Searching for dark absorption with direct detection experiments

Journal of High Energy Physics, Jun 2017

We consider the absorption by bound electrons of dark matter in the form of dark photons and axion-like particles, as well as of dark photons from the Sun, in current and next-generation direct detection experiments. Experiments sensitive to electron recoils can detect such particles with masses between a few eV to more than 10 keV. For dark photon dark matter, we update a previous bound based on XENON10 data and derive new bounds based on data from XENON100 and CDMSlite. We find these experiments to disfavor previously allowed parameter space. Moreover, we derive sensitivity projections for SuperCDMS at SNOLAB for silicon and germanium targets, as well as for various possible experiments with scintillating targets (cesium iodide, sodium iodide, and gallium arsenide). The projected sensitivity can probe large new regions of parameter space. For axion-like particles, the same current direction detection data improves on previously known direct-detection constraints but does not bound new parameter space beyond known stellar cooling bounds. However, projected sensitivities of the upcoming SuperCDMS SNOLAB using germanium can go beyond these and even probe parameter space consistent with possible hints from the white dwarf luminosity function. We find similar results for dark photons from the sun. For all cases, direct-detection experiments can have unprecedented sensitivity to dark-sector particles.

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Searching for dark absorption with direct detection experiments

JHE Searching for dark absorption with direct detection experiments Tien-Tien Yu 0 1 2 3 6 Itay M. Bloch 0 1 2 5 6 Rouven Essig 0 1 2 3 6 Kohsaku Tobioka 0 1 2 4 5 6 Tomer Volansky 0 1 2 5 6 Stony Brook 0 1 2 6 NY 0 1 2 6 U.S.A. 0 1 2 6 Open Access 0 1 2 6 c The Authors. 0 1 2 6 0 nd similar results for dark 1 Rehovot 7610001 , Israel 2 Tel-Aviv 69978 , Israel 3 C.N. Yang Institute for Theoretical Physics, Stony Brook University 4 Department of Particle Physics and Astrophysics, Weizmann Institute of Science 5 Raymond and Beverly Sackler School of Physics and Astronomy, Tel-Aviv University 6 [9] M. Betz , F. Caspers, M. Gasior, M. Thumm and S.W. Rieger , First results of the CERN We consider the absorption by bound electrons of dark matter in the form of dark photons and axion-like particles, as well as of dark photons from the Sun, in current and next-generation direct detection experiments. Experiments sensitive to electron recoils can detect such particles with masses between a few eV to more than 10 keV. For dark photon dark matter, we update a previous bound based on XENON10 data and derive new bounds based on data from XENON100 and CDMSlite. We nd these experiments to disfavor previously allowed parameter space. Moreover, we derive sensitivity projections for SuperCDMS at SNOLAB for silicon and germanium targets, as well as for various possible experiments with scintillating targets (cesium iodide, sodium iodide, and gallium arsenide). The projected sensitivity can probe large new regions of parameter space. For axionlike particles, the same current direction detection data improves on previously known direct-detection constraints but does not bound new parameter space beyond known stellar cooling bounds. However, projected sensitivities of the upcoming SuperCDMS SNOLAB using germanium can go beyond these and even probe parameter space consistent with possible hints from the white dwarf luminosity function. We photons from the sun. For all cases, direct-detection experiments can have unprecedented sensitivity to dark-sector particles. experiments; Dark matter; Dark Matter and Double Beta Decay (experiments) 1 Introduction Axion-like particles ALPs as dark matter ALPs from the Sun Dark photons as dark matter Dark photons from the Sun Analysis of current and future experiments A Rates for ALPs and dark photons in various materials B Solar A0 production Signi cant experimental evidence and theoretical considerations suggest that the Standard Model (SM) of particle physics is incomplete. In particular, there is compelling evidence for the existence of dark matter (DM). Determining the identity of the DM particle is one of the most important problems in particle physics today. Many DM candidates exist, with Weakly Interacting Massive Particles (WIMPs) being the most studied. Numerous direct-detection experiments probe for the elastic scattering of WIMPs (and other DM candidates) o ordinary matter [1]. However, it is entirely plausible that DM is composed of a (pseudo)scalar or a vector boson that can be absorbed in a material, thereby depositing its full kinetic and rest-mass energy. DM in the form of axions, axion-like particles (ALPs), and dark photons can have this property, see e.g. [2]. This paper focuses on the search of such DM candidates with existing and upcoming directdetection experiments. For other types of searches see e.g. [3{13]. The axion was originally introduced as a solution to the strong CP problem [14, 15]. This pseudoscalar may couple in a model-dependent manner to the SM axial current, Peccei-Quinn [16] symmetry is broken. More generally, the spontaneous breaking of any global symmetry combined with a small explicit breaking can lead to low-mass ALPs with a similar interaction. This interaction allows the axion or ALP to be absorbed in a target material, rather than scattering o it. The case of absorption in bound electrons, known as the axioelectric e ect, has been originally suggested in [17, 18]. Numerous direct-detection experiments have conducted searches assuming that the ALP constitutes the galactic DM or is produced in the Sun, including CoGeNT [19], CDMS [20], EDELWEISS [21], XENON100 [22], and KIMS [23]. Here we show that other existing data can improve on these bounds and provide projections for future experiments. We nd that future experiments could go beyond the strong stellar constraints. Another simple DM candidate, present in many extensions of the SM, is the dark photon, which we denote as A0. The A0 is a massive vector boson that can couple weakly to ordinary matter through kinetic mixing [24{28]. Signi cant ongoing experimental e ort searches for an A0, see e.g. [29{31]. An intriguing possibility, however, is that the A0 itself is su ciently stable to play the role of the DM particle [32{34]. In this case, relic A0s may be absorbed by ordinary matter in direct-detection experiments, just like an ALP. Initial studies of this possibility were presented in [35]. We derive new direct-detection bounds that are more constraining than existing limits, and provide projected sensitivities. We focus on the sensitivity gain achieved when lowering the threshold of directdetection experiments. Traditionally, these experiments have been optimized to probe WIMPs with masses above O(GeV), requiring sensitivity to nuclear recoil energies above O(keV). However, many viable DM candidates have masses well below the GeV-scale, which can only be probed with new techniques [36]. DM scattering o electrons is one such technique suggested in [36]. Not only does this probe sub-GeV DM scattering, but a lower ionization threshold translates directly into a lower mass threshold for absorbing XENON10's sensitivity to single electrons [37] allows them to probe the lowest electron recoil energies, but without distinguishing signal from background events. This data was used to place a constraint on DM as light as a few MeV scattering o electrons [38] and on A0 DM as light as the xenon ionization energy of 12.1 eV [35]. We revisit the latter limit below. More recently, the XENON100 experiment published a search for events with four or more electrons [39]. We show that this data improves constraints on sub-keV ALPs and A0 DM. We also show that a CDMSlite dataset sensitive to O(20) electrons [40] sets the strongest constraints for some A0 DM masses. Future experiments are expected to achieve improved sensitivity to even lower DM masses by lowering the threshold down to O(eV) and having a reduced background. A key to this improvement is the use of semiconductors or scintillators, which have band gaps of O(few eV). The sensitivity gained by developing ultra-sensitive detectors able to probe one or a few electrons in semiconductors (e.g. SuperCDMS [40], DAMIC [41, 42]), scintillators, or two-dimensional targets has been studied in the context of sub-GeV DM scattering in [36, 43{47]. Below we study the prospects of such experiments to detect or constrain ALPs and A0s. Superconducting targets may probe even lower masses, either by DM scattering [48, 49] or DM absorption [50], in the future. Moreover, while beyond the scope of this paper, chemical-bond breaking could be used to probe nuclear recoils down to 10 MeV [51, 52], while super uid helium could probe nuclear recoils down to keV [53]. These would probe di erent couplings from the models considered here. Finally, axions, ALPs, and dark photons may exist without constituting a signi cant fraction of the DM energy density. If su ciently light, they would be produced in the Sun and can again be absorbed in direct-detection experiments. We will discuss constraints from existing experiments and provide projections for upcoming experiments. Since ALPs are dominantly produced in the Sun with keV energies, no gain is achieved by experiments lowering their threshold. Instead, we focus on dark photons, which are produced also at low energies, so that lower thresholds could potentially probe beyond existing constraints. Axion-like particles The original \QCD" axion has a strict relation between its mass, ma, and decay constant, fa. More generally, however, ALPs may be described by independent ma and fa. Moreover, ALPs have model-dependent coupling to gluons, photons, and SM fermions. We are interested here in possible interactions with electrons, de ned minimally by La = In the absence of additional couplings, the model dependence in gaee can be absorbed in the gaee versus ma parameter space. The ALP interaction with electrons is responsible for the \axioelectric" e ect [2, 17, 18, 54]. In analogy to the photoelectric e ect, axions may be absorbed by bound electrons, Direct detection experiments search, in part, for ionized electrons and therefore o er an opportunity to search for ALPs via their coupling to electrons. The ALP absorption cross section, AE, may be related to the photoelectric absorption cross section, PE, as 1 v2=3 where E and va are the ALP's energy and velocity, respectively [2, 17, 18]. We infer that the rate of absorption of non-relativistic ALP DM on an electron (na AEva, where na is the ALP number density) has negligible dependence on va. The absorption spectrum is thus very narrow, helping to di erentiate signal from background. PE(E) from Henke et al. [55, 56] and the Handbook of Optical Constants of Solids [57{60]. Discrepancies of O(50%) exist between di erent theoretical predictions for PE(E) below 300 eV. The main sources of theory uncertainties arise from approximating the correlation between di erent electrons. We use the Henke calculations, which are favored by experimental data [61], for energies above 30 eV. Below 30 eV, we use the measurements compiled in [57{60]. Axions and ALPs may play the role of DM. The QCD axion is produced either thermally or through the misalignment mechanism and the decay of cosmological defects (for a review see e.g. [62, 63]). By tuning the initial misalignment angle or through late-time entropy dumping, axion DM has masses . 20 eV. Non-thermal production through defects may allow for heavier axions; quite generally though, its mass must be . eV for it to be cold DM. Hot axions also require masses . eV. ALPs could obtain the correct relic density in a wider mass range, via the misalignment mechanism, or by thermal or non-thermal production. It is thus interesting to constrain ALPs without assuming a speci c production mechanism. The ALP absorption rate on RALPs = 1:9 for a local DM energy density of DM = 0:4 GeV=cm3. ALPs from the Sun Independent of their relic density, ALPs may be produced in the Sun. The ux has been calculated in [64] and is largest at O(keV) energies. Solar ALPs can be detected in direct-detection experiments [17, 18]. Limits on gaee versus the ALP mass have been published by several direct detection experiments, among the strongest of which is the XENON100 collaboration analysis that used an S1 (primary scintillation) trigger with a 2 keV threshold [22]. The more recent S2-only analysis [39], which searched for events with an ionization-only (S2) signal, has a much lower experimental threshold and is a larger dataset than that used in [22]. However, the solar ALP drops rapidly at low energies, while the S2-only analysis includes more background events due to the lower threshold. Therefore, the S2-only dataset produces a weaker bound than found in [22]. Future xenon-based experiments can improve on these limits due to their large exposures and low expected backgrounds (see e.g. [65], although exposures & 100 tonyears are required to probe beyond white-dwarf cooling limits). Conversely, lowering the experimental threshold without a signi cant gain in exposure over XENON100 [22] only provides a negligible increase in sensitivity due to the rapidly falling solar ux below O(keV) energies. Since this paper is focused on discussing the potential gain from low-threshold experiments, we do not present the solar ALP limits. The A0 is a hypothetical massive vector boson of a broken (dark) gauge group U(1)D that may kinetically mix with the SM hypercharge [24, 25]. At low energies, this mixing is dominantly between the A0 and the SM photon. The relevant interactions are where vA0 is the dark photon velocity [2]. The resulting absorption rate is given by (for DM = 0:4 GeV/cm3) 2 PE(E = mA0 ) : We include in-medium e ects. These are especially important for the detection of solar A0s, so we discuss them in the next subsection. For A0 DM, the absorption rates are a ected only by . 10% for mA0 & 100 eV, although for mA0 . 100 eV, the rate for xenon (germanium, silicon) is changed by a factor of 0.2{2.0 (1.0{1.15, 1.0{1.8). Dark photons from the Sun Direct detection experiments are sensitive to A0s from the Sun [66, 67]. The detection of these solar A0s is analogous to that of A0 DM, but here the in-medium e ects of the A0 in the Sun may signi cantly a ect production and detection. We focus on the Stuckelberg case, and summarize the e ect as described in [66, 68]. The matrix element for A0 absorption in a medium is where is the kinetic-mixing parameter, mA0 is the A0 mass, and F is the SM photon eld strength. The mass mA0 can have two origins: a Stuckelberg mass, which predicts no additional degrees of freedom, or a Higgs mechanism, where the U(1)D is broken spontaneously by an extra Higgs eld. This distinction is irrelevant for A0 DM detection, but important for detecting A0 from the Sun. For the latter, we only consider a Stuckelberg mass. Dark photons as dark matter For su ciently small and mA0 below twice the electron mass, the A0 decay lifetime can be longer than the age of the Universe, allowing for the A0 to constitute all the DM. Various production mechanisms exist, see e.g. [32{34]. The absorption of A0 DM can be modeled as the absorption of a massive non-relativistic particle with coupling e to electrons. We can then write the A0 absorption cross-section in terms of PE, A0 (EA0 = mA0 ) vA0 ' 2 PE(E = mA0 ) ; M(A0T;L + i ! f ) = Here i and f denote the initial and nal electron state and T;L are the transverse (T) and longitudinal (L) in-medium polarization functions de ned by e2hJem; Jemi = which encode the correlation function inside the medium. The absorption rate of the transverse and longitudinal modes of a solar A0 in the detector frame is where the e ective mixing angle is T = !2(1 L = (!2 For an isotropic and non-magnetic material with a refractive index nrefr, one has where q = (!; ~q) is the A0 4-momentum, with nrefr = 1 Here nA denotes the density of atoms of type A, = 2 ~c=! is the wavelength of a photon atomic scattering factors f1;2 are given in [55, 56] The database [56] has f1 above 30 eV and f2 above 10 eV. To obtain f1Xe for energies below 30 eV, we use Re[nrefr] in gure 3 of [35]. The ux of solar dark photons is given by = 1 AU is the Earth-Sun distance and R = 6:96 108 m is Sun's radius. We consider the resonant production of the transverse and longitudinal modes, which gives the dominant component of the ux. The spectrum of the total number of events is where V; T are the detector's ducial volume and exposure time, respectively. The ratio of the ionization rate to the absorption rate, Br, is unity for the energies of interest. The spectrum has a sharp rise at the mass threshold because of the resonant production of the Analysis of current and future experiments In this section, we describe how we derive bounds on ALPs and A0s using existing data from XENON10 [37], XENON100 [39], and CDMSlite [40], and how we derive projections for possible future experiments. The low-mass reach of an experiment for ALP DM or A0 DM absorption depends on the type of material used as a target. Xenon-based targets have a low-mass reach of 12.1 eV, while semiconductor- or scintillator-based experiments have a band gap, which is lower by about an order of magnitude, allowing such experiments to lower sensitivity down eV DM masses. To extract optimal limits, we need to take into account the spectral shape of the signal in the detector, which depends not only on the detector energy resolution. In particular, an electron that absorbs an ALP or A0 (the `primary' electron) is ejected from the atom, and its resulting kinetic energy (total minus binding) is converted to ionizing additional atoms, which can lead to several additional (`secondary') electrons. The number of secondary electrons produced uctuates from event to event. Their variance and average number can be predicted in a given experiment, either by modeling the secondary interactions or through measurements. For a highly energetic primary electron, this variance usually dominates over the detector resolution. We estimate below, either directly from the experimental data or from theoretical expectations, the variance and detector resolution (adding them in quadrature if needed), and smear the theoretical spectrum accordingly. To derive a conservative limit, we choose a single bin containing 95% of the signal and demand that it is less than the data at 90% C.L.. For the solar A0 signal, we use a slightly simpler procedure, since the signal is distributed over a wider energy range: we rst calculate the energy range (from mA0 upwards) that contains 95% of the signal, then smear the rst and last bins, and require the signal to be less than the data at 90% C.L. in the resulting (smeared) energy range. The energy range we can use is limited by the ionization threshold in xenon (> 12:1 eV) and by the band gap for the other materials (see below eq. (2.2)). Note that the atomic data for NaI does not extend below 30 eV, and so we do not go below 30 eV. Current experiments We consider the following existing datasets: XENON10: the XENON10 experiment uses xenon as the target material and presented results using 15 kg-days of data with an S2 (ionization signal) trigger threshold set to a single electron [37]. Previous work used these data to derive conservative bounds on subGeV DM [38], solar A0 [66], and A0 DM [35]. In the latter two, the signal was required to be less than the total number of observed S2 events with 80 electrons, without taking into account spectral information. Here we use the spectral information to place more stringent bounds, except near threshold ( 30 eV), where the bound is in reality slightly weaker. We take the signal e ciency times acceptance to be 0.92 for events with 2 electrons. For single electrons, the e ciency is only about 0.5, and we require the signal event rate to be less than 23.4 events/kg/day [38]. The number of secondary ionization electrons is challenging to calculate precisely, but we model it as discussed in [38]. Each electron produces 6:2 photoelectrons (PE), which are the actual signal seen by the XENON10 detector and de nes the detector resolution. In [37], the data has already been converted from the number of PEs to the number of electrons. In gure 1 (top), we reproduce the spectrum of events (orange histogram) shown in gure 2 of [37]. We also show (in blue) a hypothetical DM ALP/A0 signal, with the normalization chosen at the 90% C.L. limit, at two di erent masses (100 eV and 1100 eV). The pink and magenta gaussian lines show the signal shape that plot shows the data as the number of observed electrons, while bottom plot shows the data as the number of photoelectrons (PE) on the bottom axis and the corresponding number of electrons observed signal shape for two di erent DM (ALP or A0) masses. The blue histogram shows the signal distribution in terms of the number of electrons produced in the detector. Varying the secondary ionization model produces di erent signal shapes and thus di erent limits. We show the signal shape in pink and magenta that produces, respectively, the worst and best limit for the same two DM masses when varying the secondary ionization model. Numbers next to the blue curves give the 90% C.L. upper bound times e ciency on the number of signal events for those masses. produces the worst and best limit, respectively, for the same two DM masses when varying the secondary ionization model. In gure 2, we show examples of solar A0 spectra. Since the signal spans a wider energy range, the uncertainty in the secondary ionization modeling should be small, and we do not include the uncertainty. XENON100: the XENON100 collaboration presented S2-only data with events down to 80 PE, corresponding to about 4 electrons, in [39]. The data listing the number of PE for each event is reproduced in gure 1 (bottom), where we bin the data in 1 PE bins. The mean number of PE generated by one electron is 19:7 0:3, with a width of 6:9 PE/electron [39]. Our signal calculation includes the acceptance and trigger e ciencies from [39]. As for XENON10 above, we show a hypothetical DM ALP/A0 signal in blue at two di erent masses (150 eV and 570 eV). The pink and magenta gaussian lines again show, for the same two DM masses, the signal shape that produces the worst and best limit when varying the secondary ionization model. We derive the bound on solar A0s as discussed above. The bound on sub-GeV DM scattering o electrons based on this data will be presented in [69]. CDMSlite: the SuperCDMS collaboration reported results with a threshold of 56 eV from a low-threshold \CDMSlite" run in [40]. To derive a bound, we take the data from gure 3 of [40] and apply all e ciencies and cuts (note that gure 3 is not corrected for the trigger e ciency). We treat the energy range 56 eV{140 eV as a single bin, taking the data from table I. The total resolution of the signal (combining the experimental resolution and uctuations in the number of secondary electrons) can be inferred from in a xenon detector. The combined transverse and longitudinal modes (solid ) and the longitudinal case is cut o by the xenon ionization energy, 12:1 eV. the observed electron capture peaks. Thus we adopt =18.24 eV (30.68 eV, 101 eV) for the mass ranges [56 eV, 160 eV] ([160 eV, 1.3 keV], [1.3 keV, 10.37 keV]). Future experiments Future experiments are expected to have lower thresholds, allowing them to probe lower DM masses. To derive projections, we take into account expected physics backgrounds, but assume zero detector dark counts, i.e. no spontaneous creation of electron-hole pairs that mimic a signal. SuperCDMS SNOLAB HV Ge and Si: the next-generation SuperCDMS experiment will be deployed at SNOLAB. A high-voltage (HV) version will attempt to use a large bias voltage to achieve the same Luke-Neganov-phonons ampli cation of an ionization signal as the previous CDMSlite versions. Ultra-sensitive phonon detectors may achieve sensitivity down to single electrons, allowing the threshold to be given by the band gap (0.67 eV for Ge, 1.1 eV for Si). The projected exposures are 20 (10) kg-years for germanium (silicon) [1]. Our projections assume the following backgrounds: { Germanium: we use a preliminary estimate by the SuperCDMS collaboration in [70]. This estimate includes backgrounds from tritium -decay (a at background on a loglog plot between 1 eV to a few keV), various x-ray lines produced by cosmogenic activation, and solar neutrinos (for exposures of . few kg-years, these are only relevant at electron recoil energies . 10 eV). We assume that an electron in germanium absorbs a DM particle of mass mDM, which must be larger than the band gap Eg. The kinetic energy (Ee Eg = mDM this primary electron is converted into additional electron-hole pairs. The mean total number of electron-hole pairs created is hQ(Ee)i = 1 + b(Ee Eg)="ec ; ee at Ee = 1 keV [eV] width ( ee) at ionization energies of 1 keV from eq. (4.2) in various semiconductors (Si, Ge) and scintillators (NaI, CsI, GaAs). where "e is the mean energy per electron-hole pair and byc rounds y down to the nearest integer. Fluctuations around the mean are given by the Fano factor, F Q2 is the variance. To calculate the width in the signal's energy, we add in quadrature the energy resolution of the phonon detectors, t, and the uctuations in Q2=hQi, the number of secondary electron-hole pairs, Here, Vb is the bias voltage, so one electron-hole pair will produce a total phonon Ee = mDM, this equation becomes ee = "e For large enough Q, the signal shape is dominated by the Fano factor. varies less than 50% over almost two orders in magnitude in energy [71], measurements of F at energies below 200 eV would be desirable. We take t = 10 eV, although the precise value makes a di erence only at the lowest masses above the band gap when Eg for germanium (and other elements) in table 1 [73, 74]. We list ee(1 keV) as an example of the signal widths for various elements. { Silicon: silicon does not have the same low-energy x-ray lines produced by cosmogenic activation. However, 32Si is a relatively large contaminant and a low-energy -emitter. Together with tritium, this produces a at background at energies . 10 keV, which we take to be 350 events/keVee/kg/year based on preliminary estimates by the Suonly important just above the band gap. See table 1 for the values of "e, Eg, and ee(1 keV) for silicon.1 Scintillators (NaI, CsI, and GaAs): we show projections for hypothetical future experiments using scintillating targets with sensitivity down to one or more photons. Such experiments have been argued also to have great potential to sub-GeV DM scattering o electrons [47]. We make projections for three scintillating targets, sodium iodide, cesium iodide, and gallium arsenide, although other possibilities exist [47]. We assume a at -decay background of 350 events/keVee/kg/year as for the silicon projection above. We do not consider any x-ray lines activated by cosmogenics. We ignore the t term in eq. (4.2), and use the values listed in table 1 (see also [47]). Existing constraints and projected sensitivities are shown for ALP (A0) DM on the left (right) of gure 3. For ALPs, we see that the newly derived direct detection bounds from XENON100 and CDMSlite partially improve on published bounds from CoGeNT, CDMS, XENON100, EDELWEISS, and KIMS [19{23]. However, we see that these are weaker than stellar cooling bounds [35, 68, 78{82]. Prospective searches, especially SuperCDMS SNOLAB HV with germanium, could improve by a factor of a few beyond the stellar cooling constraints. Intriguingly, this includes probing part of the region consistent with a possible hint for anomalous energy loss in white dwarf stars [78, 82{84]. If the ALP mass is less than the kinetic energy of electrons, which is 3T =2 1:3 keV, it will contribute to the cooling of the white dwarf [78, 79]. For A0 DM, we have derived several constraints that go beyond the constraints from the anomalous energy loss in the Sun, horizontal-branch (HB) stars, and red-giant stars [35] (see also [66, 68, 80]). First, we have updated the bound derived in [35] based on the lowthreshold XENON10 data [37]. We nd that this bound disfavors a gap of several hundred eV in mass between the Sun and HB cooling bounds. Existing CDMSlite and XENON100 data disfavor additional parameter space beyond this. Prospective searches by SuperCDMS SNOLAB HV using germanium or silicon targets can probe up to more than an order of beyond existing constraints for sub-keV A0 masses. New experiments using scintillator targets with sensitivity to one or a few photons can have similar reach. We show the results for the solar A0 in gure 4. We nd that existing constraints from XENON10, XENON100, and CDMSlite are weaker than the stellar cooling constraints [68, 78{80]. We checked that data from KIMS [23], in the absence of a background model, provides a weaker bound (not shown) than CDMSlite. However, the future SuperCDMS, as well as possible scintillating target experiments, can signi cantly improve on existing bounds for A0 masses below 1Future versions of the DAMIC experiments [42, 76], using so-called \Skipper CCDs", could allow them to reduce their threshold to near the band gap of silicon [77]. The expected exposure is about 100 gramyears, less than the expected SuperCDMS exposure for silicon. Our projections for SuperCDMS can be rescaled easily to get projections for DAMIC. like particle (ALP) dark matter (left ) and dark-photon (A0) dark matter (right ), assuming that the ALP/A0 constitutes all the dark matter. Colored regions show constraints from XENON10, XENON100, and CDMSlite, as derived in this work, as well as the DAMIC results for A0 from [85]. Shaded bands around XENON10 and XENON100 limits show how the bound varies when changing the modeling of the secondary ionization in xenon. Deep- and light-purple solid lines show projected 90% C.L. sensitivities for SuperCDMS SNOLAB HV using either Ge (20 kg-years) or Si (10 kgyears) targets, respectively. Yellow, orange, and green solid lines show projected sensitivities for hypothetical experiments with the scintillating targets CsI, NaI, and GaAs, assuming an exposure of 10 kg-years. All projections assume a realistic background model discussed in the text, but zero dark counts to achieve sensitivity to low-energy electron recoils. In-medium e ects are included for all A0 constraints and projections. Shaded gray regions show known constraints from anomalous cooling of the Sun, red giant stars (RG), white dwarf stars (WD), and/or horizontal branch stars (HB), which are independent of the ALP or A0 relic density. Also shown (left ) are the combined bounds from XENON100 [22], EDELWEISS [21], CDMS [20], and CoGeNT [19]; and (right ) a bound derived in [35] based on XENON100 data from 2014 [22]. Shaded orange region in left plot is consistent with an ALP possibly explaining the white dwarf luminosity function. photons and ALPs. Acknowledgments photons (A0) with a Stuckelberg mass from the Sun via resonant production (including in-medium e ects). Colored regions show constraints from XENON10, XENON100, and CDMSlite, as derived in this work. Deep- and light-purple solid lines show projected 90% C.L. sensitivities for SuperCDMS SNOLAB HV using either Ge (20 kg-years) or Si (10 kg-years) targets, respectively. Yellow, orange, and green solid lines show projected sensitivities for hypothetical experiments with the scintillating targets CsI, NaI, and GaAs, assuming an exposure of 10 kg-years. All projections assume a realistic background model discussed in the text, but zero dark counts to achieve sensitivity to low-energy electron recoils. Shaded gray regions show constraint from anomalous cooling of the Sun. Also shown (dotted line) is the bound derived in [66] based on XENON10 data [22]. Signi cant progress is being made in lowering the threshold of DM direct detection experiments. In this work, we studied the implications of this progress for searching for dark absorption. We nd that planned and possible new experiments, using semiconducting and scintillating targets, can signi cantly improve on existing constrains, even probing well beyond astrophysical bounds. This provides unprecedented sensitivity to low-mass dark While completing this work, we became aware of [86], which considers We thank Haipeng An, Ranny Budnik, Lauren Hsu, Maxim Pospelov, Josef Pradler, and Javier Redondo for useful discussions or correspondence. R.E. is supported by the DoE Early Career research program DESC0008061 and through a Sloan Foundation Research Fellowship. T.-T.Y. is also supported by grant DESC0008061. T.V. is supported by the European Research Council (ERC) under the EU Horizon 2020 Programme (ERC-CoG2015 - Proposal n. 682676 LDMThExp), by the PAZI foundation, by the German-Israeli [ ] [ ] and Xe and CsI have very similar behaviors due to similar refractive indices. Foundation (grant No. I-1283- 303.7/2014) and by the I-CORE Program of the Planning Budgeting Committee and the Israel Science Foundation (grant No. 1937/12). Rates for ALPs and dark photons in various materials In gure 5, we show the couplings and gaee for detecting 1 event/kg/year in a variety of materials for the A0 and ALP models, respectively, as a function of mass. Note that these calculations do not depend on any background model. Therefore, one can easily translate these rates into a projection for a given background model or a limit for a given set of Solar A0 production A detailed description of the solar dark photon production and absorption rate can be found in ref. [66]. We describe the relevant equations for the Stueckelberg case here. The production rate is given by where T;L is the e ective mixing angle de ned in eq. (3.7). Inside the Sun the photon self energy is given by T = !p2; T;L = e !=T ) aTb;Ls; L = !p2(m2A0 =!2); = 2T;L d!dV inside the Sun, and me is the electron mass [67]. There are two primary production modes for the dark photon: resonant production and bremsstrahlung. ' 300 eV, the dominant contribution to the ux is longitudinal resonant production. The resonant production rate is given as e!=T ) e!=T (!p) e!=T (!p) d!p2=dr !p=! d!p2=dr !p=mA0 Using eq. 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Itay M. Bloch, Rouven Essig, Kohsaku Tobioka, Tomer Volansky, Tien-Tien Yu. Searching for dark absorption with direct detection experiments, Journal of High Energy Physics, 2017, 1-21, DOI: 10.1007/JHEP06(2017)087