europhyscs

BULLETIN OF THE EUROPEAN PHYSICAL SOCIETYn e w s Volume 5 Number 4 April 1974

Introduction The experimental evidence obtained Search for Neutral Currents recently in Gargamelle, a large heavy liquid bubble chamber, at CERN sup­ in Gargamelle porting the existence of neutral weak currents may have a capital influence P. Musset and A. Rousset, CERN on the present scheme of the particle P. Musset, Maître de Recherches at the CNRS, and A. Rousset, world. Generally particle physicists Professor at the Ecole des Mines, in Paris are presently doing assume that everything occuring in research at CERN, Geneva. our universe is based on four types of interactions between elementary parts of matter called particles. The recent teractions. Let us consider the weak ments at National Accelerator Labo­ experiments at CERN appear to link and the electromagnetic ones. The ratory, USA (NAL). If the two types of two types of basic interactions. weak interaction theory, first intro­ interactions are so closely related, GRAVITATION is the most common duced by Fermi and developed in why not try to have a unified scheme interaction. Whilst the weakest of 1957 makes use of a close analogy in which the two kinds of interactions known interactions, it is well known with electromagnetism by using the would be considered as different at the macroscopic level. At micro­ concept of “current". aspects of the same interaction ? scopic level this interaction however The electromagnetic current in This was attempted at an early produces such small effects that no quantum mechanics is carried by a stage, before 1960, mainly on the basis experiment with individual particles is charged particle which remains of generality : why not a neutral weak feasible. charged after the interaction, so that current, besides the charged weak WEAK INTERACTIONS are very the current is said to be neutral. For current, which permits the unification much stronger (1030) than gravitation example in the scattering of an elec­ of weak and electromagnetic Inter­ in the GeV-range. They apparently act tron on a proton (e p e~p) the actions. In that case a neutral heavy as point-like interactions, and give rise lepton (e-) has the same charge be­ boson also exists. Then the observable to the well known β-decay of the fore and after the interaction. The photon and the hypothetical neutral nucleon. For about 15 years the neu­ lagrangian density of the interaction is heavy boson are two orthonomal com­ trino, which is only sensitive to weak then given by the product of the cur­ binations of two basic fields, which interactions, has been used as a pro­ rent times the electromagnetic po­ are the Yang-Mills fields of the theory. jectile on nucleons to study the cha­ tential. By analogy the weak current The Yang-Mills theory in which the racteristic properties of the weak in­ is carried by a charged (or neutral) gauge invariance is related to the teraction at different energies. particle, which transforms into a neu­ existence of such basic fields under­ ELECTROMAGNETIC INTERAC­ tral (or charged) particle, and gives lies these ideas. TIONS are about 109 times stronger rise to a charged current. For example Gauge theory than weak interactions. They are the a neutrino interacts with a neutron giving an electron and a proton In recent years the interest for these interactions known with greatest pre­ considerations was renewed by at­ cision, both experimentally and theo­ (v + n e“ + p), where the final lepton (e~) has a charge different tempts of theoreticians to cure the retically. The theory of quantum elec­ diseases of the weak interaction trodynamics has been widely tested, from that of the incoming lepton (v). A charged current is assumed to in­ theory. In the Fermi theory the cal­ to few ppm, and found everywhere in culation of cross sections at high agreement with observations. teract with another conjugate charged current in the same way as the elec­ energy (300 GeV in the centre of mass) STRONG INTERACTIONS are about violates the strong condition of uni- 100 times stronger than electro­ tromagnetic current is interacting with magnetic interactions. A great amount the electromagnetic field. of experimental data has been accu­ In the case of electromagnetism, the Gontents mulated in the low energy and in the interactions may be seen, and are Search for Neutral Currents high energy regions.-Nevertheless, no actually mathematically described as in Gargamelle...... 1 global scheme with a sufficient degree due to the exchange of virtual pho­ EPS Council Meeting .... 4 of generality to be considered as final tons (Fig. 1). In the case of weak in­ European High Pressure Re­ theory has come forward. teractions, it might be that another search Group...... 5 type of boson is exchanged, a charged Towards unification of interactions European Solar Radio Astro­ one, which has to be heavy, and is nomers ...... 6 It is a constant aim of many theore­ really heavier than our present ma­ Society News...... 7 ticians to try to find a unified ap­ chines are able to produce, i.e. a few proach for the different types of in­ GeV according to the latest experi­ M eetings...... 8

1 dieted by the simplest model. Never­ Charged weak Neutral electr. Neutral weak theless, the experimental search was current current current pursued for neutral currents at any level of intensity. The Gargamelle bubble chamber and the neutrino beam Gargamelle is a large heavy liquid bubble chamber built at the Depart­ ment Saturne at Saclay and in ope­ ration since 1971 in the neutrino beam at CERN. The body of the chamber is a cylinder, 4.8 m long and 1.8 m in Fig. 1: The three processes v + n e~ + p, e~ + p e~ + p, and v + P —► \ + P are diameter. The mass of the useful sen­ believed to proceed via exchange of the intermediate charged boson W+, the photon γ, the neutral sitive liquids is 10 tons of Freon boson Z° respectively. (CF3 Br). The chamber is particularly adapted to the detection of neutrino tarity. The Fermi theory is adequate actions were not allowed to proceed interactions because the large mass to describe low-energy interactions via neutral currents. That this was not of the detector compensates for the but otherwise diverges. the case was moreover known with a very small cross-section of the neu­ In electromagnetism, a renormali­ high degree of accurary e.g. the decay trinos (10~38 cm2 at 1 GeV). In addition zation procedure resolves these pro­ induced via a neutral current of the the large dimensions of the visible blems, using the gauge invariance strange meson Ki° -*■ μ+ μ” was known volume of Gargamelle permit a good which applies since the mass of the to be < 1CT8 that of the decay identification of the secondary parti­ photon is zero. Again in analogy with K+ -*■ μ+ν induced via a charged cles emerging from neutrino interac­ electromagnetism, the gauge inva­ current. tions : an electron gives a characte­ riance is introduced, and the bosons For the strangeness-conserving ristic spiral track in the magnetic field which are massless become massive neutral current, very little was known (20 000 G), hadrons (meson or nucleon) through spontaneous breakdown of before the Gargamelle experiment interact in the liquid with a probability symmetry. started. Nevertheless, during the first larger than 80%, a neutral pion is In such schemes, the “divergen­ part of the experiment, first results on recognized by the materialisation of cies” are removed provided new phe­ processes possibly induced by neutral two gammas of the decay π° -»■ γ + γ. nomena exist : neutral currents, as currents were published. The elastic The neutrinos themselves are pro­ in the Weinberg-Salam-Ward model, scattering on electrons duced in a special experimental area or heavy leptons, as in the Georgi- ve + e' -*■ ve + e" was studied at of the CERN proton synchrotron (PS), Glashow, or Lee-Prentki-Zumino mo­ the Savannah nuclear reactor, and the the neutrino tunnel. Some 5.1012 pro­ dels. Also both neutral currents and one neutral pion production tons of the accelerator are extracted heavy leptons could co-exist. Various v + nucleus -»■ v + π° + nucleus every 2 seconds at the maximum other models of that sort have been μ μ energy of 26 GeV and hit a beryllium constructed. was studied at the Brookhaven acce­ lerator. target, producing about 1013 mesons. A large fraction of these mesons de­ Experimental situation before For some time, the consistency of the Gargamelle experiment cay during their flight along the 70 the results was believed to be evi­ metres of the tunnel, yielding a muon It was actually known for a long dence against the possible existence and a neutrino (π+ μ+ + v ). An iron time that strangeness-changing inter­ of neutral currents at the level pre- shielding, 22 m thick, stops all the par­ ticles except the neutrinos. A flux of about 1011 neutrinos each 2 seconds is passing through Gargamelle located just behind the shielding. On average, one picture out of ten contains a neutrino interaction. The analysis of the experiment is done through European collaboration of the Universities of Aachen, Brus­ sels, Milan, Orsay, the University Col­ lege London, the Ecole Polytechnique Paris, and CERN. The charged current events and the neutral current events Most of the charged current interac­ tions of neutrinos (CC) contain in the final state (Fig. 2) a charged lepton, according to the reaction : v + N μ- + hadrons μ The emerging muon is identified by absence of interactions in the liquid. This reaction is of the charged cur­ rent type because the electric charge Fig. 2: The non-visible neutrino beam is coming from the left. All particles interact except the of the lepton is modified in the inter­ muon, giving rise to the longest track. action (v -> μ“). M

2 search for the purely leptonic reac­ tions : V + -*■ V + e~ μ _ μ V + e y + e μ μ In the simplest theory, the cross section for these processes is ex­ pected to be very small (~ 1CT41 cm2 at 1 GeV). Such an event should ma­ nifest itself by the appearance of an isolated electron emitted at zero angle along the neutrino beam direction. A special scanning for these events on 5.105 pictures with neutrinos and 106 pictures with antineutrinos has given respectively 0 and 2 events. A small background is expected from the ve contamination of the beam through the reaction ve + n e~ + p in which the proton is not visible. The estima­ tion of this background shows that in the antineutrino exposure of the order of 0.1 event is expected. But in the same number of pictures theory pre­ Fig. 3: All particles interact giving stars or π°. dicts about 3 events of the type V + e~ V + e'. Therefore the In a neutral current interaction (NC) event (CC). A sample of these CC μ μ the electric charge of the lepton is events was selected applying the two observed events (Fig. 4) are not expected to be unchanged after the same criteria to both the complete NC likely to be attributed to background, reaction : candidate and to the hadronic part of and can also be considered as a V + N V + hadrons a CC event. The cross sections are in serious indication of the existence of μ μ The outgoing neutrino is never de­ the ratio the neutral currents. tected, and therefore all the visible = 0.22 ± 0.04 for neutrinos and Conclusion \CC) V In the two reactions studied for the secondaries in this reaction have to /w?\ = 0.43 ± 0.12 for antineutrinos. be identified as hadrons. This is pos­ VCC t V first time evidence is found for the sible because interactions with nu­ These ratios are relatively large. existence of neutral currents. The in­ cleons are unambiguous signatures of Also of interest is the fact that the tensity of the observed phenomena can hadrons. A search for such events has magnitudes of the signal in neutrino easily be compared to the simplest given 102 and 64 candidates respecti­ and antineutrino exposures are both theory. This permits the renormaliza­ vely in the neutrino and antineutrino compatible with the same value of the tion program and unifies at the same exposures (Fig. 3). Weinberg parameter of the most naïve time the scheme of electromagnetic theoretical model. and weak interactions. We are hope­ Nevertheless one has to make sure ful that these results will be consoli­ that the signal cannot be simulated dated through future experimental by background events. An obvious The leptonic mode work. If this is the case, then a great background could be due to neutrons Another way to prove the possible impulse will be given to this field of coming from the PS or coming from existence of neutral currents is the physics in the years to come. neutrino interactions in the material around the chamber. A detailed study of this background shows that no more than 15% of the signal can be explained that way. Besides, the spa­ tial energetic angular distributions of these 166 events are very similar to the analogue distributions of the charged current events. The interpretation From a strict experimental point of view, one can state that the above is evidence for neutrino-like interac­ tions in which no charged lepton is produced. To interprete the results via neutral current interactions is pre­ sently the most plausible interpreta­ tion ; however because the outgoing neutrino is not detected, this interpre­ tation cannot be considered as uni­ que. To estimate the cross section of this new process (NC) a comparison was made with the charged current Fig. 4: A very clear electron track is produced at an angle of few degrees from the beam direction.

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