EXPERI ME NTS WIT H HI G H-E NER GY NEUTRI N O BEA MS Nobel Lect ure, Dece mber 8, 1988 b y J ACK STEI NBER GER CE R N, Geneva, S witzerland, and Scuola Nor male Superiore, Pisa, Italy 1. I NTR O D UCTI O N High-energy ne utrino bea ms have fo un d intensive an d varie d a p plication in p arti cl e p h ysi cs e x p eri m e nt ati o n i n t h e l ast d e c a d es. T his r e vi e w is c o n- str ai n e d t o a f e w of t h e m ost fr uitf ul e x a m pl es: t h e dis c o v er y of n e utr al c urrents, the meas ure ment of the Weinberg angle, the st u dy of weak c urre nts a n d t he co nse q ue nt test of t he electro wea k t heor y, t he st u d y of n ucleo n q uark str uct ure, an d the testing of quantu m chro mo dyna mics ( Q C D). Ot her st u dies s uc h as t he pro d uctio n of “ pro m pt” ne utri nos, t he s e ar c h f or fi nit e n e utri n o m ass es a n d n e utri n o os cill ati o ns, t h e s e ar c h f or hea vy le pto ns or ot her ne w particles, or t he meas ure me nt of proto n a n d ne utro n str uct ure f u nctio ns, elastic a n d pse u doelastic cross-sectio ns, a n d ot her excl usive processes, are not disc usse d here. Ne utri no ex peri me nts have bee n p urs ue d vigoro usly at t he Brook have n Natio nal Laboratory, at Fer milab, an d at C E R N. It is fair to say that they have ma de large contrib u- tions to o ur un derstan ding of particle physics. 2. NE UTRI N O BE A MS Present ne utrino bea ms are pro d uce d in fo ur ste ps: i) pro d uction of secon- dar y ha dro ns i n t he collisio n of hi g h-e ner g y proto ns o n a fixe d tar get; ii) mo ment u m (charge) selection an d foc using of the ha drons; iii) passage of t he bea m t hro ug h a n (e vac uate d) decay regio n, lo ng e no ug h to per mit a s ubstantial fraction of the ha drons to decay; iv) absor ption of the re maining h a dr o ns a n d t h e m u o ns t h at ar e pr o d u c e d al o n g wit h t h e ‘ n e utri n os i n a s hiel d of a de q uate t hic k ness. T he t wo-bo d y deca ys an d acc o u nt f or ~ 97 % of t he ne utri no fl ux i n prese nt bea ms. Positive ha drons pro d uce ne utrinos, negative ha drons pro d uce a nti ne utri nos. Fig ures la a n d lb gi ve a n i m pressio n of t he t wo ha dro n bea m-for ming options that are available, si de by si de, at C E R N: a conven- tional, so-calle d narro w-ban d bea m ( N B B), an d an achro matic, Van der Meer horn-foc use d, wi de-ban d bea m ( W B B). The ne utrino s pectra pro- d uce d b y t hese t w o bea ms are ver y differe nt, as s h o w n i n Fi g ure 2. T he 4 8 8 Physics 1988 Fi g ure 1 a Sketch of narro w-band and wide-band neutrino bea m layouts at C E R N, sho wing disposition of pri mary target, focusing ele ments, decay region, shielding, and monitoring de vices. Fi g ur e 1 b Vie w of the neutrino bea m tunnel at the C E R N S P S in 1976, before operations began. T h e N B B li n e is s e e n i n t h e c e ntr e; o n t h e ri g ht is t h e p uls e tr a nsf or m er f or t h e W B B h or n, b ut t h e h or n its elf, d esti n e d f or t h e p e d est al o n t h e l eft, is n ot y et i nst all e d. At t h e f ar e n d, t h e 2. 5 m dia meter titaniu m windo w of the evacuated decay region can be seen. 4 8 9 Fi g u r e 2 Neutrino and antineutrino energy spectra, calculated for the horn-focused W B B and the more conventional N B B. W B Bs are c haracterize d b y hi g h i nte nsit y, a stee p ( ge nerall y u n desira ble) energy fall-off, an d a s ubstantial conta mination of wrong-“sign” ne utrinos. T h e N B Bs h a v e l o w er i nt e nsit y, a fl at e n er g y d e p e n d e n c e i n t h e c o ntri b u- tion fro m each of the t wo decays, and s mall wrong-sign background. They also ha ve t he i m porta nt feat ure t hat t he e ner g y of t he ne utri no ca n be kno wn, s ubject to a t wofol d π - K dichoto my, if the decay angle is kno wn. In general this can be inferre d fro m the i m pact para meter of the event in the detect or. 3. DETE CT ORS The lo w cross-sections of ne utrinos are reflecte d in t wo general feat ures of ne utri n o detect ors: i) t hey are massi ve; ii) t he tar get ser ves also as detector. In the seventies, the most s uccessf ul detectors were large b ubble cha mbers. T h e m ost s pl e n di d of t h es e w er e t h e cr y o g e ni c d e vi c es b uilt at C E R N a n d Fer mila b, eac h wit h a vol u me of ~ 1 5 m 3 , i n l ar g e m a g n eti c fi el ds, a n d ca pa ble of o perati n g wit h li q ui d h y dro ge n, de uteri u m, or neo n. A pict ure of a ty pical ne utrino event in the C E R N cha mber is sho wn in Fig ure 3. It is a n exa m ple of t he “c harge d-c urre nt” ( C C) reactio n v + N → µ − + ha dro ns. Ho wever, one of the major discoveries at C E R N was ma de not in this but in a lar g e Fr e o n-fill e d b u b bl e c h a m b er, aff e cti o n at el y c all e d G ar g a m ell e. T h e a cti v e v ol u m e w as a c yli n d er 4. 8 m l o n g a n d 1. 9 m i n di a m et er, f or a v ol u m e of a b o ut 13 m 3 , i nsi d e a m a g n et pr o d u ci n g a fi el d of 2 T. Fi g ur e 4 gi v es so me i m pressio n of its size. The bubble cha mber has no w been largely replace d by detectors base d on electro nic detectio n met ho ds. As a n exa m ple, I me ntio n here t he C D HS P h ysics 1988 Figure 3 A typical neutrino event as ob- Fi g ure 4 Pre parati o n of t he i nteri or of t he served in the Big European Bubble Cha m- 1 3 m 3 bubble cha mber Garga melle, later to b er ( B E B C) fill e d wit h n e o n at t h e C E R N b e fill e d wit h Fr e o n.