9 research outputs found
Direct Detection of Strongly Interacting Sub-GeV Dark Matter via Electron Recoils
We consider direct-detection searches for sub-GeV dark matter via electron
scatterings in the presence of large interactions between dark and ordinary
matter. Scatterings both on electrons and nuclei in the Earth's crust,
atmosphere, and shielding material attenuate the expected local dark matter
flux at a terrestrial detector, so that such experiments lose sensitivity to
dark matter above some critical cross section. We study various models,
including dark matter interacting with a heavy and ultralight dark photon,
through an electric dipole moment, and exclusively with electrons. For a
dark-photon mediator and an electric dipole interaction, the dark
matter-electron scattering cross-section is directly linked to the dark
matter-nucleus cross section, and nuclear interactions typically dominate the
attenuation process. We determine the exclusion bands for the different
dark-matter models from several experiments - SENSEI, CDMS-HVeV, XENON10,
XENON100, and DarkSide-50 - using a combination of Monte Carlo simulations and
analytic estimates. We also derive projected sensitivities for a detector
located at different depths and for a range of exposures, and calculate the
projected sensitivity for SENSEI at SNOLAB and DAMIC-M at Modane. Finally, we
discuss the reach to high cross sections and the modulation signature of a
small balloon- and satellite-borne detector sensitive to electron recoils, such
as a Skipper-CCD. Such a detector could potentially probe unconstrained
parameter space at high cross sections for a sub-dominant component of dark
matter interacting with a massive, but ultralight, dark photon.Comment: 40 pages, 12 figures. Code available at
https://github.com/temken/DaMaSCUS-CRUST and
https://doi.org/10.5281/zenodo.2846401 . v2: matches published versio
Solar Neutrinos as a Signal and Background in Direct-Detection Experiments Searching for Sub-GeV Dark Matter With Electron Recoils
Direct-detection experiments sensitive to low-energy electron recoils from
sub-GeV dark matter (DM) interactions will also be sensitive to solar neutrinos
via coherent neutrino-nucleus scattering (CNS), since the recoiling nucleus can
produce a small ionization signal. Solar neutrinos constitute both an
interesting signal in their own right and a potential background to a DM search
that cannot be controlled or reduced by improved shielding, material
purification and handling, or improved detector design. We explore these two
possibilities in detail for semiconductor (Si and Ge) and Xe targets,
considering several possibilities for the unmeasured ionization efficiency at
low energies. For DM-electron-scattering searches, neutrinos start being an
important background for exposures larger than ~1-10 kg-years in Si and Ge, and
for exposures larger than ~0.1-1 kg-year in Xe. For the absorption of bosonic
DM (dark photons and axion-like particles) by electrons, neutrinos are most
relevant for masses below ~1 keV and again slightly more important in Xe.
Treating the neutrinos as a signal, we find that the CNS of B-8 neutrinos can
be observed with ~2 sigma significance with exposures of ~2, 7, and 20 kg-years
in Xe, Ge, and Si, respectively, assuming there are no other backgrounds. We
give an example for how this would constrain non-standard neutrino
interactions. Neutrino components at lower energy can only be detected if the
ionization efficiency is sufficiently large. In this case, observing pep
neutrinos via CNS requires exposures ~10-100 kg-years in Si or Ge (~1000
kg-years in Xe), and observing CNO neutrinos would require an order of
magnitude more exposure. Only Si could potentially detect Be-7 neutrinos. These
measurements would allow for a direct measurement of the electron-neutrino
survival probability over a wide energy range.Comment: 17 pages + refs, 15 figures, 4 tables. v3 minor corrections. Scaling
of Fig. 9 corrected. Minor corrections to Fig. 4,7,8 and 15. Conclusions
unchange
Anharmonic effects in nuclear recoils from sub-GeV dark matter
Direct detection experiments are looking for nuclear recoils from scattering
of sub-GeV dark matter (DM) in crystals, and have thresholds as low as ~ 10 eV
or DM masses of ~ 100 MeV. Future experiments are aiming for even lower
thresholds. At such low energies, the free nuclear recoil prescription breaks
down, and the relevant final states are phonons in the crystal. Scattering
rates into single as well as multiple phonons have already been computed for a
harmonic crystal. However, crystals typically exhibit some anharmonicity, which
can significantly impact scattering rates in certain kinematic regimes. In this
work, we estimate the impact of anharmonic effects on scattering rates for DM
in the mass range ~ 1-10 MeV, where the details of multiphonon production are
most important. Using a simple model of a nucleus in a bound potential, we find
that anharmonicity can modify the scattering rates by up to two orders of
magnitude for DM masses of O(MeV). However, such effects are primarily present
at high energies where the rates are suppressed, and thus only relevant for
very large DM cross sections. We show that anharmonic effects are negligible
for masses larger than ~ 10 MeV.Comment: 30 pages, 12 figures, 1 table, 56 reference
Doped Semiconductor Devices for sub-MeV Dark Matter Detection
Dopant atoms in semiconductors can be ionized with meV energy
depositions, allowing for the design of low-threshold detectors. We propose
using doped semiconductor targets to search for sub-MeV dark matter scattering
or sub-eV dark matter absorption on electrons. Currently unconstrained cross
sections could be tested with a 1 g-day exposure in a doped detector with
backgrounds at the level of existing pure semiconductor detectors, but
improvements would be needed to probe the freeze-in target. We discuss the
corresponding technological requirements and lay out a possible detector
design.Comment: 12 pages, 7 figure
The Migdal effect in semiconductors for dark matter with masses below ∼ 100 MeV
Abstract Dark matter scattering off a nucleus has a small probability of inducing an observable ionization through the inelastic excitation of an electron, called the Migdal effect. We use an effective field theory to extend the computation of the Migdal effect in semiconductors to regions of small momentum transfer to the nucleus, where the final state of the nucleus is no longer well described by a plane wave. Our analytical result can be fully quantified by the measurable dynamic structure factor of the semiconductor, which accounts for the vibrational degrees of freedom (phonons) in a crystal. We show that, due to the sum rules obeyed by the structure factor, the inclusive Migdal rate and the shape of the electron recoil spectrum is well captured by approximating the nuclei in the crystal as free ions; however, the exclusive differential rate with respect to energy depositions to the crystal depends on the phonon dynamics encoded in the dynamic structure function of the specific material. Our results now allow the Migdal effect in semiconductors to be evaluated even for the lightest dark matter candidates (m χ ≳ 1 MeV) that can kinematically excite electrons
Exploring New Physics with O(keV) Electron Recoils in Direct Detection Experiments
Motivated by the recent XENON1T results, we explore various new physics models that can be discovered through searches for electron recoils in O(keV)-threshold direct-detection experiments. First, we consider the absorption of light bosons, either as dark matter relics or being produced directly in the Sun. In the latter case, we find that keV mass bosons produced in the Sun provide an adequate fit to the data but are excluded by stellar cooling constraints. We address this tension by introducing a novel Chameleon-like axion model, which can explain the excess while evading the stellar bounds. We find that absorption of bosonic dark matter provides a viable explanation for the excess only if the dark matter is a dark photon or an axion. In the latter case, photophobic axion couplings are necessary to avoid X-ray constraints. Second, we analyze models of dark matter-electron scattering to determine which models might explain the excess. Standard scattering of dark matter with electrons is generically in conflict with data from lower-threshold experiments. Momentum-dependent interactions with a heavy mediator can fit the data with dark matter mass heavier than a GeV but are generically in tension with collider constraints. Next, we consider dark matter consisting of two (or more) states that have a small mass splitting. The exothermic (down)scattering of the heavier state to the lighter state can fit the data for keV mass splittings. Finally, we consider a subcomponent of dark matter that is accelerated by scattering off cosmic rays, finding that dark matter interacting though an O(100 keV)-mass mediator can fit the data. The cross sections required in this scenario are, however, typically challenged by complementary probes of the light mediator. Throughout our study, we implement an unbinned Monte Carlo analysis and use an improved energy reconstruction of the XENON1T events
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Exploring new physics with O(keV) electron recoils in direct detection experiments
Motivated by the recent XENON1T results, we explore various new physics models that can be discovered through searches for electron recoils in O(keV)-threshold direct-detection experiments. First, we consider the absorption of axion-like particles, dark photons, and scalars, either as dark matter relics or being produced directly in the Sun. In the latter case, we find that keV mass bosons produced in the Sun provide an adequate fit to the data but are excluded by stellar cooling constraints. We address this tension by introducing a novel Chameleon-like axion model, which can explain the excess while evading the stellar bounds. We find that absorption of bosonic dark matter provides a viable explanation for the excess only if the dark matter is a dark photon or an axion. In the latter case, photophobic axion couplings are necessary to avoid X-ray constraints. Second, we analyze models of dark matter-electron scattering to determine which models might explain the excess. Standard scattering of dark matter with electrons is generically in conflict with data from lower-threshold experiments. Momentum-dependent interactions with a heavy mediator can fit the data with dark matter mass heavier than a GeV but are generically in tension with collider constraints. Next, we consider dark matter consisting of two (or more) states that have a small mass splitting. The exothermic (down)scattering of the heavier state to the lighter state can fit the data for keV mass splittings. Finally, we consider a subcomponent of dark matter that is accelerated by scattering off cosmic rays, finding that dark matter interacting though an O(100 keV)-mass mediator can fit the data. The cross sections required in this scenario are, however, typically challenged by complementary probes of the light mediator. Throughout our study, we implement an unbinned Monte Carlo analysis and use an improved energy reconstruction of the XENON1T events