Caputo, A., Liu, H. W., Mishra-Sharma, S., Pospelov, M., Ruderman, J. T., & Urbano, A. (2021). Edges and Endpoints in 21-cm Observations from Resonant Photon Production. Phys. Rev. Lett., 127(1), 011102–7pp.
Abstract: We introduce a novel class of signatures-spectral edges and end points-in 21-cm measurements resulting from interactions between the standard and dark sectors. Within the context of a kinetically mixed dark photon, we demonstrate how resonant dark photon-to-photon conversions can imprint distinctive spectral features in the observed 21-cm brightness temperature, with implications for current, upcoming, and proposed experiments targeting the cosmic dawn and the dark ages. These signatures open up a qualitatively new way to look for physics beyond the Standard Model using 21-cm observations.
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Caputo, A., Liu, H. W., Mishra-Sharma, S., & Ruderman, J. T. (2020). Modeling dark photon oscillations in our inhomogeneous Universe. Phys. Rev. D, 102(10), 103533–26pp.
Abstract: A dark photon may kinetically mix with the Standard Model photon, leading to observable cosmological signatures. The mixing is resonantly enhanced when the dark photon mass matches the primordial plasma frequency, which depends sensitively on the underlying spatial distribution of electrons. Crucially, inhomogeneities in this distribution can have a significant impact on the nature of resonant conversions. We develop and describe, for the first time, a general analytic formalism to treat resonant oscillations in the presence of inhomogeneities. Our formalism follows from the theory of level crossings of random fields and only requires knowledge of the one-point probability density function (PDF) of the underlying electron number density fluctuations. We validate our formalism using simulations and illustrate the photon-to-dark photon conversion probability for several different choices of PDFs that are used to characterize the low-redshift Universe.
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Caputo, A., Liu, H. W., Mishra-Sharma, S., & Ruderman, J. T. (2020). Dark Photon Oscillations in Our Inhomogeneous Universe. Phys. Rev. Lett., 125(22), 221303–8pp.
Abstract: A dark photon kinetically mixing with the ordinary photon represents one of the simplest viable extensions to the standard model, and would induce oscillations with observable imprints on cosmology. Oscillations are resonantly enhanced if the dark photon mass equals the ordinary photon plasma mass, which tracks the free electron number density. Previous studies have assumed a homogeneous Universe; in this Letter, we introduce for the first time an analytic formalism for treating resonant oscillations in the presence of inhomogeneities of the photon plasma mass. We apply our formalism to determine constraints from cosmic microwave background photons oscillating into dark photons, and from heating of the primordial plasma due to dark photon dark matter converting into low-energy photons. Including the effect of inhomogeneities demonstrates that prior homogeneous constraints are not conservative, and simultaneously extends current experimental limits into a vast new parameter space.
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Caputo, A., Millar, A. J., & Vitagliano, E. (2020). Revisiting longitudinal plasmon-axion conversion in external magnetic fields. Phys. Rev. D, 101(12), 123004–13pp.
Abstract: In the presence of an external magnetic field, the axion and the photon mix. In particular, the dispersion relation of a longitudinal plasmon always crosses the dispersion relation of the axion (for small axion masses), thus leading to a resonant conversion. Using thermal field theory, we concisely derive the axion emission rate, applying it to astrophysical and laboratory scenarios. For the Sun, depending on the magnetic field profile, plasmon-axion conversion can dominate over Primakoff production at low energies (less than or similar to 200 eV). This both provides a new axion source for future helioscopes and, in the event of discovery, would probe the magnetic field structure of the Sun. In the case of white dwarfs (WDs), plasmon-axion conversion provides a pure photon coupling probe of the axion, which may contribute significantly for low-mass WDs. Finally, we rederive and confirm the axion absorption rate of the recently proposed plasma haloscopes.
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Caputo, A., Pena-Garay, C., & Witte, S. J. (2018). Looking for axion dark matter in dwarf spheroidal galaxies. Phys. Rev. D, 98(8), 083024–6pp.
Abstract: We study the extent to which the decay of cold dark matter axions can be probed with forthcoming radio telescopes such as the Square Kilometer Array (SKA). In particular, we focus on signals arising from dwarf spheroidal galaxies, where astrophysical uncertainties are reduced and the expected magnetic field strengths are such that signals arising from axion decay may dominate over axion-photon conversion in a magnetic field. We show that with similar to 100 hr of observing time, SKA could improve current sensitivity by 2-3 orders of magnitude-potentially obtaining sufficient sensitivity to begin probing the decay of cold dark matter axions.
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Caputo, A., Regis, M., & Taoso, M. (2020). Searching for sterile neutrino with X-ray intensity mapping. J. Cosmol. Astropart. Phys., 03(3), 001–21pp.
Abstract: The cosmological X-ray emission associated to the possible radiative decay of sterile neutrinos is composed by a collection of lines at different energies. For a given mass, each line corresponds to a given redshift. In this work, we cross correlate such line emission with catalogs of galaxies tracing the dark matter distribution at different redshifts. We derive observational prospects by correlating the X-ray sky that will be probed by the eROSITA and Athena missions with current and near future photometric and spectroscopic galaxy surveys. A relevant and unexplored fraction of the parameter space of sterile neutrinos can be probed by this technique.
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Caputo, A., Regis, M., Taoso, M., & Witte, S. J. (2019). Detecting the stimulated decay of axions at radio frequencies. J. Cosmol. Astropart. Phys., 03(3), 027–22pp.
Abstract: Assuming axion-like particles account for the entirety of the dark matter in the Universe, we study the possibility of detecting their decay into photons at radio frequencies. We discuss different astrophysical targets, such as dwarf spheroidal galaxies, the Galactic Center and halo, and galaxy clusters. The presence of an ambient radiation field leads to a stimulated enhancement of the decay rate; depending on the environment and the mass of the axion, the effect of stimulated emission may amplify the photon flux by serval orders of magnitude. For axion-photon couplings allowed by astrophysical and laboratory constraints (and possibly favored by stellar cooling), we find the signal to be within the reach of next-generation radio telescopes such as the Square Kilometer Array.
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Caputo, A., & Reig, M. (2019). Cosmic implications of a low-scale solution to the axion domain wall problem. Phys. Rev. D, 100(6), 063530–10pp.
Abstract: The post-inflationary breaking of Peccei-Quinn (PQ) symmetry can lead to the cosmic domain wall catastrophe. In this paper we show how to avoid domain walls by implementing the instanton interference effect with a new interaction which itself breaks PQ symmetry and confines at an energy scale smaller than Lambda(QCD). We give a general description of the mechanism and consider its cosmological implications and constraints within a minimal model. Contrary to other mechanisms, we do not require an inverse phase transition or fine-tuned bias terms. Incidentally, the mechanism leads to the introduction of new self-interacting dark matter candidates and the possibility of producing gravitational waves in the frequency range of SKA. Unless a fine-tuned hidden sector is introduced, the mechanism predicts a QCD axion in the mass range 1-15 meV.
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Caputo, A., Sberna, L., Frias, M., Blas, D., Pani, P., Shao, L. J., et al. (2019). Constraints on millicharged dark matter and axionlike particles from timing of radio waves. Phys. Rev. D, 100(6), 063515–7pp.
Abstract: We derive constraints on millicharged dark matter and axionlike particles using pulsar timing and fast radio burst observations. For dark matter particles of charge epsilon e, the constraint from time of arrival (TOA) of waves is epsilon/m(milli) less than or similar to 10(-8) eV(-1), for masses m(milli) greater than or similar to 10(-6) eV. For axionlike particles, the polarization of the signals from pulsars yields a bound in the axial coupling g/ m(a) less than or similar to 10(-13) Gev(-1)/(10(-22) eV),for m(a) less than or similar to 10(-19) eV. Both bounds scale as (rho/rho(dm))(1/2 )for fractions of the total dark matter energy density rho(dm). We make a precise study of these bounds using TOA from several pulsars, FRB 121102, and polarization measurements of PSR J0437 – 4715. Our results rule out a new region of the parameter space for these dark matter models.
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Caputo, A., Sberna, L., Toubiana, A., Babak, S., Barausse, E., Marsat, S., et al. (2020). Gravitational-wave Detection and Parameter Estimation for Accreting Black-hole Binaries and Their Electromagnetic Counterpart. Astrophys. J., 892(2), 90–13pp.
Abstract: We study the impact of gas accretion on the orbital evolution of black-hole binaries initially at large separation in the band of the planned Laser Interferometer Space Antenna (LISA). We focus on two sources: (i).stellar-origin black-hole binaries.(SOBHBs) that can migrate from the LISA band to the band of ground-based gravitational-wave (GW) observatories within weeks/months; and (ii) intermediate-mass black-hole binaries.(IMBHBs) in the LISA band only. Because of the large number of observable GW cycles, the phase evolution of these systems needs to be modeled to great accuracy to avoid biasing the estimation of the source parameters. Accretion affects the GW phase at negative (-4) post-Newtonian order, being thus dominant for binaries at large separations. Accretion at the Eddington or at super-Eddington rate will leave a detectable imprint on the dynamics of SOBHBs. For super-Eddington rates and a 10 yr mission, a multiwavelength strategy with LISA and a ground-based interferometer can detect about 10 (a few) SOBHB events for which the accretion rate can be measured at 50% (10%) level. In all cases, the sky position can be identified within much less than 0.4 deg(2) uncertainty. Likewise, accretion at greater than or similar to 100% of the Eddington rate can be measured in IMBHBs up to redshift z approximate to 0.1, and the position of these sources can be identified within less than 0.01 deg(2) uncertainty. Altogether, a detection of SOBHBs or IMBHBs would allow for targeted searches of electromagnetic counterparts to black-hole mergers in gas-rich environments with future X-ray detectors (such as Athena) and/or radio observatories (such as SKA).
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