**Higgs boson**, the missing piece in the

**Standard Model (SM)**puzzle, has finally been announced by the

**ATLAS**and

**CMS**collaborations at

**CERN**.

The (experimental) hunt started in 1989 at CERN with the Large Electron-Positron (LEP) collider and ended in the same place, but with the Large Hadron Collider (LHC), 23 years later.

For those of you interested in details, slides from the presentations given by representatives of the collaborations can be found at http://indico.cern.ch/conferenceDisplay.py?confId=197461

What follows may not be technically entirely correct but it is meant for people not working in particle physics to have an idea of how results are produced. Still a basic understanding of statistics is required.

For consistency, only results from ATLAS will be shown, however the reader must be aware that very similar plots have been released by CMS.

The SM is a very successful theory, tested up to an extraordinary level of precision in a variety of physics processes, which describes the interaction of fundamental particles (gravity excluded). The SM formulation is based on particular symmetries of the space-time. Unfortunately, in order to satisfy these symmetries, all the particles in the theory are required to be massless! This obviously clashes with what we see around, atoms, made of protons, neutrons and electrons clearly have a mass. Therefore, various mechanisms have been hypothesize over the years in order to allow for non-zero masses, or, in other words, for the initial symmetry to be broken.

A particularly elegant mechanism, the Higgs boson mechanism, was introduced by

**Peter Higgs**in 1964. In its simplest form the mechanism can explain the masses of the known elementary particles with a small price to pay in terms of complexity of the theory: the introduction of only one extra, massive, spin 0 particle, the

**Higgs Boson**.

In science, the simplest solution of a problem is often the correct one, hence the Higgs mechanism has been "adopted" by almost all the particle physics community as one of the "mainstream" physics theories to test. This pushed researchers from all over the world to join their efforts and make more and more powerful particle accelerators, to prove the existence of the Higgs boson.

Within the standard model framework, the probability of the Higgs undergoing a decay into certain particles (branching ratios) can be precisely calculated as a function of its mass, as summarised in the following plot

Branching ratios for a SM Higgs decays as a function of its mass. The grey rectangle roughly corresponds to the mass region of the newly discovered resonance. |

^{+}l

^{-})(Z → l

^{+}l

^{-}).

As an example, the invariant mass distribution for the H → (Z → l

^{+}l

^{-})(Z → l

^{+}l

^{-}) search at ATLAS is shown below

The exclusion plots for Higgs searches at ATLAS. |

The dotted line is the estimated 95% CL limit that can be put by on the cross section of the SM Higgs and the green and yellow bands correspond to the 1 and 2 σ uncertainty on this estimation respectively. In short, if that line is below 1, it means that the experiment could, in principle, exclude a SM-like Higgs with that mass. The solid lines is the same as the dotted one but it comes from data and it is strictly related to the number of events seen in a given mass range. The discrepancy at ~125 GeV between the dotted and solid lines means simply that, looking at the data, a SM Higgs cannot be excluded, even if our experiment had the capability to do it if it was not there.

If you find this oversimplified description of the plot non-satisfactory, a more detailed explanation can be found at http://www.atlas.ch/news/2011/simplified-plots.html.

Let's finally explain what a 5 σ discovery means. When a physicist looks at the mass distribution and see a signal-like peak usually asks himself the question: "What is the probability for the background to have a statistical fluctuation as large as the peak I see?". In other words, what is the the probability that I'm just looking to an unfortunate fluctuation of the background instead of a signal?

Neglecting the details of how this probability is calculated, its value and evolution with time is shown in the plot below (y -axis on the left side of the plot) as a function of the Higgs boson mass

P-value, and corresponding sigmas, for a background fluctuation as large as the one seen at different times as a function of the Higgs mass. |

What's next?

Well, again this would require a separate post...

From the practical point of view, the next step after drinking all the bottles of 20 years old champagne and recovering from the hangover, is to measure the properties of the "supposed to be Higgs" particle, such as spin and branching ratios. This kind of study will tell us whether what we see is really a Higgs boson, whether it is really compatible with the SM, or whether we have a strong hint of new physics (by far the most exciting scenario for physicists)...

My colleagues will forgive me for anything wrong I may have written and help me to correct it, right?