Higgs: the beginning of the exploration

Even in the most specialized circles, the new particle discovered in July is not called the “Higgs boson” yet. Physicists still hesitate to give it this name because they want to be sure that its properties fit with those predicted by the Higgs theory. This week, at the HCP conference in Kyoto, CMS and ATLAS presented their latest results: a more refined data analysis has produced a more accurate value of the mass of the particle and has started to show decay channels never before observed.


Since July, both ATLAS and CMS have been working on extending the capabilities of their data analysis. The large teams of physicists have studied in even greater detail all the signals and the information that can be extracted from the data. Following this collective effort, both collaborations have recently presented an update on the mass of the new particle, together with several new measurements which are starting to unveil its properties.

ATLAS has analysed a large sample of 13 fb-1 at 8 TeV energy. The large statistics will allow the collaboration to study physics processes – such as the decays of the new particle into two photons or into four leptons – which drive many of the property measurements. A preliminary observation of new decay channels - such as those of the new boson into two taus or into two b quarks - was reported for the first time in Kyoto. Globally, the results are compatible with the Standard Model Higgs expectations. The updated value of the mass is mX = 126 GeV.

Very similar results were presented by CMS. The larger statistics are giving more precision to the July results. The mass of the new particle measured by CMS is:
mX = 125.8 ± 0.4(stat) ± 0.4(syst) GeV. The signal now has a significance of 6.8 sigmas. The analysis is also showing that the new particle seems to be approaching the characteristics expected for the Higgs boson, and its decays into fermions (such as tau particles) are starting to gain significance. The couplings of the new boson are well within 2 standard deviations from the values predicted by the Standard Model.

The analysis will continue as the experiments move from discovery to a phase of measurements and in-depth studies. The collaborations plan to present updated results based on the larger data sample in the coming months.

by Antonella Del Rosso