Image courtesy of CERN
Di-photon (γγ) invariant mass distribution for the CMS data of 2011 and 2012 (black points with error bars). The solid red line shows the fit result for signal plus background; the dashed red line shows only the background The discrepancy between the two is likely the signature of the Higgs boson.
After more than two decades of intensive search, physicists may have at last found the long sought-after Higgs boson. On July 4, 2012, the ATLAS and CMS experiments at the Large Hadron Collider (LHC) at CERN jointly announced their results to the world. Both experiments observed a new particle in the mass region around 125-126 GeV.
The Higgs boson is the last elementary particle predicted by the Standard Model. Since first postulated in the 1960s, it has been long –sought after. If the new particle proves to be the Higgs, then the mechanism by which elementary particles obtain mass—what physicists call electroweak symmetry breaking—may at last be understood.
The Standard Model of Particle Physics predicts that the Higgs boson, once made, will decay in a number of ways. Some of these decay modes are easier to observe at the LHC than others, in particular the mode in which the Higgs decays to two photons and the mode in which it decays two pairs of electrons or muons or one pair of each. These decay modes are most easily observed because their backgrounds—events that may look like they came from the decay of a Higgs but don’t—are very well understood. (Other modes were investigated as well.) Any events above the number of background events then may be statistically significant and a signature for a new particle. The accompanying figure shows CMS data for the di-photon decay mode. One can easily observe the excess of events around 125-126 GeV. Both ATLAS and CMS have similar plots for other decay modes as well. If the new particle proves to be the Higgs boson then this implies the existence of the Higgs field as the mechanism by which gauge bosons, such as the W and Z bosons, and quarks and charged leptons gain rest mass. The Higgs field has been likened to a giant vat of molasses spread throughout the universe through which particles wade. If a specific elementary particle is heavier than another, then it is more strongly coupled to the Higgs field. The strength of this interaction decides the particles mass. If the new particle is not the Higgs, then some other explanation for electroweak symmetry breaking must be found.
Srini Rajagopalan, BNL
Greg Landsberg, Brown University
Basic research: Office of Science High Energy Physics program
“Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC,” ATLAS Collaboration, Phys. Lett. B 716 1-29 (2012).
“Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC,” CMS Collaboration, Phys. Lett. B 716 30-61 (2012).
University, DOE Laboratory
Non-DOE Interagency Collaboration, International Collaboration