Abstract Particle physics today faces the challenge of explaining the mystery of dark matter, the origin of matter over anti-matter in the Universe, the origin of the neutrino masses, the apparent fine-tuning of the electro-weak scale, and many other aspects of fundamental physics. Perhaps the most striking frontier to emerge in the search for answers involves new physics at mass scales comparable to familiar matter, below the GeV-scale, or even radically below, down to sub-eV scales, and with very feeble interaction strength. New theoretical ideas to address dark matter and other fundamental questions predict such feebly interacting particles (FIPs) at these scales, and indeed, existing data provide numerous hints for such possibility. A vibrant experimental program to discover such physics is under way, guided by a systematic theoretical approach firmly grounded on the underlying principles of the Standard Model. This document represents the report of the FIPs 2022 workshop, held at CERN between the 17 and 21 October 2022 and aims to give an overview of these efforts, their motivations, and the decadal goals that animate the community involved in the search for FIPs. 

Feebly-interacting particles: FIPs 2022 Workshop Report

This paper provides an overview of experiments and facilities for accelerator-based dark matter searches as part of the US Community Study on the Future of Particle Physics (Snowmass 2021). Companion white papers to this paper present the physics drivers: thermal dark matter, visible dark portals, and new flavors and rich dark sectors. Submitted to the Proceedings of the US Community Study on the Future of Particle Physics (Snowmass 2021) 1 ar X iv :2 20 6. 04 22 0v 1 [ he pex ] 9 J un 2 02 2

Experiments and Facilities for Accelerator-Based Dark Sector Searches

The design of a primary electron beam facility at CERN is described. It re-enables the SPS as an electron accelerator, and leverages the development invested in CLIC technology for its injector and as accelerator R&D infrastructure. The facility would be relevant for several of the key priorities in the 2020 update of the European Strategy for Particle Physics, such as an e+e- Higgs factory, accelerator R&D, dark sector physics, and neutrino physics. In addition, it could serve experiments in nuclear physics. The electron beam delivered by this facility would provide access to light dark matter production significantly beyond the targets predicted by a thermal dark matter origin, and for natures of dark matter particles that are not accessible by direct detection experiments. It would also enable electro-nuclear measurements crucial for precise modelling the energy dependence of neutrino-nucleus interactions, which is needed to precisely measure neutrino oscillations as a function of energy. The implementation of the facility is the natural next step in the development of X-band high-gradient acceleration technology, a key technology for compact and cost-effective electron/positron linacs. It would also become the only facility with multi-GeV drive bunches and truly independent electron witness bunches for plasma wakefield acceleration. The facility would be used for the development and studies of a large number of components and phenomena for a future e+e- Higgs and electroweak factory as the first stage of a next circular collider at CERN, and its cavities in the SPS would be the same type as foreseen for such a future collider. The operation of the SPS with electrons would train a new generation of CERN staff on circular electron accelerators. The facility could be made operational in about five years and would operate in parallel and without interference with Run 4 at the LHC.

Conceptual Design Report -- eSPS

Fixed target missing-momentum experiments such as LDMX and ${\mathrm{M}}^{3}$ are powerful probes of light dark matter and other light, weakly coupled particles beyond the Standard Model (SM). Such experiments involve $\ensuremath{\sim}10\text{ }\text{ }\mathrm{GeV}$ beam particles whose energy and momentum are individually measured before and after passing through a suitably thin target. If new states are radiatively produced in the target, the recoiling beam particle loses a large fraction of its initial momentum, and no SM particles are observed in a downstream veto detector. We explore how such experiments can use kinematic variables and experimental parameters, such as beam energy and polarization, to measure properties of the radiated particles and discriminate between models if a signal is discovered. In particular, the transverse momentum of recoiling particles is shown to be a powerful tool to measure the masses of new radiated states, offering significantly better discriminating ability compared to the recoil energy alone. We further illustrate how variations in beam energy, polarization, and lepton flavor (i.e., electron or muon) can be used to disentangle the possible the Lorentz structure of the new interactions.

Characterizing dark matter signals with missing momentum experiments

We present a model explaining both the $4.2\ensuremath{\sigma}$ muon $g\ensuremath{-}2$ anomaly and the relic density of dark matter (DM) in which DM interacts with the Standard Model (SM) via a scalar portal boson $\ensuremath{\varphi}$ carrying both dark and SM leptonic numbers, and mediating a nondiagonal interaction between the electron and muon that allows $e\ensuremath{\leftrightarrow}\ensuremath{\mu}$ transitions. The $\ensuremath{\varphi}$ could be produced in high-energy electron scattering off a target nuclei in the reaction $eZ\ensuremath{\rightarrow}\ensuremath{\mu}Z\ensuremath{\varphi}$ followed by the prompt invisible decay $\ensuremath{\varphi}\ensuremath{\rightarrow}\mathrm{DM}$ particles and searched for in events with large missing energy accompanied by a single outgoing muon in the final state. Interestingly, several events with a similar signature have been observed in a data sample of $\ensuremath{\simeq}3\ifmmode\times\else\texttimes\fi{}{10}^{11}$ electrons on target collected during 2016-2018 for the search for light dark matter in the NA64 experiment at the CERN SPS [D. Banerjee et al. (NA64 Collaboration), Phys. Rev. Lett. 123, 121801 (2019).]. Attributing so far these events to background allows us to set first constraints on the $\ensuremath{\varphi}$ mass and couplings while leaving at the same time decisively probing the origin of these events and a large fraction of the remaining parameter space to a near exiting future with the upgraded NA64 detector or other planned experiments. 

Leptonic scalar portal: Origin of muon <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>g</mml:mi><mml:mo>−</mml:mo><mml:mn>2</mml:mn></mml:math> anomaly and dark matter?

Fixed-target experiments using primary electron beams can be powerful discovery tools for light dark matter in the sub-GeV mass range. The Light Dark Matter eXperiment (LDMX) is designed to measure missing momentum in high-rate electron fixed-target reactions with beam energies of 4 GeV to 16 GeV. A prerequisite for achieving several important sensitivity milestones is the capability to efficiently reject backgrounds associated with few-GeV bremsstrahlung, by twelve orders of magnitude, while maintaining high efficiency for signal. The primary challenge arises from events with photo-nuclear reactions faking the missing-momentum property of a dark matter signal. We present a methodology developed for the LDMX detector concept that is capable of the required rejection. By employing a detailed Geant4-based model of the detector response, we demonstrate that the sampling calorimetry proposed for LDMX can achieve better than 10−13 rejection of few-GeV photons. This suggests that the luminosity-limited sensitivity of LDMX can be realized at 4 GeV and higher beam energies.

/pdf/a-high-efficiency-photon-veto-for-the-light-dark-matter-19vjcr1coq.pdf

A high efficiency photon veto for the Light Dark Matter eXperiment

Fixed-target experiments using primary electron beams can be powerful discovery tools for light dark matter in the sub-GeV mass range. The Light Dark Matter eXperiment (LDMX) is designed to measure missing momentum in high-rate electron fixed-target reactions with beam energies of 4 GeV to 16 GeV. A prerequisite for achieving several important sensitivity milestones is the capability to efficiently reject backgrounds associated with few-GeV bremsstrahlung, by twelve orders of magnitude, while maintaining high efficiency for signal. The primary challenge arises from events with photo-nuclear reactions faking the missing-momentum property of a dark matter signal. We present a methodology developed for the LDMX detector concept that is capable of the required rejection. By employing a detailed GEANT4-based model of the detector response, we demonstrate that the sampling calorimetry proposed for LDMX can achieve better than $10^{-13}$ rejection of few-GeV photons. This suggests that the luminosity-limited sensitivity of LDMX can be realized at 4 GeV and higher beam energies.

A High Efficiency Photon Veto for the Light Dark Matter eXperiment

We report results from thin films of novel biomaterials based on natural minerals, never before synthesized in the laboratory using primarily non-toxic and environmentally friendly materials and characterized optically. These biomaterial films have high indices of refraction and would be a natural and toxicologically safe material to use for large-area optical sensing of molecules, including toxic industrial molecules. Adhering modest concentrations of molecules in solution (water/humidity, ethanol, glucose, ammonia, etc.) to the surface of a Fabry-Perot cavity is shown experimentally to sufficiently alter the index of refraction and thickness of the Fabry-Perot films to enable detection of the molecules via optical methods (reflectance, ellipsometry, transmission, etc.). We report laboratory sensing of 3 types of molecules in solution with controlled high-quality Fabry-Perot cavities. We discuss different and better biominerals to use and discern potential applications. 

Novel environmentally-safe, non-toxic, and supply-chain-resilient biomaterials for optical detection of molecules

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A high efficiency photon veto for the Light Dark Matter eXperiment

A High Efficiency Photon Veto for the Light Dark Matter eXperiment

Novel environmentally-safe, non-toxic, and supply-chain-resilient biomaterials for optical detection of molecules