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	~~Alyson~~ is ~~what my~~ [~~http~~:/~~/modenpeople~~.~~co.kr/modn/qna/292291 cheap psychic readings~~] ~~spouse loves~~ to ~~call me but I don~~'~~t like when people use my full title~~. ~~Ohio~~ [http://~~bigpolis~~.~~com~~/~~blogs/post/6503 telephone psychic~~] is ~~exactly where my house is~~ but ~~my husband wants us~~ to ~~transfer~~. ~~What me and my family members love~~ is to ~~climb~~ but ~~I'm thinking on beginning some thing new. Credit authorising is how he makes cash~~.<br><br>~~Also visit my web site:~~ [~~http~~:~~//www~~.~~herandkingscounty~~.~~com/content/information~~-and-~~facts~~-~~you~~-~~must~~-~~know~~-~~about-hobbies authentic psychic readings~~]		{{technical\|date=July 2012}}
			{{Refimprove\|date=September 2009}}
			[[Image:Tau-decay-fcnc.png\|thumb\|right\|
			''Above'': Highly suppressed [[tau (particle)\|tau]] [[particle decay\|decay]] via '''flavor-changing neutral current''' at one-loop order in the [[Standard Model]].<br>
			''Below'': [[Beyond the Standard Model]] tau decay via flavor-changing neutral current mediated by a new ''S'' boson.]]

			In [[theoretical physics]], '''flavor-changing neutral currents''' (FCNCs) are hypothetical expressions that change the [[flavor (particle physics)\|flavor]] of a [[fermion]] current without altering its [[electric charge]]. If they occur in nature (as reflected by [[Lagrangian]] interaction terms), these processes may induce phenomena that have not yet been observed in experiment. Flavor-changing neutral currents may occur in the [[Standard Model]] beyond the [[Feynman diagram\|tree level]], but they are highly suppressed by the [[GIM mechanism]].

			FCNCs are generically predicted by theories that attempt to go beyond the Standard Model, such as the models of [[supersymmetry]] or [[technicolor (physics)\|technicolor]]. Their suppression is necessary for an agreement with observations, making FCNCs important in model-building.

			==Example==

			Consider a [[toy model]] in which an undiscovered [[boson]] ''S'' may couple both to the [[electron]] as well as the [[tau (particle)\|tau]] via the term
			:<math>S\bar\psi_e\psi_\tau</math>
			Since the electron and the tau have equal charges, the electric charge of ''S'' clearly must vanish to respect the conservation of electric charge. A [[Feynman diagram]] with ''S'' as the intermediate particle is able to convert a tau into an electron (plus some neutral decay products of the ''S''). The [[MEG experiment]]<ref>[http://meg.web.psi.ch MEG experiment website]</ref> at the [[Paul Scherrer Institute]] near [[Zurich]] will search for a similar process, in which an [[muon\|antimuon]] decays to a [[photon]] and an [[positron\|antielectron]]. In the Standard Model, such a process proceeds only by emission and re-absorption of a charged [[W boson]], which changes the tau into a [[neutrino]] and then an electron, emitting a [[photon]] to [[conservation of energy\|conserve energy]] and [[conservation of momentum\|momentum]].

			In most cases of interest, the boson involved is not a new boson ''S'' but the [[Z boson]] itself.<ref>FCNCs involving the [[photon]] cannot occur at zero [[momentum transfer]]s because of the unbroken electromagnetic [[gauge symmetry]]; as such, FCNCs involving the photon at a non-zero momentum transfer are relatively suppressed compared to FCNCs involving the Z boson.</ref> This can occur if the coupling to weak neutral currents is (slightly) non-universal. The dominant universal coupling to the Z boson does not change flavor, but sub-dominant non-universal contributions can.

			FCNCs involving the Z boson for the down-type [[quark]]s at zero momentum transfer are usually parameterized by the [[effective action]] term
			:<math>\frac{g}{2\cos \theta_W}\overline{d}_{L\alpha}U_{\alpha\beta}\gamma^\mu d_{L\beta}Z_\mu</math>

			[[Image:FCNC.svg\|frame\|left\|An example of a hypothetical (i.e., not yet observed) flavor-changing neutral current process in the [[Minimal Supersymmetric Standard Model]]. A [[strange quark]] emits a [[Gaugino\|bino]], turning into a sdown-type [[squark]], which then emits a Z boson and reabsorbs the bino, turning into a down quark. If the MSSM squark masses are flavor-violating, such a process can occur.]]
			This particular example of FCNC is often studied the most because we have some fairly strong constraints coming from the decay of [[B meson\|B<sup>0</sup> mesons]] in [[Belle experiment\|Belle]] and [[BaBar]]. The off-diagonal entries of ''U'' parameterizes the FCNCs and current constraints restrict them to be less than one part in a thousand for \|''U''<sub>bs</sub>\|. The contribution coming from the one-loop SM corrections are actually dominant, but the experiments are precise enough to measure slight deviations from the SM prediction.

			Experiments tend to focus on flavor-changing neutral currents as opposed to [[flavor-changing charged current]]s, because the [[weak neutral current]] (Z boson) does not change flavor in the Standard Model proper at the tree level whereas the weak charged currents ([[W boson]]s) do. New physics in charged current events would be swamped by more numerous W boson interactions; new physics in the neutral current would not be masked by a large effect due to ordinary Standard Model physics.

			==References==
			{{reflist}}

			[[Category:Particle physics]]

en>Omnipaedista at 08:13, 20 September 2011

2011-09-20T08:13:39Z

New page

Alyson is what my [http://modenpeople.co.kr/modn/qna/292291 cheap psychic readings] spouse loves to call me but I don't like when people use my full title. Ohio [http://bigpolis.com/blogs/post/6503 telephone psychic] is exactly where my house is but my husband wants us to transfer. What me and my family members love is to climb but I'm thinking on beginning some thing new. Credit authorising is how he makes cash.<br><br>Also visit my web site: [http://www.herandkingscounty.com/content/information-and-facts-you-must-know-about-hobbies authentic psychic readings]

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en>Addbot: Bot: Migrating 1 interwiki links, now provided by Wikidata on d:q3700189

en>Omnipaedista at 08:13, 20 September 2011