The Higgs boson in the H → ZZ(*) → 4l decay channel with the ATLAS detector at the LHC

Abstract

The Standard Model of particle physics is a very successful theory that describes the known matter in terms of its elementary constituents and their interactions. It describes and uni es three out of the four fundamental interactions: the electromagnetic interaction, responsible for the interactions between charged particles; the weak interaction, responsible for the existence of atomic nuclei; and the strong interaction, responsible for binding quarks together to form protons and neutrons and consequently nuclei. The gravitational interaction acts on macroscopic scale and it cannot be uni ed with the other three forces in the Standard Model theoretical framework. In the Standard Model particles acquire mass through the Higgs-Brout-Englert mechanism, usually called the Higgs mechanism, which postulates the existence of a scalar eld, the Higgs eld. Then, the gauge bosons and the fermions acquire mass by interacting with the Higgs eld. This leads to the emergence of a physical scalar particle, the Higgs boson. It has been hunted for decades in di erent experiments at both the LEP and Tevatron colliders without having any experimental evidence. The search continued when the world's largest proton-proton collider, the Large Hadron Collider, started its operations. The rst evidence of the Higgs boson was achieved in July 4th 2012 when the collaborations of the ATLAS and CMS experiments announced the observation of a new particle with a mass around 125 GeV, with a combined signi cance of more than 5 standard deviations. The analyses presented in this thesis focus on the search for the Standard Model Higgs boson in the decay channel H ! ZZ( ) ! 4l in the context of the ATLAS experiment at the LHC. This decay channel is one of the most sensitive one and provides a clean nale state signature and the possibility to fully reconstruct the Higgs mass with excellent detector resolution. Only muons and electrons are considered in the nal state. Electrons are reconstructed and identi ed by combining the information from the Inner Detector and the calorimeter, whereas for the muons the Inner Detector and the Muon Spectrometer are used. In particular, since the 4 nal state is the most promising one for measuring the Higgs-boson mass, it is very important to know the muon performance of the ATLAS detector. In order to achieve this, the muon momentum resolution of the ATLAS detector has been studied using Z decays in two muons using data-driven techniques. The muon momentum resolution as a function of the muon pT is provided for both i the Inner Detector and the Muon Spectrometer. Then, the observation of a new particle in the search for the Standard Model Higgs boson in the H ! ZZ( ) ! 4l decay channel has been presented. The analysis has been performed using 4:8 fb1 and for 5:8 fb1 of p-p collision data at p s = 7 TeV and at p s = 8 TeV, respectively. An excess of data events over the background-only prediction has been observed at mH = 125 GeV with a local p0 of 0.018%, corresponding at 3.6 standard deviations. The combination with the other searches of SM Higgs boson in ATLAS has been presented. The signi cance of the combined excess at mH = 126:5 GeV reached 6 standard deviations, with an expected value in the presence of a SM Higgs boson signal at that mass of 4.9 standard deviations. The resulting estimate for the mass of the new observed particle is mH = 126:0 0:4 (stat) 0:4 (syst) GeV, while the observed best- t signal strength parameter , de ned as the ratio of the observed and expected number of events, is = 1:4 0:3 which is consistent with a Standard Model Higgs boson. The analysis in the H ! ZZ( ) ! 4l decay channel has been updated with the full 2011 and 2012 datasets corresponding to a total integrated luminosity of about 25 fb1 of p-p collision data. Using the full available dataset, the excess of events is found around mH = 124:3 GeV with a local p0 value of 2:7 1011, corresponding to a signi cance of 6.6 : the single channel discovery is therefore reached. The value of the estimated mass is mH = 124:3+0:6 0:5 (stat) +0:50:3 (syst) GeV, while the signal strength at this best t value for mH is = 1:7+0:5 0:4. To verify if the new discovered particle is the Standard Model Higgs boson or not, its spin and parity have been measured. In order to test the Standard Model hypothesis, spin zero and even parity, against other spin and parity, JP , hypotheses, a multivariate technique using Boosted Decision Trees has been developed. The JP states explored in this analysis are spin 0, 1 and 2 with even and odd parity. Expect the 2 hypothesis, which appears to be preferred by the data when compared to the 0+ hypothesis, the 0, 1+ , 1 and 2+ hypotheses are excluded at the 97.8%, 99.8%, 94% and 83.2% CLs con dence levels in favour of the SM 0+ hypothesis. The Higgs-like boson is therefore found to be compatible with the SM hypothesis. The results obtained in the search in the H ! ZZ( ) ! 4l decay channel have been combined with the results from the other two most sensitive decay channels in ATLAS, H ! and the H ! WW ! l l . In the combination, the excess of events has been found at mH = 125:5 GeV with a local signi cance of 10 . The mass and the signal strength are measured to be mH = 125:5 0:2 (stat) +0:5 0:6 (syst) GeV and = 1:33 0:14 (stat) 0:15 (syst), respectively. The measurements of the spin-parity properties of the new boson performed using the three decay channels have been combined too. The Standard Model hypothesis, when compared to alternative spin-parity hypotheses (JP = 0, 1+, 1, 2+ m, 2), has been found strongly favoured: all the other hypotheses have been excluded with a con dence level above 97.8%.