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Concept
Isospin faible
Résumé
In particle physics, weak isospin is a quantum number relating to the electrically charged part of the weak interaction: Particles with half-integer weak isospin can interact with the _W boson+- bosons; particles with zero weak isospin do not.
Weak isospin is a construct parallel to the idea of isospin under the strong interaction. Weak isospin is usually given the symbol T or I, with the third component written as T_3 or I_3.
It can be understood as the eigenvalue of a charge operator.
T_3 is more important than T; typically "weak isospin" is used as short form of the proper term "3rd component of weak isospin".
The weak isospin conservation law relates to the conservation of weak interactions conserve T_3. It is also conserved by the electromagnetic and strong interactions. However, interaction with the Higgs field does not conserve T_3, as directly seen by propagation of fermions, mixing chiralities by dint of their mass terms resulting from their Higgs couplings. Since the Higgs field vacuum expectation value is nonzero, particles interact with this field all the time even in vacuum. Interaction with the Higgs field changes particles' weak isospin (and weak hypercharge). Only a specific combination of them, (electric charge), is conserved.
Fermions with negative chirality (also called "left-handed" fermions) have and can be grouped into doublets with that behave the same way under the weak interaction. By convention, electrically charged fermions are assigned with the same sign as their electric charge.
For example, up-type quarks (u, c, t) have and always transform into down-type quarks (d, s, b), which have and vice versa. On the other hand, a quark never decays weakly into a quark of the same Something similar happens with left-handed leptons, which exist as doublets containing a charged lepton (_Electron-, _Muon-, _Tau-) with and a neutrino (_Electron neutrino, _Muon neutrino, _Tau neutrino) with In all cases, the corresponding anti-fermion has reversed chirality ("right-handed" antifermion) and reversed sign
Fermions with positive chirality ("right-handed" fermions) and anti-fermions with negative chirality ("left-handed" anti-fermions) have and form singlets that do not undergo charged weak interactions.
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