Analysing the energy spectrum
The energy of each event is measured over a period of months, and the energy spectrum is obtained by plotting the number of events as a function of energy (blue data points). Note that particles with the same mass will lead to events with a range of energies depending on the details of the collision between the particle and the nucleus. Moreover, the event energy is always much less than the WIMP mass: it is typically less than about 60 keV, while the WIMP mass must be at least 34 GeV c2. The expected energy spectrum is a product of the underlying interaction rate, an exponential function that depends on the WIMP mass, and the resolution and threshold of the detector. To determine the upper limit on the interaction rate for a given mass, the rate is increased until the predicted spectrum coincides with the observed one. This provides one data point on a plot like figure 2. The mass is then changed and the process in repeated. The spectrum for low-mass WIMPs (green line) falls off faster than that for high-mass WIMPs (red line), so different upper limits are set on the interaction rate for each mass.