ATLAS and CMS experiments shed light on Higgs properties
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.
All of the measured properties are in agreement with the predictions of the Standard Model and will become the reference for new analyses in the coming months, enabling the search for new physics phenomena. This follows the best measurement of the mass of the Higgs boson, published in May 2015 after a combined analysis by the two collaborations.
"The Higgs boson is a fantastic new tool to test the Standard Model of particle physics and study the Brout-Englert-Higgs mechanism that gives mass to elementary particles," said CERN Director General Rolf Heuer. "There is much benefit in combining the results of large experiments to reach the high precision needed for the next breakthrough in our field. By doing so, we achieve what for a single experiment, would have meant running for at least two more years." There are different ways to produce a Higgs boson, and different ways for a Higgs boson to decay to other particles. For example, according to the Standard Model, the theory that describes best forces and particles, when a Higgs boson is produced, it should decay immediately in about 58 percent of cases into a bottom quark and a bottom antiquark. By combining their results, ATLAS and CMS determined with the best precision to date the rates of the most common decays.
Such precision measurements of decay rates are crucially important as they are directly linked to the strength of the interaction of the Higgs particle with other elementary particles, as well as to their masses. Therefore, the study of its decays is essential in determining the nature of the discovered boson. Any deviation in the measured rates compared to those predicted by the Standard Model would bring into question the Brout-Englert-Higgs mechanism and possibly open the door to new physics beyond the Standard Model. "This is a big step forward, both for the mechanics of the combinations and in our measurement precision," said ATLAS Spokesperson Dave Charlton. "As an example, from the combined results the decay of the Higgs boson to tau particles is now observed with more than 5 sigma significance, which was not possible from CMS or ATLAS alone."
"Combining results from two large experiments was a real challenge as such analysis involves over 4200 parameters that represent systematic uncertainties," said CMS Spokesperson Tiziano Camporesi. "With such a result and the flow of new data at the new energy level at the LHC, we are in a good position to look at the Higgs boson from every possible angle."
Nikhef's contribution
Nikhef is a member of the ATLAS experiment, one of the detectors at the Large Hadron Collider (LHC) at CERN in Geneva. Wouter Verkerke is joint programme leader of the Dutch contribution to ATLAS: "This new result from ATLAS and CMS demonstrates that the recently discovered Higgs particle provides a fantastic new laboratory for research into the fundamental properties of nature: which mechanism explains the origin of the mass of elementary particles? This result consists of dozens of different measurements from two experiments that have now been combined and interpreted in a consistent manner. This has never been done before on such a scale and with so much eye for detail and it offers a wealth of information. In this way the Higgs particle is providing us with a 'look under the bonnet' of nature."
One of the driving forces behind this ATLAS/CMS combination at ATLAS is Nikhef researcher Stefan Gadatsch who gained his doctorate cum laude in June for research that included the ATLAS input of the aforementioned results. And Wouter Verkerke is the convener of the ATLAS-Higgs combination subgroup that coordinates the ATLAS side of this result. The foundation of the mechanics of the combination that Dave Charlton talks about in the press release is the RooFit framework that Verkerke has developed and elaborated over the past 15 years and which is used by both ATLAS and CMS for the statistical analysis.
Further information
For further information please see the CERN website or Nikhef website.
Science Communication Department Nikhef, +31 (0)20 592 50 75 or Wouter Verkerke, +31 (0)20 592 51 34