Record Measurement of the Number of Proton Collisions at Large Hadron Collider at CERN
Uppsala researchers have together with physicists from among others KTH and Stockholm University carried out the most precise measurement of the so called luminosity at the ATLAS experiment at the particle accelerator Large Hadron Collider at CERN in Switzerland.
The Large Hadron Collider, LHC, at CERN is the world’s largest particle accelerator. It accelerates protons up to almost the speed of light and these are later collided at extremely high energies. In the collisions, new particles are created and these are detected with the help of large detectors, like for example the ATLAS detector. In this way, the universe’s smallest components, the elementary particles, and the forces with which these interact can be mapped.
Several physicists at Uppsala University work with analysing data from the ATLAS detector together with around 3 000 other researchers, students and engineers from the whole world. In recent years, the scientists have measured the luminosity, that is the number of proton collisions created and detected during a certain time period, of the data set which was collected between 2015 and 2018 during the so called Run2.
“The luminosity is used, among other things, to calculate the probability that an elementary particle studied at the LHC will be created in a proton collision. These probabilities are called cross sections and can be used in comparisons with other experimental results and theoretical predictions and is one of the foundations of particle physics,” says Giulia Ripellino, postdoctoral fellow at the department of physics at Uppsala University.
The researchers have measured the luminosity in ATLAS with a number of different methods and carefully estimated all systematical errors that could have affected the measurements. Among other things, they have studied how effective ATLAS is to detect proton collisions and how the settings of the accelerator affects the luminosity.
“The ATLAS collaboration has delivered the most precise luminosity measurement ever at a proton collider, with an uncertainty of only 0.83%. This can be compared with the luminosity measurement made at ATLAS for LHC Run1, which had an uncertainty of 1.9%,” says Giulia Ripellino.
This research has been a large part of Giulia Ripellino’s doctoral thesis which was made at KTH. The developed methods will be of great help during the ongoing LHC Run3 and will contribute to that the researchers may achieve similar precision in future measurements. This will hopefully contribute to more interesting discoveries in particle physics in the future.
ATLAS Collaboration, Luminosity determination in pp collisions at s√=13 TeV using the ATLAS detector at the LHC. arXiv: https://doi.org/10.48550/arXiv.2212.09379 (Dec 20, 2022).
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Translation: Johan Wall