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The innovative MINERvA project at Fermilab, which employs the NuMI beam, has accomplished the first accurate representation of a proton utilizing neutrinos in place of light as the primary imaging instrument. As the fundamental constituents of atomic nuclei, protons and neutrons consist of quarks and gluons that engage in potent interactions with one another. The intensity of these interactions makes it difficult to determine the structure of protons and neutrons through theoretical computations.
As a result, researchers turn to experimental techniques to unveil their structure. Neutrino studies employ targets made of nuclei containing multiple protons and neutrons held together, complicating the extraction of information regarding proton structure from these observations. By scattering neutrinos off the protons found within the hydrogen atom nuclei in the MINERvA detector, scientists have offered the first neutrino-based measurements of this structure using unbound protons.
Several large-scale neutrino experiments, such as DUNE and the Sanford Underground Research Facility, are currently under construction. These studies will enable the precise determination of neutrino properties, shedding light on the role neutrinos played in shaping our Universe. Such experiments necessitate a comprehensive understanding of neutrino interactions with the heavy nuclei present, like argon in DUNE’s case.
Differentiating between the effects of nuclear binding and neutrino scattering from neutrons or protons is necessary to develop a hypothesis for these interactions. By measuring the properties of free protons, MINERvA’s findings will contribute to the development of more holistic theories surrounding neutrino interactions. The main challenge in the measurements detailed in this recent study is that the hydrogen in MINERvA’s detector is chemically blended with carbon atoms at a 50:50 ratio in the plastic. Carbon atoms contain six protons, resulting in a more prominent carbon background reaction.
The researchers devised an innovative method to gauge the outgoing neutron’s direction in the reaction where an anti-muon neutrino interacts with a proton, generating an anti-muon and neutron. With the separation of the two reaction types made possible by this method, residual backgrounds can be examined in a neutrino beam, where a reaction on hydrogen atoms is not possible, using the same parallel reaction. This structural measurement is understood as the axial vector form factor of the proton, a technical term for the structure unveiled by neutrino scattering, which can be used as input for neutrino reaction predictions.