# Weishuang Linda Xu 许蔚爽

## Research

My research has focused mostly on aspects of dark matter phenomenology, but I am broadly interested in all aspects of BSM physics, inclined towards a cosmological/astrophysical toolkit.

A technical abstract is included for each of the publications below, as well as a high-level overview that should be accessible to more general audiences.

### Publications

[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

High Level Overview: An extremely well-studied class of dark matter (DM) models are Weakly Interacting Massive Particles (WIMPs), a set of typically nuclei-mass set of candidates that are typically stablized (that is, prevented from decaying) by some new physics symmetry. One of the foremost properties WIMPs can share is annihilation into Standard Model (SM) particles (where two DM particles turn into two SM particles, like photons or electrons). The idea of a thermal relic further links the strength of this annihilation with the amount of DM observable today, so the abundance we measure today fixes the amount of annihilation to expect.

Since the thing that this type of DM does is annihilate, one of the best ways to search for it is to search for signatures of DM annihilation—cosmic and gamma rays—in the local universe, since we know there's a lot of DM around in our galaxy. The main problem with most of these approaches is that it's difficult to tell whether the cosmic rays we end up seeing come from dark matter or some high energy astrophysical event. One potential solution is to narrow the scope to cosmic rays that are extremely rare in astophysical events but are comparatively less suppressed in DM annihilations, and heavy antinuclei are the place to go for this. Antideuterons, the antimatter counterpart to deuterium nuclei, are the first step on this path.

In this paper, we present detection prospects for hidden sector dark matter models via cosmic ray antideuterons. Hidden Sector Dark Matter is a class of models where the communication between dark and light sector particles occurs via some intermediary invisible particle: for instance, the annihilation process occurs via the dark matter annihilating into these new intermediaries, and those intermediaries annihilating or decaying into SM particles. This type of interaction effectively shuts out any other type of observable particle interaction other than annihilation, so if dark matter is hidden sector, we would need indirect detection, and quite possibly need probes like antideuterons to detect its interactions. So it's good to know that detection prospects are fairly optimistic!

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

High Level Overview: While a plethora of ground-based, astrophysical, and cosmological experiments have been developed to search for potential interactions between us and dark matter (DM), another intriguing possibility is that dark matter only interacts with itself and not (or only gravitationally) with us. These types of dark matter models—Self-Interacting Dark Matter—are inaccessible via direct (contemporary) particle experiment, but nonetheless can offer richly interesting phenomenology.

One of the best ways to search for truly dark (but not cold or collisionless) dark matter is by looking at its distribution at small scales: generically thermal contact between dark matter particles will modify the formation of the dense clumps needed to source the formation of galaxies—we can't see dark matter clumps, but dark matter clumps drive gas&star clumps, and we can see those! Dwarf galaxies, especially, are a great place to look for deviations in the collisionless-ness of dark matter, since they represent structure at very small scales and tend to be relatively gasless—stars are pretty collisionless so one expects them to faithfully trace out the underlying DM distributions.

The wrinkle in this plan is that theoretical predictions of dwarf galaxy systems, especially for exotic dark matter models, are expensive to get and often flawed. In this paper, we settle for the next best thing for now: we compare real dwarf galaxies observed around the Milky Way with those simulated assuming the null hypothesis of cold collisionless dark matter (CDM). Previous work has pointed out discrepancies in the profiles, number, size, and diversity of CDM-simulated galaxies vs real observed ones; in this paper we point out a new one: galaxy shape. Each of these could point to potential new physics, or to the inability of either observation or simulation to accurately reflect its intended underlying sample. Either way, worth keeping an eye on!

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

High Level Overview: A key type of potential interaction that one might consider between Standard Model (SM) particles and Dark Matter (DM) particles is elastic scattering—a type of interaction where two particles exchange momentum via a “collision”, but the types of particle at play are unchanged. Elastic scattering is extremely ubiquitous in particle theories of dark matter, so model-agnostic constraints on simply the amount of dark matter scattering allowed by the data can be very powerful, broadly applicable to many theories at once.

In this paper we place constraints on the allowed amount of elastic scattering between DM and protons using measurements of Cosmic Microwave Background and Large-Scale Structure measurements. The main cosmological observable of DM-proton scattering is that the momentum transfer between the dark and light sectors heats up the dark matter fluid, giving it some thermal pressure so that it clumps less efficiently, at least on short distance scales. The clumping of dark matter sources the growth of all cosmic structure, so the ultimate result is that less Stuff gets formed at small scales. We can look for this signature in the data, or the lack thereof, and learn something about the nature of DM interactions.

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.

## Teaching

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.