ELSEVIER Nuclear Physics B (Proc. Suppl.) 87 (2000) 67-69

PROCEEDINGS SUPPLEMENTS

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Dark Matter search with the DAMA experiments

R. BernabeP, P. Belli a, R. Cerulli a, F. Montecchia ~, M. Amato b, G. Ignesti ~, A. Incicchitti b, D. Prosperi b, C. J. Dai c, H.L. He r, H.H. Kuang r, J.M. Ma r

Dipartimento di Fisica, H Universitd di Roma and I.N.F.N. Sezione di Roma2, Italy; b Dipartimento di Fisica, Universith di Roma and I.N.F.N. Sezione di Roma, Italy. ~ IHEP, Chinese Academy, P.O. Box 918//3, Beijing 100039, China.

DAMA experiments are running deep underground in the Grail Sasso National Laboratory of I.N.F.N. Results on various rare event searches and, in particular, on WIMP annual modulation signature have been achieved.

1. E X P E R I M E N T A L A C T I V I T I E S

DAMA is devoted to searches for rare events by developing and using low radioactive scintil- lators; its main aim is the search for relic parti- cles embedded in the galactic halo mainly by in- vestigating the annual modulation of the WIMP "wind" which should continuously hit the Earth. The success of this kind of search depends on the possibility to build large mass detectors with high intrinsic radiopurity; they are needed both to sig- nificantly search for candidates with extremely re- duced rate (such as the neutralino) and to exploit peculiarities of the WIMP "wind" in order to un- ambiguously select a signal from the background. The relevance of performing experiments with a proper signature is clear.

In the following we will briefly summarize only few points related to the DAMA WIMP searches, carried out by: i) CaF2(Eu) detectors; ii) ~_ 2 1 liquid Xenon (LXe) pure scintillator; iii) ~_ 100 kg highly radiopure NaI(T1) set-up. A relevant part of the experimental DAMA activity regards the ~_ 100 kg NaI(T1) DAMA set-up. This set-up, its radiopurity and its main performances have been described in detail in ref. [1], while some comments related to radiopure NaI(T1) detectors can be found e.g. in ref. [1-4].

Recently a new RK:D to obtain very effective chemical and/or physical radiopurification of NaI and TlI powders has been carried out satisfacto- rily; on that basis, few weeks after this conference the construction of 16 (9.7 kg each) new higher

radiopure NaI(T1) detectors has been funded. Studies on possible future applications of

large mass highly radiopure NaI(T1) set-ups are also carried out [2,5]. Moreover, searches for /3/3, charge-non-conserving and Pauli-exclusion- principle-violating processes [6-9] are performed as well as other approaches to Dark Matter search [10].

2. E X C L U S I O N P L O T S

Significant exclusion plots on WIMP-proton elastic scattering have been achieved by exploit- ing also the pulse shape discrimination between recoils and electromagnetic background both in NaI(T1) [11] and in LXe [12]. Similar sensitivities have been reached in both cases. Quantitative investigations on possible systematic effects have been presented in both papers (see e.g. fig. 7 in ref. [11] and the text in ref. [12]) and dis- cussed in details at several conferences; further- more, some general considerations can be found e.g. in ref. [1,13]. Finally, new results also for spin-dependent coupled WIMP with CaF~(Eu) scintillators have been recently achieved [6].

3. A N N U A L M O D U L A T I O N S E A R C H

The main aim of the _ 100 kg NaI(T1) set- up is the investigation of the annual modulation of the counting rate induced by a possible relic WIMP contribution. This modulation due to the Earth's motion around the Sun is a good signa- ture, because it requires that all of the following

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68 R. Bernabei et al./Nuclear Physics B (Proc. Suppl.) 87 (2000) 67-69

Table 1 Summary of the present DAMA results on the annual modulation signature (see text) [14,15].

running period statistics (kg • d)

DAMA/NaI-I+2

DAMA/NaI-1 3363.8 winter + 1185.2 summer DAMA/NaI-2 14962 (1/2 november - ~ end of July)

19511

Mw (GeV) ~io (pb) C.L.

(7.0+~!~)10 -6 98.5% (7.0+~:~)10 -6 99.6%

specifications are satisfied: i) presence in the rate of a modulated part varying as a cosine function; ii) with proper period (1 year); iii) with proper phase (about 2 june); iv) only in a defined low en- ergy region; v) for single "hit" events in a multi- detector set-up (that is, events where only one detector is firing, being negligible the probability that a WIMP will interact in more than one); vi) with modulated amplitude in the region of max- imal sensitivity < 7% [1,3,14,15]. We take this opportunity to remark that much in advance of this conference not only the measured counting rate between 2 and 20 keV with 1 keV bin was reported [11,14,15], but also those: i) between 3.5 and 6 keV with 0.25 keV bins [9]; ii) between "" 1 and 10 keV with 0.25 keV bins [4,16,17]; iii) in higher energy regions (1 keV bins) [8]. We remark that our low energy distributions are always re- ferred to single hit events; therefore, low energy Compton events or X rays/Auger electrons cor- related with higher energy escaping gammas are vetoed by coincidences in the nearby crystals; this is obviously impossible when a single detector is used. The behaviour we observe is consistent with our present knowledge of the identified residual contaminants in the set-up, with the behaviour of the light response of our detectors for inter- nal keV radiation and with the 2-20 keV spec- tra induced in our detectors by external 241Am source and by Compton electrons [1,16]. Further, various specific considerations on the realization and/or comparison of radiopure NaI(T1) detec- tors have been addressed e.g. in ref. [1-3,13,16].

At time of this conference results obtained by analysing two data sets: Dama/NaI-1 and DAMA/NaI-2 (total statistics of 19511 kg.day) [14,15] were available. Deep investigations of all the stability parameters can be found in [1,3,4,14- 16,19]; in particular, in ref. [1] all the details are

1 0 " 4 . . . . t . . . . 1 . . . . I . . . . I . . . .

10-5 .El

~IO-B

I0-7 . . . I . , , , I . . . . I . ~ , . i . . . . ,

~o ~oo ~so 2oo 25o 3oo

r"lw [GeV)

Figure 1. Effect of the uncertainties in v0 and v~sc on the DAMA allowed region: the solid contour represents the region at 2~r C.L. for a SI coupled candidate with v0 = 220 km/s and vest = 650 km/s. The dashed line represents the 2a con- tour from the same data when varying v0 between 170 and 270 km/s and ves~ between 450 and 650 km/s[18]. The quantity ~ is the local neutralino matter density in units of 0.3 GeV/cm 3.

given for the various components of the overall efficiency, whose stability has been discussed at various conferences[4,16,19].

The results achieved so far by the DAMA inves- tigations of the annual modulation signature are summarized in Table 1; the quoted results use the values given in ref.[ll,14,15] for the relevant as- trophysical and nuclear physics quantities; in par- ticular, the local velocity is there: v0 = 220 km/s. In fig. 1 the effect of the uncertainties in v0 and in the escape velocity v~c on the DAMA allowed region is shown; it enlarges the allowed masses: from 30 GeV to 130 GeV (1 ~r C.L.). This is fur- ther extended from 30 GeV to 180 GeV (1 g C.L.) when including possible dark halo rotation [18]. Finally, to offer a visual aid, the model indepen- dent residual rate in the lowest energy interval as a function of the time elapsed since January, 1-st of the first year of data taking is shown in fig.

R. Bernabei et al./Nuclear Physics B (Proc. Suppl.) 87 (2000) 67-69 69

~, 0.1

~ o.o5

& 0

o 3

,'9_ -0 .05

-0.1 ;, I , a I 400 600

.~ - D~A/Nal-2 i - ~

i

i i i

i I I i I I I

800 1000

time (doys)

Figure 2. Model independent residual rate in the lowest energy interval as a function of the time (see text).

2. The expected behaviour of a WIMP signal is a cosine function with minimum roughly at the dashed vertical lines and with maximum roughly at the dotted ones. The complete time and energy correlation data analysis favours the presence of a similar modulation at 99.6% C.L. [4,16]. As it can be easily understood, our result is not in con- tradiction with any of the results obtained in the less sensitive and/or less controlled (as it can be inferred from the papers) data taking of ref. [20] as discussed in all details in [19,21]. The inter- pretation of our data in terms of relic neutralinos is discussed in details in ref. [22].

Finally, we remind that the DAMA/NaI-2 data have been recently analysed also in terms of rate diurnal variation with sidereal time [17]. This ef- fect would be induced by the Earth's daily rota- tion and would be appreciable in case of high cross section SI coupled candidates. This investigation has excluded - at the given C.L. - new regions for high cross section candidates not previously ex- plored; in particular, it has shown that the possi- ble signal pointed out by the annual modulation studies would account for a halo fraction > 10 -3 [17].

The careful investigation of the annual modu- lation effect over several cycles is in progress.

R E F E R E N C E S

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