lepton in the interaction, for example: Similarly, a down-type quark (d with a charge of −) can be converted into an up-type quark (u, with a charge of +), by emitting a boson or by absorbing a boson. More precisely, the down-type quark becomes a quantum superposition of up-type quarks: that is to say, it has a possibility of becoming any one of the three up-type quarks, with the probabilities given in the CKM matrix tables. Conversely, an up-type quark can emit a boson, or absorb a boson, and thereby be converted into a down-type quark, for example: The W boson is unstable so will rapidly decay, with a very short lifetime. For example: Decay of a W boson to other products can happen, with varying probabilities. In the so-called beta decay of a neutron (see picture, above), a down quark within the neutron emits a virtual boson and is thereby converted into an up quark, converting the neutron into a proton. Because of the limited energy involved in the process (i.e., the mass difference between the down quark and the up quark), the virtual boson can only carry sufficient energy to produce an electron and an electron-antineutrino – the two lowest-possible masses among its prospective decay products. At the quark level, the process can be represented as: Neutral-current interaction In neutral current interactions, a quark or a lepton (e.g., an electron or a muon) emits or absorbs a neutral boson. For example: Like the bosons, the boson also decays rapidly, for example: Unlike the charged-current interaction, whose selection rules are strictly limited by chirality, electric charge, weak isospin, the neutral-current interaction can cause any two fermions in the standard model to deflect: Either particles or anti-particles, with any electric charge, and both left- and right-chirality, although the strength of the interaction differs. The quantum number weak charge () serves the same role in the neutral current interaction with the that electric charge (, with no subscript) does in the electromagnetic interaction: It quantifies the vector part of the interaction. Its value is given by: Since the weak mixing angle the parenthetic expression with its value varying slightly with the momentum difference (called “running”) between the particles involved. Hence since by convention and for all fermions involved in the weak interaction The weak charge of charged leptons is then close to zero, so these mostly interact with the boson through