What is the Standard Model of particle physics, and what does it leave unexplained?

An atom is not the last stop in the search for matter’s parts. It contains electrons and a nucleus. The nucleus contains protons and neutrons, which have quarks inside them. The Standard Model describes these basic particles and how they interact. It is more than a chart of names. Its rules let physicists calculate results that experiments can test.

The matter particles fall into two groups. Quarks help make protons and neutrons. Leptons include electrons and neutrinos. An electron is familiar from atoms and electric currents. A neutrino has no electric charge and interacts so rarely that most pass through matter without leaving a trace.

The model also describes three forces. Electromagnetism governs electric charge and light. The strong force holds quarks together. The weak force takes part in particle changes, including some radioactive processes. Gravity is missing from this list. It also leaves dark matter unidentified and does not explain dark energy or the cosmic matter excess.

The Higgs field has another role. Its interactions give mass to basic particles such as electrons and quarks. Scientists found the Higgs boson, a particle associated with that field, in 2012. This was a major successful test. It did not complete every part of physics or explain all the mass of a proton in one step.

Neutrinos show the difference between a successful theory and a finished one. Experiments find that their detected types can change as they travel. This requires differences in masses, with at least some masses above zero. The original simplest Standard Model treated them as massless. Extending the rules can include their masses, but why those masses are so tiny remains unknown. The theory’s successes and this gap are both part of an honest account.