Particle Physics

The LHCb detector was designed to be the dedicated heavy-flavor physics experiment at the LHC, and has been the world's premier lab for studying processes where the net quark content changes for several years. These studies permit observing virtual contributions from beyond the SM particles up to very high mass scales, potentially (greatly) exceeding the direct reach of the LHC.  I will summarize the constraints placed on high-mass BSM physics by such studies, and also highlight a few interesting anomalies.  I will also discuss direct searches for low-mass BSM physics, and a selection of novel SM studies.  Finally, future prospects will be presented. 

The vacuum energy changes when cosmological phase transitions take place, or in environments with high temperatures or chemical potentials.  The propagation of primordial gravity waves is affected through the trace anomaly, and eras where the vacuum energy dominates can lead tofeatures in the gravity wave spectrum.  High density phases of QCD in neutron stars, where the vacuum is rearranged, are also a novel probe:  The mass-radius relationships for neutron stars is sensitive to the equation of state appearing in the Einstein equations, which may depend on new dynamics associated with vacuum energy.  Such observations could potentially rule out large classes of explanations for the smallness of the currently observed cosmological constant.

I discuss a new proposal for nonperturbatively defining chiral gauge theories, something that has resisted previous attempts. The proposal is a well defined field theoretic framework that contains mirror fermions with very soft form factors, which allows them to decouple, as well as ordinary fermions with conventional couplings.  The construction makes use of an extra dimension, which localizes chiral zeromodes on the boundaries, and a four dimensional gauge field extended into the bulk via classical gradient flow.  After explaining how this setup works, I consider open questions concerning the flow of topological gauge configurations, as well as possible exotic phenomenology in the Standard Model lurking at the low energy frontier.

We argue that solutions to the strong CP problem motivate different searches for TeV scale physics at the LHC than are currently being emphasized. We present two solutions to the strong CP problem that require the existence of new colored particles with masses below 10 TeV. New motivated searches at the LHC would provide a strong constraint on these solutions to the strong CP problem.

I consider the Standard Model as an effective field theory (EFT) at the electroweak scale $v$. At the scale $f\geq v$ I assume a new, strong interaction that breaks the electroweak symmetry dynamically. The Higgs boson arises as a composite pseudo-Nambu-Goldstone boson in these scenarios and is therefore naturally light $(m_{h}\sim v)$. Based on these assumptions and the value of $\xi=v^{2}/f^{2}$, I explain the systematics that governs the effective expansion:\\

For $\xi=\mathcal{O}(1)$ the effective theory is given by a loop expansion, equivalent to an expansion in chiral dimensions (similar to chiral perturbation theory). I will briefly discuss the operators that arise at next-to-leading order ($\mathcal{O}(f^{2} / \Lambda^{2})\simeq \mathcal{O}(1/16\pi^{2})$). On the other hand, in the decoupling limit where $\xi\rightarrow 0$, an expansion in canonical dimension is recovered. The case where $\xi$ is small but non-zero is of phenomenological interest. It leads to a double expansion in $\xi$ and $1/16\pi^{2}$, which captures the expected corrections of a strongly-interacting light Higgs to the Standard Model in a systematic way. \\

Further, I will apply the leading order chiral Lagrangian to current LHC Higgs data. I will show that this gives a QFT justification of the $\kappa$-framework, which is currently used as phenomenological signal-strength parametrization by the experiments at the LHC. I will also present a fit of the leading order chiral Lagrangian to the LHC Higgs data.

It has been known for a long time that quadratic gravity, which generalizes Einstein gravity with quadratic curvature terms, is renormalizable and asymptotically free in the UV.  However the theory is afflicted with a ghost problem if the perturbative spectrum is taken seriously. We explore the possibility that the dimensional scale of Einstein-Hilbert term is far smaller than the scale where the dimensionless gravitational couplings become strong. The propagation of the gravitational degrees of freedom can change character at this strong interaction scale. Lattice QCD studies show a particular suppression of gluon propagator in the IR, which removes the perturbative gluon from the physical spectrum. We propose that the same fate can apply to the spin-2 ghost. The Planck mass is associated with the strong dynamics scale below which the normal Einstein description can emerge. In this picture both the UV and IR limits have weakly coupled descriptions, similar to perturbative QCD and the chiral Lagrangian. Some implications of a small mass ratio in the theory are considered.

Quantum superpositions of matter are unusually sensitive to decoherence by tiny momentum transfers, in a way that can be made precise with a new diffusion standard quantum limit.  Upcoming matter interferometers will produce unprecedented spatial superpositions of over a million nucleons. What sorts of dark matter scattering events could be seen in these experiments as anomalous decoherence?  We show that it is extremely weak but medium range interaction between matter and dark matter that would be most visible, such as scattering through a Yukawa potential. We construct toy models for these interactions, discuss existing constraints, and delineate the expected sensitivity of forthcoming experiments.  In particular, the OTIMA interferometer developing at the University of Vienna will directly probe many orders of magnitude of parameter space, and the proposed MAQRO satellite experiment would be vastly more sensitive yet.  This is a multidisciplinary talk that will be accessible to a non-specialized audience.

The search for physics beyond the Standard Model at the LHC is largely oriented towards new particles associated with solutions to the electroweak hierarchy problem. While the precise character of these partner states may vary from model to model, they typically possess large QCD production rates favorable for detection at hadron colliders. Null results in searches for partner particles during Run 1 of the LHC have placed the idea of electroweak naturalness under increasing strain. In this talk I'll discuss a broad class of natural theories where the new degrees of freedom relevant for naturalness lie in hidden sectors and are largely unconstrained by LHC data. Rather than rendering electroweak naturalness untestable, they give rise to entirely new signs of naturalness at the LHC, including displaced decays and other exotic signatures. They also furnish a variety of viable dark matter candidates testable at current and future experiments.

 I  will first analytically show a simple, yet subtle "invariance" of two-body decay kinematics for the case of a massless  daughter and a mother particle which is unpolarized and has a *generic* boost distribution in the laboratory frame. Namely, the laboratory frame energy distribution of the massless decay product has a peak, whose location is identical to the  (fixed) energy of that particle in the rest frame of the corresponding mother particle. In turn, this value of the energy is a simple function of the other masses involved in the decay.
As a proof of principle of the usefulness of this observation, I will then apply it for measuring  the mass of the top quark at the LHC, using simulated data (including experimental  effects). In fact, CMS collaboration (in CMS-PAS-TOP-15-002) has recently implemented our method for measuring the top quark mass! Finally, I will show how it can be used to measure all the superpartner masses in a cascade decay chain of the gluino.

Twin Higgs theories attempt to solve the little hierarchy problem - why has the LHC not yet observed new states stabilising the EW scale? - by introducing a SM-neutral twin sector, related to the SM by an approximate Z_2 symmetry. The physical Higgs is then a PNGB mixture from both sectors, and acts as a portal between them. In this talk, I will discuss the cosmology of minimal (“fraternal”) Twin Higgs models. Higgs portal interactions establish thermal equilibrium between the SM and twin sector at high temperatures, giving a thermal history with possibilities for both symmetric dark matter (through a “twin WIMP miracle”), and asymmetric dark matter (from twin QCD states which naturally have GeV-scale masses). More generally, relic abundances of cosmologically stable states place constraints on the parameters of the theory, and twin sector phase transitions need to be considered.

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.