6 Neutralino Project Overview and Limits

The range and breadth of experimental work now underway world-wide makes it very difficult to maintain an up-to-date and complete catalogue of activities. Table 3 presents an overview of experiments currently going on, but is far from exhaustive.

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Table 3: Experiment Overview.

Experiment	Type	Target	Quenching	Mass
			factor	[kg]
Heidelberg/Moscow	I	Ge		2.88	165
HDMS	I	Ge	0.25	0.20	–
GENIUS	I	Ge		1000	–
TANDAR/USC/	I	Ge	–	1.033	831
PNL/Zaragoza	S	NaI		32
USC PNL Zaragoza	I	Ge
COSME/TWIN
Neuchatel Caltech	I	Ge
PSI
	S	NaI(Tl)	Na(0.3)	6	1500
UKDMC	S	Xe	I(0.08)	6	–
	S/I	Xe	Xe(0.2)	20	–
	S	NaI(Tl)	Ca(0.08)	115	30000
DAMA	S		F(0.12)	0.37	10
	S	Xe	Xe(0.65)	6.5	823
ELEGANT-V	S	NaI(Tl)		662	241630
ELEGANT-VI	S			8
Saclay	S	NaI
Amherst UCB	O	Mica	–	–
SIMPLE	O	Freon		1	0.190
(CERN Lisbon Paris)
Montreal Chalk River	O	F, Cl		1
Tokyo Dark Matter	P	LiF	1	0.168
Search
Milano	P			7
ROSEBUD	P	Sapphire		0.100
CRREST	P	Sapphire		0.262
CMDS	P/I	Ge		0.262
EDELWEISS	P/I	Ge		0.900
Orpheus	O	Sn		0.032
Salopard	O	Sn		0.100

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Figure 14, adapted from [23], shows the published upper limits on the nucleon-WIMP scattering cross-section for coherent and axial coupling respectively, as they were in 1996. In both cases, the best limits came from the large mass DAMA NaI experiments with the UKDMC NaI experiment close behind. For the coherent interactions the germanium results are comparable to the UKDMC NaI. Finally, it is intriguing that both the DAMA and UK NaI experiments have low-level signal effects that do not appear to be consistent with -ray backgrounds. The UKDMC experiment, using pulse-shape analysis, revealed a family of short-time-constant events [120], which are even faster than their neutron induced recoil events. The derived ‘recoil spectrum’ for these events falls with energy as expected for WIMPs (but the implied WIMP mass is rather high), and there is even some hint of an annual modulation. However, the effect has now been shown to be a spurious surface effect [125]. The DAMA experiment has an annual modulation signal, which has persisted in a consistent way through a complete rebuild of the experiment [18] and with data accumulated over a four year period [16]. From their data it is possible to delineate an allowed region in coherent cross-section parameter space that contains cosmologically interesting combinations of MSSM parameters, as shown in Figure 15.

Finally, Figure 16 shows the two most recent limits plots. The left hand plot comes from the CDMS collaboration [3], which runs a hybrid bolometer/ionisation experiment. The limit they have produced almost excludes completely the DAMA coherent cross-section result (shown as the solid grey region). However, CDMS operates in a shallow site and experiences an ambient neutron background. Their limit relies on the assumption that the circled events shown in Figure 12 are indeed due to ambient neutrons. The numbers are consistent with this and there is a cross-check using multi-site multiple neutron scattering events. However, with such small numbers the result cannot be decisive. Recently, the EDELWEISS experiment has produced an upper limit that is comparable to CDMS for high WIMP masses [14]. The technique is similar to CDMS, but has the advantage of no detectable neutron background as it is in a deeper site. Unfortunately, it has a higher energy threshold at present and work is ongoing to improve this. So we are left with a tantalising claim of a WIMP annual modulation signal from DAMA, which they have defended despite much concern over possibilities of spurious annual modulation effects, and an almost exclusive limit from CDMS, which is statistically arguable. The right-hand plot in Figure 16 shows a plot from the SIMPLE [38] experiment using superheated droplets. The target has non-zero spin; they present results in terms of limits to a pure spin-dependent cross-section. The plot shows a compilation of limits from all the other experiments that have reported to date. In the bottom left-hand corner of this plot can be seen the “tip of the iceberg” of MSSM predictions for spin-dependent cross-sections.

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 3 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].

The need for more sensitive and more powerful experiments is clear as we start to impinge more and more on the allowed neutralino parameter space and as experiments begin to reveal features at levels never before investigated. The need for multiple experiments to confirm the results of others is also clear.