Particle physics and the Standard Model.

Step 1 of 6

Quarks

Every atom has a nucleus of protons and neutrons, and for a long time these looked like the smallest pieces of matter. They are not. A proton is a tiny bag, about 1.7 femtometres across, with three quarks inside: two up quarks and one down quark.

Quarks carry fractions of the electron’s charge: an up quark has $+\tfrac{2}{3}$⁠ and a down quark $-\tfrac{1}{3}$⁠. In the proton: $+\tfrac{2}{3} + \tfrac{2}{3} - \tfrac{1}{3} = +1$$+\tfrac{2}{3} + \tfrac{2}{3} - \tfrac{1}{3} =$$+1$⁠. The readings beside the Scene do this sum for whichever particle you choose under Particle. Particles made of quarks are called hadrons.

Step 2 of 6

Where the mass comes from

Look at the bar under the readings. A proton weighs 938 MeV, in the units particle physicists use for mass. The thin gold sliver at its start is the quarks’ own mass.

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Heavy quarks

There are six kinds of quark: up, down, strange, charm, bottom and top. The J/psi is a meson: one quark and one antiquark, its partner from antimatter. The proton, neutron, lambda and omega are baryons, three quarks each. Pull a quark out is fixed at rest.

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The string

The force that holds quarks together is the strong force, carried by gluons. Electric and gravitational forces weaken as two bodies move apart. The strong force between quarks does not: beyond a few tenths of a femtometre the gluon field gathers into a narrow tube, and every extra femtometre of it adds the same energy, like a rubber band that never goes slack. Here Pull a quark out is at 1 fm: an up quark is dragged a femtometre away, and the tag shows the energy in the tube.

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Keep pulling

When a particle collider knocks a quark out of a proton, the quark comes out as a jet, a spray of new hadrons. The Particle is fixed at Proton.

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The Standard Model

Zoom out to the Standard Model is at Standard Model: each ball is one of the elementary particles. Height is mass, on a log scale; depth is electric charge; and the columns are generations, I, II and III. The quarks are purple, and the leptons green: the electron, its heavier cousins the muon and tau, and three neutrinos. On the right are the force carriers, the gluon, photon, W and Z, and the Higgs boson, found at CERN in 2012. Seventeen balls in all: that is the Standard Model.