The inventory of Swiss particle physics has just opened a new register for a new kind of research: thanks to EPFL assistant professor Radoslav Marchevski, it can add kaon physics to its list. The Bulgarian scientist, who did his undergraduate studies in Sofia, Bulgaria and his graduate studies in Mainz, Germany, joined the ranks of Swiss academia at the beginning of the year and brought his specialty particle with him.
If you ask Radoslav Marchevski, kaons are the stars amongst all the things that can be made of quarks. Quarks and antiquarks, to be precise, because kaons always consist of a pair of these. Kaons can stress-test the Standard Model of Particle Physics to the extreme, and the smallish, ultra-precise experiments that study them have a relative fast turnaround, offering scientists to take part in all the stages of the experiment from conception via construction to data-taking (and proposing the next generation of experiments). Marchevski is part of the NA62 collaboration, an experiment of some 200 scientist located at CERN, so now Switzerland is part of it, too. “We can measure extremely rare processes with unprecedented precision by profiting from a long line of kaon experiments at CERN,” says Marchevski. The scope may be smaller than at the gigantic multi-purpose LHC experiments, but the physics is fascinating.
Physicists have been trying for decades to prove that the underlying and thus far very reliable Standard Model is incomplete. Stress-testing it, they hope, will reveal rare processes of interactions between particles that deviate from prediction, identifying gaps in the theory and revealing new phenomena. And even if they don’t find these gaps, they are still making an invaluable contribution to the understanding of the rules that govern our Universe by placing ever-tighter limits on where and how frequently these potential rare processes occur.
The specialty of NA62 is the analysis of the rare case of a charged kaon decaying into a charged pion and a neutrino-anti neutrino pair. “It’s a fantastic process,” Marchevski raves. It requires a strange quark to transform into a down quark, which is officially forbidden by the Standard Model. “Interactions that change the quark flavour without changing its electric charge are not allowed. That’s why such interactions need to happen in loop processes – the quarks must change flavour two times. These loops are rare, but if there is new physics lurking in the quark sector, we are sensitive to it.”
The experiment studies the spray of particles that are created when a beam of protons from CERN’s SPS accelerator is smashed into a beryllium target. A setup of highly sensitive detectors can reconstruct the particles’ paths in high-resolution 3D images with precise time information. After hitting the target, the experiment selects specific charged particles from the beam, identifies them, and measures their momentum with a silicon pixel detector. The kaons in the beam fly on through a 80-metre vacuum vessel, decay in flight, and the outcoming track is measured in a magnetic spectrometer, while the subsequent RICH detector, electromagnetic and hadronic calorimeter, and muon veto detector provide excellent muon-pion identification.
The hadronic calorimeter is the part of the experiment that Marchevski has been working on during his PhD studies at the University of Mainz in Germany. At EPFL, his new team is heavily involved in the silicon pixel detector and physics data analysis. The requirements at NA62 are very similar to those needed for the Hi-Luminosity LHC, so exchange between small specialist experiments and new LHC experiments is almost guaranteed. Marchevski will also contribute to the kaon program of LHCb, which he joined at the beginning of the year and where he hopes to be able to study short-lived neutral kaons. “With kaons, Switzerland, and EPFL in particular, now has the complete flavour physics competency and we can stay flexible if something pops up in the data,” he says.
NA62, which is set to take data until 2025, is the latest experiment in a long line of kaon experiments that provide ever more challenging measurements and put stronger limits on possible new physics, alternating between neutral and charged kaons. The quest for better precision demands that NA62 pushes the intensity frontier and thus state-of-the-art technology. The next, ever more precise and ambitious experiment is already taking shape in scientists’ minds.
Author: Barbara Warmbein for CHIPP
Swiss Institute of Particle Physics (CHIPP)
c/o Prof. Dr. Ben Kilminster
Department of Physics