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 
Real scalar - ΔNeff = 0.027
Weyl fermion - ΔNeff = 0.047 (Dirac - 0.094)
Vector field - ΔNeff = 0.054

Neffmin.png

(figure from arXiv:1303.5379)

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

PhaseShift.png

Forecasts for CMB Stage IV
Planck 2015 versus forecasts with 2 different marginalizations (see arXiv:1508.06342)

Neff.png