Search For A Vector Boson Resonance Particle In The VZâ†'vv(|-)qq(|-) Channel With The ATLAS Detector

On 8 October 2013, Nobel Price for Physics was awarded jointly to Francios Englert and Peter Higgs for the theoretical discovery of Higgs. The Higgs boson is the quantum excitation of Higgs field, from where the particles gained mass. Experimentally, the ATLAS and CMS collaboration announced that they have found a new particle at mass around 125 GeV independently, this new particle is regarded as the standard model Higgs boson.The ATLAS is a general purpose detector on large hadron collider. It records the information of secondaries produced by the particle collision, and is intended to investigate many different types of physics, from the confirmation of the standard model to the search of new physics. So far the most important result is the discovery of the said Higgs boson, however there are still a lot of questions remain, one of them is that if the discovered Higgs boson is the standard model one.To test if the new particle is the standard model Higgs boson or not, we use the data collected by ATLAS at Run I to search a high mass Higgs boson decaying to ZZ pair, and one of the Z boson decays to quark pair, the other decays to two neutrinos. The mass range is from 400 GeV to 1 TeV. Since the neutrinos are undetected, we take advantage of the principle of momentum conservation in center of mass system. Assuming the neutrinos are balanced with other objects, hence we can calculated the ETmiss as the vectorial sum of all the other objects’transverse momentum. Due to the color confinement, the quarks can not be isolated and therefore not detected. They will appear as jets (cluster of hadrons) through the hadronization in the detector. After obtaining the kinematic information of ETmiss and jets, we can construct the transverse mass of the system, if there exists a resonance particle, it will come up with a bump in the spectrum.The data collected by ATLAS in Run I at center-of-mass energy at (?)s= 8 TeV corresponds to a total integrated luminosity of 20.3 fb-1. It provides a great opportunity for the search of new particles. If we can find the high mass Higgs boson, then the 125 GeV particle can be disapproved to be the standard model Higgs boson. Apart from the standard model, in some beyond standard model theories, like the two-Higgs-Doublet Model (2HDM), Electro-Weak Singlet (EWS) and etc., some new particles are predicted. So within the same framework, these theorites can be tested.The Run II data collecting at ATLAS starts from 2015, the center-of-mass energy of LHC is increased to 13 TeV, with an integrated luminosity of 3.2 fb-1 so far. The higher energy made it in favor of the heavier particle search, hence the search range is expanded to 1 TeV-3 TeV. The Run I high mass Higgs search is continued, in the meantime, taking the same or similar channel, the results are interpreted into new models, including the Graviton in Warped Extra Dimensions of the Randall-Sundrum Model [1] and heavy vector boson W’predicted by Heavy Vector Triplet model (HVT) [2].No significant deviation from the standard model prediction has been observed, so we set the 95% confidence upper limit on each model.The scheduled integrated luminosity at Run II is around 100 fb-1, we believe with such amount of data, the new physics can be revealed in the near future. What is more, the ATLAS and CMS have already shown a promising result of a possible particle at mass around 750 GeV through the γγ decay channel, we will follow on the new search with more statistics.During my Ph.D period, this analysis is one of the main areas I have been involved. As said above, it is divided into two stages, the high mass Higgs boson search in Run I and diboson resonance search in Run II. The main contributions I have made include the software framework developing, data processing, selec-tion optimization, systematics study, and the statistical interpretation. To further improve the result, I also participated the combination of this analysis and other channels. The Run I result has been published on EPJC [3], and the Run II result has been published as a CONF NOTE, the papar is under way.Besides, I was also involved in the Higgs boson to bb search, it is aiming to find the bb decay of the standard model Higgs boson in associated (W/Z)H production. The analysis consists of three channels(called 0/1/2 lepton channel) depending on the number of charge leptons from the vector boson decay, and I mainly worked on the 0 lepton channel, including the QCD background estimation, mass resolution study, and the input providing. The result has been published on JHEP [4].I’m qualified as the ATLAS author because of the service work on the Tile Calorimeter performance study using the muon data from the p-p collision. And another important work is the global monitoring, I’m responsiable for the main-taining the software package and the on-call help for the data quality shifter in the control room.