Standard Model

One of the most surprising revelations of 20th-century physics was just how many elementary particles exist in the universe. The idea of fundamental, indivisible building blocks goes all the way back to ancient Greek atomism, but it wasn’t until the 1900s that physicists began probing what actually happens inside matter at the smallest scales. What they found changed everything you thought you knew about the physical world.

Quantum physics predicts exactly 18 types of elementary particles, 17 of which have been confirmed by experiment (including the Higgs boson, detected at CERN in 2012). The remaining particle, the graviton, is still theoretical. Searching for it and testing the limits of the Standard Model is one of the great ongoing quests in particle physics.

The Standard Model of Particle Physics

The Standard Model sits at the heart of modern physics. It describes three of the four fundamental forces of nature (electromagnetic, weak nuclear, and strong nuclear) along with the particles that carry those forces, called gauge bosons. You’ll notice gravity is missing from the list. That’s because physicists haven’t yet figured out how to fold gravity into the quantum framework, though many are working on it.

Think of the Standard Model as a periodic table for particle physics. Just as the periodic table organizes chemical elements by their properties, the Standard Model organizes all known elementary particles by their mass, charge, and spin. Once you understand its structure, the relationships between particles start to make intuitive sense.

Groups of Particles

Particle physicists love organizing particles into groups, and for good reason. These classifications tell you how particles behave and interact. Here’s the full breakdown you need to understand.

Elementary Particles are the smallest constituents of matter and energy. As far as we can tell, they aren’t made from combinations of anything smaller. They come in two broad families:

• Fermions – These are particles with a half-integer spin (-1/2, 1/2, 3/2, etc.). Fermions make up the matter you see around you. They obey the Pauli exclusion principle, which means no two identical fermions can occupy the same quantum state. This is why matter takes up space.
  • Quarks – A class of fermion that combines to form heavier particles called hadrons (like protons and neutrons). There are 6 types, organized into three generations:
    • Up Quark
    • Charm Quark
    • Top Quark
    • Down Quark
    • Strange Quark
    • Bottom Quark
  • Leptons – A class of fermion that doesn’t feel the strong nuclear force. You’re most familiar with the electron, but there are 6 leptons total:
    • Electron
    • Electron Neutrino
    • Muon
    • Muon Neutrino
    • Tau
    • Tau Neutrino
• Bosons – Bosons have integer spin values (0, 1, 2, etc.) and serve as the force carriers in quantum field theory. Unlike fermions, multiple bosons can occupy the same quantum state, which is why laser light (made of photons) behaves so differently from matter.
  • Photon – carries the electromagnetic force
  • W Boson – carries the weak nuclear force
  • Z Boson – carries the weak nuclear force
  • Gluon – carries the strong nuclear force
  • Higgs Boson – gives particles their mass via the Higgs field, confirmed experimentally in 2012 at CERN
  • Graviton – theoretically predicted to carry gravity in a quantum theory, but not part of the Standard Model and not yet detected

Composite Particles are built from combinations of elementary particles. You encounter these every day without realizing it, because atoms themselves are composite particles.

• Hadrons – Particles made up of multiple quarks bound together by the strong force.
  • Baryons (fermions) – made of three quarks
    • Nucleons – protons & neutrons, the building blocks of atomic nuclei
    • Hyperons – short-lived particles that contain at least one strange quark
  • Mesons (bosons) – made of one quark and one antiquark
• Atomic Nuclei – protons and neutrons bind together through the strong force to form the atomic nucleus.
• Atoms – the basic chemical building blocks, composed of electrons orbiting a nucleus of protons and neutrons.
• Molecules – complex structures of multiple atoms bonded together. Understanding how atoms bond into molecules is the foundation of modern chemistry.

A Note on Particle Classification

If you find all these names confusing, you’re in good company. Here’s a trick that might help: think of how biological classification works. A human is simultaneously a primate, a mammal, and a vertebrate. In the same way, a proton is simultaneously a baryon, a hadron, and a fermion. Each label tells you something different about the particle’s properties.

The tricky part is that particle physics names sound far more alike than biology names do. Mixing up “boson” and “baryon” is much easier than confusing “primate” and “invertebrate.” There’s no shortcut here. You just have to study each group carefully and pay attention to which name is being used in context.

Matter and Forces: Fermions and Bosons

Every elementary particle in physics is classified as either a fermion or a boson. The distinction comes down to a quantum property called spin, which is a kind of intrinsic angular momentum that particles carry even when they aren’t physically rotating. Quantum physics shows that this spin is always quantized, meaning it can only take specific values.

A fermion (named after Enrico Fermi) has half-integer spin (1/2, 3/2, etc.), while a boson (named after Satyendra Nath Bose) has integer spin (0, 1, 2, etc.). This difference in spin leads to fundamentally different behavior. Fermions obey the Pauli exclusion principle and resist being squeezed into the same state, which is why solid matter exists. Bosons have no such restriction, which is why you can pack unlimited photons into the same space.

Simple addition of spins gives you two useful rules to remember:

• Combining an odd number of fermions produces a fermion (the total spin remains a half-integer)
• Combining an even number of fermions produces a boson (the total spin becomes an integer)

This is why a proton (made of three quarks, all fermions) is itself a fermion, while a meson (made of two quarks) is a boson.

Breaking Down Matter: Quarks and Leptons

When you strip matter down to its most basic components, you find two families of particles: quarks and leptons. Both are fermions, which means all bosons in the universe are built from even combinations of these fundamental ingredients.

Quarks interact through all four fundamental forces: gravity, electromagnetism, the weak nuclear force, and the strong nuclear force. You’ll never find a quark on its own in nature. They always bind together to form composite particles called hadrons. Hadrons split into two sub-groups: mesons (which are bosons, made of one quark and one antiquark) and baryons (which are fermions, made of three quarks). Protons and neutrons are baryons, meaning they’re made of three quarks arranged so that the total spin is a half-integer.

Leptons are different. They don’t feel the strong nuclear force at all, which means they never form hadrons. There are three “flavors” of charged leptons: the electron, the muon, and the tau. Each comes paired with a nearly massless neutral partner called a neutrino. So you get the electron and electron-neutrino as one pair, the muon and muon-neutrino as another, and the tau and tau-neutrino as the third. These pairs are called “weak doublets” because they interact through the weak nuclear force.