Editing Weak interaction (section)

== Interaction types ==
There are two types of weak interaction (called ''[[Feynman diagram|vertices]]''). The first type is called the "[[charged-current interaction]]" because the ''weakly interacting'' fermions form a [[Mathematical formulation of the Standard Model#The charged and neutral current couplings and Fermi theory|current]] with total ''electric'' charge that is nonzero.  The second type is called the "[[neutral-current interaction]]" because the ''weakly interacting'' fermions form a [[Mathematical formulation of the Standard Model#The charged and neutral current couplings and Fermi theory|current]] with total ''electric'' charge of zero. It is responsible for the (rare) deflection of [[neutrino]]s. The two types of interaction follow different [[selection rule]]s. This naming convention is often misunderstood to label the electric charge of the [[W and Z boson|{{math|W}} and {{math|Z}} boson]]s, however the naming convention predates the concept of the mediator bosons, and clearly (at least in name) labels the charge of the current (formed from the fermions), not necessarily the bosons.{{efn|The exchange of a virtual {{math|W}} boson can be equally well thought of as (say) the emission of a {{math|W}}{{sup|+}} or the absorption of a {{math|W}}{{sup|−}}; that is, for time on the vertical co‑ordinate axis, as a {{math|W}}{{sup|+}} from left to right, or equivalently as a {{math|W}}{{sup|−}} from right to left.}}

=== Charged-current interaction ===
{{main|Charged current}}
[[File:Beta Negative Decay.svg|thumb|right|200px|The [[Feynman diagram]] for beta-minus decay of a [[neutron]] ({{math|n {{=}} udd}}) into a [[proton]] ({{math|p {{=}} udu}}), [[electron]] ({{math|e{{sup|−}}}}), and [[neutrino|electron anti-neutrino]] {{math|{{overline|ν}}{{sub|e}}}}, via a charged vector boson ({{math|{{SubatomicParticle|W boson-}}}}). ]]
In one type of charged current interaction, a charged [[lepton]] (such as an [[electron]] or a [[muon]], having a charge of −1) can absorb a [[W boson|{{math|{{SubatomicParticle|W boson+}}}}&nbsp;boson]] (a particle with a charge of +1) and be thereby converted into a corresponding [[neutrino]] (with a charge of 0), where the type ("flavour") of neutrino (electron {{math|ν{{sub|e}}}}, muon {{math|ν{{sub|μ}}}}, or tau {{math|ν{{sub|τ}}}}) is the same as the type of lepton in the interaction, for example:
: <math> \mu^- + \mathrm{W}^+ \to \nu_\mu </math>
Similarly, a down-type [[quark]] ({{math|d}}, {{math|s}}, or {{math|b}}, with a charge of {{sfrac|−| 1 |3}}) can be converted into an up-type quark ({{math|u}}, {{math|c}}, or {{math|t}}, with a charge of {{sfrac|+| 2 |3}}), by emitting a {{SubatomicParticle|W boson-}}&nbsp;boson or by absorbing a {{math|{{SubatomicParticle|W boson+}}}}&nbsp;boson. More precisely, the down-type quark becomes a [[quantum superposition]] of up-type quarks: that is to say, it has a possibility of becoming any one of the three up-type quarks, with the probabilities given in the [[CKM matrix]] tables. Conversely, an up-type quark can emit a {{math|{{SubatomicParticle|W boson+}}}}&nbsp;boson, or absorb a {{math|{{SubatomicParticle|W boson-}}}}&nbsp;boson, and thereby be converted into a down-type quark, for example:
: <math>\begin{align}
                 \mathrm{d} &\to \mathrm{u} + \mathrm{W}^- \\
  \mathrm{d} + \mathrm{W}^+ &\to \mathrm{u} \\
                 \mathrm{c} &\to \mathrm{s} + \mathrm{W}^+ \\
  \mathrm{c} + \mathrm{W}^- &\to \mathrm{s}
\end{align}</math>

The W&nbsp;boson is unstable so will rapidly decay, with a very short lifetime. For example:
: <math>\begin{align}
  \mathrm{W}^- &\to \mathrm{e}^- + \bar\nu_\mathrm{e} ~ \\
  \mathrm{W}^+ &\to \mathrm{e}^+ + \nu_\mathrm{e} ~
\end{align}</math>

Decay of a W&nbsp;boson to other products can happen, with varying probabilities.<ref name="PDG2">{{cite journal |author1=Nakamura, K. |display-authors=etal |collaboration=[[Particle Data Group]] |year=2010 |title=Gauge and Higgs Bosons |journal=[[Journal of Physics G]] |volume=37 |issue=7A |page=075021 |doi = 10.1088/0954-3899/37/7a/075021 |bibcode = 2010JPhG...37g5021N |url=http://pdg.lbl.gov/2010/tables/rpp2010-sum-gauge-higgs-bosons.pdf}}</ref>

In the so-called [[beta decay]] of a neutron (see picture, above), a down quark within the neutron emits a [[Virtual particle|virtual]] {{math|{{SubatomicParticle|W boson-}}}} boson and is thereby converted into an up quark, converting the neutron into a proton. Because of the limited energy involved in the process (i.e., the mass difference between the down quark and the up quark), the virtual {{math|{{SubatomicParticle|W boson-}}}} boson can only carry sufficient energy to produce an electron and an electron-antineutrino – the two lowest-possible masses among its prospective decay products.<ref name=PDG3>
{{cite journal
 |author1=Nakamura, K.
 |display-authors=etal
 |collaboration=[[Particle Data Group]]
 |year=2010
 |title={{math| {{SubatomicParticle|neutron}} }}
 |journal=[[Journal of Physics G]]
 |volume=37  |issue=7A  |page=7
 |doi=10.1088/0954-3899/37/7a/075021
 |bibcode=2010JPhG...37g5021N
 |url=http://pdg.lbl.gov/2010/listings/rpp2010-list-n.pdf
}}
</ref>
At the quark level, the process can be represented as:
: <math> \mathrm{d} \to \mathrm{u} + \mathrm{e}^- + \bar\nu_\mathrm{e} ~</math>

=== 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.