2 DAMIC AT SNOLAB The low-energy spectrum in DAMIC at SNOLAB Alvaro E. Chavarria1for the DAMIC Collaboration

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2 DAMIC AT SNOLAB

The low-energy spectrum in DAMIC at SNOLAB

Alvaro E. Chavarria1?for the DAMIC Collaboration

1Center for Experimental Nuclear Physics and Astrophysics, University of Washington, Seattle,

United States

* chavarri@uw.edu

October 4, 2022

14th International Conference on Identiﬁcation of Dark Matter

Vienna, Austria, 18-22 July 2022

doi:10.21468/SciPostPhysProc.?

Abstract

The DAMIC experiment employs large-area, thick charge-coupled devices (CCDs) to search

for the interactions of low-mass dark matter particles in the galactic halo with silicon atoms

in the CCD target. From 2017 to 2019, DAMIC collected data with a seven-CCD array

(40-gram target) installed in the SNOLAB underground laboratory. We report dark-matter

search results, including a conspicuous excess of events above the background model below

200 eVee, whose origin remains unknown. We present details of the published spectral anal-

ysis, and update on the deployment of skipper CCDs to perform a more precise measurement

by early 2023.

1 Introduction

The DAMIC experiment at SNOLAB employs the bulk silicon of scientiﬁc charge-coupled devices

(CCDs) as a target for interactions of particle dark matter (DM) from the galactic halo. The low

pixel readout noise of 1.6 e−R.M.S., combined with extremely low leakage current of a few e−

per mm2·day, provides DAMIC CCDs with sensitivity to the small ionization signals from recoiling

electrons or nuclei following the interactions of low-mass DM particles.

2 DAMIC at SNOLAB

DAMIC CCDs were developed by Berkeley Lab’s Microsystems Laboratory and fabricated by Tele-

dyne DALSA. The devices feature a rectangular array of pixels, each of size 15×15 µm2, and a

fully-depleted active region of 675 µm. Other details of the CCD design and fabrication process

can be found in Ref. [1]. Several arrangements of CCDs were deployed in the DAMIC cryostat

since 2012, with the ﬁnal installation of seven 16 Mpix CCDs in 2017, for a total silicon target

mass of 40 g. Details of the DAMIC setup at SNOLAB are presented in Refs. [2,3].

arXiv:2210.00587v1 [astro-ph.CO] 2 Oct 2022

2 DAMIC AT SNOLAB



σxy σxy z

∝

z y x

σx z

∝

1x100:

a) b) c)

Ionization Fully active

region

Pixel array

15 µm

675 µm

b) c)

Figure 1: a) Sketch of a low-energy particle ionizing the CCD active region. b) Diffusion

of charge as it drifts toward the pixel array. c) The spread of the cluster of charge on the

pixel array is positively correlated with the depth of the interaction.

DAMIC data consists of images that contain a two-dimensional projection of all charge gen-

erated in the active region of the CCDs throughout an image exposure (typically 8 h). Particles

generate free charges (e-h pairs) in the CCD active region by ionization (Fig. 1a), with one free

e-h pair generated on average for every 3.8 eV of kinetic energy deposited by a recoiling elec-

tron. For a recoiling nucleus, the charge yield is lower and non-linear, and was directly calibrated

in Ref. [4]. The free charges are then drifted by an electric ﬁeld toward the pixel array. Since

charge diffuses laterally as it drifts, interactions that occur deeper in the CCD bulk lead to more

diffuse charge clusters (Fig. 1b). The spread of the charge in the image (x-yplane) can then be

used to reconstruct the depth (z) of an interaction in the CCD active region (Fig. 1c). Clustering

algorithms are run on DAMIC images to identify clusters of pixels with charge above noise and

reconstruct the deposited energy and (x,y,z)location of particle interactions in the bulk silicon.

Details on the CCD response, image cleanup and processing are also presented in Refs. [2,3].

Dark matter searches in DAMIC are performed by comparing the charge (energy) distribu-

tions of individual pixels or pixel clusters against a background model that includes instrumental

noise and ionizing backgrounds from natural radioactivity. Ref. [3]provides all details on the

construction of the radioactive background model, including the extensive radioassay program of

all components. The background model was constrained and validated with several independent

measurements of radiocontaminants in the detector, e.g., surface/bulk 210Pb and bulk 32Si, that

were performed with the CCDs themselves by searching for spatio-temporal correlations between

decays [5].

The main science results from DAMIC are:

• The ﬁrst search for DM interactions that produce as little as a one e-h pair in silicon, resulting

in the ﬁrst exclusion limit on the absorption of hidden photons [6]with masses as small as

1.2 eV c−2[7].

• Exclusion limits on the scattering of hidden-sector DM particles [8]with masses as small as

0.5 MeV c−2with electrons [9].

• The most sensitive direct search for weakly interacting massive particles (WIMPs) [10–12]

with masses in the range 1–9 GeV c−2[13]scattering with silicon nuclei. This result sig-

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2DAMICATSNOLABThelow-energyspectruminDAMICatSNOLABAlvaroE.Chavarria1?fortheDAMICCollaboration1CenterforExperimentalNuclearPhysicsandAstrophysics,UniversityofWashington,Seattle,UnitedStates*chavarri@uw.eduOctober4,202214thInternationalConferenceonIdenticationofDarkMatterVienna,Austria,18-22July2022d...

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2 DAMIC AT SNOLAB The low-energy spectrum in DAMIC at SNOLAB Alvaro E. Chavarria1for the DAMIC Collaboration

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