Astroparticle Physics

228 10 Big Bang Nucleosynthesishad already found that there could be at most one additionalneutrino family [21] (see also Chap. 2). The interplay betweenthese two different determinations of N ν played animportant rôle in alerting particle physicists to the relevanceof cosmology. Although the example that has just been dis-cussed is for the number of neutrino families, the same argumentsapply to any particles that would contribute to g ∗ suchas to affect the neutron freeze-out temperature. Thus theabundances of light elements provide important constraintsfor any theory involving new particles that would contributesignificantly to the energy density during the BBN era.impact of cosmologyon particle physics10.8 Problems1. Equation (10.4) shows thattT 2 ≈ 0.74 s MeV 2 .How can this numerical result be derived from the previousequations?2. In the determination of the neutron-to-proton ratio thecross section for the (n ↔ p) conversion reactionν e + n → e − + pwas estimated from〈σ v〉≈G 2 F T 2 .How can the T 2 dependence of the cross section be motivated?3. The measurement of the total width Γ Z of the Z resonanceat LEP led to a precise determination of the numberof light neutrinos N ν . How can N ν be obtained fromthe Z width?The solution of this problem requires some intricate detailsof elementary particle physics.