Flavor changing neutral current

Flavor changing neutral currents (English for flavor changing neutral currents ), short FCNC are a phenomenon of physics , in which quark or leptons of different generations of the same electric charge by emission of an electrically neutral Eichbosons can interconvert. FCNC do not exist as a fundamental interaction in the Standard Model of elementary particle physics ; they only appear in higher-order diagrams and are therefore strongly suppressed. Some theories beyond the Standard Model predict new interactions that allow fundamental FCNC.

Standard model

In the standard model there are three electrically neutral gauge bosons: the photon as a mediator of quantum electrodynamics , the gluon for quantum chromodynamics and the Z boson as one of the three bosons of the weak interaction . Photon and gluon couple to the mass eigenstates of the particles and do not lead to any change in the flavor. The Z boson, on the other hand, couples to the eigenstates of the weak interaction. These are a superposition of the intrinsic mass states, so that in principle a flavor change appears possible. The reason why this does not occur is due to the glass show Iliopolus Maiani mechanism . In 1970 Glashow, Iliopolus and Maiani postulated the existence of a fourth quark, the charm quark, so that the neutral current of the Z boson is diagonal both in the space of the eigenstates of mass and in the space of the eigenstates of the weak interaction. Let the mass eigenstates of the Down or Strange quark and the eigenstates of the weak interaction be: ${\displaystyle d,s}$ ${\displaystyle d_{C},s_{C}}$ ${\displaystyle {\bar {d}}d+{\bar {s}}s={\bar {d}}_{C}d_{C}+{\bar {s}}_{C}s_{C}}$

In higher orders, FCNC are not prohibited. These are induced by a W loop, as the charged currents of the W bosons in the weak interaction always lead to a change in flavor and can change the quark generation due to the mixture of states. Due to the higher order, these processes in the quark sector are already suppressed by the factor ( is the fine structure constant ). In the leptonic sector, since the mass eigenstates of the charged leptons are also the eigenstates of the weak interaction, a neutrino oscillation must occur. ${\displaystyle \alpha ^{2}\approx 1/137^{2}}$ ${\displaystyle \alpha }$

Experimental search

The Mu3e experiment is currently planned at the Paul Scherrer Institute , which will investigate the occurrence of FCNC and determine its probability. The decay channel that the Mu3e experiment is looking for is one in which an antimuon net transforms into a positron with the emission of an electron-positron pair. In the Standard Model, the predicted branching ratio of this muon decay is less than 10 ⁻¹² ; such a process only occurs with less than a trillion decays. ${\displaystyle \mu ^{+}\to e^{+}\gamma ^{*}\!\!/\!Z^{*}\to e^{+}e^{+}e^{-}}$

Individual evidence

1. ↑ cf. z. B. Kaori Fuyuto, Wei-Shu Hou and Masaya Kohda: Z 'induced FCNC decays of top, beauty and strange quarks . In: Phys Rev D . tape 93 , no. 5 , 2016, p. 054021-1-054021-19 , doi : 10.1103 / PhysRevD.93.054021 (English).
2. ↑ Ian JR Aitchison and Anthony JG Hey: Gauge Theories in Particle Physics . 2nd Edition. Institute of Physics Publishing, Bristol 1989, ISBN 0-85274-329-7 , pp. 366-371 (English).
3. ^ The Mu3e Experiment. Retrieved March 8, 2019 . 
4. ↑ PDG booklet. (PDF) Retrieved March 8, 2019 . 

literature

• T. Morii, CS Lim and SN Mukherjee: The Physics of the Standard Model and Beyond . World Scientific Publishing, Singapore 2004, ISBN 981-02-4571-8 , pp. 215-247 (English).