Strategy Recommendations

Formulated as 21 individual recommendations, the goals that APPEC aspires to achieve in the decade ahead as laid out in the latest roadmap are presented below. They are grouped into three categories:
• scientific issues;
• organisational issues; and
• societal issues.

Scientific issues

Large-scale multi-messenger infrastructures
To improve understanding of our Universe, APPEC identified as a very high priority those research infrastructures that exploit all confirmed high-energy ‘messengers’ (cosmic particles that can provide vital insights into the Universe and how it functions). These messengers include gamma rays, neutrinos, cosmic rays and gravitational waves. European coordination is essential to ensuring timely implementation of such infrastructures and enabling Europe to retain its scientific leadership in this field.

1. High-energy gamma rays
Through the use of ground-based gamma ray telescopes (e.g. HESS and MAGIC) and key participation in satellite missions such as Fermi, Europe has played a leading and pioneering role in establishing high-energy gamma rays as an ideal messenger to enable exploration of the extreme Universe – as demonstrated by the astonishing number of gamma-ray sources discovered in recent years. The next-generation European-led, ESFRI-listed global project will be the Cherenkov Telescope Array (CTA), which has excellent discovery potential ranging from astrophysics to fundamental physics. The CTA is expected to start full operation as an observatory in 2023.

APPEC fully supports the CTA collaboration in order to secure the funding for its timely, cost-effective realisation and the subsequent long-term operation of this observatory covering both northern and southern hemispheres.

2. High-energy neutrinos
IceCube’s first observation of PeV-scale cosmic neutrinos in 2013 has opened an entirely new window onto our Universe: neutrino astronomy. As well as presenting the opportunity to resolve neutrinos’ mass hierarchy by studying atmospheric neutrinos, this led ESFRI to include KM3NeT 2.0 in its 2016 roadmap, with operation anticipated to commence in 2020. Within the Global Neutrino Network (GNN), the IceCube, KM3NeT and BaikalGVD collaborations already join forces to provide a network of large-volume detectors viewing both northern and southern hemispheres and to exploit efficiently the full discovery potential inherent in neutrino astronomy.

For the northern hemisphere (including Baikal GVD), APPEC strongly endorses the KM3NeT collaboration’s ambitions to realise, by 2020: (i) a large-volume telescope with optimal angular resolution for high-energy neutrino astronomy; and (ii) a dedicated detector optimised for low energy neutrinos, primarily aiming to resolve the neutrino mass hierarchy. For the southern hemisphere, APPEC looks forward to a positive decision in the US regarding IceCube-Gen2.

3. High-energy cosmic rays
The Pierre Auger Observatory is the world’s largest, most sensitive ground-based air-shower detector. Understanding the evident flux suppression observed at the highest energies requires good mass resolution of primary cosmic rays: are they predominantly light nuclei (protons) or heavy nuclei (like iron)? This is the missing key to deciding whether the observed cut-off is due to particles being limited in energy because of interactions with the CMB, or to cosmic accelerators ‘running out of steam’ to accelerate particles. The Auger collaboration will install additional particle detectors (AugerPrime) to measure simultaneously the electron and muon content of air showers, in order to help determine the mass of primary cosmic rays. This upgrade will also deepen understanding of hadronic showers and interactions at centre-of-mass energies above those accessible at the LHC.

APPEC strongly supports the Auger collaboration’s installation of AugerPrime by 2019. At the same time, APPEC urges the community to continue R&D on alternative technologies that are cost-effective and provide a 100% (day and night) duty cycle so that, ultimately, the full sky can be observed using very large observatories.

4. Gravitational waves
The first direct observations of gravitational waves by the LIGO-Virgo consortium have revealed a scientific treasure trove. Multi-solar-mass black holes coalescing within seconds into one larger black hole and simultaneously radiating the equivalent of a few solar masses of energy as gravitational waves are now an established fact; they also provide unprecedented tests of General Relativity. Another new, revolutionary window onto our Universe has therefore now opened: gravitational-wave astronomy. In this field, the laboratories that host gravitational-wave antennas play a crucial role by developing new technologies to increase detection efficiencies further. The incredibly high precision in monitoring free-falling objects in space recently achieved by ESA’s LISA Pathfinder mission is an important step towards complementary (low-frequency) space-based gravitational-wave astronomy.

With its global partners and in consultation with the Gravitational Wave International Committee (GWIC), APPEC will define timelines for upgrades of existing as well as next generation ground-based interferometers. APPEC strongly supports further actions strengthening the collaboration between gravitational-wave laboratories. It also strongly supports Europe’s next-generation ground based interferometer, the Einstein Telescope (ET) project, in developing the required technology and acquiring ESFRI status. In the field of space-based interferometry, APPEC strongly supports the European LISA proposal.

Medium-scale Dark Matter and neutrino experiments
APPEC considers as its core assets the diverse, often ultra-precise and invariably ingenious suite of medium-scale laboratory experiments targeted at the discovery of extremely rare processes. These include experiments to detect the scattering of Dark Matter particles and neutrino-less double-beta decay, and direct measurement of neutrino mass using single-beta decay. Collectively, these searches must be pursued to the level of discovery, unless prevented by an irreducible background or an unrealistically high demand for capital investment.

5. Dark Matter
Elucidating the nature of Dark Matter is a key priority at the leading tip of astroparticle physics. Among the plethora of subatomic particles proposed to explain the Dark Matter content of our Universe, one category stands out: the Weakly Interacting Massive Particle (WIMP). WIMPs arise naturally, for instance, in supersymmetric extensions of the Standard Model of particle physics. Many experiments located in deep-underground laboratories are searching for WIMP interactions. For masses in excess of a few GeV, the best sensitivity to WIMPs is reached with detectors that use ultrapure liquid noble-gas targets; such detectors include XENON1T (using 3.5 tons of xenon) and DEAP (using 3.6 tons of argon), which both started operating in 2016. Their sensitivity can be further enhanced by increasing the target mass. A suite of smaller-scale experiments is exploring, in particular, low-mass WIMPs and other Dark Matter hypotheses such as those based on dark photons and axions.

APPEC encourages the continuation of a diverse and vibrant programme (including experiments as well as detector R&D) searching for WIMPs and non-WIMP Dark Matter. With its global partners, APPEC aims to converge around 2019 on a strategy aimed at realising worldwide at least one ‘ultimate’ Dark Matter detector based on xenon (in the order of 50 tons) and one based on argon (in the order of 300 tons), as advocated respectively by DARWIN and Argo.

6. Neutrino mass and nature
Despite all previous efforts, some of the neutrino’s very fundamental characteristics remain unknown. Notably, these include neutrino mass and whether the neutrino is its own anti-particle or not (in other words, whether it is a Majorana-type particle or a Dirac-type particle). Both of these issues can be explored by studying the beta decay of selected isotopes. Single-beta decay allows direct kinematical inference of neutrino mass; first results from the world-leading KATRIN experiment in Germany are eagerly awaited. The double-beta decay of, for instance, germanium, tellurium or xenon, meanwhile, is used to probe physics beyond the Standard Model in a unique way by searching for decays without neutrinos. This process is only allowed if neutrinos are Majoranatype particles and its observation would not only reveal the neutrino’s nature and pinpoint its mass but also demonstrate violation of lepton number. Among the various experiments worldwide
searching for neutrino-less double-beta decay, European experiments such as GERDA (focusing on germanium), CUORE (tellurium) and NEXT (xenon) are some of the most competitive.

APPEC strongly supports the present range of direct neutrino-mass measurements and searches for neutrino-less double-beta decay. Guided by the results of experiments currently in operation and in consultation with its global partners, APPEC intends to converge on a roadmap for the next generation of experiments
into neutrino mass and nature by 2020.