Just in Time

A new paper I was involved with came out today which documents some of the instrumental work that kept me busy during the two years I spent as a CERN fellow.


The paper documents the performance of the tiny 1mm x 1mm sensors pictured above: they are called LGADs (low-gain avalanche diodes), and we are going to use them to build a new part of the ATLAS detector, to be installed around 2027.

LGADs may not look like much, but they can do something amazing: they can distinguish signals which are separated in time by only a few tens of picoseconds. To give you a sense of what that means, if you scale 40 picoseconds up to one second, the one second scales up to around 800 years.

We plan to use these sensors (nearly two million of them to be exact) to build one of the first-ever high-granularity timing detectors, which will be critical for the next phase of the LHC, where the collisions will be so numerous that it won’t be able to unpick one from the other using spatial information alone.

In this particular paper, we put arrays of these LGADs into one of the “test beams” at CERN and at DESY in Hamburg. The idea is to test that under “realistic” beam conditions, you really can reach the required resolution, all while meeting a set of other specifications.

The great news from the paper is that we showed that the most recent versions of these LGADs meet all the specifications for the detector owe are trying to build: that brings us one step closer to actually making project into a real, physical object. The fist of its kind!

A plot from the paper, showing that all the sensors we tested, when operated at the right voltage, can meet the minimum time resolution we need (70 picoseconds) and actually can achieve resolutions of around 40ps.

On a personal note, I only started working on instrumentation in 2021 when I joined CERN: before that, my experience in physics was purely data analysis. Working on R&D for detectors has been a huge amount of fun, and uses a very different part of one’s brain. Instead of “are we using the correct statistical model?” it’s more like “how can I glue these components together without damaging them?” or “Can I hack this oscilloscope to use it as a makeshift data acquisition system?” In my new job at Clermont-Ferrand, although based away from CERN, I am still working on this instrumentation project, now designing a system to perform final checks on the assembled detector before it is lowered into the ATLAS experimental cavern in 2027.

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