What do you know about fundamental particles?

When I recently moved to Edinburgh I was so dazzled by the city. At the time, I was staying in a hotel while doing a flat-hunting for a place to live. The hotel was just next to the Holyrood palace, where Queen Elizabeth presumably spends a couple of weeks in summer and, actually, she was staying over then. On my way to work I had to walk by the palace every day. I even started recognising some of the guards.

Oddly enough, around these dates Trump was visiting Scotland too. But, I never got to meet nor the Queen nor Trump, instead I coincidentally ran into Nobel Prize Winner Peter Higgs. I was so thrilled. I called my family to share the excitement. I told them that I had met a Physics Nobel Prize winner.

Usually, people do not really know much about who wins the Nobel Prize in Physics. I started explaining that I had met the Physicist who is associated to the discovery of the Higgs boson (better known as the “particle of God”).

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Explaining the details of particle physics and Higgs’ contribution is rather challenging. Let me start by saying that the Standard Model of particles is a highly successful theory of Physics [1] because it describes the most fundamental particles known to us, and not only the particles but also their interactions. Maybe you have heard that there are four fundamental forces in nature (meaning in the Universe and all that exists). These are the gravitational force, the electromagnetic force, the nuclear strong force and the weak interaction.  Understanding them is essential for us to be able to describe nature in various limits. For instance, we can describe the state of the Universe millions and millions of years after the Big Bang. Here is a brief description of each of the forces so you can follow the importance of the Standard Model.

Gravitational force is the one we are most familiarised with. It is the one that keep us bounded to the earth, and therefore we experienced it all the time. Physics is currently lacking of a self-consistent theoretical framework to describe this force since we have not found “a particle” to be held responsable for the force of gravity, instead we use general relativity. I would like to write about general relativity at some point but for now let’s keep on.

The electromagnetic force is another familiar one for all of us. We use it everyday, using electricity to generate artificial light. We also experience it from nature, most life on Earth benefit from sunlight, which is a form of an electromagnetic field. To be able to describe the electromagnetic force we need the presence of some particles such as electrons or photons. If we make a survey among the general population asking what the most fundamental particles in nature are, the answers will likely be around atoms, electrons, protons, neutros, photons, etc. These particles are overall familiar to us, but they are not fundamental anymore, as you will now see.

The main problem comes when we try to explain the other two remaining forces: the strong and weak interactions. At some point in the history of humanity (hehe), physicists weren’t able to find a suitable framework for this. First let me tell you briefly the definitions for the strong and weak interactions. In the model of an atom, you may remember that protons and neutrons are tied to the nucleus. The force that keep them tied together is called the strong nuclear force. This force is stronger that the electromagnetic force. It is why regardless of the electric charge of protons or the lack of charge of neutrons, they at bounded to the nucleus. The weak interaction is the one involved, for instance, in radioactivity (e.g. beta decay).  When physicists were trying to explain these forces, they came up with a theory that needed the existence of a myriad of other (smaller) particles. To make this idea work, they proposed the quarks model, which says that all the already known sub-atomic particles are not that fundamental after all, and instead all of them are made by quarks. Today we know that we account for six quarks named as up, down, strange, charm, top and bottom, and they are classified according to their spin (a concept related to the intrinsic angular momentum of the particles).

It is easy to get lost from this point on. Things start to get a bit messy. In the following diagram you can see a visually-friendly classification of all the particles that make up for the standard model. Please notice that in the diagram, they show not only quarks but leptons, fermions, bosons (W,Z and the Higgs) and the graviton. Notice that the electron (e-) is a lepton unlike protons (p+) and neutrons (n0), which are made by quarks. But the three of them (p+, n0 and e-) are fermions. Take a moment to analyse it better.

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The Standard Model of Particle Physics
(Credit: CERN)

Going back to the main problem we had, our issues with developing a suitable theory for the description of the strong and weak interactions. This theory had to be able to describe them without messing up with the electromagnetic and gravitational forces. Also, where does Peter Higgs’ contribution fit in all this?

Well, to be able to explain the weak interaction physicists needed the role of a new kind of massless boson. The scientific community at the time new that such particle could not exist because otherwise they would have found it already, supposedly. So, what happened? A series of articles from several researchers, Peter Higgs included, proposed an explanation for the masses of the bosons (now, the W and Z) responsible for the weak interaction. Moreover, they proposed a theory that could unify the weak interaction with the electromagnetic force. At the time of the publication, the idea was completely rejected on grounds that it was irrelevant for Physics. This turned out to play in favour of Peter Higgs since once the community realised about the relevance of these papers, they started calling it the Higgs theory (which refers not only to the Higgs boson but also to the Higgs field and the Higgs mechanism). However, there were a lot of other people involved in the the development of the Standard Model, so, calling it the Higgs boson might come across as unfair.

I know at this point you all may have even more questions, and reality is that a proper university course of particle physics is probably necessary for us to explain all the details of this theory. So, what exactly is the boson, field and mechanism of Higgs? Although the answer is not straightforward, there is a main message you can take away: the Higgs boson and Higgs field explain why particles have mass. This is, there is a field that permeates the space, and the particles that interact with it have mass and the ones that do not interact with it, do not have mass [2]. The Higgs boson is the last piece of the jigsaw that makes the Standard Model of Particles complete, and allows Physics to explain the mass of particles; which makes the theory highly successful.

In July 2012, the particle accelerator laboratory – CERN – announced that a new particle was found, and it was consistent with the characteristics of the Higgs Boson. Today there is still some uncertainty on whether the particle is the Higgs boson described by the Standard Model, or if we just found a new particle out of it. This is the discovery that made Peter Higgs win the Nobel Prize in 2013, for which he is now “famous”.

The end 🙂 Molto interesting, right?

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