Higgs Boson: The Theorist and the Experimentalist

Professors Tao Han and James Mueller complement each other. Both physicists in Pitt’s Kenneth P. Dietrich School of Arts and Sciences, the two have conducted research—for decades—on the Higgs boson, a particle thought to be the fundamental building block of the universe.

Han is the theorist; Mueller is the experimentalist. A close working relationship, they say, is common in physics. Theoretical physicists serve as guides, indicating where such particles as the Higgs might exist. And the experimentalists—as their name suggests—conduct experiments in the field.

But Han and Mueller are only two members of one very large team. Han, along with Pitt Professors Ayres Frietas and Adam Leibovich, are among hundreds of theorists trying to predict new phenomena and interpret experimental results with mathematical tools. 

Mueller, along with Pitt Professors Joseph Boudreau, Vittorio Paolone, Vladimir Savinov, and Professor Emeritus Bill Cleland are among the 3,000 physicists who have worked with ATLAS, one of the two detectors at the Large Hadron Collider, housed at the European Organization for Nuclear Research (CERN) in Switzerland. Pitt has played—and continues to play—a significant role with ATLAS, which is used to understand the interactions among elementary particles at the world’s highest energies (similar to what happened after the Big Bang). 

Despite its discovery, questions still remain about the Higgs boson and what it means to our universe. Below, Han and Mueller uncover the history of the Higgs boson, detailing how it may transform the way we live.

Q: What exactly is the Higgs boson? 

Han: Finding the Higgs was a revolutionary discovery. It was the result of 50 years of work by theorists and 25 years of work by experimentalists. The Higgs field is around us all of the time. It permeates the universe as a vacuum background. It creates a little drag, like walking in water. You can’t see it, but, if you strike it, like they do at the Large Hadron Collider, you can find it. You see the decaying products of the particle—not the actual particle itself because it disappears too fast.

Mueller: By combining these decaying products, we can reconstruct the properties of the parent particle and whether those properties are consistent with what is expected for the Higgs Boson.

Q: How might the Higgs boson transform the way we live?

Han: High-energy physics, or elementary particle physics, is a research branch seeking a deep understanding of nature, using the smallest particle building blocks and studying their interactions. It is driven by human curiosity and the pursuit for pure knowledge. The discovery of the Higgs boson is like a newborn baby: No one can predict the usefulness at this early stage, but that’s our future.

Mueller: Some of the technical work we do in our research can affect the general population. For instance, in order to facilitate communication between thousands of particle physicists around the world who were working with CERN data, British computer scientist Tim Berners-Lee developed the “world wide web.” Now this seems to be used by people everywhere to watch cat videos.

Q: How is Pitt involved with Higgs boson research?

Han: We, the theorists, predict the Higgs boson’s properties, calculate its signal rates, propose strategies to search for the particle, and record our observations. Many of our proposals were adopted in the experimental searches for the Higgs discovery.

Mueller: The Pitt ATLAS group has contributed to the experiment in many ways. We designed and produced electronic components for the online trigger system, which is designed to select, in real time, interesting physics events. We also wrote the software needed for turning those electronic signals into final, identified particles, and we also reconstructed their kinematic (motion) quantities.

Q: Can you explain in greater detail what happens at the Large Hadron Collider?

Mueller: The Large Hadron Collider was introduced in 2008 and is the world’s largest scientific instrument. It’s a 27-kilometer tunnel accelerator with four detectors, and the temperature inside is colder than space. Particles in the cylinder move between six and 18 miles per hour below the speed of light. With this collider, scientists force particles to collide so we can study the remnants. There is one Higgs collision every 10 seconds, but there are a total of 400 million collisions per second in the Large Hadron Cylinder. So, it’s like finding a needle in a haystack.

Han: Actually, it’s worse than finding a needle in a haystack. It’s sort of like throwing a stone in the water and watching the ripples. You shake up the vacuum, and pop the Higgs out to see the ripples.

Mueller: The electronics produced by the Pitt group were crucial for making real-time decisions regarding which of those collisions were worth analyzing. It isn’t just faculty contributing to this. We must give credit to the work of Pitt employees like Joseph Rabel from the Department of Chemistry’s Electronics Shop and George Zuk from the Department of Physics and Astronomy’s Electronics Shop. There have also been countless graduate students and undergraduates who worked on our project, and their work was vital to its success. Professor Cleland officially retired around 15 years ago, but he is still working in the lab every day. 

Q: Did you expect the discovery of the Higgs in 2012?

Han: When the Higgs announcement was made, I was actually leading a conference in Beijing. We all knew an announcement was coming, but we didn’t know if it would be a trivial update for the search or a statement of the discovery. I had a hunch. So, on my way to the conference I stopped by the liquor store and bought some champagne to bet on it. It’s rare to have such a scale of excitement worldwide. It was the most exciting moment in my professional career.

Mueller: As a member of ATLAS, I knew what we were going to announce at the conference. It was hard to keep quiet when asked by my theorist friends to confirm or deny rumors they had heard.

Q: What’s up next? Are there other particles we’re still searching for?

Han: Even though the Higgs has been discovered, there are still so many unknown questions about our universe. For example, why does the Higgs behave as it is? Does it have siblings, cousins, relatives, I mean, other related particles that we haven’t seen? What is dark matter? What is dark energy? And any relationship with the Higgs? The pursuit for deeper understanding of nature goes on.

Mueller: We also need to study in detail the already discovered particles to determine if their properties are fully consistent with our current theories, or whether there are signals of new physics to be found there. The experiment will resume data taking in 2015, with double the energy. This will allow us to extend our reach in searching for phenomena beyond what we know so far. 

As CERN improves the Large Hadron Collider, we need to improve the ability of ATLAS to record the potentially interesting data. Pitt faculty, staff, and students are currently working to upgrade our electronics so we can keep up with the expected increased data rate.