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

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics


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.
Description Several papers probing vector boson scattering (VBS) at the Large Hadron Collider were published or are about to be submitted to peer-reviewed journals.

The most precise fiducial and differential cross-sections of sam-sign WW production in association with two jets were published, alongside with searches for new physics using Effective Field Theory (EFT) approaches as well a dedicated models (Georgi-Machacek H++). Futhermore, results of searches for Heavy Neutrinos in same-sign WW events were published.

The longitudinally polarized fraction of same-sign WW scattering at the LHC was studied. It is crucial to examine the unitarisation mechanism of the VBS amplitude through Higgs and possible new physics. A Deep Neural Network approach was taken to extract the longitudinally polarised fraction.
Exploitation Route The tools developed during this grant will be useful to other LHC collaborators and myself, in order to observe longitudinally-polarised VBS produciton in same-sign WW events, something which is currently out of reach but which will become possible with improved techniques as well as more data from the LHC.
Sectors Other

Description Expanding the precision timing frontier for particle tracking at hadron colliders
Amount £129,634 (GBP)
Funding ID ST/W005735/1 
Organisation Science and Technologies Facilities Council (STFC) 
Sector Public
Country United Kingdom
Start 12/2021 
End 03/2022
Description Expanding the timing frontier: precision timing for particle tracking and identification
Amount £287,886 (GBP)
Funding ID ST/X005062/1 
Organisation Science and Technologies Facilities Council (STFC) 
Sector Public
Country United Kingdom
Start 11/2022 
End 03/2023
Description VBSCan COST Network 
Organisation European Cooperation in Science and Technology (COST)
Country Belgium 
Sector Public 
PI Contribution My contribution covers the experimental aspects of probing vector boson scattering with the ATLAS detector.
Collaborator Contribution The main goal of the VBSCan project is to investigate the Vector Boson Scattering (VBS) process and its implications for the Standard Model, by coordinating existing theoretical and experimental efforts in the area and by best exploiting hadron colliders data, thereby laying the groundwork for long-term studies of the subject and creating a solidly interconnected community of VBS experts.
Impact Vector Boson Scattering Processes: Status and Prospects Report number: CP3-21-14, DESY-21-064, IFJPAN-IV-2021-8, PITT-PACC-2106, VBSCAN-PUB-04-21 ssWW modelling in ATLAS and CMS Report number: ATL-PHYS-PUB-2020-026
Start Year 2020
Description ATLAS Youtube Live: Exploring electroweak phenomena at the ATLAS Experiment - Live talk with Dr. Karolos Potamianos 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact This is an ATLAS Youtube Live talk (+Q&A) on the mechanism of Electroweak Symmetry Breaking and it oulines my research in the wider context of particle physics and that of the ATLAS experiment. At the time of writing (March 6, 2022), this talk had 883 views. It is expected that the audience be wider, as it will remain available in replay.
Year(s) Of Engagement Activity 2022
Description ATLAS hunts for new physics in the scattering of W bosons 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact This physics brief describes two of the physics analyses I was involved in and describes the work to a general audience.
Year(s) Of Engagement Activity 2023