I am interested in what particle physics can tell us
about the early universe and vice versa.
Some of the fundamental questions that we try to address are:
- What is the dark matter that makes up the majority of mass in the universe? How is it produced and how can it be detected?
- Why is there more matter than antimatter? How can this asymmetry arise in the early universe?
In both cases, we study modifications of the Standard Model
that include new particles and interactions, examining their consistency
against existing observations and designing ways of testing them in the
future. Specifically, I focus on the following directions (for a complete list of papers see
Accelerator Probes of Light Dark Sectors
The nature of dark matter is unknown, but certain particle physics models make specific predictions for how strongly
it can interact with Standard Model particles. Remarkably, many of these scenarios can be definitively tested with
future fixed-target and collider experiments. Together with colleagues, I have been building the
physics case for a new "missing momentum" experiment, LDMX, that can probe many
compelling dark matter models and look for other new physics. LDMX will search for production of
dark matter particles in electron-nucleus collisions by measuring the momentum of the scattered electron and
identifying events with anomalous momentum transfer and no other activity in the detector. Fixed-target experiments like LDMX are incredibly
powerful probes of light particles (say, lighter than a proton), but it is hard to produce heavier ones. For this reason, we also
develop new ideas for analyses at higher energy colliders, like the LHC.
Cosmology of Light Relics
Many models of physics beyond the Standard Model that seek to fix
one of its deficiencies (e.g., lack of dark matter or neutrino masses) feature new light,
very feebly interacting particles. These may be very difficult to probe with the accelerators
described above, but they can still leave an imprint on cosmological observables such as the
Cosmic Microwave Background or the distribution of matter in the universe. I am working to understand
whether this data leaves room for non-standard physics in the context of specific models.
An interesting question that I have been exploring with collaborators is whether new MeV-scale
particles can be in thermal equilibrium with the Standard Model until late times. We
recently showed that, despite common lore, this is possible, while being compatible with current data.
Even though this requires very feeble couplings, such a comsology leaves observable imprints on the distribution
of cosmic neutrinos and dark matter.
This is an exciting and generic feature of particle physics models of dark matter: the microphysics imprints itself on the
cosmological evolution of enormous length scales. I am interested in using the multitude of astrophysical data to learn about the particle nature of dark matter.
The recent discovery of the Higgs boson has answered many questions we had about the
Standard Model, but it also posed several more. While its properties are roughly
consistent with the simplest possbility of a single scalar Higgs particle, several puzzling features have emerged. One
is the apparent metastability of the electroweak vacuum. Is this an accident? Or was the other
vacuum populated during the evolution of the universe? Does it serve a purpose, in, e.g.,
generating other physical scales?
Another mystery is the nature of the electroweak
phase transition in the early universe. In many extensions of the minimal Higgs sector, this
phase transition can actually generate the observed baryon asymmetry, thereby fixing
another shortcoming of the Standard Model.
Together with colleagues, I showed that models that realize this first order transition also modify Higgs properties in
a measurable way. With the discovery of gravitational waves,
we have, for the first time, the possbility of directly observing traces of this early period in the evolution of the universe.
I am working to understand whether such signals can be correlated with observables in terrestrial experiments, like the LHC,
to learn about the early universe.