Low-mass halo perturbations in strong gravitational lenses at redshift z ∼ 0.5 are consistent with CDM

We use a sample of 17 strong gravitational lens systems from the BELLS GALLERY survey to quantify the amount of low-mass dark matter haloes within the lensing galaxies and along their lines of sight, and to constrain the properties of dark matter. Based on a detection criterion of 10σ, we report no significant detection in any of the lenses. Using the sensitivity function at the 10σ level, we have calculated the predicted number of detectable cold dark matter (CDM) line-of-sight haloes to be μ_l = 1.17 ± 1.08, in agreement with our null detection. Assuming a detection sensitivity that improved to the level implied by a 5σ threshold, the expected number of detectable line-of-sight haloes rises to μ_l = 9.0 ± 3.0. Whilst the current data find zero detections at this sensitivity level (which has a probability of P^{5σ }_{CDM}(n_det=0) = 0.0001 and would be in strong tension with the CDM framework), we find that such a low-detection threshold leads to many spurious detections and non-detections and therefore the current lack of detections is unreliable and requires data with improved sensitivity. Combining this sample with a subsample of 11 SLACS lenses, we constrain the half-mode mass to be log (M_hm) < 12.26 at the 2σ level. The latter is consistent with resonantly produced sterile neutrino masses m_s < 0.8 keV at any value of the lepton asymmetry at the 2σ level.