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Image : ESO

The symmetries of the very big and very small

EPFL graduate Guillaume Pietrzyk wins CHIPP PhD prize

Guillaume Pietrzyk, CHIPP prizewinner 2022
Image : Gaëlle Khreich IJCLab

We’ve all heard stories of how people have changed their lives because of the pandemic – apart from those whose lives were changed by the loss of a loved one, of course. Some people finally learned how to play the guitar, some quit their jobs, some simply stopped cutting their hair. And some solved tricky physics problems because lockdown gave them a chance to change perspective. This happened to EPFL PhD student Guillaume Pietrzyk – and he won this year’s CHIPP prize for his solution! (He is also one of three winners of the LHCb thesis awards, given every year to students with great theses who also made exceptional contributions to LHCb beyond their main thesis topic.)

Guillaume Pietrzyk, who is of French-Polish origin, had been working on this problem for nearly two years under thesis supervisor Fred Blanc. His task was to check whether the LHCb detector can measure a specific parameter involving charm quarks called yCP, a parameter connected to the oscillation of a particle to its antiparticle. It’s an extremely tricky experimental problem, and the matrix-based mathematical approach he and his colleagues had been following for months wasn’t working. “It was quite hard,” Pietrzyk remembers. “I was nearing the end of my PhD and it looked like I wouldn’t be able to show any cool new results.”

And then COVID started and suddenly everybody was locked up at home. “I could just be alone and think about the problem,” he says. “We had just come up with a new technique – I call it matching – and I really wanted to make it work. And then suddenly I knew what the solution was. It was like opening a door and suddenly you’re on a highway.” Finally, everything fell into place and Guillaume could show that the yCP parameter can indeed be measured.

And what does this parameter do? What is it good for? It adds a significant amount of knowledge to everything we know about how particles behave and why the Universe is the way it is – basically, particle physics’ fundamental quest – through a better understanding of D mesons, also known as charm particles. We know that everything we see is made of matter, and we also know that matter and antimatter must have been created in equal amounts at the big bang. The LHCb detector and the scientists working on it have specialised in finding tiny differences between matter and antimatter particles that might explain the mystery of the missing antimatter.

One area of research is into particle oscillations, “a fun phenomenon of quantum mechanics,” as Guillaume Pietrzyk calls it. Some particles can oscillate between being a matter and an antimatter particle, and they do it with vastly different behaviors. The Bs mesons (composed of a strange quark and a beauty antiquark) oscillate extremely fast, about 3 million million times per second. On the other side of the spectrum, the oscillation of a D0 meson (composed of a charm quark and up antiquark) is an extremely rare process to observe. Most of these charm particles disappear before being able to oscillate into their antimatter counterpart. Therefore, lots of these particles need to be seen inside the LHCb detector to describe precisely their oscillation properties. This is where the measurement of Guillaume’s parameter, yCP, comes into play. A precise measurement of yCP for D0 mesons allows to improve significantly the scientific knowledge of charm oscillations. With the help of the methods described in Guillaume ’s thesis, they will be much more easy to spot in past and future collisions.

When the 28-year-old started studying, he didn’t think he would end up in particle physics. In high school he started reading books and watching YouTube videos about astrophysics and thought that that was what he wanted do – study the Universe to find out more about its rules and its fate. In his third year at EPFL, at the time a little disillusioned with his chosen subject, he took a course on high-energy physics given by Olivier Schneider from LHCb, and he was hooked. “It was super fascinating,” he recounts, so from one day to the next he changed direction and went from the very big to the extremely small. He did his Master’s on LHCb data and it felt very natural to him to also go on to do a PhD.

He wanted to specialise in data analysis and ended up doing so, but the traditional PhD process requires students to do some “hardware work” as well, and so Pietrzyk built fibre mats for the new LHCb SciFi tracker. “It was very different from what I was used to, but it was really nice – I learned a lot!” He found the analysis topic that earned him the PhD prize by walking around the lab at EPFL, talking to everybody and picking the topic that seemed like the biggest challenge to him. The idea came from an Italian researcher at EPFL. Measuring the yCP parameter for D0 mesons was a “fancy idea” that nobody had thought of before, so the PhD student’s competitiveness was tickled. “I like to explore the unknown,” Guillaume smiles.

He grew up in the area around CERN and is actually the second generation of LHCb researchers in his family – his father is a well-known physicist in the collaboration and very active in outreach. He obviously passed on the passion for sharing fascinating research to his son, who started the LHCb Instagram account in 2017 and still manages LHCb social media together with two colleagues. He is very active on twitter – give him a follow to receive all the latest science news and HEP chatter.

And you may even catch him on stage: during his PhD he joined the very active improvisation scene in Lausanne. “That wasn’t only an amazing amount of fun, it also taught me loads,” he explains. “Giving presentations, for example, seems much less scary when you’ve had the improv experience of being up on stage, by nature of the art form completely unprepared, and trying to make the audience laugh and enjoy itself. He is keen to take this hobby up again where he is now – a postdoc at the lab Irène Curie Joliot (IJCLab) in Orsay near Paris, studying rare decays of B mesons on the hunt for new physics.

Author: Barbara Warmbein

Guillaume Pietrzyk, CHIPP prizewinner 2022 at CERN
Guillaume Pietrzyk, CHIPP prizewinner 2022 at CERNImage : B. Warmbein


  • Physique des particules élémentaires


Swiss Institute of Particle Physics (CHIPP)
c/o Prof. Dr. Michele Weber
Université de Berne
Laboratory for High Energy Physics LHEP
Sidlerstrasse 5
3012 Berne