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Concept
Weak charge
Résumé
In nuclear physics and atomic physics, weak charge refers to the Standard Model weak interaction coupling of a particle to the Z boson. For example, for any given nuclear isotope, the total weak charge is approximately −0.99 per neutron, and +0.07 per proton. It also shows an effect of parity violation during electron scattering.
This same term is sometimes also used to refer to other, distinct quantities, such as weak isospin, weak hypercharge, or the vector coupling of a fermion to the Z boson (i.e. the coupling strength of weak neutral currents).
Electroweak interaction
The formula for the weak charge is derived from the Standard Model, and is given by
where is the weak charge,
is the weak isospin,
is the weak mixing angle, and is the electric charge.
The approximation for the weak charge is usually valid, since the weak mixing angle typically is and and a discrepancy of only a little more than
This relation only directly applies to quarks and leptons (fundamental particles), since weak isospin is not clearly defined for composite particles, such as protons and neutrons, partly due to weak isospin not being conserved. One can set the weak isospin of the proton to + 1/2 and of the neutron to − 1/2, in order to obtain approximate value for the weak charge. Equivalently, one can sum up the weak charges of the constituent quarks to get the same result.
Thus the calculated weak charge for the neutron is
The weak charge for the proton calculated using the above formula and a weak mixing angle of 29° is
a very small value, similar to the nearly zero weak charge of charged leptons (see the table below).
Corrections arise when doing the full theoretical calculation for nucleons, however. Specifically, when evaluating Feynman diagrams beyond the tree level (i.e. diagrams containing loops), the weak mixing angle becomes dependent on the momentum scale due to the running of coupling constants, and due to the fact that nucleons are composite particles.
Source officielle
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