Light Particle Solution to the Cosmic Lithium Problem.

Area of Science:

• Cosmology
• Particle Physics
• Nuclear Astrophysics

Background:

• The observed abundance of Lithium-7 (⁷Li) in the universe is significantly lower than predicted by standard Big Bang nucleosynthesis (BBN) models.
• This discrepancy, known as the lithium problem, suggests potential modifications to the standard cosmological model or BBN.

Purpose of the Study:

• To investigate whether the introduction of light, electrically neutral particles (X) with substantial nucleon interactions can resolve the cosmological lithium problem.
• To determine the parameter space for such particles (mass, lifetime, interaction strength) that reconciles predicted and observed abundances of light elements.

Main Methods:

• Incorporating X-initiated reactions into the standard BBN framework.
• Analyzing the impact of these reactions on the freeze-out abundances of Lithium-7 (⁷Li) and Beryllium-7 (⁷Be).
• Constraining particle properties (mass, lifetime, interaction cross-sections) by comparing model predictions with observed abundances of Deuterium (D) and Helium (He).

Main Results:

• A reduction in the freeze-out abundance of ⁷Li+⁷Be is achieved by including X-initiated reactions.
• The lithium problem can be solved without affecting precisely measured D and He abundances.
• Specific conditions for particle mass (1.6 MeV ≤ mX ≤ 20 MeV), lifetime (few 100s ≤ τX ≤ 10⁴ s), and interaction rates (nXσabs v ~ H) were identified.

Conclusions:

• The presence of light, neutral particles (X) with specific properties offers a viable solution to the cosmological lithium problem.
• Axionlike particles (X=a) with couplings to d quarks (f_d⁻¹ ~ TeV⁻¹) are a potential realization of this scenario.
• These axionlike particles could be detectable at intensity frontier experiments.