Testing the Short Strip Module for HL-LHC

The ATLAS Inner Tracker (ITk) is a crucial part of the ATLAS experiment at CERN, designed to improve the detection of particles produced in high-energy collisions. As part of the High-Luminosity Large Hadron Collider (HL-LHC) upgrade, the ITk will use advanced silicon strip modules. One important component of this upgrade is the Short Strip (SS) module, which needs to be tested to ensure it can perform well under various conditions.

This study focuses on testing the SS module using electron beams of specific energy levels. The goal is to assess how well the module works at different temperatures, both warm and cold. Understanding its performance in these conditions is vital for planning future experiments and ensuring the module will operate effectively during the HL-LHC.

#Background on ATLAS ITk Upgrade

The Large Hadron Collider (LHC) is set to undergo an upgrade that will significantly increase its capabilities. This upgrade will allow the LHC to produce many more particle collisions, enhancing its ability to discover new physics. To support this increased activity, the ATLAS detector is being upgraded to the ITk system, which will replace much of the existing inner detector.

The new ITk system will consist of multiple layers of silicon detectors, including both pixel and strip detectors. These detectors are designed to withstand the higher radiation levels expected in the HL-LHC environment while providing improved precision and speed in data collection.

#Importance of Temperature in Detector Performance

The performance of the ITk strip modules can vary based on temperature. During operation at the HL-LHC, the modules will be kept at cold temperatures to reduce radiation damage. Thus, testing their performance at both warm and cold temperatures is essential. The measurements taken during these tests will provide insights into how the modules behave under operational conditions.

#Experimental Setup

The tests were conducted at the DESY-II Testbeam Facility in Hamburg, where a beam of electrons was generated. The electrons were tuned to energies of 5.4 GeV and 5.8 GeV for the tests. The SS module was placed in a controlled environment to analyze its response under different temperatures.

A special cooling box was used to maintain the module at cold temperatures, while warm tests were conducted at room temperature. Additionally, external detectors were employed to accurately track the paths of the electrons hitting the module. This setup allowed for precise measurements of both the module’s electronic noise and the signals it generated in response to the electron beam.

#Measuring Performance

#Noise Measurement

One of the critical aspects of detector performance is the level of noise in the system. Noise can interfere with the detection of real signals from particles and affect the overall efficiency of the module. The noise levels were assessed by conducting pedestal runs without the beam and injecting known amounts of charge into the system.

Two different sets of measurements were performed: one without any charge injection and another with a slight charge of 0.2 femtocoulombs (fC). The results indicated that the noise was consistently lower at cold temperatures compared to warm conditions. This reduction in noise would help maintain the integrity of the signals the module detects, particularly in the challenging environment of HL-LHC.

#Charge Detection Efficiency

Detection efficiency is another critical measure. It defines how many times the module accurately detects a hit when a charged particle passes through it. The performance was evaluated based on whether the hit was registered within a specific number of strips adjacent to the reconstructed track of the incoming electrons.

The location where the particle hits the strip also influences detection efficiency. When a particle strikes the center of a strip, it typically results in a stronger signal, leading to a higher chance of detection. However, if the hit occurs too close to the edge of a strip, the signal may spread to adjacent strips, making it less likely to trigger a detection in any one strip.

#Pulse Shape Reconstruction

Understanding the shape of the signal pulse produced by the module is vital for effective data readout. The induced current from a charged particle produces a pulse that is then shaped and read by the system. The timing of sampling this pulse is critical; if not sampled at the right moment, the reading may not accurately reflect the signal's strength.

To ensure the timing was correct, multiple delay settings were tested to see how they affected the recorded pulse shape. These measurements were used to create a representation of the pulse over time, demonstrating how the signal behaves at different temperatures.

#Results

The results from the testing provided several key insights into the performance of the SS module.

#Noise Performance

The noise levels measured at cold and warm temperatures showed a distinct difference. At cold temperatures, the noise measurements were significantly lower, which is advantageous for ensuring the signal-to-noise ratio remains high. This is crucial for operating efficiently in the high-luminosity environment of the HL-LHC.

#Detection Efficiency

Detection efficiency results indicated that the module maintained high efficiency in both cold and warm conditions, with values well above the target of 99%. This was achieved despite the challenges posed by varying temperatures. The effects of different hit locations-center versus edge-were also characterized, confirming that efficiency was optimally maintained with appropriate thresholds.

#Signal Measurements

The amount of charge collected from the hits was consistent across temperatures, with no significant differences observed. This is encouraging as it suggests that temperature fluctuations do not adversely affect the signal strength, which is a desirable quality for operating in the HL-LHC environment.

#Pulse Shape Analysis

The pulse shape analysis showed that the pulses from the electronics were faster at cold temperatures. This speed is vital to minimize the effects of pile-up during high collision rates expected at HL-LHC. The shape and duration of the pulses agreed well with theoretical predictions.

#Conclusion

The successful measurement and comparison of the Short Strip module’s performance at varying temperatures provide confidence in its deployment in the HL-LHC. With reduced noise, high detection efficiency, and stable signal measurements, the module is well-positioned to handle the increased demands of high-luminosity operations.

These findings not only validate the design of the module but also highlight the importance of environmental factors on detector performance. As testing continues, the data collected will be essential for ensuring the upgraded ATLAS detector meets the high standards required for future discoveries in particle physics.