CRESST: First results with the phonon-light technique

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CRESST: First results

with the phonon-light technique

F. Petricca

^a,

∗ , G. Angloher

^a

, C. Bucci

^d

, P. Christ

^a

, C. Cozzini

^b

, F. von Feilitzsch

^c

, D. Hauff

^a

, S. Henry

^b

, T. Jagemann

^e

,

J. Jochum

^e

, H. Kraus

^b

, B. Majorovits

^b

, J. Ninkovi´c

^a

, W. Potzel

^c

, F. Pr¨obst

^a

, Y. Ramachers

^f

, W. Rau

^c

, W. Seidel

^a

,

M. Stark

^c

, L. Stodolsky

^a

, W. Westphal

^c

and H. Wulandari

^c

aMPI für Physik, Föhringer Ring 6, 80805 München, Germany

bUniversity of Oxford, Department of Physics, Oxford OX1 3RH, U.K.

cTechnische Universit¨at M¨unchen, Physik Department, 85747 Garching, Germany

dLaboratori Nazionali del Gran Sasso, I-67010 Assergi, Italy

eEberhard-Karls-Universität Tübingen, D-72076 Tübingen, Germany

fUniversity of Warwick, Coventry CV4 7AL, U.K.

Abstract

We present first significant limits on WIMP dark matter by the phonon-light technique, where combined phonon and light signals from a scintillating cryogenic detector are used to suppress the non-nuclear recoil background. The performance of the detectors developed for the second phase of the CRESST experiment will be dis- cussed and data collected with two prototype modules in a short run, corresponding to a net exposure of 20.5 kg days, will be presented.

Key words: WIMPs, Dark Matter, Low temperature detectors PACS:, 95.35.+d; 24.40.-n; 29.40.mc

1 Introduction

The aim of the CRESST experiment is to detect WIMP dark matter par- ticles via their elastic scattering on

∗ Corresponding author. Tel.: +49 (0)89 32354309

Email address: petricca@mppmu.mpg.de

(F. Petricca).

target nuclei in the absorber of a cryogenic detector. For the second phase of the experiment currently in preparation, cryogenic detectors based on scintillating CaWO₄ crys- tals have been developed. When supplemented with cryogenic light detectors these provide an efficient discrimination of nuclear recoils from radioactive background down to re-

Preprint submitted to Elsevier Science 8th November 2005

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coil energies of about 10 keV by the simultaneous measurement of scintillation light and phonons ([1] [2] [3]).

For coherently interacting WIMPs, due to the presence of a factor A² in the cross section, massive nuclei are desired. In the case of CRESST tungsten nuclei are the favoured target for which the expected recoil energies extend up to about 40 keV.

Data used to set dark matter limits were collected with two detector modules during a running period of 53 days from January to March 2004 at the Laboratori Nazionali del Gran Sasso, corresponding to a net exposure of 20.5 kg days. The experimental facility is described in detail in [4]. A neutron shield and a muon veto to be installed for the final setup were not present for the data presented here.

2 Detectors

Detector modules consist of a 300 g scintillating CaWO₄ crystal of cylin- drical shape, operated as a cryogenic calorimeter (phonon channel) and a separate much smaller cryogenic calorimeter optimized for the detec- tion of scintillation photons (light channel). Both detectors are enclosed in a housing of highly reflective foil and are read out by optimized tungsten superconducting phase tran- sition thermometers operated at a temperature of about 10 mK. The characteristics of thermometers developed for phonon and light chan- nels are described in [3] and [5].

We obtained in the phonon chan-

nel an energy resolution of 1 keV (FWHM) for 46.5 keVγ’s and 7 keV for 2.3 MeV α’s. The energy resolu- tion of the light channel observed in dark matter runs reflects not only the performance of the thermometer, but also fluctuations in light produc- tion and collection. Using the low energy data from a²³²Th calibration we observed an almost linear energy dependence of the light channel resolution [3]. This may result from a slight dependence of the light output on the position of the energy deposition in the CaWO₄ crystal. The electronic noise of light detectors and hence the energy resolution referred to energy deposition in CaWO₄, is of 2 keV for the best of the two modules, comparing favourably with photomultipliers.

3 Results

Data from the dark matter run are presented in figure 1. The vertical axis gives the light to phonon energy ratio, while the horizontal axis is the phonon energy.

Within the acceptance region for O, Ca and W nuclear recoils in the phonon light plane we have 16 events from the two modules in the energy range between 12 and 40 keV, corresponding to a rate of (0.87±0.22) events/(kg day). This is compatible with the rate expected from our unshielded neutron background; moreover most of these events lie in the region of the phonon- light plane anticipated for neutron- induced recoils.

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Figure 1. Low energy event distribution from the data sample collected in the dark matter run for the two modules. The regions below the upper dashed lines contain 90% of O, Ca and W nuclear recoils while the lower ones represent the 10% boundary. The solid lines represent the 90%

boundary for tungsten recoils only. The vertical lines delimit the energy range used in the analysis.

Ignoring the neutron background and attributing all 16 events to WIMP interactions, we can set conservative upper limits for the coherent WIMP scattering cross section (dashed line in figure 2).

A particularly strong limit for WIMPs with coherent scattering results from selecting the region corresponding to tungsten recoils, where the best module shows zero events, corresponding to a poissonian upper limit (90% CL) for the W recoil rate of 0.35 events/(kg day). The dif- ference between the two modules is due to the better resolution of the light detector for the module with no events.

Figure 2. Spin independent WIMP-nucleon cross section limits at 90% C.L. as a function of the WIMP mass obtained from the dark matter run. The dashed line is obtained attributing all observed nuclear recoils to WIMP interactions whereas the solid line is obtained considering tungsten recoils only for the module Daisy/BE13.

4 Conclusions and perspec- tives

The present results were obtained without a neutron moderator and the sensitivity appears to be limited by the neutron background when all nuclear recoils in CaWO4 are at- tributed to WIMP interactions. The method of identifying the neutron background opens important pos- sibilities for improving the experimental sensitivity in addition to the improvement expected by increasing the target mass to 10 kg. To reach this goal the measurement were halted to proceed with the upgrade of the experimental setup.

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Acknowledgements

This work was partially supported by the DFG SFB 375 “Teilchen- Astrophysik”, the EU Network HPRN-CT-2002-00322 “Applied Cryodetectors” and PPARC.

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