# UMICH-2015: CMB-Lensing break-out session 2

From CMB-S4 wiki

Blake - CMB delensing, CMB lensing cross correlated with galaxy surveys, CMB lensing cross correlated with optical weak lensing, parameter forecasts, instrument requirements, survey requirements, systematic challenges (20 min)

## Delensing Details

Blake, ~10 min, File:Delensing.pdf Delensing intro

- When is delensing necessary?
- Can external delensing contribute anything?
- With delensing, is it worth having different noise levels on two different experiments (high and low-resolution)?

Questions for discussion

- Algorithms - Quadratic Estimator / Iterative / Sampling
- What is the best algorithm to use?

- Systematics and Delensing
- Dust foregrounds and sources - how do they propagate through the delensing procedure? How much do they matter?
- Instrumental systematics, higher order biases, LSS non-Gaussianity - any concerns for delensing?

- Analysis of Delensed Maps
- What are the statistical and noise properties of the delensed maps? How to construct likelihoods?

## CMB Lensing Cross-correlations

Blake, ~10 min

- weak lensing calibration: cross-correlations with LSST / Euclid weak lensing
- how precise are the correlations?
- is this level of multiplicative bias calibration interesting?
- are intrinsic alignments a limiting factor?

- clustering from correlations with DESI/Euclid/LSST data
- how well can biases be modeled and constrained?
- can dark matter clustering be recovered sufficiently cleanly to, e.g., detect neutrino mass?
- what are the key science drivers in extragalactic astrophysics?

- other surveys
- would overlap with external datasets gain from a northern site?