R. Núñez-Lagos

Neutrinoless double β-decay in76Ge

SummaryWe have attempted to look for the neutrinoless double beta-decay of76Ge (Jπ = 0+) leading to the first excited state (Jπ = 2+) of76Se. By using a multicoincidence set-up between a Ge(Li) detector and seven NaI detectors, a lower limitT1/2>2·1021 y, at 68% confidence level, has been obtained. The current theoretical situation for the76Ge double-beta-decay half-life has been also reviewed.RiassuntoSi cerca di trovare il doppio decadimento beta senza neutrini del76Ge (Jπ = 0+) che porta al primo stato eccitato (Jπ = 2+) di76Se. Usando un dispositivo di coincidenza multipla tra un rivelatore Ge(Li) e sette rivelatori NaI, si è ottenuto un, limite inferioreT1/2>2·1021 y ad un livello di confidenza del 68%. È stata anche riesaminata la situazione teorica corrente per la vita media del doppio decadimento beta nel76Ge.РезюмеМы пытаемся найти двойной бета-распад без участия нейтрино76Ge (Jπ = 0+), приводящий к первому возбужденном\ состоянию (Jπ = 2+)76Se. Используя систему совпадений между Ge(Li) детектором и семяю NaI детекторами, получается нижний пределT1/2>2·1021 лет, со степенью достоверности 68%. Анализируется существующая теоретическая ситуация для времени двойного бетараспада76Ge.

Neutrinoless double beta decay in76Ge for transitions to excited states of76Se

SummaryNeutrinoless double beta decays of76Ge (Jπ=0+) leading to the excited states (Jπ=21+,E=559.1 keV andJπ=02+,E=1122.3 keV) of76Se have been investigated. After some theoretical considerations about the information one can extract from the experimental data, we report on a measurement made with a Ge (Li) detector and nine NaI detectors in a 4π geometry. The following limits have been obtained after 1079 hours, of statisticsT1/2(0+→21+)≥5.5·1021 y andT1/2(0+→02+)≥2.3·1021 y (at 68% confidence level).RiassuntoSono stati studiati i doppi decadimenti beta senza neutrini di76Ge (Jπ=0+) che portano agli stati eccitati (Jπ=21+,E=559.1 keV eJπ=02+,E=1122.3 keV) di76Se. Dopo alcune considerazioni teoriche sulle informazioni che si possono trarre dai dati sperimentali si riportano le misurazioni effettuate con un rivelatore a Ge(Li) e nove rivelatori a NaI in una geometria 4π. Sono stati, ottenuti i limiti seguenti dopo 1079 ore, di statisticheT1/2(0+→21+)≥5.5·1021 y eT1/2(0+→02+)≥2.3·1021 y (ad un livello di confidenza del 68%).

A dynamical mechanism for the two-neutrino double beta decay

SummaryTo describe the physics of the two-neutrino (2v) double beta decay we propose the mechanism of one-neutral-pion exchange between nucléons as the dominant strong process to substitute the usual closure approximation which averages over intermediate nuclear states. Assuming that the corresponding nuclear matrix element for128Te →128Xe is equal to that for130Te →130Xe, we find a ratio of their half-lifes in the 2ν case essentially equal to that obtained in the conventional approach,i.e. somehow lower than the recent experimental results of Kirstenet al., which would imply no lepton number violation. For the half-lifes of both isotopes we get, within some approximations, rough estimates which lie rather close to the experimental values.RiassuntoPer descrivere la fisica del decadimento doppio beta a due neutrini (2ν) si propone il meccanismo di scambio di un pione neutro tra nucleoni come il processo forte dominante per sostituire la consueta approssimazione di chiusura ehe fa la media degli stati nuoleari intermedi. Ipotizzando che l’elemento matriciale nucleare corrispondente per128Te →128Xe sia uguale a quello per130Te →130Xe, si trova un rapporto delle loro vite medie nel caso a 2ν essenzialmente uguale a quello ottenuto nell’approceio convenzionale, cioè in qualche modo inferiore ai risultati sperimentali recenti di Kirstenet al., che non implicherebbe la violazione del numero leptonico. Per le vite medie di entrambi gli isotopi otteniamo, entro una certa approssimazione, stime grossolane che si trovano abbastanza vicine ai valori sperimentali.РезюмеДля описания физики двух-нейтринного (2v) двойного бета-распада мы предлагаем механизм обмена одним нейтральным пионом между нуклонами, как доминирующий силсный процесс, чтобы заменить обычное приближение, в котором проводится усреднение по всем промежуточным ядерным состояниям. Предполагая, что соответствуиющий ядерный матричый элемент для128Те →128Хе равен матричному элементу для130Те →130Хе, мы находим, что отношеное их перидов полураспада в случае (2ν) оказывается равным отношению, полученному в общепринятом подходе, т.е. меньше, чем недавние экспериментальные результаты Кирстена и др., которые указывают на отсутствие нарумения числа лептонов. Для периодов полураспада обоих изотопов, в рамках техже приближений, мы получаем грубые оценки, которые лежат значительно ближе к эксперименталвным величинам.

Feynman rules for processes involving composite bound systems

A set of Feynman rules is developed to deal with any Bethe-Salpeter model description of baryons and mesons interacting through any effective current-current interaction. The rules follow from the Mandelstam generalisation of the Gell-Mann-Low method to compute matrix elements and are used to evaluate the Delta T=1/2 enhancement in the process K to 2 pi .

Results of a search of the neutrinoless decay of76Ge to the first excited state of76Se in the Canfranc tunnel

SummaryTo further investigate a small, unexplained coincidence effect close to the region where a neutrinoless, 0+→2+76Ge double beta decay should be expected which we found in a previous experiment in the Frejus tunnel, a new experiment, with improved background and efficiency conditions, has been performed in the Canfranc tunnel. It is conclude that aftert=6062.5 hours of counting time the Frejus peak has been rejected as a neutrinoless double beta effect at the confidence level of 95%.

A search on the parity-forbidden alpha width of theEx=4.753 MeV (Jπ,T=0+, 1) level of18F

SummaryResults on the parity-nonconserving alpha width ΛαPNC of the (Jπ,T=0+, 1)Ex=4.753 MeV level of18F are reported. This level is tentatively fed by the14N(α, γ)18F reaction and the absence of de-excitation γ-ray allows us to set an upper limit of ΛαPNC⩽1.2·10−5 eV.RiassuntoSi riportano i risultati dell’ampiezza di alfa ΛαPNC che non conserva la parità del livello (Jπ,T=0+, 1)Ex=4.753 MeV di18F. Si tenta di alimentare quel livello mediante la reazione14N(α, γ)18F e l’assenza di un raggio gamma di diseccitazione permette di fissare un limite superiore di ΛαPNC⩽1.2·10−5 eV.

A reply to the comments of J. Bustoet al. on our paper

Although we do not want to continue a sterile controversy initiated by the preceding Comment by Busto et al. to our paper(l), we are forced to reply to a series of statements made in such a Comment, which need to be rectified. It was clearly stated in the Acknowledgements of our paper(l) that ,,the experiment was performed in collaboration, with the Bordeaux group, and thanks were given to be the staff of the Frejus Laboratory. As a matter of fact, the collaboration on 2~ decay was triggered by the Zaragoza group. The analysis of the data was made, as agreed, independently by both groups. We found a small coincidence (~peak,~) effect at about the energies where a neutrinoless double beta decay of 76Ge to the first (2 +) excited state of 76Se, in coincidence with the 76Se(2+)~76Se(0+) de-excitation photon, should be expected. After a thorough analysis of the data (see ref. (1.2)) we conclude not to make any claim of having positively found a neutrinoless double beta decay. The reasons were exposed at length in the above references, but we would like to stress that, besides the small statistical significance (2.5~) of the peak, this effect has not been seen by any of the other double beta germanium experiments, in

Indication of Neutrinoless Double Beta Decay of 76Ge to the First Excited State of 76Se. Results of the Frejus Experiment

In the present paper we report on the experimental results of a search for the neutrinoless double beta decay of 76Ge, whose decay scheme is shown in fig. 1.

Neutrinoless double

As is well known the detection of the neutrinoless double beta decay of nuclei — would indicate the violation of lepton number and would provide significant info mation about the character, possible mass and chirality couplings of the neutrino. Several direct experiments aiming to detect such a rare event have been recently carried out or are being done. We present here the preliminary results of an — attempt to search for the neutrinoless double beta decay of 7 6Ge, currently being done in the Frejus tunnel. The experiment (which uses as source and detector an — assembly of 4 Germanium crystal [1]) investigates the transition from the ground state 0+ of the 7 6Ge to the ground state 0+ of 7 6Se, as well as to the excited sta te 2+ (559.1 KeV), of 7 6Se, looking for a gaussian shaped peak at E2β=2040.7 KeV in the anticoincidence spectrum, and at E=1481.6 KeV in the coincidence spectrum 2β/γ (Eγ=559.1 KeV), respectively, by surrounding the 4-Ge detector with a 4π — assembly of NaI scintillators. Transition to the 0 2 + (1122.3 KeV) excited state is also investigated.

\beta

Abstract The effects induced by the Z-boson exchange in the hyperfine structure and Lamb shift of some fundamental bound systems is analyzed. The hfs of muonium appears as a promising test of the Weinberg-Salam theiry.