Summary
A high sensitivity experiment to search for the neutrinoless conversion of a muon into an electron {ie261-01}, where {ie261-02} is a Cu nucleus) has been carried out at the CERN Synchro-cyclotron. The space correlation between the muon and the electron was « seen » in a spark chamber operated as a target in the muon beam. The time correlation was observed on an oscilloscope trace which also contained the pulse from a large NaI(Tl) crystal, used to measure the energy of the electron (103.8 MeV in the case of the above mentioned process occurring coherently in the Cu plates of the chamber). Nearly 2.4 billion μ−-mesons were stopped during the main run. From the scanning of the film of the chamber (two 90° stereo-views) and of the corresponding oscilloscope pictures obtained in this run, four events have been found fulfilling all requirements of time correlation, space correlation and correct energyrelease in the Nal counter. It is shown, however, that ∼5±2 is the expected number of events of spurious nature. Hence, if the neutrinoless coherent nuclear capture of a meson exists at all, its branching ratio to the ordinary capture is concluded to be less than 2.2 .10−7 with 90% confidence level. This negative result fits into the recent discovery of two kinds of neutrinos.
Riassunto
Un esperimento di alta sensibilità per rivelare la conversione di un muone in un elettrone senza emissione di neutrini {ie282-01}, dove {ie282-02} è un nuoleo di Cu) è stato eseguito al sincrociclotrone del CERN. Si osservava la correlazione spaziale fia il muone e Felettrone in una camera a scintilla usata come « target » in un fascio di muoni. La correlazione temporale veniva osservata sulla traccia di un oscillografo ; su quest’ultima veniva anche inviato l’impulso proveniente da un grande cristallo di Nal(Tl), usato per misurare l’energia dell’elettrone (103.8 MeV nel caso che il suddetto processo avvenga « coerentemente » nelle lastre di Cu della camera). Nel corso delle misure sono stati frenati circa 2.4 miliardi di muoni negativi. In seguito all’analisi delle fotografle della camera (due fotografle a 90°) e dell’oscillografo, si sono trovati quattro eventi soddisfacenti tutti i requisiti di correlazione temporale e spaziale, con inoltre la giusta perdita di energia nel contatore a Nal. Tuttavia si mostra che il numero di aspettazione degli eventi di natura spuria è 5±2. Si conclude quindi che, se la cattura coerente di un muone senza emissione di neutroni esiste, il suo « branching ratio » rispetto alla cattura ordinaria è minore di 2.2-10−7 con un livello di confldenza del 90%. Questo risultato negativo è in accordo con la récente scoperta dell’esistenza di due tipi di neutrini.
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CERN Visiting Professor, on leave of absence from the University of Rome.
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Conforto, G., Conversi, M., di Lella, L. et al. Search for neutrinoless coherent nuclear capture of μ -mesons. Nuovo Cim 26, 261–282 (1962). https://doi.org/10.1007/BF02787041
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DOI: https://doi.org/10.1007/BF02787041