The Mass of the Higgs Boson

Abstract: Abstract Within the Standard Model of particle physics, there was only one undiscovered fundamental particle. This was the Higgs boson. In July 2012, the announcement was made that a Higgs like particle had been discovered by the ATLAS Collaboration and the CMS Collaboration at CERN. The aim of this Bachelor’s thesis was to use data collected by the ATLAS detector facility to estimate the mass of the Higgs boson. The estimation was done with consideration to one decay channel, namely when the Higgs boson decays into two Z bosons, and each Z boson subsequently decays into two muons of opposite charge. Setting constraints on the four-muon system kinematical data yielded a selection of possible Higgs candidates. A histogram was created of the invariant masses of the possible Higgs bosons. The masses of the intermediate step Z bosons were also put in a histogram. Next, an estimation of the background processes was made by forming new four-muon systems, by combining Z bosons from different events. This yielded a mass spectrum that was later normalized to the same integral as the original resulting mass spectrum. Finally, the estimated background was subtracted from the original results, to yield a histogram without background. Both the results with and without the estimated background subtracted displayed a prominent peak in the mass range 120 GeV/c2 – 125 GeV/c2. When the background was subtracted, the amplitude of this peak was about twice the amplitude of any other bin, and it was one of the few bins that did not include the “zero events registered” value within its error bars. This is a strong indication that a particle with a mass within this range was detected, and the range is exceptionally close to the mass of the Higgs boson reported by the ATLAS and CMS Collaborations; 126 GeV/c2.