Explained | The decades-long search for a rare decay of the Higgs boson

Last week, physicists working with the Large Hadron Collider (LHC) particle buster at CERN in Europe reported that they had detected a Higgs boson decaying into a Z boson particle and a photon. This is a very rare decay process that tells us important things about the Higgs boson and our universe.

The Higgs boson is a type of boson, a subatomic force-carrying particle. It carries the force that a particle experiences when it moves through an energy field, called the Higgs field, which is believed to be present throughout the universe. For example, when an electron interacts with the Higgs field, the effects it experiences are said to be due to its interaction with the Higgs bosons.

What is the Higgs boson?

An electron is a subatomic particle with mass. How did this mass arise? How can we tell that an electron has less mass than a proton, or that a photon has no mass at all? The answer lies in the Higgs boson. The stronger a particle’s interaction with the Higgs boson, the more mass it has. This is why electrons have some mass, protons have more, and neutrons have only a little more than protons, and so on. A Higgs boson can also interact with another Higgs boson: this is how we know that its mass is greater than that of protons or neutrons.

Since all matter in the universe is made up of these particles, understanding how strongly each type couples to the Higgs bosons, along with understanding the properties of the Higgs bosons themselves, can tell us a lot about the universe itself. The latter is why the new result is notable.

Photons, particles of light, have no mass because they do not interact with the Higgs bosons. So a question should arise: how did a Higgs boson decay into a Z boson and a photon if it doesn’t interact with photons? This is a good question whose answer lies in spacetime.

What are virtual particles?

According to quantum field theory, which is the theory physicists use to study these interactions, space at the subatomic level is not empty. It’s filled with virtual particles, which are particles that rapidly pop in and out of existence. They can’t be detected directly, but according to physicists their effects sometimes linger.

The LHC creates a Higgs boson by accelerating billions of highly energetic protons in a head-on collision, releasing a huge amount of energy which condenses into different particles. When a Higgs boson is created in this hot soup, it has a fleeting interaction with virtual particles that creates a Z boson and a photon.

What’s the new result?

Because it is so heavy, the Higgs boson is an unstable particle that decays into lighter particles. We can’t always tell what combination of particles it will decay into. However, the theory describing the properties of fundamental particles has clearly predicted the probability that it will take a certain path.

For example, this theory, called the Standard Model, states that a Higgs boson will decay into a Z boson and a photon 0.1% of the time. This means that the LHC must have created at least 1,000 Higgs bosons to be able to detect one that decays into a Z boson and a photon.

The Z boson also happens to be unstable. According to Martin Bauer, an associate professor at the Institute for Particle Physics Phenomenology, Durham University, Z bosons decay into two muons about 3% of the time. If the LHC’s detectors were looking for a pair of muons plus a photon created simultaneously, Dr. Bauer estimated that the LHC would have had to create at least 30,000 Higgs bosons to observe the decay just once.

This is why, even though the Higgs boson was discovered more than a decade ago at the LHC, it’s only now that physicists are confirming this decay path.

Is it a new discovery?

The two detectors that announced the new measure, called ATLAS and CMS, had in fact searched and found the decay even earlier (in 2018 and 2020). This time, however, the two teams merged their data, collected between 2015 and 2018, and as a result significantly increased the statistical accuracy and scope of their searches, according to a CERN statement.

This significance is not yet high enough for the teams to say that a Higgs boson has decayed into a Z boson and a photon with 100% certainty, reflecting the rarity of the decay path.

What is the Standard Model?

Why do physicists go to such great lengths to spot decay in the first place? This is because the Standard Model predicts that the Higgs boson will take this path 0.1% of the time if its mass is 125 billion eV/c2 (a unit of mass used for subatomic particles).

The Standard Model has made many accurate predictions but it cannot explain what dark matter is or, indeed, why the Higgs boson is so heavy. Testing its predictions as precisely as possible is one way for physicists to find out if there are any cracks in the Model through which they can validate new theories in physics.

For example, some theories predict a higher rate of decay through this pathway; if the LHC and its detectors find experimental proof, the new theories could open up a new realm of science.

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