Dark Matter, Millicharges, Axion and Scalar Particles, Gauge Bosons, and Other New Physics with LDMX (1807.01730v2)

Abstract: The proposed LDMX experiment would provide roughly a meter-long region of instrumented tracking and calorimetry that acts as a beam stop for multi-GeV electrons in which each electron is tagged and its evolution measured. This would offer an unprecedented opportunity to access both collider-invisible and ultra-short lifetime decays of new particles produced in electron (or muon)-nuclear fixed-target collisions. In this paper, we show that the missing momentum channel and displaced decay signals in such an experiment could provide world-leading sensitivity to sub-GeV dark matter, millicharged particles, and visibly or invisibly decaying axions, scalars, dark photons, and a range of other new physics scenarios.

Overview of Dark Matter and New Physics Exploration with LDMX

The paper "Dark Matter, Millicharges, Axion and Scalar Particles, Gauge Bosons, and Other New Physics with LDMX" explores the potentials of the Light Dark Matter eXperiment (LDMX) in investigating various aspects of dark matter, new physics particles, and forces. The authors specifically focus on how LDMX could lead to significant advancements in sensitivity towards sub-GeV dark matter candidates, millicharged particles, axion-like particles, scalar particles, gauge bosons, and other beyond the Standard Model physics scenarios.

Experimental Setup and Proposed Capability

LDMX is designed to analyze electron-nuclear fixed-target collisions in a high-luminosity environment, with electron beams ranging between 4 GeV and 16 GeV. This setup enables tracking and calorimetric measurements of electron interactions in an unprecedented manner. The primary experimental channel is missing momentum — where the electron loses substantial energy with minimal calorimeter activity — allowing insight into invisible or ultra-short-lived decay products from potential new physics particles.

Sensitivity to Dark Matter Models

Several classes of dark matter models are put forth, highlighting thermal relics, asymmetric dark matter, and strongly interacting models. A key strength of LDMX is in its ability to test models where annihilation reactions to sub-GeV mass states are invisible, showcasing sensitivity that could provide vital evidence for novel dark sector interactions.

For kinetically mixed dark photons, various spin configurations of dark matter are analyzed, demonstrating that LDMX could explore interaction strengths needed for thermal relic targets over a broad mass range. The paper further discusses extensions to other mediators, such as those that couple through $B-L$, $L_i-L_j$, and other vector interactions. LDMX promises an advance in sensitivity, both theoretically through new calculations and practically via missing momentum and displaced decay signal detection.

Millicharged Particles and Analysis of New Physics Mediators

LDMX also holds the potential to lead in sensitivity for millicharged particle detection, where small electromagnetic charges challenge fixed-target experiments to discern tiny interaction cross sections. This capability may bridge gaps left by earlier experiments that could not effectively probe the low-charge parameter space.

The authors extend the discussion to include potential mediator particles that are not necessarily tied directly to the dominant WIMP or alternative dark matter paradigms. These mediators could be spin-0 scalars or pseudo-scalars, and axion-like particles that decay preferences and interacting strengths provide both missing momentum and visible detection channels.

Implications and Theoretical Insights

Through showcasing projected sensitivities of LDMX, along with comparisons to Belle II and ongoing direct detection experiments, the work brings into focus how LDMX complements these efforts by addressing currently unexplored parameter spaces. This broadens theoretical implications, laying groundwork for potentially transformative discoveries in sub-GeV scale physics.

Consideration of the missing energy and visible decay possibilities with LDMX also hints at intriguing possibilities for severely constrained models, such as those motivated by the $(g-2)_{\mu}$ anomaly and cosmologically interesting freeze-in scenarios with heavy dark photons. The authors emphasize that extensive exploration of these spaces will help connect terrestrial experimental results with cosmological observations — a critical aspect in understanding the full picture of dark matter and new physics forces.

Concluding Remarks and Future Outlook

While proposing expansions into higher luminosity regimes and modified methodologies like muon-beam variants, the paper sets a profound tone for future research and collaboration between experimental and theoretical frontiers. The insights provided by LDMX can substantially impact theoretical models and guide the direction of subsequent exploratory experiments in the domain of sub-GeV new physics, potentially reshaping our understanding of fundamental interactions. This is vital not just for confirming the existence of dark matter but also for uncovering broader aspects of physics that remain hidden beyond the Standard Model.