Probing the Standard Model via boosted W/Z boson precision measurements at the Large Hadron Collider

• Applicant: Dr. Chris Malena Delitzsch
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 469666862

The Standard Model of particle physics (SM) describes the elementary particles, fermions and the fundamental interactions between them, which are mediated via gauge bosons. Its predictions have been tested with high precision at various powerful particle accelerators, with no significant deviations observed so far. The last missing piece of the SM, the Higgs boson was discovered at the Large Hadron Collider (LHC) in 2012. The Higgs boson allows fermions and gauge bosons to acquire mass without breaking local gauge invariance. The measurement of the Higgs properties and its coupling to SM particles is of utmost importance as any deviations from its prediction could hint at new physics beyond the SM which might be out of reach at the LHC via direct searches. The coupling of the Higgs boson to the top quark (top Yukawa coupling, yt) is of special interest due its large predicted value. The top Yukawa coupling can be measured via direct and indirect measurements at the LHC. These measurements have to rely however on assumptions that no physics beyond the SM exists or have small cross-sections (ttH and tH). In addition, final states with quarks are dominated by large uncertainties of the background processes. Therefore, a new approach will be pursued in this proposal to measure yt without relying on the on-shell Higgs production. This so-called "Higgs without Higgs" approach allows for a complementary measurement of yt focussing on the high transverse momentum regime. It relies on the fact that any modifications to the top Yukawa coupling would result in an increase of the production rate of a specific process which leads to a final states with one forward jet, one top quark and two longitudinally polarised vector bosons. The focus lies on the hadronic decay of the vector bosons and the top quark as the hadronic decay modes represent the majority of the branching fraction. The analysis depends on the successful identification of vector bosons and top quarks as large-radius jets. Their inner structure will be studied to suppress the background. Measurements involving jets are often associated with large uncertainties. Therefore, cross-section measurements are first performed in V+jets and diboson events to strengthen our understanding of jet formation and their inner structure. These measurements constitute an important test of the SM and will be used to tune Monte Carlo generators. In addition, the precise measurement of the energy deposits within the large-radius jets is crucial for the proposed measurements. The topoclustering algorithm, used to reconstruct hadronic signals in the calorimeter, will be modified using machine learning techniques to improve jet reconstruction under the high pile-up scenarios expected for Run-3 at the LHC and to enhance the potential to identify jets containing the hadronic decay of vector bosons and top quarks. The measurements will be performed using proton-proton collisions recorded with the ATLAS detector.

• DFG Programme: Independent Junior Research Groups