Neutrinoless double beta decay

Radioactive decays are a well known phenomenon. In a beta decay, a neutron in the nucleus decays into a proton emitting an electron and an anti-neutrino. Some nuclei cannot decay this way because the new nucleus would have more mass than the old one. In that case, two neutrons have to decay together, as shown in the following figure:

As it is hard for neutrons to communicate and agree on such a dead, such decays are extremely rare. However, they are considered normal and are observed. This can be done with germanium detectors. The GERDA and MAJORANA collaboration currently work with germanium detectors. The GERDA collaboration has published a value for the half-life for normal double beta decay in germanium of T(1/2) = (1.84 +0.14 -0.10) x 10^{21} years [1]. This has to be compared to the age of the universe of “only” about 1.3 x 10^{10} years.

The observed decays show a spectrum of the energy of the two electrons which is compatible with two anti-neutrinos taking some of the energy released in the decay away. If no neutrinos would be emitted, all the energy would be carried by the electrons and all event would show exactly the same energy. The search for such a peak in the energy spectrum is the way to look for neutrinoless double beta decay. Such a decay is possible, if the neutrino is its own anti-particle. In this case, one neutron emits the anti-neutrino but the other neutron considers it a neutrino and absorbs it.

It would be a fundamental discovery to observe neutrinoless double beta decay. If the neutrino is its own anti-particle this could be part of the explanation why after the big bang some matter survived but no anti-matter. However, even if they can, it is much harder for two neutrons to match up properly to exchange a neutrino than to just decay together. Therefore, the decay rate is expected to be several orders of magnitude lower than for normal double beta decay, i.e. the half-life is orders of magnitude larger. The best experimental limit in germanium has also been set by the GERDA collaboration as T(1/2) > 5.3 · 10^{25} years (90% C.L.) [2]. In the future, the search is to be extended by several orders of magnitude. This will happen using large amounts of germanium and also xenon.

The research done within the framework of GEMARDARC will contribute to the better understanding of the germanium detectors used for the future germanium-based experiment LEGEND and will, thus, help to develop the concepts and strategies for this experiment. Some members of the LEGEND collaboration are also members of GEMARDARC PIRE and provide opportunities to be part of this research.

[1] J. Phys. G: Nucl. Part. Phys. 40 (2013) 035110 (arXiv:1212.3210)

[2] Nature 554 (201) 47-52 (arXiv:1703.00570)

Dark matter

There seems to be a lot more in the universe than can be seen. The normal matter that “our world” is made of seems to be only a small part of it. The first indication that the observed matter cannot be all there is came from the study of galaxy clusters: there is apparantly not enough matter to hold them together. The galaxies move about so fast that the clusters should disperse.

Then came the observation that the stars at the edge of large spiral galaxies circle their galaxies too fast. This indicates that these galaxies have very massive halos which are invisible, i.e. “dark”.

Stars move as fast at the edge of the galaxy as further in. They would be slower, if only the visible matter was there.

And finally came the mapping of the dark masses with gravitational lensing.

In Einstein’s description of gravity, any mass bends space-time. Thus, even the path of a massless particle like a photon, i.e. light, gets bent. This can result in observing the same object multiple times.

A typical example of this phenomenon is an “Einstein cross”.

Spectral analysis shows that the five images show the same object. A single object is sometimes seen as an arc or even as a complete ring. This depends on the distribution of the masses bending the light.

Thus, the observation of multiple images of many far away objects makes it possible to estimate the amount and distribution of mass between the far away objects and us. It is like a gigantic puzzle. In the end, a three dimensional map of the dark universe emerges.

The amazing thing is that there is about five times more dark mass in the universe than there is normal mass.

Normally the dark mass is seen to surround the normal mass. However, one exception has been observed. The bullet cluster is a very special case.

Two galaxy clusters collided and the visible matter interacted to form one cluster. The dark matter components of the two clusters did not interact and separated again. They were mapped and are shown in blue. The normal matter is dominated by hot gas shown in pink. The bullet cluster is generally seen as THE evidence for dark mass.

As we cannot see this dark mass, we call whatever it is made of “dark matter”. Particle physics provides a couple of hypothetical candiates for dark matter. One of them is the “weakly interacting massive particle”, WIMP. This particular candidate has been searched for by many collaborations and in many ways. One approach is to place germanium detectors in a laboratory deep underground, protected from all natural radioactivity. In such an environment, a germanium detector does not produce background signals and it is possible to wait for the signal from an interaction of a WIMP in the crystal. So far, nobody has been able to identify such a signal. Thus, it is only possible to excludes WIMPs with certain masses and certain propabilities to interact.

Such exclusions only make sense, if we assume that dark matter is present around and in our earth. As the stars at the edge of our galaxy also move faster than the visible mass would suggest, this is believed to be a good assumption.

One of the experiments searching for WIMPs is the “China Dark Matter EXperiment”, CDEX. Some Members of CDEX are also members of GEMADARK PIRE and visits are possible.

Neutrino Electromagnetic Interactions

Neutrino oscillation experiments provide compelling evidence on finite neutrino masses and mixings. It is expected that these would lead to anomalous couplings between the neutrinos and the photons (Figure 1). The studies of neutrino electromagnetic interactions [1] are promising avenues to probe these possibilities.

Figure 1. Studies of neutrino electromagnetic processes probe the possible existence of couplings between the neutrino and the photon. Neutrino magnetic moments parametrize the case when the incoming and outgoing neutrinos have opposite helicities (as depicted), while neutrino milli-charge and charge radius describe one where the spin states are conserved.

Intrinsic neutrino properties such as magnetic moments (μ_ν) [2], milli-charge (q_ν) [3] and charge radius (<r_ν²>) are the realizations of anomalous neutrino electromagnetic effects. Their experimental manifestations are the deviations of the integral and differential cross-sections in neutrino-atom interactions:

ν + A → ν + A⁺ + e^-

relative to those due to Standard Model electroweak processes. In particular, both ν_ν and q_ν would provide cross-section enhancement as well as distinct spectral features at low (<10 keV) energy transfer or equivalently the measureable energy in an interaction (Figure 1). These distinct features have also be adopted in placing constraints on sterile neutrinos as dark matter [4].

Germanium detectors, with their low threshold and excellent energy resolution, are optimal in the studies of neutrino electromagnetic effects [5]. Some of the most sensitive studies have been obtained with germanium detectors using reactor neutrinos at the Kuo-Sheng Reactor Neutrino Laboratory in Taiwan by the TEXONO group, a collaborating partner or the PIRE-GEMADARC research program. The expected improvement of the germanium detector technologies in detection threshold, energy resolution and intrinsic background in the course of the PIRE-GEMADARC program will further extend the sensitivity reach in these investigations.

Figure 2: Schematics of neutrino electromagnetic interaction with matter. Interactions with changes in the neutrino helicity states probe neutrino magnetic moments (as depicted) while those without changes are due to neutrino milli-charge and charge radius. The observable signatures are derived from the measurements of the final-state photons and electrons.

Figure 3: Measureable differential spectra for various neutrino interactions with matter using reactor neutrinos at typical configurations – black and blue lines correspond to the Standard Model neutrino-electron and –nucleus elastic scatterings; red, magenta add green lines denote those due to neutrino magnetic moments, milli-charge and charge radius at the specified values, respectively.

References:

1. C. Giunti, A. Studenikin, Rev. Mod. Phys. 87 (2015) 531 (and references therein).
2. H.T. Wong, et al. (TEXONO Collaboration),Phys. Rev. D75 (2007) 012001; A.G. Beda, et al. (GEMMA Collaboration), Phys. Part. Nucl. Lett. 10 (2013) 139.
3. J.W. Chen, et al., Phys. Rev. D90 (2014) 011301(R).
4. J.W. Chen, et al., Phys. Rev. D.93 (2016) 093012.
5. J.W. Chen, et al., Phys. Rev. D.91 (2015) 013005; A.K. Soma et al. (TEXONO Collaboration), Nucl. Instrum. Meth. A836 (2016) 67.

Coherent Elastic ν-Nucleus Scattering (CEvNS)

A bullet can distroy an apple, while a billiard ball can only strike another without tearing it apart. This difference is caused by the difference in the energies of the incident objects. A very energetic incident object tends to interact with only a small part of the target, while a less energetic one tends to interact with the target as a whole.

Similarly, a high energy neutrino, ν, tends to interact with individual nucleons in a nucleus, while a low energy ν can interact with a nucleus as a whole. The latter process is called the Coherent Elastic ν-Nucleus Scattering, or CEvNS in short.

The existence of CEvNS was predicted long time ago [1, 2], but it was only observed very recently in the COHERENT experiment [3]. The CEvNS process is important in the evolution of astronomical objects [4]. It can also be used to probe non-standard neutrino interactions [5], sterile neutrinos [6] and nuclear structures [7, 8], as well as to monitor the activity of a reactor [9]. Many experiments along with COHERENT have been performed or proposed to detect CEvNS and to study neutrino physics beyond the Standard Model, for example, MINER [10], CONNIE [11], TEXONO [12], and RED [13], etc. The major difficulty is that the energies of recoiled nuclei are so small that detectors with extremely low energy thresholds are needed. PIRE collaborators are in the leading institutes of the MINER and TEXONO experiments.

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[3] Akimov, D. et al., Observation of Coherent Elastic Neutrino-Nucleus Scattering, Science, August 3, 2017, eaao0990. https://doi.org/10.1126/science.aao0990.

[4] H.-T. Janka, K. Langanke, A. Marek, G. Martinez-Pinedo and B. Mueller, Theory of Core-Collapse Supernovae, Phys. Rept. 442 (2007) 38 [astro-ph/0612072].

[5] J. Barranco, O.G. Miranda and T.I. Rashba, Probing new physics with coherent neutrino scattering off nuclei, JHEP 12 (2005) 021.

[6] J.A. Formaggio, E. Figueroa-Feliciano and A.J. Anderson, Sterile Neutrinos, Coherent Scattering and Oscillometry Measurements with Low-temperature Bolometers, Phys. Rev. D 85 (2012) 013009 [arXiv:1107.3512].

[7] K. Patton, J. Engel, G.C. McLaughlin and N. Schunck, Neutrino-nucleus coherent scattering as a probe of neutron density distributions, Phys. Rev. C 86 (2012) 024612 [arXiv:1207.0693].

[8] K.M. Patton, G.C. McLaughlin and K. Scholberg, Prospects for using coherent elastic neutrino-nucleus scattering to measure the nuclear neutron form factor, Int. J. Mod. Phys. E 22 (2013) 1330013.

[9] A. Drukier and L. Stodolsky, Principles and Applications of a Neutral Current Detector for Neutrino Physics and Astronomy, Phys. Rev. D 30 (1984) 2295.

[10] G. Agnolet et al., Background studies for the MINER Coherent Neutrino Scattering reactor experiment, Nucl. Instrum. Meth. A, 853:53-60, May 2017. ISSN 0168-9002. doi:10.1016/j.nima.2017.02.024.

[11] A. Aguilar-Arevalo et al., The CONNIE experiment, Journal of Physics: Conference Series, 761(1):012057, 2016. ISSN 1742-6596. doi:10.1088/1742-6596/761/1/012057.

[12] TEXONO collaboration, S. Kerman, V. Sharma, M. Deniz, H.T. Wong, J.W. Chen, H.B. Li et al., Coherency in Neutrino-Nucleus Elastic Scattering, Phys. Rev. D 93 (2016) 113006 [arXiv:1603.08786].

[13] D.Y. Akimov et al., RED-100 detector for the first observation of the elastic coherent neutrino scattering off xenon nuclei, J. Phys. Conf. Ser. 675 (2016) 012016.