High-Precision Measurement of D($γ$, $n$)$p$ Photodisintegration Reaction and Implications for Big-Bang Nucleosynthesis

Abstract

We report on a high-precision measurement of the D($γ$,\,$n$)$p$ photodisintegration reaction at the newly commissioned Shanghai Laser Electron Gamma Source (SLEGS), employing a quasi-monochromatic $γ$-ray beam from Laser Compton Scattering. The cross sections were determined over $E_γ$=2.327--7.089 MeV, achieving up to a factor of 2.2 improvement in precision near the neutron separation threshold. Combined with previous data in a global Markov chain Monte Carlo (MCMC) analysis using dibaryon effective field theory, we obtained the unprecedentedly precise $p$($n$,\,$γ$)D cross sections and thermonuclear rate, with a precision up to $\approx$4 times higher than previous evaluations. Implemented in a standard Big-Bang Nucleosynthesis (BBN) framework, this new rate decreases uncertainty of the key cosmological parameter of baryon density $Ω_b h^2$ by up to $\approx$16\% relative to the LUNA result. A residual $\approx$1.2$σ$ tension between $Ω_b h^2$ constrained from primordial D/H observations and CMB measurements persists, highlighting the need for improved $dd$ reaction rates and offering potential hints of new physics beyond the standard model of cosmology.

Figures (4)

• Figure 1: The 4$\pi\cdot A_0$ fit to the folded cross sections (median values) of D($\gamma$, $n$)p measured with SLEGS. The experimental data and and fitted ones are indicated by the solid points and circles, respectively. The solid curve connecting the fitted data points is just for guiding the eyes.
• Figure 2: (a) Photoneutron cross sections of D($\gamma$, $n$)$p$ reaction. The present SLEGS results are shown as a red line, with the associated uncertainties indicated by a colored band. The previous data bishop50bire85more89gra92hara03uts15 are shown for comparison; (b) Ratios of cross sections between the present results (as the reference) and previous ones. See more details in the text.
• Figure 3: MCMC fits to experimental data available in following four channels: (a) $np$ scattering cross section nn-online; (b) $np$ capture cross section multiplied by neutron speed suzuki95nagai97; (c) photon analyzing power $\Sigma$($\theta$) for $d\vec{\gamma}$$\rightarrow$$np$ process sch00tor03; (d) cross section of D($\gamma$, $n$)$p$bire85more89gra92; (e) folded data of D($\gamma$, $n$)$p$ obtained at SLEGS. See more details in Ref. ando06.
• Figure 4: BBN constraints on $\Omega_bh^2$ calculated for three different sets of $dd$ rates Pisanti:2020efzgom17coc15, with 1$\sigma$ error bar indicated. The constraints using observed D/H values of Cooke et al.coo18 and of Navas et al.ParticleDataGroup:2024cfk are shown for comparison. The Planck CMB observation Planck:2018vyg is indicated by the gray band.