Experiments at Fermilab are advancing an intriguing story that began three decades ago, with investigations of coherent neutrino interactions that produce pions yet leave the target nucleus unscathed.

When neutrinos scatter coherently off an entire nucleus, the exchange of a Z0 or W± boson can lead to the production of a pion with the same charge. The first observations of such interactions came in the early 1980s from the Aachen–Padova experiment at CERN’s Proton Synchrotron, followed by an analysis of earlier data from Gargamelle. A handful of other experiments at CERN, Fermilab and Serpukhov provided additional measurements before the end of the 1990s. These experiments determined interaction cross-sections for high-energy neutrinos (5–100 GeV), which were in good agreement with the model of Deiter Rein and Lalit Sehgal of Aachen. Published shortly after the first measurements were made, their model is still used in some Monte Carlo simulations.

More recently, the SciBooNE and K2K collaborations attempted to measure the coherent production of charged pions at lower neutrino energies (less than 2 GeV). However, they found no evidence of the interaction, and published upper limits below Rein and Sehgal’s original estimation. These results, together with recent observations of coherent production of neutral pions by the MiniBooNE and NOMAD collaborations, have now motivated renewed interest and new models of coherent pion production.

In the NuMI beamline at Fermilab – which has a peak energy of 3.5 GeV and energies beyond 20 GeV – coherent charged-current pion production accounts for only 1% of all of the ways that a neutrino can interact. Nevertheless, both the ArgoNeuT and MINERvA collaborations have now successfully measured the cross-sections for charged-current pion production by recording the interactions of neutrinos and antineutrinos.

ArgoNeuT uses a liquid-argon time-projection chamber (TPC), and has results for coherent interactions of antineutrinos and neutrinos at mean energies of 3.6 GeV and 9.6 GeV, respectively (Acciarri et al. 2014). A very limited exposure produced only 30 candidates for coherent interactions of antineutrinos and 24 for neutrinos (figure 1), but a measurement was possible thanks to the high resolution and precise calorimetry achieved by the TPC. It is the first time that this interaction has been measured in a liquid-argon detector. ArgoNeuT’s results agree with the state-of-the-art theoretical predictions (figure 2), but its small detector size (<0.5 tonnes) limits the precision of the measurements.

MINERvA uses a fine-grained scintillator tracker to fully reconstruct and select the coherent interactions in a model-independent analysis. With 770 antineutrino and 1628 neutrino candidates, this experiment measured the cross-section as a function of incident antineutrino and neutrino energy (figure 2). The measured spectrum and angle of the coherently produced pions are not consistent with models used by oscillation experiments (Higuera et al. 2014), and they will be used to correct those models.

The techniques developed during both the ArgoNeuT and MINERvA analyses will be used by larger liquid-argon experiments, such as MicroBooNE, that are part of the new short-baseline neutrino programme at Fermilab. While these experiments will focus on neutrino oscillations and the search for new physics, they will also provide more insight into coherent pion production.