Meet the particles: The standard model of physics

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Meet the particles: The standard model of physics
Particle accellerators like the Large Hadron Collider enable physicists to hunt for particles. (Jupiterimages/ Images)

The discovery of a Higgs boson will not only be seen as the crowning moment of the Standard Model of particle physics but also the opening of an exciting new chapter in fundamental science

— Professor Jim Virdee – Imperial College London

The quest to understand the fundamental properties and interactions of matter in the universe has continued throughout human history. From the ancient thinkers who first questioned whether there were fundamental, indivisible building blocks of matter at all to the modern particle physicists who aim to solve sub-atomic mysteries by smashing protons together in domineering particle accelerators, all of humanity’s ideas about physics have lead to the Standard Model. Everything we know about the fundamental nature of the universe can be effectively summarised with nothing other than a collection of eccentrically-named particles. Well, almost everything…

Matter particles

The particles of the standard model come in two basic types: bosons and fermions. Fermions make up the matter we experience on a day to day basis – the “stuff” we can see and interact with. They can’t be in the same place at the same time – just like a human can’t occupy the exact same location as a wall. There are twelve fermions, which are further divided into two groups – quarks and leptons. You may have never heard of a quark – but you’re actually made of them. The protons and neutrons in the nuclei of atoms are actually quark-structures, and electrons are the poster-particles for the lepton group.

Quark is a notably unusual word, and the names only get weirder from there on. There are three “generations” of quarks, which get progressively heavier and rarer. The first generation includes the “up” and “down” quarks, and varying combinations of these form protons and neutrons. For example, a proton is made from two up quarks and one down quark. The remaining two generations are made from the fatter, elusive quarks: the strange, charm, top and bottom quarks. These can’t form structures for very long at all: the strange quark is part of the lambda particle, for example, which exists for less than a billionth of a second.

The difference between the leptons and the quarks is basically that quarks like to huddle together, whereas leptons are independent. The electron is the most familiar, and it’s a part of the first generation of leptons. As you might expect from the quarks, the later generations, the muon and the tau particles, are essentially the same but heavier. The other three leptons are neutrinos – the electron neutrino, muon neutrino and tau neutrino – which have almost imperceptibly low mass and no electric charge.

Force particles

Fermions in the Standard Model account for the matter we see around us, but something else is needed to explain effects like electromagnetism, the strong nuclear force which holds protons together in the nucleus of atoms and radioactive decays. The matter in itself doesn’t explain everything, so “bosons,” or force-carrying particles, enable the effects to be explained. The remaining particles carry forces of nature, and only one is even remotely familiar in day to day life.

The photon is the only force particle we can experience as humans. It’s the force-carrier for electromagnetism, but we see it as light. This force is responsible for charge (up quarks have a 2/3 positive charge and electrons have a minus 1 charge, for example), so the interactions between protons and electrons, for example, are actually a result of the exchange of photons. Gluons carry the strong nuclear force, which literally “glues” protons together stronger than electromagnetism can cause them to repel – allowing the creation of atoms. The W and Z bosons control the weak nuclear force, which explains how neutrons can decay into protons.

The Higgs boson is the most famous boson, after the announcement of its discovery at the Large Hadron Collider. The “Higgs” mechanism gives some of the fundamental particles mass, but leaves others – like the photon and gluon – massless. This was previously a huge problem for particle physics, because before then there was no explanation for fermions and some bosons having mass. Dr. Matthias Neubert – professor of particle physics at Johannes Gutenberg University – explains that “the discovery of the Higgs boson represents a milestone in the exploration of the fundamental interactions of elementary particles.”

What’s missing?

Meet the particles: The standard model of physics
(lineartestpilot/iStock/Getty Images)

Despite the success of the Standard Model, the Higgs boson does nothing other than add another piece to a still incomplete theory. For one, there is no explanation of gravity at a subatomic scale, with the theorised (but never detected) graviton not even holding a place in the model. Since the effects of the comparatively weak force of gravity are negligible at quantum scales, this is usually ignored, but it’s so important at larger scales that many physicists believe a “grand unifying theory” is required to accurately describe nature.

There are also additional problems for the Standard Model, as physicist Dr. Volker Büscher explains, “the Standard Model has no explanation for the so-called dark matter, so it does not describe the entire universe – there is a lot that remains to be understood.” This problem with the standard model has led to other ideas, the most well-known of which is string theory – which posits that matter is actually made up of miniscule loops of energy vibrating across 11 dimensions.

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