Periodic Reporting for period 1 - LightBosons (Search for Higgs boson decays into Light Bosons in Boosted Hadronic final states)
Okres sprawozdawczy: 2019-05-01 do 2021-04-30
The goal of this project was to search for new beyond the Standard Model (SM) physics in the Higgs sector, through a novel search for anomalous decays of the Higgs boson into a Z boson and an undiscovered light neutral boson. The necessity for such new physics is motivated by several fundamental questions, such as the unexplained nature of the dark matter, that corresponds to 85% of the matter of the universe, and the origin of the fermion mass hierarchy, which spans several orders of magnitude in mass. The SM offers no explanations for these important questions. Several ideas put forward to explain these puzzles, such as those involving extended Higgs sectors, also predict the existence of new light neutral bosons, often denoted a. Such bosons are expected to be produced in the decays of the observed Higgs boson, either in pairs, H -> aa, or in association with a Z boson, H -> Za. While the H -> aa channel had been searched for at the LHC, until the completion of this project, the H -> Za channel represented a highly viable yet largely under-investigated probe of potential new physics in the Higgs sector requiring urgent attention. In this project a direct search was performed for the the H -> Za decay with proton-proton collision data collected by the ATLAS experiment during Run 2 of the CERN Large Hadron Collider. The experimental strategy was designed to maximise the sensitivity by targeting boosted hadronic decays of the a boson. This goal was achieved by the development of several novel tools. No excess of events over the expected background was found, and an upper limit on the fraction of these potential decays of the Higgs boson of around 30% was set at 95% confidence level. The results of this seminal study were published in Phys. Rev. Lett. 125 (2020) 22 (arXiv:2004.01678).
The project designed, optimised and implemented a novel approach to reconstruct and identify boosted hadronic decays of a neutral boson with a mass in the range of 0.5 – 4 GeV. The approach centred around reconstructing the decay as a calorimeter jet and the ensemble of reconstructed charged particle tracks associated with the jet was used as the basis for a novel multivariate identification algorithm to discriminate between signal decays and background sources. This algorithm was deployed to perform the first search for the H -> Za (a -> hadrons) process using a dataset of proton-proton collisions collected by the ATLAS experiment at the CERN Large Hadron Collider. No statistically significant signal was observed and 95% CL exclusion limits on the product of the cross-section times branching fraction for the H -> Za (a -> hadrons) process were derived. These limits, when interpreted in terms of the branching fraction for the H -> Za (a -> hadrons) decay chain (assuming SM Higgs boson production) reach down to O(30%). The fellow also contributed to the production and testing of silicon electronics for the forthcoming ATLAS tracking detector (ITk) for the High Luminosity LHC programme. In particular, the fellow led the commissioning of the QA test procedures to monitor the radiation hardness of the pre-production silicon micro-strip detectors for the ITk strip tracking detector.
While no signal for the H -> Za (a -> hadrons) process was observed, the importance of the result of this project centres around the constraints it imposes on the phase space of possible new physics scenarios and the first demonstration of the experimental potential in this channel. This pioneering work will serve as a important milestone towards further searches for an extended Higgs sector in challenging hadronic final states and guide future studies with the datasets expected to be delivered during LHC Run 3 and 4. During the course of this project, a number of experimental challenges were identified and overcome. Furthermore, strategies to address the challenges which remain are also under development.