Target dependence of the annual modulation in direct dark matter searches (1504.06772v2)

Abstract: Due to Earth's revolution around the Sun, the expected scattering rate in direct dark matter searches is annually modulated. This modulation is expected to differ between experiments when given as a function of recoil energy $E_\text{R}$, e.g. due to the gravitational focusing effect of the Sun. A better variable to compare results among experiments employing different targets is the minimum speed $v_\text{min}$ a dark matter particle must have to impart a recoil energy $E_\text{R}$ to a target nucleus. It is widely believed that the modulation expressed as a function of $v_\text{min}$ is common to all experiments, irrespective of the dark matter distribution. We point out that the annual modulation as a function of $v_\text{min}$, and in particular the times at which the rate is maximum and minimum, could be very different depending on the detector material. This would be an indication of a scattering cross section with non-factorizable velocity and target material dependence. Observing an annual modulation with at least two different target elements would be necessary to identify this type of cross section.