The ATLAS team from the Clermont-Ferrand Physics Laboratory (LPC) took part in beam tests to measure the performance of silicon sensors measuring around 1 mm x 1 mm. These sensors, called LGADs (low-gain avalanche diodes), are capable of doing something extraordinary: they can distinguish signals that are separated in time by only a few tens of picoseconds. To give an idea of what this means, if we reduce 40 picoseconds to one second, one second is equivalent to about 800 years.
The LPC team plans, in an international collaboration, to use this technology to build a new part of the ATLAS detector at CERN's Large Hadron Collider (LHC), to help search for new particles and making precise measurements of the properties of the Standard Model of particle physics. Indeed, nearly two million LGADs will be assembled to build one of the very first high-granularity time detectors, which will be essential for the next phase of the LHC, where collisions will be so numerous that it will be impossible to distinguish between them, using only spatial information.
The members of the ATLAS team wanted to study these LGADs when they are introduced into beams of particles (pions or electrons) at CERN (Geneva, Switzerland) and DESY (Hamburg, Germany). The idea is to verify that, under realistic beam conditions, it is actually possible to achieve the required temporal resolution, while respecting all the specifications for the ATLAS youth project. The LPC team played a key role in coordinating and setting up the beam tests, as well as analyzing the data, and collaborated with other institutes in France (Laboratoire Irène Joliot Curie in Paris) and abroad (Taiwan, Netherlands, Germany, Spain, China, Switzerland and Morocco) to achieve this goal.
The bet is won: the measurements made during these tests prove that the most recent versions of these LGADs meet all the specifications of the detector that the team is trying to build: the project is therefore a big step forward towards the realization of this detector at another temporal granularity, which will be the first of its kind.