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ATLAS and CMS experiments shed light on Higgs properties

September 04, 2015
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Three years after the announcement of the discovery of a new particle, the so-called Higgs boson, the ATLAS and CMS Collaborations present for the first time combined measurements of many of its properties, at the third annual Large Hadron Collider Physics Conference (LHCP 2015). By combining their analyses of the data collected in 2011 and 2012, ATLAS and CMS draw the sharpest picture yet of this novel boson. The new results provide in particular the best precision on its production and decay and on how it interacts with other particles. The Standard Model of  particle physics predicts a coupling strength of the Higgs boson to all other particles proportional to their mass – a pattern, shown in the Figure above, that is consistent with the measurements from ATLAS and CMS.

The SFU Higgs physics analysis efforts (groups of O’Neil, Stelzer and Vetterli) have centered on the Higgs decay into tau particles and W bosons with a focus on multivariate analysis techniques published for the tau (JHEP 04 (2015) 117) and WW (PRD 92, 012006 (2015)) analyses earlier this year. With the new combined results from ATLAS and CMS, the decay of the Higgs boson to tau particles is now observed with more than 5 sigma significance. Similarly, the production of the Higgs boson through fusion of W bosons is now observed with more than 5 sigma significance. All of the measured properties are in agreement with the predictions of the Standard Model and are entering a new level of precision enabling the search for new physics phenomena during the 2nd LHC run at higher energy, which is ongoing since June 2015.

Measurement of the Higgs boson coupling-strength to fermions (matter particles) and vector bosons (force mediating particles) using data from the ATLAS and CMS experiment. The combined result is the most precise to date. The star indicates the prediction from the Standard Model of particle physics.


SFU physicists are also active in testing and building new components of ATLAS to maintain and enhance the discovery potential of the experiment during the scheduled LHC operation at much higher collision rates in the year 2020 and beyond. The Canadian Foundation for Innovation has recently supported a consortium from Carleton, McGill, SFU and TRIUMF to replace the present first muon detector station of ATLAS in the forward regions with the so-called New Small Wheels (NSWs). The NSWs are fast, high precision muon tracking detectors with a diameter of approximately 9m (“small” compared to the ATLAS Big Wheels that span about 22m). SFU researchers, together with collaborators from Canada and Israel have successfully tested the first full sized prototype NSW chambers at a beam test experiment at Fermilab.

Muon spectrometer in the forward region of the ATLAS experiment showing the present muon small wheel. (Bottom) Test of the first full size New Small Wheel detector component by Canadian and Israeli collaborators at Fermilab.