Dark Photon Mass Research Articles

Measurement of B(J/ψ→η′e+e−) and search for a dark photon

Using a data sample of $(1310.6\pm7.0)\times10^{6}$ $J/\psi$ decay events collected with the BESIII detector at BEPCII, we study the electromagnetic Dalitz decay $J/\psi \to \eta' e^+e^-$ with two dominant $\eta'$ decay modes, $\eta' \to \gamma \pi^+ \pi^-$ and $\eta' \to \pi^+\pi^-\eta$. The branching fraction is determined to be $\mathcal{B}(J/\psi \to \eta' e^+e^-) = (6.59\pm0.07\pm0.17) \times 10^{-5}$, which improves in precision by a factor of 2 over the previous BESIII measurement. A search for the dark photon ($\gamma '$) is performed via $J/\psi \to\eta' \gamma ', \gamma' \to e^{+}e^{-}$. Excluding the $\omega$ and $\phi$ mass regions, no significant signal is observed in the mass range from 0.1 to 2.1 GeV/$c^{2}$. We set upper limits at the 90\% confidence level on $\mathcal{B}(J/\psi \to \eta' \gamma ')\times\mathcal{B}(\gamma ' \to e^+e^-)$, $\mathcal{B}(J/\psi \to\eta' \gamma'$) and the mixing strength as a function of dark photon mass. This is among the first searches for dark photons in charmonium decays.

Dark tridents at off-axis liquid argon neutrino detectors

We present dark tridents, a new channel for exploring dark sectors in short-baseline neutrino experiments. Dark tridents are clean, distinct events where, like neutrino tridents, the scattering of a very weakly coupled particle leads to the production of a lepton-antilepton pair. Dark trident production occurs in models where long-lived dark-sector particles are produced along with the neutrinos in a beam-dump environment and interact with neutrino detectors downstream, producing an on-shell boson which decays into a pair of charged leptons. We focus on a simple model where the dark matter particle interacts with the standard model exclusively through a dark photon, and concentrate on the region of parameter space where the dark photon mass is smaller than twice that of the dark matter particle and hence decays exclusively into standard-model particles. We compute event rates and discuss search strategies for dark tridents from dark matter at the current and upcoming liquid argon detectors aligned with the Booster beam at Fermilab — MicroBooNE, SBND, and ICARUS — assuming the dark sector particles are produced off-axis in the higher energy NuMI beam. We find that MicroBooNE has already recorded enough data to be competitive with existing bounds on this dark sector model, and that new regions of parameter space will be probed with future data and experiments.

Dark photon portal dark matter with the 21-cm anomaly

A strong absorption profile was reported by the EDGES Collaboration, which indicates the hydrogen gas being colder than expected. It could be signatures of non-gravitational interactions between normal matter and dark matter (DM), and a possible explanation is that a small fraction of millicharged DM scatters with normal matter, with the DM mass in tens of MeV. To obtain the small fraction of millicharged DM and meanwhile being tolerant as regards the constraints, the dark photon portal scalar and vector millicharged DM are explored in this paper. We consider the mass of dark photon to be slightly above twice the millicharged DM mass, and thus the millicharged DM predominantly annihilates in p-wave during the freeze-out period, with the annihilation being enhanced near the resonance. The dark photon mainly decays into millicharged DM, and couplings of the dark photon with SM particles could be allowed by the lepton collision experiments. The corresponding parameter spaces are derived. Future lepton collision experiments can be employed to search for millicharged DM via the production of the invisible dark photon.

Characterization and performance of PADME’s Cherenkov-based small-angle calorimeter

The PADME experiment, at the Laboratori Nazionali di Frascati (LNF), in Italy, will search for invisible decays of the hypothetical dark photon via the process e+e−→γA′, where the A′ escapes detection. The dark photon mass range sensitivity in a first phase will be 1 to 24 MeV. We report here on performance measurements and simulation studies of a prototype of the Small-Angle Calorimeter, a component of PADME’s detector dedicated to rejecting 2- and 3-gamma backgrounds. The crucial requirement is a timing resolution of less than 200 ps, which is satisfied by the choice of PbF2 crystals and the newly released Hamamatsu R13478UV photomultiplier tubes (PMTs). We find a timing resolution of 81 ps (with double-peak separation resolution of 1.8 ns) and a single-crystal energy resolution of 10% at 550 MeV with light yield of 2.05 photo-electrons per MeV, using 100 to 400 MeV electrons at the Beam Test Facility of LNF. We also propose the investigation of a two-PMT solution coupled to a single PbF2 crystal for higher-energy applications, which has potentially attractive features.

Signatures of dark Higgs boson in light fermionic dark matter scenarios

Thermal dark matter scenarios based on light (sub-GeV) fermions typically require the presence of an extra dark sector containing both a massive dark photon along with a dark Higgs boson. The latter typically generates both the dark photon mass and an additional mass term for the dark sector fermions. This simple setup has both rich phenomenology and bright detection prospects at high-intensity accelerator experiments. We point out that in addition to the well studied pseudo-Dirac regime, this model can achieve the correct relic density in three different scenarios, and examine in details their properties and experimental prospects. We emphasize in particular the effect of the dark Higgs boson on both detection prospects and cosmological bounds.

Kinetic mixing, dark photons and extra dimensions. Part II: fermionic dark matter

Extra dimensions can be very useful tools when constructing new physics models. Previously, we began investigating toy models for the 5-D analog of the kinetic mixing/vector portal scenario where the interactions of bulk dark matter with the brane-localized fields of the Standard Model are mediated by a massive U(1)D dark photon also living in the bulk. In that setup, where the dark matter was taken to be a complex scalar, a number of nice features were obtained such as U(1)D breaking by boundary conditions without the introduction of a dark Higgs field, the absence of potentially troublesome SM Higgs-dark singlet mixing, also by boundary conditions, the natural similarity of the dark matter and dark photon masses and the decoupling of the heavy gauge Kaluza-Klein states from the Standard Model. In the present paper we extend this approach by examining the more complex cases of Dirac and Majorana fermionic dark matter. In particular, we discuss a new mechanism that can occur in 5-D (but not in 4-D) that allows for light Dirac dark matter in the ∼ 100 MeV mass range, even though it has an s-wave annihilation into Standard Model fields, by avoiding the strong constraints that arise from both the CMB and 21 cm data. This mechanism makes use of the presence of the Kaluza-Klein excitations of the dark photon to extremize the increase in the annihilation cross section usually obtained via resonant enhancement. In the Majorana dark matter case, we explore the possibility of a direct s-channel dark matter pair-annihilation process producing the observed relic density, due to the general presence of parity-violating dark matter interactions, without employing the usual co-annihilation mechanism which is naturally suppressed in this 5-D setup.

Supernova 1987A constraints on sub-GeV dark sectors, millicharged particles, the QCD axion, and an axion-like particle

We consider the constraints from Supernova 1987A on particles with small couplings to the Standard Model. We discuss a model with a fermion coupled to a dark photon, with various mass relations in the dark sector; millicharged particles; dark-sector fermions with inelastic transitions; the hadronic QCD axion; and an axion-like particle that couples to Standard Model fermions with couplings proportional to their mass. In the fermion cases, we develop a new diagnostic for assessing when such a particle is trapped at large mixing angles. Our bounds for a fermion coupled to a dark photon constrain small couplings and masses ≲ 200 MeV, and do not decouple for low fermion masses. They exclude parameter space that is otherwise unconstrained by existing accelerator-based and direct-detection searches. In addition, our bounds are complementary to proposed laboratory searches for sub-GeV dark matter, and do not constrain several benchmark-model targets in parameter space for which the dark matter obtains the correct relic abundance from interactions with the Standard Model. For a millicharged particle, we exclude charges between 10−9–few×10−6 in units of the electron charge, also for masses ≲ 200 MeV; this excludes parameter space to higher millicharges and masses than previous bounds. For the QCD axion and an axion-like particle, we apply several updated nuclear physics calculations and include the energy dependence of the optical depth to accurately account for energy loss at large couplings. These corrections allow us to rule out a hadronic axion of mass between 0.1 and a few hundred eV, or equivalently to put a bound on the scale of Peccei-Quinn symmetry breaking between a few×104 and 108 GeV, closing the hadronic axion window. For an axion-like particle, our bounds disfavor decay constants between a few×105 GeV up to a few×108 GeV, for a mass ≲ 200 MeV. In all cases, our bounds differ from previous work by more than an order of magnitude across the entire parameter space. We also provide estimated systematic errors due to the uncertainties of the progenitor.

Combined limit on the production of a light gauge boson decaying into μ+μ− and π+π−

We searched for the μ+μ− decay of a light vector gauge boson, also known as dark photon, in the e+e−→μ+μ−γISR process by means of the Initial State Radiation (ISR) method. We used 1.93 fb−1 of data collected by the KLOE experiment at the DAΦNE ϕ-factory. No structures have been observed over the irreducible μ+μ− background. A 90% CL limit on the ratio ε2=α′/α between the dark coupling constant and the fine structure constant of 3×10−6–2×10−7 has been set in the dark photon mass region between 519 MeV and 973 MeV. This new limit has been combined with the published result obtained investigating the hypothesis of the dark photon decaying into hadrons in e+e−→π+π−γISR events. The combined 90% CL limit increases the sensitivity especially in the ρ–ω interference region and excludes ε2 greater than (13−2)×10−7. For dark photon masses greater than 600 MeV the combined limit is lower than 8 ×10−7 resulting more stringent than present constraints from other experiments.

Photoproduction of Triplets on Free Electrons and the Search for the Dark Photon

We have investigated the photoproduction of triplets on free electrons, γe– → e+e–e–, in which dark photon A′ can be formed as an intermediate state with subsequent decay into an e+e– pair. This effect appears as a result of the so-called kinetic mixing and is characterized by the small parameter describing the intensity of the interaction of the dark photon with charged Standard Model (SM) particles in terms of electric charge e. The search for a manifestation of A′ in this process is advantageous since the background to the A′ signal is purely electrodynamic and, hence, can be calculated with the required accuracy. We calculate this background taking into account the identity of final electrons. As regards A′, its contribution is taken into account only in Compton-type diagrams (of virtual Compton scattering) in which a virtual dark photon is a time-like particle and its propagator has the form of a Breit–Wigner resonance. It is only in the vicinity of resonance that A′ can be manifested. We calculate the invariant mass distribution of both e+e– pairs formed in the process and analyze the kinematic region in which relatively small squares of momenta transferred from the target electron to the formed electrons are excluded. In these conditions, the contribution to the differential cross section from Compton-type diagrams is not suppressed relative to the contribution of the remaining (Borsellino) diagrams. A number of limitations on parameter depending on the dark photon mass and the statistics (number) of events are obtained for a special method of gathering events in which the invariant mass of one e+e– pair remains fixed and of the other pair is scanned.

Dark decay of the neutron

New decay channels for the neutron into dark matter plus other particles have been suggested for explaining a long-standing discrepancy between the neutron lifetime measured from trapped neutrons versus those decaying in flight. Many such scenarios are already ruled out by their effects on neutron stars, and the decay into dark matter plus photon has been experimentally excluded. Here we explore the decay into a dark Dirac fermion χ and a dark photon A′, which can be consistent with all constraints if χ is a subdominant component of the dark matter. Neutron star constraints are evaded if the dark photon mass to coupling ratio is mA ′/g ′ ≲ (45 − 60) MeV, depending upon the nuclear equation of state. g′ and the kinetic mixing between U(1)′ and electromagnetism are tightly constrained by direct and indirect dark matter detection, supernova constraints, and cosmological limits.

Signatures of self-interacting dark matter in the matter power spectrum and the CMB

We consider a self-interacting dark matter model in which the massive dark photon mediating the self-interaction decays to light dark fermions to avoid over-closing the universe. We find that if the model is constrained to explain the dark matter halos inferred for spiral galaxies and galaxy clusters simultaneously, there is a strong indication that dark matter is produced asymmetrically in the early universe. It also implies the presence of dark radiation, late kinetic decoupling for dark matter, and a suppressed linear power spectrum due to dark acoustic damping. The Lyman-α forest power spectrum measurements put a strong upper limit on the damping scale and the model has little room to reduce the abundances of satellite galaxies. Future observations in the matter power spectrum and the CMB, in tandem with the impact of self-interactions in galactic halos, makes it possible to measure the gauge coupling and masses of the dark sector particles even when signals in conventional dark matter searches are absent.

Dark photon decay beyond the Euler-Heisenberg limit

We calculate the exact width for a dark photon decaying to three photons at one loop order for dark photon masses m' below the e+e- production threshold of 2m_e. We find substantial deviations from previous results derived from the lowest order Euler-Heisenberg effective Lagrangian in the range m_e < m' < 2m_e, where higher order terms in the derivative expansion are nonnegligible. This mass range is precisely where the three photon decay takes place on cosmologically relevant timescales. Our improved analysis opens a window for dark photons in the range 850 keV < m' < 2m_e, 10^-5 < epsilon < 10^-4.

Search for a heavy dark photon at future e+e\u2212 colliders

A coupling of a dark photon A′ from a U(1)A′ with the standard model (SM) particles can be generated through kinetic mixing represented by a parameter ϵ. A non-zero ϵ also induces a mixing between A′ and Z if dark photon mass mA′ is not zero. This mixing can be large when mA′ is close to mZ even if the parameter ϵ is small. Many efforts have been made to constrain the parameter ϵ for a low dark photon mass mA′ compared with the Z boson mass mZ . We study the search for dark photon in e+e− → γA′ → γμ+μ− for a dark photon mass mA′ as large as kinematically allowed at future e+e− colliders. For large mA′, care should be taken to properly treat possible large mixing between A′ and Z. We obtain sensitivities to the parameter ϵ for a wide range of dark photon mass at planed e+e− colliders, such as Circular Electron Positron Collider (CEPC), International Linear Collider (ILC) and Future Circular Collider (FCC-ee). For the dark photon mass 20 GeV ≲ mA′ ≲ 330 GeV, the 2σ exclusion limits on the mixing parameter are ϵ ≲ 10−3-10−2. The CEPC with sqrt{s}=240 GeV and FCC-ee with sqrt{s}=160 GeV are more sensitive than the constraint from current LHCb measurement once the dark photon mass mA′ ≳ 50 GeV. For mA′ ≳ 220 GeV, the sensitivity at the FCC-ee with sqrt{s}=350 GeV and 1.5 ab−1 is better than that at the 13 TeV LHC with 300 fb−1, while the sensitivity at the CEPC with sqrt{s}=240 GeV and 5ab−1 can be even better than that at 13TeV LHC with 3ab−1 for mA′ ≳ 180 GeV. We also comment on sensitivities of e+e− → γA′ with dark photon decay into several other channels at future e+e− colliders.

ForwArd Search ExpeRiment at the LHC

New physics has traditionally been expected in the high-$p_T$ region at high-energy collider experiments. If new particles are light and weakly-coupled, however, this focus may be completely misguided: light particles are typically highly concentrated within a few mrad of the beam line, allowing sensitive searches with small detectors, and even extremely weakly-coupled particles may be produced in large numbers there. We propose a new experiment, ForwArd Search ExpeRiment, or FASER, which would be placed downstream of the ATLAS or CMS interaction point (IP) in the very forward region and operated concurrently there. Two representative on-axis locations are studied: a far location, $400~\text{m}$ from the IP and just off the beam tunnel, and a near location, just $150~\text{m}$ from the IP and right behind the TAN neutral particle absorber. For each location, we examine leading neutrino- and beam-induced backgrounds. As a concrete example of light, weakly-coupled particles, we consider dark photons produced through light meson decay and proton bremsstrahlung. We find that even a relatively small and inexpensive cylindrical detector, with a radius of $\sim 10~\text{cm}$ and length of $5-10~\text{m}$, depending on the location, can discover dark photons in a large and unprobed region of parameter space with dark photon mass $m_{A'} \sim 10~\text{MeV} - 1~\text{GeV}$ and kinetic mixing parameter $\epsilon \sim 10^{-7} - 10^{-3}$. FASER will clearly also be sensitive to many other forms of new physics. We conclude with a discussion of topics for further study that will be essential for understanding FASER's feasibility, optimizing its design, and realizing its discovery potential.

Status and prospects for the PADME experiment at LNF

The PADME collaboration aims to search for signals of a light dark photon A′ using the beam of the LNF LINAC. The experiment, approved by INFN at the end of 2015, foresees to detect A′, produced in the annihilation of positrons on a thin fixed target, by searching for missing mass signals. The detector construction is ongoing and should be completed within the end of 2017 in order to allow the collection of about 1013 positrons on target which are necessary to get a 10−3 sensitivity on the mixing parameter ε up to a dark photon mass of 23.7 MeV/c2.

Search for dark photons using data from CRESST-II Phase 2

Understanding the nature and origin of dark matter is one of the most important challenges for modern particle physics. During the previous decade the sensitivities of direct dark matter searches have improved by several orders of magnitude. These experiments focus their work mainly on the search for dark-matter particles interacting with nuclei (e.g. Weakly Interacting Massive Particles, WIMPs). However, there exists a large variety of different candidates for dark-matter particles. One of these candidates, the so-called dark photon, is a long-lived vector boson with a kinetic mixing to the standard-model photon. In this work we present the preliminary results of our search for dark photons. Using data from the direct dark matter search CRESST-II Phase 2 we can improve the existing constraints for the kinetic mixing for dark-photon masses between 0.3 and 0.5 keV/c2. In addition, we also present projected sensitivities for the next phases of the CRESST-III experiment showing great potential to improve the sensitivity for dark-photon masses below 1 keV.

Updated constraints on self-interacting dark matter from Supernova 1987A

We revisit SN1987A constraints on light, hidden sector gauge bosons ("dark photons") that are coupled to the standard model through kinetic mixing with the photon. These constraints are realized because excessive bremsstrahlung radiation of the dark photon can lead to rapid cooling of the SN1987A progenitor core, in contradiction to the observed neutrinos from that event. The models we consider are of interest as phenomenological models of strongly self-interacting dark matter. We clarify several possible ambiguities in the literature and identify errors in prior analyses. We find constraints on the dark photon mixing parameter that are in rough agreement with the early estimates of Dent et al., but only because significant errors in their analyses fortuitously canceled. Our constraints are in good agreement with subsequent analyses by Rrapaj & Reddy and Hardy & Lasenby. We estimate the dark photon bremsstrahlung rate using one-pion exchange (OPE), while Rrapaj & Reddy use a soft radiation approximation (SRA) to exploit measured nuclear scattering cross sections. We find that the differences between mixing parameter constraints obtained through the OPE approximation or the SRA approximation are roughly a factor of $\sim 2-3$. Hardy & Laseby include plasma effects in their calculations finding significantly weaker constraints on dark photon mixing for dark photon masses below $\sim 10\, \mathrm{MeV}$. We do not consider plasma effects. Lastly, we point out that the properties of the SN1987A progenitor core remain somewhat uncertain and that this uncertainty alone causes uncertainty of at least a factor of $\sim 2-3$ in the excluded values of the dark photon mixing parameter. Further refinement of these estimates is unwarranted until either the interior of the SN1987A progenitor is more well understood or additional, large, and heretofore neglected effects, are identified.

Dark photon search at a circular e+e− collider

One of the interesting portals linking a dark sector and the Standard Model (SM) is the kinetic mixing between the SM [Formula: see text] field with a new dark photon [Formula: see text] from a [Formula: see text] gauge interaction. Stringent limits have been obtained for the kinetic mixing parameter [Formula: see text] through various processes. In this work, we study the possibility of searching for a dark photon interaction at a circular [Formula: see text] collider through the process [Formula: see text]. We find that the constraint on [Formula: see text] for dark photon mass in the few tens of GeV range, assuming that the [Formula: see text] invariant mass can be measured to an accuracy of 0.5% [Formula: see text], can be better than [Formula: see text] for the proposed CEPC with a 10-year running at [Formula: see text] (statistic) level, and better than [Formula: see text] for FCC-ee with even just one-year running at [Formula: see text], better than the LHCb, ATLAS, CMS experiments and other facilities can do in a similar dark photon mass range. For FCC-ee, running at [Formula: see text], the constraint can be even better.

Detecting Dark Photons with Reactor Neutrino Experiments.

We propose to search for light U(1) dark photons, A^{'}, produced via kinetically mixing with ordinary photons via the Compton-like process, γe^{-}→A^{'}e^{-}, in a nuclear reactor and detected by their interactions with the material in the active volumes of reactor neutrino experiments. We derive 95%confidence-level upper limits on ε, the A^{'}-γ mixing parameter, ε, for dark-photon masses below 1MeV of ε<1.3×10^{-5} and ε<2.1×10^{-5}, from NEOS and TEXONO experimental data, respectively. This study demonstrates the applicability of nuclear reactors as potential sources of intense fluxes of low-mass dark photons.

Superradiance in rotating stars and pulsar-timing constraints on dark photons

In the presence of massive bosonic degrees of freedom, rotational superradiance can trigger an instability that spins down black holes. This leads to peculiar gravitational-wave signatures and distribution in the spin-mass plane, which in turn can impose stringent constraints on ultralight fields. Here, we demonstrate that there is an analogous spindown effect for conducting stars. We show that rotating stars amplify low frequency electromagnetic waves, and that this effect is largest when the time scale for conduction within the star is of the order of a light crossing time. This has interesting consequences for dark photons, as massive dark photons would cause stars to spin down due to superradiant instabilities. The time scale of the spindown depends on the mass of the dark photon, and on the rotation rate, compactness, and conductivity of the star. Existing measurements of the spindown rate of pulsars place direct constraints on models of dark sectors. Our analysis suggests that dark photons of mass $m_V \sim 10^{-12}$ eV are excluded by pulsar-timing observations. These constraints also exclude superradiant instabilities triggered by dark photons as an explanation for the spin limit of observed pulsars.