Neutral current

<h2 id="in-simple-terms">In simple terms</h2>
The weak force is best known for its role in nuclear decay. It has very short range but (apart from gravity) is the only force to interact with <a href="/facts/Neutrino/yGWVfSg2">neutrinos</a>. Like other subatomic forces, the weak force is mediated via exchange particles. Perhaps the most well known of the exchange particles for the weak force is the <a href="/facts/W_boson/gAncY5XV">W particle</a> which is involved in <a href="/facts/Beta_decay/vxTBlckp">beta decay</a>. W particles have <a href="/facts/Electric_charge/xxsBfmlB">electric charge</a> – there are both positive and negative W particles – however the Z boson is also an exchange particle for the weak force but does not have any electrical charge.
Exchange of a Z boson transfers <a href="/facts/Momentum/TTkGv5eY">momentum</a>, <a href="/facts/Spin_(physics)/4Jvp7e8P">spin</a>, and <a href="/facts/Energy/wd8PrQM7">energy</a>, but leaves the interacting particles' quantum numbers unaffected – charge, <a href="/facts/Flavor_(physics)/RR7rsuTD">flavor</a>, <a href="/facts/Baryon_number/58MYGC3n">baryon number</a>, <a href="/facts/Lepton_number/vvocnLqR">lepton number</a>, etc. Because there is no transfer of electrical charge involved, exchange of Z particles is referred to as "neutral" in the phrase "neutral current". However the word "current" here has nothing to do with electricity – it simply refers to the exchange of the Z particle.<a class="footnote-ref" id="fnref:1" href="#fn:1">1</a>
The Z boson's neutral current interaction is determined by a derived quantum number called <a href="/facts/Weak_charge/m6uBSpnE">weak charge</a>, which acts similarly to <a href="/facts/Weak_isospin/uxsGzgFH">weak isospin</a> for interactions with the W bosons.

<h2 id="definition">Definition</h2>
The neutral current that gives the interaction its name is that of the interacting particles.
For example, the neutral current contribution to the νee− → νee− elastic <a href="/facts/Scattering_amplitude/TmdeAs1o">scattering amplitude</a> is

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    {\displaystyle {\mathfrak {M}}^{\mathsf {NC}}~\propto ~J_{\mu }^{\mathsf {(NC)}}(\nu _{\mathrm {e} })\;J^{{\mathsf {(NC)}}\ \mu }(\mathrm {e^{-}} )\ ,}

where the neutral currents describing the flow of the <a href="/facts/Neutrino/yGWVfSg2">neutrino</a> and of the electron are given by:<a class="footnote-ref" id="fnref:2" href="#fn:2">2</a>

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    {\displaystyle J^{{\mathsf {(NC)}}\ \mu }(f)={\bar {u}}_{f}\ \gamma ^{\mu }\ {\frac {1}{2}}\left(g_{\mathsf {V}}^{f}-g_{\mathsf {A}}^{f}\ \gamma ^{5}\right)\ u_{f}\ ,}

where:<a class="footnote-ref" id="fnref:3" href="#fn:3">3</a>

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    {\displaystyle g_{\mathsf {V}}^{f}=T_{3}(f)-2\sin ^{2}\theta _{\mathsf {W}}\ Q(f)={\frac {1}{2}}\ Q_{\mathsf {W}}(f)}

and 
 
 
 
  
 
 g
 
 
 A
 
 
 
 f
 
 
 =
 
 T
 
 3
 
 
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 f
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 {\displaystyle \ g_{\mathsf {A}}^{f}=T_{3}(f)\ }
 
 are the <a href="/facts/Chirality_(physics)/H8Xbcvzt">vector and axial couplings</a> for <a href="/facts/Fermion/LEckdDsR">fermion</a> 
 
 
 
  
 f
  
 .
 
 
 {\displaystyle \ f~.}
 
 
 
 
 
  
 
 T
 
 3
 
 
  
 
 
 {\displaystyle \ T_{3}\ }
 
 denotes the <a href="/facts/Weak_isospin/uxsGzgFH">weak isospin</a> of the fermions, Q their electric charge and 
 
 
 
  
 
 Q
 
 
 W
 
 
 
  
 
 
 {\displaystyle \ Q_{\mathsf {W}}\ }
 
 their <a href="/facts/Weak_charge/m6uBSpnE">weak charge</a>. These <a href="/facts/W_and_Z_bosons/gAncY5XV">couplings amount to essentially left chiral for neutrinos and axial for charged leptons</a>.
The Z boson can couple to any Standard Model particle, except <a href="/facts/Gluon/t6YH8I21">gluons</a> and <a href="/facts/Photon/mFNhAEzA">photons</a> (<a href="/facts/Sterile_neutrinos/5604ofcP">sterile neutrinos</a> would also be an exception, if they were found to exist). However, any interaction between two charged particles that can occur via the exchange of a virtual Z boson can also occur via the exchange of a virtual <a href="/facts/Photon/mFNhAEzA">photon</a>. Unless the interacting particles have energies on the order of the Z boson mass (91 GeV) or higher, the virtual Z boson exchange has an effect of a tiny correction, 
 
 
 
  
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 {\displaystyle \ (E/M_{\mathrm {Z} })^{2}\ ,}
 
 to the amplitude of the electromagnetic process.
Particle accelerators with energies necessary to observe neutral current interactions and to measure the mass of Z boson weren't available until 1983.
On the other hand, Z boson interactions involving <a href="/facts/Neutrino/yGWVfSg2">neutrinos</a> have distinctive signatures: They provide the only known mechanism for <a href="/facts/Elastic_scattering/51u9WlE1">elastic scattering</a> of neutrinos in matter; neutrinos are almost as likely to scatter elastically (via Z boson exchange) as inelastically (via <a href="/facts/W_boson/gAncY5XV">W boson</a> exchange), of major experimental significance, in, e.g. , the <a href="/facts/Sudbury_Neutrino_Observatory/jEqnTGaW">Sudbury Neutrino Observatory</a> experiment.
Weak neutral currents were predicted by <a href="/facts/Electroweak_theory/QATz4dYF">electroweak theory</a> developed mainly by <a href="/facts/Abdus_Salam/M4GNh8dE">Abdus Salam</a>, <a href="/facts/John_Clive_Ward/ENSr5UAQ">John Clive Ward</a>, <a href="/facts/Sheldon_Glashow/KNS7qjog">Sheldon Glashow</a> and <a href="/facts/Steven_Weinberg/G61D6kJw">Steven Weinberg</a>,<a class="footnote-ref" id="fnref:4" href="#fn:4">4</a> and confirmed shortly thereafter in 1973, in a neutrino experiment in the <a href="/facts/Gargamelle/YzC4UGPB">Gargamelle</a> <a href="/facts/Bubble_chamber/awVVhk4G">bubble chamber</a> at <a href="/facts/CERN/y1gwRchm">CERN</a>.

<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Charged_current/uJAQqxoj">Charged current</a></li>
<li><a href="/facts/Flavor_changing_neutral_current/F17nW4BD">Flavor changing neutral current</a></li>
<li><a href="/facts/Neutral_particle_oscillation/1Sz0IxF4">Neutral particle oscillation</a></li>
<li><a href="/facts/Electric_current/Dc1xdnUX">Electric current</a></li>
<li><a href="/facts/Quantum_chromodynamics/3cA2ctyb">Quantum chromodynamics</a></li>
<li><a href="/facts/Sudbury_Neutrino_Observatory/jEqnTGaW">Sudbury Neutrino Observatory#Neutral current interaction</a></li>
<li><a href="/facts/Weak_charge/m6uBSpnE">Weak charge</a></li></ul>

<h2 id="external-links">External links</h2>
<ul><li><a href="http://cerncourier.com/cws/article/cern/29168">"Discovery of weak neutral currents"</a>. CERN Courier. 3 October 2004.</li>
<li><a href="https://web.archive.org/web/20110827031552/http://public.web.cern.ch/public/en/Research/Gargamelle-en.html">"Gargamelle"</a>. CERN public web. Research. Archived from <a href="http://public.web.cern.ch/public/en/Research/Gargamelle-en.html">the original</a> on 2011-08-27. Retrieved 2011-08-27.</li>
<li>"Neutral current interaction". <a href="http://www.britannica.com/EBchecked/topic/410842/neutral-current-interaction">Britannica</a>.</li>
<li>Nave, R. <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/particles/neucur.html">"Neutral current"</a>. GSU.</li>
<li>Nieves, J.; Valverde, M.; Vicente Vacas, M.J. (2006). <a href="https://web.archive.org/web/20120121101618/http://www.actaphys.uj.edu.pl/vol37/pdf/v37p2295.pdf">"Charged and neutral current neutrino induced nucleon emission reactions"</a> (PDF). Acta Physica Polonica B. 37 (8): 2295–2301. <a href="/facts/ArXiv_(identifier)/H6EtgnBe">arXiv</a>:<a href="https://arxiv.org/abs/hep-ph/0605221">hep-ph/0605221</a>. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2006AcPPB..37.2295N">2006AcPPB..37.2295N</a>. Archived from <a href="http://www.actaphys.uj.edu.pl/vol37/pdf/v37p2295.pdf">the original</a> (PDF) on 21 January 2012.</li>
<li><a href="http://www.symmetrymagazine.org/breaking/2009/07/07/gargamelle/">"Gargamelle"</a>. Symmetry Magazine. 2009-07-07.</li>
<li>Fraser, Gordon (3 November 1998). <a href="http://cerncourier.com/cws/article/cern/27904">"Twenty-five years of neutral currents"</a>. CERN Courier. 27904. Gordon Fraser looks back at how confirmation of the existence of neutral currents ushered in a new understanding of physics.</li>
<li>Fenkart, Sanje (3 July 2023). <a href="https://cerncourier.com/a/cerns-neutrino-odyssey/">"CERN's neutrino odyssey"</a>. CERN Courier. Sanje Fenkart reounts the discovery of neutral currents in its 50 years anniversary</li>
<li>Padilla, Antonio (Tony). <a href="/facts/Brady_Haran/7oNb0Tez">Brady Haran</a> (ed.). <a href="http://www.sixtysymbols.com/videos/neutral_currents.htm">"Gargamelle and neutral currents"</a>. Sixty Symbols. <a href="/facts/University_of_Nottingham/PR8osn19">University of Nottingham</a>.</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">Nave, R. "Neutral current". GSU. <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/particles/neucur.html" target="_blank">http://hyperphysics.phy-astr.gsu.edu/hbase/particles/neucur.html</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">"Lecture 11 - Weak Interactions" (PDF). Particle Physics (course notes). University of Edinburgh. p. 7. Retrieved May 20, 2021. <a href="https://www2.ph.ed.ac.uk/~playfer/PPlect11.pdf" target="_blank">https://www2.ph.ed.ac.uk/~playfer/PPlect11.pdf</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">"Lecture 11 - Weak Interactions" (PDF). Particle Physics (course notes). University of Edinburgh. p. 7. Retrieved May 20, 2021. <a href="https://www2.ph.ed.ac.uk/~playfer/PPlect11.pdf" target="_blank">https://www2.ph.ed.ac.uk/~playfer/PPlect11.pdf</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">"The Nobel Prize in Physics 1979". Nobel Foundation. Retrieved 2008-09-10. <a href="http://www.nobel.se/physics/laureates/1979" target="_blank">http://www.nobel.se/physics/laureates/1979</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
</ol>

Neutral current open-in-new

Neutral current