A large-scale map of particle physics

Guido Altarelli — on closer inspection the Standard Model does not even look like an ultimate theory.

Most theorists will admit to being dissatisfied with the current 'textbook'picture of particle physics. Here Guido Altarelli explains why.

The so-called 'Standard Model' of particle interactions is a compre­ hensive and relatively simple way of describing the fundamental for­ ces of Nature. It consists of quan­ tum chromodynamics (QCD) — the theory of quarks and gluons inter­ acting through the 'colour' force, and the 'electroweak' unification of electromagnetism and the weak nuclear force. Add Einstein's clas­ sical theory of gravity to complete the picture. This Standard Model agrees with numbers is full of intriguing but which would be an integral all the experimental information, features — the succession of three part of a more complete theory. and with the discovery of the W 'generations' each containing two Another serious deficiency is and Z carriers of the weak nuclear kinds of quark plus one kind of the absence of any quantum pic­ force at CERN in 1983, a chapter lepton is impressive and unex­ ture of gravity. Although the force of physics closed. plained. The quantization of electric of gravity is extremely weak, it is Despite all the euphoria, the ex­ charge is not implied by any Stand­ still non-zero, so that as increasing perimental evidence for the Stand­ ard Model principle (although it energies probe deeper and deeper ard Model is not complete. Not does lead to useful cancellations into matter, a level eventually only has the sixth ('top') quark yet in the theory). should be reached where quantum to be firmly established, but at a Other features whose origins re­ gravitational effects should show more fundamental level the so- main a mystery have to be fitted up. This is the so-called 'Planck called 'Higgs mechanism', which in. These include the violation of Mass' of about 1019 GeV - ten gives the weak force carriers their parity P (left-right symmetry) and million times the energy of the mass (see March issue, page 26) charge conjugation C (particle-anti- Fermilab Tevatron. has not been tested at all. Even particle symmetry), some aspects The 'Grand Unified Theories' the widely-acclaimed electroweak of the combined CP violation in (GUT) which attempt to unify the sector has only been checked to the neutral kaon system, the con­ colour force of quarks and gluons within a few per cent, contrasting servation of baryon number and with the electroweak sector natu­ with the few parts per million pre­ of the three distinct types of lep- rally involve very high energies at cision of quantum electromagne­ tons (electron, muon and tauon), which the strengths of the colour tism. and the apparent masslessness of and electroweak forces become On closer inspection, the Stand­ their associated neutrinos. comparable. These energies are ard Model does not even look like These conservation laws are not intriguingly close to the Planck an ultimate theory. Many questions protected by a theoretical principle, mass. emerge and a host of crucial prob­ in contrast to the conservation of This enormous energy sets a lems appear. A large number of electric charge which is guaranteed new horizon for physics. Can the masses and coupling strengths by having a massless photon. They world remain basically the same have to be put in artificially. The might turn out to be violated by all the way from the Tevatron to pattern of masses and quantum some as yet unknown interactions. the Planck mass (the 'desert scen-

6 CERN Courier, April 1986 Physics monitor

ario')? The so-called 'Hierarchy spins, aligned by the combined Problem' suggests it cannot. To Polarized action of a strong magnetic field have a single field theory spanning macromolecules (2.5 T) and microwave radiation such a wide energy range brings (70 GHz), become 'frozen in' at up technical difficulties. temperatures of a fraction of a These can be sidestepped by Interactions between protons and degree kelvin. In one hour, polari­ injecting new ideas, the chief cur­ neutrons depend on the orientation zations of up to 75 per cent have rent contender being 'supersym- of their intrinsic angular momentum been achieved. metry', an elegant new classifica­ (spin). Thus by lining up the spins Their spins lined up in this way, tion of particles (see January/Fe­ of the particles (polarized targets molecules of proteins, enzymes, bruary 1983 issue, page 18). Su- and/or beams), increased informa­ polymers and bacteria have been persymmetry is a vital ingredient tion can be extracted. studied using beams of polarized of the 'string' models which have Techniques for producing polar­ thermal neutrons. The research really caught on in recent months ized targets and beams have made involves an unusual collaboration (see June 1985 issue, page 18), great progress in recent years, and between CERN, the Institut Laue- rating a two-page feature in 'Time' now provide a powerful additional Langevin, Grenoble, the Max magazine earlier this year. These physics tool. Planck Institute for Molecular Gene­ models provide a very promising CERN's experience in producing tics, Berlin, the HASYLAB synchro­ framework for unifying gravity and polarized targets has found a new tron radiation Laboratory at DESY, particle interactions. outlet in the study of biological Hamburg, and the University of A number of clues hint that if macromolecules. Ingenious techni­ Mainz, under project leader Hein- the iteration of quark and lepton ques developed for handling the rich Stuhrmann. generations has not yet stopped spins of nuclear targets have been Using neutron beams at the Ger­ at three, it should do so soon. applied to gigantic molecules man GKSS research centre at However higher energies could containing hundreds of thousands Geesthacht, the results shed new well uncover the supersymmetric or even millions of nucleons. light on the behaviour of these partners of these particles. To The method is a direct extension giant molecules and provide ano­ solve the hierarchy problem these of the dynamical nuclear polariza­ ther important example of particle new supersymmetric particles tion recipe developed at CERN. physics techniques paying divi­ should be below a few thousand The specimen is dissolved in a dends in another field of research. GeV. Other new physics could mixture of heavy water and deuter- Applications can be foreseen in come from additional degrees of ated alcohol (e.g. propanediol) and many areas of molecular biology freedom (for example left-right doped with a deuterated metallo- and in the study of polymers, glas­ symmetry, which could throw light organic complex. The nucleon ses, metals, alloys, etc. on the origin of P and C symmetry violations), or from a deeper sub­ structure within quarks and lep- tons. Despite the apparent impregna­ bility of the Standard Model so far, important new physics might be just over the horizon.

At the heart of a new technique to study the structure of large molecules by lining up the spins of the component nucleons. The specimen is held in the small cryostat between the poles of the magnet. Right is the neutron beam used as a probe and left is the tube which picks up the scattered particles.

CERN Courier, April 1986 7