One Quadrillion LHC Collisions Lead to a Rare Discovery

Today, my colleagues reported a substantial new advance in our understanding of the Higgs boson, the particle that is responsible for giving mass to fundamental subatomic particles. These results are available in two scientific papers, one published today and the other now submitted for publication.

Using the 16.5-mile-around Large Hadron Collider (LHC) at the CERN laboratory just outside of Geneva, Switzerland, we collided beams of protons to work out the rules that govern matter and energy.

ATLAS is one of two general-purpose detectors at the Large Hadron Collider (LHC)

After sifting through around a quadrillion (1015) collisions, we identified a handful of instances in which a Higgs boson was created at the same time as a top quark/antiquark pair. The combined discovery could give us an idea of how the Higgs contributes to mass and, if something is amiss, may suggest some exciting new physics. These collisions are so rare and so complex that we needed to employ the most sensitive statistical techniques and combine dozens of independent analyses to make the measurement.

The Higgs boson is a particle that allows us to characterize the Higgs field, which is an energy field that permeates the universe and gives mass to fundamental subatomic particles. It was first proposed in 1964 by several scientists, and it was observed at the CERN laboratory in 2012. In 2013, British physicist Peter Higgs and Belgian physicist Francois Englert shared the Nobel Prize in Physics for their work. Englert’s collaborator, American-born physicist Robert Brout, died a year prior to the discovery and did not share the honor, as the Prize cannot be awarded posthumously.

The top quark is the heaviest of all fundamental subatomic particles. It was discovered in 1995 at Fermilab, located just west of Chicago. It has a mass similar to that of an atom of tungsten.

This new discovery is exciting for a lot of reasons. Scientifically it is important because there are mysteries associated with the Higgs boson. While the Higgs field gives mass to fundamental subatomic particles, the Higgs boson itself also has mass. Part of that mass comes from its interaction with the Higgs field, but some of it is indirect, where the Higgs boson interacts with heavy particles (e.g. the top quark), which interact with the Higgs field and, through this connection, alters the mass of the Higgs boson. When these indirect effects are considered, the mass of the Higgs boson disagrees with expectations of that interaction. Thus, it is important to nail down the strength of the interaction between the Higgs boson and the top quark to shed some light on this mystery.

In addition, it takes highly energetic collisions to be able to simultaneously produce a top quark/antiquark pair and Higgs boson. These collisions are an ideal microscope to search for new phenomenon. This first observation is not precise enough to accomplish that, but there is more data coming. At the end of 2018, the LHC experiments will record only 3% of the total amount of data expected over the operational lifetime of the accelerator. Future measurements will be far more accurate.

Like many other physicists, I was a member of the groups that discovered the top quark in 1995, and I also participated in the discovery of the Higgs boson in 2012. It is not uncommon for researchers to have participated in all three measurements. This is the nature of modern particle physics experiments, which can have over 3,000 researchers. Both of those earlier measurements involved a handful of collisions, and now we can study ultra-rare events like the ones announced today. The LHC has only recorded about 3% of the total data it’s expected to gather over its lifetime—who knows what we’ll see as the LHC hits its stride?