Experimental Searches for Dark Matter

Abstract

There is now an enormously rich variety of experimental techniques being brought to bear on experimental searches for dark matter, covering a wide range of suggested forms for it. The existence of "dark matter", in some form or other, is inferred from a number of relatively simple observations and the problem has been known for over half a century. To explain "dark matter" is one of the foremost challenges today - the answer will be of fundamental importance to cosmologists, astrophysicists, particle physicists, and general relativists. In this article, I will give a brief review of the observational evidence (concentrating on areas of current significant activity), followed by anequally brief summary of candidate solutions for the 'dark matter'. I will then discuss experimental searches, both direct and indirect. Finally, I will offer prospects for the future.

Figure 1

Allowable parameter spaces for Ω_Λ, Ω_k, ω_b, ω_d, f_ν, τ and r. The figure is taken from [143] and the dashed lines mark the 95% confidence limits.

Figure 2

The left-hand panel from [143] shows the joint constraints on the baryonic matter and dark matter densities, together with the allowed band of baryonic density from BBN models. The right-hand panel from [42] shows the joint constraints on Ω_λ and Ω_m which result from combined use of CMB data and high-redshift supernova data.

Figure 3

Possible scale-lengths where different types of dark matter might be present, based on a similar representation which appeared in [ 30 ].

Figure 4

High-resolution N-body simulation of a galactic dark matter halo [ 90 ].

Figure 5

The rotation curve of the Milky Way. In the left-hand panel are the measured rotation speeds given by the average values from a number of measurements on different objects [ 50 ]. The right hand panel shows the various mass components that combine together to reproduce the observed curve between 5 and 25 kpc [ 72 ]. The dotted lines are the bulge and disk contributions, and the short-dashed curve is the dark matter contribution. The solid curve shows the combined effect of all three, and this is compared to the long-dashed curve which approximates the measured data in the left-hand panel below 25 kpc.

Figure 6

Total neutralino elastic scattering cross-section normalised to one nucleon for a range of neutralino models within MSSM and mSUGRA, taken from [ 79 ]. The pink area corresponds to a neutralino in a dominantly bino state, the green bounded area is dominantly higgsino. The cross-section includes both spin-independent and spin-dependent contributions, and in general the spindependent part is likely to be larger.

Figure 7

Background energy spectra for two Ge detectors of the PNL/USC/Zaragoza group taken from [ 28 ] (a — upper panel). Coherent crosssection upper limits from Ge detectors taken from [ 1 ] (b — lower panel).

Figure 8

Background rate from 428.1 days of data binned in 10-minute intervals and folded to look for daily modulation [ 1 ] (a — upper panel). Results of an annual modulation search using ^∼ 4 years of data from the DAMA experiment [ 16 ] (lower panel).

Figure 9

Differential pulse shapes from NaI for various radiation types. There is a clear difference between the functional form for high dE/dx interactions, such as nuclear recoil and alpha tracks, and γ-ray induced electron tracks.

Figure 10

Differential time constant distributions from the UK NaI experiment [106, 118] showing the measured background (solid line + data points), and neutron and γ-ray calibration distributions.

Figure 11

The two-phase xenon test chamber used by Wang [ 35 ].

Figure 12

Relative signal amplitudes for γ-rays and neutrons for two hybrid type experiments. On the left is shown the primary to secondary scintillation signal amplitudes for a two-phase xenon instrument [ 35 ]. Neutrons have large S1/S2 ratios. The right hand panel shows the ionisation versus phonon performance of the CDMS germanium bolometer [ 3 ]. γ-rays populate the plot above the solid line with nuclear recoils below it. The circled points are experimental data thought to originate from neutron recoils.

Figure 13

Relative figures of merit for the discrimination potential in NaI, cooled NaI and two-phase liquid xenon [ 117 ]. In this plot a lower figure of merit implies proportionately better performance.

Figure 14

Latest published upper limits on (a — upper panel) coherent and (b — lower panel) axial coupled WIMP-nucleon cross-sections adapted from [ 23 ].

Figure 15

The upper panel shows the region of coherent cross-section parameter space consistent with the DAMA NaI annual modulation results [16]. The four curves show the results from each individual year of the four year period shown in figure 8. The lower panel shows a scatter plot of possible MSSM models which populate the region defined by the first two years of data from [22]. Open circles are cosmologically interesting.

Figure 16

The upper panel shows the current results on the allowed coherent cross-section parameter space. The plot is from [ 14 ] and shows the CDMS 3σ upper limit [ 3 ] (dotted purple curve), the DAMA annual modulation positive detection region (blue solid curve), the DAMA upper limit from pulse shape discrimination (blue dot-dash curve), the EDELWEISS limits [ 14 ] (red curves) and the current limits from all combined germanium ionisation detectors [ 70 ] (dashed green curve). The lower panel shows an equivalent plot for the axial spin-dependent coupling cross-section. This is a composite plot produced by the SIMPLE collaboration in announcing their latest result [ 38 ].

Figure 17

Expected progress in covering MSSM parameter space from both indirect and direct search techniques over the next several years [ 49 ].