It has long been argued that the action of the Schwinger mechanism in the gauge sector of Quantum Chromodynamics leads to the generation of a gluon mass scale. Within this scenario, the analytic structure of the fundamental vertices is modified by the creation of scalar colored excitations with vanishing mass. In the limit of zero momentum transfer, these terms act as massless poles, providing the required conditions for the infrared stabilization of the gluon propagator, and producing a characteristic displacement to the associated Ward identities. In this article we offer an extensive overview of the salient notions and techniques underlying this dynamical picture. We place particular emphasis on recent developments related to the exact renormalization of the mass, the nonlinear nature of the pole equation, and the key role played by the Fredholm alternative theorem.

A review of neutron sources for large scale user facilities is provided, aimed at users of neutron sources who need to understand the characteristics and peculiarities of the different types of neutron sources in order to select the most suitable source for their needs and to optimize their experimental setups for their specific scientific requirements. To this end, we provide an overview of (i) the main nuclear processes used at user facilities to release neutrons from nuclei, namely fission, spallation, and low-energy nuclear reactions, (ii) the various possibilities to tailor the time structure and spectrum of free neutrons, from pulsing to moderation, (iii) the mechanisms to extract and transport neutron beams with desired properties in terms of flux, brilliance/brightness, spectrum, pulse shape, etc., (iii) the applications and related experimental requirements in major scientific fields, with emphasis on those with a larger user community, (iv) the technology and realization of research reactors and neutron spallation sources, and (v) the progress made in Compact Accelerator-based Neutron Sources CANS, but especially in High Current Accelerator-driven Neutron Sources HiCANS. HiCANS are the focus of the current review, as this entirely new type of facility could in the future play the role in the neutron ecosystem that national reactor-based sources have played in the past. As such, HiCANS do not aim for the highest neutron brightness, but rather for parameters such as resilience, reliability, flexibility, ease of access, minimization of radioactive waste, excellent signal-to-noise ratio and optimization of the price/performance ratio. These are key features needed to further expand the community of neutron users from science and industry to whom this review is addressed.

International audience

The Large Hadron Collider (LHC) has confirmed the Higgs mechanism to generate mass in the Standard Model (SM), making it attractive also to consider spontaneous symmetry breaking as the origin of mass for new particles in a dark sector extension of the SM. Such a dark Higgs mechanism may in particular give mass to a dark matter candidate and to the gauge boson mediating its interactions (called dark photon). In this review, we summarize the phenomenology of the resulting dark Higgs boson and discuss the corresponding search strategies with a focus on collider experiments. We consider both the case that the dark Higgs boson is heavier than the SM Higgs boson, in which case leading constraints come from direct searches for new Higgs bosons as well missing-energy searches at the LHC, and the case that the dark Higgs boson is (potentially much) lighter than the SM Higgs boson, such that the maximum sensitivity comes from electron–positron colliders and fixed-target experiments. Of particular experimental interest for both cases is the associated production of a dark Higgs boson with a dark photon, which subsequently decays into SM fermions, dark matter particles or long-lived dark sector states. We also discuss the important role of exotic decays of the SM-like Higgs boson and complementary constraints arising from early-universe cosmology, astrophysics, and direct searches for dark matter in laboratory experiments.