Millicharged particles at electron colliders

Abstract: We propose to search for millicharged particles in electron colliders operated with the center-of-mass energies at ${\cal O}$(1-10) GeV, which include Belle II, BESIII, BaBar, and also the proposed experiment STCF. We use the monophoton final state at electron colliders to probe the parameter space of millicharged particles, that is spanned by millicharge $\epsilon$ and mass $m$. We find that electron colliders have sensitivity to the previously unexplored parameter space for millicharged particles with MeV-GeV mass: $\epsilon \lesssim {\cal O}(10^{-1})$ for $0.5$ GeV $\lesssim m \lesssim 3.5$ GeV in BaBar, $\epsilon \lesssim {\cal O}(10^{-3})$ for $0.1$ GeV $\lesssim m \lesssim 1.5$ GeV in BESIII, $\epsilon \lesssim 10^{-3}-10^{-2}$ for $0.1$ GeV $\lesssim m \lesssim 4$ GeV in Belle II, and $\epsilon \lesssim {\cal O}(10^{-4})$ for $1$ MeV $\lesssim m \lesssim 1$ GeV in STCF.

• The paper introduces a novel search strategy using monophoton signatures at electron colliders to probe new regions of millicharged particle parameter space.
• It demonstrates that clean experimental environments at Belle II, BESIII, BaBar, and STCF can achieve sensitivity down to millicharges of O(10⁻⁴) across MeV–GeV masses.
• The findings bridge collider constraints with astrophysical data, reinforcing a multifaceted approach to investigate physics beyond the Standard Model.

Investigating Millicharged Particles at Electron Colliders

The study of millicharged particles (MCPs), which are theorized entities with electric charges significantly less than that of an electron, represents a promising area of exploration beyond the Standard Model (BSM) in particle physics. The research by Liang et al. proposes an innovative methodology to detect MCPs via electron colliders operating at center-of-mass energies of approximately 1-10 GeV, such as Belle II, BESIII, BaBar, and the proposed Super Tau Charm Factory (STCF). This essay examines the assertions, methodologies, and potential impacts of this research on the future of particle physics.

Methodology

The authors' investigation leverages the production of monophoton final states at electron colliders to probe the parameter space characterized by the millicharge $\epsilon$ and MCP mass $m$. The monophoton signature, consisting of a single detected photon in the final state with missing transverse energy, is ideal for its simplicity and sensitivity to novel particles like MCPs. This approach exploits the advantages of electron colliders, known for clean experimental environments and precise control over initial states, making them optimal tools for probing low mass regions with MeV-scale precision.

Key Findings

This analysis reveals that electron colliders have the capability to explore previously inaccessible ranges of the parameter space for MCPs. Specifically:

• BaBar can probe millicharge $\epsilon \lesssim {\cal O}(10^{-1})$ for masses between 0.5 GeV and 3.5 GeV.
• BESIII's sensitivity lies around $\epsilon \lesssim {\cal O}(10^{-3})$ for mass ranges between 0.1 GeV and 1.5 GeV.
• Belle II expands this reach with $\epsilon \lesssim 10^{-3}-10^{-2}$ for masses between 0.1 GeV and 4 GeV.
• The projected STCF experiment could explore down to $\epsilon \lesssim {\cal O}(10^{-4})$ for masses from 1 MeV to 1 GeV.

Discussion

The authors' work opens up significant experimental prospects. By complementing astrophysical and cosmological constraints on MCPs with collider-based searches, this research strengthens the multi-faceted approach necessary for comprehensive investigation beyond the Standard Model. The specificity and range of sensitivities outlined create a roadmap for current and future experimental efforts.

The implications extend to theoretical constructs, where MCP models are posited as candidates in explanations for dark matter and cosmic anomalies, such as the EDGES 21 cm signal, which suggests unexpected cooling mechanisms during the cosmic dawn. The constraints derived from this study will either reinforce or challenge these theoretical frameworks, driving further refinement in BSM physics models.

Future Directions

The study highlights the potential of improving detector technology to mitigate background noise effectively and enhance the sensitivity of MCP searches further. The continued operation and development of electron colliders, coupled with advances in data analysis techniques, will be critical in refining the limits of detection and potentially uncovering novel physics phenomena.

Moreover, the integration of findings from electron collider experiments with data from other experimental realms, such as neutrino detectors and astrophysical observations, will provide a more complete picture of MCPs and their role in the universe. The interplay between experimental and theoretical advancements underscores an exciting horizon in the exploration of fundamental physics. As models evolve, the search for MCPs at electron colliders will remain an essential endeavor at the intersection of experimentation and theory, offering insights that may redefine our understanding of the fabric of the universe.