The weak isospin in particle physics is a quantum number relating to the weak interaction, and parallels the idea of isospin under the strong interaction. Weak isospin is usually given the symbol T or I with the third component written asT3, Tz or I3. Weak isospin is a component of the weak hypercharge, which unifies weak interactions with electromagnetic interactions. Fermions with negative helicity (also called left-handed fermions) have T = 1⁄2 and can be grouped into doublets with T3 = ±1⁄2 that behave the same under the weak interaction. For example, "u" type quarks (u, c, t) have T3 = +1⁄2 and always transform in "d" type quarks (d, s, b) that have T3 = −1⁄2 and vice versa. On the other hand, a quark never decays into a quark of the same type. Something similar happens with left-handed leptons, which exist as doublets containing a charged lepton with T3 = −1⁄2 and a neutrino with T3 = 1⁄2. Fermions with positive chirality (also called right-handed fermions) have T = 0 and form singlets that do not undergo weak interactions. There is also a weak isospin conservation law: all weak interactions must preserve the weak isospin. [edit] Weak isospin and the W bosonsThe symmetry associated with spin is SU(2). This requires gauge bosons to transform between weak isospin charges: bosons W+, W− and W0. This implies that W bosons have a T = 1, with three different values of Tz.
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