The relationship between pixels, sequences, clusters, and hits is indicated in Figure 2, where the result of three tracks passing over a padrow is shown.
Figure 2: Pixel data for part of a padrow is shown for three tracks crossing the row. The size of a box indicates the magnitude of the ADC value of the pixel.
A pixel (a.k.a. voxel) for the TPC describes one time bucket on one pad. A sequence is a series of consecutive time buckets on a given pad with ADC values above the readout threshold. A cluster is a set of sequences on adjacent pads on a given padrow, each sequence of which overlaps in time with at least one other sequence in the cluster on an adjacent pad. A cluster may be thought of as a continuous ``land mass'' of pixels in a padrow, if altitude is given by ADC value, and threshold is sea level.
Charged particles passing through the TPC cross over the padrows at padrow crossings. The ionization left by these tracks is digitized into spatial pixels. To reconstruct the spatial positions of these padrow crossings, the cluster/hitfinder identifies and localizes regions of high charge density (ADC value) called hits. Using the above analogy, these are ``mountain peaks'' in the padrow terrain. If two (or more) tracks are spatially close together as they pass over a padrow, their ionization patterns may overlap, resulting in a single cluster. If the tracks are not too close (see merging distance, below), then more than one hit will be associated with the cluster.