MeV Scale Research Articles

Imprints of MeV scale hidden dark sector at Planck data

New light species can contribute to the number of effective relativistic degrees of freedom (Neff) at Cosmic Microwave Background (CMB) which is precisely measured by Planck. In this work, we consider an MeV scale thermally decoupled non-minimal dark sector and study the imprint of the dark sector dynamics on the measurement of Neff at the time of CMB formation. We have predicted the allowed region of model parameter space in the light of constraints arising from the measurements of both Neff and dark matter relic density by Planck. It turns out that the impact of the dark sector dynamics on Neff is significant in case of a non-hierarchical mass spectrum of the dark sector particles.

Dark energy and neutrino superfluids

We show that the neutrino mass, the dark matter and the dark energy can be explained in a unified framework, postulating a new invisible Born–Infeld field, which we name “non-linear dark photon”, undergoing a meV-scale dynamical transmutation and coupled to neutrinos. Dark energy genesis is dynamically explained as a byproduct of the dark photon condensation, inducing the bare massless neutrinos to acquire an effective mass around the meV scale. It is fascinating to contemplate the channel induced by the non-linear dark photon leading to the pairing of the non-relativistic neutrinos, hence generating a cosmological superfluid state. As a consequence, the appearance of a light neutrino composite boson is predicted, providing a good cold dark matter candidate. In particular, if our model is enriched by an extra global Lepton number U L ( 1 ) symmetry, then the neutrino pair can be identified with a composite Majoron field with intriguing phenomenological implications for the neutrinoless-double-beta-decay ( 0 ν β β ) . Our model carries interesting phenomenological implications since dark energy, dark matter and the neutrino mass are time-varying dynamical variables, as a consequence of the non-linear Born–Infeld interaction terms. Limits arising from PLANCK+SNe+BAO collaborations data are also discussed. Finally, our model allows for an inverse hierarchy of neutrino masses, with interesting implications for the JUNO experiment.

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.

Direct measurement of Ni59(n,p)Co59 and Ni59(n,α)Fe56 at fast-neutron energies from 500 keV to 10 MeV

Nuclear reaction data for neutron induced reactions on unstable nuclei are critical for a wide range of applications spanning studies of nuclear astrophysics, nuclear reactor designs, and radiochemistry diagnostics. However, nuclear data evaluations of the reaction cross sections are largely based on calculations due to the difficulty in performing this class of measurements and the resulting lack of experimental data. For neutron induced charged particle reactions at fast neutron energies, at the MeV scale, these cross section predictions are predominately driven by statistical Hauser-Feshbach calculations. In this work, we present partial and total $^{59}\mathrm{Ni}(n,p)$ and $^{59}\mathrm{Ni}(n,\ensuremath{\alpha})$ cross sections, measured directly with a radioactive $^{59}\mathrm{Ni}$ target, and compare the results to the present nuclear data evaluations. In addition, the results from this work are compared to a recent study of the $^{59}\mathrm{Ni}(n,xp)$ reaction cross section that was performed via an indirect surrogate ratio method. The expected energy trend of the cross section, based on the current work, is inconsistent with that of the surrogate work. This calls into question the reliability of that application of the surrogate ratio method and highlights the need for direct measurements on unstable nuclei, when feasible.

Gamma-Ray Imaging Using Coincident Detection of Cherenkov Photons for Medical Applications

A novel detection concept for high-energy gamma rays for medical applications has been tested using the coincident detection of the Cherenkov photons from Compton-electrons in an optically transparent radiator material. Such detection capability would facilitate imaging of the distribution of certain targeted alpha-emitter therapies and also could be applicable in range verification for proton therapy, which are both connected to prompt gamma emission. To prove the concept, a demonstrator experiment at energies below the MeV scale was conducted: measurements with a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22</sup> Na gamma source emitting two 511-keV photons were performed and the Cherenkov light from Compton and photo electrons in UV transparent polymethyl metacrylate (PMMA) was detected in coincidence. Using a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8\times 8$ </tex-math></inline-formula> silicon photomultiplier (SiPM) array and fast FPGA-based read-out electronics, efficiency in the order of 0.1 % was achieved. A strong increase up to about 3 % for higher gamma energies up to 1.5 MeV is indicated by the Monte-Carlo simulations. The number of detected Cherenkov photons per event and per channel was counted using the time over threshold (TOT) of the SiPM signal. The measured number of photons and their distribution from accumulated events are in good agreement with simulations, and the measured patterns of photon hits indicate spatial sensitivity. The ability to detect 511-keV photons with the Cherenkov light was demonstrated successfully.

Production and attenuation of cosmic-ray boosted dark matter

Light sub-GeV halo dark matter (DM) particles up-scattered by high-energy cosmic-rays (CRs) (referred to as CRDM) can be energetic and become detectable by conventional DM direct detection experiments. We perform a refined analysis on the exclusion bounds of the spin-independent DM-nucleon scattering cross section σχ p in this approach. For the exclusion lower bounds, we determine the parameter of the effective distance Deff for CRDM production using spatial-dependent CR fluxes and including the contributions from the major heavy CR nuclear species. We obtain Deff≃ 9 kpc for CRDM particles with kinetic energy above ∼ 1 GeV, which pushes the corresponding exclusion lower bounds down to σχ p∼ 4× 10-32 cm2 for DM particle mass at MeV scale and below. For the exclusion upper bounds from Earth attenuation, previous estimations neglecting the nuclear form factor leaded to typical exclusion upper bounds of σχ p∼\U0001d4aa(10-28) cm2 from the XENON1T data. Using both the analytic and numerical approaches, we show that for CRDM particles, the presence of the nuclear form factor strongly suppresses the effect of Earth attenuation. Consequently, the cross section that can be excluded by the XENON1T data can be a few orders of magnitude higher, which closes the gap in the cross sections excluded by the XENON1T experiment and that by the astrophysical measurements such that for the cosmic microwave background (CMB), galactic gas cloud cooling, and structure formation, etc..

Status and Extension of the Geant4-DNA Dielectric Models for Application to Electron Transport

The development of accurate physics models that enable track structure simulations of electrons in liquid water medium over a wide energy range, from the eV to the MeV scale, is a subject of continuous efforts due to its importance (among other things) in theoretical studies of radiation quality for application in radiotherapy and radiation protection. A few years ago, the Geant4-DNA very low-energy extension of the Geant4 Monte Carlo code had offered to users an improved set of physics models for discrete electron transport below 10 keV. In this work we present refinements to this model set and its extension to energies up to 1 MeV. Preliminary comparisons against the existing Geant4-DNA physics models with respect to total and differential ionization cross sections of electrons in liquid water are reported and discussed.

The DUNE vertical drift photon detection system

The Deep Underground Neutrino Experiment (DUNE) is a long-baseline neutrino experiment designed to mainly investigate oscillation parameters, supernova physics and proton decay. Its far detector will be composed of four liquid argon time projection chamber (LArTPC) underground modules, in South Dakota-USA, which will detect a neutrino beam produced at Fermilab, 1300 km away, where a near detector will be in place. The second DUNE far detector module, vertical drift, will be a single phase LArTPC with electron drift along the vertical axis with two volumes of 13.5 m × 6.5 m × 60 m dimensions separated by a cathode plane. The charge collection will be performed by two anode planes placed at the top and bottom of the module, each composed by stacked layers of a perforated PCB technology with electrode strips. The photon detection system (PDS) will make use of large size X-Arapuca tiles distributed over three detection planes. One plane will consist of a horizontal arrangement of double sided tiles installed on the high voltage cathode plane and two vertical planes, each placed on the longest cryostat membrane walls. A light active coverage of 14.8% over the cathode and 7.4% over the laterals should allow improvements in the low energy physics range that can be probed in DUNE, especially regarding supernova neutrinos (∼10 MeV). We present the initial characterization of the vertical drift PDS using a Monte Carlo simulation and preliminary studies on its reconstruction capabilities at the MeV scale. The information obtained with the PDS alone should allow determination of a neutrino interaction region with a precision of at least 65 cm for events with deposited energy above 5 MeV and the deposited energy can be reconstructed with precision better than 10%, both at the center of the volume.

Constraining Cosmological Phase Transitions with the Parkes Pulsar Timing Array.

A cosmological first-order phase transition is expected to produce a stochastic gravitational wave background. If the phase transition temperature is on the MeV scale, the power spectrum of the induced stochastic gravitational waves peaks around nanohertz frequencies, and can thus be probed with high-precision pulsar timing observations. We search for such a stochastic gravitational wave background with the latest data set of the Parkes Pulsar Timing Array. We find no evidence for a Hellings-Downs spatial correlation as expected for a stochastic gravitational wave background. Therefore, we present constraints on first-order phase transition model parameters. Our analysis shows that pulsar timing is particularly sensitive to the low-temperature (T∼1-100 MeV) phase transition with a duration (β/H_{*})^{-1}∼10^{-2}-10^{-1} and therefore can be used to constrain the dark and QCD phase transitions.

Luminous solar neutrinos. II. Mass-mixing portals

Solar neutrinos can be efficiently upscattered to MeV scale heavy neutral leptons (HNLs) within the Earth's mantle. HNLs can then decay to electron-positron pairs leading to energy deposition inside large-volume detectors. In this paper we consider mass-portal upscattering of solar neutrinos to HNLs of mass 20 MeV $\geq m_N \geq 2 m_e$. The large volume of the Earth compensates for the long decay-length of the HNLs leading to observable rates of $N\rightarrow \nu_\alpha e^+e^-$ in large volume detectors. We find that searches for mantle-upscattered HNLs can set the novel limits on mixing with third generation leptons, $|U_{\tau N}|$ for masses in the MeV regime; sensitivity to mixing with first- and second-generation leptons is not competitive with existing search strategies.

Chain early dark energy: A Proposal for solving the Hubble tension and explaining today’s dark energy

We propose a new model of Early Dark Energy (EDE) as a solution to the Hubble tension in cosmology, the apparent discrepancy between local measurements of the Hubble constant $H_0\simeq 74$ km s$^{-1}$ Mpc$^{-1}$ and $H_0\simeq 67$ km s$^{-1}$ Mpc$^{-1}$ inferred from the Cosmic Microwave Background (CMB). In Chain EDE, the Universe undergoes a series of first order phase transitions, starting at a high energy vacuum in a potential, and tunneling down through a chain of every lower energy metastable minima. As in all EDE models, the contribution of the vacuum energy to the total energy density of the universe is initially negligible, but reaches $\sim 10\%$ around matter-radiation equality, before cosmological data require it to redshift away quickly -- at least as fast as radiation. We indeed obtain this required behavior with a series of $N$ tunneling events, and show that for $N>600$ the phase transitions are rapid enough to allow fast percolation and thereby avoid large scale anisotropies in the CMB. We construct a specific example of Chain EDE featuring a scalar field in a quasiperiodic potential (a tilted cosine), which is ubiquitous in axion physics and, therefore, carries strong theoretical motivation. Interestingly, the energy difference between vacua can be roughly the size of today's Dark Energy (meV scale). Therefore, the end result of Chain EDE could provide a natural explanation of Dark Energy, if the tunneling becomes extremely slow in the final step before the field reaches zero (or negative) energy. We discuss a simple mechanism which can stop the scalar field in the desired minimum. Thus Chain EDE offers the exciting prospect to explain EDE and Dark Energy by the same scalar field.

Cosmology of Sub-MeV Dark Matter Freeze-In.

Dark matter (DM) could be a relic of freeze-in through a light mediator, where the DM is produced by extremely feeble, IR-dominated processes in the thermal standard model plasma. In the simplest viable models with DM lighter than 1MeV, the DM has a small effective electric charge and is born with a nonthermal phase-space distribution. This DM candidate would cause observable departures from standard cosmological evolution. In this work, we combine data from the cosmic microwave background (CMB), Lyman-α forest, quasar lensing, stellar streams, and MilkyWay satellite abundances to set limits on freeze-in DM masses up to ∼20 keV, with the exact constraint depending on whether the DM thermalizes in its own sector. We perform forecasts for the CMB-S4 experiment, the Hydrogen Epoch of Reionization Array, and the Vera Rubin Observatory, finding that freeze-in DM masses up to ∼80 keV can be explored.

Challenges for a QCD axion at the 10 MeV scale

We report on an interesting realization of the QCD axion, with mass in the range mathcal{O} (10) MeV. It has previously been shown that although this scenario is stringently constrained from multiple sources, the model remains viable for a range of parameters that leads to an explanation of the Atomki experiment anomaly. In this article we study in more detail the additional constraints proceeding from recent low energy experiments and study the compatibility of the allowed parameter space with the one leading to consistency of the most recent measurements of the electron anomalous magnetic moment and the fine structure constant. We further provide an ultraviolet completion of this axion variant and show the conditions under which it may lead to the observed quark masses and CKM mixing angles, and remain consistent with experimental constraints on the extended scalar sector appearing in this Standard Model extension. In particular, the decay of the Standard Model-like Higgs boson into two light axions may be relevant and leads to a novel Higgs boson signature that may be searched for at the LHC in the near future.

The physics potential of a reactor neutrino experiment with Skipper CCDs: measuring the weak mixing angle

We analyze in detail the physics potential of an experiment like the one recently proposed by the vIOLETA collaboration: a kilogram-scale Skipper CCD detector deployed 12 meters away from a commercial nuclear reactor core. This experiment would be able to detect coherent elastic neutrino nucleus scattering from reactor neutrinos, capitalizing on the exceptionally low ionization energy threshold of Skipper CCDs. To estimate the physics reach, we elect the measurement of the weak mixing angle as a case study. We choose a realistic benchmark experimental setup and perform variations on this benchmark to understand the role of quenching factor and its systematic uncertainties, background rate and spectral shape, total exposure, and reactor antineutrino flux uncertainty. We take full advantage of the reactor flux measurement of the Daya Bay collaboration to perform a data driven analysis which is, up to a certain extent, independent of the theoretical un- certainties on the reactor antineutrino flux. We show that, under reasonable assumptions, this experimental setup may provide a competitive measurement of the weak mixing angle at few MeV scale with neutrino-nucleus scattering.

Light dark matter from dark sector decay

We study the possibility that light dark matter can be produced with right relic density by the decay of other dark sector particles. We study this possibility in a model with right-handed neutrinos, a dark sector singlet scalar and a dark sector singlet fermion. The decay of the heavier dark sector singlet scalar gives rise to the lighter dark sector singlet fermion which serves as the dark matter candidate. We show that the right dark matter relic density can be produced by the decay of the dark sector singlet scalar. We find that the mass of the dark sector singlet fermion can be GeV scale or MeV scale and the interaction of the dark sector singlet fermion is very weak. The dark sector singlet scalar can have a mass of GeV-TeV scale and can have weak scale interactions with the Standard Model (SM) particles. So, in this scenario, dark matter has an interaction with the SM much weaker than the weak interaction, but the dark sector still has weak scale interaction with the SM.

An improved design of the readout base board of the photomultiplier tube for future PandaX dark matter experiments

The PandaX project consists of a series of xenon-based experiments that are used to search for dark matter (DM) particles and to study the fundamental properties of neutrinos. The next DM experiment PandaX-4T will be using 4 ton liquid xenon in the sensitive volume, which is nearly a factor of seven larger than that of the previous experiment PandaX-II. Due to the increasing target mass, the sensitivity of searching for both DM and neutrinoless double-beta decay (0νββ) signals in the same detector will be significantly improved. However, the typical energy of interest for 0νββ signals is at the MeV scale, which is much higher than that of most popular DM signals. In the baseline readout scheme of the photomultiplier tubes (PMTs), the dynamic range is very limited. Signals from the majority of PMTs in the top array of the detector are heavily saturated at MeV energies. This deteriorates the 0νββ search sensitivity. In this paper we report a new design of the readout base board of the PMTs for future PandaX DM experiments and present its improved performance on the dynamic range.

Discovering supernova-produced dark matter with directional detectors

Supernovae can produce vast fluxes of new particles with masses on the MeV scale, a mass scale of interest for models of light dark matter. When these new particles become diffusively trapped within the supernova, the escaping flux will emerge semirelativistic with an order-one spread in velocities. As a result, overlapping emissions from Galactic supernovae will produce an overall flux of these particles at Earth that is approximately constant in time. However, this flux is highly anisotropic and is steeply peaked towards the Galactic center. This is in contrast with the cosmological abundance of a WIMP-like dark matter which, due to the rotation of the Galaxy, appears to come from the direction of the Cygnus constellation. In this paper, we demonstrate the need for a directional detector to efficiently discriminate between a signal from a cold cosmological abundance of GeV-scale WIMPs and a signal from a hot population of supernova-produced MeV-scale dark matter.

Prospects of Migdal effect in the explanation of XENON1T electron recoil excess

The XENON1T experiment has recently announced the observation of an excess in electron recoil events at energy range of 1–7 keV with a 3.5σ signal significance over the Standard Model prediction. In this letter we sketch the prospects of explaining such an excess from Migdal ionization events with below threshold nuclear recoil energies. Interestingly, these are expected to show signal events in the ballpark energy scale of the observed excess. We demonstrate that the observed signal can be reproduced through the Migdal effect by an O(1) GeV neutron-philic dark matter having a spin-dependent coupling with the nucleus. A more optimistic scenario is explored where the Migdal ionization is driven by MeV scale boosted dark matter.

Evidence for bosonic mode coupling in electron dynamics of LiFeAs superconductor**Project supported by the National Key Research and Development Program of China (Grant Nos. 2016YFA0300300, 2016YFA0300600, 2017YFA0302900,2018YFA0704200, 2018YFA0305600, and 2019YFA0308000), the National Natural Science Foundation of China (Grant Nos. 11888101, 11922414, and 11874405), the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant Nos. XDB25000000 and XDB33010300),

Super-high resolution laser-based angle-resolved photoemission measurements are carried out on LiFeAs superconductor to investigate its electron dynamics. Three energy scales at ∼ 20 meV, ∼ 34 meV, and ∼ 55 meV are revealed for the first time in the electron self-energy both in the superconducting state and normal state. The ∼ 20 meV and ∼ 34 meV scales can be attributed to the coupling of electrons with sharp bosonic modes which are most likely phonons. These observations provide definitive evidence on the existence of mode coupling in iron-based superconductors.

Ultracold Electrons via Near-Threshold Photoemission from Single-Crystal Cu(100).

Achieving a low mean transverse energy or temperature of electrons emitted from the photocathode-based electron sources is critical to the development of next-generation and compact x-ray free electron lasers and ultrafast electron diffraction, spectroscopy, and microscopy experiments. In this Letter, we demonstrate a record low mean transverse energy of 5meV from the cryo-cooled (100) surface of copper using near-threshold photoemission. Further, we also show that the electron energy spread obtained from such a surface is less than 11.5meV, making it the smallest energy spread electron source known to date: more than an order of magnitude smaller than any existing photoemission, field emission, or thermionic emission based electron source. Our measurements also shed light on the physics of electron emission and show how the energy spread at few meV scale energies is limited by both the temperature and the vacuum density of states.