The cosmological⁷Li problem from a nuclear physics perspective

Abstract

Abstract. The primordial abundance of 7 Li as predicted by Big Bang Nucleosynthesis (BBN) is more than a factor 2 larger than what has been observed in metal-poor halo stars. Herein, we analyze the possibility that this discrepancy originates from incorrect assumptions about the nuclear reaction cross sections relevant for BBN. To do this, we introduce an efficient method to calculate the changes in the 7 Li abundance produced by arbitrary (temperature dependent) modifications of the nuclear reaction rates. Then, considering that 7 Li is mainly produced from 7 Be via the electron capture process 7 Be + e − → 7 Li + ν e , we assess the impact of the various channels of 7 Be destruction. Differently from previous analysis, we consider the role of unknown resonances by using a complete formalism which takes into account the effect of Coulomb and centrifugal barrier penetration and that does not rely on the use of the narrow-resonance approximation. As a result of this, the possibility of a nuclear physics solution to the 7 Li problem is significantly suppressed. Given the present experimental and theoretical constraints, it is unlikely that the 7 Be + n destruction rate is underestimated by the 2.5 factor required to solve the problem. We exclude, moreover, that resonant destruction in the channels 7 Be + t and 7 Be + 3 He can explain the 7 Li puzzle. New unknown resonances in 7 Be + d and 7 Be + α could potentially produce significant effects. Recent experimental results have ruled out such a possibility for 7 Be + d. On the other hand, for the 7 Be + α channel very favorable conditions are required. The possible existence of a partially suitable resonant level in 11 C is studied in the framework of a coupled-channel model and the possibility of a direct measurement is considered.

Primordial Nucleosynthesis

“…Unknown resonances that could sufficiently increase the cross sections of reactions that destroy 7 Be were not found experimentally (see e.g. [3] and references therein) and in any case would have too low strengths [31] because of the Coulomb barrier.…”

The RIB in-flight facility EXOTIC at INFN-LNL

“…Among the different processes of stellar nucleosynthesis forming elements heavier than 9 Be, the rp-capture and αp processes, occurring in explosive astrophysical environments, are those than can be investigated by using the EXOTIC RIBs. By developing a radioactive 18 Ne beam through the 3 He( 16 O, 18 Ne)n reaction, the 18 Ne(α,p) 21 Na process could be studied at astrophysical energies to provide a link between the Hot CNO cycle and the rp-process [14]. Another measurement relevant to astrophysics can be performed at EXOTIC such as the 30 P(p,γ) 31 S with a 30 P beam, essential for the production of heavy elements (from Si to Ca) in the explosion of ONe novae and in particular to explain the anomalously high 30 Si/ 28 Si rate measured in pre-solar grains of possible ONe novae origin [16].…”

“…Moreover, experiments based on the Trojan Horse Method (THM) [17] are considered. In this direction, a first experiment was performed by means of the THM for the 7 Be(n,α) 4 He reaction to shed light on the cosmological lithium-problem, of interest in the Big Bang Nucleosynthesis [18]. The first results have been presented in this conference [19].…”

Indirect techniques for astrophysical reaction rates determinations

“…The cross sections of 7 Be(p,n) 7 Li and 7 Li(p,α)α reactions are also well known [9]. Recently some authors [10][11][12][13] proposed the idea that some missed resonances in secondary destruction channels of 7 Li and 7 Be could be at the origin of the 7 Li deficit. Three promising candidates came out from their investigation: 7 Be+d, 7 Be+ 3 He and 7 Be+ 4 He.…”