Increasing evidence for Higgs particle
The short-lived Higgs particle decays in a number of different sets of final particles (decay channels) depending on its mass. Identifying decay products for a light Higgs particle (about 120 GeV) from the overwhelming background of already known processes is extremely challenging. In such cases, it is crucial to look at all possible decay channels. To help with this, scientists initiated the EU-funded project 'Understanding the origin of mass with the ATLAS experiment at the Large Hadron Collider' (HIGGS-ZAP). Specifically, the project focused on cases in which a Higgs boson was produced in association with a Z boson and its subsequent decay in bottom quarks (b-jets). Z bosons have a clear decay signature, thus offering a useful way to pick Higgs-Z events from the background. The Higgs particles were expected to decay into two b-jets that contained relatively long-lived hadrons. This helps to identify decays of Higgs particles. The innovative element of the HIGGS-ZAP approach was that the Higgs and associated Z bosons were required to have high energy. This made Higgs-Z events stand out from the background compared to the 95 % of all other potential Higgs events.Online tracking algorithms developed in the project helped to reconstruct particle tracks in the ATLAS inner detector. Based on these, an online monitoring system was used to select b-jets and leptons and to filter out irrelevant events. Project members performed the most up-to-date measurements for single b-jets and pairs produced in the Large Hadron Collider collisions for this new energy regime. This offered significant insight into proton collisions and proof-of-concept that high-energy Higgs bosons can be observed. Scientists observed for the first time high-energy Z bosons decaying into two b-jets that merged into a fat jet. Algorithms based on real-time data compared with those based on reconstruction of particle tracks showed tolerable deviation in identifying highly energetic particles decaying into b-jets. Project results were published in peer-reviewed journals and presented at an international conference. HIGGS-ZAP significantly strengthened ATLAS capabilities for Higgs measurements.
