Measurement of the axial vector form factor from antineutrino–proton scattering

Abstract

Scattering of high energy particles from nucleons probes their structure, as was done in the experiments that established the non-zero size of the proton using electron beams1. The use of charged leptons as scattering probes enables measuring the distribution of electric charges, which is encoded in the vector form factors of the nucleon2. Scattering weakly interacting neutrinos gives the opportunity to measure both vector and axial vector form factors of the nucleon, providing an additional, complementary probe of their structure. The nucleon transition axial form factor, FA, can be measured from neutrino scattering from free nucleons, νμn → μ−p and $${\bar{\nu }}_{\mu }p\to {\mu }^{+}n$$ ν ¯ μ p → μ + n , as a function of the negative four-momentum transfer squared (Q2). Up to now, FA(Q2) has been extracted from the bound nucleons in neutrino–deuterium scattering3–9, which requires uncertain nuclear corrections10. Here we report the first high-statistics measurement, to our knowledge, of the $${\bar{\nu }}_{\mu }\,p\to {\mu }^{+}n$$ ν ¯ μ p → μ + n cross-section from the hydrogen atom, using the plastic scintillator target of the MINERvA11 experiment, extracting FA from free proton targets and measuring the nucleon axial charge radius, rA, to be 0.73 ± 0.17 fm. The antineutrino–hydrogen scattering presented here can access the axial form factor without the need for nuclear theory corrections, and enables direct comparisons with the increasingly precise lattice quantum chromodynamics computations12–15. Finally, the tools developed for this analysis and the result presented are substantial advancements in our capabilities to understand the nucleon structure in the weak sector, and also help the current and future neutrino oscillation experiments16–20 to better constrain neutrino interaction models.

Isovector Axial Charge and Form Factors of Nucleons from Lattice QCD

“…A comparison of the antineutrino-nucleon charged current elastic cross sections calculated using predictions of AVFF from lattice (PNDME 23 [19]) and neutrino-deuterium analysis [20] with MINERvA measurement [17,42] is presented in Figure 9, which is reproduced from Ref. [18].…”

“…The MINERνA experiment [17] has recently shown that the axial vector form factor of the nucleon can be extracted from the charged current elastic scattering process ν µ H → µ + n in which the free proton in hydrogen (H) (part of the hydrocarbon in the target) is converted into a neutron. This opens the door to direct measurements of the nucleon axial vector form factor without the need for extraction from scattering off nuclei, whose analysis involves nuclear corrections that have unresolved systematics.…”

“…A recent comparison [18] of results for the AVFF from lattice QCD [19], the MINERνA experiment [17], and the phenomenological extraction from neutrino-deuterium data from 1980s and 1990s [20] showed that, in the near term, the best prospects for determining the AVFF will be a combination of lattice QCD calculations and MINERνA-like experiments. Lattice QCD will provide the best estimates for Q 2 ≲ 0.5 GeV 2 and be competitive with MINERνA for 0.5 ≲ Q 2 ≲ 1 GeV 2 .…”

Nucleon-Pion Spectroscopy from Sparsened Correlators

“…These finite-volume results can then be matched to infinite volume 𝑁 → Δ and non-resonant 𝑁 → 𝑁 𝜋 and 𝑁 → 𝑁 𝜋𝜋 amplitudes through generalizations of the Lellouch-Lüscher formula under active development [22][23][24][25][26][27] or used to constrain finite-volume nuclear effective theories. The same variational calculations will result in determinations of nucleon elastic axial form factors, another crucial input needed to reduce uncertainties in neutrino-nucleus cross sections [4,5,9,28,29], with the contributions from 𝑁 𝜋 excited states explicitly removed. This will provide important validation for other current and future methods of removing significant 𝑁 𝜋 excited-state contamination from lattice QCD studies of nucleon axial form factors [30][31][32][33][34][35].…”

Nucleon isovector form factors from domain-wall lattice QCD at the physical mass

“…In Lattice 2022, I reported the nucleon isovector vector-and axialvectorcurrent form factors [12] calculated jointly by LHP, RBC, and UKQCD collaborations using the 2+1-flavor dynamical domain-wall fermions lattice-QCD ensemble generated jointly by RBC and UKQCD collaborations. In this "48I" ensemble [13], the lattice spacing is set at about 0.1141(3) fm, and the lattice spatial extent is 48 spacings or about 5.4750 (14) fm. The dynamical strange and degenerate up and down quark mass values are set at their essentially physical values to provide the physical Ω mass and a degenerate pion mass of 0.1392(2) GeV.…”