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August 27, 2021

Courtesy of Professor Stefan Spanier

The High-Energy Physics group recently celebrated the installation of a detector that measures the brightness, or specifically luminosity, of the proton beam at the Large Hadron Collider (LHC). Luminosity is a central ingredient for the searches of physics beyond the standard model of particle physics.

Nimmitha Karanurathna

Nimmitha Karanurathna

Himal Acharya

Himal Acharya

Jesse Harris

Jesse Harris

This luminosity detector sits very close to the LHC beam pipe and about 1.8 m away from the region where collisions occur. It uses small silicon pixel detectors. A series of three such detectors is called a telescope. Each telescope detector has to register a hit simultaneously to call this a particle track. Hence, the instrument, which consists of 16 such telescopes, is also called the Pixel Luminosity Telescope (PLT).

Postdoctoral researcher Andres Delannoy leads the detector assembly at the CERN laboratory in Geneva, Switzerland. The detector measures the rate of particle tracks that are produced during collisions of protons, which happens every 25 nanoseconds. This rate is then used to calculate how often predicted processes such as the production of Higgs bosons can occur. In turn, once new signals are found, their measured production rate can only be compared to theory with a known luminosity value. Hence, this quantity luminosity must be measured with very high precision posing challenges to the detector technology.

Andres Delannoy, Francesco Romeo, Joanna Wanczyk, Brandon Soubasis, Stefanos Tsoukias with the finalized BRIL subsystems

Left to right: UT Postdoctoral Researcher Andres Delannoy with Francesco Romeo, Joanna Wanczyk, Brandon Soubasis, and Stefanos Tsoukias with the finalized BRIL subsystems (Credit: CERN/CMS)

The graduate students of the UT group help to make the instrument work, now that it is installed. The next step is to write software that monitors its performance during collisions and calculates corrections to the measured particle track rate. There is an efficiency at which tracks are registered, and there are some particles that do not originate from collisions and hence have to be subtracted. Graduate student Nimmitha Karanurathna uses likelihood fitting techniques to measure correction parameters from data. Graduate student Himal Acharya found a method to measure the collision point position with reconstructed tracks in the PLT. And graduate student Jesse Harris is going to explore machine learning methods to predict the performance of the instrument and necessary corrections or maintenance. The detector will have to work perfectly 24/7 to achieve the required incredibly high precision. The LHC will start collisions for new physics searches in spring of next year. The graduate students, at present, are scheduled to travel to CERN in spring 2022 when the LHC starts up.

The PLT instrument within the CMS experiment is a collaborative effort between the Vanderbilt University, Princeton University, Rutgers University, and the University of Tennessee, and the institutes CERN, DESY, and PSI, and in the US is managed by the UT Knoxville group.

Find the CERN article co-authored by Dr. Delannoy here.

And here's the video:

For more background, read Planning for years of luminosity measurements with BRIL.

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