Probing for New Physics at the LHC: Unraveling the Higgs Mechanism through Polarisation and Hadronic Decays

The Large Hadron Collider (LHC) at CERN collides protons at the highest energy produced in the laboratory. The ATLAS experiment collects 40 million collision "pictures" per second, selects a small fraction (the most interesting ones) using fast electronic systems, and stores 1 GB of data per second for further analysis. Particle physicists like myself analyse these huge amounts of data, investigating the particles that make up our Universe and studying their interactions.

In 2012, the Higgs boson was discovered, confirming a prediction of the Standard Model of particle physics (SM), a theory that accurately predicts a wide range of observed phenomena. Yet, cosmological observations suggest that the SM only explains 4% of the Universe, its "visible" part, with 96% being unknown, and for this reason called dark matter and dark energy. The goal of particle physics is to measure the SM as accurately as possible and search for physics phenomena Beyond the SM (BSM) that dark matter is possibly made of.

The vast majority of proton collisions involve its constituents: quarks and the gluons (carriers of the strong force) which hold them together. My work involves the study of the much rarer collision of (weak) vector bosons (W and Z, carriers of the electroweak force) emitted by the protons, using the LHC as a weak boson collider! These processes, known as vector boson scattering (VBS), are highly sensitive to new physics, and could shed light on undiscovered particles that don't interact with quarks and gluons. Gaining a better understanding of the electroweak force through VBS is one very promising path to solving the mystery of dark matter, that some theories predict to be a weakly-interacting particle, and separately, to understanding how fundamental particles acquire mass (and whether the Higgs boson is alone in this process). Studying the electroweak force could be key to explaining the tiny mass of the neutrinos.

But probing VBS is very challenging as these are a tiny fraction of collisions, and any subtle sign of new particles is difficult to uncover. My research focuses on techniques to extract the rare VBS events from proton-proton collision data. Using these to select only 60 events among many billions, I had a lead role in the group that observed the production of two W bosons of the same charge, a very rare process that happens once per 20 000 billion collisions (typically once per day at the LHC).

The precision of the measurements can be further improved by collecting more data. But an even bigger impact can come from new analysis techniques, such as the identification of hadronic W boson decays (when the W decays to quarks). This is very difficult to do, as many other, more frequent, processes also produce quarks. To achieve this, my research will involve complex machine learning algorithms, similar to those that allow for automatic face recognition or driver-less cars.

Also of great interest is the Higgs boson, its interaction with weak bosons, and its self-interaction (HH), even rarer than VBS and which requires an upgraded LHC. These are very important probes of the SM that could yield hints of new physics, as small deviations from the SM can have a large impact on event rates.

To improve the precision of measurements, I perform R&D on the detectors used to collect the data, in particular silicon pixel detectors, which are similar to the sensors in digital cameras. At the heart of particle detectors, they are the first that the collision products encounter. I devise and study new detector concepts to cope with the challenges of future particle colliders. High precision silicon detectors are essential to identify the collision event characteristics, the first step towards unraveling the mysteries of our Universe.

Finally, I am sensitive to data preservation so that future theories can be tested against ATLAS data, so that the work we do today can help future generations shed light on Nature.