Particle Physics

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.

The radial acceleration relation is an empirical universal scaling relation between the total gravitational field and the Newtonian acceleration generated by baryons at any given radius within spiral galaxies. In this talk, I will discuss the possibility that such a relation arises from interactions between baryons and dark matter (DM), rather than from feedback processes or modifications of gravity. Starting from this premise, I will discuss what we can infer about the nature of baryon-DM interactions. In particular, I will argue that  such interactions must depend on the local DM density in order to correctly reproduce the observed behavior of spiral and elliptical galaxies. I will briefly revisit existing phenomenological, astrophysical and cosmological constraints on baryon-DM interactions in light of the unusual density dependence of our cross-section. I will conclude by mentioning possible ways of realizing this scenario in particle physics models, and discuss a non-relativistic toy model.

Electric dipole moments are extremely sensitive probes for additional sources of CP violation in new physics models. The multi-scale problem of relating the high precision measurements with neutrons, atoms and molecules to fundamental parameters can be approached model-independently to a large extent; however, care must be taken to include the uncertainties from especially nuclear and QCD calculations properly. The resulting bounds on fundamental parameters are illustrated in the context of Two-Higgs-Doublet models.

Motivated by the exciting prospect of new wealth of information arising from the first observations of gravitational and electromagnetic radiation from the same astrophysical phenomena, the Dark Energy Survey (DES) has performed a broad range follow-up program for LIGO/Virgo events using the Dark Energy Camera (DECam). In this talk, I present the discovery of the optical transient associated with the neutron star merger GW170817 using DECam and discuss its implications for the emerging field: multi-messenger cosmology with gravitational waves and optical data.

The MAJORANA DEMONSTRATOR (MJD) is a 44-kg array of low-background germanium detectors of which 30kg is made from detectors enriched to 88% in 76Ge. MJD is operating a mile underground in the Sanford Underground Research Laboratory in Lead, SD. Its main purpose is to search for the neutrinoless double-beta decay of 76Ge and to demonstrate the technical feasibility of a tonne-scale Ge-based neutrinoless double-beta decay experiment. It is also capable of direct searches of a variety of dark matter candidates and other physics beyond the Standard Model. In this talk I will review the motivation, design and construction of the MJD, as well as recent results for a the search for bosonic dark matter using commissioning data taken in 2015. I will also discuss the current status of MJD and conclude with a discussion of future plans for MJD and a proposed tonne-scale Ge-based experiment, LEGeND.

Direct detection experiments are rapidly improving their sensitivity to weak scale Dark Matter.  A particular interesting (and minimal) possibility is that the Dark matter interacts with ordinary matter via the exchange of weak bosons: the W, Z, and Higgs.  Dark matter with substantial coupling to the Higgs boson is already under significant tension from limits on spin-independent scattering.  We comment on the power of  spin-dependent scattering as a probe of Z-mediated dark matter, both in a simple effective theory, and in the so-called Singlet-Doublet Model, which we argue is a useful benchmark.  We also review the case where the cosmology of the WIMP is dominated by co-annihilation processes, focusing on the stop co-annihilation region of the Minimal Supersymmetric Standard Model, and discuss prospects for direct detection in this case.

One of the most concrete implications of the discovery of the Higgs boson is that, in the absence of physics beyond the standard model, the long term fate of our universe can now be established through precision calculations. Are we in a metastable minimum of the Higgs potential or the true minimum? If we are in a metastable vacuum, what is its lifetime? To answer these questions, we need to understand tunneling in quantum field theory.

This talk will give an overview of the interesting history of tunneling rate calculations and all of its complications in calculating functional determinants of fluctuations around the bounce solutions. Several problems has persisted for the last four decades, and we present new solutions to these problems that enabled us to calculate exact closed-form expressions of the functional determinant. Applied to the Standard model, we then get the first-ever complete calculation of the lifetime of our universe.

Galaxy clusters represent excellent laboratories to search for Axion-Like Particles (ALPs). They contain magnetic fields which can induce quasi-sinusoidal oscillations in the X-ray spectra of AGNs situated in or behind them. Ultra-deep Chandra observations of the Perseus cluster contain over 5 x 105 counts from the central NGC1275 AGN, and represent an extraordinary dataset for ALP searches. In this talk I will describe how we used these to search for spectral irregularities from the AGN. No irregularities were found at the ~30% level, allowing us to place leading constraints on the ALP-photon mixing parameter gaγγ  <1.5 × 10−12GeV−1 for ma < 10-12 eV. I will move on to discuss the upcoming Athena X-ray Observatory, due for launch in 2028. The X-ray Integral Field Unit (X-IFU) instrument onboard will be far better able to constrain ALPs than Chandra, due to its excellent energy resolution. Using the SIXTE simulation software, we estimate that non-observation of spectral modulations for a 200ks observation of NGC1275 will constrain gaγγ  <1.5 × 10−13GeV−1 , an order of magnitude improvement over that derived from Chandra data.