1 INTRODUCTION Even Lighter Particle Dark Matter Alvaro E. Chavarria1_2

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1 INTRODUCTION

Even Lighter Particle Dark Matter

Alvaro E. Chavarria1?

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

United States

* chavarri@uw.edu

January 2, 2023

14th International Conference on Identiﬁcation of Dark Matter

Vienna, Austria, 18-22 July 2022

doi:10.21468/SciPostPhysProc.?

Abstract

We report on recent progress in the search for dark matter particles with masses from

1 MeV c−2to 1 GeV c−2. Several dark matter candidates in this mass range are expected to

generate measurable electronic-recoil signals in direct-detection experiments. We focus on

dark matter particles scattering with electrons in semiconductor detectors since they have

fundamentally the highest sensitivity due to their low ionization threshold. Charge-coupled

device (CCD) silicon detectors are the leading technology, with signiﬁcant progress expected

in the coming years. We present the status of the CCD program and brieﬂy report on other

efforts.

1 Introduction

Weakly interacting massive particles (WIMPs) [1–3]with masses mχ>1 GeV c−2have been the

leading dark matter (DM) candidate for many decades. Direct searches for their scattering with

atomic nuclei in underground detectors have progressed signiﬁcantly throughout this time, with-

out any discovery. Although the WIMP direct detection program continues unabated [4], a fraction

of the scientiﬁc community has recently turned its attention to the possibility that DM particles

are signiﬁcantly lower in mass than previously considered [5]. Theoretical studies have uncovered

several well-motivated scenarios where DM particles with mχ<1 GeV c−2can be created in the

early universe and detected today in the laboratory. This has spurred a number of experimental

searches with existing detector technologies, and motivated the development of novel technolo-

gies with improved sensitivity to the low-energy depositions from light DM particles. The leading

new technology of skipper charge-coupled devices (CCDs) is reaching maturity with the deploy-

ment of the ﬁrst sizable detectors underground, which are expected to improve in sensitivity to

light DM particles by orders of magnitude in the coming years.

arXiv:2210.05661v2 [astro-ph.CO] 30 Dec 2022

3 CCDS TO SEARCH FOR DM

2 Direct detection signals from low-mass DM particles

For DM with mχ < 1 GeV c−2, the traditional approach to search for the energy deposited in the

target by nuclear recoils from DM scattering becomes limited because the mismatch between the

mass of the DM particle and the nucleus does not allow for efﬁcient transfer of energy. Recently,

other direct-detection mechanisms have been proposed to extend sensitivity to lower DM masses.

The ﬁrst is to search for electrons or photons emitted in the DM-nucleus interaction, which can

carry a signiﬁcant fraction of the DM particle’s kinetic energy and appear above the detection

threshold. The “Migdal effect” [6]is the process where an electron from the recoiling atom is

emitted, with a probability in silicon of O(10−5)[7]. The probability of photon emission (the

“Bremsstrahlung” process) is signiﬁcantly smaller [8]. Another possibility is to look directly for

the scattering of low-mass DM particles with electrons in the target [9,10]. Atomic electrons

are lighter than nuclei and, since they have a momentum distribution, there are regions of phase

space where the electron can take a signiﬁcant fraction of the DM particle’s kinetic energy. DM-

e interactions naturally arise in hidden-sector vector-portal DM theories [11], where the DM-e

scattering is mediated by a “hidden photon.” The hidden photon itself could also constitute the

DM and be absorbed by atomic electrons [12–14]. Since all these DM signals are electronic recoils

in the target, several experiments have placed exclusion limits on sub-GeV DM from their measured

electronic-recoil spectra [15–18].

The kinetic energy of a DM particle in the galactic halo is EK∼10−6mχc2. Most often, only a

fraction of EKis transferred to the electron, with the predicted deposited-energy spectrum highly

peaked at the lowest energies. Fig. 1shows the spectrum from a 1 GeV c−2DM particle scattering

with electrons in silicon predicted by the EXCEED-DM code [19]. Most of the spectrum—even at

1 GeV c−2—is below 10 eVee, which corresponds to only a few ionized charges in silicon. Thus,

detectors capable of counting single ionized electrons (already achieved in semiconductor and

noble-element direct-detection technologies) are needed to perform sensitive searches for low-

mass DM-e interactions. Semiconductor detectors have a fundamental advantage since the ion-

ization threshold is ∼1 eV compared to >10 eV for noble elements. Furthermore, the momentum

distribution of the electrons in the semiconductor valence band are better “kinematically matched”

to the DM ﬂux [10]. For this reason, the current sensitivity of silicon CCD detectors to DM-e scat-

tering surpasses that of noble liquids for most of the parameter space (specially for smaller DM

masses and lighter mediators) even though their target exposures are orders of magnitude smaller

(O(g·day) vs. O(tonne·day)).

3 CCDs to search for DM

Charge-coupled devices (CCDs) are monolithic solid-state silicon imaging devices that measure

the free charges generated in their fully-depleted active target. Particles generate free charges (e-

h pairs) in the CCD active region by ionization, with a minimum energy to ionize an e-h pair of 1.2

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

electron. The free charges are then drifted by an electric ﬁeld toward the pixel array. Since charge

diffuses laterally with time as it drifts, the spread of the charge cluster in the image (x-yplane) is

positively correlated to the depth (z) of the interaction. Thus, CCDs can, in principle, provide the

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

spatial resolution may be limited in one or more axes depending on the readout mode and event

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1INTRODUCTIONEvenLighterParticleDarkMatterAlvaroE.Chavarria1?1CenterforExperimentalNuclearPhysicsandAstrophysics,UniversityofWashington,Seattle,UnitedStates*chavarri@uw.eduJanuary2,202314thInternationalConferenceonIdenticationofDarkMatterVienna,Austria,18-22July2022doi:10.21468/SciPostPhysProc.?Abs...

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