Weak charge


In nuclear physics and atomic physics, weak charge, or rarely neutral weak charge, refers to the Standard Model weak interaction coupling of a particle to the Z boson; it is named by analogy to electric charge, which measures coupling to the photon of electromagnatism. 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, different quantities, such as weak isospin or weak hypercharge; this article concerns the use of weak charge for a quantity that measures the degree of vector coupling of a fermion to the Z boson.

Empirical formulas

Measurements in 2017 give the weak charge of the proton as .
The weak charge may be summed in atomic nuclei, so that the predicted weak charge for Cs is 55× + 78× −73.19, while the value determined experimentally, from measurements of parity violating electron scattering, was −72.58 .
A recent study used four even-numbered isotopes of ytterbium to test the formula for weak charge, with corresponding to the number of neutrons and to the number of protons. The formula was found consistent to 0.1% accuracy using the Yb, Yb, Yb, and Yb isotopes of ytterbium.
In the ytterbium test, atoms were excited by laser light in the presence of electric and magnetic fields, and the resulting parity violation was observed. The specific transition observed was the forbidden transition from 6s S to 5d6s D. The latter state was mixed, due to weak interaction, with 6s6p P to a degree proportional to the nuclear weak charge.

Particle values

This table gives the values of the electric charge, weak isospin, weak hypercharge and the approximate Z boson coupling factors.
If the variable correction terms shown for different values are omitted, then the table's constant values for weak charge are only approximate: They happen to be exact for particles whose energies make the weak mixing angle with This value is very close to the typical angle observed in particle accelerators. The embedded formulas give more accurate values for those cases when the Weinberg angle, is known.
For brevity, the table omits antiparticles. Every particle listed has an antiparticle with identical mass and opposite charge. All non-zero signs in the table have to be reversed for antiparticles. The paired columns labeled "Left" and "Right" for fermions, have to be swapped in addition to their signs being flipped.
All left-handed fermions and right-handed antifermions have and therefore interact with the W boson. They could be referred to as "proper"-handed. Right-handed fermions and left-handed antifermions, on the other hand, have zero weak isospin and therefore do not interact with the W boson ; they could therefore be referred to as "wrong"-handed. "Proper"-handed fermions are organized into isospin doublets, while "wrong"-handed fermions are represented as isospin singlets. While "wrong"-handed particles do not interact with the W boson, all "wrong"-handed fermions known to exist do interact with the Z boson, excepting perhaps sterile neutrinos, if they exist.
"Wrong"-handed neutrinos have never been observed, but may still exist since they would be invisible to existing detectors. Sterile neutrinos, assuming they exist, are speculated to play a role in a few theoretical mechanisms that might provide neutrinos with mass. The statement above, that the interacts with all fermions, will need an exception inserted for sterile neutrinos, if they are ever detected experimentally.
Massive fermions – except neutrinos – always exist in a superposition of left-handed and right-handed states, and never in pure chiral states. This mixing is caused by interaction with the Higgs field, which acts as an infinite source and sink of weak isospin and / or hypercharge, due to its non-zero vacuum expectation value.

Theoretical basis

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

Extension to larger, composite protons and neutrons

This relation only directly applies to quarks and leptons, 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 and of the neutron to, 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 total weak charge of charged leptons.
Corrections arise when doing the full theoretical calculation for nucleons, however. Specifically, when evaluating Feynman diagrams beyond the tree level, 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.

Relation to weak hypercharge

Because weak hypercharge is given by
the weak hypercharge   , weak charge   , and electric charge are related by
or equivalently
where is the weak hypercharge for left-handed fermions and right-handed antifermions, hence
in the typical case, when the weak mixing angle is approximately 30°.

Derivation

The Standard Model coupling of fermions to the Z boson and photon is given by:
where
and the expansion uses for its basis vectors the Pauli matrices from the Weyl equation:
and
The fields for B and W boson are related to the Z boson field and electromagnetic field by
and
By combining these relations with the above equation and separating by and one obtains:
The term that is present for both left- and right-handed fermions represents the familiar electromagnetic interaction. The terms involving the Z boson depend on the chirality of the fermion, thus there are two different coupling strengths:
and
It is however more convenient to treat fermions as a single particle instead of treating left- and right-handed fermions separately. The Weyl basis is chosen for this derivation:
Thus the above expression can be written fairly compactly as:
where