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Ann Arbor, MI—A team at the University of Michigan Department of Physics, led by Professor Tom Schwarz, directly observed the interaction between the Higgs boson and the heaviest known elementary particle, the top quark.
The team was part of the ATLAS experiment, one of the four major experiments at the Large Hadron Collider at CERN, the research center in Switzerland that is home to the world’s most powerful particle accelerators.
Physicists’ current understanding of the physics of fundamental particles is encompassed in the theory of the Standard Model. In this model, there are particles that make up matter called “fermions,” such as electrons and top quarks, and particles called “bosons” responsible for their interaction, such as the photon and the Higgs boson.
For example, electrons, which are fermions, interact with each other through the photon, which is a boson. The researchers quantify how strongly a given boson interacts with particles as the “coupling.” The electric charge is an example of the coupling of electromagnetism to charged particles, like electrons. Similar to the electric charge’s relationship to electromagnetism, the mass of a particle tells us how it couples to the Higgs boson—the more massive the particle, the stronger the coupling.
Though the top quark plays a very important role in the production of the Higgs boson, it has been difficult to isolate their relationship from other physics processes. This new measurement directly searches for Higgs bosons being produced in association with the top quark, by searching for both particles in a collision.
“It’s a very rare process, but with the amount and value of the data provided by the LHC, the use of advanced machine-learning techniques, and the many hard-working and clever scientists from the ATLAS and CMS collaborations, we have been able to observe this important physics,” Schwarz said.
In addition to Schwarz, the U-M team performing the data analysis includes postdoctoral researcher Allison McCarn Deiana, soon to be faculty at Southern Methodist University; graduate students Rachel Hyneman and Garrett Merz; and Michigan undergraduate student Noah Zipper. Zipper spent a semester working on the ATLAS experiment as part of the CERN semester abroad program, started at Michigan by the late professor Homer Neal and funded by the Lounsbury Foundation. Professor Jianming Qian and Professor Bing Zhou, along with graduate students Zhi Zheng, Zirui Wang and Rongkun Wang also contributed to this work in different Higgs boson decay channels.
Time-lapse animation showing the increasing ttH signal in the diphoton mass spectrum as more data are included in the measurement. (Credit: ATLAS Collaboration/CERN)