Toward a solution to the missing lithium problem

If there is something missing that should be there, what is the cause? ― It has been known for decades that there is a significant discrepancy between the theoretical and observed lithium abundance in the early universe; the primordial ⁷Li abundance is overestimated and/or underobserved by a factor of 3 – 4. Such an inconsistency is called the “cosmological lithium problem (CLP)”, although the standard Big Bang theory is known as a very “successful” model in comparison with most of other observational facts.

Researchers now found experimental evidence that the theoretical ⁷Li abundance should be corrected downward by about 10%. This is not yet a complete solution, but a necessary contribution for the further theoretical approaches to the CLP solution with slightly less dauting tasks.

The first lithium is created during the Big Bang nucleosynthesis (BBN) period as well as hydrogen and helium from a jumble of protons and neutrons. The BBN is a complex network of nuclear reactions, and the abundance of a nuclide depends not only on the reaction directly to produce it but also on the reaction to destroy it and often on other reactions along the way. For example, the abundance of ⁷Li is mainly dominated by the production and reduction processes of ⁷Be. This falls in the context of the primordial nucleosynthesis studied performed by the ASFIN group and in particular to the investigations on ⁷Be destruction which has been examined in several papers.

Project Assistant Professor Seiya Hayakawa from the Center for Nuclear Study – the University of Tokyo collaborating with the nuclear astrophysics group ASFIN of the INFN – LNS, Sungkyunkwan University, and others leads an experimental project aiming at measuring the nuclear reactions responsible for the ⁷Be reduction in BBN. Recently, there are several attempts to measure these reactions ⁷Be(n, p)⁷Li and ⁷Be(n, α)⁴He by other research groups, but the data in the BBN-relevant energy region were still somewhat scarce.

These reactions are very difficult to observe directly since both ⁷Be and neutron are unstable. The research group proposed to use deuteron as a target instead of a bare neutron, and a ⁷Be beam produced by the CRIB (Center-for-Nuclear-Study Radioactive Ion Beam separator, https://www.cns.s.u-tokyo.ac.jp/crib/crib-new/home-en/index.html). This is a unique technique known as the Trojan Horse method, developed by the nuclear astrophysics group at INFN – LNS. By this method, the deuteron is like the Trojan horse in Greek myth, and the neutron is the soldier who sneaks into the impregnable city of Troy, namely, the reactions of interest, ⁷Be(n, p)⁷Li and ⁷Be(n, α)⁴He. The experimental result shows a significant contribution of the transition to the first excited state of ⁷Li for the first time, which offers a further ⁷Be reduction during the BBN and ultimately less ⁷Li abundance by about 10%.

Experimental setup and conceptual diagram of the Trojan Horse method

Link to the original paper: https://iopscience.iop.org/article/10.3847/2041-8213/ac061f

Press release of The University of Tokyo (in Japanese): https://www.s.u-tokyo.ac.jp/ja/info/7456/

Press release of The University of Tokyo (in English): https://www.u-tokyo.ac.jp/focus/en/press/z0508_00184.html

For more details:

Seiya Hayakawa
Center for Nuclear Study, The University of Tokyo
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Marco La Cognata
INFN – LNS
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