arXiv subjects

Josef Pradler

Publications and source records attributed to Josef Pradler.

At least 55 records · Page 3Linked to original sources

arXiv2019

Stellar probes of dark sector-photon interactions

Electromagnetically neutral dark sector particles may directly couple to the photon through higher dimensional effective operators. Considering electric and magnetic dipole moment, anapole moment, and charge radius interactions, we derive constraints from stellar energy loss in the Sun, horizontal branch and red giant stars, as well as from cooling of the proto-neutron star of SN1987A. We provide the exact formula for in-medium photon-mediated pair production to leading order in the dark coupling, and compute the energy loss rates explicitly for the most important processes, including a careful discussion on resonances and potential double counting between the processes. Stringent limits for dark states with masses below $3\,$keV ($40\,$MeV) arise from red giant stars (SN1987A), implying an effective lower mass-scale of approximately $10^9\,$GeV ($10^7\,$GeV) for mass-dimension five, and $100\,$GeV ($2.5\,$TeV) for mass-dimension six operators as long as dark states stream freely; for the proto-neutron star, the trapping of dark states is also evaluated. Together with direct limits previously derived by us in Chu et al. (2018), this provides the first comprehensive overview of the viability of effective electromagnetic dark-state interactions below the GeV mass-scale.

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arXiv2019

Light dark states with electromagnetic form factors

New particles $χ$ that are electrically neutral but couple to the electromagnetic current via higher-dimensional operators and that are sufficiently light, at or below the GeV-mass scale, can be produced in pairs in a number of dedicated high-intensity experiments. In this work we consider the production of $χ$ through magnetic- and electric-dipole moments as well as through anapole moment and charge radius interactions in electron beams. We derive new constraints from BaBar, NA64 and mQ and forecast the future sensitivity on the existence of such states, from Belle-II, LDMX and BDX. We present for the first time a detailed treatment of the off-shell production of photons in electron beams with subsequent decay into a $χ\barχ$ pair in a 2-to-4 process. These direct limits are then compared to the effects on SM precision observables, as well as to bounds from flavor physics and high energy colliders. Finally, we consider the possibility that $χ$ is dark matter and study ensuing astrophysical and cosmological constraints. We find that a combination of all considered probes rule out $χ$ particles with mass-dimension five and six photon interactions as dark matter when assuming a standard freeze-out abundance.

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arXiv2018

Dark radiation: 21cm signals and laboratory tests

It is entirely possible that our Universe is filled with dark radiation, such as SM neutrinos or new physics states, that are sourced by the decay of dark matter with cosmologically long lifetime. If non-thermal neutrinos produced such way carry sufficient energy, they can leave a detectable imprint in experiments designed to search for very weakly interacting particles: dark matter and underground neutrino experiments. Conversely, a very soft non-thermal population of dark photons sourced this way is allowed to exceed the number density of CMB photons by many orders of magnitude without being in conflict with current bounds. Equipped with a small probability of conversion into ordinary photons, the scenario becomes testable through the cosmological 21cm signal.

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arXiv2018

Cosmological beam dump: constraints on dark scalars mixed with the Higgs boson

Precision cosmology provides a sensitive probe of extremely weakly coupled states due to thermal freeze-in production, with subsequent decays impacting physics during well-tested cosmological epochs. We explore the cosmological implications of the freeze-in production of a new scalar $S$ via the super-renormalizable Higgs portal. If the mass of $S$ is at or below the electroweak scale, peak freeze-in production occurs during the electroweak epoch. We improve the calculation of the freeze-in abundance by including all relevant QCD and electroweak production channels. The resulting abundance and subsequent decay of $S$ is constrained by a combination of X-ray data, cosmic microwave background anisotropies and spectral distortions, $N_{\rm eff}$, and the consistency of BBN with observations. These probes constrain technically natural couplings for such scalars from $m_S \sim$ keV all the way to $m_S \sim 100$ GeV. The ensuing constraints are similar in spirit to typical beam bump limits, but extend to much smaller couplings, down to mixing angles as small as $θ_{Sh} \sim 10^{-16}$, and to masses all the way to the electroweak scale.

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arXiv2018

Directly Detecting MeV-scale Dark Matter via Solar Reflection

If dark matter (DM) particles are lighter than a few MeV/$c^2$ and can scatter off electrons, their interaction within the solar interior results in a considerable hardening of the spectrum of galactic dark matter received on Earth. For a large range of the mass vs. cross section parameter space, $\{m_e, σ_e\}$, the "reflected" component of the DM flux is far more energetic than the endpoint of the ambient galactic DM energy distribution, making it detectable with existing DM detectors sensitive to an energy deposition of $10-10^3$ eV. After numerically simulating the small reflected component of the DM flux, we calculate its subsequent signal due to scattering on detector electrons, deriving new constraints on $σ_e$ in the MeV and sub-MeV range using existing data from the XENON10/100, LUX, PandaX-II, and XENON1T experiments, as well as making projections for future low threshold direct detection experiments.

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arXiv2018

New Physics in the Rayleigh-Jeans Tail of the CMB

We show that despite stringent constraints on the shape of the main part of the CMB spectrum, there is considerable room for its modification within its Rayleigh-Jeans (RJ) end, $ω\ll T_{\rm CMB}$. We construct explicit New Physics models that give an order one (or larger) increase of photon count in the RJ tail, which can be tested by existing and upcoming experiments aiming to detect the cosmological 21 cm emission/absorption signal. This class of models stipulates the decay of unstable particles to dark photons, $A'$, that have a small mass, $m_{A'} \sim 10^{-14} - 10^{-9}$ eV, non-vanishing mixing angle $ε$ with electromagnetism, and energies much smaller than $T_{\rm CMB}$. The non-thermal number density of dark photons can be many orders of magnitude above the number density of CMB photons, and even a small probability of $A'\to A$ oscillations, for values of as small as $ε\sim 10^{-9}$, can significantly increase the number of RJ photons. In particular, we show that resonant oscillations of dark photons into regular photons in the interval of redshifts $20 < z < 1700$ can be invoked as an explanation of the recent tentative observation of a stronger-than-expected absorption signal of 21 cm photons. We present a model that realizes this possibility, where milli-eV mass dark matter decays to dark photons, with a lifetime longer than the age of the Universe.

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arXiv2018

Signatures of Dark Radiation in Neutrino and Dark Matter Detectors

We consider the generic possibility that the Universe's energy budget includes some form of relativistic or semi-relativistic dark radiation (DR) with non-gravitational interactions with Standard Model (SM) particles. Such dark radiation may consist of SM singlets or a non-thermal, energetic component of neutrinos. If such DR is created at a relatively recent epoch, it can carry sufficient energy to leave a detectable imprint in experiments designed to search for very weakly interacting particles: dark matter and underground neutrino experiments. We analyze this possibility in some generality, assuming that the interactive dark radiation is sourced by late decays of an unstable particle, potentially a component of dark matter, and considering a variety of possible interactions between the dark radiation and SM particles. Concentrating on the sub-GeV energy region, we derive constraints on different forms of DR using the results of the most sensitive neutrino and dark matter direct detection experiments. In particular, for interacting dark radiation carrying a typical momentum of $\sim30$~MeV$/c$, both types of experiments provide competitive constraints. This study also demonstrates that non-standard sources of neutrino emission (e.g. via dark matter decay) are capable of creating a "neutrino floor" for dark matter direct detection that is closer to current bounds than is expected from standard neutrino sources.

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arXiv2017

Simply split SIMPs

Dark Matter which interacts strongly with itself, but only feebly with the Standard Model is a possibility that has been entertained to solve apparent small-scale structure problems that are pertinent to the non-interacting cold Dark Matter paradigm. In this paper, we study the simple case in which the self-scattering rate today is regulated by kinematics and/or the abundance ratio, through the mass-splitting of nearly degenerate pseudo-Dirac fermions $χ_1$ and $χ_2$ or real scalars $ϕ_1$ and $ϕ_2$. We calculate the relic density of these states in a scenario where self-scattering proceeds through off-diagonal couplings with a vector particle $V$ (Dark Photon) and where the abundance is set through number-depleting 4-to-2 reactions in the hidden sector, or, alternatively, via freeze-in. We study the implications of the considered models and their prospect of solving astrophysical small-scale structure problems. We also show how the introduction of the (meta-)stable heavier state may be probed in future dark matter searches.

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arXiv2017

Probing sub-GeV Dark Matter with conventional detectors

The direct detection of Dark Matter particles with mass below the GeV-scale is hampered by soft nuclear recoil energies and finite detector thresholds. For a given maximum relative velocity, the kinematics of elastic Dark Matter nucleus scattering sets a principal limit on detectability. Here we propose to bypass the kinematic limitations by considering the inelastic channel of photon emission in the nuclear recoil. Our proposed method allows to set the first limits on Dark Matter below 500 MeV in the plane of Dark Matter mass and cross section with nucleons. In situations where a Dark Matter-electron coupling is suppressed, Bremsstrahlung may constitute the only path to probe low-mass Dark Matter awaiting new detector technologies with lowered recoil energy thresholds.

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arXiv2016

Cosmological tests of an axiverse-inspired quintessence field

Inspired by the string axiverse idea, it has been suggested that the recent transition from decelerated to accelerated cosmic expansion is driven by an axion-like quintessence field with a sub-Planckian decay constant. The scenario requires that the axion field be rather near the maximum of its potential, but is less finely tuned than other explanations of cosmic acceleration. The model is parametrized by an axion decay constant $f$, the axion mass $m$, and an initial misalignment angle $|θ_i|$ which is close to $π$. In order to determine the $m$ and $θ_{i}$ values consistent with observations, these parameters are mapped onto observables: the Hubble parameter $H(z)$ at and angular diameter distance $d_{A}(z)$ to redshift $z= 0.57$, as well as the angular sound horizon of the cosmic microwave background (CMB). Measurements of the baryon acoustic oscillation (BAO) scale at $z\simeq 0.57$ by the BOSS survey and Planck measurements of CMB temperature anisotropies are then used to probe the $\left\{m,f,θ_i\right\}$ parameter space. With current data, CMB constraints are the most powerful, allowing a fraction of only $\sim 0.2$ of the parameter-space volume. Measurements of the BAO scale made using the SPHEREx or SKA experiments could go further, observationally distinguishing all but $\sim 10^{-2}$ or $\sim 10^{-5}$ of the parameter-space volume (allowed by simple priors) from the $Λ$CDM model.

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arXiv2016

A light particle solution to the cosmic lithium problem

We point out that the cosmological abundance of ${}^7$Li can be reduced down to observed values if during its formation Big Bang Nucleosynthesis is modified by the presence of light electrically neutral particles $X$ that have substantial interactions with nucleons. We find that the lithium problem can be solved without affecting the precisely measured abundances of deuterium and helium if the following conditions are satisfied: the mass and lifetimes of such particles are bounded by $ 1.6~{\rm MeV}\leq m_X \leq 20~{\rm MeV}$ and $ {\rm few}~100~{\rm s} \lesssim τ_X \lesssim 10^4~{\rm s}$, and the abundance times the absorption cross section by either deuterium or ${}^7$Be are comparable to the Hubble rate, $n_X σ_{\rm abs} v \sim H$, at the time of ${}^7$Be formation. We include $X$-initiated reactions into the primordial nucleosynthesis framework, observe that it leads to a substantial reduction of the freeze-out abundances of ${}^7$Li+${}^7$Be, and find specific model realizations of this scenario. Concentrating on the axion-like-particle case, $X=a$, we show that all these conditions can be satisifed if the coupling to $d$-quarks is in the range of $f_d^{-1} \sim {\rm TeV}^{-1}$, which can be probed at intensity frontier experiments.

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arXiv2015

Direct detection prospects of dark vectors with xenon-based dark matter experiments

Dark matter experiments primarily search for the scattering of WIMPs on target nuclei of well shielded underground detectors. The results from liquid scintillator experiments furthermore provide precise probes of very light and very weakly coupled particles that may be absorbed by electrons. In these proceedings we summarize previously obtained constraints on long-lived dark matter vector particles $V$ (dark photons) in the $0.01-100$ keV mass range. In addition, we provide a first projected sensitivity reach for the upcoming XENON1T dark matter search to detect dark photons.

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arXiv2015

Direct Detection Constraints on Dark Photon Dark Matter

Dark matter detectors built primarily to probe elastic scattering of WIMPs on nuclei are also precise probes of light, weakly coupled particles that may be absorbed by the detector material. In this paper, we derive constraints on the minimal model of dark matter comprised of long-lived vector states V (dark photons) in the 0.01-100 keV mass range. The absence of an ionization signal in direct detection experiments such as XENON10 and XENON100 places a very strong constraint on the dark photon mixing angle, down to $O(10^{-15})$, assuming that dark photons comprise the dominant fraction of dark matter. This sensitivity to dark photon dark matter exceeds the indirect bounds derived from stellar energy loss considerations over a significant fraction of the available mass range. We also revisit indirect constraints from $V\to 3γ$ decay and show that limits from modifications to the cosmological ionization history are comparable to the updated limits from the diffuse gamma-ray flux.

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arXiv2015

Nucleosynthesis constraints on the faint vector portal

New Abelian U(1)' gauge bosons $V_μ$ can couple to the Standard Model through mixing of the associated field strength tensor $V_{μν}$ with the one from hypercharge, $F_{μν}^Y$. Here we consider early Universe sensitivity to this vector portal and show that the effective mixing parameter with the photon, $κ$, is being probed for vector masses in the GeV ballpark down to values $10^{-10} \lesssim κ\lesssim 10^{-14}$ where no terrestrial probes exist. The ensuing constraints are based on a detailed calculation of the vector relic abundance and an in-depth analysis of relevant nucleosynthesis processes.

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arXiv2014

Dark energy from the string axiverse

String theories suggest the existence of a plethora of axion-like fields with masses spread over a huge number of decades. Here we show that these ideas lend themselves to a model of quintessence with no super-Planckian field excursions and in which all dimensionless numbers are order unity. The scenario addresses the "why now" problem, i.e., why has accelerated expansion begun only recently, by suggesting that the onset of dark-energy domination occurs randomly with a slowly decreasing probability per unit logarithmic interval in cosmic time. The standard axion potential requires us to postulate a rapid decay of most of the axion fields that do no become dark energy. The need for these decays is averted, though, with the introduction of a slightly modified axion potential. In either case, a Universe like ours arises in roughly 1 in 100 universes. The scenario may have a host of observable consequences.

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arXiv2014

Cosmological Constraints on Very Dark Photons

We explore the cosmological consequences of kinetically mixed dark photons with a mass between 1 MeV and 10 GeV, and an effective electromagnetic fine structure constant as small as $10^{-38}$. We calculate the freeze-in abundance of these dark photons in the early Universe and explore the impact of late decays on BBN and the CMB. This leads to new constraints on the parameter space of mass $m_V$ vs kinetic mixing parameter $κ$.

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arXiv2014

Silk damping at a redshift of a billion: a new limit on small-scale adiabatic perturbations

We study the dissipation of small-scale adiabatic perturbations at early times when the Universe is hotter than T ~ 0.5 keV. When the wavelength falls below the damping scale 1/kD, the acoustic modes diffuse and thermalize, causing entropy production. Before neutrino decoupling, kD is primarily set by the neutrino shear viscosity, and we study the effect of acoustic damping on the relic neutrino number, primordial nucleosynthesis, dark-matter freeze-out, and baryogenesis. This sets a new limit on the amplitude of primordial fluctuations of DeltaR^2 < 0.007 at 10^4/Mpc< k < 10^5/Mpc and a model dependent limit of DeltaR^2 < 0.3 at k < 10^{20-25}/Mpc.

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arXiv2014

Observing dark photon with dark matter detectors

Motivated by various kinds of new physics models, a new light neutral vector boson (the dark photon) connected to the Standard Model of particle physics only through the kinetic mixing with the U(1)_Y factor has been studied extensively. Various kinds of experiments are proposed to detect it. For the dark photon with a mass smaller than 10 keV, it can be copiously produced inside the Sun, and then be detected by the detectors on the earth. We show that with the S2 only analysis, the result from XENON10 experiment provides the up-to-date most stringent limit on the region 10^{-5} eV < m_V < 10^3 eV: kappa m_V < 3 times 10^{-12} eV for the Stueckelberg dark photon model, where kappa is the kinetic mixing and m_V is the mass of the dark photon. If there is a light Higgs boson accompanied with the dark photon, the sensitivity of XENON10 experiment is kappa times e' < 10^{-13}, which is still second to the constraints from the lifetime of horizontal branch starts which dictates kappa times e' < 3 times10^{-14}.

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