PARTICLE PHYSICS

DESY theorist Aditya Pathak has been awarded a Starting Grant from the European Research Council (ERC) ― one of the most coveted European research grants for early-career researchers – for his research project TOPMASS. Starting in September 2025, Pathak will lead a group that will work towards strengthening the precision of measurements of the top quark’s mass. His research will also address longstanding problems associated with the description of “hadronization”, the process by which energetic quarks and gluons produced in particle collisions bind together into hadrons (the family of particles that comprise most famously the atomic nucleus). A precise determination of the top quark mass and a systematic treatment of hadronization is crucial for many other precision measurements in particle physics and is currently still heavily influenced by an over-reliance on simulations of particle collisions. Pathak’s winning proposal aims to eliminate this dependence and make the mass of the top quark more concretely known.

With the renowned Starting Grants, the ERC supports talented young scientists with 1.5 million euros over a period of five years so that they can realise their outstanding research projects and build up their own teams. The application process for ERC Starting Grants is highly competitive: in the 2024-call year, around 14% of applications were successful, with 494 researchers selected out of 3,474 proposals. The ERC funded the TOPMASS project (full title: “A new paradigm for high-precision top mass and jet substructure measurements at the LHC”) for its consideration of a major bottleneck in the search for new physics. The top quark, which was discovered by the Fermilab experiments D0 and CDF in 1995, is among the most massive particles described by the Standard Model of particle physics. Since it is one of the superheavy third-generation particles in the model, it tends to decay into other particles rapidly, impeding the ability to study it directly.

Existing theoretical strategies, while providing means of measuring the mass of the top quark, have several major weaknesses that TOPMASS hopes to resolve. This includes an over-reliance on a statistical probability method called Monte Carlo simulations and the need for modelling how energetic quarks and gluons produced from the top quark decay would be bound into hadrons. Hadrons, of which protons and neutrons are the most well-known examples, are held together by the strong force, mediated by particles discovered at DESY in 1979 called gluons, but our understanding of how they self-assemble is limited. This means that the all-important transition from quarks and gluons into hadrons, a fundamental property of the strong force, is not as clear as it should be, impacting all kinds of measurements performed in high energy colliders.

Pathak’s past research results have shown the potential for an alternative to both of these measurement roadblocks. He has provided a new method of accessing the top mass with high precision without relying on Monte Carlo simulations. The method exploits another heavy particle produced in the intermediate stages of the top quark decay, the W boson, discovered in 1983. “My work draws inspiration from cosmology, where distances between galaxies are not directly measured, but only indirectly using the so-called ‘Standard Candles’, pulsating stars whose intrinsic luminosity is strongly related to their pulsation period”, Pathak says. “In the same way, the intermediate W boson in the top quark decay chain, whose mass has been known to a significantly higher precision, is a gift from nature as it provides the much-needed ‘standard candle’ for measuring the top mass.” Pathak will build on this work through TOPMASS, as well as fleshing out a new way of describing the impact of the transition of quarks and gluons into hadrons on on various measurements in particle colliders. These developments, once completed and shown to be experimentally viable, can be used to give an even clearer picture of a fundamental process at the root of the generation of matter as we know it.

On that note, better knowledge of the top quark mass can tell us much about the fate of the universe. The stability the vacuum we live in, which due to quantum phenomena is not at all empty but rather more akin to a roiling sea of energy, is commonly used in particle physics to predict the future properties of our evolving universe. TOPMASS can therefore deliver a crucial input for such predictions as well.

“TOPMASS is a creative way of measuring the mass of the most massive fundamental particle we know of in our universe,” says DESY Director for Particle Physics Beate Heinemann. “I am thankful that the ERC recognised his innovative ideas and awarded him this grant, and I wish him all the best for achieving groundbreaking results.”