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The Standard Model of elementary particles represents our knowledge of the microscopic world. It describes the matter constituents (quarks and leptons) and their interactions (mediated by bosons), which are the electromagnetic, the weak, and the strong interactions.

Among all these particles, the Higgs boson still represents a very peculiar case. It is the second heaviest known elementary particle (mass of 125 GeV) after the top quark (175 GeV).

The ideal instrument for measuring the Higgs boson properties is a particle collider. The Large Hadron Collider (LHC), situated nearby Geneva, between France and Switzerland, is the largest proton-proton collider ever built on Earth. It consists of a 27 km circumference ring, where proton beams are smashed at a centre-of-mass energy of 13 TeV (99.999999% of speed of light). At the LHC, 40 Million collisions / second occurs, providing an enormous amount of data. Thanks to these data, ATLAS and CMS experiments discovered the missing piece of the Standard Model, the Higgs boson, in 2012.

During a collision, the energy is so high that protons are "broken" into their fundamental components, i.e. quarks and gluons, which can interact together, producing particles that we don't observe in our everyday life, such as the top quark. The production of a top quark is, by the way, a relatively "rare" phenomenon, since there are other physical processes that occur more often, such as those initiated by strong interaction, producing lighter quarks (such as up, down, strange quarks). In high-energy physics, we speak about the cross-section of a process. We say that the top quark production has a smaller cross-section than one of the productions of light quarks.

The experimental consequence is that distinguishing the decay products of a top quark from a light quark can be extremely difficult, due to the quite larger probability to occur of the latter phenomenon.

Experimental signature of the Higgs boson in a particle detector

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