Why Does Z Boson Only Decay to Fermion-Antifermion Pairs?

In summary, the Z boson only decays to fermion-antifermion pairs because quarks and leptons occur in SU(2) doublets and the CKM matrix is unitary, which suppresses Flavor Changing Neutral Currents (FCNC) through the GIM Mechanism. This is because the Z is flavor-diagonal and only the charged weak interaction violates flavor. At tree-level, the Z-boson couplings to quark-anti-quark are flavor-diagonal even after rotating to their mass eigenstate basis. However, at the one-loop level, the Z boson could potentially have flavor-changing decays to a strange + anti-bottom, but no such decay has been observed yet.
  • #1
copernicus1
99
0
Why does the Z boson only decay to fermion-antifermion pairs? I'd just like to understand the basic reason why something like Z --> anti-down, strange wouldn't work. This would conserve charge. It obviously wouldn't conserve strangeness, but the weak interaction doesn't, so I'm just wondering why this decay is forbidden.

Thanks!
 
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  • #2
In a nutshell, because quarks and leptons occur in SU(2) doublets and because the CKM matrix is unitary. Key words to google are FCNC (Flavor Changing Neutral Currents) and the GIM Mechanism which suppresses them.

For a simple explicit demonstration that they exactly cancel in tree order, see pp 144-146 here.
 
  • #3
Because the Z is flavor-diagonal. Only the charged weak interaction violates flavor, because that links a quark to a different antiquark. The Z (and the photon) link it to the same antiquark.
 
  • #4
Indeed at tree-level the Z-boson couplings to quark-anti-quark are flavour-diagonal even after you rotate the quarks to their mass eigenstate basis. This is because the neutral Z must couple to two down-type quarks or two up-type quarks and the combined unitary rotations cancel. The charged W boson couples to one up-type quark and one down-type quark, the combination of the two unitary rotations of up and down quarks results in the CKM matrix.

Now I know that the process Bs --> muon + anti-muon, recently discovered at the LHC, comes mostly from a Z penguin with a W in the loop. This is a rare flavour changing process. Does this mean that at the one-loop level that the Z boson should also have flavour-changing decays to a strange + anti-bottom?
 
  • #5
jkp said:
Does this mean that at the one-loop level that the Z boson should also have flavour-changing decays to a strange + anti-bottom?
I would expect those, but according to the http://pdglive.lbl.gov/Rsummary.brl?nodein=S044&inscript=Y&fsizein=1&wholedec0=Y , no such decay was found yet.
 
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1. Why does the Z boson only decay to fermion-antifermion pairs?

The Z boson is a neutral particle and follows the law of conservation of energy and momentum. Therefore, it can only decay into particles with equal mass and opposite charge, which are fermion-antifermion pairs.

2. What is the significance of the Z boson decaying into fermion-antifermion pairs?

The decay of the Z boson into fermion-antifermion pairs is an important process in particle physics as it helps in understanding the nature of the weak interaction. It also provides evidence for the existence of the Higgs boson and helps in determining its mass.

3. Can the Z boson decay into other particles besides fermion-antifermion pairs?

Yes, the Z boson can also decay into other particles such as neutrinos, which are not fermions. However, this is a very rare process and occurs only in certain circumstances, such as during the decay of the top quark.

4. How is the decay of the Z boson into fermion-antifermion pairs observed in experiments?

In experiments, the decay of the Z boson into fermion-antifermion pairs can be observed by detecting the particles produced in the decay and measuring their properties such as mass, charge, and energy. This information is then used to confirm the decay process and study its properties.

5. Are there any other particles that can only decay into fermion-antifermion pairs?

Yes, the W boson, which is also a carrier particle of the weak interaction, can also only decay into fermion-antifermion pairs. This is because the W boson and the Z boson are both bosons and follow the same laws of conservation in their decays.

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