x-cut Cosmic Shear: Optimally Removing Sensitivity to Baryonic and
  Nonlinear Physics with an Application to the Dark Energy Survey Year 1 Shear
  Data
  We present a new method, called $x$-cut cosmic shear, which optimally removes
sensitivity to poorly modeled scales from the two-point cosmic shear signal. We
show that the $x$-cut cosmic shear covariance matrix can be computed from the
correlation function covariance matrix in a few minutes, enabling a likelihood
analysis at virtually no additional computational cost. Further we show how to
generalize $x$-cut cosmic shear to galaxy-galaxy lensing. Performing an $x$-cut
cosmic shear analysis of the Dark Energy Survey Year 1 (DESY1) shear data, we
reduce the error on $S_8= \sigma_8 (\Omega_m / 0.3) ^ {0.5}$ by $32 \%$
relative to a correlation function analysis with the same priors and angular
scale cut criterion, while showing our constraints are robust to different
baryonic feedback models. Largely driven by information at small angular
scales, our result, $S_8= 0.734 \pm 0.026$, yields a $2.6 \sigma$ tension with
the Planck Legacy analysis of the cosmic microwave background. As well as
alleviating baryonic modelling uncertainties, our method can be used to
optimally constrain a large number of theories of modified gravity where
computational limitations make it infeasible to model the power spectrum down
to extremely small scales. The key parts of our code are made publicly
available.
