On 4 July 2012, two experiments at CERN’s Large Hadron Collider announced the discovery of a particle in the mass region around 125 GeV/c2. For the physics community, this date is holy. It marked the discovery of the last missing piece of the Standard Model, a theory that unites three of the four fundamental forces of nature. The Standard Model is our best attempt, backed by dozens of experiments, to explain the universe’s composition. However, it took decades to develop the Standard Model of physics and discover the particles predicted by it.

This article is more of a story of the discovery of the Higgs boson. I understand that most of the people reading this article aren’t physicists. Hence, I have tried to explain everything in a simple way.

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 were aware of 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. Instead, they are further composed of quarks.

Particle Zoo
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 MannGlashowSalamFeynman, 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.

4 fundamental forces

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 with each other. The four Maxwell’s equations (they are named after him although he didn’t formulate them) describe the electromagnetic force.

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 | Higgs Boson

Look at the above chart carefully. It contains two types of particles: Fermions (yellow and cyan) and bosons (Purple and Blue). 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. But you can read this article to understand the concept of spin in quantum mechanics.

Now, look at the fermions. The ones colored yellow 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 cyan-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 purple-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.

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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.’ 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 this 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.

Experimental background

The probability of producing Higgs bosons resulting from two beams of accelerated particles is very low (1 in 10 billion at the highest energy collider until now). It has a short half-life and detectors are not usually able to detect its decay. The final, detectable particles resulting from a collision can be linked to different initial conditions, which means even more data should be analyzed before one is sure to have found evidence of the Higgs boson. Advanced computing facilities were built to manage all the data recorded (25 petabytes per year as of 2012), sent in 36 countries. The Large Hadron Collider at CERN was built for the same purpose since the probability of producing this particle is higher in very high energy collisions.

Narrowing Down The Search

The first experimental search for the Higgs boson was conducted at the Large Electron-Positron Collider at CERN. Extensive research in the 1990s showed no evidence of this particle, which led scientists to believe higher energies were required. Furthermore, the boson they were looking for had a mass greater than 144.4 GeV/c2. The next step was reproducing the experiment at the Tevatron at Fermilab, which was then the highest energy collider.

CERn LHC Images
Image: Anja Sjostrom

Final analysis showed no evidence of the respective boson within a mass range of 147 GeV/c2 and 180 GeV/c2, but a slight possibility to find it around 130 GeV/c2. In 2010, the Large Hadron Collider took over. This detection system, still the biggest and most powerful ever built, was designed to perform collisions of particles with energy up to 14 TeV.

More in particle physics

Discovery at CERN

Initial data from CERN included some deviations “too large to be neglected,” which led the two teams working independently at two different detectors (ATLAS and CMS) to think they discovered the boson. Further research allowed both teams to find a previously unknown particle – a 125.3 ± 0.6 GeV/c2 boson for CMS and a 126.0 ± 0.6 GeV/c2 boson for ATLAS. These results were found independently and had a standard deviation of less than 5 sigma, implying that the experiments were valid and incontestable proofs of the existence of the Higgs boson.  

Final Confirmation of the Higgs boson

In 2013, the team at CERN declared, “CMS and ATLAS have compared several options for the spin-parity of this particle. These all prefer no spin and even parity. This, coupled with the measured interactions of the new particle with other particles, strongly indicates that it is a Higgs boson.” To this day, so many new experiments have been created and carried out that the existence of the Higgs boson is no longer a theory but a fact.

Why is the Higgs boson called the God Particle?

If you talk to a physicist about the Higgs boson by referring to it as the God Particle, you’re almost guaranteed to elicit a wince, a grimace, or at the very least, a flash of mild annoyance. The reason being ‘God’ doesn’t exist in the equations of physics, and Higgs boson’s discovery has nothing to do with spirituality!

The reason behind this name is that physicist Leon Lederman referred to the Higgs as the “Goddamn Particle.” The nickname was meant to poke fun at how difficult it was to discover this particle: 40 years and $13.25 billion.

“The Goddamn Particle” was supposed to be the title of Lederman’s book that came out in the 1990s and was wildly popular for a book about physics. However, his publishers weren’t exactly on board with that phrasing, so the title was changed to “The God Particle.”

Unfortunately, the publisher’s version of the nickname stuck, and physicists are not happy about it. Vivek Sharma, a physicist at University of California, San Diego, said, “I am not particularly religious, but I find the term an ‘in your face’ affront to those who [are].”

The Higgs boson is the quantum excitation of the Higgs field that is omnipresent in space-time. Through the Higgs mechanism, particles get mass. The fancy nickname of this particle doesn’t even explain a single property of the Higgs boson. Instead, a Masson would have been a better name! Agree?

Learn astrophysics at home

Did you always want to learn how the universe works? 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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