Volume 168B, number 1,2 PHYSICS LETTERS 27 February 1986 ATOMIC ENHANCEMENTS IN THE DETECTION OF WEAKLY INTERACTING PARTICLES Savas DIMOPOULOS ~. Glenn D. STARKMAN : t'hrsws Department, Stanford Untt'ersit)', Stanford. ('A 94305, USA and Bryan W. LY'N'N ~ Stanford l, mear Accelerator Center. Theory Group, Stanford. CA 94305. USA Received 23 November 1985 Proposed bolometric and supercolloidal detectors can measure energy depositions of the order of atomic energies. At these energies, atomic bound state effects lead to great enhancements in the detection of weakly interacting particles. As an example, we show that solar axions could give event rates 10~-106 times larger than published neutrino detector design capabilities. Thus, relatively small detectors nught see solar axions. Introduction. There are arguments in both theo- retical elementary particle physics and astrophysics for the proliferation of neutral weakly interacting particles. On the theoretical side, gauge theories sug- gest the existence of many new particles: neutrinos, axions [ 1] ~t, etc. Experimentally, astronomical ob- servations suggest that the mass of the universe is dom- inated by dark matter which might be made of neu- tral weakly interacting particles. Because these parti- cles tend to interact so weakly, we must rely on some enhancement mechanisms to detect them. For exam- ple, one suggested enhancement mechanism for de- tecting neutrinos relies on their coherent nuclear scat- tering [3-7] ; this works well for neutrinos if the mo- mentum transfers involved are less than 100 MeV. Until now, detectors for weakly interacting parti- cles, such as neutrinos, have been limited by high minimum energy deposition thresholds. However, re- i Work supported by National Science Foundation Grant PHY 83-10654. A.P. Sloan Foundation Fellow. 2 National Science and Engineering Research Council (Canada).Postgraduate Scholar. Work supported by DOE contract DE-AC03-76SFO0515. ,1 Other axion detectors have been proposed in ref. [2]. cent progress in experimental techniques [4,6] has made feasible the measurement of energy depositions as small as atomic energies using supercolloidal and bolometric detectors. In this paper, we show that, at these energies, atomic bound state effects lead to great enhancements (~106) in the detection rates of weakly interacting particles. As a simple example we work out the ionization of atoms by axions. In ref. [8], we consider similar enhancements for neutrino detection. Atomic enhancements are quite familiar. A well- known example is the photoelectric effect, shown in fig. 1. The photoelectric cross section per unit mass of, for example, silicon is 234 times larger than that of hydrogen around I keV photon energies. This is because silicon has electrons with binding energies of 1 keV; similar enhancements occur in any atom with keV electron binding energies. Enhancements similar to those in the photoelectric effect occur for the ionization of atoms by absorption of axions. We call this process, depicted in fig. 2, the axioelectric effect or axionization. We expect such ef- fects to be large for solar axions because their energy is comparable to atomic energies. We show that the 145