We examine the uncertainties in the spin-independent and -dependent elastic scattering cross sections of supersymmetric dark matter particles on protons and neutrons, particularly those that arise due to uncertainties in quark masses and hadronic matrix elements that are related to the xnucleon a term and the spin content of the nucleon. By far the largest single uncertainty is that in spin-independent scattering induced by our ignorance of the (MOW) matrix elements linked to the r-nucleon a term, which affects the ratio of cross sections on proton and neutron targets as well as their absolute values. This uncertainty is already impacting the interpretations of experimental searches for cold dark matter. We plead for an experimental campaign to determine better the Jrnucleon a term. Uncertainties in the spin content of the proton affect significantly, but less strongly,the calculation of rates used in indirect searches.
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