LHC Experiments Join Forces to Zoom in on the Higgs Boson

The following news release is being issued today by CERN, the European laboratory that hosts the Large Hadron Collider (LHC). Physicists from the U.S. Department of Energy's Brookhaven National Laboratory play essential roles in the operation of ATLAS, one of the large particle physics experiments at the LHC.

Higgs event enlarge

Event display of a H -> 2e2mu candidate event with m(4l) = 122.6 (123.9) GeV without (with) Z mass constraint. The masses of the lepton pairs are 87.9 GeV and 19.6 GeV. The event was recorded by ATLAS on 18-Jun-2012, 11:07:47 CEST in run number 205113 as event number 12611816. Muon tracks are colored red, electron tracks and clusters in the LAr calorimeter are colored green. The larger inset shows a zoom into the tracking detector. The smaller inset shows a zoom into the vertex region, indicating that the 4 leptons originate from the same primary vertex. Image courtesy CERN.

Geneva—Today during the 50th session of "Rencontres de Moriond" in La Thuile Italy, ATLAS and CMS presented for the first time a combination of their results on the mass of the Higgs boson. The combined mass of the Higgs boson is mH = 125.09 ± 0.24 (0.21 stat. ± 0.11 syst.) GeV, which corresponds to a measurement precision of better than 0.2 percent. The Higgs boson is an essential ingredient of the Standard Model of particle physics, the theory that describes all known elementary particles and their interactions. The Brout-Englert-Higgs mechanism, through which the existence of the Higgs boson was predicted, is believed to give mass to all elementary particles. It is the most precise measurement of the Higgs boson mass yet and among the most precise measurements performed at the LHC to date. 

"Collaboration is really part of our organization's DNA," said CERN Director General Rolf Heuer. "I'm delighted to see so many brilliant physicists from ATLAS and CMS joining forces for the very first time to obtain this important measurement at the LHC." 

The Higgs boson decays into various different particles. For this measurement, results on the two decay channels that best reveal the mass of the Higgs boson have been combined (Higgs boson decaying to two photons and to four leptons, leptons being electron or muon here). Each experiment has found a few hundred events in the Higgs-to-photons channel and a few tens in the Higgs-to-leptons channel, using the data collected at the LHC in 2011 and 2012 at centre-of-mass energies of 7 and 8 TeV, having examined about 4000 trillion proton-proton collisions. The two collaborations worked together and reviewed the analyses and their combination. Experts of the analyses and of the different parts of the detectors that play a major role in this measurement were closely involved. 

"The Higgs Boson was discovered at the LHC in 2012 and the study of its properties has just begun. By sharing efforts between ATLAS and CMS, we are going to understand this fascinating particle in more detail and study its behaviour," said CMS spokesperson Tiziano Camporesi.

"CMS and ATLAS use different detector technologies and different detailed analyses to determine the Higgs mass. The measurements made by the experiments are quite consistent, and we have learnt a lot by working together, which stands us in good stead for further combinations," said ATLAS spokesperson Dave Charlton.

ATLAS detector enlarge

The ATLAS experiment at CERN. Courtesy of CERN.

The Standard Model does not predict the mass of the Higgs boson itself and therefore it must be measured experimentally. However, once supplied with a Higgs mass, the Standard Model does make predictions for all the other properties of the Higgs boson, which can then be tested by the experiments. This mass combination represents the first step towards a combination of other measurements of Higgs boson properties, which will involve also the other decays.

"While we are just getting ready to restart the LHC, it is admirable to notice the precision already achieved by the two experiments and the compatibility of their results. This is very promising for LHC Run 2," said CERN Director of Research Sergio Bertolucci. 

This result was achieved by bringing together physicists of the ATLAS and CMS collaborations, representing together more than 5,000 scientists from over 50 different countries.

Up to now, increasingly precise measurements from the two experiments have established that all observed properties of the Higgs boson, including its spin, parity and interactions with other particles are consistent with the Standard Model Higgs boson.  With the upcoming combination of other Run 1 Higgs results from the two experiments and with higher energy and more collisions to come during LHC Run 2, physicists expect to increase even more the precision of the Higgs boson mass and explore in more detail the particle's properties. During Run 2, they will be able to combine their results promptly and thus increase the LHC's sensitivity to effects that could hint at new physics beyond the Standard Model.

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