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% as heavy as 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 comes from various sources, one of the most important being 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 known as the Higgs boson, similar to how an electron is an excitation of the electron field.

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

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 precisely, 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 seems natural, but all other similar particles are much lighter, making the top quark seem like an oddball.”

The Universe as We Know It

The Higgs boson’s mass is also a subject of interest because it is more massive than expected. This means the Higgs field is more energy-laden than anticipated, which implies that the universe is more energetic than expected. Physicists have calculated these expectations and believe their calculations are accurate. So, why does the Higgs field have so much energy?

Physicists have 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 has potential energy today and could become more stable by shedding some of it. This could happen in two ways: either by gaining some energy first before losing it, like climbing a mountain to reach a deeper valley, or through quantum tunneling, where the field’s potential energy tunnels through the mountain to reach the valley.

This is why Stephen Hawking said in 2016 that the Higgs boson could spell the “end of the universe” as we know it. Even a slight increase in the Higgs field’s strength could destroy the atoms of most chemical elements, wiping out stars, galaxies, and life on Earth. However, other physicists quickly pointed out that the frequency of such a tunneling event is 1 in 10^100 years.

The Higgs boson’s mass of 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, being the most massive particle, plays a crucial role in this picture.

“Measuring the top quark mass precisely 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 the Tevatron particle accelerator in the US, initially 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, updating the mass to 174.98 GeV/c^2 in 2014. On June 27, scientists at the Large Hadron Collider (LHC) in Europe reported the most precise measurement yet: 172.52 GeV/c^2.

Measuring the top quark’s mass is challenging due to its extremely short lifespan. Particle accelerators produce a hot soup of particles, and 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 properties.

Scientists rely on sophisticated mathematical models and advanced technologies to improve their measurements. As engineers enhance these devices, the accuracy of the physicists’ results also improves.

Researchers will now use the top quark’s mass measurement to refine our understanding of the universe’s particles. Some will continue to seek even more precise values. According to Dr. Raj, accurately measuring the top quark’s mass is also crucial for determining whether another particle with a similar mass might be hidden in the data.