Search for the Higgs boson decaying to b-quark pairs in 13/14 TeV pp collisions at ATLAS/LHC
IDPASC Portugal - PHD Programme 2016
2016 / 2017
Experimental Particle Physics
Patricia Conde Muino
Faculdade de Ciências - Universidade de Lisboa
Laboratório de Instrumentação e Física Experimental de Partículas
One of the main physics motivations to build the Large Hadron Collider (LHC) at CERN was the search for the last missing piece of the Standard Model of Particle Physics (SM), the Higgs boson, responsible for the mass of the fundamental particles. The discovery of a Higgs boson at ATLAS and CMS, in 2012, followed by the first measurements of is properties, was the first big success of the LHC, that motivated the 2013 Nobel prize awarded to Peter Higgs and François Englert. The Higgs boson, however, continues to be the least known particle of the SM and the main subject of an intense research effort at the LHC. Up to now, its mass, its spin and CP quantum numbers and the coupling to the vector bosons (W, Z and photon) have been measure with reasonable accuracy, but its couplings to fermions, and in particular to quarks, have not being probed directly. A distinctive property of the Higgs boson is that it couples to fermions through the Yukawa coupling proportional to the fermion mass, so some of the most important measurements to be done now are the couplings to the different fermions. In this project we propose to study one of the channels that is essential to measure the Higgs couplings to b-quarks, the associated production of the Higgs to a W boson, when the Higgs decays to b-quark pairs. Being the decay channel with the highest branching ratio, the H->bb is at the same time one of the most difficult ones in a hadron collider, due to very large backgrounds. The associated production of a Higgs with a W boson provides, through the semileptonic decay of the W boson, a good way to reduce the background and therefore observe the Higgs decaying to b-quark pairs. This project involves the analysis of the 13/14 TeV pp collisions that ATLAS will collect during the run II. Analysis algorithms that use the standard ATLAS software tools will be developed to search for the Higgs decaying into b-quark pairs in the associated production channel. At this energy regime, the most sensitive phase space region is the one in which the Higgs boson has a very large transverse momentum, so the two b-jets produced in its decay are very collimated and form a single large jet. This is, in addition, the most sensitive region to new physics effects in the Higgs to b-quark pairs decay. The student will apply jet substructure techniques to identify this kind of events and study its combination with multi-variate analysis techniques in order to improve the current sensitivity of the analysis. The student will be immerse in the international environment of the ATLAS experiment, contributing to the ATLAS data taking, detector calibration and physics analysis activities, with frequent trips to CERN to participate in shifts, in collaboration meetings and discuss his/her work with other colleagues.