How does a quark emit a W boson?

Question

When a quark changes from up to down, or vice versa, how does it emit a W boson? Aren't the W and Z bosons much more massive?

Answer 1

Yes, the W boson is very massive, and what is actually emitted in neutron decay is a virtual W boson. In quantum mechanics, rules of conservation of energy can be violated for short periods of time. Virtual particles are common in subatomic interactions, if you don't see the word "virtual", note that physicists will sometimes refer to them as "particles off the mass shell".

Virtual particles are not just mathematical fictions. If a virtual W boson were to interact with another particle during its short existance, it could exchange enough energy/momentum with that particle to become a real boson itself (albeit one that will still decay in a short time because of the finite lifetime of real W bosons).

Virtual particles are absolutely necessary in particle physics in order to model the behaviour correctly. Even the attraction of positively and negatively charged objects is explained by the exchange of virtual photons.

Answer 2

Particle Physics does not match Standard Phyiscs so the conservation of mass may not apply.

According to Department of Physics, Case Western Reserve University, Cleveland, OH: http://www.citebase.org/cgi-bin/fulltext?format=application/pdf&identifier=oai:arXiv.org:gr-qc/0604072
"Similarly, in the context of any grand unified theory (GUT), baryon-number and lepton-number violating processes would be expected to be generic since the quarks and leptons of a given family would be members of the same representation of the GUT gauge group. The GUT gauge bosons would therefore mediate transformations between quarks and leptons, just as SU(2) gauge bosons mediate transformations between up and down type quarks or between charged leptons and their associated neutrinos. Thus, while neither electroweak instanton mediated processes nor GUT lepto-quark boson mediated processes have yet been observed, B and L violating processes, such as proton decay, are expected to occur at low energies, albeit with extremely low probability. Moreover, there is strong circumstantial evidence for B violation – the overwhelming predominance of baryons over anti-baryons in the universe. Non-conservation of baryon number is one of the three key ingredients in most models of baryogenesis [2]."

Or maybe it is because of the actions of the Y boson.

Answer 3

The Standard Model contains both fermionic and bosonic fundamental particles. Fermions are particles which possess half-integer spin and obey the Pauli exclusion principle, which states that no fermions can share the same quantum state. Bosons possess integer spin and do not obey the Pauli exclusion principle. Informally speaking, fermions are particles of matter and bosons are particles that transmit forces.