ELSEVIER Nuclear Physics B (Proc. Suppl.) 91 (2001) 361-367 SU LmENTS ~w~.elsevi~.~l/locate/npe

Searching for signals from the Dark Universe by DAMA at Gran Sasso R. Bernabei a, P. BellP, R. CerullP, F. Montecchia ~, M. Amato b, G. Ignesti b, A. Incicchitti b, D. Prosperi b, C. J. Dai c, H.L. He c, H.H. Kuang c, J.M. Ma c

Dipartimento di Fisica, II Universitk di Roma and I.N.F.N. Sezione di Roma2, Italy;

b Dipartimento di Fisica, Universit& di Roma and I.N.F.N. Sezione di Roma, Italy.

IHEP, Chinese Academy, P.O. Box 918/3, Beijing 100039, China.

The DAMA set-ups are running deep underground in the Gran Sasso National Laboratory of the I.N.F.N.. Results on various rare event searches have been achieved. In particular, the results obtained in the investigation of the WIMP annual modulation signature obtained in four annual cycles (total statistics: 57986 kg • day) are summarized.

1. S H O R T O U T L O O K O F T H E D A M A E X P E R I M E N T A L A C T I V I T I E S

DAMA is dedicated to searches for rare events by developing and using low radioactive scintilla- tors; its main aim is the search for relic particles embedded in the galactic halo.

The DAMA WIMP searches have been car- ried out so far by means of the: i) _~ 100 kg highly radiopure NaI(T1) set-up[l]; ii) ~ 2 1 liq- uid Xenon pure scintillator [2]; iii) CaF2 (Eu) low background detectors[3]. Studies on possible fu- ture applications of large mass highly radiopure NaI(T1) set-ups have been also carried out [4].

Finally, several results on other rare processes have been obtained by analysing the distributions measured by the experiments in the whole en- ergy spectrum or by performing dedicated mea- surements in the installation named "R&D" [5].

According to the results obtained by a new dedicated R&D exploiting chemical/physical ra- diopurification procedures for selected powders, we are in progress to increase the exposed mass up to _~ 250 kg.

In the following, only the results achieved so far

in the investigation of the WIMP annual modula- tion signature by means of the _ 100 kg NaI(T1) set-up will be shortly summarized.

2. G E N E R A L I T I E S

Due to the known intrinsic uncertainties in the comparison among the results obtained by differ- ent experiments (mainly when different target- nuclei and/or different techniques are used), it is necessary to realize experiments offering their own effective distinctive signature. For this pur- pose we investigate the so-called annual modula- tion signature, which is based on the annual mod- ulation of the WIMP rate induced by the Earth's motion around the Sun [1,6]. This signature is quite strong since it requires the satisfaction of all the following specifications: i) presence in the rate of a modulated part varying as a cosine func- tion; ii) with proper period (1 year); iii) with proper phase (about 2 june); iv) only in a well- defined low energy region; v) for those events in which only one detector of many actually "fires", since the probability of a WIMP multiple scatter- ing is negligible; vi) with modulation amplitude in the region of maximal sensitivity < 7%. To fake this signature possible systematics must also satisfy all these six requirements; therefore, for

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362 R. Bernabei et aI./NucIear Physics B (Proc. Suppl.) 91 (2001) 361-367

some other effect to mimic it is highly unlikely. The detailed description of the ~ 100 kg

NaI(T1) setup, of its radiopurity and of its per- formances has been given in Ref. [1]. Here we only recall that the detectors used in the annual modulation studies are nine 9.70 kg NaI(T1) scin- tillators especially built for this purpose. The bare NaI(T1) crystals are encapsulated in suit- ably radiopure Cu housings; 10 cm long Tetrasil- B light guides act as optical windows on the two end faces of the crystals and are coupled to EMI9265-B53/FL photomultipliers (PMT). The two PMTs work in coincidence and collect light at single photoelectron threshold, while the software energy threshold has been cautiously taken at 2 keV [1]. The detectors are inside a low radioactive sealed copper box installed in the center of a low radioactive Cu/Pb/Cd- foils/polyethylene/paraffin shield. The copper box is maintained in a high purity (HP) Nitrogen atmosphere in slightly overpressure with respect to the external environment. Furthermore, also the whole shield is sealed and maintained in the HP Nitrogen atmosphere. The installation is air conditioned. On the top of the shield a glove-box (also maintained in the HP Nitrogen atmosphere) is directly connected to the inner Cu box, housing the detectors, through Cu pipes. The pipes are filled with low radioactive Cu bars, which can be removed to allow the insertion of source holders for calibrating the detectors in the same running condition, without any contact with external air.

The only data treatment, which is performed on the raw data, is to eliminate obvious noise events (sharply decreasing with the increase of the number of available photoelectrons) present below _ 10 keV [1]. The noise in our experiment is given by PMT fast single photoelectrons with decay times of order of tens ns, while the "physi- cal" (scintillation) pulses have decay times of or- der of hundreds ns. The large difference in de- cay times, the good performances of the electronic chain and the relatively large number of available photoelectrons (5.5-7.5 photoel./keV, depending on the detector) allows an effective noise rejection [1]. As mentioned in ref. [1] several variables can be built by using the pulse information recorded over 3250 ns by a Transient Digitizer. In partic-

ular, for each considered energy bin, we plot the r ~_ A r e a ( f r o m 0 ns f;o 50 ns)

A r e a ( f r o m 0 ns to 100 ns) value versus the X A r e a ( f r o m 100 ns to 600 ns)

: A r e a ( f r o m o n s ~o 600 ns) value calculated for each event. In this X, Y plane the slow scintilla- tion pulses are grouped roughly around (X _ 0.7, Y _ 0.5) well separated from the noise popula- tion which is grouped around small X and high Y values (see ref. [1]). To select the scintillation pulses an acceptance window in X, Y is applied. Since the statistical spread of the two populations in the X, Y plane becomes larger when decreas- ing the number of available photoelectrons and also the S/N decreases, smaller acceptance win- dows become necessary to maintain the same full noise rejection power (as necessary also e.g. for correct pulse shape discrimination, PSD, analy- ses). According to standard procedures, the win- dow acceptance for scintillation pulses is deter- mined by applying the same window to the scin- tillation data induced - in the same energy bin - by an external source of suitable strength [1]. En- ergy spectra in various energy regions have been published [1,5]. We recall here only that our low energy spectra refers always to single hit events (that is each detector had all the others in the same installation as veto).

3. RESULTS ON T H E I N V E S T I G A T I O N F O R T H E W I M P A N N U A L M O D U - LATION S I G N A T U R E

Results on the data collected during four an- nual cycles (see Table 11 ) have been released so far [1].

An immediate evidence of the presence of an- nual modulation in the lowest energy experimen- tal rate during the considered four annual cycles is given by the single hit events residual rate in the cumulative 2-6 keV energy interval as a func- tion of the time (see fig. 1) [1]; this is completely model independent. In fact, the X 2 test on the data of fig. 1 disfavours the hypothesis of unmod- ulated behaviour giving a probability of 4 .10 -4

1The DAMA/NaI-0 running period is also listed there. Its data were analysed in terms of PSD, obtaining upper limit on recoils [1], which has been accounted in the particular model dependent search for a candidate performed in ref. [1].

R. Bernabei et al./Nuclear Physics B (Proc. Suppl.) 91 (2001) 361-367 3 6 3

Table 1 Released data sets. [1]

period statistics (kgday)

DAMA/NAI-1 4549

DAMA/NaI-2 14962

DAMA/NaI-3 22455

DAMA/NaI-4 16020

Total statistics 57986

+ DAMA/NaI-0 limits on recoils by PSD

~ 0.1

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u

-~ O

m

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i DA~ Nal-I ~"

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, ; r i 1 1 1000

DA MA/:: " DAMM:: Nal-3 : ' ~ Nai-4 ~ "

1

, ~ , , ' , I ,

1500

t ime (doys )

Figure 1. Model independent residual rate for single hit events, in the 2-6 keV cumulative en- ergy interval, as a function of the time elapsed since January 1-st of the first year of data tak- ing. 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 [1].

( x 2 / d . o . f . = 48/20), while fitting these residuals with the function A- cos[w • (t - to)] (integrated in each of the considered time bin), one gets T = 2~ = (1.00 ± 0.01) year, when fixing to at 152.5 days and to = (144 4- 13) days, when fixing T at 1 year. Similar results, but with larger errors, are

found in case all the three parameters are kept free. As it is evident the period and the phase fully agree with the ones expected for a WIMP induced effect. This model independent analy- sis gives evidence for the possible presence of a WIMP signal independently from the nature of the WIMP and from its interaction type with or- dinary matter. This is further supported by the detailed investigation carried out on possible sys- tematics able to mimic such a signature.

The investigation on possible systematic effects has been discussed in refl [1,7]. Here we only summarize some of the main arguments related to the two periods more recently released.

Fig. 2 offer the general behaviours with time of several parameters acquired with the produc- tion. In particular, sizeable temperature varia- tions could only induce a light output variation, which is negligible considering: i) that around our operating temperature, the average slope of the light output is % -0.2%/°C; ii) the energy res- olution of these detectors in the keV range; iii) the role of the intrinsic and routine calibrations [1]. Moreover, a time correlation analysis of the temperature data gives a modulation amplitude (considering the same period and phase as for WIMPs) compatible with zero: (0.021 ± 0.046) °C and (0.064 ± 0.058) °C for DAMA/NaI-3 and DAMA/NaI-4 respectively. Therefore, a temper- ature effect can be excluded.

Furthermore, although our detectors are ex- cluded from environmental air, we examined the behaviour of the external Radon level with time; the fitted cosine annual modulation amplitude with the WIMP expected phase results (0.14 d: 0.25) Bq/m 3 and (0.12 + 0.20) Bq/m 3, for the two periods respectively; they are consistent with zero. Therefore, a Radon effect can be excluded firstly because our detectors are maintained in HP Nitrogen atmosphere, then because the exter- nal Radon itself does not show any modulation. Moreover, in any case, a modulation induced by Radon would fail some of the six requirements of the annual modulation signature, inducing e.g. modulation also in other energy regions than the one of interest for WIMP induced recoils, which has not been observed in the data.

The distribution of the total hardware rate of

364 R. Bernabei et al./Nuclear Physics B (Proc. Suppl.) 91 (2001) 361-367

40

30

20

tO

0

2O

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tlme (de,)}

" I . . . . I . . . . I . . . . I"

; . . . . !. :.: ,.1 .... I .... I .... I~ 300 400 500 600

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-4

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,,I,,, I,,, I,,, 300 400 500 600

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o o I o . . . . oo oo oo .... i: !J;I o o H ' ' . . . .

700 800 go0 700 800 go0 tlrne (day] tlme [da V]

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,", I ,'.", " . . . . ,I . . . . I, _4-,,t . . . . I . . . . I, 700 800 go0 700 800 gO0

t:lme {day) tzrne (day)

Figure 2. Behaviour of monitored parameters during the DAMA/NaL3 (a) and DAMA/NaI-4 (b) running periods. Here T is the operating temperature, R n is the environmental Radon level from which the detectors are excluded, Flux identifies the HP N2 flux in the inner Cu box maintained in slight overpressure and RH is the total hardware rate (that is the rate of the OR of the nine detectors) above a single photoelectron. The time scale here starts from January, 1 st 1997.

the nine detectors (RH) above a single photoelec- tron threshold, which is largely determined by the noise, has also been investigated; it shows a Gaus- sian behaviour with a = 0.6% for DAMA/NaI-3 and a = 0.4% for DAMA/NaI-4, values which are in agreement with those expected on the ba- sis of simple statistical arguments. By fitting the time behaviour of RH with time considering both data periods and including a WIMP-like modu- lated term, we obtain absence of modulation and, therefore, an upper limit on the relative modula- tion amplitude of the noise has been derived to be % 2 • 10 -3 (90% C.L.). As an example, even in a worst purely hypothetical case of a 10% contam- ination of residual noise - after filtering - in the counting rate, the noise contribution to the mod- ulation amplitude in the lowest energy bins would be < 1% of the observed modulation amplitude.

The stability of the measured energy distribu-

tion in energy regions not involved in the Dark Matter direct detection, during the data taking, has been investigated. It allows to exclude that a modulation detected in the lowest energy region could be ascribed to a background modulation. In fact, the background in the lowest energy re- gion is expected to be essentially due to "Comp- ton" electrons, X-rays a~d/or Auger electrons, muon induced events, etc., which are strictly cor- related with the events in the higher energy part of the spectrum; therefore, if a detected modula- tion with time in the lowest energy region would be due to a variation of the background (instead of the possible signal) with time, an equal or higher (sometimes much higher) variation in the higher energy region should be present. For this purpose, we have investigated the rate integrated above 90 keV, Rg0, as a function of the time dur- ing the DAMA/NaI-3 and DAMA/NaI-4 running

R. Bernabei et al./Nuclear Physics B (Proc. Suppl.) 91 (2001) 361-367 365

periods. The distributions of the percentage vari- ations of Rg0 with respect to their mean values for all the crystals during the whole DAMA/NaI-3 and DAMA/NaI-4 running periods show cumu- lative Gaussian behaviours with cr ~ 1% which are accounted by the expected statistical spread arising from the sampling time used for the rate evaluation, excluding any significant background variation. Moreover, fitting the behaviour of this higher energy rate with time, adding also a term modulated according to a cosine function with 1 year period and 152.5 day phase (as expected for a WIMP signal), values -(0.11 ± 0.33) cpd/kg and -(0.35 + 0.32) cpd/kg have been found for the modulation amplitudes of the two periods; they are consistent with zero. This excludes the pres- ence of a background modulation in the whole energy spectrum at a level much lower than the effect found in the lowest energy region; in fact, otherwise the modulated term of Rg0 should be of order of tens cpd/kg, that is ~ 100 standard deviations far away from the measured value.

Furthermore, a similar analysis can be per- formed in the energy region just above the first pole of the Iodine form factor: again, modulation amplitudes of the counting rate statistically com- patible with zero have been found. In fact, in the energy region 10-20 keV they are: -(0.0044 ± 0.0044) cpd/kg/keV and -(0.0071 ± 0.0044) cpd/kg/keV for the two periods. Performing sim- ilar calculations for the (6 - 10) keV energy inter- val one gets: -(0.0017 ± 0.0037) cpd/kg/keV for the two periods together.

In long term running conditions, the knowl- edge of the energy scale is assured by periodi- cal calibration with 241Am source and by con- tinuously monitoring within the same production data (grouping them every _ 7 days) the position and resolution of the 21°Pb peak (46.5 keV)[1]. As in refs. [1], the distribution of the relative variations of the energy calibration factors, esti- mated from the position of the 21°Pb peak for all the 9 detectors during both DAMA/NaI-3 and DAMA/NaI-4 taken without any correction, has been investigated; it shows a Ganssian behaviour with a = (0.95-4-0.04)%. Since the results of the routine calibrations axe obviously properly taken into account in the data analysis, such a

result allows to conclude that the energy calibra- tion factors for each detector are known with an uncertainty < 1%. Due to the relatively poor en- ergy resolution of the detectors at low energy, this could give rise only to an additional relative en- ergy spread ~< 10 -4 in the lowest energy region and < 10 -3 at 20 keV, which is totally negligible.

The stability of the efficiencies over the whole data taking periods has been even investigated. Their possible time variation depends essentially on the stability of the so-called cut efficiencies (see e.g. ref. [1]). The latter have been reg- ularly measured by dedicated calibrations (see above and in ref. [1]). In particular, the per- centage variations of the efficiency values in the (2-8) keV energy interval considering 2 keV bins have been studied; they show a Gaussian distri- bution with a = 0.6% and 0.5% for DAMA/NaI- 3 and DAMA/NaI-4, respectively. Moreover, we have verified that the time behaviour of these per- centage variations does not show any modulation with period and phase expected for WIMP signal. In particular, in the (2-4) keV energy interval a modulation amplitude equal to (1.0 + 1.0) • 10 -3 is found for the two periods all together, while in the (4-6) keV the result is (0.1 :t= 0.7). 10 -3, they are both consistent with zero. Similar results are obtained in other energy bins.

The investigations summarized above of known sources of possible systematic effects, which could affect the energy spectrum, credit a percentage systematic error of order of ~ 10 -3. Moreover, the results on the analysis of Rgo exclude at an even more stringent level the presence of a pos- sible overall background modulation (excluding also significant contribution e.g. from possibly surviving neutrons from the environment). Fi- nally, no contribution can arise from environmen- tal Radon, as the detectors are isolated from it (in addition, a time correlation analysis offers, as mentioned, a Radon modulated contribution compatible with zero). Moreover, no known sys- tematics able to satisfy all the same six require- ments for a WIMP induced effect - as it is nec- essary to fake the signature - has been found so far.

As regards possible competing physical pro-

366 R. Bernabei et al./Nuclear Physics B (Proc. Suppl.) 91 (2001) 361-367

cesses we found up to now only the muon mod- ulation reported in ref. [8], but it would give in our set-up modulation amplitudes < < 10 -4 cpd/kg/keV, that is much smaller than we ob- serve. Moreover, it will also fail some of the six re- quirements necessary to mimic the signature (e.g. it would yield a modulation in R9o which is not observed). So it can be safely ignored.

For the sake of completeness, we recall that ev- idence for the anomalous events with decay-time shorter than recoils (as observed e.g. by the UK Collaboration) has never been found in our data by using the pulse shape discrimination 2.

As a result of the model independent approach and of the investigation of known systematic ef- fects we can conclude that - as mentioned above - the presence of an annual modulation compati- ble with a WIMP presence in the Galactic halo is candidate by the data independently on its nature and coupling with ordinary matter.

As a further step in order to investigate the nature of a possible candidate, a full time and energy correlation analysis of the data has to be performed and it is necessary to assume a par- ticular model, which would require not only the choice e.g. of a particular coupling, of a particular velocity distribution, but also to fix every needed parameter on a certain value. According to ref. [1], the events between 2 and 20 keV have been analysed by using the standard maximum likeli- hood method considering in particular a candi- date with spin-independent interaction and mass above 30 GeV (for details see ref. [1]). In the min- imization procedure the WIMP mass has been varied up to 10 TeV. Totally similar results are achieved when using different approaches as X 2 method, Feldman and Cousins method, etc..

As usual in particle Dark Matter direct searches standard hypotheses have been used in the calculations of the distribution of astrophys- ical velocities [1], while a detailed investigation of the effects induced by their uncertainties [9] has been performed in ref. [10]. The model de-

2As it can be easily u n d e r s t o o d and as it h a s been shown also in thos conference, e.g. the P S D m e t h o d we used in ref. [1] was sensi t ive to s imilar events; if present , t hey would have given rise to recoil cand ida tes which were not observed.

pendent results of every cycle independently are consistent; therefore, a global analysis has been carried out. In addition, also the consistency with the results of DAMA/NaI-0 [1] has been re- quired. The obtained 90% C.L. allowed region is shown in fig. 3. A dedicated discussion quanti- tatively comparing this particular model depen- dent result with the model independent one has been carried out in ref. [1]. The theoretical im- plications of the observed effect in terms of neu- tralino with dominant spin-independent interac- tion and mass above 30 GeV has been discussed in ref. [11-13], the case for an heavy neutrino of the fourth family has instead been introduced in ref. [14]. The inclusion of present uncertainties

10-4

10-5

10-6

I I .... I I I ,L,

irl rl

i0-7 q . , r . . . . I I , , t . . . . 10 20 50 100 200 500 1000

H w ~GeV]

Figure 3. Regions allowed at 3a C.L. by the com- plete global analysis: i) for vo = 220 km/s (dotted contour); ii) when accounting for vo uncertainty (170 km/s < v0 < 270 km/s; continuous con- tour); iii) when considering also a possible bulk halo rotation as in Ref. [10] (dashed contour). The consistency with the results of ref. [1] has been required here.

on some nuclear and particle physics parameters would enlarge these regions; full estimates are in progress. As an example, let us notice that, as for the case of v0 and possible bulk rotation, uncer- tainties can also arise from standard nuclear and

R. Bernabei et al./Nuclear Physics B (Proc. Suppl.) 91 (2001) 361-367 367

particle physics assumptions used in the evalua- tion of the expected unmodulated term and of the modulation amplitude, Sin. We have also men- tioned the case of the form factor, which depends on the nuclear radius and on the thickness param- eter of the nuclear surface [18]. Varying e.g. by 20% the values of the latter parameters with re- spect to standard assumptions [18], the locations of the minimum move toward slightly larger Mw and lower ~ap, while the calculated Sm in the 2-6 keV energy interval increases by about 15%.

4. C O N C L U S I O N S

To further investigate the observed effect, the data of a fifth annual cycle, available at end of August 2000, will be analysed. Moreover, a sig- nificant upgrading of the electronics and DAQ of the _ 100 kg NaI(T1) set-up is in progress as well as the work to build new radiopure NaI(Tt) de- tectors to increase the experimental sensitivity.

Finally investigation on the model framework and on the role of the uncertainties associated to the parameters to be used in model dependent calculations are in progress.

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