Scientists at the world’s largest physics experiment have reported the most precise measurement of the most massive subatomic particle we know. The discovery sounds esoteric, but it would not be an understatement to say that it has implications for the entire universe.
The Greek philosopher Empedocles hypothesized 2,400 years ago that matter could be broken down 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.
The top quark
Rather than a ‘smaller’ particle, contemporary particle physicists are concerned with the elusive particle.
More energetic particles often decay into less energetic ones. The greater the difference in energy between that of 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 scientists have found to date is the top quark.
It is 10 times heavier than a molecule of water, about three times heavier than an atom of copper and 95% heavier than a whole molecule of caffeine.
As a result, the top quark is so unstable that it can explode 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 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 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 considered to be an excitation of an ‘electron field’.
All these areas relate to each other in specific ways. When the ‘electron field’ interacts with the Higgs field at energies much smaller than 100 GeV, for example, the electron particle will gain mass. The same applies to 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.) Elucidating 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 more strongly with it. By measuring the mass of the top quark as precisely as possible, then physicists can learn a lot about the Higgs boson as well.
“Physicists are intrigued by the quark’s high mass because there is something special about it,” said Nirmal Raj, particle theorist and assistant professor at the Indian Institute of Science, Bengaluru. Hindu. “On the one hand, it is the closest to the mass of the Higgs boson, which is what would be ‘naturally’ expected before we measure it. On the other hand, all the others [particles like it] are much, much lighter, making one wonder if the top quark is actually a strange ball, not a ‘natural’ species.
The universe as we know it
But the rabbit hole goes deeper.
Physicists are keen to study the Higgs boson also because of its mass, which it gains 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 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 of how the Higgs field first formed (at the birth of the universe). If they are right, there is a small but non-zero chance that someday in the future, the field could go through some kind of self-regulation that reduces its energy and modifies the universe in drastic ways.
They know that the field has some potential energy today and there is a way they can shed some of it to have less and become more stable. There are two ways to reach this steady state. One is for the field to first gain some energy before losing it, and more, like climbing one side of a mountain to enter a deeper valley on the other. The other is if an event called quantum tunneling occurs, where the potential energy of the field will ‘tunnel’ through the mountain instead of climbing over it and falling into the valley there.
This is why Stephen Hawking said in 2016, the Higgs boson could spell “the end of the universe” as we know it. Even if the Higgs field is slightly stronger than it is 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 quickly said the frequency of the tunneling event was 1 in 10100 years.
Higgs boson mass – 126 GeV/c2 (a unit used for subatomic particles) – is also nearly sufficient to maintain the universe in its current state; anything else and “the end” would happen. Such a fine-tuned value is certainly curious, and physicists would like to know 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 closest friend of the Higgs boson.
“Measuring the mass of the top quark has implications for exactly whether our universe will come out of existence,” said Dr. Raj.
Finding the top quark
Physicists discovered the top quark in 1995 at a US particle accelerator called the Tevatron, measuring its mass at 151-197 GeV/c.2. Tevatron closed in 2011; physicists continued to analyze the data he had collected and updated the value three years later to 174.98 GeV/c2. Other experiments and research groups provided more accurate values over time. On June 27, physicists at the Large Hadron Collider (LHC) in Europe reported the most accurate figure yet: 172.52 GeV/c2.
Measuring the mass of a top quark is difficult when its lifetime is about 10-25 seconds. Typically, a particle chopper will produce an ultra-hot soup of particles. If a top quark is present in this soup, it will quickly decay into specific sets of lighter particles. Detectors take care of these events and when they occur they track and record their properties. Finally, computers collect this data and physicists analyze it rebuild physical properties of the top quark.
Scientists learn what to expect at each point in this process based on sophisticated mathematical models and must deal with many uncertainties. Many of the equipment used in these cars also include state-of-the-art technology; when the engineers improve them further, the results of the physicists also improve so much.
Now researchers will incorporate the measurement of the top quark’s mass into calculations that inform our understanding of the particles of our universe. Some of them will use it to search for an even more accurate value. According to Dr. Raj, measuring the mass of the top quark accurately is also key to knowing if any other particles with mass close to that of the top quark might be hidden in the data.
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