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. We no w kno w that his pre monitory visio n is e mbo die d i n t he electro weak t heory of Glas ho w, Wei nberg a n d Sala m [ 6], w hi c h will b e dis c uss e d i n d et ail l at er i n t his l e ct ur e. It is w ort h q u oti n g Kl ei n’s vi e w dir e ctl y: C. R u b bi a 2 4 3

‘ T he r ole of these particles, a nd their properties, bei ng si milar to those of the photo ns, we m a y per h a ps call t he m “electr o- p h ot o ns ” ( n a mel y electric all y c h ar ge d p h ot o ns). ’

I n t he prese nt lect ure I s hall follo w to day’s pre vale nt notatio n of W + a n d W - for t hese particles-fro m ‘ wea k’ [7] - alt ho u g h o ne m ust reco g nize t hat Klei n’s defi nitio n is no w m uc h more perti ne nt. T he basic Fey n ma n diagra ms of reactio n (2) are t he o nes s ho w n i n Fig. 2a. T he ne w, di me nsio nless co u pli ng co nsta nt g is t he n i ntro d uce d, relate d to f o r q 2 < < T h e V - A nat ure of t he Fer mi i nteractio n r e q uir es t h at t h e s pi n J of t h e W p arti cl e b e 1. It is w ort h r e m ar ki n g t h at i n Klei n’s pa per, i n a nal o g y t o t he p h ot o n, J = 1 a n d g = a. T h e a p p a r e n t l y excelle nt tit of t he ne utri n o data t o t he f o ur-fer mi o n p oi nt-li ke i nteracti o n ( Fi g. 1) i n dicates t hat m w is ver y lar ge ( ≥60 Ge V /c 2 ) a n d is c o m pati ble wit h

2. Pr o d ucti o n of W p articles Direct pro d uctio n of W particles follo we d b y t heir deca y i nto t he electro n- ne utri no is s ho w n i n Fig. 2b. T he ce ntreof- mass e nergy i n t he q uark-a nti- q uark collisio n m ust be large e no ug h, na mely T he cross-sectio n aro u n d t he reso na nce will follo w a c haracteristic Breit- Wig ner s ha pe, re mi nis- ce nt of n uclear p hysics ex peri me nts. T he cross-sectio n is easily calc ulate d:

w h er e is t he re d uce d q uark wavele ngt h i n t he ce ntre of mass. Q uark a n d a ntiq uark m ust ha ve i de ntical colo urs. T he i nitial-state wi dt h

- 7 3 4 . 5 x 1 0 m ( Ge V) calc ulate d fro m G F is s ur prisi n gl y wi de: na mel y, for G e V / c 2 as pre dicte d b y S U(2) x U(1) t he or y, Me V. T he t ot al wi dt h de pe n ds o n t he n u mber of q uark a n d le pto n ge neratio ns. Taki ng

N q = 3 a n d a g ai n f or - 1 0 0 G e V, w e fi n d G e V. At t he pea k of t he res o na nce,

w h er e is t h e br a n c hi n g r ati o f or t h e i n c o mi n g c h a n n el. Of co urse q uark-a ntiq uark collisio ns ca n not be realize d directly si nce free q uar ks are n ot a vaila ble. T he cl osest s u bstit ute is t o use c ollisi o ns bet wee n protons an d anti protons. The fraction of n ucleon mo ment u m carrie d by the q uar ks a n d a nti q uar ks i n a proto n is s ho w n i n Fi g. 3. Beca use of t he prese nce of a ntiq uarks, proto n- proto n collisio ns also ca n be efficie ntly use d to pro d uce W particles. Ho we ver, a sig nifica ntly greater bea m e nergy is nee de d a n d t here is n o wa y of i de ntif yi n g t he directi o ns of t he i nc o mi n g q uar k a n d a nti q uar k. As we s hall see, t his a mbig uity will pre ve nt t he obser vatio n of i m porta nt asy m me- tries associate d wit h parity ( P) a n d c harge ( C) violatio n of weak i nteractio ns. T he ce ntre-of- mass e nergy i n t he q uark-a ntiq uark collisio n is r el at e d t o b y t h e w ell- k n o w n f or m ul a, 2 4 4 Physics 1984

2 Fi g. 3. Str u ct ur e f u n cti o ns F 2 , x F 3 , a n d measured in different experi ments, for fixed Q vers us x, pl ott e d ass u mi n g T h e el e ctr o m a g n eti c str u ct ur e f u n cti o n measured by the E M C ( European Muon Collaboration) and the BFP [ Berkeley ( L B L) - F N A L- Princeton] is co mpared wit h t h e c h ar g e d- c urr e nt str u ct ur e f u n cti o n using the 18/5 factor fro m the average charge squared of the quarks. No correction has been applied for the difference bet ween the strange and char m 2 s e a q u ar ks, s o t h e i nt er pr et ati o n is F 2 = x[ q t - 3/ 5(s + - c - C)]. (I n t his Q ra n ge, is d e pl et e d b y a si mil ar a m o u nt d u e t o c h ar m t hr es h ol d eff e cts i n t h e tr a nsiti o n c.) T he a nti q u ar k distri b uti o n m e as ur e d fr o m a nti n e utri n o s c att eri n g is T h e s oli d li n es 0. 5 5 3. 2 8 0. 5 5 ( 1- x) 3. 2 , Relative nor- h a v e t h e f or ms: F 2 = 3 . 9 x ( 1- x) +1.1(1-x) , x F 3 = 3 . 6 x m ali z ati o n f a ct ors h a v e b e e n fitt e d t o o pti mi z e a gr e e m e nt b et w e e n t h e diff er e nt d at a s ets, a n d absolute changes have been arbitrarily chosen as indicated. [ References: C D HS-- H. Abra mo wicz et al., Z. P h ys. C 1 7, 283 (1983); C CF R R-F. Sciulli, private co m munication; E M C-J.J. Aubert et al., P h ys. L ett. 1 0 5 B, 322 (1981); and A. Ed wards, private co m munication; B F P-- A. R. Clark et al., P h ys. R e v. L ett. 5 1, 1 8 2 6 ( 1 9 8 3); and P. Meyers, Ph. D. Thesis, L B L-17108 (1983), Univ. of C alif., B er k el e y. C o urt es y J. C arr, L B L.]

N ote t hat acc or di n g t o Fi g. 3, i n or der t o e ns ure t he c orrect c orrelati o n bet wee n t he q uar k of t he proto n (a n d t he a nti q uar k of t he a nti proto n) t he e n er g y s h o ul d b e s u c h t h at ≥ 0 . 2 5 Therefore there is one broad opti mu m C. R u b bi a 2 4 5

Fig. 4a, b. Production cross-sections of inter mediate vector bosons for proton-antiproton collisions. The mass is para metrized with τ = N ot e i n Fi g. 4 a t h e s m all pr o b a bilit y of wr o n g q u ar k- a nti q u ar k a s si g n m e nt s. T h e pri nt s i n Fi g. 4 b r el at e t o m a s s pr e di cti o n s f or t h e S U(2) x U(1) model. e nergy ra nge for t he proto n-a nti proto n collisio ns for a gi ve n W mass. For

2 m w = 8 0 G e V / c , Ge V. T he pro d uctio n cross-sectio n for t he pr ocess

± ± ± W + X , W → e + v e

( w here X de notes t he frag me ntatio n of s pectator parto ns) ca n be easily e val uate d by fol di ng t he narro w reso na nce wi dt h o ver t he p a n d mo mentu m distri b uti o ns ( Fi g. 4). F or m w = 82 Ge V /c a n d Ge V, o ne fi n ds 1 0 - 3 3 c m 2 .

3. Proto n-a ntiproto n collisio ns T he o nl y practical wa y of ac hie vi n g ce ntre-of- mass e ner gies of t he or der of 500 G e V is t o c olli d e b e a ms of pr ot o ns a n d a nti pr ot o ns [ 8]. F or a l o n g ti m e s u c h a n i dea ha d bee n co nsi dere d as u n practical beca use of t he lo w de nsity of bea ms w he n use d as tar gets. 2 4 6 Physics 1984

b

Fi g. 4 C. R u b bi a 2 4 7

fr o m Ref. [9]. Pr ot o ns (100 Ge V /c) are perio dically extracte d i n s hort b ursts a n d pro d uce 3.5 Ge V /c a nti proto ns, w hic h are acc u m ulate d a n d c o ol e d i n t h e s m all st a c ki n g ri n g. T h e n ar e r ei nj e ct e d i n a n R F b u c k et of t h e m ai n ri n g a n d accelerate d to to p e ner g y. T he y colli de hea d o n a gai nst a b u nc h fille d wit h proto ns of e q ual e ner g y a n d rotati n g i n t he o p posite directio n.

T he rate R of e ve nts of cr oss-secti o n for t wo co u nter-rotati ng bea m

b unches colli ding hea d on, with freq uency a n d n l a n d n 2 p articl es, is

w h er e is t h e ( c o m m o n) b e a m r a di us, a n d t h e n u m eri c al f a ct or l / 4 t a k es i nt o acco u nt t he i ntegratio n over Ga ussia n profiles. For o ur ex peri me nt, ty pically c m a n d c m 2 . T heref ore a n d a ver y l ar g e pro d uct is nee de d to o verco me t he ‘geo metry’ effect. The sche me use d in the present ex peri mental progra m me has been dis- c uss e d b y R u b bi a, Cli n e a n d M cI nt yr e [ 9] a n d is s h o w n i n Fi g. 5. It m a k es us e of the existing 400 Ge V C E R N Proton Synchrotron ( PS) [10], s uitably mo di- fie d i n or der to be able to store co u nter-rotati ng b u nc hes of proto ns a n d a nti proto ns at a n e nergy of 270 Ge V per bea m. A nti proto ns are pro d uce d by c ollisi o ns of 2 6 G e V / c pr ot o ns fr o m t h e P S o nt o a s oli d t ar g et. A c c u m ul ati o n i n a s mall 3.5 Ge V /c st ora ge ri n g is f oll o we d b y st oc hastic c o oli n g [ll] t o co m press p hase s pace. I n Table 1 t he para meters of Ref. [9] are gi ve n. Ta ki n g into acco unt that the original pro posal was for m ulate d for another machine, na mel y t he Fer mila b s y nc hrotro n ( Bata via, Ill.) t he y are q uite close to t he co n ditio ns realise d i n t he S PS co nversio n. Details of t he acc u m ulatio n of anti protons are describe d in the acco m panying lect ure by Si mon van der M e er. The C E R N ex peri ments with proton-anti proton collisions have been the first, a n d so far t he o nl y, exa m ple of usi n g a stora ge ri n g i n w hic h b u nc he d proto ns a n d a nti proto ns colli de hea d o n. Alt ho ug h t he C E R N C olli der uses b u nched b e a ms, as d o t h e colli ders, t he p hase-s pace da m pi ng d ue to synchrotron ra diation is no w absent. F urt her more, si nce a nti proto ns are 2 4 8 Physics 1984

T a bl e 1. List of p ar a m et ers (fr o m R ef. [ 9])

1. M AI N RI N G (Fer milab) - Bea m mo mentu m 2 5 0 ( 4 0 0) G e V/ c - Equivalent laboratory energy for 133 (341) T e V - Accelerating and bunching frequency 5 3. 1 4 M H z - Har monic nu mber 1113 - R F p e a k v olt a g e/t ur n 3. 3x 1 0 6 v - Residual gas pressure < 0 . 5 x 1 0 - 7 T o r r - B et a f u n cti o ns at i nt er a cti o n p oi nt 3. 5 m - M o m e nt u m c o m p a cti o n at i nt. p oi nt - 0 m - 1 2 I n v ari a nt e mitt a n c es ( N p = 1 0 ) - l o n git u di n al 3 e V.s - tra ns verse 5 0 π 1 0 - 6 r a d . m - Bunch length 2. 3 m - D esi g n l u mi n osit y 5 x 1 0 2 9 ( 8 x 1 0") c m - 2 s - 1

2. A N TIP R O T O N S O U R C E (Stochastic Cooling [11] - N o mi n al st or e d mo mentu m 3. 5 G e V/ c - Circu mference of ring 1 0 0 m - M o m e n t u m a c c e p t a n c e 0. 0 2 - Betatron acceptances 1 0 0 π 1 0 - 6 r a d . m - Band width of mo mentu m stochastic cooling 4 0 0 M H z - Maxi mu m stochastic accelerating RF voltage 3 0 0 0 v - B a n d wi dt h of b et atr o n st o c h asti c c o oli n g 2 0 0 M H z

1 0 Final invariant e mittances ( N p = 3 x 1 0 ) - l o n git u di n al 0. 5 e V.s - tra ns verse 1 0 π 1 0 - 6 r a d . m

scarce, o ne has to o perate t he colli der i n co n ditio ns of relati vel y lar ge bea m- bea m i nteractio ns, w hic h is not t he case for t he co nti n uo us proto n bea ms of the previo usly o perate d Intersecting Storage Rings (IS R) at C E R N [12]. One of t he most re mar ka ble res ults of t he Colli der has probably bee n t he fact t hat it has o perate d at s uc h hi g h l u mi nosit y, w hic h i n t ur n mea ns a lar ge bea m-bea m t u ne s hift. I n t he early days of co nstr uctio n, very serio us co ncer n ha d bee n voice d regar di ng t he i nstability of t he bea ms d ue to bea m-bea m interaction. The bea m-bea m force can be a p proxi mate d as a perio dic s ucces- sio n of extre mel y no n-li near pote ntial kic ks. It is ex pecte d to excite a co nti n- u u m of reso na nces of t he stora ge ri n g w hic h has, i n pri nci ple, t he de nsit y of ratio nal n u mbers. Re d uce d to bare esse ntials, we ca n co nsi der t he case of a weak a nti proto n bea m colli di ng hea d o n wit h a stro ngly b u nc he d proto n bea m. T he i ncre me nt, d ue t o t he a n g ular kic k ∆ x’, of t he acti o n i n varia nt W = of a n a nti pr ot o n is a n d t his c a n b e e x pr ess e d i n t er ms of t h e ‘t u n e s hift’, A Q as If w e n o w ass u me t hat t he s uccessive kicks are ra n do mize d, t he seco n d ter m of ∆ W a vera ges t o zer o, a n d we get =

F o r t h e d e s i g n l u m i n o s i t y w e n e e d A Q - 0 . 0 0 3 , l e a d i n g t o W ) = 7 . 1 x 1 0 - 4 . T his is a ver y lar ge n u m ber i n dee d, gi vi n g a n e-f ol d i n- C. R u b bi a

Fi g. 6. Maxi mu m allo wed bea m-bea m tune-shift para meter, XI- Y, as a function of energy of the el e ctr o n- p ositr o n c olli d er S P E A R. O n e c a n s e e a dr a m ati c dr o p i n t h e all o w e d t u n e s hift at l o w er energies, as a consequence of the reduced synchrotron da mping. Extrapolation to the case of proton-antiproton collisions where the da mping is absent and therefore the da mping ti me is c o nst a nt, is t o b e i d e ntifi e d wit h t h e b e a m lif eti m e, p er mitti n g a n i nfi nit esi m al t u n e s hift a n d therefore to an unpractical lu minosity. cr e as e of W i n o nl y l / 7. 1 x 1 0 - 4 = 1. 4 1 x 1 0 3 kicks! T herefore t he o nly reaso n w h y t he a nti proto n motio n re mai ns sta ble is beca use t hese stro n g kic ks are not r a n d o m b ut p eri o di c, a n d t h e b e a m h as a l o n g ‘ m e m or y’ w hi c h all o ws t h e m t o be a d de d co here ntly rat her t ha n at ra n do m. Off-reso na nce, t he effects of t hese kic ks t he n ca ncel o n t he a vera ge, gi vi n g a n o verall zero a m plit u de gro wt h. T he bea m- bea m effects are ver y diffic ult, if not i m possi ble, to e val uate t heoretical- l y, si n c e t hi s a pri ori p urely deter mi nistic proble m ca n ex hibit stoc hastic be havio ur a n d irreversible diff usio n-like c haracteristics. A meas ure me nt at t he electro n- positro n colli der S P E A R at Sta nfor d ha d f urt her aggravate d t he ge neral co ncer n abo ut t he viability of t he c olli der sc he me. Re d uci n g t he e ner g y of t he electr o n c olli der ( Fi g. 6) res ulte d i n a s maller val ue of t he maxi m u m allo we d t u ne s hift, i nter prete d as bei ng d ue to the re duce d synchrotron ra diation da mping. Equating the nee de d bea m life- ti m e f or t h e colli der ( w here da m pi ng is abse nt) wit h t he extra polate d d a m p i n g t i m e o f a n e + e - colli der gives a maxi m u m allo we d t une shift 1 0 - 5 ÷ 1 0 - 6 , w hic h is catastro p hically lo w. T his bleak pre dictio n was n ot c o nfir me d b y t he ex perie nce at t he c olli der, w here per cr ossi n g, a n d six crossi ngs are ro uti nely ac hieve d wit h a bea m l u mi nosity lifeti me a p proac hi ng o ne day. W hat, t he n, is t he reaso n for s uc h a striki ng co ntra dic- tion bet ween ex peri ments with protons an d those with electrons? The differ- 2 5 0 Physics 1984 e nce is ca use d b y t he prese nce of s y nc hr otr o n ra diati o n i n t he latter case. T he e missio n of sy nc hrotro n p hoto ns is a major so urce of q uick ra n do mizatio n bet wee n crossi ngs a n d lea ds to a ra pi d deterioratio n of t he bea m e mitta nce. Fort unately, the sa me pheno menon also provi des us with an effective da m ping mec ha nis m. T he colli der works beca use bot h t he ra n do mizi ng a n d t he da m ping mechanis ms are absent. This un us ually favo urable co mbination of effects has e ns ure d t hat colli ders ha ve beco me viable de vices. T he y ha ve t he pote ntial for s ubsta ntial i m prove me nts i n t he f ut ure. T he acc u m ulatio n of more a nti proto ns wo ul d per mit us to obtai n a s ubsta ntially larger l u mi nosity, a n d a project is u n der way at C E R N w hic h is ex pecte d to be able to deli ver eno ugh anti protons to acc u m ulate, i n o ne si n gle day, t he i ntegrate d l u mi nosity o n w hic h t he res ults prese nte d i n t his lect ure ha ve bee n base d ( ~ 100 nb - 1 ).

4. The detectio n method T he process we wa nt to obser ve is t he o ne re prese nte d i n Fig. 2b, na mely

± ± ± W + X , W e + v e , ( 3) w here X re prese nts t he s u m of t he debris fro m t he i nteractio ns of t he ot her proto ns (s pectators). Alt ho ug h t he detectio n of hig h-e nergy electro ns is rela- tively straightfor war d, the observation of ne utrino e mission is unco m mon in colli ding-bea m ex peri ments. The probability of secon dary interactions of the ne utri no i n a ny co ncei vable a p parat us is i nfi nitesi mal. We m ust t herefore rely o n ki ne matics i n or der to si g nal its e missio n i n directl y. T his is ac hie ve d wit h a n a p pro priately desig ne d detector [13] w hi c h is u nif or ml y s e nsiti v e, o v er t h e w h ole s oli d a n gle, t o all t he c har ge d or ne utral i nteracti n g de bris pr o d uce d b y t he c ollisi o n. Si nce c ollisi o ns are o bser ve d i n t he ce ntre of mass, a si g nifica nt mo ment u m i mbalance may signal the presence of one or more non-interacting particles, pres u mably ne utrinos. The metho d can be conveniently i m ple mente d with calori meters, since their e nergy res po nse ca n be ma de rat her u nifor m for differe nt i nci de nt particles. Calori metr y is also i deall y s uite d to t he acc urate meas ure me nt of t he e ner g y of the acco m panying high-energy electron for process (3). Energy de positions ( Fi g. 7) i n i n di vi d ual cells, Ei, are co n verte d i nto a n e ner g y flo w vector w here is t h e u nit v e ct or p oi nti n g fr o m t h e c ollisi o n p oi nt t o (t h e ce ntre of) t he cell. T he n, f or relati vistic particles a n d f or a n i deal cal ori meter res po nse pro vi de d no no n-i nteracti ng particle is e mitte d. T he s u m c o v ers t h e w h ol e s oli d a n gl e. I n r e alit y t h er e ar e fi nit e r esi d u es t o t h e s u m: T his q ua ntity is calle d t he ‘ missi ng e nergy’ vector. Ob vio usly i n t he case of a ne utri n o e missi o n, I n t he case of pr ocess (3) t he effect is partic ularl y s pectac ular, si nce i n t he ce ntre of mass of t he W t he neutrino mo mentu m is v er y l ar g e. T he practical realizatio n of s uc h a detector [14] is s ho w n i n Fi g. 8a. After mo ment u m analysis in a large-i mage drift cha mber in a horizontal magnetic fiel d of 7000 G orie nte d nor mal to t he bea m directio ns, six co nce ntric sets of fi nely seg me nte d calori meters (Fig. 8 b) s urro u n d t he collisio n poi nt, do w n to C. R u b bi a 2 5 1

CONSTRUCTION OF ENERGY VECTORS

Fig. 7. Principal diagra m for constructing energy vectors and the missing energy of the event

a n gles of 0.2º wit h res pect to t he bea m directio ns. T he o peratio n of t hese calori meters is s ho w n sc he maticall y i n Fi g. 9a. T he first fo ur se g me nts are sa n d wic hes of lea d a n d sci ntillator, i n w hic h electro ns are ra pi dly absorbe d ( Fi g. 9 b), f oll o w e d b y t w o s e cti o ns of ir o n /s ci ntill at or s a n d wi c h ( w hi c h is als o. t he ret ur n yoke of t he mag netic fiel d). All ha dro ns are co m pletely absorbe d wit hi n t hese calori meters. M uo ns are detecte d by eig ht pla nes of large drift c ha mbers w hic h e nclose t he w hole detector vol u me. If o ne or more m uo ns are detecte d, their mo menta, meas ure d by magnetic c urvat ure, m ust be a d de d ‘by h a n d’ t o t h e e n er g y fl o w v e ct or. The perfor mance of the energy flo w meas ure ment has been teste d with 2 5 2 Physics 1984

b)

MA GNETIC CURVATURE ELE MENTARY S OLID AN GLE CR OSSIN G I C. R u b bi a 2 5 3

FRACTION OF ENERGY DEPOSITED Fi g. 9. a) S c h e m a of a n el e m e nt ar y s oli d- a n gl e c ell. Aft er f o ur s e g m e nts of l e a d/s ci ntill at or s a n d wi c h, t h er e ar e t w o el e m e nts of ir o n/s ci ntill at or s a n d wi c h, w hi c h is als o t h e m a g n eti c fi el d return loop. b) Energy depositions for high-energy pions and electrons. The nature of the particle c a n b e dis cri mi n at e d l o o ki n g at t h e tr a nsiti o n c ur v e.

sta n dar d collisio ns ( mi ni m u m bias). Fig. 10 s ho ws ho w well t he vertical co m ponent of the missing-energy vector is observe d for mini m u m bias events. T he missi ng e nergy resol utio n for eac h tra nsverse co m po ne nt ca n be para metrize d as w here , i n u nits of G e V, is t h e s c al ar s u m of t he tra ns verse co m po ne nts of t he e nergy flo w T he sa me para metri- zatio n also hol ds for e ve nts w hic h co ntai n hig h tra ns verse mo me nt u m jets, a n d for w hic h t he detector no n- u nifor mities are more critical si nce e nergy de posi- ti o n is hi g hl y l o c ali z e d ( Fi g. 1 1). T h e r es ol uti o n f u n cti o n is s h o w n i n Fi g. 1 2, w here t he missi n g e ner g y for t wo-jet e ve nts is s ho w n alo n g wit h a Mo nte Carlo calc ulatio n of t he ex pecte d distrib utio n base d o n t he ex pecte d be ha vio ur of t he calori meters as deter mine d by test-bea m data an d the meas ure d frag mentation f u ncti o ns of jets. F or a t y pi c al e v e nt wit h G e V, we meas ure the transverse co m po- n e nts of t o a b o ut 4 The longitudinal co mponent of the mo mentu m b al a n c e will n ot b e us e d i n t h e pr es e nt a n al ysis si n c e, i n s pit e of t h e s m all n ess of t h e wi n d o w t hr o u g h w hi c h t h e b e a m pi p es p ass e ner getic particles q uite ofte n esca pe t hro ug h t he a pert ure. 2 5 4 Physics 1984

Fi g. 1 0. Scatter-plot of the vertical co mponent of missing transverse energy versus the total transverse energy observed in all calori meter cells.

5. Observatio n of t he e + v si g nal T h e observation by t he U Al Collaboratio n [15] of t he c harge d i nter me diate vector boson was re porte d in a pa per p ublishe d in Febr uary 1983, follo we d s hortly by a parallel pa per fro m t he U A2 Collaboratio n [16]. Mass val ues were g i v e n : m 2 2 w = ( 8 0 ± 5 ) G e V / c ( U A1) a n d G e V / c ( U A2). Si nce t he n, t he ex peri me ntal sa m ples have bee n co nsi derably i ncrease d, a n d o ne ca n no w procee d much further in un derstan ding the pheno menon. In particular, t he assig n me nt of t he eve nts to reactio n (3) ca n no w be prove d rat her t ha n post ulate d. We s hall follo w here t he a nal ysis of t he U Al e ve nts [17]. O ur res ults are base d o n a n i nte grate d l u mi nosit y of 0.136 p b - 1 . W e first perfor me d a n i ncl usive searc h for hig h-e nergy isolate d electro ns. T he trigger selectio n re q uire d t he prese nce of a n e nergy de positio n cl uster i n t he electro- ma g netic cal ori meters at a n gles lar ger t ha n wit h tra ns verse e ner g y i n excess of 1 0 G e V. I n t h e e v e nt r e c o nstr u cti o n t his t hr es h ol d w as i n cr e as e d t o 1 5 G e V, lea di n g to abo ut 1. 5 x 1 0 5 bea m-bea m collision events. B y re q uiri n g t he prese nce of a n ass ociate d, is olate d trac k wit h Ge V/c i n t he ce ntral detect or, we re d uce d t he sa m ple b y a fact or of a b o ut 100. Next, a maxi m u m energy de position (leakage) of 600 Me V was allo we d in the ha dron calori meter cells after t he electro ma g netic co u nters, lea di n g to a sa m ple of 346 C. R u b bi a 2 5 5

Fi g. 1 1. Missing-energy resolution for mini mu m-bias and jet events e ve nts. We t he n classifie d e ve nts accor di ng to w het her t here was pro mi ne nt jet a cti vit y. We f o u n d t hat i n 291 e ve nts t here was a clearl y visi ble jet wit hi n a n azi m ut hal a ngle co ne o p posite to t he ‘electro n’ trac k. T hese e ve nts were str o n gl y c o nta mi nate d b y jet-jet e ve nts i n w hic h o ne jet fa ke d t he electro n si g nat ure a n d ha d to be rejecte d. We were left wit h 55 e ve nts wit ho ut a ny jet, or wit h a jet not back-to-back wit h t he ‘electro n’ wit hi n 30”. T hese eve nts ha d a very clea n electro n sig nat ure (Fig. 13) a n d a perfect matc hi ng bet wee n t he poi nt of electro n i nci de nce a n d t he ce ntroi d i n t he s ho wer detec- 2 5 6 Physics 1984

Fig. 12. Transverse energy balance observed for a sa mple of t wo-jet events. To convert the h ori z o nt al s c al e t o t h e n u m b er of st a n d ar d d e vi ati o ns ( n), us e t h e r el ati o ns hi p Variables have been chosen in such a way as to transfor m a Gaussian basic response of the calori meters into a li n e ar pl ot. T h e c o nti n u o us li n e is t h e r es ult of a c al c ul ati o n b as e d o n t h e e x p e ct e d c al ori m et er responses, as measured with test-bea m particles. C. R u b bi a 2 5 7

b)

Fi g. 1 3. Distrib utio ns s ho wi ng t he q uality of t he electro n sig nat ure: a) The energy de position in the ha dron calori meter cells behin d the 27 ra diation lengths (r. l.) of t he e. m. s ho wer detector. b) T h e fr a cti o n of t h e el e ctr o n e n er g y d e p osit e d i n t h e f o urt h s a m pli n g ( 6 r.l. d e e p, aft er 1 8 r.l. co n verter) of t he e. m. s ho wer detector. T he c ur ve is t he ex pecte d distrib utio n fro m test-bea m data. c) As distrib utio n (b) b ut for t he first sa m pli ng of t he e. m. s ho wer detector (first 6 r.l.). 2 5 8 Physics 1984

Fi g. 1 4. T h e distri b uti o n of t h e missi n g tr a ns v ers e e n er g y f or t h os e e v e nts i n w hi c h t h er e is a si n gl e electr o n wit h E T > 1 5 G e V, a n d n o c o pl a n ar jet a cti vit y. T h e c ur v e r e pr es e nts t h e r es ol uti o n f u nctio n for no missi ng e nergy nor malize d to t he t hree lo west missi ng-e nergy eve nts. tors, f urt her s u p porti ng t he abse nce of co m posite overla ps of a c harge d track a n d ne utral ex pecte d fr o m jets. T he b ulk of t hese e ve nts was c haracterize d by t he prese nce of ne utri no e missio n, si g nalle d b y a si g nifica nt missi n g e ner g y (see Fi g. 14). Accor di n g to the ex peri mental energy resol utions, at most the three lo west missing-energy eve nts were co m patible wit h no ne utri no e missio n. T hey were excl u de d by t he c ut >15 Ge V. We were t he n left wit h 52 e ve nts. I n or der to e ns ure t he best acc uracy i n t he electro n e nergy deter mi natio n, o nl y t hose e ve nts were retai ne d i n w hic h t he electro n trac k hit t he electro ma g- C. R u b bi a 2 5 9

Fi g. 15a. Missing transverse energy squared versus T for all verified events which have m or e t h a n 4 St. d e v. fr o m z er o f or all e v e nts wit h e + v decays re moved. The events are l a b ell e d a c c or di n g t o t h eir t o p ol o g y. netic detectors more than ±15° a way fro m their to p an d botto m e dges. The sa m ple was t he n re d uce d to 43 e ve nts. A n alter nati ve selecti o n was carrie d o ut, base d o n t he i ncl usi ve prese nce of a si g nifi c a nt mi s si n g e n er g y [ 1 8]. T hi s i s ill u str at e d i n Fi g. 1 5 a, w h er e all e v e nt s wit h missi n g e ner g y i n excess of 4 sta n dar d de viatio ns are s ho w n. O ne ca n see t hat pre vio usly selecte d electro n e ve nts are fo u n d as a s ubset of t he sa m ple. Ho wever, a significant n u mber of a d ditional events (t wenty-seven) were also recor de d, i n w hic h t here was eit her a jet or a n electro ma g netic cl uster i nstea d of t he is olate d electr o ns ( Fi g. 15 b). E vi de ntl y t he i ncl usi ve missi n g-e ner g y defi nitio n i m plies a broa der class of p hysical p he no me na (Fig. 16 c) t ha n t he si m ple e + v deca y ( Fi gs. 16a, b). As t he st u d y of t hese e ve nts [19] is be y o n d t he sc o pe of t his lect ure, it will n ot be p urs ue d a n y f urt her. We pr ocee de d t o a detaile d i n vesti gati o n of t he e ve nts i n or der t o el uci date t heir p hysical origi n. T he large missi ng e nergy observe d i n all of t he m was i nter prete d as bei ng d ue to t he e missio n of o ne or se veral no n-i nteracti ng ne utri nos. A very stro ng correlatio n i n a ngle a n d e nergy was obser ve d (i n t he pla ne nor mal to t he colli di ng bea ms, w here it co ul d be deter mi ne d acc urately), 2 6 0 Physics 1984

A

A

Fi g. 1 5 b.

wit h t he corres po n di ng electro n q ua ntities, i n a c haracteristic back-to-back c o nfi g ur ati o n e x p e ct e d fr o m t h e d e c a y of a m assi v e, sl o w p arti cl e ( Fi gs. 1 7 a, b) . T his s u g g est e d a c o m m o n p h ysi c al ori gi n f or t h e el e ctr o n a n d f or o n e or s e v er al ne utri n os. I n or der to ha ve a better u n dersta n di n g of t he tra ns verse motio n of t he electro n- ne utri no(s) syste m, we st u die d t he ex peri me ntal distrib utio n of t he resultant transverse mo mentu m obtaine d by a d ding the ne utrino(s) an d electro n mo me nta (Fig. 18). T he average val ue was Ge V /c. Five e ve nts w hic h ha d a visi ble jet ha d als o t he hi g hest val ues of Tra nsverse mo ment u m balance was al most exactly restore d when the vector mo ment u m of t he jet was a d de d. T he ex peri me ntal distrib utio n was i n goo d agree me nt wit h the many theoretical expectations fro m quantu m chro modyna mics ( Q C D) for t he pro d uctio n of a massive state via t he Drell- Ya n q uark-a ntiq uark a n ni hila- ti o n [ 2 0]. T h e s m all fr a cti o n ( 1 0 %) of e v e nts wit h a j et w er e t h e n e x pl ai n e d as har d gl uo n bre msstra hl u n g i n t he i nitial state. Se veral differe nt h y pot heses o n t he p h ysical ori gi n of t he e ve nts were teste d by looki ng at ki ne matical q ua ntities co nstr ucte d fro m t he tra nsverse variables of t he electro n a n d t he ne utri no(s). We retai ne d t wo possibilities, na mely: i) t he t wo-bo dy decay of a massive particle i nto t he electro n a n d o ne ne utri no, C. R u b bi a 2 6 1

Fi g. 1 6 b. T h e s a m e a s pi ct ur e ( a), e x c e pt t h at n o w o nl y p arti cl e s wit h Ge V/c and c al ori m et ers wit h E T >1 Ge V are sho wn. 2 6 2 Physics 1984

Fi g. 1 6 c. E v e nt of t h e t y p e, j et + missi n g e n er g y. O nl y tr a c ks wit h p T > 1. 5 G e V/ c a n d c ells wit h

E T > 1.0 Ge V are displayed.

e + v e ; a n d ii) t he t hree-bo dy decay i nto t wo, or possibly more, ne utri nos a n d t h e el e ctr o n. It c a n b e s e e n fr o m Fi gs. 1 9 a a n d 1 9 b t h at h y p ot h esis (i) is stro ngly favo ure d. At t his stage, t he ex peri me nt co ul d not disti ng uis h bet wee n o ne or se veral closel y s pace d massi ve states. Wit h t he hel p of a sa m ple of isolate d ha dro ns at lar ge tra ns verse mo me nta, we esti mate d i n detail t he possible so urces of backgro u n d co mi ng fro m or di- nary ha dro nic i nteractio ns, a n d we co ncl u de d t hat t hey were negligible ( C O.5 e ve nts). ( For more details o n bac k gro u n d, we refer t he rea der to Ref. 20.) Ho wever, we ex pect to get so me backgro u n d eve nts fro m ot her decays of t he W, n a m el y:

or C. R u b bi a 2 6 3

Fi g. 17a. T wo-di mensional plot of the transverse co mponents of the missing energy (neutrino m o m e nt u m). E v e nts h a v e b e e n r ot at e d t o bri n g t h e el e ctr o n dir e cti o n t o p oi nt al o n g t h e v erti c al axis. The striking back-to-back configuration of the electron-neutrino syste m is apparent.

T hese e ve nts were ex pecte d to co ntrib ute at o nly t he lo w- p T p a rt of t h e electro n s pectr u m, a n d co ul d e ve n be eli mi nate d i n a more restricti ve sa m ple. A val ue of t he W mass ca n be extracte d fr o m t he data i n a n u m ber of wa ys: i) It can be obtaine d fro m the incl usive transverse mo ment u m distrib ution of t h e el e ctr o ns ( Fi g. 1 9 a), b ut t h e dr a w b a c k of t his t e c h ni q u e is t h at t h e transverse mo ment u m of the W particle m ust be kno wn. Taking the Q C D pre dictio ns [21], in reasonable agree ment with experi ment, we obtaine d m w=(80.5±0.5) Ge V/c 2 . ii) We ca n defi ne a tra ns verse mass variable, ( 1 wit h

t he pro perty m T ≤ m w , w here t he eq uality wo ul d hol d for o nly t hose eve nts wit h no lo ngit u di nal mo me nt u m co m po ne nts. Fitti ng Fig. 19b to a Physics 1984

Electr o n tra nsverse e ner gy ( Ge V)

Fi g. 1 7 b. Correlation bet ween the electron and neutrino transverse energies. The neutrino co mpo- n e nt al o n g t h e el e ctr o n dir e cti o n is pl ott e d a g ai nst t h e el e ctr o n tr a ns v ers e ener g y.

co m mon val ue of the mass was done al most in de pen dently of the trans-

2 verse m oti o n of t he W particles, m w Ge V/c . It s h o ul d b e n ot e d t hat t he l o wer part of t he distri b uti o n i n was sli g htl y affecte d b y decays an d other backgro un ds. iii) We ca n defi ne a n e n ha nce d tra nsverse mass distrib utio n, selecti ng o nly eve nts i n w hic h t he decay ki ne matics is largely do mi nate d by t he tra ns- verse varia ble wit h t he si m ple c uts Ge V /c. T he res ulta nt distrib utio n (Fig. 19c) t he n s ho we d a relatively narro w peak at a p proxi- mately 76 Ge V /c 2 . Mo del- depen dent corrections no w only contrib ute d to

t he differe nce bet wee n t his average mass val ue a n d t he fitte d m w v al u e, 2 m w = (80.9±1.5) Ge V /c . A n i nt er esti n g u p p er li mit t o t h e wi dt h of t h e W was also deri ve d fro m t he distrib utio n, na mely Ge V/c 2 ( 9 0 % co nfi de nce le vel). C. R u b bi a 2 6 5

Fi g. 1 8. T he tra nsverse mo me nt u m distrib utio n of t he W derive d fro m o ur eve nts usi ng t he electro n a n d missi ng tra nsverse e nergy vectors. T he hig hest e v e nts h a v e a visi bl e jet (s h o w n i n bl a c k i n t he fi g ure). T he data are co m pare d wit h t he t heoretical pre dictio ns for W pro d uctio n base d o n Q C D ( R ef. [ 2 1]). 2 6 6 Physics 1984

Fi g. 1 9 a. T he electr o n tra ns verse e ner g y distri b uti o n. T he t w o c ur ves s h o w t he res ults of a lit of t he e n ha nce d tra nsverse mass distrib utio n to t he hy pot heses a n d T h e first h y p ot hesis is clearl y preferre d.

The three mass deter minations gave very si milar res ults. We preferre d to r et ai n t h e r es ult of m et h o d (iii), si n c e w e b eli e v e d it t o b e t h e l e ast aff e ct e d b y s yste matic effects, e ve n if it ga ve t he lar gest statistical error. T wo i m porta nt co ntri b utio ns ha d to be a d de d to t he statistical errors: i) Co u nter-to-co u nter calibratio ns. T h e y w er e esti m at e d t o b e 4 % r. m.s. I n t h e deter mi natio n of t he W mass t his effect was greatl y atte n uate d to a ne gli gible le vel, si nce ma n y differe nt ele me nts co ntrib ute d to t he e ve nt s a m pl e. ii) C ali br ati o n of t he a bs ol ute e ner g y sc ale. T his was esti mate d t o be %, a n d of co urse affects bot h t he Z 0 an d the W sa m ples by the sa me m ulti plicative f act or. O nce t he decay reactio n was establis he d, t he lo ngit u di nal mo- me nt u m of t he electro n- ne utri no syste m was deter mi ne d wit h a t wofol d a mbig uity for the un meas ure d longit u dinal co m ponent of the ne utrino mo men- t u m. The overall infor mation of the event was use d to establish mo ment u m a n d e nergy co nservatio n bo u n ds i n or der to resolve t his a mbig uity i n 70 % of t he cases. Most of t he re mai ni ng e ve nts ha d sol utio ns w hic h were q uite close, C:. R u b bi a 2 6 7

Fi g. 1 9 b. The distribution of the transverse mass derived fro m the measured electron and neutrino vectors. The t wo curves sho w the results of a lit to the hypotheses e + v a n d e +v +v.

Fi g. 1 9 c. The enhanced electron-neutrino transverse mass distribution (see text). The t wo curves sho w the results of a fit to the hypotheses W e + v a n d X e + v + v . 2 6 8 P hysics 1984

Fi g. 2 0 a. T h e fr a cti o n al b e a m e n er g y x w carried by the W. The curve is the prediction obtained by assu ming that the W has been produced by f usi o n. N ote t hat i n ge neral t here are t w o

kine matic solutions for x w (see text), which are resolved in 70 % of the events by consideration of the energy flo w in the rest of the event. Where this a mbiguity has been resolved, the preferred

kine matic solution has been the one with the lo west x w . In the 30 % of the events where the

a mbiguity is not resolved, the lo west x w solution has therefore been chosen. a n d t he p hysical co ncl usio ns were nearly t he sa me for bot h sol utio ns. T he fractional bea m energy x w c arri e d b y t h e W p arti cl e is s h o w n i n Fi g. 2 0 a, a n d it a p pears to be i n excelle nt a gree me nt wit h t he h y pot hesis of W pro d uctio n i n a n ni hilatio n [22]. Usi ng t he well-k no w n relatio ns a n d we deter mi ne d t he releva nt parto n distrib utio ns i n t he proto n a n d a nti pr ot o n. It ca n be see n t hat t he distri b uti o ns are i n excelle nt a gree me nt wit h t he ex pecte d x distrib utio ns for q uarks a n d a nti q uarks i n t he proto n a n d a nti proto n, res pecti vely (Figs. 20b a n d 20c). Co ntrib utio ns of t he u a n d d q uarks were also neatly se parate d by looki ng at t he c harges of pro d uce d W e ve nts, si nce W + a n d W - ( Fi gs. 20 d a n d 20e).

6. Observatio n of the parity (charge co nj ugatio n) violatio n, a nd deter mi natio n of t he s pi n of t he W p article O ne of t he m ost rele va nt pr o perties of wea k i nteracti o ns is t he vi olati o n of parity a n d c harge co nj ugatio n. Evi de ntly t he W particle, i n or der to me diate C. R u b bi a

ANTIPROTON ANTIQUARKS

Fi g. 2 0 6. The x-distribution of the proton quarks producing the W by qq fusion. The curve is the pr e di cti o n a s s u mi n g q q f u si o n. Fi g. 2 0c. T he sa me as Fi g. 2 0 b f or t he a nti pr ot o n q uar ks.

wea k processes, m ust also ex hibit t hese pro perties. F urt her more, as alrea dy me ntio ne d, t he V- A nat ure of t he fo ur-fer mio n i nteractio n i m plies t he assig n- me nt J = 1 f or its s pi n. B ot h of t hese pr o perties m ust be verifie d ex peri me ntall y. A c c or di n g t o t h e V- A t h e or y, weak i nteractio ns s ho ul d act as a lo ngit u di nal polarizer of t he W particles, si nce q uar ks (a nti q uar ks) are pro vi de d b y t he proto n (a nti proto n) bea m. Like wise, decay a ng ular distrib utio ns fro m a polar- izer are ex pecte d to ha ve a large asy m metry, w hic h acts as a polarizatio n analyser. A strong back war d-for war d asy m metry is therefore ex pecte d, in w hic h electro ns ( positro ns) prefer to be e mitte d i n t he directio n of t he proto n (a nti proto n). I n or der to st u dy t his effect i n de pe n de ntly of W- pro d uctio n mec ha nis ms, we have looke d at t he a ng ular distrib utio n of t he e missio n a ngle of t he electro n ( positro n) wit h res pect to t he proto n (a nti proto n) directio n i n t he W ce ntre of mass. O nly e ve nts wit h no reco nstr uctio n a mbig uity ca n be us e d. W e v erifi e d t h at t his d o es n ot bi as t h e distri b uti o n i n t h e v ari a bl e c os Accor di ng to t he ex pectatio ns of V- A t heory t he distrib utio n s ho ul d be of t he t y p e i n excelle nt agree me nt wit h t he ex peri me ntal data (Fig. 21). T he parit y violatio n para meters a n d t he s pi n of t he W particle ca n be 2 7 0 P hysics 1984

Fi g. 2 0 d. T h e s a m e as Fi g. 2 0 b b ut f or quarks in the proton (antiproton). Fi g. 2 0e. T he sa me as Fi g. 2 0 b b ut f or d(ii) q uar ks i n t he pr ot o n (a nti pr ot o n). deter mi ne d directl y. It has bee n s h o w n b y Jac o b [23] t hat f or a particle of ar bitrar y s pi n J, o ne ex pects = w here ( µ) a n d are t he gl o bal helicit y of t he pr o d ucti o n s yste m ( u d) a n d of t he decay syste m (e v), res pecti vely. For V- A, we t he n ha ve = = - 1, J = 1, l e a di n g t o t h e m a xi m al v al u e 〈c os F or J = O it is o b vi o us t h at ( c os a n d for a n y ot her s pi n v a l u e 〈c o s θ * ≤ l /6. Ex peri me ntally we fi n d 〈c o s =0.5±0.1, w hic h s u p ports b ot h t he J = 1 assi g n me nt a n d maxi mal helicit y states at pro d uctio n a n d d e c a y. N ot e t h at t h e c h oi c e of si g n = ( h) = ± 1 c a n n ot b e s e p ar at e d, i. e. rig ht- a n d left- ha n de d c urre nts, bot h at pro d uctio n a n d decay, ca n not be resolve d witho ut a polarization meas ure ment.

7. T ot al cr oss-secti o n a n d li mits t o hi g her m ass W’s The integrate d l u minosity of the ex peri ment was 136 nb - 1 , a n d it is k n o w n t o a b o ut ± 15 % u ncertai nt y. I n or der t o get a clea n W e v e sa m ple we selecte d 47 e ve nts wit h Ge V /c. T he conta mination in the sa m ple was esti mate d to be 2±2 events. The event acce ptance was co m p ute d to be 0.65, pri maril y beca use of i) t he Ge V /cc ut (0.80); ii) t he jet vet o re q uire- C. R u b bi a 2 7 1

Fi g. 2 1. The angular distribution of the electron e mission angle i n t h e r est fr a m e of t h e W aft er correction for experi mental acceptance. Only those events have been used in which the electron charge is deter mined and the kine matic a mbiguity (see text) has been resolved. The latter require ment has been corrected for in the acceptance calculation.

me nt wit hi n (0.96 ±0.02); iii) t he electro n-trac k isolatio n re q uire- me nt (0.90±0.07); a n d i v) t he acce pta nce of e ve nts d ue to geo metry (0.94±0.03). T he cross-sectio n was t he n

= 0.53 ±0.08 ( ±0.09) n b, w here t he last err or ta kes i nt o acc o u nt s yste matic err ors. T his val ue is i n excelle nt agree me nt wit h t he ex pectatio ns for t he Sta n dar d Mo del [22]: n b. N o e ve nt wit h or i n excess of t he ex pecte d distri b utio n for e ve nts was o bser ve d. T his res ult ca n be use d t o set a li mit t o t he p ossi ble existe nce of very massive W-like objects ( W’) decayi ng i nto electro n- ne utri no p air s. W e f o u n d B) pb at 90 % co nfi de nce le vel, corres po n di ng to 170 Ge V /c 2 , when stan dar d co u plings an d q uark distrib utions were use d to e val uate t he cross-sectio ns. 2 7 2 P hysics 1984

Fi g. 22. Exa mples of decay modes of the W particle: a ) b) c) d) t + b F o r t h e e v e n t s o f t y p e (d), one can reconstruct the invariant masses of the W particle and of the decaying t-quark jet ( Fi g. 2 2 e).

8. U ni vers alit y of t he W c o u pli n g T h e W fi el d s h o ul d e x hi bit a u ni v ers al c o u pli n g str e n gt h f or all t h e f u n d a m e n- t al l e pt o n d o u bl ets a n d all t h e q u ar k d o u bl ets. T his i m pli es - a p art fr o m s m all p hase-s pace correctio ns - e q uality of t he bra nc hi ng ratios of t he decay pro- cesses ( 4 a) ( 4 b) ( 4c)

Li ke wise, i n t he case of t he q uar k deca y c ha n nels ( 4 d)

c s C , ( 4 e)

t b C , ( 4f)

w here t is t he sixt h q uar k (t o p q uar k) pr o vi de d it exists wit hi n t he ki ne matic ra nge of reactio n (4f). Neglecti ng p hase-s pace correctio ns, w hic h are probably C. R u b bi a 2 7 3

Fi g. 2 2 6. i m porta nt for reactio n (4f), we ex pect e q ualit y of t he bra nc hi n g ratios, wit h a n o verall factor of 3 of e n ha nce me nt wit h res pect to le pto nic c ha n nels [(4a) to ( 4 c)] d u e t o c ol o u r c o u nti n g. T he s u bscri pt C i n c ha n nels (4 d) t o (4f) i n dicates t he prese nce of t he Cabibbo mixi ng. Reactio ns (4a) a n d (4 d) are i m plie d b y t he res ults of Sectio n 5. Reactio ns (4 b), (4c), a n d (4e) ha ve bee n observe d, a n d wit hi n abo ut ±20 % t hey a p pear to have t he correct bra nc hi ng ratios. So me e ve nts w hic h are belie ve d to be e vi de nce for t he process (4f) ha ve also bee n re porte d [24]. T he y are i nter prete d for t he reactio n

(l electro n or m uo n).

T h e a n d b c q uar ks are ‘ ha dro nize d’ i nto jets. Data are ro u g hl y co nsiste nt wit h G e V / c 2 . E xa m ples of reacti o ns (4 b), (4 c), (4 e), a n d (4f) are s h o w n i n Fi gs. 2 2 a- d, r es p e cti v el y. T heref ore, wit hi n t he li mite d statistics t here is e vi de nce f or u ni versalit y. 2 7 4 P hysics 1984

Fi g. 2 2 c.

9. Ca n we derive weak i nteractio ns fro m W-particle observatio ns? A n u mber of pro perties of weak i nteractio ns as deter mi ne d by lo w-e nergy ex peri me nts ca n no w be ex plai ne d as a co nseq ue nce of t he ex peri me ntally observe d pro perties of t he W particles. I n dee d we k no w t hat W ± m ust co u ple to vale nce q uarks at pro d uctio n a n d to (e v ) p airs at d e c a y, w hi c h i m pli es t h e existe nce of t he beta- deca y processes a n d

T he mass val ue m w a n d t he cross-sectio n meas ure me nt ca n t he n be use d to - 5 - 2 calc ulate G F , t h e F e r m i c o u p l i n g c o n s t a n t : G F = ( 1 . 2 ± 0 . 1 ) x 1 0 G e V . T h us t he W- pole sat urates t he observe d weak i nteractio n rate. T he i nteractio n m ust be vect or si nce J = 1, a n d parit y is maxi mall y vi olate d si nce = ± 1. T he o nly missi ng ele me nt is t he se paratio n bet wee n V + A a n d V- A alter nati ves. For t his p ur pose a polarizatio n meas ure me nt is nee de d. It may be a c c o m plis h e d i n t h e n e ar f ut ur e b y st u d yi n g, f or i nst a n c e, t h e d e c a y W a n d u si n g t h e τ decay as t he polarizatio n a nalyzer or pro d uci ng i nter me diate vector bosons (I V Bs) with longit u dinally polarize d protons. T he u ni versality of co u pli ngs a n d t he decay mo des of particles of differe nt fla v o urs i nt o differe nt le pt o n fa milies ca n als o be ex pecte d o n t he basis of t he obser vatio ns of t he ot her decay mo des of t he W particles.

1 0. Observatio n of the ne utral boso n Z 0 We exte n de d o ur searc h to t he ne utral part ner Z 0 , res po nsi ble for ne utral c urre nts. As i n o ur previo us work, pro d uctio n of I V Bs was ac hieve d wit h pr ot o n-a nti pr ot o n c ollisi o ns at Ge V i n t he U Al detector, exce pt C. R u b bi a 2 7 5

Fi g. 2 2 d. 276 P hysics 1984

t hat we no w searc he d for electro n a n d m uo n pairs rat her t ha n for electro n- -ne utrino coinci dence. The process is then

Z 0 + X , Z 0 → e + + e - or µ + µ -.

T his reacti o n is a p pr oxi matel y a fact or of 10 less fre q ue nt t ha n t he c orres p o n d- i ng W ± le pto nic decay c ha n nels. A fe w eve nts of t his ty pe were t herefore ex pecte d i n o ur m uo n or electro n sa m ples. E vi de nce for t he existe nce of t he Z 0 i n t he ra nge of masses accessible to t he U Al ex peri me nt has also bee n deri ve d fro m weak-electro magnetic interference experi ments at the highest P E T R A e nergies, w here deviatio ns fro m poi nt-like ex pectatio ns have bee n re porte d ( Fi g. 2 3). We first l o o ke d at e ve nts of t he t y pe [ 2 5, 2 6]. As i n t h e c as e of t h e W ± searc h, a n electro n sig nat ure was defi ne d as a localize d e nergy de positio n i n t wo co nti g uo us cells of t he electro ma g netic detectors wit h Er >25 Ge V, a n d a s mall (or no) e nergy de positio n Me V) i n t he C. R u b bi a 2 7 7

Fi g. 2 3. Experi mental evidence for a weak-electro magnetic interference effect in the process e + e - + at hi g h-e ner g y c olli di n g bea ms. It ca n be see n t hat data are better fitte d if t he presence of a finite mass m Z propagator is assu med. ha dron calori meters i m me diately behin d the m. The isolation req uire ment was define d as the absence of charge d tracks with mo menta a d ding u p to more than 3 Ge V /c of transverse mo ment u m an d pointing to war ds the electron cl uster cells. T he effects of t he s uccessi ve c uts o n t he i n varia nt electro n-elec- tr o n mass are s h o w n i n Fi g. 24. F o ur e + e - eve nts s urvive d c uts, co nsiste nt wit h a co m mo n val ue of (e + e -) i nvaria nt mass. O ne of t hese eve nts is s ho w n i n Fi gs. 25 a n d 26. As ca n be see n fr o m t he e ner g y de p ositi o n pl ots ( Fi g. 27), t he do mi na nt feat ure of t he fo ur eve nts is t wo very pro mi ne nt electro mag netic e nergy de positio ns. All eve nts a p pear to bala nce t he visible total tra nsverse e nergy co m po ne nts; na mely, t here is no evi de nce for t he e missio n of e nergetic ne utri nos. Exce pt for t he o ne trac k of e ve nt D w hic h tra vels at less t ha n 15” parallel t o t he ma g netic fiel d, all trac ks are s h o w n i n Fi g. 28, w here t he mo menta meas ure d in the central detector are co m pare d with the energy de positio n i n t he electro ma g netic calori meters. All trac ks b ut o ne ha ve co nsist- ent energy an d mo mentu m measure ments. The negative track of event C sho ws a val ue of (9±1) Ge V /c, m uch s maller than the corres pon ding de posi- tio n of (49±2) Ge V. T his e ve nt ca n be i nter prete d as t he li kel y e missio n of a har d ‘ photon’ acco m panying the electron. T he sa me feat ures are a p pare nt also fro m t he e ve nts i n w hic h a pair of m uo ns [27] were e mitte d. A s har p peak (Fig. 29) is visible for hig h- mass di m uo ns. Wit hi n t he statistical acc uracy t he eve nts are i nco m patible wit h a d ditional ne utrino e mission. They are all co mpatible with a co m mon mass v al u e:

( = Physics 1984

G e V; b) as a b o v e, a n d a tr a c k wit h Ge V /c a n d projectio n le n gt h of more t ha n 1 c m p oi nti n g t o t h e cl ust er. I n a d diti o n, a s m all e n er g y d e p ositi o n i n t h e h a dr o n c al ori m et ers i m m e di at el y b e hi n d ( < 0. 8 G e V) e ns ur es t h e el e ctr o n si g n at ur e. Is ol ati o n is r e q uir e d wit h Ge V /c for all ot her tracks poi nti ng to t he cl uster. c) T he seco n d cl uster also has a n isolate d track. co nsiste nt wit h t he val ue meas ure d for Z 0 → e + e -:

w here t he first err or acc o u nts f or t he statistical err or a n d t he sec o n d f or t he u ncertai nty of t he o verall e nergy scale of t he calori meters. T he a verage val ue f or t h e ni n e Z 0 eve nts fo u n d i n t he U Al ex peri me nt is Ge V/c 2 , w here t he error i ncl u des syste matic u ncertai nties. C. R u b bi a 2 7 9

Fi g. 25. E ve nt dis pla y. All reco nstr ucte d vertex-associate d trac ks a n d all calori meter hits are dis pla ye d.

Fi g. 2 6 T h e s a m e as Fi g. 2 5, b ut t hr es h ol ds ar e r ais e d t o Ge V /c for c harge d tracks a n d G e V f or c al ori m et er hits. W e r e m ar k t h at o nl y t h e el e ctr o n p air s ur vi v es t h es e mil d c uts. P hysics 1984

Fig. 27. Electro magnetic energy depositions at angles wit h r es p e ct t o t h e b e a m dir e cti o n f or t h e f o ur el e ctr o n p air s.

Negative tracks Positive tracks

Fig. 28. Magnetic deflection in 1/p units co mpared with the inverse of the energy deposited in the electro magnetic calori meters. Ideally, all electrons should lie on the l/ E = 1/p line. C. R u b bi a 2 8 1

Fig. 29. Invariant mass distribution of dilepton events fro m U Al and U A2 experi ments. A clear peak is visible at a mass of about 95 Ge V/c 2 .

T he i ntegrate d l u mi nosity for t he prese nt data sa m ple is 108 nb - 1 , wit h a n esti mate d u ncertai nty of 15 %. Wit h t he geo metrical acce pta nce of 0.37, t he cross-sectio n, calc ulate d usi n g t he fo ur e ve nts, is

w h er e t h e l ast err or i n cl u d es t h e s yst e m ati cs fr o m t h e a c c e pt a n c e a n d fr o m t h e l u mi nosity. T his val ue is i n goo d agree me nt bot h wit h Sta n dar d Mo del p r e d i c t i o n s [ 2 2 ] a n d w i t h o u r r e s u l t s f o r Z 0 + e +e - , n a m e l y pb. Fro m t he electro n a n d t he m uo n c ha n nels we obtai n t he a verage cross-sectio n of

5 8 ± 2 1( ± 9) p b.

1 1. Co mpari ng theory with experi me nt T he ex peri me nts disc usse d i n t he previo us sectio n have s ho w n t hat t he W particle has m ost of t he pr o perties re q uire d i n or der t o be t he carrier of wea k i nteractio ns. T he prese nce of a narro w dile pto n peak has bee n see n aro u n d 95 Ge V /c 2 . Rates a n d feat ures of t he e ve nts are co nsiste nt wit h t he hy pot hesis 2 8 2 P hysics 1984

T a b l e 2 . W ± a n d Z 0 para meters fro m the U Al and U A2 experi ments

U A I U A 2

e v) 5 2 ” 3 7 b 2 8 0 . 9 ± 1 . 5 ± 2 . 4 8 3 . 1 ± 1 . 9 ± 1 . 3 m w ( G e V / c ) (90 % C L) G e V ( n b ) 0 . 5 3 ± 0 . 0 8 ± 0 . 0 9 0 . 5 3 ± 0 . 1 0 ± 0 . 1 0

µ v) 1 4

2 + 6 m w ( G e V / c ) 8 1. 0 - 7 ( n b ) 0 . 6 7 ± 0 . 1 7 ± 0 . 1 5

N ( Z 0 + e + e - ) 2 9 2 . 7 ± 1 . 7 ± 1 . 4 m z 0 ( G e V / c ) 9 5 . 6 ± 1 . 4 ± 2 . 9 ( 9 0 % C L) S 8 . 5 G e V S 6 . 5 G e V ( n b ) 0 . 0 5 ± 0 . 0 2 ± 0 . 0 0 9 0 . 1 1 + 0 . 0 4 + 0 . 0 2

8 5 . 6 ± 6 . 3 ( n b ) 0 . 1 0 5 + 0 . 0 5 + 0 . 1 5 0.226±0.015 0.216±0.010±0.007

c os 0.968±0.045 1.02 ±0.06

1 5 G e V / c G e V / c c ( E , > 2 0 G e V )

t hat t he ne utral part ner of t he W ± has i n dee d bee n obser ve d. At prese nt t he statistics are not s ufficie nt to test t he for m of t he i nteractio n ex peri me ntall y; neit her has parity violatio n bee n detecte d. Ho we ver, t he precise val ues of t he masses of Z 0 a n d W ± n o w a vaila ble c o nstit ute a critical test of t he i dea of u nificatio n bet wee n weak a n d electro mag netic forces, a n d i n partic ular of t he pre dicti o ns of t he S U(2) X U(1) t heory of Glas ho w, Wei nberg a n d Sala m [6]. A caref ul acco u nt of s yste matic errors is nee de d i n or der to e val uate a n a vera ge bet ween the mass deter mination for the t wo colli der ex peri ments, U A1 an d U A2 [28]. Table 2 s u m marizes all ex peri me ntal i nfor matio n relate d to W ± a n d Z 0 . T he c harge d vector boso n mass is

= ( 8 0. 9 ± 1. 5) G e V / c 2 (statistical err ors o nl y),

t o w hic h a 3 % e ner g y scale u ncertai nt y m ust be a d de d. I n t his re p ort a val ue f or t h e Z 0 m a s s, Ge V/c 2 , has been given. Neglecting syste m- atic errors, a mass val ue is fo u n d wit h so me w hat s maller errors:

= ( 9 5. 6 ± 1. 4) G e V / c 2 (statistical err ors o nl y),

to w hic h t he sa me scale u ncertai nty as t hat for t he W ± a p plies. The q uote d err ors i n cl u d e: i) t h e n e utr al wi dt h of t h e Z 0 p e a k, w hi c h is f o u n d t o b e G e V / c 2 (90 % co nfi de nce le vel); ii) t he ex peri me ntal resol utio n of co u nters; a n d iii) t he r. m.s. s prea d bet wee n calibratio n co nsta nts of i n di vi d ual ele me nts. A l l o w e d a t o n e l o o p l e v e l - /

Fi g. 30. Co mparison bet ween the Standard Model and the experi mental results ( U A1 and U A2 co mbined). Theory is fro m Ref. [29].

It s ho ul d be re marke d t hat t he masses of t he I V Bs ha ve t he follo wi ng pr e di cti o n:

m w = = w here t he val ue Ar re prese nts t he effect of t he hi g her-or der ra diati ve correc- tio ns, a n d t he seco n d eq uatio n ca n be use d as a defi nitio n of t he Wei nberg a n g l e Si nce G F a n d a are k n o w n, c a n b e e l i m i n a t e d b e t w e e n e q uatio ns:

1

A = (37.2810±0.0003) Ge V.

Ra diati ve correctio ns are q uite lar ge [29] a n d detecta ble at t he prese nt le vel of acc urac y. Calc ulati o ns of or der l n m) gi v e t h e f oll o wi n g r es ult:

∆ r = 0.0696±0.0020, 2 8 4 P hysics 1984 w hic h is i nse nsiti ve to t he para meters

= 0. 2 1 7,

2 2 = 40 Ge V /c , m b = 5 Ge V /c .

T he mai n effect ca n be u n derstoo d as bei ng a r u n ni ng co u pli ng co nsta nt, n a m el y:

a = 1 /137.035962, at q 2 = 0,

a = 1 / 1 3 7. 5, at q 2 =

I n Fi g. 30 we ha ve plotte d m Z a gai nst m w . T he elli ptical s ha pe of t he err ors r efl e cts t h e u n c ert ai nt y i n t h e e n er g y s c al e. It c a n b e s e e n t h at t h er e is e x c ell e nt agree me nt wit h t he ex pectatio ns of t he S U(2)x U(1) Sta n dar d Mo del [29].

2 We ca n t he n extract t he re nor malize d val ue of si n at mass scale m w .

I nserti ng t he val ue of m w o ne fi n ds = 0.220 ±0.009,

I n excelle nt agree me nt wit h t he re nor malize d val ue of de d uce d fro m ne utral-c urre nt ex peri me nts. Usi ng t he i nfor matio n of t he Z 0 mass, o ne ca n deter mi ne t he para meter r el at e d i m m e di at el y t o t h e is os pi n of t h e Hi g gs p arti cl e:

Usi ng t he ex peri me ntal val ues, o ne fi n ds

= 1.000±0.036, i n perfect agree me nt wit h t he pre dictio n of 1 f or a Hi g gs d o u blet. Let us p oi nt o ut t hat 9 de viates fr o m 1 at m ost b y 3 %, o wi n g t o ra diati ve c orrecti o ns i nvolvi ng possible ne w fer mio n ge neratio ns. T he prese nt val ue see ms to i n di- c at e n o s u c h n e w f er mi o n f a mili es. We co ncl u de t hat, wit hi n errors, t he observe d ex peri me ntal val ues are co m pletely co m patible with the S U(2)x U(1) mo del, th us s u p porting the hy pot hesis of a u nifie d electro weak i nteractio n.

A C K N O W L E D G E M E N T S

T his l e ct ur e is b as e d o n t h e w or k of t h e U Al C oll a b or ati o n t e a m, a n d I w o ul d like to ex press my a p preciation of their re markable achieve ments which have le d to so ma ny exciti ng res ults. At prese nt t he follo wi ng perso ns are me mbers of t he collaboratio n: G. Ar niso n, A. Astb ur y, B. A ubert, C. Bacci, A. Beza g uet, R. K. B o c k, T. J. V. B o w c o c k, M. C al v etti, P. C at z, P. C e n ni ni, S. C e ntr o, F. Cera di ni, S. Citt oli n, D. Cli ne, C. C oc het, J. C olas, M. C or de n, D. Dall ma n, C. R u b bi a 2 8 5

D. D a u, M. D e B e er, M. D ell a N e gr a, M. D e m o uli n, D. D e n e gri, A. Di ci a c ci o, D. Di Bito nto, L. Dobrzy nski, J. D. Do well, K. Eggert, E. Eise n ha n dler, N. Ellis, P. Er h ar d, H. F aiss n er, M. Fi n c k e, G. F o nt ai n e, R. Fr e y, R. Fr ü h wirt h, J. G ar v e y, S. G e er, C. G h e s q ui er e, P. G h e z, K. L. Gi b o ni, W. R. Gi b s o n, Y. Gira u d- Hera u d, A. Giverna u d, A. Goni dec, G. Grayer, T. Hansl- Kozanecka, W. J. Haynes, L. O. Hertzberger, C. Ho dges, D. Hoff mann, H. Hoff mann, D. J. H olt h ui z e n, R. J. H o m er, A. H o n m a, W. J a n k, G. J or at, P. I. P. K al m us, V. Kari maki, R. Keeler, I. Ke nyo n, A. Ker na n, R. Ki n n u ne n, W. Koza necki, D. Kr y n, F. Laca va, J. P. La u gier, J. P. Lees, H. Le h ma n n, R. Le uc hs, A. Le ve q ue, D. Li n gli n, E. Locci, J. J. Malosse, T. Mar kie wicz, G. Ma uri n, T. Mc Mahon, J. P. Men dib ur u, M. N. Minar d, M. Moha m ma di, M. Moricca, K. Morga n, H. M uir hea d, F. M uller, A. K. Na n di, L. Na u ma n n, A. Norto n, A. Or ki n- L e c o urt ois, L. P a ol u zi, F. P a uss, G. Pi a n o M ort ari, E. Pi et ari n e n, M. Pi mi ä, J. P. P ort e, E. R a d er m a c h er, J. R a n s d ell, H. R eit hl er, J. P. R e v o 1, J. Ric h, M. Rijsse n bee k, C. R o berts, J. R o hlf, P. R ossi, C. R u b bia, B. Sa d o ulet, G. Saj ot, G. Sal vi, G. Sal vi ni, J. Sass, J. Sa u draix, A. Sa v o y- Na varr o, D. S c hi n z el, W. S c ott, T. P. S h a h, D. S mit h, M. S pir o, J. Str a u s s, J. Str e et s, K. S u morok, F. Szo ncso, C. Tao, G. T ho m pso n, J. Ti m mer, E. Tsc heslog, J. T u o mi ni e mi, B. V a n Eij k, J. P. Vi all e, J. Vr a n a, V. V uill e mi n, H. D. W a hl, P. Wat ki ns, J. Wils o n, R. Wils o n, C. E. W ulz, Y. G. Xie, M. Y vert a n d E. Z urfl u h.

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