Physics Research: Higgs Analysis

The primary goal of the ATLAS Experiment was the search for a particle known as the Higgs Boson. This particle was postulated by our leading theory of sub-atomic physics, the Standard Model (SM), as the reason ordinary matter has mass. In July 4th, 2012, the discovery of a particle consistent with the SM Higgs Boson was announced (link:http://press.web.cern.ch/press-releases/2012/07/cern-experiments-observe-particle-consistent-long-sought-higgs-boson)!  This discovery heralded the end of the Discovery era, and start of the Measurement era at the LHC, where the properties of this newly discovered Boson will be measured with the highest precision. Measurement of the properties of the Higgs boson is a powerful probe into our understanding of the SM and sub-atomic physics, as well as detector performance.

Studies are are being undertaken by graduate students Kate Whalen, James Lacey, Graham Cree, and David Di Valentino to measure the properties of the newly discovered Higgs Boson. These studies analyze the decays of the Higgs into four electrons or muons (via Z bosons), or two photons (via fermion loop).  These studies have focused on (i) measuring backgrounds produced by non-Higgs interactions which mimic the Higgs (KW), (ii) measuring cross sections by searching for specific production modes involving quark jets, called Vector Boson Fusion (JL+DDV) (iii) measuring the mass of the Higgs through models of how the detector responds to individual decay products (GC). An understanding of all aspects of the newly found boson is crucial to determining its role in the sub-atomic nature of matter.

 

P-value plot for 2011 and 2012 data of Higgs decaying in two Z bosons, then four leptons (electrons or muons). The p-value is the probability of having observed the data seen in ATLAS, if there was no Higgs. In this case the lowest point of the p-value is approximately 2*10^-11, or a 2 in 100 billion chance.

Figure 1: P-value plot for 2011 and 2012 data of Higgs decaying in two Z bosons, then four leptons (electrons or muons). The p-value is the probability of having observed the data seen in ATLAS, if there was no Higgs. In this case the lowest point of the p-value is approximately 2*10^-11, or a 2 in 100 billion chance.


P-value plot for 2011 and 2012 data of Higgs decaying into two photons. In this case the p-value is approximately 1 in 10 trillion.

Figure 2: P-value plot for 2011 and 2012 data of Higgs decaying into two photons. In this case the p-value is approximately 1 in 10 trillion.


Vector Boson Fusion event observed in the Higgs to two photon decay channel. The signature of these events is two Jets (brown cones) and two photons (yellow boxes), both detected by their showers in the calorimeters.

Figure 3: Vector Boson Fusion event observed in the Higgs to two photon decay channel. The signature of these events is two Jets (brown cones) and two photons (yellow boxes), both detected by their showers in the calorimeters.