Dark Photon Research Articles

New pathways to the relic abundance of vector-portal dark matter

We fully explore the thermal freezeout histories of a vector-portal dark matter model, in the region of parameter space in which the ratio of masses of the dark photon $A^{\prime}$ and dark matter $\chi$ is in the range $1 \lesssim m_{A^{\prime}}/m_{\chi} \lesssim 2$. In this region $2 \rightarrow 2$ and $3 \rightarrow 2$ annihilation processes within the dark sector, as well as processes that transfer energy between the dark sector and the Standard Model, play important roles in controlling the thermal freezeout of the dark matter. We carefully track the temperatures of all species, relaxing the assumption of previous studies that the dark and Standard Model sectors remain in thermal equilibrium throughout dark matter freezeout. Our calculations reveal a rich set of novel pathways which lead to the observed relic density of dark matter, and we develop a simple analytic understanding of these different regimes. The viable parameter space in our model provides a target for future experiments searching for light (MeV-GeV) dark matter, and includes regions where the dark matter self-interaction cross section is large enough to affect the small-scale structure of galaxies.

Self-interacting dark baryons

Using results from lattice QCD, it is possible to quantitatively design models of dark baryons leading to velocity-dependent self-interaction cross sections that match the values needed for solving small-scale structure problems of standard cold dark matter. However it is not obvious that the main dark matter component in such models will be nucleons rather than large nuclei, or dark pions or atoms, whose scattering properties would be different. We first identify the parameters of a dark SU(3) sector analogous to QCD---the confinement scale $\mathrm{\ensuremath{\Lambda}}$ and pion mass ${m}_{\ensuremath{\pi}}$---needed to reproduce desired self-interaction cross sections. Then we show that these values can generically be compatible with the absence of a sufficiently stable deuteron bound state, and hence leading to no heavier nuclei, thus establishing the consistency of the scenario for self-interacting dark nucleons. The range of dark photon masses needed to avoid dominant pion or atomic dark matter is determined, as well as allowed values for the kinetic mixing parameter. The dark proton might be detected directly in future searches, by dark photon exchange.

New thermal relic targets for inelastic vector-portal dark matter

We examine the vector-portal inelastic dark matter (DM) model with DM mass $m_\chi$ and dark photon mass $m_{A'}$, in the `forbidden dark matter' regime where $1 \lesssim m_{A'}/m_\chi \lesssim 2$, carefully tracking the dark sector temperature throughout freezeout. The inelastic nature of the dark sector relaxes the stringent cosmic microwave background (CMB) and self-interaction constraints compared to symmetric DM models. We determine the CMB limits on both annihilations involving excited states and annihilation into $e^+e^-$ through initial-state-radiation of an $A'$, as well as limits on the DM self-scattering, which proceeds at the one-loop level. The unconstrained parameter space serves as an ideal target for accelerator $A'$ searches, and provides a DM self-interaction cross section that is large enough to observably impact small-scale structure.

Asymmetric dark matter may not be light

It is often said that asymmetric dark matter is light compared to typical weakly interacting massive particles. Here we point out a simple scheme with a neutrino portal and $\mathcal{O}(60 \text{ GeV})$ asymmetric dark matter which may be ''added'' to any standard baryogenesis scenario. The dark sector contains a copy of the Standard Model gauge group, as well as (at least) one matter family, Higgs, and right-handed neutrino. After baryogenesis, some lepton asymmetry is transferred to the dark sector through the neutrino portal where dark sphalerons convert it into a dark baryon asymmetry. Dark hadrons form asymmetric dark matter and may be directly detected due to the vector portal. Surprisingly, even dark anti-neutrons may be directly detected if they have a sizeable electric dipole moment. The dark photons visibly decay at current and future experiments which probe complementary parameter space to dark matter direct detection searches. Exotic Higgs decays are excellent signals at future $e^+ e^-$ Higgs factories.

Manifesting hidden dynamics of a sub-component dark matter

We emphasize the distinctive cosmological dynamics in multi-component dark-matter scenarios and their impact on probing a sub-dominant component of dark matter. The dynamics originate from the conversion among different dark-matter components. We find that the temperature of the self-interacting sub-component dark matter is significantly enhanced by the dark-matter annihilation into the sub-component. The same annihilation sharply increases the required annihilation cross section for the sub-component as we consider a smaller relative abundance fraction among the dark-matter species. Because of the enhanced temperature and couplings of the sub-component, contrary to a naive expectation, the sub-component with smaller abundance fractions tends to be disfavored by dark-matter direct/indirect-detection experiments and cosmological observations. We demonstrate this by taking a dark photon portal scenario for the sub-component at the sub-GeV mass scale. For the abundance fraction ≳ 10 %, the enhanced temperature of the sub-component is subject to warm dark matter constraints which are complementary to the parameter space probed by accelerator-based experiments. Smaller abundance fractions tend to be disfavored by accelerator-based experiments due to the enhanced coupling to Standard Model particles.

Vortex String Formation in Black Hole Superradiance of a Dark Photon with the Higgs Mechanism.

Black hole superradiance, which only relies on gravitational interactions, can provide a powerful probe of the existence of ultralight bosons that are weakly coupled to ordinary matter. However, as a boson cloud grows through superradiance, nonlinear effects from interactions with itself or other fields may become important. As a representative example of this, we use nonlinear evolutions to study black hole superradiance of a vector boson that attains a mass, via a coupling to a complex scalar, through the Higgs mechanism. For the cases considered, we find that the superradiant instability can lead to a transient period where the scalar field reaches its symmetry restoration value, leading to the formation of closed vortex strings, the temporary disruption of the exponential growth of the cloud, and an explosive outburst of energy. After the cloud loses sufficient mass, the superradiant growth resumes, and the cycle repeats. Thus, the black hole will be spun down but, potentially, at a much lower rate compared to when nonlinear effects are unimportant and with the liberated energy going primarily into bosonic radiation instead of gravitational waves.

(In)equivalence of metric-affine and metric effective field theories

In a geometrical approach to gravity the metric and the (gravitational) connection can be independent and one deals with metric-affine theories. We construct the most general action of metric-affine effective field theories, including a generic matter sector, where the connection does not carry additional dynamical fields. Among other things, this helps in identifying the complement set of effective field theories where there are other dynamical fields, which can have an interesting phenomenology. Within the latter set, we study in detail a vast class where the Holst invariant (the contraction of the curvature with the Levi-Civita antisymmetric tensor) is a dynamical pseudoscalar. In the Einstein–Cartan case (where the connection is metric compatible and fermions can be introduced) we also comment on the possible phenomenological role of dynamical dark photons from torsion and compute interactions of the above-mentioned pseudoscalar with a generic matter sector and the metric. Finally, we show that in an arbitrary realistic metric-affine theory featuring a generic matter sector the equivalence principle always emerges at low energies without the need to postulate it.

Quantum simulations of dark sector showers

We consider dark sector scenarios where dark matter is accompanied by a dark photon and multiple-flavor dark fermions charged under the dark gauge group. We study quantum interference effects in dark sector jets, where multiple dark photons are emitted from high-energy dark fermions. We perform fully quantum simulations of dark sector showers and compare the results against those of the classical Monte-Carlo simulations. We find important differences in probability distributions of dark photon countings between quantum and classical computations. When the number of dark-fermion flavors is large, we find significant enhancements in large numbers of dark photon emissions. Such enhancements can provide distinguishing signals for our scenarios at particle colliders.

Constraints on the dark photon from parity violation and the W mass

We present an analysis of the experimental data for parity-violating electron scattering (PVES) and atomic parity-violation, including the effects of a dark photon. We derive the favored region of dark photon parameter space, which provides a good description of the experimental data from the Qweak Collaboration and the Jefferson Lab PVDIS Collaboration and simultaneously relieves the tension between the neutron skin thickness determined in the PREX-II experiment and nuclear-model predictions. In addition, we extract the parameter region required to explain the latest W-boson mass anomaly. Our results indicate that a heavy dark photon with mass above the Z boson mass is favored, while other sources of new physics beyond the Standard Model in addition to the dark photon would also be expected.

Using Artificial Intelligence in the Reconstruction of Signals from the PADME Electromagnetic Calorimeter

The PADME apparatus was built at the Frascati National Laboratory of INFN to search for a dark photon (A′) produced via the process e+e−→A′γ. The central component of the PADME detector is an electromagnetic calorimeter composed of 616 BGO crystals dedicated to the measurement of the energy and position of the final state photons. The high beam particle multiplicity over a short bunch duration requires reliable identification and measurement of overlapping signals. A regression machine-learning-based algorithm has been developed to disentangle with high efficiency close-in-time events and precisely reconstruct the amplitude of the hits and the time with sub-nanosecond resolution. The performance of the algorithm and the sequence of improvements leading to the achieved results are presented and discussed.

Inelastic dark matter from dark Higgs boson decays at FASER

We consider inelastic dark matter scenarios with dark photon mediator and a dark Higgs boson. The dark Higgs boson spontaneously breaks the gauge symmetry associated with the dark photon, and gives the mass to the dark photon and the mass difference to dark particles. Such a dark Higgs boson can decay into the dark particles and hence can be another source of the dark particles at collider experiments. We analyze the sensitivity to decays of the excited state into the dark matter and charged particles at the FASER 2 experiment in fermion and scalar inelastic dark matter scenarios. We consider two mass spectra as illustrating examples in which the excited state can be produced only through the decay of dark Higgs boson. We show that unprobed parameter region can be explored in fermion dark matter scenario for the illustrating mass spectra.

Effective field theory of Stückelberg vector bosons

We explore the effective field theory of a vector field ${X}^{\ensuremath{\mu}}$ that has a St\"uckelberg mass. The absence of a gauge symmetry for ${X}^{\ensuremath{\mu}}$ implies Lorentz-invariant operators are constructed directly from ${X}^{\ensuremath{\mu}}$. Beyond the kinetic and mass terms, allowed interactions at the renormalizable level include ${X}_{\ensuremath{\mu}}{X}^{\ensuremath{\mu}}{H}^{\ifmmode\dagger\else\textdagger\fi{}}H$, $({X}_{\ensuremath{\mu}}{X}^{\ensuremath{\mu}}{)}^{2}$, and ${X}_{\ensuremath{\mu}}{j}^{\ensuremath{\mu}}$, where ${j}^{\ensuremath{\mu}}$ is a global current of the SM or of a hidden sector. We show that all of these interactions lead to scattering amplitudes that grow with powers of $\sqrt{s}/{m}_{X}$, except for the case of ${X}_{\ensuremath{\mu}}{j}^{\ensuremath{\mu}}$ where ${j}^{\ensuremath{\mu}}$ is a nonanomalous global current. The latter is well known when $X$ is identified as a dark photon coupled to the electromagnetic current, often written equivalently as kinetic mixing between $X$ and the photon. The power counting for the energy growth of the scattering amplitudes is facilitated by isolating the longitudinal enhancement. We examine in detail the interaction with an anomalous global vector current ${X}_{\ensuremath{\mu}}{j}_{\mathrm{anom}}^{\ensuremath{\mu}}$, carefully isolating the finite contribution to the fermion triangle diagram. We calculate the longitudinally-enhanced observables $Z\ensuremath{\rightarrow}X\ensuremath{\gamma}$ (when ${m}_{X}<{m}_{Z}$), $f\overline{f}\ensuremath{\rightarrow}X\ensuremath{\gamma}$, and $Z\ensuremath{\gamma}\ensuremath{\rightarrow}Z\ensuremath{\gamma}$ when $X$ couples to the baryon number current. Introducing a ``fake'' gauge-invariance by writing ${X}^{\ensuremath{\mu}}={A}^{\ensuremath{\mu}}\ensuremath{-}{\ensuremath{\partial}}^{\ensuremath{\mu}}\ensuremath{\pi}/{m}_{X}$, the would-be gauge anomaly associated with ${A}_{\ensuremath{\mu}}{j}_{\mathrm{anom}}^{\ensuremath{\mu}}$ is canceled by ${j}_{\mathrm{anom}}^{\ensuremath{\mu}}{\ensuremath{\partial}}_{\ensuremath{\mu}}\ensuremath{\pi}/{m}_{X}$; this is the four-dimensional Green-Schwarz anomaly-cancellation mechanism at work. Our analysis demonstrates there is a much larger set of possible interactions that an EFT with a St\"uckelberg vector field can have, revealing scattering amplitudes that grow with energy. The growth of these amplitudes can be tamed by a dark Higgs sector, but this requires dark Higgs boson interactions (and reintroduces fine-tuning in the dark Higgs sector) that can be separated from $X$ interactions only in the limit $g\ensuremath{\ll}1$.

Atmospheric resonant production for light dark sectors

Cosmic ray atmospheric showers provide an effective environment for the production of MeV-scale dark sector particles. We show that, when available, the resonant annihilation of positrons from the shower on atmospheric electrons is the dominant production mechanism by more than an order of magnitude. We provide a quantitative example based on dark photon production and update existing constraints on a corresponding light dark matter model from kilotons neutrino experiments and xenon-based direct detection experiments.

Dark photon kinetic mixing effects for the CDF W -mass measurement

A new $U(1)_X$ gauge boson $X$ primarily interacting with a dark sector can have renormalizable kinetic mixing with the standard model (SM) $U(1)_Y$ gauge boson $Y$. This mixing besides introduces interactions of dark photon and dark sector with SM particles, it also modifies interactions among SM particles. The modified interactions can be casted into the oblique $S$, $T$ and $U$ parameters. We find that with the dark photon mass larger than the $Z$ boson mass, the kinetic mixing effects can reduce the tension of the W mass excess problem reported recently by CDF from $7\sigma$ deviation to within $3 \sigma$ compared with theory prediction. If there is non-abelian kinetic mixing between $U(1)_X$ and $SU(2)_L$ gauge bosons, in simple renormalizable models of this type a triplet Higgs is required to generate the mixing. We find that this triplet with a vacuum expectation value of order 5 GeV can naturally explain the W mass excess.

Linear sigma dark matter

We present a model of self-interacting dark matter based on QCD-like theories and inspired by the proximity of a0(980 ± 20) to the Koverline{K} (990) threshold. Dark matter is comprised of dark pions which self-scatter via the σ resonance close to the ππ threshold. While the linear sigma model serves as a qualitative guide, a fully unitary description of the scattering in the strongly coupled regime is given by effective range theory. The introduction of a kinetically mixed dark photon allows the dark pion to either freeze-out or -in. We study the viable parameter space which explains the observed relic abundance while evading all current constraints. Searches for dark matter self interactions at different scales, (in)direct detection signals, and (in)visibly-decaying dark photons will test this model in the near future.

Mono-γ production of a dark vector at future e + e − colliders* *Supported by the Innovation Capability Support Program of Shaanxi Province (2021KJXX-47), the Double First Class start-up fund (WF220442604), the Shanghai Pujiang Program (20PJ1407800), National Natural Science Foundation of China (12090064), as well as Chinese Academy of Sciences Center for Excellence in Particle Physics (CCEPP)

The associated production of a dark particle and photon, represented as a mono-γ event, is a promising channel to probe particle content and dynamics in the dark sector. In this study, we investigate the properties of the mono-γ production of vector dark matter at future colliders. Photon-like and Pauli operators as well as triple gauge boson interactions involving dark matter are considered in the framework of effective field theory. We show that, in comparison with the Pauli operator, the triple gauge boson couplings are significantly more interesting in high energy colliders. Beam polarization effects are also analyzed, and we show that the experimental sensitivities cannot be significantly enhanced because of the smaller luminosity.

Search for Milli-Charged particles from the Sun at IceCube

It is assumed that heavy dark matter particles ϕ with O(TeV) mass captured by the Sun may decay to relativistic light milli-charged particles (MCPs). These MCPs could be measured by the IceCube detector. The massless hidden photon model was taken for MCPs to interact with nuclei, so that the numbers and fluxes of expected MCPs and neutrinos may be evaluated at IceCube. Based on the assumption that no events are observed at IceCube in 6 years, the corresponding upper limits on MCP fluxes were calculated at 90% C. L. These results indicated that MCPs could be directly detected in the secondaries’ energy range O(100GeV)-O(10TeV) at IceCube, when ϵ2 ≳ 10−10. And a new region of 0.6 MeV < mMCP < 10 MeV and 6 × 10−6 < ϵ ≲ 10−4 is ruled out in the mMCP-ϵ plane with 6 years of IceCube data.

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.

FASER’s Electromagnetic Calorimeter Test Beam Studies

FASER, or the Forward Search Experiment, is a new experiment at CERN designed to complement the LHC’s ongoing physics program, extending its discovery potential to light and weakly interacting particles that may be produced copiously at the LHC in the far-forward region. New particles targeted by FASER, such as long-lived dark photons or axion-like particles, are characterised by a signature with two oppositely charged tracks or two photons in the multi-TeV range that emanate from a common vertex inside the detector. The full detector was successfully installed in March 2021 in an LHC side tunnel 480 m downstream from the interaction point in the ATLAS detector. FASER is planned to be operational for LHC Run 3. The experiment is composed of a silicon-strip tracking-based spectrometer using three dipole magnets with a 20 cm aperture, supplemented by four scintillator stations and an electromagnetic calorimeter. The FASER electromagnetic calorimeter is constructed from four spare LHCb calorimeter modules. The modules are of the Shashlik type with interleaved scintillator and lead plates that result in 25 radiation lengths and 1% energy resolution for TeV electromagnetic showers. In 2021, a test beam campaign was carried out using one of the CERN SPS beam lines to set up the calibration of the FASER calorimeter system in preparation for physics data taking. The relative calorimeter response to electrons with energies between 10 and 300 GeV, as well as high energy muons and pions, has been measured under various high voltage settings and beam positions. The measured calorimeter resolution, energy calibration, and particle identification capabilities are presented.

Kinetic mixing, dark Higgs triplets, and MW

The kinetic mixing (KM) portal is a popular mechanism that allows light dark matter (DM) in the mass range below $\ensuremath{\sim}1\text{ }\text{ }\mathrm{GeV}$ to achieve the observed relic density by thermal means and can be effectively described by only a few parameters, e.g., $\ensuremath{\epsilon}$, the strength of this KM. In the simplest setup, the Standard Model (SM) $U(1{)}_{Y}$ hypercharge gauge boson and a $U(1{)}_{D}$ dark photon (DP), which only couples to fields in the dark sector, experience KM via loops of portal matter (PM) fields which have both SM and dark charges, thus generating a small coupling between us and dark matter (DM). However, if one wishes to understand the underlying physics behind this idea in a deeper fashion we need to take a step upward to a more UV-complete picture. Meanwhile, CDFII has measured a value for the $W$-boson mass which lies significantly above SM expectations. In this paper, we speculate that this shift in the $W$'s mass may be related to the $\ensuremath{\sim}1\text{ }\text{ }\mathrm{GeV}$ mass of the DP within a framework of scalar PM that leads to phenomenologically interesting values of $\ensuremath{\epsilon}$ via non-Abelian KM due to the existence of an $SU(2{)}_{L}$, $Y=0$, complex Higgs triplet which carries a nonzero value of the $U(1{)}_{D}$ dark charge. Possible gauge boson plus missing energy signatures of this scenario that can appear at the LHC and elsewhere are examined. Indeed, with modest assumptions, all of the new scalar PM states are predicted to have masses below roughly $\ensuremath{\simeq}630\text{ }\text{ }\mathrm{GeV}$ and so should be at least kinematically accessible. The HL-LHC will very likely to be able to explore all of this model's allowed parameter space.