Method to search for the triple-neutron state in an electron scattering experiment

TL;DR

This work proposes and analyzes a novel electron-scattering method to search for the trineutron $^3n$ via the reaction $^4 ext{He}(e,e'pπ^{+})^3n$ at MAMI-A1, leveraging triple-coincidence measurements to reconstruct the missing-mass spectrum. A detailed MC study combines reaction dynamics, detector acceptances, and energy-loss corrections to estimate a $^3n$ production rate of about 1.5 events per day and a missing-mass resolution near 0.4 MeV, suggesting that ~16 days of beam time could yield a $>5σ$ signal for a narrow width around 0.9 MeV. The paper also outlines calibration strategies and discusses extensions to future facilities and alternative reaction channels, highlighting the potential of electron-beam experiments to illuminate multineutron states. Overall, this approach provides a concrete, experimentally feasible path to observe or constrain $^3n$ with high precision missing-mass spectroscopy.

Abstract

An electron scattering experiment to search for the trineutron state $^3n$ by reaction ${\rm ^4He}(e,~e'pπ^{+})^{3}n$ is designed for the A1 facility at Mainzer Microtron. The detailed principles, setup, and simulation of this experiment are presented. With the momenta of the scattered electron, the produced proton and $π^+$ from the reaction measured by three spectrometers with their triple coincidence, the missing mass spectrum of $^3n$ can be obtained. The production rate of $^3n$ based on the cross section of the reaction and a MC simulation is estimated to be about 1.5 per day, which can provide a confidence level of the signal greater than 5$σ$ with a beam time longer than 16 days. According to a MC simulation that evaluates the energy losses of particles in materials and the performance of three spectrometers, the estimated resolution and the predicted shape of the missing mass spectrum are presented. This work provides a new experimental concept for the search for multineutron states in future experiments with an electron beam.

Figures (5)

• Figure 1: Image of the experiment setup in A1 hall. The red, blue, and green spectrometers are SpekA, B, and C, respectively.
• Figure 2: Illustration of the principle of the reaction ${\rm ^4He}(e,~e'p\pi^{+})^{3}n$.
• Figure 3: From left to right: total energy loss distributions of scattered electron, the produced proton, and $\pi^+$ in the material of experimental setup. The blue histograms represent the real energy losses while the red ones are from corrections.
• Figure 4: Reconstructed missing mass spectrum of $^3n$ minus the three neutron mass threshold from MC simulations. Upper panel: a simulation with 0 MeV $^3n$ width and a large statistics. The black curve represents a Gaussian fit. Lower panel: a simulation with statistics evaluated for a 16 days beam time and 0.9 MeV $^3n$ width.
• Figure 5: Reconstructed missing mass spectrum of $^{12}$C from SpekA minus the mass of its ground state with 420 MeV electron beam. The red curves represent the fits for each peak.