Searching for cavities of various densities in the Earth’s crust with a low-energy ν̄e β-beam

We propose searching for deep underground cavities of different densities in the Earth’s crust using a long-baseline νe disappearance experiment, realized through a low-energy β-beam with highly-enhanced luminosity. We focus on four cases: cavities with densities close to that of water, iron-banded formations, heavier mineral deposits, and regions of abnormal charge accumulation that have been posited to appear prior to the occurrence of an intense earthquake. The sensitivity to identify cavities attains confidence levels (C.L.s) higher than 3σ and 5σ for exposure times of three months and 1.5 years, respectively, and cavity densities below 1 g cm−3 or above 5 g cm−3, with widths greater than 200 km. We reconstruct the cavity density, width, and position, assuming one of them known while keeping the other two free. We obtain large allowed regions that improve as the cavity density differs more from the Earth’s mean density. Furthermore, we demonstrate that the knowledge of the cavity density is important to obtain O(10%) error on the width. Finally, we introduce an observable to quantify the presence of a cavity by changing the orientation of the νe beam, with which we are able to identify the presence of a cavity at the 2σ to 5σ C.L.