Particle physicists working at CERN have observed a rare decay of the Higgs boson, expanding our understanding of the quantum universe. Scientists have found evidence of the massive particle decaying into two leptons and a photon. The decay of the Higgs boson into two photons was previously known. But what are leptons? How have scientists ‘observed’ this decay? And what does it tell us about the Higgs boson? Many of you might not be an expert in particle physics; hence, I have explained everything right from the beginning in this article.

Higgs boson and the Standard Model

Plethora of particles

You don’t have to be a rocket scientist to understand the basics of the Higgs boson and the Standard Model. More than a hundred years ago, the first sub-atomic particle was discovered. We called it an electron. As scientists probed deeper into the matter, we discovered two more particles: protons and neutrons. By 1947, physicists knew seven elementary particles: electron, photon, proton, neutron, kaon, and meson. An elementary particle isn’t composed of any smaller particles. It was several years later when we found that protons and neutrons aren’t elementary. They are further composed of quarks.

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Image: Tim Jones/HETDEX

Anyways, the couple of decades that followed the year 1950 left theoretical particle physics in a mess. Experimentalists were happy. Each year, they were discovering several elementary particles. In the 1950s, there was a joke among physicists that the year’s Nobel Prize for Physics must go to a researcher who did not discover a new particle! Theoretical physicists were in trouble because they didn’t know how to group these particles. They had to come up with a theory. Else, they would keep drowning in the soup of elementary particles.

The 1960s saw triumphs in theoretical particle physics. Efforts of scientists such as Gell-Mann, Glashow, Salam, Feynman, Weinberg, and many others made theoretical particle physics clean and elegant. But, besides that, there was one more thing physicists were trying to achieve: unifying the fundamental forces of nature.

The four fundamental forces

According to physicists, everything you see around is a result of four fundamental forces. The first is gravity. It is responsible for the motion of astronomical objects. For example, the Moon goes around the Earth; the Earth goes around the Sun; the Sun revolves around the galactic center. The force that governs the motion of celestial objects is gravity. It also keeps you grounded. The theory that explains gravity is the general theory of relativity given by Einstein. However, in this remarkably successful theory, gravity is described as a curvature of spacetime instead of a force.

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The second is the electromagnetic force. For quite a long time, electricity and magnetism were thought to be different forces. But, thanks to experimentalists who loved playing with currents and magnets, the relationship between the two was discovered. Later, Scottish physicist James Clerk Maxwell brought together the four equations that show how the two forces are mingled. The four Maxwell’s equations (they are named after him although he didn’t formulate them) describe the electromagnetic force.

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The third on the list is the weak nuclear force responsible for the radioactive decay of atoms. The theory of the weak force is Fermi’s theory of beta decay. Finally, we have the strong nuclear force that binds the nucleus (and ultimately matter) together.

The Standard Model

After years of brainstorming and hard work, physicists successfully combined three of the four fundamental forces into a complex theory called the Standard Model. The three forces are the electromagnetic force, the weak force, and the strong force. The elementary particles of the standard model are given below.

Standard Model of Physics

Look at the above chart carefully. It contains two types of particles: Fermions (purple and green) and bosons (Red and Yellow). All the fermions have a half-integer spin, and the bosons have an integral spin. If you don’t know the concept of spin, don’t worry. It’s not required here. Now, look at the fermions. The ones colored purple are called quarks. There are six types of quarks. Different combinations of quarks and anti-quarks make up different composite particles. For example, two up quarks (uu) and a down quark (d) make up a proton (remember I told you proton isn’t an elementary particle. Now you know why).

The green-colored elementary particles are the six leptons: electron, muon, tau, and the three neutrinos. Quarks make up the protons and the neutrons that further make up the nucleus. Electrons revolving around the nucleus form atoms. So, quarks and leptons are basically the building blocks of the universe. However, the picture is incomplete without the bosons. There are four red-colored bosons given in the above diagram. They are the force carriers. The photon mediates the electromagnetic force, the gluon mediates the strong force (it ‘glues’ the quarks together), and finally, the W and the Z bosons are the carriers of the weak force.

The Higgs boson does not intermediate any of the four fundamental forces. Roughly speaking, this heavyweight of the standard model gives mass to all the other particles. The universe is thought to be filled with a quantum field called the Higgs field. As the particles travel in this Higgs field, they interact with it. The more the particle interacts with this omnipresent field, the heavier it becomes. The particles that do not interact with the Higgs field, such as photons, are massless.

Here’s a simple daily life analogy to understand this mechanism. Imagine you are walking on a road. You walk easily. Your legs feel no resistance. But what happens when you try to walk in a pool which is, say, filled up to your waist. You have to put extra effort to overcome the ‘drag’. The same is the case with the particles. The water is the Higgs field, and your legs are the particles. This is a crude analogy, but it serves the purpose of explaining the mechanism.

The Higgs boson

The quantum excitation of the Higgs field is known as the Higgs boson. Theorized in the 1960s, it took more than 50 years and about $14 billion to search for the exotic particle. Finally, on July 4, 2012, scientists working at CERN’s Large Hadron Collider announced they had discovered the Higgs boson, the last missing piece of the Standard Model of physics.

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The Dalitz Decay

The recently observed rare decay of the Higgs boson into a photon and a pair of leptons is known as the Dalitz decay. In this type of decay, after its uber-short life, the Higgs boson quickly turns into one photon and what scientists call a “virtual photon.” That “virtual photon,” also known as an “off-shell photon,” then immediately turns into something like, in this case, two leptons. This “virtual photon” has a tiny non-zero mass, while regular photons are completely massless, James Beacham, a particle physicist with the ATLAS experiment at the LHC, told 

While scientists have predicted that this type of decay should exist with the Higgs boson, this new detection is “the first hint of evidence of this very rare decay mode of the Higgs boson,” Beacham said. 

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However, he added, the team likely won’t be able to directly observe the rare decay until they upgrade the facilities for the upcoming High-Luminosity LHC program (which will come following the LHC Run 3. The data used for this study was collected during Run 2, the second running period for the collider that began in 2015 and ended in 2018. Run 3 will begin in March 2022.)

By studying rare decays like this, researchers can explore the possibility of new physics that stretches beyond the Standard Model. The Standard Model explains many things about our physical universe, but it doesn’t include gravity or dark matter, Beacham said. Dark matter, which emits no light and cannot be directly observed, is thought to make up about 80% of all matter in the known universe, but scientists do not yet know exactly what it is.

Now, don’t get too excited. This paper “does not give us new information yet about the Higgs portal into the ‘dark sector,'” Beacham told But “this paper proves that we can look for sporadic things like this, quite handily,” he said, which pushes the search forward overall.

Learn astrophysics at home

Did you always want to learn how the universe works? Then read our 30-article Basics of Astrophysics series absolutely free of cost. From the popular topics such as stars, galaxies, and black holes to the detailed concepts of the subject like the concept of magnitude, the Hertzsprung Russell diagram, redshift, etc., there is something for everyone in this series. All the articles are given here. Happy reading!

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