Particle Physics

In this talk, I will detail two ways to search for low-mass axion dark matter using cosmic microwave background (CMB) polarization measurements. These appear, in particular, to be some of the most promising ways to directly detect fuzzy dark matter. Axion dark matter causes rotation of the polarization of light passing through it. This gives rise to two novel phenomena in the CMB. First, the late-time oscillations of the axion field today cause the CMB polarization to oscillate in phase across the entire sky. Second, the early-time oscillations of the axion field wash out the polarization produced at last-scattering, reducing the polarized fraction (TE and EE power spectra) compared to the standard prediction. Since the axion field is oscillating, the common (static) ‘cosmic birefringence’ search is not appropriate for axion dark matter. These two phenomena can be used to search for axion dark matter at the lighter end of the mass range, with a reach several orders of magnitude beyond current constraints. I will present a limit from the washout effect using existing Planck results, and discuss the significant future discovery potential for CMB detectors searching in particular for the oscillating effect.

[1903.02666]

Bosonic ultra-light dark matter (ULDM) would form cored density distributions at the centres of galaxies. These cores admit analytic description as the lowest energy bound state solution ("soliton") of the Schrödinger-Poisson equations. Numerical simulations of ULDM galactic halos found empirical scaling relations between the mass of the large-scale host halo and the mass of the central soliton. 
We connect the simulation results of different groups to basic properties of the soliton. Importantly, simulations imply that the specific kinetic energy in the soliton and in the host halo should be approximately equal. This relation predicts that the peak circular velocity, measured for the host halo in the outskirts of the galaxy, should repeat itself in the central region. Contrasting this prediction to the measured rotation curves of low surface-brightness galaxies, we show that ULDM in the mass range m ~ 10⁻²²-10⁻²¹ eV, which has been invoked as a possible solution of small-scale puzzles of ΛCDM, is in tension with the data. We discuss the potential of detailed dynamical modelling of the Milky Way and other well-resolved galaxies to probe ULDM up to m ~ 10⁻¹⁹ eV.

I will introduce my recent works on the phenomenology of solutions to the strong CP problem, QCD axion and Parity. I will first describe the production of the QCD axion in the early universe and show that the dark matter abundance is naturally reproduced for a wide range of the parameter space. I will then show a tight relation between the Parity solution, dark matter direct detection, the proton decay, and the precise measurements of the standard model parameters.

Mirror sectors -- hidden sectors that are approximate copies of the Standard Model -- are a generic prediction of many models, notably the Mirror Twin Higgs model. Such models can have a rich cosmology and many interesting detection signatures beyond the realm of colliders. In this talk, I will focus on the possibility that mirror matter can form stars which undergo mirror nuclear fusion in their cores. I will discuss the mechanisms by which these objects can emit Standard Model light and estimate their luminosity and prospects for their detection.

The minimal Standard Model running of the gauge couplings gives us a hint of a Grand Unified Theory (GUT) at M_U ~ 10^14 GeV — a scale, however, too high to probe directly via collider searches. Fortunately, since the inflationary Hubble scale H can be as high as  5 x 10^13 GeV ~ M_U, such GUT scale states can be cosmologically produced during inflation and contribute to primordial non-Gaussianity (NG).

In this talk, I will explore the possibility of doing on-shell, mass-spin spectroscopy of GUT-states by studying such NG contributions in an extra-dimensional framework of orbifold GUTs. I will identify an interesting regime where the extra dimension is stabilized close to the onset of a horizon and find that the KK gravitons and KK gauge bosons can mediate observable NG providing a direct probe of orbifold GUTs.

Axion-like particles are a broad class of dark matter candidates which are expected to behave as a coherent, classical field with a weak coupling to photons. Research into the detectability of these particles with laser interferometers has recently revealed a number of promising experimental designs. Inspired by these ideas, we propose the Axion Detection with Birefringent Cavities (ADBC) experiment, a new axion interferometry concept using a cavity that exhibits birefringence between its two, linearly-polarized laser eigenmodes. This experimental concept overcomes several limitations of the designs currently in the literature, and can be practically realized in the form of a simple bowtie cavity with tunable mirror angles. Our design thereby increases the sensitivity to the axion-photon coupling over a wide range of axion masses.

The Twin Higgs model is an attractive solution to the little Hierarchy problem with top partners that are neutral under SM gauge charges. The framework is consistent with the null result of LHC colored top partner searches while offering many alternative discovery channels. Depending on model details, the phenomenology looks very different: either spectacular long-lived particle signals at colliders, or a plethora of unusual cosmological and astrophysical signatures via the existence of a predictive hidden sector. I will examine the latter possibility, and describe how the asymmetrically reheated Mirror Twin Higgs provides a predictive framework for a highly motivated and highly non-trivial interacting dark sector, with correlated signals in the CMB, Large Scale Structure, and direct detection searches, as well as higgs precision measurements at colliders. This provides a vivid example of the collider-cosmology complementarity, and motivates a variety of new astrophysical searches that are motivated by the hierarchy problem.

Laser spectroscopy of muonic hydrogen [1,2] yielded a proton rms charge radius which is 4% (or ~6 sigmas) smaller than the CODATA value [3]. This discrepancy is now called the "proton radius puzzle" [4].
Also the deuteron charge radius from muonic deuterium [5] is 6 sigmas smaller than the
CODATA value, but consistent with the smaller proton inside the deuteron.
These smaller charge radii, when combined with precision measurements of the 1S-2S transitions in regular (electronic) hydrogen [6] and deuterium [7], yield a 6 sigmas smaller value of the Rydberg constant [8], compared to the CODATA value.
In this talk I will report about a new measurement of the Rydberg constant from the 2S-4P transition in regular hydrogen performed in Garching [9], which supports the smaller, "muonic" value. More recently, however, a new measurement of the 1S-3S transition in Paris confirmed the larger proton radius [10].
Several new measurements, such as hydrogen from Toronto, elastic electron scattering at lower Q², and new results from electronic and muonic helium will help understand the proton radius puzzle.

Hidden sectors are motivated by a range of phenomena unexplained by the Standard Model, such as dark matter, neutrino masses, and the baryon asymmetry. Hidden sector particles below the weak scale can be copiously produced at high-energy and intensity-frontier experiments, but may have evaded detection with current searches.
I will discuss new searches I am doing with the BaBar collaboration that are probing unexplored, well-motivated territory for hidden sectors. I will also show how signatures such as displaced vertices motivate comprehensive, complementary searches at low- and high-energy experiments.

New scalar bosons that are uncharged under the Standard Model have many phenomenological applications, and in particular may serve as mediators between the dark and visible sectors. Such scalars are often taken to have Higgs-like couplings to Standard Model fermions, in order to evade constraints from bounds on flavor-changing neutral currents.
We describe an alternative approach, based on an effective field theory approach in which a new scalar preferentially couples to a single fermion mass eigenstate. In such theories, we show through a spurion analysis that small flavor changing couplings are still technically natural. Examples of our framework include a muon-philic scalar that can explain the observed anomalous magnetic moment of the muon and an up-specific scalar mediator to a dark sector, whose phenomenologies we discuss in detail.

In recent years the Standard Model Effective Field Theory (SMEFT) has emerged as a well defined and systematically improvable theory to study the constraints on physics beyond the Standard Model. This formalism explains old mysteries in interpreting LEP data and offers a field theory framework to combine such data with LHC measurements of the properties of the Higgs. We discuss the current status of these combined studies. The lack of evidence for new physics resonances at LHC has also encouraged new approaches to explaining the observed value of the electroweak scale, in the presence of physics beyond the Standard Model. One such approach is the Neutrino Option, that generates the electroweak scale from an underlying Majorana scale, when Neutrino masses come about due to a seesaw mechanism. We discuss how this approach is embedded into the SMEFT.

Repeated null-results at dark matter experiments targeted at WIMP masses, have resulted in the spotlight shifting to lighter dark matter and more exotic WIMP candidates. In this talk I shall present the rich level structure of molecules and nuclei as a tool to explore MeV scale dark matter and dark forces. I will also present a novel detector concept that supplies energy to dark matter, thus accessing inelastic dark matter parameter space.