This article is based on the blog “ Birth of a new particle" by Jon Butterworth of University College London and the ATLAS collaboration
In making these plots, the ATLAS physicists start with the hypothesis that proton-proton collisions in the LHC can produce Higgs bosons, which in turn decay to other particles. For the animated plot at the far right, they searched through the data they collected, focusing on proton-proton collisions that yielded pairs of high-energy photons, whose energies they measured with calorimeters in the ATLAS detector.
If the two photons were produced by the transformation of a single particle, the mass of that particle can be calculated from the energy of the photons and the angle between their two directions. In most cases, the photons came from other sources, so the mass value forms a smooth "background" – the number of photon pairs expected for a given number of proton-proton collisions at a given energy. The expected background level is shown as the blue line on the plot.
If there is a new particle, more photon pairs than expected appear above the background at a mass corresponding to the particle’s mass. Watch the animation of the plot on the far right here to see a bump eventually emerge out of the background data – the "signal" from a Higgs-like particle in the mass region around 126 GeV. The black dots show the data and reveal how a small but significant effect can emerge as more and more data are analysed.
See more animated plots from the ATLAS collaboration.