Search

We review the most important models of light dark matter and discuss the NA64 experiment aimed at searching for hypothetical particles, including dark matter, in the mass range ⩽ O (1) GeV with the use of electron and muon beams at the Super Proton Synchrotron (SPS) accelerator at CERN. We consider the methods and results of searches in the NA64 and other accelerator experiments and also discuss their further prospects.

The QCD axion or axion-like particles are candidates of dark matter of the universe. On the other hand, axion-like excitations exist in certain condensed matter systems, which implies that there can be interactions of dark matter particles with condensed matter axions. We discuss the relationship between the condensed matter axion and a collective spin-wave excitation in an anti-ferromagnetic insulator at the quantum level. The conversion rate of the light dark matter, such as the elementary particle axion or hidden photon, into the condensed matter axion is estimated for the discovery of the dark matter signals., 32 pages, 3 figures

Delayed single- and few-electron emissions plague dual-phase time projection chambers, limiting their potential to search for light-mass dark matter. This paper examines the origins of these events in the XENON1T experiment. Characterization of the intensity of delayed electron backgrounds shows that the resulting emissions are correlated, in time and position, with high-energy events and can effectively be vetoed. In this work we extend previous S2-only analyses down to a single electron. From this analysis, after removing the correlated backgrounds, we observe rates < 30 events/(electron*kg*day) in the region of interest spanning 1 to 5 electrons. We derive 90% confidence upper limits for dark matter-electron scattering, first direct limits on the electric dipole, magnetic dipole, and anapole interactions, and bosonic dark matter models, where we exclude new parameter space for dark photons and solar dark photons., 20 pages, 17 figures

This minireview aim is to outline the main experimental results related to the search for LDM (light dark matter) at NA64 experiment at CERN. We discuss current results and the NA64 perspectives. Also we discuss the problem of the origin of the photon-dark photon mixing term $$\tfrac{\varepsilon }{2}{{F}^{{\mu \nu }}}F_{{\mu \nu }}^{'}$$ and its connection with loop corrections.

We revisit the scenario of a massive spin-2 particle as the mediator for communicating between dark matter of arbitrary spin and the Standard Model. Taking the general couplings of the spin-2 particle in the effective theory, we discuss the thermal production mechanisms for dark matter with various channels and the dark matter self-scattering. For WIMP and light dark matter cases, we impose the relic density condition and various experimental constraints from direct and indirect detections, precision measurements as well as collider experiments. We show that it is important to include the annihilation of dark matter into a pair of spin-2 particles in both allowed and forbidden regimes, thus opening up the consistent parameter space for dark matter. The benchmark models of the spin-2 mediator are presented in the context of the warped extra dimension and compared to the simplified models., Comment: 39 pages, 8 figures, v2: references and unitarity bounds added, v3: Accepted for publication in European Physical Journal C

We discuss a novel approach for directional, light dark matter searches inspired by the high precision position measurements achieved in gravitational wave detectors. If dark matter interacts with ordinary matter, movable masses are subject to an effect similar to Brownian motion induced by the scattering with dark matter particles which exhibits certain characteristics and could be observed. We provide estimates for the sensitivity of a hypothetical experiment looking for that motion. Interestingly, if successful, our approach would allow to constrain the local distribution of dark matter momentum., 12 pages, 3 figures; version published in EPJ C

We present a unified theoretical framework for computing spin-independent direct detection rates via various channels relevant for sub-GeV dark matter -- nuclear recoils, electron transitions and single phonon excitations. Despite the very different physics involved, in each case the rate factorizes into the particle-level matrix element squared, and an integral over a target material- and channel-specific dynamic structure factor. We show how the dynamic structure factor can be derived in all three cases following the same procedure, and extend previous results in the literature in several aspects. For electron transitions, we incorporate directional dependence and point out potential daily modulation signals in anisotropic target materials. For single phonon excitations, we present a new derivation of the rate formula from first principles for generic spin-independent couplings, and include the first calculation of phonon excitation through electron couplings. We also discuss the interplay between single phonon excitations and nuclear recoils, and clarify the role of Umklapp processes, which can dominate the single phonon production rate for dark matter heavier than an MeV. Our results highlight the complementarity between various search channels in probing different kinematic regimes of dark matter scattering, and provide a common reference to connect dark matter theories with ongoing and future direct detection experiments., 39 pages, 6 figures, minor updates to constraint projections in figures with PhonoDark v1.1.0 (https://phonodark.caltech.edu)

Discovering the nature of dark matter (DM) remains one of the most important outstanding questions in particle physics. While the astrophysical evidence for its existence continues to accumulate, we know very little about its fundamental constituents, and how it connects to the Standard Model. Terrestrial direct detection experiments offer a unique experimental perspective. Detection of a signal would be the first evidence for non-gravitational interactions between DM and ordinary matter, a crucial clue in understanding the particle nature of DM. Moreover detection in a laboratory is less susceptible to astrophysical uncertainties which accompany indirect detection strategies, and offer a wider vantage than colliders with the ability to swap target materials and search for modulation effects. In this dissertation I will discuss searching for DM with the current state-of-the-art direct detection experiments based on electronic excitations, as well as a potential direction for future experiments based on phonon and magnon excitations. Previous generations of direct detection experiments utilized nuclear recoil to search for DM particles. While this process is well suited for DM candidates with masses above typical nuclear masses, ℴ(GeV), sensitivity drops precipitously for lighter DM. New physical processes must be utilized to facilitate the search for well-motivated light, sub-GeV, DM candidates. Electronic excitations are one such process which can probe DM candidates which have enough energy to excite electrons across the band gap. I will discuss many aspects of this DM-induced excitation rate which were developed in this work: the most advanced first-principles calculations of DM scattering and absorption signals using density functional theory (DFT) input, daily modulation effects in anisotropic crystal targets, comparisons of a wide variety of potential detector targets, theoretical development of a non-relativistic effective field theory (NR EFT) to aid in the calculation of DM absorption rates, as well as a study of interactions in cutting edge small gap, spin-orbit coupled targets. While direct detection experiments using electronic excitations are currently underway, to reach even lower DM masses, which do not carry enough energy to excite states across the band gap, new ideas must be explored. Collective excitations, such as phonons and magnons, exist below the electronic band gap and offer an exciting future for direct detection experiments. Detectors searching for single phonon excitations are currently in development, whereas those based on magnons are in their infancy. Similar to the electronic excitations we will discuss a myriad of topics involving single phonon and magnon excitations: an EFT of DM-collective excitation scattering for general UV theories, potential uses for detection of axion DM, as well as advanced first-principles calculations, detailed study of the directional detectability in anisotropic crystal targets, and comparisons across a variety of candidate target materials.

The current most stringent constraints for the existence of sub-GeV dark matter coupling to Standard Model via a massive vector boson $A^\prime$ were set by the NA64 experiment for the mass region $m_{A^\prime}\lesssim 250$ MeV, by analyzing data from the interaction of $2.84\cdot10^{11}$ 100-GeV electrons with an active thick target and searching for missing-energy events. In this work, by including $A^\prime$ production via secondary positron annihilation with atomic electrons, we extend these limits in the $200$-$300$ MeV region by almost an order of magnitude, touching for the first time the dark matter relic density constrained parameter combinations. Our new results demonstrate the power of the resonant annihilation process in missing energy dark-matter searches, paving the road to future dedicated $e^+$ beam efforts.

We extend the calculation of dark matter direct detection rates via electronic transitions in general dielectric crystal targets, combining state-of-the-art density functional theory calculations of electronic band structures and wave functions near the band gap, with semi-analytic approximations to include additional states farther away from the band gap. We show, in particular, the importance of all-electron reconstruction for recovering large momentum components of electronic wave functions, which, together with the inclusion of additional states, has a significant impact on direct detection rates, especially for heavy mediator models and at $\mathcal{O}(10\,\text{eV})$ and higher energy depositions. Applying our framework to silicon and germanium (that have been established already as sensitive dark matter detectors), we find that our extended calculations can appreciably change the detection prospects. Our calculational framework is implemented in an open-source program $\texttt{EXCEED-DM}$ (EXtended Calculation of Electronic Excitations for Direct detection of Dark Matter), to be released in an upcoming publication., Comment: 34 pages, 15 figures

The field of dark matter detection is a highly visible and highly competitive one. In this paper, we propose recommendations for presenting dark matter direct detection results particularly suited for weak-scale dark matter searches, although we believe the spirit of the recommendations can apply more broadly to searches for other dark matter candidates, such as very light dark matter or axions. To translate experimental data into a final published result, direct detection collaborations must make a series of choices in their analysis, ranging from how to model astrophysical parameters to how to make statistical inferences based on observed data. While many collaborations follow a standard set of recommendations in some areas, for example the expected flux of dark matter particles (to a large degree based on a paper from Lewin and Smith in 1995), in other areas, particularly in statistical inference, they have taken different approaches, often from result to result by the same collaboration. We set out a number of recommendations on how to apply the now commonly used Profile Likelihood Ratio method to direct detection data. In addition, updated recommendations for the Standard Halo Model astrophysical parameters and relevant neutrino fluxes are provided. The authors of this note include members of the DAMIC, DarkSide, DARWIN, DEAP, LZ, NEWS-G, PandaX, PICO, SBC, SENSEI, SuperCDMS, and XENON collaborations, and these collaborations provided input to the recommendations laid out here. Wide-spread adoption of these recommendations will make it easier to compare and combine future dark matter results., 20 pages, 8 figures, 4 tables, version 2 to match publication

Ultra-light dark matter (ULDM) is a class of dark matter models (DM) where DM is composed by bosons with masses ranging from $10^{-24}\, \mathrm{eV} < m < \mathrm{eV}$. These models have been receiving a lot of attention in the past few years given their interesting property of forming a Bose-Einstein condensate (BEC) or a superfluid on galactic scales. BEC and superfluidity are one of the most striking quantum mechanical phenomena manifest on macroscopic scales, and upon condensation, the particles behave as a single coherent state, described by the wavefunction of the condensate. The idea is that condensation takes place inside galaxies while outside DM behaves like a normal cold particle DM. This wave nature of DM on galactic scales that arise upon condensation can address some of the curiosities of the behaviour of DM on small scales while maintaining the successes of LCDM on large scales. There are many models in the literature that describe a DM component that condenses in galaxies. In this review, we are going to describe those models and classify them according to the different ways they achieve condensation. For that, we review the phenomena of BEC and superfluidity, and apply this knowledge to the DM in order to explain their construction and phenomenology. We describe the small scale challenges these models aim to solve and how ULDM alleviates them. These models present a rich phenomenology that is manifest in different astrophysical consequences. We review here the astrophysical and cosmological tests used to constrain those models, together with new and future observations that promise to test these models in different regimes. We finalize by showing some predictions that are a consequence of the wave nature of this component, like vortices and interference, that could represent a smoking gun in the search of these rich and interesting alternative class of DM. (Abridged), Invited review for The Astronomy and Astrophysics Review. Version accepted for publication. Comments are welcome

We discuss two complementary strategies to search for light dark matter (LDM) exploiting the positron beam possibly available in the future at Jefferson Laboratory. LDM is a new compelling hypothesis that identifies dark matter with new sub-GeV "hidden sector" states, neutral under standard model interactions and interacting with our world through a new force. Accelerator-based searches at the intensity frontier are uniquely suited to explore it. Thanks to the high intensity and the high energy of the CEBAF (Continuous Electron Beam Accelerator Facility) beam, and relying on a novel LDM production mechanism via positron annihilation on target atomic electrons, the proposed strategies will allow us to explore new regions in the LDM parameters space, thoroughly probing the LDM hypothesis as well as more general hidden sector scenarios., 11 pages, 8 figures. Prepared for submission in EPJA. This work supersedes the corresponding contribution in arXiv:2007.15081. Added aknowledgement to Jefferson Lab

We study the prospects of characterising Dark Matter at colliders using Machine Learning (ML) techniques. We focus on the monojet and missing transverse energy (MET) channel and propose a set of benchmark models for the study: a typical WIMP Dark Matter candidate in the form of a SUSY neutralino, a pseudo-Goldstone impostor in the shape of an Axion-Like Particle, and a light Dark Matter impostor whose interactions are mediated by a heavy particle. All these benchmarks are tensioned against each other, and against the main SM background ($Z$+jets). Our analysis uses both the leading-order kinematic features as well as the information of an additional hard jet. We explore different representations of the data, from a simple event data sample with values of kinematic variables fed into a Logistic Regression algorithm or a Fully Connected Neural Network, to a transformation of the data into images related to probability distributions, fed to Deep and Convolutional Neural Networks. We also study the robustness of our method against including detector effects, dropping kinematic variables, or changing the number of events per image. In the case of signals with more combinatorial possibilities (events with more than one hard jet), the most crucial data features are selected by performing a Principal Component Analysis. We compare the performance of all these methods, and find that using the 2D images of the combined information of multiple events significantly improves the discrimination performance., Comment: 21 figures, 6 tables, minor revision

We report constraints on light dark matter through its interactions with shell electrons in the PandaX-II liquid xenon detector with a total 46.9 tonne$\cdot$day exposure. To effectively search for these very low energy electron recoils, ionization-only signals are selected from the data. 1821 candidates are identified within ionization signal range between 50 to 75 photoelectrons, corresponding to a mean electronic recoil energy from 0.08 to 0.15 keV. The 90% C.L. exclusion limit on the scattering cross section between the dark matter and electron is calculated based on Poisson statistics. Under the assumption of point interaction, we provide the world's most stringent limit within the dark matter mass range from 15 to 30 $\rm MeV/c^2$, with the corresponding cross section from $2.5\times10^{-37}$ to $3.1\times10^{-38}$ cm$^2$., 6 pages, 5 figures, 2 tables

The inelastic dark matter model is one kind of popular models for the light dark matter (DM) below $O(1)$ GeV. If the mass splitting between DM excited and ground states is small enough, the co-annihilation becomes the dominant channel for thermal relic density and the DM excited state can be long-lived at the collider scale. We study scalar and fermion inelastic dark matter models for $ {\cal O}(1) $ GeV DM at Belle II with $ U(1)_D $ dark gauge symmetry broken into its $Z_2$ subgroup. We focus on dilepton displaced vertex signatures from decays of the DM excited state. With the help of precise displaced vertex detection ability at Belle II, we can explore the DM spin, mass and mass splitting between DM excited and ground states. Especially, we show scalar and fermion DM candidates can be discriminated and the mass and mass splitting of DM sector can be determined within the percentage of deviation for some benchmark points. Furthermore, the allowed parameter space to explain the excess of muon $(g-2)_\mu$ is also studied and it can be covered in our displaced vertex analysis during the early stage of Belle II experiment., Comment: 42 pages, 13 figures, and 5 tables, version published in JHEP

We present the analysis of a new signature for light dark matter detection with superfluid $^{4}\mathrm{He}$: the emission of three phonons. We show that, in a region of mass below the MeV, the kinematics of this process can offer a way to reconstruct the dark matter interaction vertex while providing background rejection via coincidence requirements and directionality. We develop all the theoretical tools to deal with such an observable, and compute the associated differential distributions.

We use Super-K data to place new strong limits on interactions of sub-GeV Dark Matter (DM) with nuclei, that rely on the DM flux inevitably induced by cosmic-ray upscatterings. We derive analogous sensitivities at Hyper-K and DUNE and compare them with others, e.g. at JUNO. Using simplified models, we find that our proposal tests genuinely new parameter space, allowed both by theoretical consistency and by other direct detection experiments, cosmology, meson decays and our recast of monojet. Our results thus motivate and shape a new physics case for any large volume detector sensitive to nuclear recoils., 22 pages, 5 figures, submission to SciPost

The possibility of light dark matter (DM) annihilating through a dark photon (DP) which kinetically mixes (KM) with the Standard Model (SM) hypercharge field is a very attractive scenario. For DM in the interesting mass range below $\sim 1$ GeV, it is well known that bounds from the CMB provide a very strong model building constraint forcing the DM annihilation cross section to be roughly 3 orders of magnitude below that needed to reproduce the observed relic density. Under most circumstances this removes the possibility of an $s$-wave annihilation process for DM in this mass range as would be the case, e.g., if the DM were a Dirac fermion. In an extra-dimensional setup explored previously, it was found that the $s$-channel exchange of multiple gauge bosons could simultaneously encompass a suppressed annihilation cross section during the CMB era while also producing a sufficiently large annihilation rate during freeze-out to recover the DM relic density. In this paper, we analyze more globally the necessary requirements for this mechanism to work successfully and then realize them within the context of a simple model with two `dark' gauge bosons having masses of a similar magnitude and whose contributions to the annihilation amplitude destructively interfere. We show that if the DM mass threshold lies appropriately in the saddle region of this destructive interference between the two resonance humps it then becomes possible to satisfy these requirements simultaneously provided several ancillary conditions are met. The multiple constraints on the parameter space of this setup are then explored in detail to identify the phenomenologically successful regions., 26 pages, 12 figs; figure and discussion added, typos removed

We consider light dark matter candidates originated from the evaporation of Schwarzschild primordial black holes, with masses in the range $10^{-5}-10^9$ g. These candidates are beyond Standard Model particles with negligible couplings to the other particles, so that they interact only gravitationally. Belonging to the category of warm dark matter, they nevertheless spoil structure formation, with a softer impact for increasing values of the candidate spin. Requiring such candidates to fully account for the observed dark matter, we find that the scenario of black hole domination is ruled out for all spin values up to 2. For the scenario of radiation domination, we derive upper limits on the parameter $\beta$ (the primordial black hole energy density at formation over the radiation one), which are less stringent the higher the candidate spin is., Comment: 27 pages, 8 figures

The direct searches for heavy scalar dark matter with a mass of order 100 GeV are much more sensitive than for light dark matter of order 1 GeV. The question arises whether dark matter could be light and has escaped detection so far. We study a simple extension of the Standard Model with two additional real singlets. We show that this simple extension may provide the observed relic dark matter density, does neither disturb big-bang nucleosynthesis nor the cosmic microwave background radiation observations and fulfills the conditions of clumping behavior for different sizes of galaxies. The potential of one Standard Model-like Higgs-boson doublet and the two singlets gives rise to a changed Higgs phenomenology, in particular, an enhanced invisible Higgs-boson decay rate is expected, detectable by missing transversal momentum searches at the ATLAS and CMS experiments at CERN., Comment: 9 pages, 3 figures, new version adapted to journal version

Light dark matter (DM), defined here as having a mass between 1 MeV and about 1 GeV, is an interesting possibility both theoretically and phenomenologically, at one of the frontiers of current progress in the field of DM searches. Its indirect detection via gamma rays is challenged by the scarcity of experiments in the MeV–GeV region. We look therefore at lower-energy x-ray data from the INTEGRAL telescope, and compare them with the predicted DM flux. We derive bounds which are competitive with existing ones from other techniques. Crucially, we include the contribution from inverse Compton scattering on galactic radiation fields and the cosmic microwave background, which leads to much stronger constraints than in previous studies for DM masses above 20 MeV., Funded by SCOAP3., We also acknowledge the following organizations for support: European Research Council (ERC) under the EU Seventh Framework Programme (Grant No. FP7/2007-2013)/ERC Starting Grant (Agreement No. 278234—“NEWDARK” project); CNRS 80|Prime grant (“DAMEFER” project) (M. C.); Departments of Excellence grant awarded by the Italian Ministry of Education, University and Research (MIUR) (N. F. and E. P.); Research grant of the Università Italo-Francese, under Bando Vinci 2020 (E. P.); Research grant The Dark Universe: A Synergic Multimessenger Approach, Grant No. 2017X7X85K funded by the Italian Ministry of Education, University and Research (MIUR); Research grant The Anisotropic Dark Universe, Grant No. CSTO161409, funded by Compagnia di Sanpaolo and University of Torino; Research grant TASP (Theoretical Astroparticle Physics) funded by Istituto Nazionale di Fisica Nucleare (INFN) (N. F. and E. P.); Spanish Agencia Estatal de Investigación (AEI, MICIU) for the support to the Unidad de Excelencia María de Maeztu Instituto de Física de Cantabria, Ref. No. MDM-2017-0765 (B. J. K.).

The singlet majoron model of seesaw neutrino mass is appended by one dark Majorana fermion singlet $\chi$ with $L=2$ and one dark complex scalar singlet $\zeta$ with $L=1$. This simple setup allows $\chi$ to obtain a small radiative mass anchored by the same heavy right-handed neutrinos, whereas the one-loop decay of the standard-model Higgs boson to $\chi \chi + \bar{\chi} \bar{\chi}$ provides the freeze-in mechanism for $\chi$ to be the light dark matter of the Universe., Comment: 11 pages, 5 figures

Germanium ionization detectors with their unique features and diverse applications in fundamental research are novel candidates for the search of exotic particles. The TEXONO Collaboration aims to progressively improve the sensitivities toward exotic particles like low energy neutrinos, light dark matter candidates, and relativistic millicharged particles at the Kuo-Sheng Neutrino Laboratory (KSNL) in Taiwan. Relativistic millicharged particles (\(\chi _q\)) have been proposed in various extensions to the Standard Model of particle physics. We present the direct constraints on \(\chi _q\) with low threshold point-contact germanium detectors under the scenarios of \(\chi _q\) produced at (i) nuclear power reactors, (ii) as products of cosmic-ray interactions, and (iii) as dark matter particle accelerated by supernova shock. The atomic ionization cross sections of \(\chi _q\) with matter are derived with the equivalent photon approximation. Smoking-gun signatures with significant enhancement in the differential cross section are identified. We also report results from searches of pseudoscalar and vector bosonic super-weakly interacting massive particles (super-WIMP) using 314.15 kg days of data from an n-type Point-Contact Germanium detector.

We investigate the diurnal modulation of the event rate for dark matter scattering on solid targets arising from the directionally dependent defect creation threshold energy. In particular, we quantify how this effect would help in separating dark matter signal from the neutrino background. We perform a benchmark analysis for a germanium detector and compute how the reach of the experiment is affected by including the timing information of the scattering events. We observe that for light dark matter just above the detection threshold the magnitude of the annual modulation is enhanced. In this mass range using either the annual or diurnal modulation information provides a similar gain in the reach of the experiment, while the additional reach from using both effects remains modest. Furthermore, we demonstrate that if the background contains a feature exhibiting an annual modulation similar to the one observed by DAMA experiment, the diurnal modulation provides for an additional handle to separate dark matter signal from the background., Comment: 10 pages, 9 figures

Particle physics experiments rely extensively on computing and data services, making e-infrastructure an integral part of the research collaboration. Constructing and operating distributed computing can however be challenging for a smaller-scale collaboration. The Light Dark Matter eXperiment (LDMX) is a planned small-scale accelerator-based experiment to search for dark matter in the sub-GeV mass region. Finalizing the design of the detector relies on Monte-Carlo simulation of expected physics processes. A distributed computing pilot project was proposed to better utilize available resources at the collaborating institutes, and to improve scalability and reproducibility. This paper outlines the chosen lightweight distributed solution, presenting requirements, the component integration steps, and the experiences using a pilot system for tests with large-scale simulations. The system leverages existing technologies wherever possible, minimizing the need for software development, and deploys only non-intrusive components at the participating sites. The pilot proved that integrating existing components can dramatically reduce the effort needed to build and operate a distributed e-infrastructure, making it attainable even for smaller research collaborations., Comment: 10 pages, 4 figures, Submitted to 25th International Conference on Computing in High-Energy and Nuclear Physics (vCHEP 2021)

We report the results of a search for a new vector boson ($$ A'$$ A ′ ) decaying into two dark matter particles $$\chi _1 \chi _2$$ χ 1 χ 2 of different mass. The heavier $$\chi _2$$ χ 2 particle subsequently decays to $$\chi _1$$ χ 1 and an off-shell Dark Photon $$ A'^* \rightarrow e^+e^-$$ A ′ ∗ → e + e - . For a sufficiently large mass splitting, this model can explain in terms of new physics the recently confirmed discrepancy observed in the muon anomalous magnetic moment at Fermilab. Remarkably, it also predicts the observed yield of thermal dark matter relic abundance. A detailed Monte-Carlo simulation was used to determine the signal yield and detection efficiency for this channel in the NA64 setup. The results were obtained re-analyzing the previous NA64 searches for an invisible decay $$A'\rightarrow \chi \overline{\chi }$$ A ′ → χ χ ¯ and axion-like or pseudo-scalar particles $$a \rightarrow \gamma \gamma $$ a → γ γ . With this method, we exclude a significant portion of the parameter space justifying the muon g-2 anomaly and being compatible with the observed dark matter relic density for $$A'$$ A ′ masses from 2$$m_e$$ m e up to 390 MeV and mixing parameter $$\varepsilon $$ ε between $$3\times 10^{-5}$$ 3 × 10 - 5 and $$2\times 10^{-2}$$ 2 × 10 - 2 .