Session C3: Particle Physics II

Future Colliders

The LHC is expected to run with energies of at least 13 TeV for the next 20 years. While this will provide years of interesting proton-proton data, the field is exploring a number of other collider facilities and technologies. These include lepton colliders, Higgs factories, and very high energy hadron colliders. I will quickly review some of the main efforts, paying most attention to how they fit into the experimental and physics landscape.   

The moment of truth for natural supersymmetry?

Supersymmetry is still one of the main contenders for generating the new physics needed to solve the many shortcomings of the Standard Model (SM). However, with the pressing absence of any new physics discoveries at the LHC, viable supersymmetric extensions of the SM are progressively being pushed towards unnatural regimes in parameter space. Scenarios with light, higgsino-like electroweakinos with nearly degenerate masses are strongly motivated by naturalness arguments, while still avoiding the reach of conventional supersymmetry searches. I will give a brief introduction into the phenomenology of such scenarios and the role that naturalness plays here, and will then cover a strategy for investigating such scenarios during Run 2 of the LHC and beyond. By looking for signatures with soft leptons in events with a hard jet and sizable missing transverse energy, sufficient sensitivity can be reached to probe largely unexplored regions of parameter space during Run 2 of the LHC and beyond.   

Graph duality and AdS/CFT

Recent mathematical identities between the branching graphs of the unitary groups and the branching graphs of the symmetric groups have been discovered. They go beyond the celebrated Schur-Weyl duality. Schur-Weyl duality was found to have a deep connection with the AdS/CFT correspondence, which states the equivalence between String Theory in AdS backgrounds and specific superconformal field theories. In this talk we will investigate the relation between these new mathematical identities and the AdS/CFT correspondence by identifying each side of the identities with AdS processes and CFT processes respectively. The holographic interpretation of identities involving unitary and symmetric groups is indeed a new methodology for approaching the AdS/CFT correspondence.   

Using Neural Networks to Separate Signal from Background with Real Missing Transverse Energy

Deep neural networks are an exciting technique for approaching classification problems in Machine Learning. This research investigated the potential application of deep neural networks to the ATLAS missing transverse energy trigger. We trained neural networks to distinguish between associated Higgs production signal events and minimum bias background events. We investigated neural networks trained on high level input features and found an overall improvement in performance compared with a one dimensional missing transverse energy threshold. By treating the calorimeter as an image, we were also able to train deep convolutional neural networks~to classify the events based entirely on low level input~features. The performance achieved using the low level features was marginally poorer than the performance achieved on the high level input features.   

Building a strong event discriminant using the Matrix Element Method for the $\lowercase{t\bar{t}}H$ search with the ATLAS detector

The first run of the Large Hadron Collider at CERN has brought experimental confirmation of Higgs boson production via two out of the four main predicted mechanisms. Of major importance and so far unobserved is the $t\bar{t}H$ process, where a Higgs boson is produced in association with a pair of top quarks; it allows for a direct measurement of the top quark Yukawa coupling. This talk discusses the case where the Higgs boson decays to $b\bar{b}$ and a single lepton is present in the decay products of the top quarks.\\ The Matrix Element Method provides a strong discriminant between $t\bar{t}H$ events and the overwhelming $t\bar{t} + b\bar{b}$ background. Its probabilistic approach relies on calculating likelihoods of observed events being consistent with $t\bar{t}H$ and $t\bar{t} + b\bar{b}$ hypotheses. Focus of the talk is a new implementation of this method, which supports calculation of the matrix elements on both GPUs and the traditionally used CPUs. Significant performance improvements are achieved, which help deal with the extreme computational demand. The current discrimination power is presented, and possible approximations in the matrix element calculations and their effects in the context of the Run II search for $t\bar{t}H$ with the ATLAS detector are discussed.   

Chiral Sum Rules from QCD Current Algebra on a Null-Plane

As basic relations correlating physical cross sections to low-energy constants (e.g., the pion decay constant $f_\pi$ and axial coupling $g_A$ in the case of pion-nucleon scattering), sum rules have the potential to illuminate strong interaction physics over a large sweep of hadronic momenta. In this talk, I will briefly overview a recent unified derivation of a wide class of sum rules obtained using the chiral current algebra of QCD formulated on a light-like hyperplane (i.e., Dirac's null-plane). In the end, I will show that our formulation, when married to the technology of dispersion relations, allows us to write novel constraints on the Regge behavior of scattering amplitudes for various processes, including Compton scattering and pion electroproduction off the nucleon.