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Seth Zenz | Imperial College London | UK

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It’s Jet Quenching… and I Helped!

The ATLAS experiment has released a major new result in the past few hours, and I’m very excited about it because I helped! A public preprint of our paper, already accepted for publication by Physical Review Letters, is here. The result is that we’ve seen striking signs of a phenomenon called jet quenching in heavy ion collisions, in which hadronic jets get spread out and lose energy by interacting with the ultra hot and dense state of matter known as quark-gluon plasma. I’m exhausted, so I won’t try to explain it better than the CERN press release:

The ATLAS and CMS experiments play to the strength of their detectors, which both have very powerful and hermetic energy measuring capability. This allows them to measure jets of particles that emerge from collisions. Jets are formed as the basic constituents of nuclear matter, quarks and gluons, fly away from the collision point. In proton collisions, jets usually appear in pairs, emerging back to back. However, in heavy ion collisions the jets interact in the tumultuous conditions of the hot dense medium. This leads to a very characteristic signal, known as jet quenching, in which the energy of the jets can be severely degraded, signalling interactions with the medium more intense than ever seen before. Jet quenching is a powerful tool for studying the behaviour of the plasma in detail.

“ATLAS is the first experiment to report direct observation of jet quenching,” said ATLAS Spokesperson Fabiola Gianotti. “The excellent capabilities of ATLAS to determine jet energies enabled us to observe a striking imbalance in energies of pairs of jets, where one jet is almost completely absorbed by the medium. It’s a very exciting result of which the Collaboration is proud, obtained in a very short time thanks in particular to the dedication and enthusiasm of young scientists.”

I worked on this paper for only 4 or 5 days, meeting with a few other enthusiastic “young scientists” on Skype almost continuously in order to do a cross-check of the results using track jets. (I explained a bit about track jets when I explained my thesis topic a while back. This work wasn’t part of my thesis, but it’s an application of the same technique.) Our work ended up becoming three sentences on page four of the paper:

The analysis was independently corroborated by a study of “track jets”, reconstructed with [Inner Detector] tracks of pT > 4 GeV using the same jet algorithms. . . . A similar asymmetry effect is also observed with track jets. The jet energy scale and underlying event subtraction were also validated by correlating calorimeter and track-based jet measurements.

My immediate partners on this part of the work were David Miller and Zach Marshall, but I don’t want you to think just because I’m blogging about it that we were the critical people working on this paper. Obviously there were many other cross-checks of this result, all done through equally hard work in an equally short time, and then of course there are the Heavy Ion experts who did the core measurement itself! We only did a small part of it all, but we worked hard and we’re proud.

ATLAS as a whole is proud. We have won some and lost some this year in the competition with our colleagues at the CMS detector to make new measurements and discoveries first. This time, we won, and it’s a great feeling to end the year with.

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