Particle Physics

Microwave Kinetic Inductance Detectors, or MKIDs, are superconducting detector arrays that can measure the energy and arrival time of individual optical through near-IR photons without read noise or dark counts.  I will discuss our recent work and results from the first two MKID Integral Field Spectrographs (IFSs) for high contrast imaging, DARKNESS/SDC at the Palomar 200" and MEC/SCExAO on Subaru. I will then look at the future of MKIDs and their potential on 30-m class telescopes like the TMT, and explore a fascinating new application in the detection of light scalar dark matter. 

I show that dark matter abundance can be set by the decoupling of inelastic scatterings instead of annihilations. Coscattering points to dark matter that is exponentially lighter than the weak scale and has a suppressed annihilation rate, avoiding constraints from indirect detection. The late decays of the states into which dark matter upscatters, can lead to observable distortions to the blackbody spectrum of the cosmic microwave background. 

In the last decades, advances in the level of precision in controlling atomic and optical systems opened up the low-energy precision frontier to fundamental physics tests. Exploitation of quantum entanglement in such systems to further improve the sensitivity of certain existing approaches is currently an active field of research. Drawing from the experiments in our lab, in this talk I will focus on the properties, generation and usage of a particular set of entangled states called spin squeezed states.

The space-based gravitational wave detector LISA may be able to detect gravitational waves from a first order phase transition at the electroweak scale. Acoustic waves produced during the transition are largely responsible for the resulting signal. I will present results from a large campaign of simulations studying such phase transitions, determining the spectral shape of the gravitational wave power spectrum with unparalleled accuracy. Measuring a cosmological stochastic background could place constraints on the phase transition parameters, such as the nucleation rate and temperature, and therefore provide important information about physics beyond the Standard Model. However, better understanding of the source, as well as the underlying theories of physics beyond the Standard Model, is required before the launch of LISA. I will outline how this understanding can be developed.

I will discuss the calculation of the axion dark matter relic abundance produced by strings and domain walls in the early universe. These objects appear if the global symmetry that the axion is associated with is unbroken at the end of inflation, and in this scenario there is, in principle, a unique prediction for the axion dark matter mass. I will present results from numerical simulations that indicate that the density of strings may be significantly larger than previously thought, leading to a corresponding change in the required axion dark matter mass. I will also discuss the difficulties in computing the energy spectrum of axions produced in the physically relevant regime, and the remaining uncertainties and challenges.

If a component of the dark matter has dissipative interactions, it could collapse to form a thin dark disk in our Galaxy coincident with the baryonic disk. It has been suggested that dark disks could explain a variety of observed phenomena, including periodic comet impacts. Using the first data release from the Gaia mission, we search for a dark disk via its effect on stellar kinematics in the Milky Way. I will present new limits on the presence of a thin dark matter disk, as well as measurements on the matter density in the solar neighborhood.

I will discuss a class of models in which thermal dark matter is lighter than an MeV. If dark matter thermalizes with the Standard Model below the temperature of neutrino-photon decoupling, constraints from measurements of the effective number of neutrino species are alleviated. This framework motivates new experiments in the direct search for sub-MeV thermal dark matter and light force carriers.

We suggest that two-to-two dark matter fusion may be the relaxation process that resolves the small-scale structure problems of the cold collisionless dark matter paradigm. In order for the fusion cross section to scale correctly across many decades of astrophysical masses from dwarf galaxies to galaxy clusters, we require the fractional binding energy released to be greater than v^n ~ [10^{-(2-3)}]^n, where n=1,2 depends on local dark sector chemistry. The size of the dark-sector interaction cross sections must be sigma ~ 0.1-1 barn, moderately larger than for Standard Model deuteron fusion, indicating a dark nuclear scale Lambda ~ O(100 MeV). Dark fusion firmly predicts constant sigma v below the characteristic velocities of galaxy clusters. Observations of the inner structure of galaxy groups with velocity dispersion of several hundred kilometer per second, of which a handful have been identified, could differentiate dark fusion from a dark photon model.

Primordial black hole (PBH), which can be naturally produced in the early universe, remains a promising dark matter candidate . It can merge with a supermassive black hole (SMBH) in the center of a galaxy and generate gravitational wave (GW) signals in the favored frequency region of LISA-like experiments. In this work, we initiate the study on the event rate calculation for such extreme mass ratio inspirals (EMRI). Including the sensitivities of various proposed GW detectors, we find that such experiments offer a novel and outstanding tool to test the scenario where PBH constitutes (fraction of) dark matter. The PBH energy density fraction of DM (fPBH) could potentially be explored as small as 10^−3∼10^−4. Further, LISA has the capability to search for PBH mass upto 10^−2∼10^−1 solar mass. Other proposed GW experiments can probe lower PBH mass regime.

Standard Model particles account for a small fraction of the matter content of the universe.  If the remaining dark matter (DM) was ever in thermal equilibrium  with itself or with the Standard Model (SM) sector, there must  exist interactions that allowed its number density to be depleted  to its present value.  An interesting possibility for achieving this arises in scenarios  where the DM is composed of ``pions'' of a QCD-like dark sector.  The leading number changing process in these theories is the annihilation of three pions into two pions, which heats the pion bath relative to the SM. Consistency with observations of large and small scale structure in the universe requires a non-zero coupling with SM states, suggesting a multitude of experimental probes. I will review the cosmological production of DM in these strongly interacting dark sectors and discuss the prospects for discovering them at fixed target experiments.

As a child, Quebec native Pauline Gagnon dreamed of understanding what the universe was really made of.

As an adult, she studied exactly that, working at the largest experiment ever built, CERN’s Large Hadron Collider. In her role as a Senior Research Scientist, based at Indiana University and working at CERN, she searched for dark matter particles in the decays of the famous Higgs boson, in the form of hypothetical particles called dark photons.

Now retired from active research, Gagnon is dedicated to inspiring other curious minds of all ages to ponder the same big questions that fascinated her as a child. Having worked in the CERN Communications group, she is adept at explaining the complex science of particle physics in engaging, comprehensible ways. She has delivered nearly 100 presentations to audiences in nine countries on three continents.

Gagnon’s popular science book, Who Cares about Particle Physics: Making Sense of the Higgs boson, the LHC and CERN, not only explains current issues in particle physics but also explores the importance of fundamental physics in shaping not only our understanding of the universe, but in shaping society as well.

In her Perimeter Institute Public Lecture, Gagnon will explore the incredible (and improbable!) feats of ingenuity and cooperation that have led to one of humanity’s greatest experiments, and why such pioneering research, albeit “useless” in terms of everyday practicality, has changed the way we live and is vital to our collective future.