ATLAS releases first open data from heavy-ion collisions
13 December 2024 | By
The ATLAS Collaboration has released its first open data of heavy-ion collisions for research purposes. This dataset features lead-ion (Pb-Pb) collisions at an energy of 5 TeV per nucleon pair, recorded in 2015 as part of the Large Hadron Collider’s second operation period (LHC Run 2). Today’s release is the highest energy heavy-ion collision data ever made publicly available and the first LHC Run 2 heavy-ion open dataset. Researchers worldwide can now explore over four terabytes of open data – comprising 221 million collision events – unlocking new opportunities for scientific discussions.
Heavy-ion collisions, such as those between lead nuclei, are markedly different from proton-proton collisions. “Their massive size leads to very different types of interactions,” says Zach Marshall of the ATLAS Open Data team. “While colliding protons is akin to smashing two garbage cans together, colliding lead ions is what happens when you drive two garbage trucks at each other.”
Unlike colliding trucks, when lead ions smash together they can create a droplet of quark-gluon plasma (QGP). This is the state of matter believed to have existed in the early universe, shortly after the Big Bang. As this droplet of plasma cools and evaporates, it leaves behind a distinct energy signature in the ATLAS experiment that researchers can analyse.
“Using the new open dataset,” continues Zach, “researchers can study these energy signatures to learn how the QGP forms and evaporates, as well as the intricate dynamics of particle interactions within this extreme environment.” In particular, this dataset offers the opportunity to study jet suppression, a phenomenon where high-energy particle jets lose energy as they pass through the QGP. Such suppression provides researchers with a window into the properties of the quark-gluon plasma.
Today’s release is the highest energy heavy-ion collision data ever made publicly available and the first LHC Run 2 heavy-ion open dataset.
Releasing heavy-ion open data posed unique challenges for the ATLAS Open Data team. “Unlike proton-proton data, we needed to include lots of extra collision event information – from charged particle tracks to energy measurements in our far-forward detectors – as that is where details about the QGP’s behaviour are hidden,” explains Zach. “To achieve this, we developed a new open data format (DAOD_HION14), which includes all this information but can also be studied with the same tools as our proton-proton datasets.”
Today’s release features “Minimum Bias” heavy-ion data, which includes a wide range of collision events (see image banner) from central, head-on collisions to glancing, ultra-peripheral interactions. Next year, ATLAS will release “Hard Probes” heavy-ion data collected in 2015, focusing on collision events triggered by high-energy particles such as Z bosons or top-quark pairs. To help researchers understand the unique characteristics of each open dataset, the release is also accompanied by documentation and simulated data.
Earlier this year, ATLAS made public over 65 TB of proton-proton collisions in its first open dataset for research. Today’s release adds to this legacy, comprising approximately 20% of ATLAS’ total Pb-Pb dataset, and more releases are already in the pipeline. This commitment to openness reflects core values of the high-energy physics community: fostering accessibility, enhancing reproducibility, and driving scientific progress.
As ATLAS continues to make its data publicly available, the potential for collaboration grows. Researchers worldwide are invited to explore this new dataset and join the exploration of the universe’s most extreme conditions.
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