Nucleon axial form factor: lattice QCD versus deuterium-target data
In plain words
How strongly a neutrino scatters off a single proton or neutron depends on a function called the axial form factor, which describes the nucleon's weak-charge distribution. Lattice QCD (computer simulations of the strong force) gives a form factor that falls off more slowly than old bubble-chamber data on deuterium suggest.
Precise statement
Determine $F_A(Q^{2})$ for $0<Q^{2}<1\ \mathrm{GeV}^{2}$ to 5 percent and resolve why lattice QCD results exceed z-expansion fits to neutrino-deuterium bubble-chamber data by roughly 20 to 30 percent at $Q^{2}\sim 0.5\ \text{to}\ 1\ \mathrm{GeV}^{2}$, which implies quasielastic cross sections higher by a similar amount. The answer identifies whether the discrepancy lies in lattice systematics (excited states, continuum and volume extrapolation), in deuteron corrections and normalization of the old data, or in both, using the antineutrino-hydrogen measurement of MINERvA (2023) as an independent check.
What would settle it
Lattice results from several groups with full systematic budgets agreeing with new high-statistics neutrino-hydrogen or antineutrino-hydrogen data to 5 percent over $Q^{2} < 1\ \mathrm{GeV}^{2}$.
Status in the literature
Unverified note
A 2025-2026 analysis of elementary-target data found significant tension between hydrogen and deuterium data and concluded that deuterium extractions underestimate both the central value and the uncertainty of $F_A$.