Dark Matter Search in Missing Energy Events with NA64
The paper documents the efforts of the NA64 collaboration in their search for sub-GeV dark matter production mediated by a hypothesized vector boson, often referred to as the dark photon, denoted as $A'$. Conducted at the CERN SPS, the experiment employed an active beam dump approach to scrutinize potential missing energy events arising from 100 GeV electron interactions. Despite the anticipation surrounding possible findings, the analysis of data covering an extensive period from 2016 to 2018—comprising $2.84\times10{11}$ electrons on target—yielded no substantiating evidence for processes involving dark photon production. Nevertheless, the experiment succeeded in establishing unprecedented constraints on the $A'$ mixing strength with photons and characterizing the parameter space relevant to both scalar and fermionic dark matter within the mass range $\lesssim 0.2$ GeV.
The underlying theory for this research suggests that, in addition to gravitational interactions, a new force might exist between dark and visible matter mediated by a dark photon. The dark photon could be a candidate in the sub-GeV mass regime and couple to the Standard Model (SM) through kinetic mixing with ordinary photons. The formalism is encapsulated in the Lagrangian extended by the dark sector, conveying the interaction dynamics through kinetic mixing and introducing parameters such as the mixing strength $\epsilon$. The research broadly interprets that, under the assumption of the $A'$ being dominant in the dark sector, it primarily undergoes decay into lighter dark matter states, provided they exist with masses $m_\chi < m_{A'}/2$.
The NA64 experiment capitalized on a strategically designed setup that incorporated a fixed-target approach using a high-intensity electron beam. The setup facilitated identification of potential dark photon events via missing energy signatures—specifically, scenarios where a substantial discrepancy emerges between the expected and measured electron energy in the ECAL, attributable to the hypothetical $A'$ production. A novel and critical aspect of the experiment was its sensitivity reliance predominantly on $\epsilon2$, a factor associated primarily with the production rather than decay, marking an advancement in dark force detection methodologies.
The findings place the most stringent direct constraints to date on the $A'$ mixing strength within the stipulated mass range. Notably, the paper discusses the formulation of a constraint on dark matter models by linking the mixing strength constraints with dark matter relic density assumptions. These theoretical assumptions guide the search and interpretation, while projecting implications on scalar, pseudo-Dirac, and Majorana light dark matter scenarios. Given the absence of signal evidence within the data and consequent establishment of upper bounds on the coupling strength $\epsilon$, these results significantly constrain regions of the parameter space pertinent to dark matter models and have implications for ongoing and future dark matter detection efforts.
Future prospects involve detector upgrades to further refine sensitivity and potentially expand the search for additional mass ranges or interaction mechanisms. This paper exemplifies an innovative stride in the utilization of fixed-target experiments for dark matter searches, highlighting the adaptability and scalability of the NA64 methodology for probing fundamental physics questions surrounding dark forces, and fostering subsequent advancements in experimental particle physics.