Focus on Michael Knichel

Virginia Greco

At the Quark Matter 2018 Conference, which took place last May in Venice (Italy), Michael Knichel presented recent results of his ‘analysis of the apparent nuclear modification in peripheral 5.02 TeV Pb-Pb collisions with ALICE' and was assigned the Elsevier Young Scientist Award for the best experimental talk, ex aequo with Barbara Trzeciak.

Michael studied physics in Germany at the Technische Universit?t (TU) Darmstadt and, during that period, he spent an exchange year in Saskatoon (Canada), where he attended the University of Saskatchewan. He joined ALICE already in 2008, when he also had the occasion to go for the first time to CERN and stay there for a couple of months. “Back then, I worked mainly on hardware and detector development,” Michael comments. “In particular, I participated in assembling a test setup for the ALICE TPC.

After he had concluded his Master studies, Michael enrolled for a PhD at the TU Darmstadt to work at GSI with Peter Braun-Munzinger, who had already supervised him during his Master thesis. “My research was focused on charged particle spectra in pp, p-Pb and Pb-Pb collisions and they were the first measurements of this kind performed in ALICE,” he explains. When he defended his PhD thesis, in 2014, Michael was already a postdoc fellow at the University of Heidelberg, where he continued working on the same topic.

In April 2017, he moved to CERN thanks to its fellowship programme and joined the ALICE Physics Group. There, he carried out the analysis that he presented at the last Quark Matter conference, in a talk that earned him the Elsevier Young Scientist Award.

In particular, at QM2018 Michael reported on recent measurements of the nuclear modification factor RAA in peripheral heavy ion collisions, based on an idea of Andreas Morsch and Constantin Loizides, who suggested that what seems to be parton energy loss observed in peripheral Pb-Pb collisions (which results in RAA< 1), might simply be a bias from the measurement itself. “This idea is very tempting,” explains Michael, “because if we did not require jet quenching to describe the Pb-Pb peripheral events, we would have consistency with what we see in smaller systems, namely pp and p-Pb collisions. Many other observables – like flow, modifications of the particle composition and so on – show a continuous behavior in p-Pb, in peripheral Pb-Pb, and even in high multiplicity pp collisions, if measured as a function of the charged particle multiplicity, and everything is nicely consistent. On the contrary, the parton energy loss seems to be very different in the various systems.”

The results do not rule out the presence of energy loss in peripheral collisions,” he clarifies, “but it seems that there is no need for a large effect.

As Michael explains, taking data – for example – of oxygen-oxygen or carbon-carbon collisions would be very useful, because one could look for parton energy loss and jet quenching in these systems, which have a similar number of nucleon-nucleon collisions and similar multiplicity as the p-Pb ones but very different geometry.

Last year there was indeed a short run of different ion collisions: Xe-Xe. “Unfortunately,” he highlights, “xenon is not really a small system, so it is rather similar to Pb-Pb in terms of parton energy loss.Nevertheless, the bias is expected to be larger and I would like to extend the studies that we have already performed on those Xe-Xe events to include peripheral collisions.

At the moment, Michael's research activity is focused on this topic, but he is also working on other measurements that can be performed in ALICE. “For example,” he explains “there are data sets of pp collisions including the Zero Degree Calorimeters (ZDC) to be analyzed.” The ZDC are detectors located in the tunnel, far from the collision point, which allow measuring the energy of the nucleons that, in heavy ion collisions, do not take part in the event and fly away along the beam line. “We could investigate if we can use this data to make some centrality selection in pp collisions and, maybe, find a way to measure jet quenching in pp collisions,” he continues. “I would also like to perform measurements on the charged sigma baryon or something else involving the particle identification, since this is really a matter for which ALICE is unique.

The direction that his work will take in the future also depends on the job opportunities that will come up next year, at the end of his fellowship: “The most natural choice for me would be to continue working in research, since I like this kind of job and the atmosphere in scientific collaborations; possibly I would remain in ALICE. But I cannot exclude a priori other possibilities, such as changing experiment or even leaving research. Country-wise, I would prefer to stay in Europe; but I could also reconsider, if a good offer arises.

In addition to research itself, Michael also enjoys teaching and supervising students: “It is someway a learning experience for me as well,” he explains, “because, when you teach basic courses, you look back at things that you studied long time before, but with much wider knowledge and experience. Hence, you understand those concepts at a deeper level and see things that you could not think of when you were an undergraduate or graduate student.

When he does not work, Michael likes listening to music, going to the movies and practicing yoga. He also enjoys travelling, whenever he can, and is really fond of mountaineering: hiking, climbing and skiing. “But I haven't been doing it a lot lately,” he confesses laughing.

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