LBNL-2016: Systematic Errors and Forecasting for Inflation and Lensing

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Inputs to Inflation Forecasting Machinery

It is necessary that the forecasts for CMB-S4 science reach should be actually borne out in the final results.

To ensure that this is the case here is a proposal for the features of "S4 compliant projections".

1) Avoid ab initio calculations which start from per detector NET, number of detectors and nominal run time. Instead use as input actual N_l noise spectra taken from existing experiments (derived from full season Q/U maps), and apply simple scaling for relative numbers of detectors and integration time. This automatically builds in all "real world" inefficiencies. (Although one might still be concerned about correlated noise causing failure of N_det scaling.) This was at some level agreed at the Ann Arbor meeting.

2) Assume that the ultimate proof that an apparent signal is not systematic in origin will come from the data itself - unknown systematics should be assumed to be as large as null tests can prove them not to be - i.e. the noise uncertainty. We may wish to consider stronger criteria which push towards high signal to noise in the map - because systematics are often "obvious" when viewed in a map with s/n>=1 per mode. This "systematics penalty" should be built into the forecasting machinery as an adjustable constraint so that its effect on the instrument/survey design can be probed.

3) All projections of course need to include realistic foreground removal (critical for inflation projections).

We would like multiple sets of forecasts so we can check and cross-compare. But they should all respect the above requirements before being taken seriously.

Systematics Figure from BICEP paper

Notes on parallel (compiled by TC)

  • Even N_l is not adequate to describe achieved noise (Borrill)
    • Could use multiple map-level noise realizations (Kovac)
  • Need to know how low you can go in ell from a given platform (Bond)
    • Limited by atmosphere or systematics? (Pryke)
  • Do we need two different kinds of forecasting: "proven" and "desired," both clearly labeled? (Kovac)
    • Most importantly, need to avoid apples-to-oranges comparisons between forecasts. (Pryke)
  • Need at least a baseline grid of platform configurations. (Hlozek)
  • Consensus: We need to agree on the inputs to forecasting.
  • What about foreground bias to de-lensing. (Baccigalupi & others)
  • Consider forecasts for r=0 and r=some fiducial value and/or adaptive observing strategy? (Kovac)

Lensing systematics and forecasting

This session will discuss the impact of systematic errors (as well as complications for forecasting) from instrument, modeling, or atmosphere.

Instrumental systematics and their impact on lensing measurements (Meng Su): 10+5

LSS non-Gaussianity and higher order lensing biases (M. Schmittfull): 5+5

Lensing Systematics - Plenary Summary