Particle Physics

I will discuss the appeal of pseudo-Goldstone bosons (pGBs) for the generation of scales in Early Universe cosmology. In particular, I will demonstrate how in Goldstone Inflation a pGB inflaton can solve the hierarchy problem of inflation (the tension between the Lyth bound and the inflationary scale as preferred by CMB anisotropies), while avoiding the problems with trans-Planckian scales that are typically associated with related models. A simple model based on the coset SU(4)/Sp(4) realises both the Higgs doublet and an inflaton singlet as Goldstone modes. A single setup can then give rise to Goldstone Inflation and the dynamical generation of the electroweak scale through a composite Higgs, thus also addressing the EW hierarchy problem. I will discuss perturbative reheating in this model, and show how it naturally connects to both EW physics and a UV completion. If time permits I will address our current studies on non-perturbative reheating and the possibility of electroweak baryogenesis in this setup. 

The continued lack of definitive signals at direct detection experiments places many models of weakly interacting dark matter into tension. Direct detection is naturally suppressed in models where the dark matter co-annihilates with another particle in the early universe.  The cosmology, direct detection, and LHC signals of such models can often be well understood by considering only the most relevant low-energy degrees of freedom.  We draw lessons for the Minimal Supersymmetric Standard Model.

A new analysis of electron-proton scattering data (those published in 2010 by the Mainz A1 collaboration and previous world compilations) to determine the proton electric and magnetic radii is presented. The analysis enforces model-independent constraints of form factor analyticity and investigates a wide range of possible systematic effects. Employing standard models for radiative corrections, our improved analysis yields proton electric radii for the Mainz and world data sets that are consistent, although a simple combination yields a value r_E = 0.904(15) fm that is 4-sigma larger than the CREMA muonic hydrogen determination. Remaining possible deficiencies are discussed that, if addressed, could reconcile the values from muonic hydrogen spectroscopy and ep-scattering.

Extensions of the Standard Model (SM) Higgs sector often predict the existence of new vacua and can feature novel patterns of symmetry breaking in the early universe. In this talk, I will discuss the implications of such scenarios for electroweak scale cosmology, baryogenesis, and Higgs phenomenology. I will focus on two classes of models, one involving a gauge singlet scalar field and the other an inert SU(2) doublet scalar. The former can give rise to a strong first-order electroweak phase transition, but also to rapidly expanding bubbles which can pose a problem for viable electroweak baryogenesis. In the latter, electroweak symmetry can be spontaneously broken by a first-order phase transition to an exotic vacuum, allowing for successful electroweak baryogenesis decoupled from the Standard Model-like Higgs field. Nevertheless, this class of scenarios generically predicts modifications of the SM-like Higgs couplings within reach of present-day collider experiments.

The  QCD axion is a curious Dark Matter candidate, having a mass like the neutrino, but behaving as Cold Dark Matter. I will review how this occurs, and discuss the interesting question  of whether WIMPs could be distinguished from axions with Large Scale Structure data.

Identifying the nature of dark  matter is one of the most challenging problems in physics. There is a general consensus that dark matter is a weakly interacting particle and predominantly cold, yet the Cold Dark Matter (CDM) hypothesis remains to be verified.  I will show that next cosmological surveys could play a leading role in understanding the dark matter microphysics.

In this talk I’ll discuss some of the recent developments in precision physics which will be useful for extracting the best physics results we can from LHC run II. I’ll mostly focus on a specific example regarding anomalous interactions of the Higgs boson.

Massive neutral fermions can be realized in Nature as Majorana particles. Are neutrinos Majorana particles? Our most accepted theoretical prejudice can be verified by searching for neutrinoless double-beta decay. I will overview the current knowledge of the neutrino mass spectrum and discuss theoretical scenarios where cosmological data can contribute to resolve this challenging question. Some cosmological observables sensitive to neutrino masses are outlined.

There have been recent claims that the weak gravity conjecture (WGC) rules out multi-field natural inflation. I review these claims and then show how 2-field natural inflation can be consistent with even the most stringent form of WGC. I also discuss my recent attempt at numerically proving the WGC via the conformal bootstrap.

 Super-massive black holes that grow at the center of dark matter halos distort the dark matter within their zone of influence into a steep density spike.  This spike can give rise to strong enhancements of standard indirect detection signals, and can lead to qualitatively new windows onto the physics of the early universe.   I will talk about potential dark matter signals from the Milky Way's central black hole, some astrophysical caveats, and the possible use of black holes as dark matter accelerators.

Recent landmark measurement of the muonic hydrogen Lamb shift generated more questions than answers, as it stands in a sharp disagreement with what was predicted based on known properties of muons and protons. It adds on top of the existing anomalies in the muon sector (discrepancy in g-2 and in radiative muon capture). I will critically review some suggestions for the new physics explanations of these anomalies, and describe their implications. One of the outstanding effects, not tested for muons, is the parity violation in the neutral current channel, which has proven to be an extremely difficult problem, and where the enhancement relative to the standard model prediction is still possible. Following my suggestion for a new way to approach this measurement, this summer the Paul Scherrer Institute in Switzerland will conduct a preliminary muonic atom experiment, tentatively called mu-ARC.

Freeze-in is a general and calculable mechanism for dark matter production in the early universe. Assuming a standard cosmological history, such a framework predicts metastable particles with a lifetime generically too long to observe their decays in a collider environment. In this talk I will report work in progress where we consider alternative cosmologies, where entropy dumped in the primordial Standard Model plasma leads to shorter lifetime for the metastable particles in order to reproduce the observed dark matter density. Famous examples are moduli decays in SUSY theories and inflationary reheating. Remarkably, for a large region of the parameter space the decay lengths are in the displaced vertex range and they can be observable at present and future colliders.