Editing Weak interaction (section)

=== Neutral-current interaction ===
{{main|Neutral current}}
In [[neutral current]] interactions, a [[quark]] or a [[lepton]] (e.g., an [[electron]] or a [[muon]]) emits or absorbs a neutral [[Z boson|{{math|Z}} boson]]. For example:
: <math> \mathrm{e}^- \to \mathrm{e}^- + \mathrm{Z}^0</math>

Like the {{math|{{SubatomicParticle|W boson+-}}}}&nbsp;bosons, the {{math|{{SubatomicParticle|Z boson0}}}}&nbsp;boson also decays rapidly,<ref name="PDG2"/> for example:
: <math> \mathrm{Z}^0 \to \mathrm{b} + \bar \mathrm{b} </math>

Unlike the charged-current interaction, whose selection rules are strictly limited by chirality, electric charge, {{nowrap|and / or}} weak isospin, the neutral-current {{math| {{SubatomicParticle|Z boson0}} }} interaction can cause any two fermions in the standard model to deflect: Either particles or anti-particles, with any electric charge, and both left- and right-chirality, although the strength of the interaction differs.{{efn|
The only fermions which the {{math|{{SubatomicParticle|Z boson0}}}} does ''not'' interact with at all are the hypothetical [[sterile neutrino|"sterile" neutrinos]]: Left-chiral anti-neutrinos and right-chiral neutrinos. They are called "sterile" because they would not interact with any Standard Model particle, except perhaps the [[Higgs boson]]. So far they remain entirely a conjecture: As of October&nbsp;2021, no such neutrinos are known to actually exist.
: 
: "[[MicroBooNE]] has made a very comprehensive exploration through multiple types of interactions, and multiple analysis and reconstruction techniques", says co-spokesperson [[Bonnie Fleming]] of Yale. "They all tell us the same thing, and that gives us very high confidence in our results that we are not seeing a hint of a sterile neutrino."<ref name=CERN-2021-10-28/>
: 
: ...&nbsp;"eV-scale sterile neutrinos no longer appear to be experimentally motivated, and never solved any outstanding problems in the Standard Model", says theorist Mikhail Shaposhnikov of EPFL. "But GeV-to-keV-scale sterile neutrinos – so-called Majorana fermions – are well motivated theoretically and do not contradict any existing experiment."<ref name=CERN-2021-10-28>
{{cite news
 |first=Mark |last=Rayner 
 |title=MicroBooNE sees no hint of a sterile neutrino
 |date=28 October 2021
 |periodical=CERN Courier
 |url=https://cerncourier.com/a/microboone-sees-no-hint-of-a-sterile-neutrino/
 |access-date=2021-11-09
}}
</ref>
}}

{{anchor|weak_charge_anchor}}The quantum number ''[[weak charge]]'' ({{mvar|Q}}{{sub|{{sc|w}}}}) serves the same role in the neutral current interaction with the {{math| {{subatomic Particle|Z boson0}} }} that electric charge ({{mvar|Q}}, with no subscript) does in the [[electromagnetic interaction]]: It quantifies the vector part of the interaction. Its value is given by:<ref name=dzuba>
{{cite journal
 |first1=V. A. |last1=Dzuba         |first2=J. C. |last2=Berengut
 |first3=V. V. |last3=Flambaum      |first4=B.   |last4=Roberts
 |year=2012
 |title=Revisiting parity non-conservation in cesium
 |journal=Physical Review Letters
 |volume=109  |issue=20  |page=203003
 |doi=10.1103/PhysRevLett.109.203003  |arxiv=1207.5864
 |pmid=23215482  |bibcode=2012PhRvL.109t3003D  |s2cid=27741778
}}
</ref>
: <math> Q_\mathsf{w} = 2 \, T_3 - 4 \, Q \, \sin^2\theta_\mathsf{w} = 2 \, T_3 - Q + (1 - 4 \, \sin^2\theta_\mathsf{w}) \, Q ~.</math>
Since the [[weak mixing angle]] {{tmath|1= \theta_\mathsf{w} \approx 29^\circ }}, the parenthetic expression {{tmath|1= (1 - 4 \, \sin^2\theta_\mathsf{w}) \approx 0.060 }}, with its value [[Renormalization group|varying slightly with the momentum difference (called "''running''")]] between the particles involved. Hence 
: <math>\ Q_\mathsf{w} \approx 2 \ T_3 - Q = \sgn(Q)\ \big(1 - |Q|\big)\ ,</math>
since by convention {{tmath|1= \sgn T_3 \equiv \sgn Q }}, and for all fermions involved in the weak interaction {{tmath|1= T_3 = \pm\tfrac{1}{2} }}. The weak charge of charged leptons is then close to zero, so these mostly interact with the {{math|Z}}&nbsp;boson through the axial coupling.