COMPARISON OF WEAR INTERACTION THEORY WITH EXPERIMENT*

A comprehensive review of experimental data related to the Standard Model of Weak interactions is presented. Charged current processes and neutral current reactions are described and compared with theory. Alternative models are discussed and the extension of the Standard Model to six quarks and the effects of CP violation are considered.


INTRODUCTION
It la the purpose ot tMi review to ate hew w«U the Dilating experlMhtal dace agree with the standard wtak Interaction theory, A literal interpretation of this task would be clearly beyond the intended scope of this work; accordingly ease dec Is ton* need to be evade ee to the material to be included. In outline these choices, my guiding principle nm* been co discuss only these data uhlch test the heart of ths standard model ulthout having to rely too such on var ious peripheral assumptions, in this spirit I tend to exclude, for example, the various prediction* sbout the purely hadronle eeak de cays* since the expected accuracy of these predictions can he under stood only in the framework of rather Involved QCD calculations i) The agreement, or lack thereof, between (be data and the predictions. IT'.' reprpiunt in this ease aera a taat of the calculatlonal techniques than of fundamental weak Interaction theory. In the same apirlt I tend to ignore various nuclear physics experineato vhoae Interpre tation to obscured by the uncertainties having to do with the nuclear •otrlx eleaent calculation*.
The vain body of this review shall concentrate on what I consi der the throe cornerstones of today's standard Model: charged current phenomenology as rtlstad to the V-A theory, 2 ' neutral corrent phenomenology In Ibt fraaauork of the Glashow-1ielnbere-5alaa aodel, ' and the comparison of the char* picture tilth the Glaolwlllopoalos-Haianl aodel, 4  To the extent postlblo, I would like to take • pedogogical and historical approach Co this review, ttiat 1 aeon here. In that I will try to indlciite eva swell as possible What specific aspect of to* standard nocel la tested by a given experiment and where does this prediction c«w frost* 7*i addition, 1 vill try to a certain extent to follow the historical development of the twin ideas. The develop ment of physic* does not, however, always take » logical course -•me of the recent work attacks stellar eu>-«tla.s that were origi nally confronted by expartaents 20 year* ago bo. in a different svbf ield. A T and v decay c laparls^ft Is one gaod example of auch a situation. In these eases, I sfe&ll violate the history In the Interest of a more rational logical structure.
To conclude those Introductory remarks, I should acknowledge several recent excellent reviews, more limited in subject natter

i.e. an intermediate vector boson that is heavy en the IMI scale of the experiments in question.
c) 3 lepton fesdlieo with similar structure d) lepton conservation law e) "universality" between u weak interaction processes and nucleoli 0 decays. The exact meaning of universality will be developed as we go on.

teptonlc Reactions
Next The ether aaawireaMe *** of parasefctra hec to 4a vith the polarization of the electron eoltted in tha anon decay. Hot* ftpecif-IcatUy one can weesure both the loagltfldlnal polarisation as tHrtl «• rb* wo coatponanta of the trensveree polarization, one of tfhlefe is forbidden by tlae reversal Invariant:*. iht value* of the** 1 polari zation coaooneots as of the tlae i»f tin »» Vancouver Conference ace mwamrUed in Table II 5"e» void* of explanation aw needed fesoriins £*»» trattsverae pelsri-2ation cooponente. The theoretical valuoi of thoan eompsnonta can be expressed in terns of a, a', fi, B', A iwi % parameter! (a, ft', e. $' and A Just like 4t are functions of th* different coupling con stants) and are functions of both electron energy and angle of eoisslen of electron with reificet tu SWOB spin direction. Tims it i& ware meaningful to fit thai* polarisation data in tarns of the above parameters rather than quota t«« absolute valu* if th« polari zation, lite values ejtcteo resreceot fit* under the «ssustpUoft of total cancellation of scalar and paeudoacalar coupling <a » o* " 0). The aaxiana poseitOe value that </* <«d a 7A) can tafce i* 0.24.
We can now write a phenonenologlcal current reaponslble for th* en part of the charge retention HanlltonUn of u decay a* V-(l + c)A, and ask what ere the experimental 1lefts m e. I would like to em phasise that since we are vwtaa. the charge retention forawliaa, thia question is not equivalent to the prablcn of poMible existence of m beevy lnteraecllatc vector boeoo with rich: handed coupling. The Halts Oct on t by volwsa' 6 ' of n, EP , and *£ are displayed in ft vector t mi ccmwx coupling coafflcieote ttwt occur In tmon decay BaaUtottian <see lef, * for explicit ferulae), THt» shay Mtute directiy the fora of interaction reapoosible far the decay. Thi spectrum is quite aensttlvo to the value of o a* can he. Keen (torn Fig. 2a; m the other nana the epoetin* ten involvoin*, ti la multiplied by *-/*,. sad ttma the spectrum i# affected very little as one varies n over it* full al lowed range fw» -1 to +1 (|u Fig. 21*). Only at vary low elec tron energies f la there any eentltivity to th« v»l«t *f t(.
To take tat* account the correlation between muon tpin direction asd the elastics eeisentuc v«ctor t 2 aara paiiniteri are required, 5 and S. Tha forwar la re loud to the magnitude of tha forward backward aayBBsatvy; tha Utter paramotri*<i tha differ ence in nomeni'uia ipictrum of the electrons emitted at different =.igl«s. The ttoat recant pub lished valoai of theea ear ameers are llGt«4 to TaMc  experimentally one always SMMSUTSS fix F , t liat tha product of these 2 omentitis* In tha  result* ere retber prelielnary at this Mag* but they lead us to balieve that better laboratory cateexJBar.es en double ft decay can taach ua aoBetblng fwndfwtal about wet* interactlone.
The T decay phtnessua provide as not only « autaae of testing the hypothesis that T virb v r 9 neutrino fern e third leptonlc doublet but also allow one to repeat amy of tha v-oacay studies at higher eatrsfes. ihe T situation bu recently been reviewed ceaorebeoeiveiy by rerl and east recent result* tuwe been emaurlxad at this confer-•occ by Faldaro. To eTold duplinatlon* 1 aboil onto briefly eouuer-«e die points e»« salient to the tbeae of thle review.
In the conventional picture* »oe 1 decay ceo be described by the generalised Feyeaen diagraa of F-i,. 6, 1» tens of this diagram, the Most important conclusions can be euarariiad es follows; a) The i appears to couple to tha atoa internediata vector hatm that is responsible for other weak Intaractlena. This state ment is faaeed en tha fact that ill tht aeavurad branching ratios • %

Hadrowic Procoaeoe
So fw the oiecoaaion baa boon United to the lepeooit part of the weak current. Turning now to tba hadronlc sector, the early experinente indicated that V-A alto IHH to bft operative there* but the complication* doe to strong interactions sake a etralghtforuerd fonallso ojore difficult. By opplying, bnvever» rather general principle* or by resorting to a specific aodel, these difficulties can be overcone to a large extent and accurate predictiene are possible. I vovld lilw next to tun to BOB* of to* confrontations of the charged current weak Interaction theory with too experlacnt in the hadronic sector.
Historically the conserved vector current theory and the related isoeriplet currant hypethfttia have baan the fine truly successful link between tha weak and olactronagntCic Interactions. By placing tha vector weak interaction charged tun ant In tha •*•* multiplet with the laovactor part of tha electromagnetic current, it provided an explanation or lack of ranornalizatlon off acta and pre dicted the exiatance of aoae direct waak lntaraetiona between various particles. The latter hypothaais aliened one to calculate preciaely (except for snail electromagnetic correction!) the aatrix elements for a variety of ptocaaaaa involving hastens• One of the trtst cele brated of these predictions vai the 0 decay of tha pion, i.e* « + * *' e\ which according to the CVC should occur with a oiaiscule branching ratio of J .065 x W" 8 . A ne« oBparlnent «t UUffF ettasuriftg this branching ratio o/totM 4 uceltetaeey result of (1.02 1 .«6)xlO , representing already • slgnlflccaC Isproveaant over the old world overage.
Anwhw recent, and quite different toot of the CVC hypothesis, Involves the Mavurejemt of the branching ratio t* •* e*» . lb* decay rate for this process can be related via CVC to the annihilation cross section e e" * p D . Tha latest axpniaeatal noshex of 3 *! BR (T* * p*v) -(21.6 * 1.8(etat)s 3.6(*y*t))2 agrees vary vail with the anet up to date prediction of (21.5 * 1,5)X, X would Ilk* to ton now to • trlef discussion of the partes (or quark) BOdtl which has had soae rewrkahla successes Jn predicting the beherior of badrons in tens of structure composed of eleuentary constituents. As •»* shall see later on, the quark approach has bean renrkablj' svcceaci'MJ. In linking cht theory with experiment In the field ef neutral current phenomena. Hera I want to address mynlf •pacifically to the idea that quark* snake up a V-A charged currint of the um kind M tlw leptoas, and th.ta the knowledge of h&dron ccapositlcn can lead to soae very specific experiaettt&l predictions. 9 , the Cabibbo angle, is a free peractoter co be deceratnea try the experiment, end whose theoretics! prediction repreeanta en isnortsnt challenge to all higher synaatry models.

If tha quark part of tha current la pure V-A then oe have ex plicit prediction*: v -quark (or v -antlqua;k)&catterlt>g,t do/dy -constant v -enark (or v -«ntiquark)scetteringr do/dy -(1-y) . for cht V+A quark current the predictions are simply interchanged (y li tha atandard inelasticity paraaitur). Thus if nucleons
The formalism has been since extended first to the second doob-Jel, containing the charm quark and wore recently to the slx-anaife world by the addition of 3 sore paraneters. However, even in the 6 quark picture, the original prediction! involving u, d, and • quarks, as veil at purely leptonic processesi differ from the 6 quark pre dictions only in second order of quantifies that appear to be small experimentally. Thus for the purpose of present discussion we Shall stick with a single parameter formalism.
The strength of ns-sl transitions (I.e. sin « c ) can be •assured in 2 independent ways i.e. a> The KSj decays (both neutral and charged) represent pure vector transitions and hei.ee the 5V, breaking effects are supposed to * 2 be small as ona extrapolates the Dallti plot density t*-> q "0.
(Adeaallo-Catto theorem) 3 . 8 * Thus the decay ntt coupled Vlth tfctt foro factor determination can yield the value «f sin* • b) the barymic r-enileptooic decays arc ceapletely pnrsnstriied by th* D/F ratio In the axial-vector oatrJJC element* and the Cablbbo angle v c (provided that one use* C"X to obtain behavior of Mae of the ton factors and assumes absence of second class currants). Thus a global fit tw neutron and bypcroo decay data will yield these 2 parameters.
Schrwcb and Hong h-*e recently pexf otned am analysis ' of the above data to obtain   fig. 12. (The first reaction h*a additional 2 diagram* with ¥ and t" in the t channel), in principle theaa raactiona can yield information on 3 different cou-plin| eonatanta, ecanonly referred to aa hyy, h^, and h^. which In th* standard Bodei and aaaunisg lepton universality reduce to-Nv-V k (1 2 1

Tfea Istareat to that* raaetiMa tram tha point of view of thi> review KM In tha fact tttat they *re acoaitive to the interference effects batwato Y and Z° diagram, aa atom In
•MA"^1'

Tha ten muUlpllad by h w atiowa upr ' in the expression for total araia section ei a percentage change away from the predict lor ef ent photon exchange dlairaa and rlics linearly with a. h^A gives rlt* to parity violating affaeta lltta tion lero helicity of outgoing laptonl and croaa •action dependence on tht hellcity of the incident electron or poiitron, finally h., will manifest Itself as a forward backward aayamatry of the outgoing leptons of a given charge.
Baildaa tha Intrinsic difficulty, connected vith the measurement ef tha firat 2 affaeta thay are expected to be very small in the •tandatd nodal. Thin la dua to the fact that they are predicted to 2 vanllh If Bin 8 • 0.23, which appear* to lie very near the experi mental value. Thua It !• not aurprlclng that none of these effect;have baan obaarvad BH yat. Thar* appears now, however, evidence for pretence of Ron aero h.. taru, Tha ioat con vincing evidence conea froa tha obaarvad aiya-•ctry In tha reaction • + a 4 U + V Tii* result*, on that reac tion, aa nail aa th* relatad T r" cosnMl, praaantad 5 *' at tha loan fiflt 12* The two interfering diagrams to a 4 *" •* u JI~ 2?

>4
Conference are ouaMXlsed in Table IV

Semihadranlc neutral Current Reactions: v-Hadron Interactions
These TO<MIM praicnt additional complication that la not present In purely laptonlc interactions,, I.e. the fee that hadrona »"e complex structures. Thus we have to rely on the quark model and sonet ises other theoretical assumptlone to make the connection be tween theoretical frelictions and experlBemtal reality. Fortunately, the structure functions of the mtcleoas are known now quite well in the region of interest aa Is the fraction and composition of quazfcaatiqwerfc sea. Serious theoretical and calculation*! difficulties still remain: questions regarding exclusive plon production channels and size of atcolt physics effects are acne of the example* illustra tive of this point.    Toe first 2 charged currants and the first 2 neutral' currents >r« "oW" currents that have already been discussed. Furthermore Ot ds current Is also an "old" current whose absence was the ralson d'etre (or the GIN awchsninn. Accordingly 1 shsll limit ny diacusaion to the other S currents, emphasizing aelnly the comparison of the experinental data with the CIH predictions. One should note that the extension to the 6 quark node ( In euBsery, the spectroscopy and interactions of the charm particle* are in good agreement with the CIM nodal. In eeveral sectors, however, there exists great deal of rooa for experimental improvement.

EXTENSION TO 6 QUARKS
In a remarkable paper, written before the dloeovery of the c quark, Kobsysahi and Maakaua 5> urgued thee within the alaplcat SU(2) iU(l) model (i.e. only one Uigg* doublet) with only a quarks, there it aft natural way to generate CP violation. They eboved that out of tbe peaoibl* wave to ham CP vlelaiien witbfa this wedel, ws* to enlarge the quack population to 6. Mtnla> tibia craaework tht charged carta**, weals' fee written an J + * (Set) uWj where II la a unitary uatrin ttiat defines the nixing of the MIS eleanstatea. For the 3 x 3 dlneneioeaHty II ia characterised by 3 Euler*llke angles and one phase. Specifically tl can be written ' C 3 a 1 .^j 2 "VaVW** "•iWw"' Thia tenant becan aorc attractive aa T lepton sained respectability, insofar that equality of quark and iapteo populations (with conven tional charge sssignaents) la one way to remove the triangle anoaa-Ilea. The sehene becaue the "niw orthodoxy" with the discovery ' of T at Feral Ub ind ita subsequent eonfiraation ' at DES¥. X would like to review In thia chspttr the question aa to hew veil this "»CH orthodoxy" ia supported by the experimental data. tie Bay first «ak how well it the eaiatence «i thia new doublet established-There ia now reasonably good clrcuastsutlal evidence that new flavor haa been produced In the e + e" annihilations) from the T spectroscopy, ** the excess electron' 2 * and k*on 93 ' production at the 48 f cute* and the value of * at high enerev. 7<) On the other hand* It ia not clear that any unaabiguous nat<M beauty b signal haa bees seen ee yet. ' In auBBarv, however, t think seat people would agree that the evidence for existence of a b quark is quite good.
Unat about the top quark, t ? £BHU detectors have searched for the t quark up to the blgheet energies of that ring and aee no evi dence of t production. ' this placea an upper Unit of s> t * IB cev> Should this be * waves of worry to the advocates of the $ quark •odel? the theoretical eatiaetea are uncertain, but it appear* that t quark mass of around 40 CeV would sot be toe surprising. 9 *) A fair statement to sake would probably ba to My chat aw though there is no experimental evidence for the t qnark, neither do the present searches speck strongly against the existence of a (t b> doublet. I turn next to the possible alternative multiple* vhcre the la»l factor Is a slowly varying function of e, and quark staaeee. Since no one knows how to calculate I, the above equation ean be viewed as a oesns of measuring ( (provided that there are no other contributions to cr violations). That will be passible once better knowledge ef 0. and e will be tVAilable.
It doeb appear that the ratio |t*/(! can he calculated within the K-H model, and the consensus of different calrulatlons la that 112) 1/500 $ lt'/t| * 1/Jn .
I.e. an the verge of experimental detectebtllty. As for the dipole stonent, however, the prediction fs fat away from the prea«>nt limits, nseely One could alao aak a general quest ioni Do ve hove an ultinatr theory of weak interactions! I do not want to answer it, but vauld like to remind the reader about the existence of the large number of free parametera in the theory* J CHS nodal paraoetere (a, £ p » aln 6> w ) JO K-M aatrlx and quark naa* parameter* 10 lepton aoetor parameters 1 naaa of Btgge scaler I Biaaber »f generations _1 BUBbar »t Higgv doublets 26 BIBIBUB total nvtbar of parameters.

S3
If th> nvapcr of Blftfi* datfblot*, lutthia of c«Mritlon«, or the •Ice of the Muft* trouft *r* lorgar. the mother of ptfflgwtwo em •row to a significantly higher number. Whether • CtMOry with M many paraaetimi can b« called truly fttndavencal la it l«wt partly m •ubjpctlw question, that cannot ht anawcrao' on any abaoluta scale. Hoat of us, however» would probably iniMr It In toe neyol ivc. it