Interlude: We have a Higgs !/?/*

CERN sent out a press release two days ago announcing the discovery of a new particle! The CERN press release is kind of boring and doesn't have any pictures. The statements from the ATLAS and CMS experiments are exciting and have lots of pictures!

This is my favorite one:

Di-photon (γγ) invariant mass distribution for the CMS data of 2011 and 2012 (black points with error bars). The data are weighted by the signal to background ratio for each sub-category of events. The solid red line shows the fit result for signal plus background; the dashed red line shows only the background.

Here is where I try to explain the graph clearly. It is a histogram, meaning it just counts events. The vertical axis keeps track of how many times an event has happened. The horizontal axis is particle mass*. So, for example, we see about 500 140 GeV particles, and about 1200 110 GeV particles.

What is this particle? We expect the Higgs boson to decay into two photons. The combined energy of the two photons is the same as the energy of the initial Higgs boson, because energy is conserved. This is the signal (S in the plot). We also expect other particle decays to produce two photons. This is the background (B in the plot). This happens very frequently at low energies, and less frequently at higher energies.

If we had just the background, we would expect a smooth decay, but what's going on at 125 GeV? Something is producing lots extra two-photon events! This must be because a new particle is being created at that energy! It produced two photons, so it must be a boson!

Technically, that's all we know so far about the new particle - it's new, it has a mass of about 125 GeV, and it's a boson - but it's very likely the Higgs boson that we've been looking for!

*As I explain in one of the following paragraphs, it's really the combined energy of the two photons but from conservation of energy we know that this must equal the mass of the particle that produced them.