Why Are Scientists Searching for the Higgs Boson’s Closest Companion?

Scientists involved in the world’s largest physics experiment have achieved the most accurate measurement yet of the heaviest subatomic particle known to us. While this discovery might seem highly specialized, it has significant implications for our understanding of the universe.

The Quest for Smaller Particles

Around 2,400 years ago, the Greek philosopher Empedocles theorized that matter could be divided into smaller and smaller pieces until only air, earth, fire, and water remained. In the early 20th century, physicists began breaking down matter into even smaller components, discovering a multitude of subatomic particles.

The Top Quark

Modern particle physicists are more interested in elusive particles rather than just smaller ones. High-energy particles often decay into lower-energy ones. The greater the energy difference between a particle and its decay products, the shorter its lifespan. According to the mass-energy equivalence principle, a more massive particle is also more energetic. The top quark is the most massive particle discovered so far.

The top quark is incredibly heavy—10 times heavier than a water molecule, about three times as heavy as a copper atom, and 95% the mass of a caffeine molecule. Due to its mass, the top quark is extremely unstable and can decay into lighter particles in less than 10^-25 seconds.

The Importance of the Top Quark’s Mass

A particle’s mass is the sum of contributions from various sources. One crucial source for all elementary particles is the Higgs field, which permeates the entire universe. A field can be thought of as a sea of energy, and particles are excitations within this field. For example, an excitation of the Higgs field is called the Higgs boson, similar to how an electron is an excitation of the electron field.

These fields interact with each other in specific ways. For instance, when the electron field interacts with the Higgs field at energies below 100 GeV, the electron gains some mass. This principle applies to other elementary particles as well. François Englert and Peter Higgs won the 2013 Nobel Prize in Physics for elucidating this mechanism.

The top quark is the most massive subatomic particle because it interacts most strongly with the Higgs boson. By measuring the top quark’s mass as accurately as possible, physicists can gain valuable insights into the Higgs boson.

“Physicists are intrigued by the top quark mass because it is peculiar,” said Nirmal Raj, a particle theorist and assistant professor at the Indian Institute of Science, Bengaluru. “It is the closest to the Higgs boson’s mass, which is what one would naturally expect. However, all other similar particles are much lighter, making one wonder if the top quark is an oddball.”

The Universe as We Know It

The intrigue doesn’t stop there. Physicists are also interested in the Higgs boson because of its own mass, which it gains by interacting with other Higgs bosons. The Higgs boson is more massive than expected, indicating that the Higgs field is more energy-laden than anticipated. Since the Higgs field permeates the universe, this means the universe is more energetic than expected. Physicists are puzzled by why the Higgs field has so much energy.

There is a theory about how the Higgs field formed at the birth of the universe. If this theory is correct, there is a small chance that the field could undergo a self-adjustment in the future, reducing its energy and drastically altering the universe.

The field currently has some potential energy, and there are two ways it could become more stable. One way is for the field to gain some energy before losing it and becoming more stable, like climbing a mountain to reach a deeper valley. The other way is through quantum tunneling, where the field’s potential energy would tunnel through the mountain instead of climbing over it, dropping into the valley beyond.

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 becomes slightly stronger, the atoms of most chemical elements would be destroyed, taking stars, galaxies, and life on Earth with them. However, while Hawking was technically correct, other physicists quickly pointed out that the frequency of such a tunneling event is 1 in 10^100 years.

The Higgs boson’s mass—126 GeV/c^2—is just enough to keep the universe in its current state. Any deviation could lead to the “end.” This finely tuned value is curious, and physicists want to understand the natural processes that contribute to it. The top quark plays a role in this picture by being the most massive particle, essentially the Higgs boson’s closest companion.

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

Discovering the Top Quark

Physicists discovered the top quark in 1995 at a particle accelerator in the US called the Tevatron, measuring its mass to be between 151-197 GeV/c^2. The Tevatron was shut down in 2011, but physicists continued to analyze its data and updated the value to 174.98 GeV/c^2 in 2014. 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.

Measuring the top quark’s mass is challenging due to its extremely short lifespan of around 10^-25 seconds. Typically, a particle accelerator produces an ultra-hot soup of particles. If a top quark is present, it quickly decays into specific groups of lighter particles. Detectors track and record these events, and computers analyze the data to reconstruct the top quark’s physical properties.

Scientists rely on sophisticated mathematical models to predict what to expect at each stage of this process and must deal with many uncertainties. The devices used in these experiments incorporate state-of-the-art technologies, and improvements in these devices lead to more accurate results.

Researchers will now incorporate the top quark’s mass measurement into calculations that enhance our understanding of the universe’s particles. Some will use it to search for an even more precise value. According to Dr. Raj, precisely measuring the top quark’s mass is also crucial for determining whether another particle with a mass close to that of the top quark is hiding in the data.