Track and energy reconstruction algorithms for a time projection chamber with orthogonal fields

In this work, we describe the development of track- and energy-reconstruction algorithms for atypical Time Projection Chambers (TPCs) that will be used at the Institute of Experimental and Applied Physics, Czech Technical University in Prague, to search for the anomalous internal pair creation reported by the ATOMKI group. These chambers operate with an inhomogeneous toroidal magnetic field oriented orthogonally to the electric field; we therefore refer to them as Orthogonal-Field TPCs (OFTPCs). Although this configuration distorts the drift of ionization electrons and complicates the resulting electron and positron trajectories, it also offers several practical advantages: a mechanically simple construction with a small number of electronic channels, containment of the tracks and their secondaries within a single detector segment, and—because the magnetic field bends the trajectories in the drift (timing) direction—the possibility of substituting fine timing for fine pad segmentation. We present the most effective of several tested approaches, which employs a simulated ionization-electron drift map for track reconstruction and a Runge–Kutta-based fit for energy reconstruction. Using simulations, we demonstrate that—under idealized conditions, namely an ideal charge readout with no amplification and no noise and with known initial track positions and directions, and neglecting multiple scattering in the gas—it is possible to achieve a fitted Gaussian width (sigma) better than 1% in relative energy for both electrons and positrons, after applying corrections for systematic effects that depend on the track parameters.