UCSD-2019: Cross-Cut: Delensing

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General charge to all parallels:

We need to use the October collaboration meeting to advance our preparations for CD-1. Right away, we need to:

  • Identify key decisions that must be made (and justified) prior to CD-1,
  • Make progress on (or actually make) those decisions,
  • Lay out a timeline and process for making each decision, consistent with the post-decision work and internal reviews that will be needed to complete preparations for CD-1,
  • Ensure that those timelines and processes are understood and supported by the collaboration, and that we (together) believe we can follow them.

Specific charge to this delensing cross-cut session:

  • For the de-lensing LAT, what are the benefits and costs of variation in angular resolution (dish size) for legacy survey science goals?
  • What is minimum dish size / throughput needed for delensing? Would multiple smaller telescopes be better than one bigger one? (both r + Neff)
  • What are the effects of foregrounds on delensing (both for r and Neff)

Remote Attendance



1) On-going work related to delensing (50 min - 1hr):

2) What are some of the questions that need answering for moving CMB-S4 to CD-1 readiness in delensing (1hr):

  • How map non-idealities impact delensing efficiency File:File.pdf (Toshiya Namikawa)
  • Discussion on instrumental systematics: need inputs from all! (Kimmy Wu)
  • What inputs are needed in order to address session charges discussion


  • Coulton: Delensing for fNL
    • Q: why is bias from using internal delensing smaller in bispectrum vs in power spectrum?
    • When using f(NL) = 0 sims, no bias; now there is ~10% bias
    • Q: Have you considered foregrounds?
  • Toshiya: Anisotropic window + inhomogeneous noise effects
    • How much do you have to worry about missing modes in the delensing LAT?
    • Can S4 use the wide area LAT/SO LAT to fill out the delensing LAT missing modes due to scan patterns from the South Pole? We should not rely on external experiments for our mission critical tasks.
    • Lots of back and forth about how much these effects affect S4.
  • Marius: forward foreground modeling
    • Q: is this linear? Get all orders in phi, lensing operation is linear
    • Incorporate foregrounds in Bayesian framework first pass with poisson sources 1-halo term modeling
    • Q: does this work better than bias-hardening? Probably, if model is accurate. ES: in bias-hardening; you subtract the form of the point sources no matter what and incur a noise penalty though a more optimal step is to weight the bias and the increased variance. Here it's likely more optimal.
    • Q: How well do you have to know the statistics of the foregrounds? Need to be able to get the model correctly
    • Q: How to use this to constraint number counts for low-flux limits? have to choose dN/dS model in this procedure, or can sample dN/dS.
    • Q: How do you know the sims are right? The same question that everyone is trying to answer
    • Q: How does this bridge to more standard analysis? Can remove low ell B modes in input map, and generate phi instead of sample parameters.
  • Delensing for Neff - Selim
    • N_eff vs Yp degeneracy breaking when delensing is incorporated
    • TE gives the best constraints for N_eff
    • Q:Maybe gradient inversion is better for the intermediate scales?
    • When Yp not fixed to BBN consistency, sigma(N_eff) degrades to ~0.15?
    • How much, without marginalizing over Yp, does delensing help in sigma(N_eff)? Not very much. With Y_p, get ~30% improvement
    • This is an update to Joel’s forecast in which the iterative code to include all the other estimators (previously it's EB only).
  • Discussion regarding the session charges:
    • Do we weight the cluster/transient science goals the same as r science goal when designing the delensing LAT? Yes. So we have to do a joint optimization of sky/beam/freq bands for all three goals.
    • The sky coverage is likely driven by the SAT survey. The beam size is likely driven by cluster/transient needs. Frequency distribution needs more study from both the delensing side and the cluster side.
    • What is minimum dish size / throughput needed for delensing? See Fig 68 in DSR. Do we need more detailed studies?
    • Bonus question from Tom Crawford: "how much do you gain in sigma(r) by matching the degree-scale weight map vs. uniformly covering the full nonzero-weight region?”
    • Foreground impacts are discussed through the presentations.