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 ΔN_eff > 0.027

For thermal decoupling above 100 GeV 

a real scalar produces ΔN_eff = 0.027

a Weyl fermion produces ΔN_eff = 0.047 (Dirac : 0.094)

a vector field produces ΔN_eff = 0.054

With 10⁶ detectors and f_sky = 0.75, forecasts of σ_Neff =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, during or after BBN), decaying Dark matter

How well can be break degeneracies?

E.g. Want to separate N_eff^CMB from Y_p

Forecasts with both varying give σ_Neff = 0.048 and σ_Yp = 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 N_eff, 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) ?

Is it a good direct probe of BSM physics (as in the case of neutrino masses)

Forecasts for CMB Stage IV

Planck 2015 versus forecasts with 2 different marginalizations (see arXiv:1508.06342)