UMICH-2015: Neutrino and Light Relativisic Species break-out session 1
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- Clear science target - A massless field in thermal equilibrium with the Standard model leads to ΔNeff > 0.027
- For thermal decoupling above 100 GeV
- a real scalar produces ΔNeff = 0.027
- a Weyl fermion produces ΔNeff = 0.047 (Dirac : 0.094)
- a vector field produces ΔNeff = 0.054
- With 106 detectors and fsky = 0.75, forecasts of σNeff =0.013 (see e.g. arXiv
- 1402.4108).
- Model independent theory motivation - Is this realistic experimentally?
- What are the trade-offs for other science goals to achieve this sensitivity
- What is missing from the forecast that could significantly reduce the sensitivity?
- More general science targets -- axion-like particles, late decays of massive fields (before or after BBN), decaying Dark matter
- How well can be break degeneracies?
- E.g. Want to separate NeffCMB from Yp
- Forecasts with both varying give σNeff = 0.048 and σYp = 0.0027 (see arXiv:1508.06342)
- In principle allows us to distinguish effects at recombination from changes to BBN
- What role does CMB lensing play?
- For Neff, delensing E-modes improves constraints significantly (sharpens peaks - improves phase shift measurement)
- Can it help break degeneracies in other models (e.g. decaying dark matter) ?
- Planck 2015 versus CMB Stage IV (2 different marginalizations)
