Why are scientists looking for the Higgs boson’s closest friend?

Scientists at the world’s largest physics experiment have reported the most precise measurement yet of the most massive subatomic particle we know of. The discovery may seem esoteric, but it would not be an exaggeration to say that it has implications for the entire universe.

2,400 years ago, the Greek philosopher Empedocles theorized that matter could be broken down into smaller and smaller pieces until only air, earth, fire, and water remained. Since the early 20th century, physicists have broken matter down into smaller and smaller pieces to find many different subatomic particles, enough to fill a zoo.

The top quark

Rather than focusing on a “smaller” particle, contemporary particle physicists are interested in elusive particles.

Higher-energy particles often decay into lower-energy particles. The greater the energy difference between a particle and its decay products, the less time the particle exists in its original form and the faster it decays. According to mass-energy equivalence, a more massive particle is also a more energetic particle. And the most massive particle that scientists have discovered to date is the top quark.

It is ten times heavier than a water molecule, about three times heavier than a copper atom, and 95% heavier than a complete caffeine molecule.

As a result, the top quark is so unstable that it could break into lighter, more stable particles in less than 10⁻²⁵ seconds.

The mass of the top quark is very important in physics. The mass of a particle is equal to the sum of the masses from several sources. An important source for all elementary particles is the Higgs field, which permeates the entire universe. A “field” is like a sea of ​​energy, and excitations in the field are called particles. So, for example, an excitation of the Higgs field is called the Higgs boson, just as an electron can be thought of as an excitation of an “electron field.”

All of these fields interact in specific ways. When the “electron field” interacts with the Higgs field at energies well below 100 GeV, for example, the electron particle acquires a certain mass. The same is true for other elementary particles. (The GeV, or gigaelectronvolt, is a unit of energy used in the context of subatomic particles: 1 joule = 6.24 billion GeV.) The elucidation of this mechanism earned François Englert and Peter Higgs the 2013 Nobel Prize in Physics.

The top quark is the most massive subatomic particle because the Higgs bosons interact most strongly with it. By measuring the mass of the top quark as precisely as possible, physicists can also learn a lot about the Higgs boson.

“Physicists are intrigued by the top quark mass because there is something special about it,” said Nirmal Raj, a particle theorist and assistant professor at the Indian Institute of Science, Bengaluru. The Hindu“On the one hand, it is the one that comes closest to the mass of the Higgs boson, which is what we would “naturally” expect before measuring it. On the other hand, all other particles like it are much, much lighter, which leads us to wonder if the top quark is actually a strange species, and not a “natural” species.

The Universe as We Know It

But the rabbit hole goes even deeper.

Physicists are also interested in studying the Higgs boson because of its mass, which it gains by interacting with other Higgs bosons. It is important to note that the Higgs boson is more massive than expected, which means that the Higgs field is more energetic than expected. And because it permeates the universe, we can say that the universe is more energetic than expected. This “expectation” comes from calculations made by physicists, and they have no reason to believe that they are wrong. Why does the Higgs field have so much energy?

Physicists also have a theory about how the Higgs field originally formed (at the birth of the universe). If they are right, there is a small but non-zero probability that someday in the future the field could undergo some sort of self-adjustment that would reduce its energy and drastically alter the universe.

They know that the field has some potential energy today, and that there is a way to lose some of it to have less and become more stable. There are two ways to reach this stable state. One is for the field to first gain energy and then lose more, like climbing up a mountainside and into a deeper valley on the other side. The other possibility is for an event called quantum tunneling to occur, whereby the potential energy of the field would “tunnel” through the mountain instead of having to climb up it and fall into the valley beyond.

That’s why Stephen Hawking said in 2016 that the Higgs boson could mean the “end of the universe” as we know it. Even if the Higgs field were slightly stronger than it is now, the atoms of most chemical elements would be destroyed, taking with them stars, galaxies, and life on Earth. But while Hawking was technically right, other physicists were quick to say that the frequency of tunneling was 1 in 10¹⁰⁰ years.

The mass of the Higgs boson — 126 GeV/c² (unit used for subatomic particles) — is also just enough to keep the universe in its current state; any other value and the “end” would occur. Such a finely tuned value is obviously curious, and physicists would like to know what natural processes contribute to it. The top quark is included in this picture because it is the most massive particle, in a sense the closest friend of the Higgs boson.

“Precisely measuring the mass of the top quark has implications for whether our universe will tunnel out,” Dr. Raj said.

Find the top quark

Physicists discovered the top quark in 1995 at a particle accelerator in the United States called the Tevatron, measuring its mass at 151-197 GeV/c.²The Tevatron was shut down in 2011; physicists continued to analyze the data it had collected and updated the value three years later to 174.98 GeV/c². Other experiments and research groups have provided more precise values ​​over time. On June 27, physicists at the Large Hadron Collider (LHC) in Europe reported the most precise figure yet: 172.52 GeV/c.².

It is difficult to measure the mass of a top quark when its lifetime is about 10^-25 seconds. Typically, a particle crusher produces an ultra-hot soup of particles. If a top quark is present in this soup, it quickly decays into specific groups of lighter particles. Detectors monitor these events and, when they occur, track and record their properties. Finally, computers collect this data and physicists analyze it to rebuild the physical properties of the top quark.

Scientists learn what to expect at each step of this process based on sophisticated mathematical models and must deal with many uncertainties. Many of the devices used in these machines also incorporate cutting-edge technology; as engineers improve them further, physicists’ results improve accordingly.

The researchers will now incorporate the top quark mass measurement into their calculations that will help us understand the particles in our universe. Some of them will also use it to search for an even more precise value. According to Dr. Raj, precisely measuring the top quark mass is also essential to knowing whether another particle with a mass close to that of the top quark could be hiding in the data.