Research
Dark Matter Indirect Detection
Despite the overwhelming amount of cosmological and astrophysical evidence for the particle nature of dark matter, we know very little information on the microscopic properties of that particle. But there's good reason to believe dark matter might interact weakly with the Standard Model: for example, that interaction might be related to how it was produced in the early universe. One component of my research is focused on searching for the indirect detection of the possible decay and annihilation products of dark matter.
The 3.5 keV line
Excitingly, in 2014, an unexplained emission line at 3.5 keV appeared in galaxy and galaxy cluster X-ray spectra. Such an emission line could have been the first signal of the particle nature of dark matter. However, my collaborators and I were able to show in [arXiv:1812.06976], using all of the XMM-Newton data ever taken, that the absence of the line in the Milky Way galaxy was inconsistent with a decaying dark matter origin. This result left open the question of what the 3.5 keV line is. We addressed this question in [arXiv:2309.03254], in which we reanalyzed the observations that formed the bulk of the evidence for a line. Surprisingly, we were unable to reproduce all but one of the analyses, finding no evidence for a line at all. Our results point to errors in the original works, which may be related to the use of local optimizers. Even in the remaining analysis, we demonstrated that the evidence is not robust to tests for mismodeling. We are forced to conclude that not only is the 3.5 keV line not originating from dark matter decay, but that there never was robust evidence for a line near 3.5 keV in the first place. We propose and perform robust analyses in [arXiv:2102.02207, arXiv:2305.17160].
Higgsino Annihilation
Thermal WIMPs are another well-motivated candidate for dark matter, arising as e.g. the lightest superpartner in supersymmetry. Null searches for superpartners at the LHC suggest that nature may implement a split nature of SUSY, where the scalars have large masses. Furthermore, while Wino DM is now excluded by indirect detection, higgsino dark matter Split-SUSY is to-date unprobed. In particular, higgsinos cannot scatter inelastically through Z-exchange and are below the neutrino floor in direct detection experiments. In [arXiv:2207.10090], we showed that the Fermi gamma-ray telescope has strong sensitivity to higgsino annihilation in the Galactic Center due to the continuum arising from the WW and ZZ annihilation channels, which had previously been overlooked in favor of the line signal. Although the line signal is cleaner, the continuum flux is much larger than that of the line. We found a 2-sigma preference for a higgsino dark matter annihilation signal that could be consistent with the thermal scenario under some realistic dark matter profiles, and will be further testable in the future at CTA.
Probing the QCD Axion
The QCD axion is a hypothetical pseudoscalar particle whose existence would solve the Strong CP problem of particle physics. Its mass could be in a ~ten order-of-magnitude wide range.
Axions, in an astrophysical context, are particles much like neutrinos in that they are weakly interacting and very light particles that can be produced in extreme astrophysical environments, for instance inside stars. Therefore, axions can affect stellar evolution in a wide variety of objects. In [arXiv:2111.09892], I set an upper limit on the axion mass using observations of neutron star cooling. This limit is on firmer statistical ground and subject to less astrophysical uncertainty than other constraints from stellar cooling.
Axions from Neutron Stars
The QCD axion is notoriously difficult to probe, but string theory predicts a large number of lighter axions that may be more strongly coupled to the Standard Model. Axions have additional phenomenology over that of neutrinos in that they can oscillate into photons in the presence of a magnetic field. Then, we can imagine that axions produced in the hot cores of neutron stars may free-stream out of the star, and convert into photons in the magnetosphere, with energies of a few keV.
Most excitingly, in [arXiv:1910.02956, arXiv:1910.04164], my collaborators and I found evidence for a hard X-ray signal that looks consistent with that scenario at the seven neutron stars known as the Magnificent Seven, which were not supposed to produce hard X-rays. It turns out that if the neutron stars have active superfluidity in their interiors, then axions may induce X-rays also at much higher energies around 10 keV. We're actively analyzing data from the NuSTAR telescope to see if this is the case. See [arXiv:1903.05088, arXiv:2104.12772] for the analogous case operating in white dwarfs and [arXiv:2008.03305] for that in Wolf-Rayet stars, where no such excesses have been observed.
Searching for Stringy Axions
The axions living in the "Axiverse" of string theory are expected to have extremely small masses. I recently showed that linear polarization measurements of magnetic white dwarfs can strongly probe low-mass axions [arXiv:2203.04319]. Although the thermal surface radiation is nearly unpolarized when it is emitted, if axions exist it can acquire a polarization as it free-streams away from the surface, because the photons polarized parallel to the magnetic field can convert into axions. Axions thus induce polarization transverse to the magnetic field of the star, which is energy-independent and can be disentangled from the highly energy-dependent polarization that is created by astrophysical processes. At the moment, the number of known highly magnetic white dwarfs is small, and the number with measurements of linear polarization is even smaller. We're currently acquiring data from spectropolarimeters on the small number of existing targets with the Lick telescope, which should have an order-of-magnitude better polarization sensitivity than that of the archival data.
