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The Elusives Enterprise: Asymmetries of the Invisible Universe

Periodic Reporting for period 1 - ELUSIVES (The Elusives Enterprise: Asymmetries of the Invisible Universe)

Reporting period: 2016-04-01 to 2018-03-31

Neutrinos and Dark Matter are the most abundant and also the most elusive building-blocks of nature because of their tenuous couplings to ordinary matter. Each particle has a mirror image with identical mass and opposite charge: its antiparticle. What is the essential nature of particles and antiparticles? This is a most fundamental open question in science. The laws of physics are almost -but not quite- symmetric for particles and antiparticles, and this could explain why the universe is made only of matter. Tiny differences detected in visible matter are largely insufficient to explain this fact, but an asymmetric behaviour of the elusives components of nature, neutrinos and/or dark matter, may explain it. The unexpected symmetric behaviour of the strong interactions also points to new physics and a new particle, the axion, which might be the origin of dark matter. Understanding the connection between the asymmetries of matter and antimatter in the visible and invisible world is the main goal of this project.
The scientific progress of the project has been excellent, leading by now to 280 scientific publications in neutrino physics, dark matter (DM) physics and their connection. The project ESR appear as (co)authors of 23 of them.

Some of the highlights include the intriguing 2σ hints for leptonic CP violation confirmed in detailed global analyses of neutrino oscillation data by our Nu-Fit group ( Our team has carefully analysed the latest data relevant for sterile neutrinos and we set new very stringent constraints on this scenario. Furthermore, members of our team who are experts in analyzing cosmology data have provided some of the best results in this field in the topic of the sum of neutrino masses.

The Majorana character of neutrinos would have profound implications in particle physics. Our team includes members of the three leading experiments in Europe: GERDA, CUORE and NEXT that are searching for neutrinoless double beta decay using three isotopes Germanium, Telurium and XENON.

The possibility that the neutrino masses might originate from new heavy states that are within reach in present of future experiments has been actively pursued. In particular, it has been shown that LHC and other future colliders, such as the ILC or FCC, could improve their sensitivity by searching for the powerful signal of displaced decays of the neutrino mass mediators. Neutrinos could also be portals to other type of new light physics states although elusive due their very weak interactions, and its impact on future neutrino experiments such as DUNE has been explored.

Our team has in addition considered various possibilities for the origin of PeV neutrinos of extraterrestrial origin detected by Icecube and has also evaluated the flavour and angular distribution of neutrinos emerging from supernovas.

Another important area of research has been the phenomenological and theoretical implications of axions or axion-like particles, that provide viable dark matter (DM) candidates, and could also solve the strong CP problem. This is particularly timely in light of the new experiments such as ADMX that will have sensitivity to QCD axions. An important new result from our team is the evaluation of the contribution of axion-like particles to lepton dipole moments. Connections between axion particles and flavour in the context of minimal flavour violation or discrete flavour symmetries have been explored. Also connections of axions to the Goldstone Higgs hypothesis (that can solve the hierarchy problem) have been analysed in detail and its LHC phenomenology worked out. Furthermore, new theoretical developments and novel signals of ALPs (axion-like-particles) for present and future colliders have been also identified by our team.

From the astrophysical point of view, axion DM might lead to interesting features, such as the formation of mini-clusters, as also considered by our team.

The most stringent bounds on the WIMP DM paradigm have been set by the XENON 1 Ton experiment, led by various members of our team. In light of these stringent bounds, variants of the WIMP DM models have been considered, particularly with the goal of establishing connections to other flavour anomalies, such as those observed in B meson semileptonic decays or in the muon anomalous magnetic moment, g-2.

The anomalies in B meson decays have received very much attention, after the LHCB results last year. Our team has carefully analysed the systematic errors involved from the theory side and has proposed various new physics explanations such as the existence of leptoquarks.

Training activities have included local training, secondments, two annual ITN events (school and workshop each year), and Elusives Virtual Institute (EVI). The Elusives Virtual Institute has organized web-based training in the field of theoretical physics, with activities including webinars, seminars, Journal Club, virtual collaboration room etc.
Elusives has by now achieved substantial beyond-the-state-of-the-art progress in neutrino physics, in particular on phenomenology of neutrino oscillations and on the experimental limits on neutrinoless double beta decay. Elusives has also pushed the theoretical and model-building frontier in neutrino physics to a new realm, with particular emphasis on the matter versus antimatter implications. On the axion and axion-like particles domain, new theoretical developments, as well as novel phenomenological signals and correlations with new physics, have been achieved which have the potential to shed important light on the crucial question of CP violation. On the experimental search for WIMP dark matter, the most stringent world bounds have been established by our team. Their impact in the understanding of different experimental anomalies observed in particle physics has been successfully explored. Innovative analysis were also carried out which are of high relevance for the present anomalies in B physics data. Furthermore, novel astrophysical and cosmological achievements have been obtained related to all these domains.

New experimental and theoretical frontiers/discoveries on the network objectives are foreseen in the next two years.

Elusives has been able to attract highly talented young people to a research career at the forefront of scientific and technological developments, has helped them to develop their full potential via a structured and coherent doctoral/early-stage training and has fostered their growth as individuals. The ITN has given them: 1) Confidence to tackle essential questions on the nature of the universe and to be the main agents of fundamental discoveries, 2) Rigorous scientific skills (solid, broad background in a specific research area) and problem-solving skills, which can be applied both to a scientific or industrial R&D questions, 3) Logical thinking, critical capacity, independence, 4) Transferable skills such as management, organisation, communication, and 5) Exceptional outreach and dissemination skills.

Elusiνes has built a unique pan-European training activity on the particle/antiparticle identity of neutrinos, DM and BSM particles. In doing so, it has highly contributed to the policy objective of maintaining and fostering the world-wide leadership that Europe owns in this domain. The non-academic sector has contributed to the doctoral/research training.