The search for the Higgs boson is one of the most important searches currently taking place at the Large Hadron Collider. The Higgs search is performed by observing all available decay channels. The Higgs was the last remaining particle from the Standard Model to be observed - with the discovery of a new 125 GeV mass particle at the Large Hadron Collider (LHC) announced to great acclaim in 2012, after eluding the physicists around the world for decades. 

The LHC highlight of 2013 was the first evidence of a fermionic decay of the Higgs boson. CoEPP researchers played a decisive role in this area. 

In the post-discovery era, analyses in general need to take into account the presence of the newly discovered state. For searches with sufficiently high resolution of additional states non-degenerate in mass, the strength of the observed state and limits on the signal strength of a potential additional state can be set independently as discussed in the next section. However in some cases— such as when a channel does not have a sufficiently fine mass resolution or when the states are nearly degenerate in mass—specific analyses need to be designed. Diglio pioneered the searches for heavy scalar boson Higgs-like in ATLAS. 

The main focus was directed towards identifying its key features, such as interactions with other particles and charge parity (CP) and parity properties. The ultimate goal of this exploration is to shed light on important details of electroweak symmetry breaking and the origin of mass. The measurements indicate that the discovered particle was indeed a Standard Model Higgs-like boson. This breakthrough experimental result led to the awarding of the 2013 Nobel Prize in Physics to theoreticians Francois Englert and Peter Higgs, who predicted the Higgs mechanism almost 50 years ago. 

(ATLAS event with two muon pairs. Video courtesy of CERN)

The properties of the Higgs boson are unambiguously defined within the Standard Model of particle physics. It is an electrically neutral particle with even parity and zero spin with couplings to other Standard Model particles proportional to their masses. Although current experimental data does not contradict Standard Model predictions, precision measurements of the Higgs are still demanded to compare SM predictions in detail. Current experimental data is not fully capable of addressing other remaining questions and, in fact, poses some new challenges. In this regard, further theoretical exploration of properties of the LHC resonance and their implications, becomes an important task.

One can identify three major directions in this research: (i) study of properties of the LHC resonance, based on already available experimental data by invoking additional theoretical considerations; (ii) more precise calculation of production and decay rates of the Higgs boson at the LHC within the Standard Model and its extensions; (iii) theoretical exploration of various extensions of the Standard Model with the aim to understand the “bigger picture” behind the observed particle. The research of CoEPP theoreticians explores all these major directions and has produced important results - some of which have been obtained in collaboration with oversees colleagues from US, Europe and China.


Fabiola Gianotti announces the ATLAS team's evidence of the Higgs boson at ICHEP 2012.