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1973-DISCOVERY of NEUTRAL CURRENTS

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1973-DISCOVERY of NEUTRAL CURRENTS Gestion INIS LAPP-EXP-Q^ m Doc. enreg. Io : /.3.[f3.yfl.H JUIN ÎWJ N" TRN : ^LaJi Destinriicn . 1973-DISCOVERY of NEUTRAL CURRENTS B.AUBERT CNRS-IN2P3 LAPP-ANNECY ^ It is difficult to speak on "1973 -Discovery of Neutral Currents" without first mentioning earlier events, mainly 1953 in Paris the July 1st, signing of the convention setting the basis for the creation of this place "CERN in Geneva", and 1963 in a trattoria of the Siena's Palio the birth of the HLBC Gargamelle, on a napkin on a warm summer day. I must also apologize that I am not able to pay any special individual tribute here to each one who acted for the good of this enterprise : engineers and technicians from the machine and the beam, the bubble chamber physicists, theorists and experimentalists, administrative personnel and scanning girls. The only names I would like to honor are A.Lagarrigue who was the real leader of the Heavy Liquid Bubble Chamber (HLBC) active community and P.Musset who acted as the convener and the driving force behind the neutral current analysis. I will spend few minutes on the status of the weak interaction theory and the search for neutral currents in the late 1960's, and on the tooling available at this time. Then I will review the analysis of the Gargamelle experiment and finally I will give a brief look on what happened elsewhere. Status of the weak interaction theory. The current theory for weak interaction, able to represent p decay, \i decay and the decays of strange particles, called the Universal Fermi Interaction (UFI), was "local", which means a priori, it does not need a propagator. All interactions were adequately described by the phenomelonogical (current x current) Lagrangian : where G is the Fermi constant and the lepton current is: Then one can write all lepton currents (ev) (v|i) or hadron currents (np) (KJC) . This description was completed with many assumptions,which were not in contradiction with experiments, such as : 1. The two component neutrino theory, (Vn ve have the helicity+1 ) 2. Lepton conservation ' 1 I 3. Absence of neutral current . 4. (ie universality ' 5. |AI| = 1/2, AS = AQ, AS = ± 1 6. Charge symmetry of AY = 0 7. CVC 8. PCAC \ - S» _ 9. CPT invariance Experimental programs in the sixties were largely based on tests of such rules . In this UFI theory , the possible existence of a neutral current was often ignored as not needed, experimentally or theoretically. The local Fermi interaction suffers several difficulties: first radiative corrections divergences, and second it is not renormalizable when we go to a higher energy domain. The next step, by analogy with the electromagnetic interaction, was to add a propagator called IVB( for Intermediate Vector Boson ). At such a point, the absence or presence of neutral currents became a question because the multiplet of IVB was in question. Based on the absence of K decay such as K° -» Ji+Ji" or H decay such as |i+ -» e++ e" + e+, the common belief was the absence of neutral currents. The difficulties faced by the IVB theory are different in detail but the overall pattern is not altered,when we want to go to higher energy we found a lack of renormalizability. To overcome the problem of renormalization , several gauge theories were proposed in the 1960's , constructed as extensions of the YANG-MILLS theory for the electromagnetic field Ajx. But in all those, neutral current, heavy leptons or both are needed. In the simplest gauge theory, GLASHOW, SALAM and WEINBERG propose only two new particles.The Lagrangian is extended. Charged currents are coupled to the charged massive field W and neutral currents are coupled to a mixture of Z and Aji '.I field.Two new coupling constants are introduced, g and g'. A mixing angle is defined , i - ] (the WEINBERG angle) by : j tan 0\v ="^" , e2 -'2 sin^ 6\v = ~2 = " S fe T6 e2 1 where — is the fine structure constant = TT13= 4JI ' *'. *• - The masses of the new particles are related to the Weinberg angle: 2 e2 2 4V2 G sin 6W e2 Z 2 42 G sin 26W and this model gives measurable predictions concerning neutrino interactions: 2 2 4 + e' -> Vn + e") = — me E[(2sin GW -I) + i sin Gw] 2K 2 — G A 1 ? 2 + + e" —» Vi1 + e') = — me E[(4 sin^ Gw) ï (2sin^ Gu/ - 1) ] n NeutralCurrent NC .1 . i. 20 . 4 Rv = Charged Current = (2 "sin 6W + 27 sm " D Neutral Current NC .1 . 2 « 20 . 4 V Charged Current = (2 ' sin 6W + T sin But because in 1970 several limits existed on neutral currents, the direct consequence was that in the proposals for new experiments, especially in neutrino physics, more emphasis was given to search for W and for heavy leptons (deviation of linear increase of the cross section and presence of dileptons events). Table I show the limit for neutral currents as measured in K decay physics, which at that time, was the main tool for probing of weak interaction theories. Table 1 K* -**e*e~ <9xlO"7 Camerini,... (1964) K* -. it* it* n~ < 3x 10"' Camerini,... (1965) K+-** w < 1,1x10"* Cline (1965) + K5 -e c" < 3.5 x 10"* Bohm (1969) 7 K1 -p*/i" < 3,1 XlO" Gjesdal (1973) , KL -»/i*/T < 1,8x10"* ClMik (1971) I0 f Kt -A«*/»" - ll*, xl0- Carither (1973) + r K1. -it e < 1,6x10"* Clark (1971) + KL -c e" <l,6xl0-* Clark (1971) In the early neutrino experiments the flux and the size of the chamber precluded a realistic ., 5**. neutral current analysis. In 1970 the result published by the 1.1 m HLBC group 1 .n-»|i-p=0.12±0.06 4) ? \ and v^ p -» v^ n 7t+/V|i p -^ ^- p Jt+ = 0.08 ± 0.04 j acted as dissuasive data, both for experimentalists and theorists. It may be worth mentioning that the 1.1m HL bubble chamber experiment group noticed a very high rate of neutron stars and ^ . called for a better understanding of the sources of so many neutrons. But the size of the chamber \ ' r did not allow for a complete understanding of this effect. H \ \ •-:.*. Nevenheless, shortly thereafter two major theoretical developments occurred, in 1970 the GlM mechanism demonstrated that the absence of AS = ' neutral weak current could be overcome and in 1971 G.'t HOOFT's proof that a class of gauge symmetric theories are renormalizable. In 1972, Weinberg remarked that his .nodel prediction for the elastic reaction : 0,15 < was not so far from the experimental limit. This led to a growing interest in the search of neutral current but balanced by the fact that if you have to choose where to put your money, you give more emphasis on the search of heavy lepton. Neutrino beam and HLBC program. In the early 1960's the first neutrino experiment with the HLBC built at Ecole Polytechnique, BP3 ( 3001), and a counter experiment gave no events. During this decade many improvements were made, mainly on the beam. In 1963-1964 the CERN 1.1m HLBC (5001 ) provided some 450 neutrino interactions.The main limiting factor was the neutrino flux. The source of neutrinos and antineutrinos are the decays of K and K produced in a pp collision. So in any neutrino beam we have a mixture of neutrino and antineutrino both from ve and V^ . The average neutrino energy, is only about 1/10 of the incident proton energy, and the spectrum for the existing accelerators was : ANL 0.5<Ev<1.5GeV CERN 2 10 NAL 5 50 One of the strengths of CERN and on what I will present as the Gargamelle experiment, is the neutrino beam. Let us review the components of such a beam. The most intense primary proton beam hits a target and produces a secondary beam of pions and kaons . A focusing device, the "Simon VAN DER MEER horn", focuses particles of a given sign and in a certain (m. momentum range. Those particles pass through a decay region where K and Jt decay, then » j through an iron shield which kills all remaining particles but muons and neutrinos. If the shield I j'.j is long enough only neutrinos remain. Monitoring devices in the shield tag the flux of muons in ^i .J depth and allow us to reconstruct the shape of the flux in energy and profile, but because of the J rapid fall-off of the flux the precise monitoring poses difficulties. The flux is always a mixture of Vn and ve, but with the v coming from Jt or from K decays having different spectra.(Fig-l ) I \ ••••M One other important tool in this search was the availability at CERN of the largest heavy liquid bubble chamber ever built. The first neutrino experiment with BP3, a chamber of 300 liters, showed that the use of heavy liquid had much potential. The drawback of a complex target instead of H is largely compensated for by the increased mass and by the particle identification capability. This was the main reason in the decision to build a 4.8 m long, 1.8 m diameter cylinder which allowed for a large fiducial volume as target with several tons of freon embedded in a large magnetic field. The two main technical challenges were the use of a large membrane for the pressure control ( the pressure must drop from 20 bars to 10 bars in 60 milliseconds, once every 1.4 seconds ) of the large volume and the optics (direct lighting with 21 flash lights and the use of 8 large angle lenses for the track-recording cameras ) to guarantee the uniformity of the illumination.
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