Fig. 1 Artist’s representation of the evolution of the universe, with time flowing to the right in the direction of the red arrow. The ⁷Li(d,n)2⁴He reaction takes place in the process of primordial nucleosynthesis at the very beginning. (Modified version of NASA’s image by HOU Suqing) 

Recently, an international research team has successfully updated the ⁷Li(d,n)2⁴He reaction rate based on latest experimental data, which removes the significant ambiguity in the cosmological lithium (Li) problem from the perspective of nuclear physics. 

Big Bang is regarded as the most successful model to describe the origination and evolution of the universe currently. However, its success has been limited by the so-called lithium problem, which refers to the fact that primordial lithium-7 abundance is overpredicted by a factor of three in comparison to the value from observation, while predictions match the observed primordial deuterium and helium abundances. 

From the perspective of nuclear physics, the accurate reaction rates of lithium destruction reactions are very crucial for accurate prediction of the primordial lithium-7 abundance and further understanding of the lithium problem. Nevertheless, as an important lithium-7 destruction reaction, the ⁷Li(d,n)2⁴He reaction has not been well studied before 2018. 

A new study published in The Astrophysical journal updated the ⁷Li(d,n)2⁴He reaction rate based on the recent experimental measurements on the three near-threshold beryllium-9 excited states. This work was conducted by an international research team, which was led by HOU Suqing at the Institute of Modern Physics (IMP), Chinese Academy of Sciences (CAS). 

Researchers found that the new ⁷Li(d,n)2⁴He rate is overall smaller than the previous estimation by about a factor of 60 at the typical temperature of the onset of primordial nucleosynthesis.  

In addition, researchers presented uncertainties of the ⁷Li(d,n)2⁴He reaction rate that are directly constrained by experiments for the first time.  

According to the study, the new results remove the significant ambiguity in the calculated lithium-7 abundance due to this reaction, which will be useful to understand the primordial lithium problem and probe exotic physics beyond the standard model.   

This work was supported by the Strategic Priority Research Program of CAS, the Youth Innovation Promotion Association of CAS and the National Natural Science Foundation of China.  

Other institutions involved in the study include University of Tokyo (Japan), National Astronomical Observatory of Japan (Japan), Beihang University (China), the University of Hull (UK), Hungarian Academy of Sciences (Hungary), Michigan State University (US), and Texas A&M University- Commerce (US). 

The paper is available at: https://doi.org/10.3847/1538-4357/ac1a11 

 Fig. 2 Total reaction rate of ⁷Li(d,n)2⁴He as a function of temperature in units of giga Kelvin where the green shaded band is its associated uncertainties. For comparison, researchers also plot the previous results from CF88 and BM93. (Image by HOU Suqing)