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

Scientists at the world’s largest physics experiment have reported the most precise measurement yet of the most massive subatomic particle known. The finding may sound esoteric, but it would be no understatement to say it has implications for the entire universe.

The Greek philosopher Empedocles surmised 2,400 years ago that matter could be broken down into smaller and smaller pieces until we were left with air, earth, fire, and water. Since the early 20th century, physicists have been breaking matter down into smaller and smaller pieces to find instead many different subatomic particles — enough to fill a zoo.

The top quark

Instead of a ‘smaller’ particle, today’s particle physicists focus on elusive particles.

More energetic particles tend to decay into particles with less energy. The greater the difference in energy between a particle and the products of its decay, the shorter 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 scientists have found so far is the top quark.

It is 10 times heavier than a water molecule, about three times as heavy as a copper atom, and 95% as heavy as an entire caffeine molecule.

As a result, the top quark is so unstable that it can decay into lighter, more stable particles in less than 10 seconds.^-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 contributed by multiple 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. In this way, for example, an excitation of the Higgs field is called the Higgs boson, just as an electron can be considered an excitation of an ‘electron field’.

All of these fields interact in specific ways. For example, when the “electron field” interacts with the Higgs field at energies much lower than 100 GeV, the electron particle will gain some mass. The same is true for other elementary particles. (GeV, or giga-electron-volt, 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 Nobel Prize in Physics in 2013.

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

“Physicists are fascinated by the mass of the top quark because there is something strange about it,” said Nirmal Raj, a particle theorist and assistant professor at the Indian Institute of Science in Bengaluru. The Hindu. “On the one hand, it is the one that is closest to the mass of the Higgs boson, which is what you would ‘naturally’ expect before measuring it. On the other hand, all the other [particles like it] are much, much lighter, making you wonder whether the top quark is actually an odd one out, and not a ‘natural’ kind.”

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 own mass, which it acquires through interactions with other Higgs bosons. Importantly, the Higgs boson is more massive than expected — meaning that the Higgs field contains more energy than expected. And because it permeates the universe, it can be said that the universe is more energetic than expected. This “expectation” comes from calculations that physicists have performed, 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 chance that the field will someday go through some kind of self-adjustment that reduces its energy and drastically changes the universe.

They know that the field today has some potential energy and that there is a way to shed some of that energy so that it has less energy and becomes more stable. There are two ways to achieve this stable state. The first is for the field to gain some energy before it loses it and more, like climbing up one side of a mountain to get to a deeper valley on the other side. The other is if there is an event called quantum tunneling, where the potential energy of the field tunnels through the mountain instead of having to climb over it and fall into the valley over there.

This is why Stephen Hawking said in 2016 that the Higgs boson could spell 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 stars, galaxies, and life on Earth with it. But while Hawking was technically correct, other physicists were quick to say that the frequency of the tunneling event was 1 part in 10¹⁰⁰ years.

The mass of the Higgs boson — 126 GeV/c² (a unit used for subatomic particles) — is also just enough to keep the universe in its current state; anything else and the “end” would happen. Such a finely tuned value is clearly odd, and physicists are interested in understanding what natural processes contribute to it. The top quark is part of this picture because it is the most massive particle, in a sense the Higgs boson’s best friend.

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

Finding the top quark

Physicists discovered the top quark in 1995 at a particle accelerator in the US, the Tevatron, and measured its mass at 151-197 GeV/c²The Tevatron was shut down in 2011; physicists continued to analyze the collected data and updated the value to 174.98 GeV/c three years later². Other experiments and research groups have come up with 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/c².

Measuring the mass of a top quark is difficult when its lifetime is about 10 years.^-25 seconds. Normally, a particle destroyer produces an ultra-hot soup of particles. If there is a top quark in this soup, it quickly decays into specific groups of lighter particles. Detectors watch for these events, and as they happen, they track and record their properties. Finally, computers collect this data and physicists analyze it to reconstruct the physical properties of the top quark.

Scientists learn what to expect at each point in this process based on advanced mathematical models and must deal with many uncertainties. Many of the devices used in these machines also contain state-of-the-art technologies; as engineers improve them, the physicists’ results improve that much.

Now, researchers will incorporate the top quark’s mass measurement into calculations that will advance our understanding of the particles in our universe. Some will also use it to search for an even more precise value. According to Dr. Raj, accurately measuring the top quark’s mass is also essential to knowing whether another particle with a mass close to that of the top quark might be hiding in the data.