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

Scientists at the world’s largest physics experiment have announced the most precise measurement yet of the most massive subatomic particle we know. The finding sounds esoteric, but it wouldn’t be an understatement to say it has universe-wide implications.

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

Top quark

Rather than “smaller” particles, contemporary particle physicists are concerned with the elusive particles.

More energetic particles often decay into less energetic particles. The greater the difference in energy between the energy of the particle and its decay products, the shorter the particle exists in its original form and the faster it decays. According to the equivalence of mass and energy, a more massive particle is also a particle with higher energy. And the most massive particle scientists have found so far is the top quark.

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

As a result, the top quark is so unstable that it could decay 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 added from multiple sources. An important source for all elementary particles is the Higgs field, which permeates the entire universe. The “field” is like a sea of ​​energy and the excitations in the field are called particles. In this way, for example, an excitation of the Higgs field is called a Higgs boson, just as an electron can be thought of as an excitation of an “electron field”.

All these fields work closely together. When the “electron field” interacts with the Higgs field at energies much lower than, say, 100 GeV, the electron particle gains some mass. The same is true for other elementary particles. (The 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 2013 Nobel Prize in Physics.

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

“The top quark matter is of interest to physicists because there is something special about it,” said Nirmal Raj, a particle theorist and assistant professor at the Indian Institute of Science in Bengaluru. Hindus. “On the one hand, it is the one closest to the mass of the Higgs boson, which one would ‘naturally’ expect before measuring it. Second, everyone else [particles like it] they are much, much lighter, so one wonders if the top quark is actually a freak, not a ‘natural’ species.”

The universe as we know it

But the rabbit hole goes deeper.

Physicists are also interested in studying the Higgs boson because of its own mass, which it acquires by interacting with other Higgs bosons. Importantly, the Higgs boson is more massive than expected – meaning the Higgs field is more energetic than expected. And because it permeates the universe, the universe can be said to be more energetic than expected. This “expectation” comes from calculations made by physicists, and they have no reason to believe they are wrong. Why does the Higgs field have so much energy?

Physicists also have a theory about how the Higgs field originally arose (at the birth of the universe). If they are right, there is a small but non-zero chance that one day in the future the field could undergo some kind of self-modification that will reduce its energy and modify the universe in drastic ways.

They know that the field has some potential energy today, and there is a way to get rid of some of it to make it less and more stable. This steady state can be reached in two ways. One is for the field to gain some energy first before losing it, and another is like climbing one side of a mountain to reach a deeper valley on the other side. The second is if an event called quantum tunneling occurs, where the field’s potential energy would “tunnel through” the mountain instead of having to climb over it and fall into the valley there.

This is 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 is slightly stronger than now, the atoms of most chemical elements will 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 tunneling frequency was 1 in 10¹⁰⁰ flight.

Higgs boson mass — 126 GeV/c² (the unit used for subatomic particles) — is also sufficient to maintain the universe in its present state; anything else and “the end” would occur. Such a fine-tuned value is obviously curious, and physicists would like to know which 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 closest friend.

“A precise measurement of the mass of the top quark has implications for whether our universe is tunneling out of existence,” said Dr. Paradise.

The search for the top quark

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

Measuring the mass of the top quark is difficult when its lifetime is around 10^-25 seconds. A particle smasher usually creates 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, they track and record their properties. Finally, computers collect this data and physicists analyze it reconstruct physical properties of the top quark.

Scientists use sophisticated mathematical models to learn what to expect at each point in the process and must contend with many uncertainties. Many of the devices used in these machines also incorporate state-of-the-art technology; as engineers refine them further, physicists’ results improve that much more.

Now researchers will incorporate measurements of the top quark’s mass into calculations that inform our understanding of the particles of our universe. Some of them will also use it to find an even more accurate value. According to Dr. An accurate measurement of the top quark’s mass is also key to knowing whether there might be another particle with a mass close to the top quark’s hiding in the data, Raje said.