Off-shell Higgs into 4 leptons using EFTs and Simulation-Based Inference & electron tracking in ATLAS (ITk)20m
The discovery of the Higgs was first made in the Higgs-to-4-lepton (H4l) decay channel, almost 10
years ago. Yet, new possibilities of analysis are still available in this channel. They lay beyond
the on-shell data by studying the off-shell Higgs, defined as having a centre-of-mass energy above
220 GeV. In particular, we shall use the framework of EFTs (Effective Field Theory) which aims to
better understand the deviations of data relative to the Standard Model. The goal is to generate
trustworthy Monte-Carlo samples for the relevant EFT operators in order to fit data and measure the
Wilson coefficients for those operators. In this work so far, we have focused on the Monte-Carlo
generation process in order to compare and validate several software versions.
The process of linking experimental data back to the theory parameters is called inference, using a
function called the likelihood. The complexity of this problem of HEP is such that the likelihood
cannot be calculated analytically: this problem is intractable. Simulation-based inference is a
promising method to extract the most information possible from the available data.
At the future HL-LHC, the ITk (Inner Tracker) is scheduled to replace and upgrade the current Inner
Detector. The ATLAS tracking software was reworked into an experiment-independent software
collaboration called ACTS (A Common Tracking Software), with the idea of future implementation of
multi-threading and Machine Learning. The ACTS is therefore being reintegrated into the Athena,
ATLAS’ software framework. Tracking is particularly challenging for the electron because of
increased brehmstrallung, as the particle loses energy as it progresses in the tracker. The Gaussian
Sum Filter (GSF) fitter, which exists in the ATLAS software, is being implemented and optimized into
ACTS. The GSF fitter from ACTS must therefore be validated and compared to the reference track
fitting algorithm in ACTS (called the CKF) and to the existing GSF fitter from ATLAS.
Polarized and unpolarized positron beams are essential for the future hadronic physics experimental program at the
Thomas Jefferson National Accelerator Facility (JLab). The main challenge is to produce high duty-cycle, and high intensity polarized positron beams. The JLab positron source uses the Polarized Electrons for Polarized Positrons (PEPPo) technique to create either a low intensity, high polarization positron beam (I > 100 nA, P=60%) or a high-intensity unpolarized positron beam (I > 3 μA ), from an intense highly polarized electron beam (I=1 mA, P=90%). The current design involves a new injector dedicated to positron production, collection, and shaping suitable for acceleration through the Continuous Electron Beam Accelerator Facility (CEBAF). The optimization of the layout and the performance of the positron source will be explored in this presentation.