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An important first step in resolving this issue was to use actual data taken from Trip-T cosmic ray events. A selection of production 6B cosmic ray data files (6,336 in total) from Run 4 were used. These were run through the basic reconstruction algorithm, Simple Track Fitter. This process ensured that only data which contributed to clearly defined single tracks was retained and any noise hits or hit channels outside of a certain physical region were dismissed. This physical region is predetermined in STF. In this particular case, this can also become a helpful diagnostic tool in finding swapped chan- nels. If a cosmic ray passes through a module in the detector in a straight line, then it is expected that all layers along the muon trajectory are hit with no discontinuities (unless a channel is dead). However, if a channel were hit in the detector but it was being recognised in the software as an alternate channel then a discontinuity or ‘hole’ would be seen in the post simple track fitter view. The corresponding channel that has been activated in the software will be disregarded by the simple track fitter as noise, as it is unlikely to occupy a position in line with the reconstructed track.

FIGURE3.2: An event display representation of a cosmic muon passing

through the ND280.

The first step in the process of determining mis-mappings was to produce plots which illustrated the number of hits for a given channel on each TFB in the ECal. After producing plots for each channel on each TFB (see Figures3.3and3.4), the next stage

was to analyse these plots to find instances of ‘Dead Channels’ and potentially mis- mapped channels. Dead channels were characterised by their lack of hits within the data sample. It was initially thought important that these were catalogued, so as not to confuse them with potentially mis-mapped channels. However, the finalised mis- mapping identification method was mostly insensitive to their presence.

FIGURE3.3: Plot showing the number of hits for each channel on TFB7 on RMM1. A clear deficit can be seen at channel 40, indicating a dead channel. The ECal group maintains a list of dead channels. The repeated slopes are due to the physical geometry of the modules and the Trip-T

cosmic trigger.

As mentioned previously, due to the physical nature of the ECal, it is unlikely that a channel would be mis-mapped with one on another TFB. Therefore, a potentially mis-mapped channel on these plots would be represented by a deficit of hits in two channels on the same TFB. This would happen as when one channel was hit in physical space, the other would be activated in the software. As these are then disregarded by simple track fitter, for summed over tracks this would manifest as a vast decrease in total number of hits for both of those channels. They would not be zero, as occasionally the activated channel would coincidentally form part of an event track and be included. At this stage these are still only potential mis-mappings, as their hit deficit could be

3.4. The Process of Determining Mis-mappings 81

due to other factors such as having a too low bias voltage or the channels having been turned off and on during the course of a run. An example is shown in Figure3.3.

FIGURE3.4: Plot showing the number of hits for each channel on TFB0 on RMM1 A clear deficit can be seen at channels 12 and 28, indicating a swap. However, this summed over data does not give a definitive indi- cation that these missing hits occurred at the same time. Although, the

fact that the deficit for both channels is similar is encouraging.

3.4.2 Plotting the Cosmic Data in a 3D Histogram

While the 2D hit count per channel plots were very useful and effective at illustrating that there is a dead channel and mis-mapped channel issue, it was time consuming to view all of them and ultimately highly inconclusive regarding mis-mappings. It was decided that hits should be plotted in bar/layer space, which also makes them more intuitive when attempting to imagine the geometry of the ECal.

Data was combined into entire modules (i.e. RMMs 0 - 11). This yielded plots which were easy to view, intuitive and not time consuming due to the number of RMMs. One important consideration at this stage was ‘hit end’ or ‘stream direction’ which retained the data of which side of a double ended bar was hit. On a TFB basis, each channel is hit from the same end and therefore there is no overlap, but when combining the data into the ‘Global’ RMM plots, it was necessary to split the data by this variable. If

both ends of a bar were represented on the same plot, it would be impossible to tell which end was at fault, and therefore which channel. In previous stages, this has been denoted by hitEnd = -1,0,1, which on the following plots is denoted by downstream, single and upstream respectively, where single refers to bars where only one end is read out. In addition some RMMs were combined to accurately represent the physical ECal Modules (RMMs 0 and 1, RMMs 5 and 6, RMMs 8 and 9). It is worth noting that all of the bars in the DSECal have double ended readout, hence the ‘single’ plot does not exist.

FIGURE 3.5: The number of hits for each channel for the entire Down- stream ECal for hits of hit end -1 in the 3D bar/layer representation. A clear deficit can be seen for a few bins, and four clear non-hit bins illus- trating dead channels in white. The even numbered layers on this plot represent bars running from left to right and the odd numbers represent bars running top to bottom, which have been combined to create this

plot.

Due to the various geometries there are 5 orientations of bars for plotting purposes. For the Barrel ECal these are upstream, downstream and single ended, and for the Downstream ECal, these are upstream right to left, downstream right to left, upstream up to down and downstream up to down. The ‘global’ module plots style is also very useful when looking at actual tracks traversing the ECal. It is intuitive to see the hit channels of a given module for a given stream as these represent reconstructed tracks. Two examples of these combined bar/layer plots are shown in Figures3.5and3.6. The

3.4. The Process of Determining Mis-mappings 83

hit distribution has a spatial variation due to the acceptance of the Trip-T cosmic trigger.

FIGURE 3.6: The number of hits for each channel for the entire Barrel Top Left Module for single ended bars. Every other layer in this view is entirely white, which represents the other orientation of the bar which is readout by other TFBs. Additionally, a number of interesting features can be seen in this plot, including dead channels (white spaces), anoma- lous or mis-mapped channels and a strange artefact hit deficit near the centre top of the plot, which is due to an intermittently bad channel in

the outermost layer of the ECal (Layer 30, Bar 52).

The artefact seen around bar 52 in Figure3.6 is due to a bad channel response in the top most layer of the plot, which has a knock on effect to reconstruction in STF. This particular deficit is not caused by mis-mapping, although it illustrates that low level issues in the detector and software can become bigger issues during and after reconstruction.