The Standard Model of Particle Physics
Click any particle to see its mass, charge, spin and lifetime, the forces it feels, how it decays and how it was discovered. Then build protons, neutrons and mesons from quarks.
Electron
Antiparticle:
What it is:
How it decays:
Discovery:
Quarks always come in colourless combinations: three quarks (red + green + blue) or a quark with an antiquark (colour + anticolour). You never see a single quark on its own.
Show the physics and the notes
Matter. Everything we are made of is built from just a few fermions (spin 1/2). The quarks feel the strong force and make up protons and neutrons: a proton is up-up-down (+2/3 + 2/3 − 1/3 = +1) and a neutron is up-down-down (+2/3 − 1/3 − 1/3 = 0). The leptons do not feel the strong force: the electron, and its heavier cousins the muon and the tau, each with a neutrino. Ordinary matter needs only the first generation (u, d, e and the electron neutrino). The other two generations are heavier copies that decay quickly, and nobody yet knows why there are three.
Forces. Forces are carried by bosons (spin 1): the gluon for the strong force (it acts on quarks and gluons, which carry “colour”), the photon for the electromagnetic force (it acts on anything with electric charge) and the W and Z for the weak force (it acts on every fermion, and it is the only force a neutrino feels apart from gravity). The photon and the gluon have no mass; the W and Z are very heavy, which is why the weak force is so short in range. Gravity acts on everything but is not part of the Standard Model.
The Higgs boson is the quantum of the Higgs field. Particles get their mass from their interaction with this field. It was the last particle of the Standard Model to be found (2012).
Mass and lifetime. The masses span about 12 orders of magnitude, from the neutrinos (less than a millionth of an electron mass) to the top quark (more than 300 000 electron masses). The heavier a particle is, the more ways it has to decay, and the shorter its life. The lifetime is the mean life: the time in which the number of particles falls to 1/e. The half-life is 0.69 of that. Quarks are never seen alone, so for them the lifetime shown is for a typical hadron that contains the quark.
Hadrons. Baryons have three quarks (baryon number +1) and mesons have a quark and an antiquark (baryon number 0). In every case the total charge is a whole number. Only the up and down quarks are in ordinary matter. The top quark decays so quickly (about 5×10−25 s) that it never forms a hadron.
The masses are from the Particle Data Group (rounded). The quark masses depend on how they are defined. The neutrino masses are upper limits: they are not zero, since neutrinos change flavour, but the values are too small to measure so far.
Learn more in the Particle physics lesson. Original idea by Aaron (Scots College), rebuilt in the Physics High style.