478 NATIJRE VOL.302 7 APRIL 1983 ------NEWSAND VIEWS------Particle physics GeV energy scale. In low-energy ex periments, such as neutron (J-decay, this 37 GeV is hidden and the force appears to be . the Z? weak ( ""w-<). But at high energies the 37 After the W boson . • GeV will be out in the open and the true from Frank Close strength a will be revealed. The 37 GeV is a measure of the mass of THIS week physicists at CERN, Geneva The abundance of the events and also the weak force quantum (the analogue of hope to begin the second round of ex their electron energy distribution are as ex electromagnetism's massless photon) - periments that could prove definitive in pected if a W boson has been produced and the W boson. In (J decay, energy isolating theW and Z bosons- the carriers has a mass of 81±5 GeV (UAI report) or momentum conservation prevents creation of the weak nuclear force. The results of 80:: 1 ~ GeV (UA2 report). This is truly ex of a 37 GeV quantum - the uncertainty the first experiments were announced in citing, for the unification of weak and principle can, however, allow it to exist January; the media announced the electromagnetic forces can be achieved if fleetingly. Indeed, its existence would be so discovery ofthe W boson, two independent the W mass is 82±2 Ge V. Pickwickian at low energies that it can groups of theorists claimed that the top Nuclear y decay is an 'electromagnetic' transmit the force for only Hr 15 em. The quark had also been seen, but the ex process; (3 decay, by contrast, is brought feeble strength and limited range of the perimentalists (who are, of course, closest about by the weak force. The probability weak force at low energies can thus both be to the data) have so far only claimed to find of neutron (3 decay into a proton, an elec understood. "electrons with large transverse energy and tron and an antineutrino is controlled by The germ of this theory was put forward missing associated energy ... which have Fermi's weak coupling constant G; elec by Julian Schwinger in 1957 and contained the signature of a two body decay ... fit tromagnetic effects, by contrast, involve y and W + , w-. Two decades of theoretical ting well the hypothesis that they are pro the fine-structure constant a. The weak work on gauge field theories (of which duced by a w+ or W- boson". The inter force, as its name implies, appears to be quantum electrodynamics is the simplest pretation is that the missing energy is car more feeble than the electromagnetic force example) and an increased awareness of the ried by an undetected neutrino, theW hav and its range is much less. In (3- decay and role of hidden symmetries (spontaneous ing decayed into the electron and the other low-energy phenomena the two symmetry breaking) in renormalizable neutrino. forces appear to be unrelated. field theories, culminated in the powerful The events are singularly impressive, yet, A quantitative measure of their relative modern theory due to Sheldon Glashow, as Norman Dombey (New Scientist 17 apparent strengths is given by the ratio of Abdus Salam and Steven Weinberg. February 1983) reminds us, "the ex G/a ::::::: I0-3 at an energy of I GeV. This The theory requires y, W + , w- and also perimentalists do not see a W per se ... a rather opaque caveat 'at 1 GeV' is impor a zo boson, the latter being the carrier of a single electron cannot identify a W with tant: G has dimensions of (energy)-2 new weak force (discovered in 1973). certainty". This caution, correctly exer whereas a is dimensionless ::::::: 11137. The Clebsch Gordan coefficients and other cised in the two experimental papers (UA1 dimensionless quantity G x (energy)2 can numerical factors cause the old 37 GeV to and UA2 collaborations Phys. Lett. 122B, be sizeable at high energy and comparable be modified as follows. The weak forces pages 103 and 476 respectively) has been with a in magnitude. have intrinsic electromagnetic strength if relaxed during recent weeks as more and It was realised that if effects due to the W+ =W-=82±2GeV more physicists received detailed first-hand absence of mirror symmetry in weak in Z=92±2GeV seminar reports about the discovery. There teractions were allowed for, then elec Other aspects of the theory relate the Wand is no doubt that the two teams have seen a tromagnetic and weak interactions could Z masses, so that if the theory is right, and new phenomenon and no alternative mech have identical intrinsic strength - provid if theW has indeed been detected at about anisms can satisfactorily explain them. ed the weak force contains an important 37 80 GeV, then the zo must exist with the cited 90 GeV mass. Cosmology The good news about zo is that it can decay into e- and e + which, being charged, are easy to detect. Hence a zo can be clean Quadrupole ly and unambiguously identified if it is pro duced in the experiment. The bad news is fades away that this type of event is expected to be only THERE is, after all, no quadrupole compo one-tenth as likely as theW events-as on nent currently detectable in the cosmic z ly nine of the latter have been seen so far, it microwave background, according to two 0 is no sur.prise that the zo is still undetected. 1 2 1- recent papers • • The result is significant
002R-