Background

If there is an interaction between light DM and ordinary matter (Figure 1a), as there has to be in the case of a thermal origin, then there necessarily is a production mechanism in accelerator-based experiments (Figure 1b), if the interaction is not electron-phobic. The most sensitive way to search for this production is to use an electron beam to produce DM in ﬁxed-target collisions. To simulate such reactions, we use a benchmark model, a dark sector QED, predicting the production of a generic mediator particle (dark photon) by a beam of electrons that interacts with the electromagnetic field around a nucleus in the target, as shown in Figure 1c. This mediator particle subsequently decays into two DM particles, c. Since the has a mass, the electron would lose most of its energy and get a transverse momentum kick.

Diagram

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In this benchmark model, we use a dark sector fine structure constant (a_D) of 0.5 and we set. These parameter choices are conservative, and in chapter II in [[i]] we discuss the experimental sensitivities when these parameters are varied.

We have recently published a design report for LDMX [[ii]], where we demonstrate the sensitivity for 4 and 8 GeV electron beams, and we have submitted an Expression of Interest to a scientific committee at CERN where we extend the beam energy to 16 GeV [i].

The use of missing energy and transverse momentum to search for invisible particles begs the question how one will know what these invisible particles are in case of discovery. This approach, however, goes back to the Pauli 1930 neutrino proposal to resolve the apparent non-conservation of energy in beta decays. It took another decade before the observation of neutrino capture. It is now generally accepted that neutrinos make up a small fraction of the DM, even though these cosmological relic neutrinos have yet to be detected. This precedent from history is important to remember: Much of the parameter space for MeV to GeV mass DM is, at present, only detectable with accelerator-based experiments because scattering and annihilation signals are strongly velocity-suppressed for the non-relativistic DM halo. Consequently, direct detection experiments can only have sensitivity for elastic scattering of DM scalar particles.

Although a discovery of new invisible particles in LDMX would not fully prove that they are the DM (or even cosmologically long-lived), it would mark the beginning of a programme of experiments to measure neutrino properties and interactions, which could further strengthen the case that they are indeed the missing DM.

A more philosophical reasoning (Occam's razor) is that if a signal is observed and is consistent with all or part of the expectation from the relic density, then the simplest explanation is that it is DM. This is because an observed signal must be BSM, and this signal can explain the main BSM phenomenon in nature, that is DM. The latter only relies on the natural hypothesis that light DM interacts with ordinary matter and can therefore be produced in accelerator-based experiments.