Still No Solution To The Mystery Of The Missing Lithium

The rate decay of beryllium into helium is too slow to explain the Universe’s missing lithium, researchers say.

AsianScientist (Mar. 28, 2017) – Although researchers have succeeded in measuring a cross section of the reaction thought to have formed all nuclei heavier than hydrogen, their results fail to explain the ‘cosmological lithium problem.’ These findings have been published in Physical Review Letters.

According to the Big Bang Theory, Big Bang nucleosynthesis (BBN) took place 10 seconds to 20 minutes after the Big Bang and produced light elements such as hydrogen, helium and lithium. For hydrogen and helium, the observed values correspond to the theoretically predicted values. However, the levels of lithium are only one third of the theoretically predicted value, an inconsistency known as the cosmological lithium problem.

One of the hypotheses to resolve this problem is the possibility that the decay of beryllium to helium (7Be+n→4He+4He reaction) occurs at a high rate. However, it has been difficult to perform experiments in the energy region related to BBN.

Using facilities at the Research Center for Nuclear Physics, Osaka University, a group of researchers led by Associate Professor Kawabata Takahiro at the Graduate School of Science, Kyoto University, Senior Visiting Scientist Kubono Shigeru at the Nishina Center, RIKEN, and Associate Professor Iwasa Naohito at the Graduate School of Science, Tohoku University, succeeded in measuring the cross section for the 7Be+n→4He+4He reaction, the probability of scattering or reaction caused by interaction between quantum mechanical particles.

To achieve this, the group came up with a method for measuring the cross section for the 4He+4He→7Be+n reaction, which is the time-reverse reaction of the 7Be+n→4He+4He reaction. By irradiating 4He2+ beams on helium gas and measuring the emitted neutrons, the researchers confirmed that the ground and first excitation states of 7Be were produced, determining the cross sections.

Based on the detailed balance principle, which depends on the time-reversal invariance of nuclear reactions, the researchers determined the cross sections for the Be+n→4He+4He reaction.

Their results showed that the cross sections for the Be+n→4He+4He reaction were ten times smaller than the theoretical estimation widely used in the BBN calculations and that the Be+n→4He+4He reaction was not large enough to solve the cosmological lithium problem, deepening the mystery of BBN.

These findings suggest that scientists should explore new physics beyond the Standard Model of Big Bang cosmology to solve the cosmological lithium problem, the researchers added.

The article can be found at: Kawabata et al. (2017) Time-Reversal Measurement of the p-Wave Cross Sections of the 7Be(n,α)4He Reaction for the Cosmological Li Problem.


Source: Osaka University; Photo: Pixabay.
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