First Direct Measurement of the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mrow><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi mathvariant="normal">H</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow><mml:mo stretchy="false">(</mml:mo><mml:mi>α</mml:mi><mml:mo>,</mml:mo><mml:mi>γ</mml:mi><mml:mo stretchy="false">)</mml:mo><mml:mmultiscripts><mml:mrow><mml:mi>Li</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>6</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow></mml:mrow></mml:math>Cross Section at Big Bang Energies and the Primordial Lithium Problem

Abstract

Recent observations of (6)Li in metal poor stars suggest a large production of this isotope during big bang nucleosynthesis (BBN). In standard BBN calculations, the (2)H(α,γ)(6)Li reaction dominates (6)Li production. This reaction has never been measured inside the BBN energy region because its cross section drops exponentially at low energy and because the electric dipole transition is strongly suppressed for the isoscalar particles (2)H and α at energies below the Coulomb barrier. Indirect measurements using the Coulomb dissociation of (6)Li only give upper limits owing to the dominance of nuclear breakup processes. Here, we report on the results of the first measurement of the (2)H(α,γ)(6)Li cross section at big bang energies. The experiment was performed deep underground at the LUNA 400 kV accelerator in Gran Sasso, Italy. The primordial (6)Li/(7)Li isotopic abundance ratio has been determined to be (1.5 ± 0.3) × 10(-5), from our experimental data and standard BBN theory. The much higher (6)Li/(7)Li values reported for halo stars will likely require a nonstandard physics explanation, as discussed in the literature.

Keywords

Affiliated Institutions

• University of Milan IT
• Helmholtz-Zentrum Dresden-Rossendorf DE
• University of Genoa IT
• Istituto Nazionale di Fisica Nucleare, Sezione di Padova IT
• University of Turin IT
• Technische Universität Dresden DE
• HUN-REN Institute for Nuclear Research HU
• Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali del Gran Sasso IT
• Collurania Teramo Observatory IT
• University of Edinburgh GB
• Istituto Nazionale di Fisica Nucleare, Sezione di Roma I IT

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