Neutrino Mass and Higgs Self-Coupling Predictions (original) (raw)
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Neutrinoless Double-Beta Decay: A Roadmap for Matching Theory to Experiment
arXiv (Cornell University), 2022
The observation of neutrino oscillations and hence non-zero neutrino masses provided a milestone in the search for physics beyond the Standard Model. But even though we now know that neutrinos are massive, the nature of neutrino masses, i.e., whether they are Dirac or Majorana, remains an open question. A smoking-gun signature of Majorana neutrinos is the observation of neutrinoless double-beta decay, a process that violates the lepton-number conservation of the Standard Model. This white paper focuses on the theoretical aspects of the neutrinoless double-beta decay program and lays out a roadmap for future developments. The roadmap is a multi-scale path starting from high-energy models of neutrinoless double-beta decay all the way to the low-energy nuclear many-body problem that needs to be solved to supplement measurements of the decay rate. The path goes through a systematic effective-field-theory description of the underlying processes at various scales and needs to be supplemented by lattice quantum chromodynamics input. The white paper also discusses the interplay between neutrinoless double-beta decay, experiments at the Large Hadron Collider and results from astrophysics and cosmology in probing simplified models of lepton-number violation at the TeV scale, and the generation of the matter-antimatter asymmetry via leptogenesis. This white paper is prepared for the topical groups TF11 (Theory of Neutrino Physics), TF05 (Lattice Gauge Theory), RF04 (Baryon and Lepton Number Violating Processes), NF03 (Beyond the Standard Model) and NF05
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Neutrinoless double beta decay and direct searches for neutrino mass
2004
* Full texts of the report of the working group. For the summary report of the APS Multidivisional Neutrino Study, 'The Neutrino Matrix', see physics/0411216 0νββ decay, independent of its rate, would show that neutrinos, unlike all the other constituents of matter, are their own antiparticles. There is no other realistic way to determine the nature-Dirac or Majorana, of massive neutrinos. This would be a discovery of major importance, with impact not only on this fundamental question, but also on the determination of the absolute neutrino mass scale, and on the pattern of neutrino masses, and possibly on the problem of CP violation in the lepton sector, associated with Majorana neutrinos. There is a consensus on this basic point which we translate into the recommendations how to proceed with experiments dedicated to the search of the 0νββ decay, and how to fund them. • To reach our conclusion, we have to consider past achievements, the size of previous experiments, and the existing proposals. There is a considerable community of physicists worldwide as well as in the US interested in pursuing the search for the 0νββ decay. Past experiments were of relatively modest size. Clearly, the scope of future experiments should be considerably larger, and will require advances in experimental techniques, larger collaborations and additional funding. In terms of m ββ , the effective neutrino Majorana mass that can be extracted from the observed 0νββ decay rate, there are three ranges of increasing sensitivity, related to known neutrino-mass scales of neutrino oscillations. • The ∼100-500 meV m ββ range corresponds to the quasi-degenerate spectrum of neutrino masses. The motivation for reaching this scale has been strengthened by the recent claim of an observation of 0νββ decay in 76 Ge; a claim that obviously requires further investigation. To reach this scale and perform reliable measurements, the size of the experiment should be approximately 200 kg of the decaying isotope, with a corresponding reduction of the background. This quasi-degenerate scale is achievable in the relatively near term, ∼ 3-5 years. Several groups with considerable US participation have well established plans to build ∼ 200-kg devices that could scale straightforwardly to 1 ton (Majorana using 76 Ge, Cuore using 130 Te, and EXO using 136 Xe). There are also other proposed experiments worldwide which offer to study a number of other isotopes and could reach similar sensitivity after further R&D. Several among them (e.g. Super-NEMO, MOON) have US participation. By making measurements in several nuclei the uncertainty arising from the nuclear matrix elements would be reduced. The development of different detection techniques, and measurements in several nuclei, is invaluable for establishing the existence (or lack thereof) of the 0νββ decay at this effective neutrino mass range. • The ∼20-55 meV range arises from the atmospheric neutrino oscillation results. Observation of m ββ at this mass scale would imply the inverted neutrino mass hierarchy or the normal-hierarchy ν mass spectrum very near the quasidegenerate region. If either this or the quasi-degenerate spectrum is established, it would be invaluable not only for the understanding of the origin of neutrino mass, but also as input to the overall neutrino physics program (long baseline oscillations, search for CP violations, search for neutrino mass in tritium beta decay and astrophysics/cosmology, etc.) To study the 20-50 meV mass range will require about 1 ton of the isotope mass, a challenge of its own. Given the importance, and the points discussed above, more than one experiment of that size is desirable. • The ∼2-5 meV range arises from the solar neutrino oscillation results and will almost certainly lead to the 0νββ decay, provided neutrinos are Majorana particles. To reach this goal will require ∼100 tons of the decaying isotope, and no current technique provides such a leap in sensitivity. • The qualitative physics results that arise from an observation of 0νββ decay are profound. Hence, the program described above is vital and fundamentally important even if the resulting m ββ would be rather uncertain in value. However, by making measurements in several nuclei the uncertainty arising from the nuclear matrix elements would be reduced. • Unlike double-beta decay, beta-decay endpoint measurements search for a kinematic effect due to neutrino mass and thus are "direct searches" for neutrino mass. This technique, which is essentially free of theoretical assumptions about neutrino properties, is not just complementary. In fact, both types of measurements will be required to fully untangle the nature of the neutrino mass. Excitingly, a very large new beta spectrometer is being built in Germany. This KATRIN experiment has a design sensitivity approaching 200 meV. If the neutrino masses are quasi-degenerate, as would be the case if the recent double-beta decay claim proves true, KATRIN will see the effect. In this case the 0νββ-decay experiments can provide, in principle, unique information about CP-violation in the lepton sector, associated with Majorana neutrinos. • Cosmology can also provide crucial information on the sum of the neutrino masses. This topic is summarized in a different section of the report, but it should be mentioned here that the next generation of measurements hope to be able to observe a sum of neutrino masses as small as 40 meV. We would like to emphasize the complementarity of the three approaches, 0νββ , β decay, and cosmology. Recommendations: We conclude that such a double-beta-decay program can be summarized as having three components and our recommendations can be summarized as follows: