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Project List » THE COSMOLOGICAL LITHIUM PROBLEM

THE COSMOLOGICAL LITHIUM PROBLEM
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Acronym: CLP
Contracting Authority: UEFISCDI
Number / Date of the contract: 133 TE / 2025-09-01
Program:
Project Manager: Haridas Pai
Partners:
Starting date / finishing date: 2025-09-01 / 2027-08-31
Project value: 473 684 RON

Abstract: Understanding the precise origin and different abundances of the elements still remains a fundamental unanswered question. An interdisciplinary branch of science named nuclear astrophysics is currently attempting to provide answers by studying the nuclear reactions involved in stellar and early Universe nucleosynthesis. This field of research emerged from the early work of Burbidge, Burbidge, Fowler, and Hoyle, who postulated a series of energy-generating processes in stars [1]. In more recent times, nuclear astrophysics has enjoyed remarkable expansion and exciting developments due to the introduction of more sophisticated stellar models, as well as increased effort applied to the experimental determination of stellar reaction rates [2, 3, 4]. However, it is still rare to find precise experimental values of nuclear reaction rates at the energies needed as inputs for stellar models, as these are almost always below the Coulomb barrier. Any of the different stages and processes of nucleosynthesis, from the Big Bang to the r-process, represent their own nuclear astrophysics pursuit. In the present project, our goal is to determine the precise nuclear reaction rates for a couple of nuclear reactions important for the Big Bang Nucleosynthesis (BBN) [5].

Objectives: Currently, the Big Bang represents the most successful cosmological theory of the Universe posited to date. According to this model, the hot and dense Universe started expanding roughly 13.8 billion years ago, passing through different spans and cooling down gradually to a present temperature of 2.73 K. Approximately 1 second after the Big Bang, however, the temperature was close to 1010 K and synthesis of the light elements 2H, 3He, 4He, and 7Li started. This process of primordial nucleosynthesis, along with the cosmic microwave background radiation detectable today, represent fundamental confirmations of the Big Bang theory.
The BBN model predicts that the Universe is composed of about 75% hydrogen, 25% helium, 0.01% of 2H and 3He, a small abundance of lithium, and a negligible amount of heavier elements [6]. For the standard BBN model, these predictions of primordial abundances depend primarily on the value of the baryon-to-photon ratio. This has been determined quite accurately from observations of the anisotropies of the cosmic microwave background. Therefore, considering that no uncertainties exist in the relevant reaction rates, the BBN model should thus reliably predict light nuclide abundances. Present predictions for the abundances of 2H, 3He, and 4He via the BBN model are consistent with the values inferred from astronomical observations. Primordial lithium abundance, however, is obtained from observations of metal-poor stars in the stellar halo (population II stars) of our galaxy. However, the BBN model predicted abundance of primordial 7Li is approximately 3-4 times higher than that deduced from observations. This serious disagreement between the abundances of observed 7Li in metal-poor halo stars and the primordial 7Li predicted by the BBN model is popularly known as the "cosmological lithium problem". Currently, this is one of the most important unresolved problems in the entire field of nuclear astrophysics.
Several avenues have been searched for solving the cosmological lithium problem over the past decade, which can be categorized broadly into three groups: astrophysical solutions, nuclear physics solutions and new physics solutions. Our approach towards addressing the problem in the present project will be from a nuclear physics point of view.
So far, the nuclear physics solutions to the cosmological lithium problem assume that the observed abundances are correct and that the Standard Model of particle physics along with our standard cosmology are valid. Instead, researchers theorize that the overestimation of the BBN model predictions with regards to lithium are due to incorrect input of the nuclear reaction rates in the BBN simulation code. Since BBN model calculations rely on experimentally determined nuclear reaction rates, their precise measurements become paramount for providing correct elemental abundances.
In the standard BBN scenario, about 95% of primordial 7Li is produced from the decay of 7Be via electron capture during the 2 months after the BBN stops. Many reactions that potentially destroy 7Be were investigated in hopes of solving the lithium problem over the past 20 years [7], however the abundance discrepancy has remained unsolved even today. At present, nuclear uncertainties still cannot be ruled out for some of the reactions that are destroying 7Li. In this regard, the 7Li(p, α)4He and 7Li(d, n)2*4He reactions are both important for 7Li destruction and we are proposing to study them at astrophysical energies using the 3 MV Tandetron accelerator of IFIN-HH.

Implementation plan:

1. Running Monte Carlo simulations for experimental optimization in terms of detector characteristics and setup configuration for the 7Li(d, n)2*4He reaction;
2. Performing the experiment for studying the 7Li(p, α)4He reaction at the 3 MV Tandetron accelerator of IFIN-HH;
3. Taking short measurements of the 7Li(d, n)2*4He reaction during the 7Li(p, α)4He experiment to check the "ELISSA" detector configurations and calculate the necessary beam time for submitting the experimental proposal to the IFIN-HH PAC;
4. Analyzing the resulting 7Li(p, α)4He data with a focus on extracting precise experimental cross-sections;
5. Presenting the proposal of the 7Li(d, n)2*4He experiment to the IFIN-HH PAC;
6. Finalizing the analysis of the 7Li(p, α)4He data along with the R-matrix calculations;
7. Presenting the results obtained from the 7Li(p, α)4He experiment at a conference;
8. Preparing and submitting for review the publication disseminating the results of the 7Li(p, α)4He experiment;
9. Performing the experiment for studying the 7Li(d, n)2*4He reaction at the 3 MV Tandetron accelerator of IFIN-HH;
10. Analyzing the resulting 7Li(d, n)2*4He data along with the R-matrix calculations;
11. Presenting the results obtained from the 7Li(d, n)2*4He experiment at a conference;
12. Preparing and submitting for review the publication disseminating the results of the 7Li(d, n)2*4He experiment.

References:
[1] Burbidge, E. Margaret, et al. "Synthesis of the elements in stars." Reviews of modern physics 29.4 (1957): 547. DOI: 10.1103/RevModPhys.29.547
[2] Matei, C., et al. "Measurement of the Cascade Transition via the First Excited State of O 16 in the C 12 ( α, α) O 16 Reaction, and Its S Factor in Stellar Helium Burning." Physical review letters 97.24 (2006): 242503. DOI: 10.1103/PhysRevLett.97.242503
[3] Tumino, A., et al. "An increase in the 12C+ 12C fusion rate from resonances at astrophysical energies." Nature 557.7707 (2018): 687-690. DOI: 10.1038/s41586-018-0149-4
[4] Zhang, Liyong, et al. "Measurement of 19F (p, α) 20Ne reaction suggests CNO breakout in first stars." Nature 610.7933 (2022): 656-660. DOI: 10.1038/s41586-022-05230-x
[5] Copi, Craig J., David N. Schramm, and Michael S. Turner. "Big-bang nucleosynthesis and the baryon density of the universe." Science 267.5195 (1995): 192-199. DOI: 10.1126/science.7809624
[6] Coc, Alain. "Primordial nucleosynthesis." Journal of Physics: Conference Series. Vol. 665. No. 1. IOP Publishing, 2016. DOI: 10.1088/1742-6596/665/1/012001
[7] Ali, Sk M., et al. "Resonance Excitations in Be 7 (d, p) Be* 8 to Address the Cosmological Lithium Problem." Physical Review Letters 128.25 (2022): 252701. DOI: 10.1103/PhysRevLett.128.252701

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