Search for Sub-GeV Dark Particles in η→π^{0}+Invisible Decay

Area of Science:

• Particle Physics
• Cosmology
• Dark Matter Research

Background:

• The nature of dark matter remains one of the most significant unsolved problems in physics.
• Searches for weakly interacting massive particles (WIMPs) have yielded null results, motivating exploration of alternative dark matter candidates like sub-GeV particles.
• The η meson provides a potential portal for discovering new light scalar particles interacting with Standard Model particles and dark matter.

Purpose of the Study:

• To conduct the first search for a dark scalar boson (S) decaying into invisible dark matter particles (χ) via the process η→π⁰S→π⁰χχ̄.
• To set limits on the properties and interactions of light scalar dark matter candidates.
• To constrain the dark matter-nucleon scattering cross section.

Main Methods:

• Analysis of (10087±44)×10⁶ J/ψ events collected by the BESIII detector at a center-of-mass energy of 3.097 GeV.
• Searched for the decay signature η→π⁰S→π⁰χχ̄ within the collected data.
• Set upper limits on branching fractions and coupling strengths for S bosons with masses from 0 to 400 MeV/c².

Main Results:

• No significant signal for the decay η→π⁰S→π⁰χχ̄ was observed.
• Upper limits on the branching fractions for this decay were established in the range (1.8–5.5)×10⁻⁵ at the 90% confidence level.
• Upper limits on the coupling strengths between the dark scalar boson S and quarks were set between (1.3–3.2)×10⁻⁵.

Conclusions:

• The study provides the most stringent constraints to date on the dark matter-nucleon scattering cross section for sub-GeV dark matter, improving previous limits by approximately five orders of magnitude.
• This research offers unique insights into the properties of light dark matter particles and their interactions.
• The results exclude a specific parameter space for dark scalar bosons and invisible dark matter, guiding future experimental searches.