" The Higgs boson is the only boson in the theory which is not a gauge boson; it has a special status in the theory, and has been the subject of some controversy. Gravitons, the bosons believed to mediate the gravitational interaction, are not accounted for in the Standard Model . There are twelve different types, or "flavours", of fermions in the Standard Model. Amongst the proton, neutron, and electron, those fermions which constitute the vast majority of matter, the Standard Model considers only the electron a fundamental particle. The proton and neutron are aggregates of smaller particles known as quarks, which are held together by the strong interaction. The fundamental fermions in the Standard Model are given in the table. Table Left handed fermions in the Standard Model Fermion (Left-handed) Symbol Electric charge Weak charge* Weak isospin Hypercharge Color charge* Mass** Generation 1 Electron e -1 2 -1/2 -1/2 1 0. Hin Import Model511 MeV Electron neutrino νe 0 2 +1/2 -1/2 1 < 50 eV Positron ec 1 1 0 1 1 0. 511 MeV Electron antineutrino 0 1 0 0 1 < 50 eV Up quark u +2/3 2 +1/2 +1/6 3 ~5 MeV *** Down quark d -1/3 2 -1/2 +1/6 3 ~10 MeV *** Anti-up antiquark uc -2/3 1 0 -2/3 ~5 MeV *** Anti-down antiquark dc +1/3 1 0 +1/3 ~10 MeV *** Generation 2 Muon μ -1 2 -1/2 -1/2 1 105. "

" Informally speaking, fermions are particles of matter and bosons are particles that transmit forces. For a detailed description of the differences between fermions and bosons, see the article on identical particles. In the Standard Model, the theory of the electroweak interaction (which describes the weak and electromagnetic interactions) is combined with the theory of quantum chromodynamics. All of these theories are gauge theories, meaning that they model the forces between fermions by coupling them to bosons which mediate (or "carry") the forces. The Lagrangian of each set of mediating bosons is invariant under a transformation called a gauge transformation, so these mediating bosons are referred to as gauge bosons. The bosons in the Standard Model are: Unsolved problems in physics: Parameters in the Standard Model: What gives rise to the Standard Model of particle physics? Why do its particle masses and coupling constants possess the values we have measured? Does the Higgs boson predicted by the model really exist? Why are there three generations of particles in the Standard Model? Is the Standard Model reality, a good approximation to reality, or fatally flawed? Photons, which mediate the electromagnetic interaction. W and Z bosons, which mediate the weak nuclear force Eight species of gluons, which mediate the strong nuclear force. Import Model Photo Six of these gluons are labelled as pairs of "colors" and "anti-colors" (for example, a gluon can carry "red" and "anti-green". ) The other two species are a more complex mix of colors and anti-colors. "