Electron Recoil Research Articles

Spin-dependent dark matter-electron interactions

Detectors with low thresholds for electron recoil open a new window to direct searches of sub-GeV dark matter (DM) candidates. In the past decade, many strong limits on DM-electron interactions have been set, but most on the one which is spin-independent (SI) of both dark matter and electron spins. In this work, we study DM-atom scattering through a spin-dependent (SD) interaction at leading order (LO), using well-benchmarked, state-of-the-art atomic many-body calculations. Exclusion limits on the SD DM-electron cross section are derived with data taken from experiments with xenon and germanium detectors at leading sensitivities. In the DM mass range of 0.1 - 10 GeV, the best limits set by the XENON1T experiment: $\sigma_e^{\textrm{(SD)}}<10^{-41}-10^{-40}\,\textrm{cm}^2$ are comparable to the ones drawn on DM-neutron and DM-proton at slightly bigger DM masses. The detector's responses to the LO SD and SI interactions are analyzed. In nonrelativistic limit, a constant ratio between them leads to an indistinguishability of the SD and SI recoil energy spectra. Relativistic calculations however show the scaling starts to break down at a few hundreds of eV, where spin-orbit effects become sizable. We discuss the prospects of disentangling the SI and SD components in DM-electron interactions via spectral shape measurements, as well as having spin-sensitive experimental signatures without SI background.

Low melting point MCP-69, MCP-96, MCP-137, and MCP-200 alloys for radiation protection in radiological and therapeutic processes

ObjectiveTo evaluate the low melting-point MCP-69, MCP-96, MCP-137, and MCP-200 alloys, and characterize them for their potential to protect from the harms associated with radiation and eliminate radiation hazards during radiological procedures and treatment of cancer. MethodsThe Klein-Nishina formula was used to calculate the electronic and atomic cross-sections of these alloys using photon beams with energies 4, 6, 9, 12, and 18 ​MeV. Energy transfer coefficients, Compton mass attenuation coefficient, mass-energy transfer coefficient, and recoil energy of electrons in the specific photon energies of 4–18 ​MeV were calculated. The alloys' effective charge number and the photon energy were key factors in determining the properties found by utilizing the Klein-Nishina formula and Compton effects. ResultsThe cross sections and energy transfer coefficients increased with the increasing effective charge number Z of the alloys and decreased as the photon energy increased. The Compton recoil of the ejected electrons was observed to have a direct relationship with photon energy, but mass-energy transfer decreased with increasing photon energy. These alloys can replace the toxic lead for environmentally cleaned radiation applications. ConclusionsThese calculations and characteristics of the MCP alloys can help further determine their viability as materials for radiation shielding, their use in safe cancer diagnosis, treatment, and environmental hazards protection.

Inelastic charged-current interactions of supernova neutrinos in two-phase liquid xenon dark matter detectors

It is known that neutrinos from supernova (SN) bursts can potentially give rise to nuclear recoil (NR) signals arising from the neutral current (NC) process of coherent elastic neutrino-nucleus scattering ($\mathrm{CE}\ensuremath{\nu}\mathrm{NS}$) interactions of the neutrinos with xenon nuclei in future, large (multiton scale) liquid xenon (LXe) detectors employed for dark matter searches, depending on the SN progenitor mass and distance to the SN. In this paper, we show that the same detectors will also be sensitive to inelastic charged-current (CC) interactions of the SN electron neutrinos (${\ensuremath{\nu}}_{e}\mathrm{CC}$) with xenon nuclei. Such interactions, while creating an electron in the final state, also leave the postinteraction target nucleus in an excited state, the subsequent deexcitation of which produces, among other particles, $\ensuremath{\gamma}$ rays and neutrons. The electron and deexcitation $\ensuremath{\gamma}$ rays will give ``electron recoil'' (ER) type signals, while the deexcitation neutrons---the so-called ``neutrino-induced neutrons'' (${\ensuremath{\nu}}_{e}\mathrm{In}$)---produce, through their multiple scattering on the xenon nuclei, further xenon nuclear recoils that will also give NR signals (in addition to those produced through the $\mathrm{CE}\ensuremath{\nu}\mathrm{NS}$ interactions). We discuss the observable scintillation and ionization signals associated with SN neutrino-induced $\mathrm{CE}\ensuremath{\nu}\mathrm{NS}$ and ${\ensuremath{\nu}}_{e}\mathrm{CC}$ events in a generic LXe detector and argue that upcoming, sufficiently large LXe detectors should be able to detect both these types of events due to neutrinos from reasonably close-by SN bursts. We also note that since the total CC-induced ER and NR signals receive contributions predominantly from ${\ensuremath{\nu}}_{e}\mathrm{CC}$ interactions while the $\mathrm{CE}\ensuremath{\nu}\mathrm{NS}$ contribution comes from NC interactions of all six species of neutrinos, identification of the ${\ensuremath{\nu}}_{e}\mathrm{CC}$ and $\mathrm{CE}\ensuremath{\nu}\mathrm{NS}$ origin events may offer the possibility of extracting useful information about the distribution of the total SN explosion energy going into different neutrino flavors.

Optimization of detector modules for measuring gamma-ray polarization in Positron Emission Tomography

Detection of γ-ray polarization in Positron Emission Tomography (PET) is as yet an unexploited feature that could be used as an additional handle to improve signal-to-background ratio in this imaging modality. The γ polarization is related to the azimuthal angle in the Compton scattering process, so the initial correlation of polarizations of the annihilation quanta translates to the correlation of the azimuthal angles in events where both annihilation photons undergo Compton scattering in respective detectors. This results in a modulated distribution of the azimuthal angle difference for true events, while this modulation is lacking for the background events. We present a comprehensive experimental study of five detector configurations based on scintillator matrices and silicon photomultipliers, suitable for measuring the azimuthal modulation. The modules consist of either GaGG:Ce or LYSO:Ce pixels with sizes varying from 1.9×1.9×20mm3 to 3×3×20mm3. The distinctive feature of the modules is that they can reconstruct the Compton scattering by detecting the recoil electron and the scattered γ in a single detector layer, which simplifies extension to larger systems. The amplitude modulation of the azimuthal angles’ difference is clearly observable in all configurations ranging from 0.26±0.01 to 0.34±0.02 depending on the event selection criteria. The results suggest that finer detector segmentation plays a leading role in achieving higher modulation factors.

Interpretation of XENON1T excess with MeV boosted dark matter * *The work of T. Li was supported by the National Natural Science Foundation of China (11975129, 12035008) and "the Fundamental Research Funds for the Central Universities", Nankai University (63196013). L.-B. Jia acknowledges support from the Longshan academic talent research supporting program of SWUST (18LZX415)

The XENON1T excess of keV electron recoil events may be induced by the scattering of electrons and long-lived particles with an MeV mass and high speed. We consider a tangible model composed of two scalar MeV dark matter (DM) particles, and , to interpret the XENON1T keV excess via boosted . A small mass splitting is introduced, and the boosted can be produced using the dark annihilation process of via a resonant scalar ϕ. electron scattering is intermediated by a vector boson X. Although the constraints from Big Bang nucleosynthesis, cosmic microwave background (CMB), and low-energy experiments set the mediated electron scattering cross section to be , the MeV scale DM with a resonance enhanced dark annihilation today can still provide sufficient boosted and induce the XENON1T keV excess. The relic density of is significantly reduced by the s-wave process , which is permitted by the constraints from CMB and 21-cm absorption. A very small relic fraction of is compatible with the stringent bounds on un-boosted -electron scattering in DM direct detection, and the -electron scattering is also allowed.

Low radioactive material screening and background control for the PandaX-4T experiment

PandaX-4T is a ton-scale dark matter direct detection experiment using a dual-phase TPC technique at the China Jinping Underground Laboratory. Various ultra-low background technologies have been developed and applied to material screening for PandaX-4T, including HPGe gamma spectroscopy, ICP-MS, NAA, radon emanation measurement system, krypton assay station, and alpha detection system. Low background materials were selected to assemble the detector. Surface treatment procedures were investigated to further suppress radioactive background. Combining measured results and Monte Carlo simulation, the total material background rates of PandaX-4T in the energy region of 1–25 keVee are estimated to be (9.9 ± 1.9) × 10−3 mDRU for electron recoil and (2.8 ± 0.6) × 10−4 mDRU for nuclear recoil. In addition, natKr in the detector is estimated to be < 8 ppt.

Revisiting the fermionic dark matter absorption on electron target

We perform a systematic study of the fermionic DM absorption interactions on electron target in the context of effective field theory. The fermionic DM absorption is not just sensitive to sub-MeV DM with efficient energy release, but also gives a unique signature with clear peak in the electron recoil spectrum whose shape is largely determined by the atomic effects. Fitting with the Xenon1T and PandaX-II data prefers DM mass at mχ = 59 keV and 105 keV, respectively, while the cut-off scale is probed up to around 1 TeV. The DM overproduction in the early Universe, the invisible decay effect on the cosmological evolution, and the visible decay signal collected by the astrophysical X(gamma)-ray observations (Insight-HXMT, NuSTAR, HEAO-1, and INTEGRAL) are thoroughly explored to constrain the DM absorption interactions. With stringent bounds on the tensor and pseudo-scalar operators, the other fermionic DM operators are of particular interest at tonne-scale direct detection experiments such as PandaX-4T, XENONnT, and LZ.

Low-energy signals from the formation of dark-matter–nucleus bound states

Dark matter particles may bind with nuclei if there exists an attractive force of sufficient strength. We show that a dark photon mediator of mass $\ensuremath{\sim}(10--100)\text{ }\text{ }\mathrm{MeV}$ that kinetically mixes with Standard Model electromagnetism at the level of $\ensuremath{\sim}{10}^{\ensuremath{-}3}$ generates keV-scale binding energies between dark matter and heavy elements, while forbidding the ability to bind with light elements. In underground direct detection experiments, the formation of such bound states liberates keV-scale energy in the form of electrons and photons, giving rise to monoenergetic electronic signals with a time structure that may contain daily and seasonal modulations. We show that data from liquid-xenon detectors provides exquisite sensitivity to this scenario, constraining the galactic abundance of such dark particles to be at most $\ensuremath{\sim}{10}^{\ensuremath{-}18}--{10}^{\ensuremath{-}12}$ of the galactic dark-matter density for masses spanning $\ensuremath{\sim}(1--{10}^{5})\text{ }\text{ }\mathrm{GeV}$. However, an exponentially small fractional abundance of these dark particles is enough to explain the observed electron recoil excess at XENON1T.

Scintillation yield from electronic and nuclear recoils in superfluid He4

Superfluid $^4$He is a promising target material for direct detection of light ($<$ 1 GeV) dark matter. Possible signal channels available for readout in this medium include prompt photons, triplet excimers, and roton and phonon quasiparticles. The relative yield of these signals has implications for the sensitivity and discrimination power of a superfluid $^4$He dark matter detector. Using a 16~cm$^3$ volume of 1.75~K superfluid $^4$He read out by six immersed photomultiplier tubes, we measured the scintillation from electronic recoils ranging between 36.3 and 185 keV$_\mathrm{ee}$, yielding a mean signal size of $1.25^{+0.03}_{-0.03}$~phe/keV$_\mathrm{ee}$, and nuclear recoils from 53.2 to 1090 keV$_\mathrm{nr}$. We compare the results of our relative scintillation yield measurements to an existing semiempirical model based on helium-helium and electron-helium interaction cross sections. We also study the behavior of delayed scintillation components as a function of recoil type and energy, a further avenue for signal discrimination in superfluid $^4$He.

Solar active-sterile neutrino conversion with atomic effects at dark matter direct detection experiments

The recent XENON1T excess can be explained by the solar active-sterile neutrino conversion with bound electrons via light mediator. Nevertheless, the atomic effects are usually omitted in the solar neutrino explanations. We systematically establish a second quantization formalism for both bound and ionized electrons to account for the atomic effects. This formalism is of great generality to incorporate various interactions for both neutrino and dark matter scatterings. Our calculation shows that the change in the cross section due to atomic effects can have important impact on the differential cross section. It is necessary to include atomic effects in the low-energy electron recoil signal at dark matter direct detection experiments even for energetic solar neutrinos. With the best-fit values to the XENON1T data, we also project the event rate at PandaX-4T, XENONnT, and LZ experiments.

Ground-state hyperfine structure of light muon-electron ions

The ground state hyperfine splitting of light muon-electron ions of lithium, beryllium, boron and helium is calculated on the basis of analytical perturbation theory in terms of small parameters of the fine structure constant and electron-muon mass ratio. The corrections of vacuum polarization, nuclear structure and recoil effects and electron vertex corrections are taken into account. The dependence of the corrections on the nucleus charge Z is studied. The obtained total values of hyperfine splitting intervals can be used for comparison with future experimental data.

Boosted self-interacting dark matter and XENON1T excess

We present a self-interacting boosted dark matter (DM) scenario as a possible explanation of the recently reported excess of electron recoil events by the XENON1T experiment. The Standard Model (SM) has been extended with two vector-like fermion singlets charged under a dark U(1)D gauge symmetry to describe the dark sector. While the presence of light vector boson mediator leads to sufficient DM self-interactions to address the small scale issues of cold dark matter, the model with sub-GeV scale DM can explain the XENON1T excess via elastic scattering of boosted DM component with electrons at the detector. Strong annihilation of DM into the light mediator leads to a suppressed thermal relic. A hybrid setup of dark freeze-out and non-thermal contribution from the late decay of a scalar can lead to correct relic abundance. We fit our model with XENON1T data and also find the final parameter space consistent with self-interaction of DM, DM-electron scattering rate, as well as astrophysical and cosmological observations. A tiny parameter space consistent with all these constraints and requirements can be further scrutinized in near-future experiments.

Study on NaI(Tl) crystal at −35 [formula omitted]C for dark matter detection

We present the response of a NaI(Tl) crystal in terms of the light yield and pulse shape characteristics at two different temperatures: 22 °C (room temperature) and −35 °C (low temperature). The light yield is measured using 59.54keV γ-rays from a 241Am source relative to the mean charge of single photoelectrons. At the low temperature, we measure a 4.7 ± 1.3% increase in the light yield compared to that at room temperature. A significantly increased decay time is also observed at the low temperature. The responses to nuclear recoil events are measured using neutrons from a 252Cf source and compared to those to electron recoil events. The measured pulse shape discrimination (PSD) power of the NaI(Tl) crystal at the low temperature is found to be improved in the entire energy range studied because of the increased light yield and the different scintillation characteristics. We also find a 9.23 ± 0.26% increased quenching factor of α-induced events, which is the light yield ratio of α recoil to electron recoil, at the low temperature. This supports the possibility of an increased quenching factor of the nuclear recoil events that are known to have similar processes of dark matter interaction. The increased light yield and the improved PSD power of nuclear recoil events enhance the sensitivity for dark matter detection via dark matter–nuclei interactions.

Relativistic theory to Compton effect for spectroscopic detector

We report here a theoretical proof of the limiting value of the velocity of target/particle responsible for various photon–electron interactions in the Compton scattering experiment which may be carried out in various applications such as gamma-ray spectroscopy. Compton scattering deals with the collision of photons with a target particle where both energy and momentum are transferred to the particle while the photon moves with reduced energy and a change of momentum. Here, the changed energy of a photon depends on the angle between the incident axis and scattered photon (θ), the rest mass of the electron, and the frequency of the photon before the collision. However, when some initial velocity is provided to an electron that is at rest initially, then in such cases, the particle obeys limiting value on velocity for a particular energetic photon to follow Compton scattering. In such cases, the changed frequency of photons also depends on the initial and final velocity of nonstationary electrons during a collision, including the angle (φ′) between the recoil electron and incident axis. Here, we have presented the derivations to calculate the recoil velocity of the nonstationary electron. These terms are the accurate solution of the Compton shift. We have tried to calculate the upper limit or limiting values of the initial velocity of the target particle in Compton scattering. Further, we have computationally demonstrated that there is a dependence of scattered photon frequency on initial velocity of target particle (u) and mass of the target particle (m) at a higher scattering angle. Finally, numerical simulations were carried out with particles such as an electron, pion (+/-, 0) to verify the greater degree of accuracy of the particles in spectroscopic detectors such as gamma-ray spectroscopy and other high energy spectroscopic techniques.

Electron recoils from terrestrial upscattering of inelastic dark matter

The growing interest in the interactions between dark matter particles and electrons has received a further boost by the observation of an excess in electron recoil events in the XENON1T experiment. Of particular interest are dark matter models in which the scattering process is inelastic, such that the ground state can upscatter into an excited state. The subsequent exothermic downscattering of such excited states on electrons can lead to observable signals in direct detection experiments and gives a good fit to the XENON1T excess. In this work, we study terrestrial upscattering, i.e. inelastic scattering of dark matter particles on nuclei in the Earth, as a plausible origin of such excited states. Using both analytical and Monte Carlo methods, we obtain detailed predictions of their density and velocity distribution. These results enable us to explore the time dependence of the flux of excited states resulting from the rotation of the Earth. For the case of XENON1T, we find the resulting daily modulation of the electron recoil signal to be at the level of 10% with a strong dependence on the dark matter mass.

Solar reflection of light dark matter with heavy mediators

The direct detection of sub-GeV dark matter particles is hampered by their low energy deposits. If the maximum deposit allowed by kinematics falls below the energy threshold of a direct detection experiment, it is unable to detect these light particles. Mechanisms that boost particles from the Galactic halo can therefore extend the sensitivity of terrestrial direct dark matter searches to lower masses. Sub-GeV and sub-MeV dark matter particles can be efficiently accelerated by colliding with thermal nuclei and electrons of the solar plasma, respectively. This process is called ``solar reflection.'' In this paper, we present a comprehensive study of solar reflection via electron and/or nuclear scatterings using Monte Carlo simulations of dark matter trajectories through the Sun. We study the properties of the boosted dark matter particles, obtain exclusion limits based on various experiments probing both electron and nuclear recoils, and derive projections for future detectors. In addition, we find and quantify a novel, distinct annual modulation signature of a potential solar reflection signal which critically depends on the anisotropies of the boosted dark matter flux ejected from the Sun. Along with this paper, we also publish the corresponding research software.

Neutrino up-scattering via the dipole portal at forward LHC detectors

The significant neutrino flux at high rapidity at the LHC motivates dedicated forward detectors to study the properties of neutrinos at TeV energies. We investigate magnetic dipole interactions between the active neutrinos and new sterile states at emulsion and liquid argon experiments that could be located in a future Forward Physics Facility (FPF) downstream of the ATLAS interaction point. The up-scattering of neutrinos off electrons produces an electron recoil signature that can probe new regions of parameter space at the High Luminosity LHC (HL-LHC), particularly for liquid argon detectors due to low momentum thresholds. We also consider the decay of the sterile neutrino through the dipole operator, which leads to a photon that could be displaced from the production vertex. FPF detectors can test sterile neutrino states as heavy as 1 GeV produced through the dipole portal, highlighting the use of high energy LHC neutrinos as probes of new physics.

Data from 14,577 cosmological objects and 14 FRBs confirm the predictions of new tired light (NTL) and lead to a new model of the IGM

Predictions by New Tired Light were tested using 14,577 objects from the NED-D compilation of redshift-independent distances. These objects give an electron number density of ne = 0.499 m −3 compared to the predicted one of ne = 0.5 m −3. In NTL the Hubble constant is given by H = 2nehre /me and, using this value for ne gives 62.5 km/s per Mpc which is very close to the accepted values. NTL predicts a linear relationship between distance and ln (1 + z) with gradient (mec/2ne hre = 1.46×1026 m). Plotting all the 14,577 points gives a straight line with gradient 1.40×1026 m – just 4% off the predicted value. Using distances from the compilation the redshift is calculated by NTL and a graph of predicted versus observed redshift is drawn. This has a gradient of 0.9756 close to the value ‘1.0’ expected in a 1:1 relationship between prediction and expected. Both graphs are linear up to redshifts of ‘9’ with no hint of relativistic effects. In NTL, there is a delay between an electron in the IGM absorbing and re-emitting a photon whereby the electron recoils (leading to the redshift). Data from FRB 121102 gives the time lag between two frequencies arriving and using the extra number of photon-electron interactions made by the longer wavelength the time delay is found. This tells us the length of the delay at each interaction as ≈ 10−10 s. Using NTL and DM the redshift of the host galaxy was calculated and found to be z = 0.143 compared to the measured value of z = 0.19 – the difference lying well within the uncertainty in DMIn NTL, DM and redshift are produced by the electrons in the IGM and so there is a direct relation between them. DMIGM = (mec/2hre ){ln(1 + z)} or DMIGM = 2470{ln(1 +z){. Plotting data from 14 localised FRBs on a graph of DM versus {ln(1 + z)} does give a straight-line graph but a selection of eight from the fourteen are colinear with a gradient of 1244 ± 147 pc cm −3 much closer t that predicted. Several hosts are said to be tentative and so we will continue to plot this graph as more and more FRBs are located. Often tired light models are discounted on the basis of an old model of the IGM as having a neutral plasma at high temperature and/or they are using Compton scatter. In NTL, recoil takes place along the line of sight so there is no blurring. Several mainstream papers show that every dust particle in the IGM is positively charged with an excess of protons due to photoionisation. This means an equal number of electrons have been released into the intervening space. On this basis the IGM is a ‘dirty plasma’ with the protons trapped on dust particles and a sea of electrons in-between. When a group of electrons come together in this way, they will arrange themselves onto a BCC lattice (Wigner-Seitz crystal). Calculations show that if we use dust density restricted by considerations of an expanding Universe there is not enough to give the ne = 0.5 m −3 found by observation but would need a dust density of ρIGM ≈ 3×10−25 kgm −3. A previous paper looked at the photoionisation of Hydrogen clouds surrounding a galaxy with the protons staying behind and forming dark matter whilst the electrons went off into the IGM to form on their crystal lattice held by mutual repulsion. The mass of dark matter surrounding the Milky Way galaxy is known and so, if this is all protons, we can find the number of protons there. An equal number of electrons will have been released into the IGM and dividing this by the average volume occupied by a galaxy gives us the ne = 1 m −3 and agrees with observation.

Electron recoil effect in electrically tunable MoSe2 monolayers

Radiative recombination of bound-exciton carrier complexes in semiconductors often occurs under simultaneous excitation of either electrons or holes to unbound states. This phenomenon, known as the electron recoil effect, manifests itself in pronounced asymmetric spectral lineshapes of the resulting emission. Monitoring the dynamics of the electron recoil effect in monolayer semiconductors offers access to transient nonequilibrium distributions of the exciton-carrier complexes and their cooling dynamics.

Neutrino backgrounds in future liquid noble element dark matter direct detection experiments

Experiments that use liquid noble gases as target materials, such as argon and xenon, play a significant role in direct detection searches for weakly interacting massive particles(-like) dark matter. As these experiments grow in size, they will soon encounter a new background to their dark matter discovery potential from neutrino scattering off nuclei and electrons in their targets. Therefore, a better understanding of this new source of background is crucial for future large-scale experiments such as ARGO and DARWIN. In this work, we study the impact of atmospheric neutrino flux uncertainties, electron recoil rejection efficiency, recoil energy sensitivity, and other related factors on the dark matter discovery reach. We also show that a significant improvement in sensitivity can potentially be obtained, at large exposures, by combining data from independent argon and xenon experiments.