Weak hypercharge

In the Standard Model of electroweak interactions of particle physics, the weak hypercharge is a quantum number relating the electric charge and the third component of weak isospin. It is frequently denoted $Y_{\mathsf {W}}$ and corresponds to the gauge symmetry U(1).^[1]^[2]

It is conserved (only terms that are overall weak-hypercharge neutral are allowed in the Lagrangian). However, one of the interactions is with the Higgs field. Since the Higgs field vacuum expectation value is nonzero, particles interact with this field all the time even in vacuum. This changes their weak hypercharge (and weak isospin $T 3$ ). Only a specific combination of them, $\ Q=T_{3}+{\tfrac {1}{2}}\,Y_{\mathsf {W}}\$ (electric charge), is conserved.

Mathematically, weak hypercharge appears similar to the Gell-Mann–Nishijima formula for the hypercharge of strong interactions (which is not conserved in weak interactions and is zero for leptons).

In the electroweak theory SU(2) transformations commute with U(1) transformations by definition and therefore U(1) charges for the elements of the SU(2) doublet (for example lefthanded up and down quarks) have to be equal. This is why U(1) cannot be identified with U(1)_em and weak hypercharge has to be introduced.^[3]^[4]

Weak hypercharge was first introduced by Sheldon Glashow in 1961.^[4]^[5]^[6]

^ Cite error: The named reference Donoghue-Golowich-Holstein-1994-DynSM was invoked but never defined (see the help page).
^ Cite error: The named reference Cheng-Li-2006-GaThElPP was invoked but never defined (see the help page).
^ Cite error: The named reference Tully-2012-Nutsh was invoked but never defined (see the help page).
^ ^a ^b Cite error: The named reference Glashow-1961-02-NucPh was invoked but never defined (see the help page).
^ Cite error: The named reference Hoddeson-Brown-Riordan-etal-1997 was invoked but never defined (see the help page).
^ Cite error: The named reference Quigg-2015-10-19-ARevNuPaSc was invoked but never defined (see the help page).

[Donoghue-Golowich-Holstein-1994-DynSM-1] Cite error: The named reference Donoghue-Golowich-Holstein-1994-DynSM was invoked but never defined (see the help page).

[Cheng-Li-2006-GaThElPP-2] Cite error: The named reference Cheng-Li-2006-GaThElPP was invoked but never defined (see the help page).

[Tully-2012-Nutsh-3] Cite error: The named reference Tully-2012-Nutsh was invoked but never defined (see the help page).

[Glashow-1961-02-NucPh-4] Cite error: The named reference Glashow-1961-02-NucPh was invoked but never defined (see the help page).

[Hoddeson-Brown-Riordan-etal-1997-5] Cite error: The named reference Hoddeson-Brown-Riordan-etal-1997 was invoked but never defined (see the help page).

[Quigg-2015-10-19-ARevNuPaSc-6] Cite error: The named reference Quigg-2015-10-19-ARevNuPaSc was invoked but never defined (see the help page).

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Flavour in particle physics
Flavour quantum numbers
Isospin: I or I₃ Charm: C Strangeness: S Topness: T Bottomness: B′
Related quantum numbers
Baryon number: B Lepton number: L Weak isospin: T or T₃ Electric charge: Q X-charge: X
Combinations
Hypercharge: Y Y = (B + S + C + B′ + T) Y = 2 (Q − I₃) Weak hypercharge: Y_W Y_W = 2 (Q − T₃) X + 2Y_W = 5 (B − L)
Flavour mixing
CKM matrix PMNS matrix Flavour complementarity
v t e