Thesis

Measurement of the Higgs decay to b-quark pairs in the associated production channel with a W boson at 13/14 TeV with the ATLAS detector at the LHC

• Call:

  IDPASC Portugal - PHD Programme 2014

• Academic Year:

  2014 /2015

• Domain:

  Experimental Particle Physics

• Supervisor:

  José Maneira

• Co-Supervisor:

  Patricia Conde Muino

• Institution:

  Faculdade de Ciências - Universidade de Lisboa

• Host Institution:

  Laboratório de Instrumentação e Física Experimental de Partículas

• Abstract:

  The Standard Model of Particle Physics (SM) is a very successful theory that describes the fundamental constituents of matter and their interactions. One of the missing pieces of the SM that has only very recently started to be probed by experiments is the Electroweak Spontaneous Symmetry Breaking mechanism through which the fundamental particles acquire mass. This mechanism foresees the existence of a new particle, called the Higgs boson. The discovery of a new particle by the ATLAS and CMS experiments at CERN, in July 2012, followed by the first measurements of its properties showing that the new particle was compatible with being a Higgs boson, were important milestones in this quest. The focus of both collaborations, ATLAS and CMS, for the near future will be the precise measurements of the Higgs boson properties in order to test wether there is only one, as predicted by the SM, or more as predicted by beyond the SM theories (like the 2 Higgs Doublet Models or Super-Symmetry). Any significative difference of the measurements with respect to the SM predictions can be a sign of new physics. 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. For the LHC run I, the sensitivity to the fermion couplings (either through the H->tau tau or H->bb decay channels or indirect constraints on the Higgs coupling to the top quark) was very limited. These measurements will be improved with the increased luminosity and higher center of mass energy expected for the run II at 13/14 TeV. The decay channel with the highest branching ratio, H->bb, is at the same time one of the most difficult ones in a hadron collider, due to very large backgrounds, but essential to measure the Higgs to b-quark coupling. The associated production channel 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. The goal of this project is to carry out an improved precision measurement, with the ATLAS/LHC Run II data, of the cross section times branching ratio for the channel where it is produced in association with a W boson, when the Higgs decays to a pair of b-quarks, an essential measurement to extract the coupling of the Higgs boson to the b-quark. The methods to reach that goal will include developing ways to deal with the increased pile-up due to the higher luminosity, and improving the Multi-Variate Analysis technique currently used. This PhD project involves the analysis of the 13/14 TeV pp collisions that ATLAS will collect during the run II (2015-17). In addition, the student will participate in the ATLAS data taking, detector calibration and physics analysis activities, with frequent trips to CERN to participate in shifts and in collaboration meetings.