Astroparticle physics is the rapidly evolving field of research that lies at the point where astronomy, particle physics and cosmology meet. Experimentally, it combines the advanced
instrumentation harnessed by particle physicists with the highest standard of imaging of the
cosmos undertaken by astronomers. Theoretically, it connects the Big Bang Model of cosmologists to the Standard Model of particle physicists; the former gives a detailed description of the evolution of the macro-cosmos while the latter describes the micro-cosmos with stunning precision. Scientifically, it aims to gain insights into longstanding enigmas at the heart of our understanding of the Universe – for example:

– The Extreme Universe: What can we learn about the cataclysmic events in our Universe by combining all of the messengers – high energy gamma rays, neutrinos, cosmic rays and gravitational waves – that we have at our disposal?

– The Dark Universe: What is the nature of Dark Matter and Dark Energy?

– Mysterious neutrinos: What are their intricate properties and what can they tell us?

– The Early Universe: What else can we learn about the Big Bang – for instance, from the
cosmic microwave background (CMB)?

Against the backdrop of the increasing complexity, extensive running time and high capital investment of the experiments operated and planned by the European astroparticle physics community, the field organised itself in 2001 with the establishment of APPEC as its coordinating body.

APPEC published a science vision, coined the ‘European Strategy for Astroparticle Physics’, in
2008 and its first prioritised roadmap in 2011.

Since then, the field has made revolutionary progress, with a highlight being the discovery of gravitational waves tracing back to one of the most energetic events in our Universe ever witnessed by humanity: the merging of two black holes. The coming decade promises to be equally successful and significant, with an impressive arsenal of cutting-edge experiments expected to start operations that will probe deeply into the scientific questions outlined above.

Competitive European participation in this dynamic and exhilarating field of research requires
careful prioritisation – notably regarding larger infrastructures – and in most cases consultation and collaboration with our global partners and colleagues working in astronomy, particle physics and cosmology. The construction costs and, most importantly, the running costs of projects must be carefully scrutinised.

APPEC’s new European Astroparticle Strategy 2017-2026 takes into account the collective
funding level expected to be available at national agencies and through the EU; as such, it not only sets out a science vision but also aims to be a resource-aware roadmap. The attribution of
resources across the various activities is indicated in the following graphic which summarises APPEC’s funding priorities in the context of global scientific ambitions. Its realisation will allow European researchers to capitalise successfully on past efforts and investments, and promises to shed bright light on the composition and mindboggling dynamics of our Universe.

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

Coherent action on all 21 recommendations will be vital to the success of the strategy set out in
this roadmap document. It can only be achieved with close cooperation between the scientific
community that APPEC represents – Europe’s astroparticle physicists – and our various national governments and funding agencies, the European Commission, our partners outside Europe, those working in the intimately connected fields of particle physics, astronomy and cosmology research, and the strong pillars that these three fields of research rely on (CERN, ESO and ESA).

Astroparticle physics is a dynamic, rapidly developing field. Its scope and focus therefore
vary slightly from one country to another. Despite this, the APPEC General Assembly adopted these recommendations by consensus at its tenth meeting, held in Stockholm in November 2016. Primarily, the scientific issues outlined below address ‘big science’ projects whose realisation hinges on a concerted multinational and often multidisciplinary strategy. Without exception, these projects build on a vibrant ecosystem of smaller-scale experiments, innovative R&D and model-building rooted in national institutes, laboratories and universities throughout Europe. Crucial to the future successes of European astroparticle physics, the continuation of this ecosystem is APPEC’s overarching priority.