Carlo Rubbia

Carlo Rubbia

EXPERI ME NTAL OBSERVATI O N OF T HE I NTER ME DIATE VECT OR B OS O NS W + , W - a n d Z 0 . Nobel lect ure, 8 Dece mber, 1984 b y C A R L O R U B B I A CE R N, C H-1211 GE NE V A 23, S witzerland 1. I ntrod uctio n I n t his lect ure I s hall descri be t he disco ver y of t he tri plet of ele me ntar y p a r t i c l e s W + , W -- , a n d Z 0 - b y far t he most massi ve ele me ntar y particles pro d uce d with accelerators u p to no w. T he y are als o belie ve d t o be t he pro pagators of the weak interaction pheno mena. On a cos mological scale, weak interactions play an absol utely f un da mental r ole. F or exa m ple, it is t he wea k pr ocess 2 + H + e + v e t hat co ntrols t he mai n b ur ni n g reactio ns i n t he s u n. T he most stri ki n g feat ure of t hese p he no me na is t heir s mall rate of occ urre nce: at t he te m perat ure a n d de nsit y at t he ce ntre of t he s u n, t his b ur ni n g process pro d uces a heat release per u nit of mass w hic h is o nly l /100 t hat of t he nat ural metabolis m of t he h u m a n b o d y. It is i n d e e d t his sl o w n ess t h at m a k es t h e m s o pr e ci o us, e ns uri n g, for i nsta nce, t he a p pro priate t her mal co n ditio ns t hat are necessary for life o n eart h. T his pro pert y is directl y relate d to t he ver y lar ge mass of t he W-fiel d q ua nta. Si nce t he f u n da me ntal discoveries of He nri Becq uerel a n d of Pierre a n d Marie C urie at t he e n d of t he last ce nt ur y, a lar ge n u m ber of beta- deca y p he no me na have bee n observe d i n n uclei. T hey all a p pear to be relate d to a pair of f un da mental reactions involving transfor mations bet ween protons an d ne utr o ns: n → p + e - + v e , ( 1) Follo wi n g Fer mi [1], t hese processes ca n be descri be d pert ur bati vel y as a poi nt i nteractio n i n vol vi n g t he pro d uct of t he fo ur partici pati n g fiel ds. Hig h-e nergy collisio ns ha ve le d to t he obser vatio n of ma ny h u n dre ds of ne w ha dro nic particle states. T hese ne w particles, w hic h are ge nerall y u nstable, a p pear to be j ust as f u n da me ntal as t he ne utro n a n d t he proto n. Most of t hese ne w particle states ex hi bit wea k i nteractio n pro perties w hic h are si milar to t hose of t he n ucleo ns. T he s pectrosco py of t hese states ca n be describe d wit h t he hel p of f u n da me ntal, poi nt-like, s pi n-1 /2 fer mio ns, t he q uarks, wit h frac- tio nal electric c harges +2 /3e a n d -1 /3e a n d t hree differe nt colo ur states. T he u niversality of t he weak p he no me na is t he n well i nter prete d as a Fer mi C. R u b bi a 2 4 1 Fig, I. The muon neutrino and antineutrino charged-current total cross-section as a function of the n e utri n o e n er g y. D at a ar e fr o m t h e P arti cl e D at a Gr o u p ( R e v. M o d. P h ys. 5 6, N o. 2, P art 2, A pril 1984) reprinted at C E R N. The lines represent the effects of the W propagator. c o u pli n g occ urri n g at t he q uar k le vel [2]. F or i nsta nce, reacti o ns (1) are act uall y d ue to t he processes + ( u) ( d) + e + v e , ( 2) w here ( u) is a +2 /3e q uar k a n d ( d) a -l /3e q uar k. ( T he brac kets i n dicate t hat particles are bo u n d.) Cabibbo has s ho w n t hat u niversality of t he weak co u- pli n g t o t h e q u ar k f a mili es is w ell u n d erst o o d, ass u mi n g t h at si g nifi c a nt mi xi n g occ urs i n t he +1 /3e q uar k states [3]. Li ke wise, t he t hree le pto nic fa milies - n a m e l y ( e , v ), ( µ, v ), a n d ( τ, v ) - e x h i b i t i d e n t i c a l w e a k i n t e r a c t i o n e µ τ be ha vi o ur, o nce t he differe nces i n masses are ta ke n i nt o acc o u nt. It is n ot kno wn if, in analogy to the Cabibbo pheno menon, mixing occ urs also a mongst t he ne utri no states ( ne utri no oscillatio ns). T his has le d to a very si m ple pert urbati ve mo del i n w hic h t here are t hree q uar k c urre nts, b uilt u p fr o m t he ( u, d c ), ( c, s c ), a n d (t, b c ) p airs (t he s ubscri pt C i n dicates Cabibbo mixi ng), a n d t hree le pto n c urre nts fro m (e, v e), ( µ, v µ ), a n d (τ, pairs. Eac h of t hese c urre nts has t he sta n dar d vector for m [ 4] J µ = f 1 ( 1 - γ 5 ) f 2 . A n y of t he pair pro d ucts of c urre nts J µ , j µ , will r el at e t o a basic fo ur-fer mio n i nteractio n occ urri ng at a stre ngt h deter mi ne d by t he universal Fer mi constant G F : 2 4 2 Physics 1984 Fig. 2a. Feyn man diagra m of virtual W exchange mediating the weak process [reaction (2)] e + Fi g. 2 b. F e y n m a n di a gr a m f or t h e dir e ct pr o d u cti o n of a W p arti cl e. N ot e t h at t h e q u ar k transfor mation has been replaced by a quark-antiquark annihilation. - 5 - 2 w h e r e G F =1.16632 x 10 G e V ( h = c = l ) . T his pert ur bati ve, p oi nt-li ke descri pti o n of wea k pr ocesses is i n excelle nt agree me nt wit h ex peri me nts, u p to t he hig hest q 2 experi ments perfor med with the high-energy ne utrino bea ms (Fig. 1). We kno w, ho wever, that s uch a pert urbative calc ulation is inco m plete an d unsatisfactory. Accor ding to q uan- t u m mec ha nics, all hig her-or der ter ms must also be included: they appear, ho wever, as q ua dratically divergent. F urther more, at centre-of- mass energies greater t ha n abo ut 300 Ge V, t he first-or der cross-sectio n violates co nservatio n of pr o b a bilit y. It was Os kar Klei n [5] w ho, i n 1938, first s u g geste d t hat t he wea k i nterac- tio ns co ul d be me diate d by massive, c harge d fiel ds. Alt ho ug h he ma de use of Y uka wa’s i dea of co nstr ucti ng a s hort-ra nge force wit h t he hel p of massive fiel d q ua nta, Klei n’s t heory establis he d also a close co n nectio n bet wee n elec- tro magnetis m an d weak interactions.

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