Spike Plots

(full approval presentation here)


In collision data we observe anomalous signals in ECAL, having the appearance of large energy deposits in a single crystal.

These signals are uniformly distributed and only in the barrel part of the calorimeter where the readout is by APD; they are not seen in the EE crystals readout by VPTs.
The rate of these anomalous signals is of order one per 103 minimum-bias events at c.m.s. of 900 GeV.

The origin of the signal is energy deposited by heavily ionizing particles in the avalanche photodiode.

Schematic and event picture:

PPT file with image

Pulse shape:

Cluster level:

Top Figure shows the oversampled pulse shape for typical and anomalous signals.
The rise time of the electronic pulse is consistent with an instantaneous signal from the APD, and not from the typical scintillation light signal in the crystal.
Bottom Figure shows the best fit of average pulse shape for a typical (green) and an anomalous (red) signal to the actual signal (blue dots) sampling of the anomalous signal.
This example pulse shape is from an anomalous signal which happens to be in time with the normal signals at 0 ns. This reflects the most challenging situation.


At the cluster level the anomalous signals appear as energy in a single crystal, while in e.m. showers the energy is typically shared between neighbouring crystals. This fact is used to tag anomalous signals. E4/E1 : Ratio of the energy deposit in one crystal (E1) over the energy in the four crystals (E4) having a common face with this crystal.
Plot shows the offline reconstructed time of the single crystal signal timing. Anomalous signals are in time with the collision event but have a much wider spread around the good timing. For anomalous signals the time reconstruction is biased due to the different pulse shape. This makes the apparent time of prompt anomalous signals about 10 ns earlier than standard signals and creates apparent structure in the tail of the anomalous signal timing. The standard signal timing is at 0 ns and the time resolution of the ECAL is better than 1 ns for signals above 1 GeV.