the right track but in need of the notion of 'asymptotic freedom', the various quark flavours all enter on serious modification or extension. It the discovery that asymptotic free an equal footing apart from their dif is not unthinkable, however, that dom is uniquely peculiar to non-Abe- fering mass parameters. The number QCD is altogether correct for a very lian gauge theories, and then to the of different flavour types, and the wide domain of strong interaction widespread focus on Quantum Chro masses, are external inputs to the physics. Internally, at least, it looks modynamics (QCD), a non-Abelian theory. The interactions among the to be fairly whole and parsimonious. gauge theory based on the SU 3 co quarks are mediated by eight mass- In order to form a reliable opinion, lour group that had already begun its less gauge bosons — the gluons. one will need not only a series of cru development from different origins. For the rest, there is only one addi cial experimental probings but also a A look at current issues of any parti tional parameter, a renormalization vast development in the art of ex cle physics research journal shows scale with the dimensions of a mass. tracting the implications of the theo that QCD now provides an ortho The basis of the QCD theory can be ry. doxy for the strong interactions. written down in one line, thanks to For the electromagnetic and weak Similarly for the weak interactions, the power of compact notation. But interactions, one deals with leptons by the late 1960s there was a whole that is of course deceptive. It is a as well as quarks. At its beginnings, new outlook based on a pulling to very long way from this one line of the unified electroweak scheme ap gether of ideas drawn from earlier formalism to all of the phenomena of peared as a rather general frame investigations of dynamical symme the strong interactions and although work, leaving open a variety of pos try breaking and, again, on the ideas the theory is very beautiful in the sible detailed realizations depending of non-Abelian gauge theory. The eyes of many beholders, it is also on the gauge group, quark and lepton new picture absorbed the phenome- ferociously difficult. content, etc. This is still the situa nologically successful current-cur The predictions extracted so far tion ; but a simple variant, the so- rent structure of the weak interac are limited, dealing for the most part called standard model has taken hold tions but, at long last, in a serious with short distance phenomena such and seems capable of assimilating all and renormalizable framework. It as deep inelastic lepton scattering, the present data. Here the quarks could incorporate the independently- electron-positron annihilation at high and leptons enter in families, each developed notion of charm and had energies and muon pair production in family on an equivalent footing. Two the great merit of predicting testable hadron collisions. families are well established and a things — neutral current interac Even within these limits, more phe- third is almost complete, awaiting tions. Both charm and neutral cur nomenological inputs, such as par- only the top quark.

406 CERN Courier, November 1981 The PLUTO detector moves into the PETRA ring in DESY in 1978, where it was able to provide valuable data to test quantum chromodynamics. After having been succeeded by the CELLO experiment, PLUTO is once more in the PETRA ring (see page 396).

(Photo DESY)

But how far beyond? There alrea dy exist visions which are simulta neously breathtaking and forebod ing. The weak interactions are so called because the forces are very tiny, at least for the domain of ener gies that have so far been achieved. Nevertheless electroweak theory unites these disparate interactions in a way that suggests that the effec tive weak forces grow with energy until the weak and electromagnetic forces become roughly comparable at energies corresponding to the masses of the weak bosons. Then there are the hadronic forces which, at present energies, are stronger still than the others. Electroweak and QCD theories sit side by side, compatible but sepa rate. Why not join them together in one grand unification picture? Ac cording to QCD ideas, the strong forces, loosely speaking, become progressively less strong with in creasing energy. On a 'grand unifica Within each family the members the weak currents and the mass and tion' picture one can make rough enter in characteristically different self-coupling strength parameter of a estimates of the characteristic ener roles. The electromagnetic forces (still undiscovered) scalar Higgs bos gy where all the interactions, strong are of course mediated by photons, on that is crucial to the theory in its and electroweak, become compara the weak ones by charged and neu present form. To many people, this ble. For the simplest versions, at tral vector bosons. The masses of Higgs particle looks somewhat artifi least, the answer is about 1015 GeV. these bosons are rather sharply pre cial — a kind of provisional stand-in Out there we would encounter all dicted in the standard model, in for deeper effects at a more funda kinds of new physics, new gauge terms of already known parameters. mental level. Altogether, although bosons, etc. The predicted masses lie below the standard electroweak model In the nearer future, we will get to 100 GeV and it is one of the exciting seems to serve very well for all the the 103 GeV region, with excitement objectives for the coming round of phenomena presently accessible to enough in pursuit of the weak bos experiments to produce, detect, and us, there is a widespread feeling that ons, the elusive Higgs or its dynami study these objects — or better yet, it is only a part of some more com cal equivalent, the top quark and per to find troubles. prehensive structure. haps still other quarks and leptons. Internally at least QCD looks to be The overall outlook comes close The immediate issue will be whether fairly whole and well structured. The to the ideal for a vigorous science: everything falls in with the standard standard electroweak model on the the proliferating phenomena of de ideas of QCD and electroweak theo other hand looks considerably less cades at last (perhaps) brought into ry. Even if it does, the standard pic whole or in final form. Spontaneous order, at least 'in principle', while at ture is in part only a framework, symmetry breaking gives the quarks the same time, there is the almost whose quark and lepton (and Higgs) and leptons their masses, but only in sure indication that we are not yet at content is still open. There will be terms of other adjustable paramet the bottom of things — that there is much to be learned even at the mod ers. There are additional parameters yet more out there beyond to be pur est 103 GeV level. as well — various mixing angles in sued. But one also wants to extend the

CERN Courier, November 1981 407 foundations and we are certainly not ly accessible 103GeV region and the I owe the following fragment of his going to build 1015 GeV colliders exciting prospects of 1015 GeV. We tory to my colleague A. Pais. It con (not even in Europe). Is there any way may still hope to get some indirect cerns Emile Nohel, the son of a Je we can get an indirect glimpse of this glimpses of that new world but the wish farmer in Czeckoslovakia, who new physics at the far lower energies evidence is bound to be limited. This entered the University of Prague in that will be accessible to us ? For that of course will not stop our theoretical 1904 and approached Anton Lamp- matter, is there anything beyond just brethren from speculating, but with fa for advice about his studies. aesthetics that suggests grand unifi out the restraining and guiding in Lampfa, Einstein's predecessor in cation? As to the latter question, fluence of data they are sure to run the chair of physics, advised Nohel there is already at least one impres amok. They often do, after all, even against going into physics because sive empirical indication: the sim when there is data. 'all the original work has been done, plest grand unification models give a There are really two questions. Is the laws have been established, and good quantitative explanation of the there in fact a 'Glashow desert' be important new developments are Weinberg angle, a parameter that is tween 103 and 1015 GeV? For the not to be expected.' well measured but that is unex vast bulk of the phenomena that will plained within the standard elec be accessible to us, do we already troweak model. have the basics in hand with QCD The simplest schemes lead to the and standard electroweak theory? expectation of baryon nonconserva- The HEPAP debaters were divided. tion and the instability of the proton. The theorists, proud of their recent The predicted lifetime is about 1031 accomplishments, tended toward years, give or take perhaps one or the affirmative view; the experimen Preparations for an underground experiment two powers of ten. A number of talists, equally proud, were in general by the Irvine/Brookhaven/Michigan group to search for proton decay and other new experiments are now under way in full of scorn. phenomena. This and similar experiments search of proton instability. The There is clearly room for caution. will put new physics ideas to the test. stakes are obviously very high. The establishment of non-zero neutrino masses, directly or through observation of neutrino oscillations, may also bear indirectly on the new physics of grand unification. Present evidence is not conclusive. More over baryon nonconservation, taken together with the violation of com bined charge and parity symmetries, opens up the possibility of dealing with one of the great outstanding problems of cosmology, the asym metry between matter and antimat ter in the content of the universe. The estimates are very crude as many of the dynamical details and paramet ers are matters of speculation, but the gross order of magnitude looks promising. Clearly, the idea of grand unifica tion out at 1015 GeV opens up vast new vistas. What is foreboding about all of this, however, is the pos sibility that there is nothing funda mentally new between the imminent

408 CERN Courier, November 1981