Section 1.6 - The Standard Model of Particle Physics

The Building Blocks of Matter

What is everything made of? Atoms? No, we learned in school that atoms are made of protons, neutrons, and electrons. Can we go further?

The Standard Model of Particle Physics

The Standard Model is our current understanding of the fundamental particles of the Universe. If there is a layer deeper than the Standard Model, we have yet to discover and describe it.

There's obviously a lot of particles, and we'll cover them all shortly, but notice that each of the particles has three quantities defined: mass, charge, and spin

• Mass is given as $\frac{eV}{c^2}$. This unusual unit is based off the mass-energy equivalence, $E = mc^2$. An electron volt $eV$ is the energy gained/lost by an electron moving across an electric potential difference of one volt. $1V = \frac{kg \cdot m^2}{A \cdot s^3}$. This is necessary due to the small scale of subatomic particles. $1\frac{eV}{c^2}=1.78\cdot10^{-36}kg$
• Charge - same concept we learned from high school
• Spin - this is a weird one, it's not normal spin (i.e. angular momentum from classical physics). This is quantum spin, a strange property of subatomic particles. See this link for the simplest explanation I can find

Key Definitions

• Quarks: fundamental particles that carry a fractional charge. As you can see, they come in 6 "flavors" - up, down, strange, charmed, top, and bottom
• Leptons: fundamental particles that carry an integer charge and are not affected by the strong force. Also come in 6 flavors
• Fermions: a broader term encompassing quarks and leptons, i.e. quarks and leptons are the two types of fermions
• Bosons: fundamental particles that carry an integer charge and are known as the "force-carriers" (more on this later)
• Baryons: subatomic particles made up of three quarks (i.e. protons and neutrons)
• Mesons: subatomic particles made up of one quark and one antiquark (more on antimatter later)

Normal Matter - Generation I

The leftmost column encompasses the particles you're probably most familiar with. Protons consist of 2 up quarks and 1 down quark, while neutrons consist of 1 up quark and 2 down quarks

 Notice how the fractional charges sum up to what we'd expect $Proton: +2/3 + 2/3 - 1/3 = +1$ $Neutron: +2/3 - 1/3 - 1/3 = 0$

Electrons are also straightforward, they're the same negatively charged particles you learned in school.

That leaves the electron neutrino. Neutrinos are chargeless leptons that have a mass so tiny they were originally thought to be massless. To this day, we still don't have a precise measurement for its mass. The neutrino is produced in many forms of radioactive decays, such as the beta decay in atomic nuclei or the thermonuclear reaction that powers stars.

Detecting a neutrino is painstakingly difficult because it barely interacts with anything. Every second, trillions of neutrinos emitted by the Sun pass through your body. Alternatively, a single neutrino could pass through a light-year of lead and only have a 50% chance of being stopped. But we knew it had to exist to maintain the law of conservation of matter. Look up the Super-Kamiokande Neutrino Observatory to learn more.

Exotic Matter - Generations II & III

You would think the up and down quarks, the electron, and the electron neutrino would be enough to constitute everything in the Universe. But for some reason, nature decided to include a heavy cousin and a super-heavy cousin of each particle. Why the triple symmetry? We don't really know. Scientists are particularly puzzled by the top quark. Why on earth is it so darn huge? (~70,000x heaver than an up quark)

These exotic fermions may be bigger, but their charge and spin are the same as their regular counterparts. Therefore, you can construct "exotic atoms" using these particles. Imagine an atom that has muons orbiting the nucleus instead of electrons!!

 Look how complicated the baryons become, just by adding the strange quark. With 6 flavors in total, possibilities abound! Source: Particle Physics: A Very Short Introduction by Frank Close

Thankfully in practice, these exotic fermions are much less stable and often decay into their more conventional counterparts

 Source: Particle Physics: A Very Short Introduction by Frank Close

Antimatter

As if there weren't enough particles already, every particle has its antimatter counterpart with opposite spin and charge. For example, antiprotons are made of two anti-up quarks and one anti-down quark. Antiparticles are denoted with a bar over their symbol, so for example, $u$ vs $\overline{u}$ for up vs anti-up. The exception is the anti-electron, which is more commonly referred to as the positron and denoted with $e^+$

Antimatter is famous for the fact that if corresponding matter and antimatter particles collide, they spectacularly annihilate in a burst of gamma rays

 In each case, the particle and its antiparticle annihilate each other, releasing a pair of high-energy gamma photons. Credit: CSIRO

There exist particles called mesons consisting of one quark and one antiquark. How do these not annihilate? Well they do, but in quantum mechanics, nothing is instantaneous, so they exist as mesons for a brief moment

The questions of why the Universe is made up almost exclusively of matter rather than antimatter is a major unsolved mystery in cosmology

Gauge Bosons - The Force Carriers

Classical physics could calculate forces to great detail but failed to explain the true nature of a force. For example, Newton discovered and mathematically defined gravity, but he himself was troubled by the fact he had no idea of its underlying nature. Quantum mechanics understands a force to be mediated by an exchange of particles (quantum bundles of energy). The gauge bosons fill that role.

• The electromagnetic force can be described as an exchange of photons

• The weak force mediates radioactive decay with the W and Z bosons (unfortunately, I'm not able to give a much deeper explanation of the weak force. For more, I recommend Particle Physics: A Very Short Introduction by Frank Close)

 The $W^-$ boson mediates beta decay, in which a neutron transforms into a proton by the emission of an electron and an antineutrino $n^0 \rightarrow p^1 + e^- +\overline{v}_e$

• The strong force bind atomic nuclei. Let me ask you a question: from high school physics, we're taught that opposite forces attract - that's what keeps negative electrons in orbit around the positive nucleus. But then how does the nucleus stay together? Shouldn't all the positive protons repel each other?

The answer is that at super close distances (the width of a nucleus), the strong force easily overpowers the protons' electromagnetic repulsion. The strong force can be described as an exchange of gluons. These have yet to be directly observed, but our best particle accelerates are on the job (to learn how particle accelerators work, watch this video. It explains it far better than I ever could). And yes, in case you were wondering, gluons are so named because they glue the nucleus together.

So we've define the force carriers for three of the four fundamental forces? What about gravity? You'll have to see the next section for the answer!

The Higgs Boson

What is the fundamental nature of mass? Why do some particles have so much of it and others none at all? What gives us mass?

Physicist Peter Higgs proposed the existence of the Higgs boson that would answer these questions in 1964, but it was not until 2013 that it was discovered, winning the 2013 Nobel Prize. The Universe is permeated by a Higgs Field that's propagated by these Higgs bosons. Some particles are heavily affected by the field and flow through it with heavy resistance, like molasses. These particles have high masses. In other words, the less a particle interacts with the Higgs field, the less its mass

Here's the best metaphor I've found (link here, credit Nigel Holmes / NYT). Think of the Higgs field like a field of snow. An old woman shuffling through the snow meets a lot of resistance. She has high mass. A skier glides through the snow. He has low mass. A bird flies over the snow. It has no mass

Book Recommendation

Particle physics can get super strange and abstract, but if you're intrigued and want to learn more, I recommend Particle Physics: A Very Short Introduction by Frank Close. It taught me basically everything you see on this page.