Weishuang Linda Xu 许蔚爽

wlxu [at] lbl.gov


My research has focused mostly on aspects of dark matter phenomenology. Despite striking evidence for the cosmic abundance of dark matter, hints for any sort of non-gravitational interaction, either with itself or us, has proved elusive. However, progress can still be made by developing model realizations of compelling ideas and thinking deeply about their experimental consequences. Cosmology and astrophysics have been very powerful here: observables such as light element abundances, cosmic microwave background anisotropies, and large-scale structure provide a strict framework for the way the univserse -- and all its perturbations therein -- must have evolved. Local observables such as precision astrometry and spectroscopy, and incoming fluxes of high energy astroparticles are also promising directions to look to for deviations from the cold collisionless hypothesis. I have worked a little on all of these things, and think often about where we should go next if we find something: as we look to a broader range of probes for hints of new physics, a lot of work needs to be done in between a (potential) discovery and actually understanding the underlying laws of nature.

Below is a list of my publications; alternatively they can be found on my Inspire page.

I also wrote The Swapland with friends from grad school. This is an april fool's paper, and therefore, my best paper.


[arXiv:2302.00008] Scalar Co-SIMP dark matter: models and sensitivities with Aditya Parikh, Juri Smirnov, and Bei Zhou

Abstract: In this work, we present UV completions of the recently proposed number-changing Co-SIMP freeze-out mechanism. In contrast to the standard cannibalistic-type dark matter picture that occurs entirely in the dark sector, the 3 \(\to\) 2 process setting the relic abundance in this case requires one Standard Model particle in the initial and final states. This prevents the dark sector from overheating and leads to rich experimental signatures. We generate the Co-SIMP interaction with a dark sector consisting of two scalars, with the mediator coupling to either nucleons or electrons. In either case, the dark matter candidate is naturally light : nucleophilic interactions favor the sub-GeV mass range and leptophilic interactions favor the sub-MeV mass range. Viable thermal models in these lighter mass regimes are particularly intriguing to study at this time, as new developments in low-threshold detector technologies will begin probing this region of parameter space. While particles in the sub-MeV regime can potentially impact light element formation and CMB decoupling, we show that a late-time phase transition opens up large fractions of parameter space. These thermal light dark matter models can instead be tested with dedicated experiments. We discuss the viable parameter space in each scenario in light of the current sensitivity of various experimental probes and projected future reach.

[arXiv:2212.07435] Search for dark matter lines at the Galactic Center with 14 years of Fermi data with Joshua Foster, Yujin Park, Benjamin Safdi, and Yotem Soreq

Abstract: Dark matter (DM) in the Milky Way halo may annihilate or decay to photons, producing monochromatic gamma rays. We search for DM-induced spectral lines using 14 years of data from the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope (Fermi-LAT) between 10 GeV and 2 TeV in the inner Milky Way leveraging both the spatial and spectral morphology of an expected signal. We present new constraints as strong as \(\langle \sigma v\rangle \leq 6\times 10^{-30}\;{\rm cm}^3/{\rm s} \) for the two-to-two annihilations and \(\tau > 10^{30}\) s for one-to-two decays, representing leading sensitivity between 10 GeV and \(\sim\)500 GeV. We consider the implications of our line-constraints on the Galactic Center excess (GCE), which is a previously-observed excess of continuum \(\sim\)GeV gamma-rays that may be explained by DM annihilation. The Higgs portal and neutralinolike DM scenarios, which have been extensively discussed as possible origins of the GCE, are constrained by our work because of the lack of observed one-loop decays to two photons. More generally, we interpret our null results in a variety of annihilating and decaying DM models, such as neutralinos, gravitinos, and glueballs, showing that in many cases the line search is more powerful than the continuum, despite the continuum annihilation being at tree level.

[arXiv:2207.11262] The inflated Chern-Simons number in spectator chromo-natural inflation with Hengameh Bagherian and Matthew Reece

Abstract: The chromo-natural inflation (CNI) scenario predicts a potentially detectable chiral gravitational wave signal, generated by a Chern-Simons coupling between a rolling scalar axion field and an SU(2) gauge field with an isotropy-preserving classical background during inflation. However, the generation of this signal requires a very large integer Chern-Simons level, which can be challenging to explain or embed in a UV-complete model. We show that this challenge persists in the phenomenologically viable spectator field CNI (S-CNI) model. Furthermore, we show that a clockwork scenario giving rise to a large integer as a product of small integers can never produce a Chern-Simons level large enough to have successful S-CNI phenomenology. We briefly discuss other constraints on the model, both in effective field theory based on partial-wave unitarity bounds and in quantum gravity based on the Weak Gravity Conjecture, which may be relevant for further explorations of alternative UV completions.

[arXiv:2207.10090] Higgsino Dark Matter Confronts 14 Years of Fermi γ-Ray Data with Christopher Dessert, Joshua Foster, Yujin Park, and Benjamin Safdi

Abstract: Thermal Higgsino dark matter (DM), with mass around 1 TeV, is a well-motivated, minimal DM scenario that arises in supersymmetric extensions of the standard model. Higgsinos may naturally be the lightest superpartners in split-supersymmetry models that decouple the scalar superpartners while keeping Higgsinos and gauginos close to the TeV scale. Higgsino DM may annihilate today to give continuum \(\gamma\)-ray emission at energies less than a TeV in addition to a linelike signature at energies equal to the mass. Previous searches for Higgsino DM, for example with the H.E.S.S. \(\gamma\)-ray telescope, have not reached the necessary sensitivity to probe the Higgsino annihilation cross section. In this work we make use of 14 years of data from the Fermi Large Area Telescope at energies above \(\sim\)10 GeV to search for the continuum emission near the Galactic Center from Higgsino annihilation. We interpret our results using DM profiles from Milky Way analog galaxies in the FIRE-2 hydrodynamic cosmological simulations. We set the strongest constraints to date on Higgsino-like DM. Our results show a mild, \(\sim 2 \sigma\) preference for Higgsino DM with a mass near the thermal Higgsino mass and, depending on the DM density profile, the expected cross section.

[arXiv:2107.09664] Cosmological constraints on light but massive relics with Julian Muñoz and Cora Dvorkin

Abstract: Many scenarios of physics beyond the standard model predict new light, weakly coupled degrees of freedom, populated in the early universe and remaining as cosmic relics today. Due to their high abundances, these relics can significantly affect the evolution of the universe. For instance, massless relics produce a shift \(\Delta N_{\rm eff}\) to the cosmic expectation of the effective number of active neutrinos. Massive relics, on the other hand, additionally become part of the cosmological dark matter in the later universe, though their light nature allows them to freely stream out of potential wells. This produces novel signatures in the large-scale structure (LSS) of the universe, suppressing matter fluctuations at small scales. We present the first general search for such light (but massive) relics (LiMRs) with cosmic microwave background (CMB) and LSS data, scanning the 2D parameter space of their masses mX and temperatures \(T_X^{(0)}\) today. In the conservative minimum-temperature (\(T_X^{(0)}=0.91\) K) scenario, we rule out Weyl (and higher-spin) fermions—such as the gravitino—with \(m_X \leq 2.3\) eV at 95% C.L., and set analogous limits of \(m_X \leq\) 11, 1.1, 1.6 eV for scalar, vector, and Dirac-fermion relics. This is the first search for LiMRs with joint CMB, weak-lensing, and full-shape galaxy data; we demonstrate that weak-lensing data is critical for breaking parameter degeneracies, while full-shape information presents a significant boost in constraining power relative to analyses with only baryon acoustic oscillation parameters. Under the combined strength of these datasets, our constraints are the tightest and most comprehensive to date.

[arXiv:2010.15129] A Closer Look at CP-Violating Higgs Portal Dark Matter as a Candidate for the GCE with Katherine Fraser and Aditya Parikh

Abstract: A statistically significant excess of gamma rays has been reported and robustly confirmed in the Galactic Center over the past decade. Large local dark matter densities suggest that this Galactic Center Excess (GCE) may be attributable to new physics, and indeed it has been shown that this signal is well-modelled by annihilations dominantly into \(b\overline{b}\) with a WIMP-scale cross section. In this paper, we consider Majorana dark matter annihilating through a Higgs portal as a candidate source for this signal, where a large CP-violation in the Higgs coupling may serve to severely suppress scattering rates. In particular, we explore the phenomenology of two minimal UV completions, a singlet-doublet model and a doublet-triplet model, and map out the available parameter space which can give a viable signal while respecting current experimental constraints.

[arXiv:2006.09395] Accurately Weighing Neutrinos with Cosmological Surveys with Nicholas DePorzio, Julian B. Muñoz, and Cora Dvorkin

Abstract: A promising avenue to measure the total, and potentially individual, mass of neutrinos consists of leveraging cosmological datasets, such as the cosmic microwave background and surveys of the large-scale structure of the universe. In order to obtain unbiased estimates of the neutrino mass, however, many effects ought to be included. Here we forecast, via a Markov Chain Monte Carlo likelihood analysis, whether measurements by two galaxy surveys: DESI and Euclid, when added to the CMB-S4 experiment, are sensitive to two effects that can alter neutrino-mass measurements. The first is the slight difference in the suppression of matter fluctuations that each neutrino-mass hierarchy generates, at fixed total mass. The second is the growth-induced scale-dependent bias (GISDB) of haloes produced by massive neutrinos. We find that near-future surveys can distinguish hierarchies with the same total mass only at the \(1\sigma\) level; thus, while these are poised to deliver a measurement of the sum of neutrino masses, they cannot significantly discern the mass of each individual neutrino in the foreseeable future. We further find that neglecting the GISDB induces up to a \(1\sigma\) overestimation of the total neutrino mass, and we show how to absorb this effect via a redshift-dependent parametrization of the scale-independent bias.

[arXiv:2006.09380] Finding eV-scale Light Relics with Cosmological Observables with Nicholas DePorzio, Julian B. Muñoz, and Cora Dvorkin

Abstract: Cosmological data provide a powerful tool in the search for physics beyond the Standard Model (SM). An interesting target are light relics, new degrees of freedom which decoupled from the SM while relativistic. Nearly massless relics contribute to the radiation energy budget, and are commonly searched through variations in the effective number \(N_{\rm eff}\) of neutrino species. Additionally, relics with masses on the eV scale (meV-10 eV) become non-relativistic before today, and thus behave as matter instead of radiation. This leaves an imprint in the clustering of the large-scale structure of the universe, as light relics have important streaming motions, mirroring the case of massive neutrinos. Here we forecast how well current and upcoming cosmological surveys can probe light massive relics (LiMRs). We consider minimal extensions to the SM by both fermionic and bosonic relic degrees of freedom. By combining current and upcoming cosmic-microwave-background and large-scale-structure surveys, we forecast the significance at which each LiMR, with different masses and temperatures, can be detected. We find that a very large coverage of parameter space will be attainable by upcoming experiments, opening the possibility of exploring uncharted territory for new physics beyond the SM.

[arXiv:1910.14669] Searching for Dark Photon Dark Matter with Cosmic Ray Antideuterons with Lisa Randall

Abstract: Low energy antideuteron detection presents a unique channel for indirect detection, targeting dark matter that annihilates into hadrons in a relatively background-free way. Since the idea was first proposed, many WIMP-type models have already been disfavored by direct detection experiments, and current constraints indicate that any thermal relic candidates likely annihilate through some hidden sector process. In this paper, we show that cosmic ray antideuteron detection experiments represent one of the best ways to search for hidden sector thermal relic dark matter, and in particular investigate a vector portal dark matter that annihilates via a massive dark photon. We find that the parameter space with thermal relic annihilation and \(m_\chi > m_{A'} \gtrsim 20 \, \mathrm{GeV}\) is largely unconstrained, and near future antideuteron experiment GAPS will be able to probe models in this space with \(m_\chi \approx m_{A'}\) up to masses of \(O(100\,\mathrm{GeV})\). Specifically the dark matter models favored by the Fermi Galactic center excess is expected to be detected or constrained at the \(5(3)-\sigma\) level assuming an optimistic (conservative) propagation model.

[arXiv:1904.08949] Testing ΛCDM With Dwarf Galaxy Morphology with Lisa Randall

Abstract: The leading tensions to the collisionless cold dark matter (CDM) paradigm are the "small-scale controversies", discrepancies between observations at the dwarf-galactic scale and their simulational counterparts. In this work we consider methods to infer 3D morphological information on Local Group dwarf spheroidals, and test the fitness of CDM+hydrodynamics simulations to the observed galaxy shapes. We find that the subpopulation of dwarf galaxies with mass-to-light ratio \(\gtrsim 100 M_\odot/L_\odot\) reflects an oblate morphology. This is discrepant with the dwarf galaxies with mass-to-light ratio \(\lesssim 100 M_\odot/L_\odot\), which reflect prolate morphologies, and more importantly with simulations of CDM-sourced galaxies which are explicitly prolate. Although more simulations and data are called for, if evidence of oblate pressure-supported stellar distributions persists, we argue that an underlying oblate non-CDM dark matter halo may be required, and present this as motivation for future studies.

[arXiv:1802.06788] Probing sub-GeV Dark Matter-Baryon Scattering with Cosmological Observables with Cora Dvorkin and Andrew Chael

Abstract: We derive new limits on the elastic scattering cross-section between baryons and dark matter using Cosmic Microwave Background data from the Planck satellite and measurements of the Lyman-alpha forest flux power spectrum from the Sloan Digital Sky Survey. Our analysis addresses generic cross sections of the form \(\sigma\propto v^n\), where \(v\) is the dark matter-baryon relative velocity, allowing for constraints on the cross section independent of specific particle physics models. We include high-\(\ell\) polarization data from Planck in our analysis, improving over previous constraints. We apply a more careful treatment of dark matter thermal evolution than previously done, allowing us to extend our constraints down to dark matter masses of \(\sim\)MeV. We show in this work that cosmological probes are complementary to current direct detection and astrophysical searches.


I'm currently one of the postdoc representatives in the Berkeley Physics DEI Committee. If you're in the Berkeley physics department and would like to talk about aspects of department climate, please feel free to reach out and schedule a chat.

Other local and non-local organizations I've enjoyed working with include the Compass Lectures (Berkeley), SAGE Summer Camp (LBL), Science in the News (Harvard), and Letters to a Pre-Scientist.


A list of courses at Harvard for which I served as a Teaching Fellow:

Spring 2021 Physics 125 Widely Applied Physics
Spring 2020 Physics 15a Mechanics and Special Relativity
Spring 2019 Physics 143a Quantum Mechanics I
Fall 2017 Physics 212 Graduate Cosmology
Spring 2017 Applied Physics 50b Electricity & Magnetism

In the summer of 2015 I served as a teaching assistant for the Summer Science Program, a nonprofit for high school students oriented towards astronomy.