Dr. Sonja Orrigo (IFIC Valencia)
The goal of several ongoing and future direct detection experiments is to discover the dark matter present in our galactic halo in the form of Weakly Interacting Massive Particles (WIMPs). These experiments attempt to isolate from various backgrounds the signal of nuclear recoils from the elastic scattering of WIMPs with the target nuclei inside the detector. The expected signal (for a given flux) depends on the WIMP mass, the WIMP-nucleon cross section and the nuclear form factor. The nuclear form factor critically determines the spectrum of the recoil nuclei. Therefore its precise determination and error estimation is crucial to establish the bounds on the WIMP-nucleon cross section from running experiments and to plan future ones. While the distribution of protons in nuclei, which determines the corresponding form factor, can be extracted precisely and, to a large extent, model independently, our present knowledge of neutron distributions is far more uncertain. The latter are particularly important in scenarios where the cross sections on protons and neutrons are different . We have performed a systematic global determination of nuclear form factors for nuclei ranging from 9Be to 209Bi using the available experimental information. We provide realistic (conservative) uncertainties for the parameters, estimating the systematic errors, and exploring possible correlations. In direct dark matter searches and related studies, it has been customary to describe the nuclear form factors using the Helm ansatz, which leads to an analytic expression for the form factor. On the other hand, the charge density distributions have been extracted from muon spectroscopy  using two-parameter Fermi (2PF) distributions. The widespread strategy to deal with this dichotomy  is to convert the 2PF parameters of Ref.  into Helm ones adopting an ad-hoc value for the nuclear thickness. We have improved this approach by fitting directly the measured nuclear radii and diffuseness [2,4]. Systematic errors related to the finite nucleon size, differences between electromagnetic and scalar form factors and the approximated compact expression adopted for the nuclear thickness are investigated. For the neutron distributions, we have used the experimental determination of the difference between neutron and proton radii using antiprotonic atoms . The systematic errors have the same sources as in the proton case plus the one from the two extreme “halo” and “skin” distributions of neutrons in nuclei. The larger errors in neutron distributions should get smaller in the future thanks to parity-violating electron scattering experiments such as PREX, CREX and QWEAK.  J. L. Feng, Ann. Rev. Astron. Astrpphys. 48 (2010) 495.  G. Fricke et al., At. Data Nucl. Data Tables 60 (1995) 177.  J. D. Lewin and P.F. Smith, Astrop. Phys. 6 (1996) 87.  H. de Vries et al., At. Data Nucl. Data Tables 36 (1987) 495.  A. Trzcinska et al., Phys. Rev. Lett. 87 (2001) 082501.