This is a background post that includes experimental data current as of results from March 2011 from the DZero experiment. It looked for "flavour-changing neutral current decays of top quark pairs, where about 30,000 of the latter have been produced in something like 300 quadrillion 1.96 TeV proton-antiproton collisions" and came up empty handed. Instead, "top quarks decay almost exclusively into bottom quarks and W bosons" without forming mesons (two quark composite particles) or baryons (three quark composite particles).
The bottom line is that the W bosons, which are charged weak force carryiing particles (they come in charge +1 and charge -1 versions and are antiparticles of each other) can change flavor (e.g. when a charm quark emits a W boson, it could turn into a down quark). But, the roughly 1/8th heavier charge zero version of the W boson called the Z boson (sometimes written with a superscript zero) does not play a part in flavor changing interactions. The starting and ending point of Z boson interactions have the same flavor. If a charm quark emits a Z boson it will remain a Z boson. If a Z boson did bring about a flavor changing interaction we would call it a flavor changing neutral current (FCNC).
There are some speculative theories about why this doesn't happen, such as the GIM mechanism (an acronymn of the surnames of the theorists who came up with it and simultaneously predicted the charm quark). But, whatever the reason, we have been smashing atoms since we discovered the Z boson in 1983 and we still haven't caught Z bosons engaged in flavor changing interactions, 28 years later. There are some multi-step processes that involve W bosons that superficially look like a flavor changing Z boson interaction would, but the theory of electroweak interactions is exceedingly precise on this point, so our ability to separate this background from the signal of a FCNC, and if FCNC's exist they must be exceedingly rare.
In other words, the only thing that can turn a particle into another particle of a different flavor as it interacts with it is a W boson. Strong nuclear force interactions (via gluons), electromagnetic interactions (via photons) and gravity (via gravitons or the geometry of space-time) don't do this trick. Neither does the Z boson. Also, it is worth noting that neither the W boson nor the Z boson change strong nuclear force color charge. W bosons do change the charge of the particles that they interact with by a factor of one plus or minus, and do convert energy into rest mass, and convert rest mass into energy (although W boson interactions conserve combined mass-energy).
What does a Z boson do? It is emitted from or absorbed by a quark or lepton. And, it decays into a fermion and its antiparticle in very precise proportions, an exact formula for which neatly matches the experimental data, although these proportions are constrained by the amount of matter-energy the Z boson has to start with (a low energy Z boson can't create higher mass pairs when it decays). Thus, "a quark or a lepton emits or absorbs a neutral Z boson. For example: e => e +Z
Like the W boson, the Z boson also decays rapidly, for example [if it has enough mass-energy]:
Z => b + anti-b."
But, a muon cannot emit a Z boson and as a result become an electron. The emission or absorbsion of a Z boson does not change the flavor of the emitting or absorbing particle.
This is worked out with more rigor than a blog post can here.
In contrast, "W bosons can decay to a lepton and neutrino or to an up-type or down type quark." Up-type quarks always decay into down-type quarks via the W boson and down-type quarks always decay into up-type quarks via the W boson. Neutrino oscillation is also a W boson driven process in the Standard Model.
The lack of FCNCs turns out to be a big deal because a large class of theories that otherwise reproduce the Standard Model in places where it has been experimentally confirmed, and extend it in places where experimental data provided no data or suggests a possible deviation from the Standard Model, have flavor-changing neutral currents that aren't found in reality. This law of physics, like the observed absence of proton decay and the observed absence of magnetic monopoles, rules out many otherwise plausible theoretical models.
The experiments don't demand that the number of FCNC interactions be zero, but they are consistent with zero (as are proton decay rates and the number of observed magnetic monopoles which have never been observed in very large data sets), and it is tempting to assume that they are exactly zero rather than very small but not quite zero in frequency, without powerful experimental motivations from some other line of evidence for doing otherwise.
Weak force interaction data also rule out fourth or higher generation particles in which neutrinos have a mass less than half of the Z boson. If such particles existed, the weak force decay rules would predict that we would have seen some of them by now. This means that a fourth generation neutrino, if one exists, is on the order of one thousand times as heavy as a third generation neutrino (if not heavier).
In electroweak unification theory, a more fundamental neutrally charged W boson and a neutrally charged B boson linearly combine to give rise to the Z and the photon, according to a two by two matrix based on the sines and cosines of what is called the the mixing angle theta sub W, a feature of the theory that explains why the photon and Z have different masses than the W+ and W- bosons in the amount seen experimentally. The W and Z also subsume two of the three predicted Standard Model Higgs bosons, leaving only one of them bare of other particles.