Building blocks

of nature

Everything in the world around us is made from small constituent pieces.

A typical water droplet has a diameter of a couple of milimetres and contains about 1,000,000,000,000,000,000,000 (one sextillion) molecules.

Water molecules are in-turn made of two Hydrogen atoms and an Oxygen atom. These are held together by the electromagnetic force.

Atoms have a nucleus made from protons and neutrons, which is orbited by electrons (with an equal number of protons and electrons).


Hydrogen is the simplest of atoms, comprising just a single proton and an electron. A Hydrogen atom has a size of about 10-15 m. In our modern picture of physical science, the proton and neutron are also made from smaller particles, called quarks. The quarks are held together by the strong nuclear force, which acts over a range of about 10-18 m. The theory that governs these very small scales is called quantum theory.

As far as we can tell there is nothing smaller than a quark.

The Standard Model

of particle physics

The Standard Model (SM) of particle physics describes the interactions of all known particles and forces (except gravity). There are 12 fundamental particles and their antiparticles: the 6 quarks, the electron and its heavier cousins the muon and tau as well as their associated neutrinos.


These particles interact with each other via the exchange of force-carrying bosons: the photon of the electromagnetic interaction; the W and Z particles of the weak interaction that are responsible for radioactive decay; and the gluons of the strong interaction that bind the quarks into the protons and neutrons that compose atomic nuclei.

The Higgs boson, which gives all particles their bare mass, was discovered in 2012 at the Large Hadron Collider (LHC) at CERN.

Protons and neutrons are built from a combination of three quarks (protons comprise two up-quarks and a down-quark, neutrons comprise two down-quarks and one up-quark). Protons and neutrons fall into a class of particle called baryons. Baryons contain three quarks and antibaryons three antiquarks.

The stong nuclear force can also bind together quark-antiquark pairs. These particles are referred to as mesons. Mesons don’t tend to hang around for very long before decaying into lighter particles (lighter mesons, electrons, muons and neutrinos) but can be studied in experiments.


of nature

Fundamental symmetries

Electromagnetism and the strong nuclear force respect these symmetries -- for example we can write down an expression for the motion of a positron in a electric and magnetic field where the positron behaves just like an electron would (but with a different electric charge).

To replace matter by antimatter we need to simultaneously apply both C and P. It turns out that the combination CP is not an exact symmetry of nature: particles and antiparticles do behave differently. In our current picture of particle physics, the weak nuclear force is responsible for this difference.


The LHCb experiment at CERN is making precise measurements of this CP symmetry violation, while the ALPHA experiment is searching for matter-antimatter differences by studying antihydrogen atoms.

Discovery of CP violation

Experimental proof that the CP symmetry can be broken in nature came in 1964 at the Brookhaven laboratory in the US. Christensen, Cronin, Fitch and Turlay observed the decay of a neutral strange meson (see above) into a pair of pions. This decay should not happen if CP is conserved in the decay. Observation of CP violation in beauty mesons came in 2002 from the BaBar experiment in the US and the Belle experiment in Japan.

Cronin and Fitch won the Noble prize for the discovery of CP violation in 1980.