High-Precision D(γ,n)p Measurement Narrows Big-Bang Nucleosynthesis Rate Uncertainties

A team of nuclear physicists has published a high-precision measurement of the deuteron photodisintegration reaction D(γ,n)p that sharpens the reaction rates used in Big-Bang nucleosynthesis calculations. The Physical Review Letters Letter reports new photoneutron cross sections measured near threshold with substantially smaller uncertainties, a result that directly reduces the nuclear physics contribution to early-universe abundance errors.

The experiment was carried out at SLEGS and sampled the threshold region finely: "The photoneutron cross sections were measured at 22 energy points near the neutron threshold." Those unfolded, monochromatic cross sections and their uncertainties are summarized in the paper’s Table 1. The authors report clear gains in precision: "Our new cross sections are up to a factor of 2.2 more precise than the previous ones."

To turn those cross sections into thermonuclear input for Big-Bang nucleosynthesis models the team evaluated capture and inverse reaction rates using dibaryon effective field theory (dEFT) combined with Bayesian sampling. "The cross sections of pp(nn, γγ)D have been evaluated by dEFT with a Markov chain Monte Carlo (MCMC) analysis, together with our new data and all other relevant experimental data," the Letter states. The evaluated thermonuclear rate in the BBN temperature regime is reported to be roughly 1.9–3.8 times more precise than prior evaluations, a fractional tightening that matters for propagation of nuclear uncertainties into predicted primordial abundances.

The paper places the new results against earlier measurements. The authors find their values to be "a factor of 1.2–2.2 more precise than Hara et al.’s data" and note that "Hara et al.’s data are ≈5.2% systematically lower than the present ones (except for their lowest energy point)," a possible effect of experimental folding or quasi-monochromatic beam treatments. The Letter argues that the new, consistent dataset can act as a better reference when evaluating the n + p ↔ D + γ cross sections needed by BBN codes.

Astrophysical implications were explored using a standard BBN model, and the paper discusses impacts on the cosmological baryon density parameter Ωb h^2. The provided excerpts do not list the numeric shifts in predicted deuterium or in Ωb h^2; readers and modelers should consult the full Letter and its Table 1 for the evaluated rates and uncertainties needed to rerun abundance calculations.

This measurement builds on a long experimental tradition in photonuclear deuteron studies, citing earlier work such as Wijesooriya et al. 2001, Liu et al. 1968, Holt 1990, Jones et al. 2000, and Afanasev & Carlson 2000. For experimentalists and modelers, the immediate practical value is clear: tighter cross sections mean smaller nuclear error bars in BBN outputs, making comparisons with cosmic microwave background baryon density measurements more decisive. The next step for the community is to incorporate the updated thermonuclear rates into BBN codes and examine whether the reduced nuclear uncertainty changes the tension, if any, between primordial abundance observations and cosmological parameters.