My first project while I was an ATLAS physicist at CERN, was to write a histogram monitoring framework to monitor the timing stability of 6700 signal channels from ATLAS calorimeter.

A calorimeter is a sub detector component that measures the energy of the subatomic particles hitting the active medium of the detector. There are many ways subatomic particles interact with matter and a calorimeter is an optimized piece of material that is designed to measure energy according to the purpose and design of the experiment.

When I started out as a Ph.D student, ATLAS experiment just began taking data after a 1-year delay. Katy, a British Ph.D student who was my predecessor in the Liquid Argon calorimeter team already has observed that the timing stability of the detector channels were a bit off between summer and winter. There is a very interesting reason for this. More about this later.

The ATLAS Liquid Argon Calorimeter.

The ATLAS liquid Argon calorimeter is a cylindrical device which is segmented into several cell like active detector regions. Each of these cells are capable of generating an analogue signal pulse due to the ionization caused by the charge particles in the Liquid Argon medium. Analog to digital converters (ADC) are used to digitize these analog signals to facilitate advanced digital processing further downstream in the data acquisition electronics of the ATLAS experiment. But here is the crucial thing, the energy of the particle that hits a calorimeter cell is proportional to the amplitude of the analog signal produced. In order to propagate this information to the digital electronics, it is important to time sampling of the peak precisely. This is achieved by synchronizing the strobing of the sampling with the LHC clock frequency.

LHC clock frequency

CERN accelerator complex that distributes the LHC clock to all the experiments. This helps the experiments to synchronize the operations of their detector with the LHC clock and thereby preparing for the arrival of the proton collisions at 40 MHz (or every 25 ns).

The clock is distributed through optical fiber cables. Typically the refractive index of these cables are between 1.4 and 1.5. This means the speed of light through these cables are reduced by 40% or 50% when compared to the light speed in vacuum. During the summer months, the thermal expansion of these materials increase the path length experienced by the clock frequency signals before reaching the ATLAS experiment. This causes our strobes to misalign a bit during the summer and winter months unless we monitor this and calibrate the strobing time.