The and bosons decay to
fermion pairs but neither the nor the bosons have sufficient energy to decay into the highest-mass
top quark. Neglecting phase space effects and higher order corrections, simple estimates of their
branching fractions can be calculated from the
coupling constants.
W bosons bosons can decay to a
lepton and antilepton (one of them charged and another neutral) or to a
quark and antiquark
of complementary types (with opposite electric charges
e and
e). The
decay width of the W boson to a quark–antiquark pair is proportional to the corresponding squared
CKM matrix element and the number of quark
colours, . The decay widths for the W boson are then proportional to: Here, , , denote the three flavours of
leptons (more exactly, the positive charged
antileptons). , , denote the three flavours of neutrinos. The other particles, starting with and , all denote
quarks and antiquarks (factor is applied). The various V_{ij} denote the corresponding
CKM matrix coefficients.{{efn|Every entry in the lepton column can also be written as three decays, e.g. for the first row, as 1, 2, 3, for every neutrino mass eigenstate, with decay widths proportional to {{tmath|1= \vert U_\text{e1} \vert^2}}, {{tmath|1= \vert U_\text{e2} \vert^2}}, {{tmath|1= \vert U_\text{e3} \vert^2 }} (
PMNS matrix elements), but experiments at present that measure the decays can't discriminate between neutrino mass eigenstates: They measure total decay width of the sum of all three processes.}}
Unitarity of the CKM matrix implies that ~ |V_\text{ud}|^2 + |V_\text{us}|^2 + |V_\text{ub}|^2 ~ = ~|V_\text{cd}|^2 + |V_\text{cs}|^2 + |V_\text{cb}|^2 = 1 ~, thus each of two quark rows Therefore, the leptonic
branching ratios of the boson are approximately \, B( \mathrm{e}^{+} \mathrm{\nu}_\mathrm{e}) = \,\, B(\mathrm{\mu}^{+} \mathrm{\nu}_\mathrm{\mu}) = \,\, B(\mathrm{\tau}^{+} \mathrm{\nu}_\mathrm{\tau}) = \, . The hadronic branching ratio is dominated by the CKM-favored and final states. The sum of the
hadronic branching ratios has been measured experimentally to be , with {{nowrap|\, B( \ell^{+} \mathrm{\nu}_\ell ) = \, .}}
Z0 boson bosons decay into a fermion and its antiparticle. As the boson is a mixture of the pre-
symmetry-breaking and bosons (see
weak mixing angle), each
vertex factor includes a factor {{tmath|1= T_3 - Q \sin^2 \,\theta_\text{W} }}, where T_3 is the third component of the
weak isospin of the fermion (the "charge" for the weak force), Q is the
electric charge of the fermion (in units of the
elementary charge), and \theta_\text{w} is the
weak mixing angle. Because the weak isospin ( T_3 ) is different for fermions of different
chirality, either
left-handed or right-handed, the coupling is different as well. The
relative strengths of each coupling can be estimated by considering that the
decay rates include the square of these factors, and all possible diagrams (e.g. sum over quark families, and left and right contributions). The results tabulated below are just estimates, since they only include tree-level interaction diagrams in the
Fermi theory. To keep the notation compact, the table uses {{tmath|1= x = \sin^2\ \theta_\text{w} \approx \tfrac{1}{4} }}.
* The impossible decay into a
top quark–antiquark pair is left out of the table. Subheadings '
and ' denote the
chirality or "handedness" of the fermions. In 2018, the CMS collaboration observed the first exclusive decay of the boson to a
ψ meson and a
lepton–antilepton pair. ==See also==