RAL, 24th September 2003
A general meeting was held at the RAL to discuss neutrinoless double beta decay, attended by about 40 people from the particle physics and nuclear physics communities.
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10:00-10:10 |
Introduction and aims of the meeting |
Ken Peach |
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10:10-10:50 |
Review of the status of neutrinoless
double beta decay |
Ruben Saakyan |
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10:50-11:30 |
NEMO-III: Status and Results |
Jenny Thomas |
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11:30-12:00 |
Super-NEMO: Plans for the next generation
NEMO experiment |
Ruben Saakyan |
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12:00-12:40 |
The COBRA (CdZnTe) project |
Kai Zuber |
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Lunch |
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14:00-14:20 |
Implications of non-linear Lorentz
Transformations for n decays |
Joao Magueijo |
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14:20-16:00 |
Discussion |
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Ken Peach outlined the background and aims of the meeting. The first aim was to find out the level of interest in neutrinoless double beta decay [0nbb] in the UK – some 40 people were present, and there were other people who unable to come to this meeting. The second aim was to decide what actions, if any, should follow the meeting, in order to enable the UK to lead, should it so wish, a double beta decay programme. It was timely because recent developments in neutrino physics indicate that the masses of the neutrinos are much less than 1eV (discarding the LSND result which, if confirmed, points to a fourth neutrino-like object with a different mass scale). While there is sensitivity to the neutrino mass in cosmological and astrophysical measurements and observations, it is vitally important to try to measure the absolute neutrino mass scale in laboratory experiments. There are two approaches - direct measurement of the electron neutrino mass through measurements near the end of the electron spectrum in (tritium) beta decay, and neutrinoless double beta decay. The KATRIN experiment currently being planned in Karlsruhe has a sensitivity of about 0.2eV, close to the upper limit deduced (with assumptions) from cosmology of about 0.25eV. However, we also need to consider how to reach sensitivities of around 0.01eV. The most promising route seems to be neutrinoless double beta decay, which can potentially reach this sensitivity for a weighted average mass for the three neutrinos. Of course, this requires that the neutrinos have a Majorana component (itself a major discovery) and it is possible that the Majorana phases could conspire to evade this limit. Achieving this level of sensitivity presents several significant experimental challenges, including evaluation of the nuclear matrix elements.
Ruben Saakyan reviewed the current status of neutrinoless double beta decay experiments. There were about thirty neutrinoless double beta candidate nuclei, but only about a dozen combined a high Q value (the rate goes as Q5, and the background from the U/Th chain decreases with energy) with a potentially low background from the “normal” 2nd order weak process. There had been tremendous progress over the past five years or so, and the current generation of experiments were achieving sensitivities that a decade ago could only be dreamt about. The latest results from NEMO-III and CUORICINO gave limits on the m2ee parameter of between 0.8 and 2.2 eV, where the range was dominated by the uncertainties in the nuclear matrix elements, although the best limit was still the Heidelberg-Moscow 76Ge limit of between 0.35-1 eV.
Jenny Thomas discussed the status of the NEMO-III experiment, including some recent results from the data taken since February 2003. The experiment used a large tracking volume immersed in a weak magnetic field to track and reconstruct the two electron tracks from decay candidates in thin foils, and calorimetry to achieve the energy resolution. The experiment was taking data, and had achieved world-leading limits for a number of nuclei.
Ruben Saakyan presented the ideas for “SuperNEMO”, with target masses (for relevant isotopes) in the 100kg range, giving sensitivities to average neutrino masses in the interesting range around 0.05 eV. The geometry of SuperNEMO was not yet decided, and was modular, permitting significant flexibility. It was not clear that a magnetic field was essential to reduce the background. Selenium seemed to be the optimum nucleus, although it was essential that neutrinoless double beta decay was observed in a number of nuclei. There had been a visit by the NEMO collaboration to Boulby, which had some interesting features – in particular, the cost of excavating a suitable cavern was relatively modest.
Kai Zuber described the work on a new double beta technique (COBRA) using Cd(Zn)Te semiconductors, which contained a number of interesting decay modes, including several double beta emitters (including one of the few double positron emitters). The R&D was at an early stage, but already with target/detectors of only a few grams, world-leading limits had been established for a few processes. Larger detectors (2-3 cm in size) were being developed. Cd(Zn)Te was a commercially available and interesting material, which combined target and detector, making it a potentially powerful technique that could scale to large volumes. Pixelised Cd(Zn)Te might combine the features of semiconductor detectors and TPCs.
Finally, Joao Magueijo discussed the
potential implications of resolving apparent inconsistencies with Lorentz
invariance for neutrino interactions in the presence of mixing. A possible
solution is that the energy and momentum of flavour eigenstates transform
according to a non-linear realization of the Lorentz group. This could have
detectable consequences in the distortions of the Kurie plot near the
end-point, and potentially elsewhere.
Although this was clearly a biased sample (after all, people had chosen to come to a meeting on the topic) it was felt that the subject (the nature of the neutrino, and the absolute mass scale) was very important, and that the UK should be among the leaders.
It was not clear which was the fastest route to the first observation, and so choosing a particular nucleus inevitably involved a high risk that the first observation would be in another nucleus. However, backing “all horses” would spread the (limited) resources too thinly, with consequential lack of profile and influence. The obvious strategy was to pursue a limited number of lines of research, but with a clear focus. The opportunities for the UK seemed to be in the NEMO/SuperNEMO experiment, exploration of the potential of Cd(Zn)Te technique and (not discussed explicitly) the possibility that the liquid Xenon expertise in UKDMC might be exploited (Xenon has 0nbb candidates). There were many nuclear physics challenges, and it was recognised that there were opportunities to working with the nuclear physics community on these aspects. There was a clear synergy with some of the technical issues (detector technology and its associated electronics including pulse shape interpretation and tracking aspects in which the UK nuclear physics community has experience) and there were also a number of interesting nuclear physics issues (nuclear matrix elements, spectroscopy and transition rates). The opportunity to collaborate with the nuclear physics community was welcomed. There was also an opportunity to investigate the use of the undergrounds facilities at Boulby for 0nbb experiments – the relatively low cost of excavation in salt was attractive, although the realistic life of experimental halls was probably about 10-15 years.
A number of “next steps” were identified.
1. There was interest in organising a phenomenology workshop in conjunction with the IPPP, to investigate the issues associated with the uncertainties in the nuclear matrix elements, physics applications and a detailed review of the implications of neutrino oscillations on a range of neutrino phenomena – the talk of Joao Magueijo had raised a number of interesting questions that deserved further consideration, and there were other physics processes (doubly charged Higgs bosons, right-handed.weak currents, R-parity violating SUSY, Leptoquarks) that could also mediate neutrinoless double beta decay.
2. A half-day IoP meeting was planned sometime next year in Sussex, which would also raise the profile of this activity, and (if it followed the IPPP workshop) could provide a forum to advertise the outcome, or (if it preceded the workshop) to advertise the opportunity.
3. It would be useful to create a UK Interest Group in 0nbb decay, to provide a focus for UK activity, and to provide an information exchange between those interested. Those who had attended this meeting would provide a natural core group. A small group was established to develop the interest group – Ruben Saakyan, Kai Zuber, Brian Fulton, Sean Paling, Sacha Davidson and James Stirling. It would be also necessary to prepare some kind of “statement of interest” or strategy to be presented to PPARC, so that it could be included as a possibility in their forward planning. EPSRC should also be approached by those interested in the nuclear physics technology developments which are required
4. In the meantime, it was clear that we should continue with the present strategy of pursuing the NEMO-III/SuperNEMO route (with exploration of the possibility that Boulby might be a suitable site for SuperNEMO) with R&D into new 0nbb technologies such as Cd(Zn)Te and perhaps Xenon.
Ken Peach
25th September 2003