A New Mechanism for Primordial Black Hole Formation from QCD Axion

We present a novel mechanism for the primordial black hole (PBH) production within the QCD axion framework. We take the case where the Peccei-Quinn symmetry breaks during inflation, resulting in a $N_{\rm DW}=1$ string-wall network that re-enters horizon sufficiently late. Therefore, closed axion domain walls naturally arising in the network are sufficiently large to collapse into PBHs. Our numerical simulation shows that $0.8\%$ of the total wall area is in the form of closed walls. Notably, this fraction is independent of any axion parameters, as its determination is firmly grounded in the principles of percolation theory. In addition, the relic abundance of dark matter is accounted for by free axions from the collapse of open walls bounded by strings. This framework yields a calculated PBH fraction of dark matter as 0.0256. The PBHs uniformly share the same mass, which spans from about $10^{-10}$ to $10^{-1}$ solar masses, corresponding to the classical QCD axion mass window $10^{-5}-10^{-2}$ eV and the re-entering horizon temperature $300-1$ MeV. Intriguingly, PBHs in this mechanism can naturally account for the gravitational-lensing events observed by the OGLE collaboration.Comment: 5 pages, 3 figure

Solar radio emissions and ultralight dark matter

Ultralight axions and dark photons are well-motivated dark matter candidates. Inside the plasma, once the mass of ultralight dark matter candidates equals the plasma frequency, they can resonantly convert into electromagnetic waves, due to the coupling between the ultralight dark matter particles and the standard model photons. The converted electromagnetic waves are monochromatic. In this article, we review the development of using radio detectors to search for ultralight dark matter conversions in the solar corona and solar wind plasma.Comment: 13 pages, 3 figures. An invited review for the special issue "Solar Radio Emissions" in the journal Univers

Domain Wall Network: A Dual Solution for Gravitational Waves and Hubble Tension?

We search for stochastic gravitational wave background (SGWB) generated by domain wall networks in the Data Release-2 of Parkes Pulsar Timing Array and find that the observed strong common power-law process can be explained by domain wall networks for the wall tension $\sigma_{\textrm{DW}}\sim (29-414~\textrm{TeV})^3$ and the wall-decay temperature $T_d\sim 20-257~\textrm{MeV}$ at $68\%$ Credible Level. Interestingly, the same parameter region can largely alleviate the Hubble tension, if the free particles generated from domain wall networks further decay into dark radiation. This coincidence that a domain wall network can simultaneously account for the nano-Hertz SGWB and Hubble tension is robust, independent of domain wall parameters and applicable to observations by other pulsar timing array collaborations in general. On the other hand, assuming that the common power-law process is not due to domain wall networks, we can put stringent constraints on the wall tension and decay temperature.Comment: 14 pages, 9 figures, 4 table

Direct detection of dark photon dark matter using radio telescopes

Dark photons can be the ultralight dark matter candidate, interacting with Standard Model particles via kinetic mixing. We propose to search for ultralight dark photon dark matter (DPDM) through the local absorption at different radio telescopes. The local DPDM can induce harmonic oscillations of electrons inside the antenna of radio telescopes. It leads to a monochromatic radio signal and can be recorded by telescope receivers. Using the observation data from the FAST telescope, the upper limit on the kinetic mixing can already reach $10^{-12}$ for DPDM oscillation frequencies at $1-1.5$ GHz, which is stronger than the cosmic microwave background constraint by about one order of magnitude. Furthermore, large-scale interferometric arrays like LOFAR and SKA1 telescopes can achieve extraordinary sensitivities for direct DPDM search from 10 MHz to 10 GHz.Comment: 5 pages, 3 figures + appendix. Match the accepted version (PRL

X-ray annual modulation observed by XMM-Newton and Axion Quark Nugget Dark Matter

The XMM-Newton observatory shows evidence with an $11 \sigma$ confidence level for seasonal variation of the X-ray background in the near-Earth environment in the 2-6 keV energy range (Fraser et al. 2014). The interpretation of the seasonal variation given in Fraser et al. 2014 was based on the assumption that solar axions convert to X-rays in the Earth's magnetic field. There are many problems with this interpretation, since the axion-photon conversion must preserve the directionality of the incoming solar axion. At the same time, this direction is avoided by the observations because the XMM-Newton's operations exclude pointing at the Sun and at the Earth. The observed seasonal variation suggests that the signal could have a dark matter origin, since it is very difficult to explain with conventional astrophysical sources. We propose an alternative explanation which involves the so-called Axion Quark Nugget (AQN) dark matter model. In our proposal, dark matter is made of AQNs, which can cross the Earth and emit high energy photons at their exit. We show that the emitted intensity and spectrum is consistent with Fraser et al. 2014, and that our calculation is not sensitive to the specific details of the model. We also find that our proposal predicts a large seasonal variation, on the level of 20-25%, much larger than conventional dark matter models (1-10%). Since the AQN emission spectrum extends up to $\sim$100 keV, well beyond the keV sensitivity of XMM-Newton, we predict the AQN contribution to the hard X-ray and $\gamma$-ray backgrounds in the Earth's environment. The Gamma-Ray Burst Monitor instrument (GBM), aboard the Fermi telescope, is sensitive to the 8 keV-40 MeV energy band. We suggest that the multi-year archival data from the GBM could be used to search for a seasonal variation in the near-Earth environment up to 100 keV as a future test of the AQN framework.Comment: 16 pages, 13 figures, submitted to MNRA

Searching for ultralight dark matter conversion in solar corona using Low Frequency Array data

Ultralight dark photons and axions are well-motivated hypothetical dark matter candidates. Both dark photon dark matter and axion dark matter can resonantly convert into electromagnetic waves in the solar corona when their mass is equal to the solar plasma frequency. The resultant electromagnetic waves appear as monochromatic signals within the radio-frequency range with an energy equal to the dark matter mass, which can be detected via radio telescopes for solar observations. Here we show our search for converted monochromatic signals in the observational data collected by the high-sensitivity Low Frequency Array (LOFAR) telescope and establish an upper limit on the kinetic mixing coupling between dark photon dark matter and photon, which can reach values as low as 10−13 within the frequency range of 30 − 80 MHz. This limit represents an improvement of approximately one order of magnitude better than the existing constraint from the cosmic microwave background observation. Additionally, we derive an upper limit on the axion-photon coupling within the same frequency range, which is better than the constraints from Light-Shining-through-a-Wall experiments while not exceeding the CERN Axion Solar Telescope (CAST) experiment or other astrophysical bounds

Axion quark nugget dark matter model : developments in model building and observations

The axion quark nugget (AQN) model was initially proposed with the motivation to explain the observed similarity between the visible and dark matter abundances in the Universe. AQNs are dense objects made of standard model quarks in color superconducting (CS) phase. AQNs can be made of matter as well as antimatter. Matter AQNs and antimatter AQNs together form the dark matter, while the disparity between them will lead to the observed matter-antimatter asymmetry. Thus, the similarity between visible and dark matter abundances can be naturally explained since they have the same origin in the AQN framework. This thesis focuses on recent developments in model building and some potential observational evidence of AQNs. First, we show how the coherent nonzero axion field in the early Universe generates the disparity between matter and antimatter AQNs. Then, we calculate the real-time evolution of an AQN from its initial state as a closed axion domain wall with baryon charge trapped inside to its final CS state. Next, we show that for the most part of axion parameter space, AQNs are the dominant part of dark matter compared to the contribution of the free axions from the misalignment mechanism. After that, we calculate the size distribution of AQNs based on percolation theory. We also demonstrate that after formation, the size distribution can survive the subsequent evolution in the early Universe. Finally, we study potential observational evidence of the AQN model, focusing on the following two phenomena: the impulsive radio events in quiet solar corona recorded by the Murchison Widefield Array and the seasonal variation of the near-Earth X-ray background observed by the XMM-Newton observatory.Science, Faculty ofPhysics and Astronomy, Department ofGraduat