Why is the W cross section so much larger than the Z cross section?

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SUMMARY

The production cross sections for W and Z bosons at hadron colliders demonstrate a significant difference, with the W cross section being approximately 10 times larger than that of the Z boson. This discrepancy is attributed to the coupling strengths and the mass difference, which is around 10%. According to the referenced paper, the ratio of the cross sections is σW/σZ ≈ 3.33, derived from the equation σW/σZ ~ |Vqq'|²/(vq² + aq²), where Vqq' is the CKM matrix element (approximately 0.97), and aq and vq represent the neutral current couplings. The discussion emphasizes that the observed values may reflect cross sections multiplied by branching fractions rather than raw cross sections alone.

PREREQUISITES
  • Understanding of particle physics, specifically W and Z bosons
  • Familiarity with hadron colliders and their operational principles
  • Knowledge of the CKM matrix and its significance in particle interactions
  • Basic grasp of cross sections and branching fractions in particle physics
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  • Research the role of the CKM matrix in particle physics and its implications for W and Z boson production
  • Study the derivation and significance of cross sections in high-energy physics experiments
  • Explore the concept of branching fractions and their impact on particle decay processes
  • Investigate the differences in coupling strengths between various particles in the Standard Model
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Particle physicists, researchers in high-energy physics, and students studying the interactions of fundamental particles will benefit from this discussion, particularly those focusing on W and Z boson production mechanisms.

JoePhysicsNut
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I had a look at the production cross sections for W/Z at hadron colliders. These differ as a function of energy with the W x-sec being consistently ~10 times larger than the Z cross sections. Why is the W cross section so much larger? I think the coupling strength is similar and the mass difference is not that large either (~10%).
 
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I don't fully understand the reasoning, but this paper gives the ratio as σW/σZ ≈ 3.33, and this book derives (Eq. 85) the ratio as

σW/σZ ~ |Vqq'|2/(vq2 + aq2)

where Vqq' ≈ 0.97 is the CKM matrix element and aq = 1/2 and vq ≈ -0.3 are the neutral current vector and axial vector couplings.

Which works out about right.
 
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What you are probably looking at are not cross-sections but cross-sections times branching fractions.
 

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