Pedagoguery

On July 4th in Geneva, at the headquarters of CERN, a press conference was held, announcing that the primary mission of the Large Hadron Collider (LHC) had been fulfilled. They had found the Higgs boson, commonly referred to as the “God particle”. What is the significance of this announcement? It all starts with one of the most successful theories in physics, the Standard Model.

The Standard Model is a mathematical model of the fundamental particles in physics. They come in two broad categories: fermions and bosons. What distinguishes one from the other? A quantum mechanical property called “spin”. Like all things in quantum mechanics, spin comes in discrete amounts. Fermions have odd multiples of ½ spin, while bosons have whole number spins. What this means from a practical standpoint is that only one fermion can occupy a single quantum state at a time, while any number of bosons can do so. Fermions come in two types: leptons and quarks. The most common types of leptons are electrons, and the most common types of quarks are the up and down quarks, which make up protons and neutrons. Thus, most normal matter is made up of fermions. Bosons, on the other hand, are force carriers: photons, which carry the electromagnetic force, gluons, which carry the strong nuclear force, and W⁺, W^-, and Z⁰ particles, which carry the weak nuclear force.

The Standard Model has been verified to the highest degree of any physical theory. However, it had one glaring flaw. It predicted that all particles would have zero mass. Clearly, this is not the case, and the mystery of mass is one that particle physicists have been struggling with. Enter Peter Higgs, a British theoretical physicist who postulated a field that existed in all space that would give particles mass when they interacted with it. The stronger the interaction, the greater the mass the particle would gain.

In quantum mechanics, most fields we deal with are vector fields. That is, they have two properties at each point in space: a strength and a direction. If you think of an electric field, for example, an electron would be accelerated toward the positive side of the field with a strength proportional to the strength of the field. However, the Higgs field, as it came to be called, is a scalar field: it has only a strength, no direction. Thus, it would not exert a force on a particle. Interactions with the Higgs field would simply serve to give the particle mass.

In quantum mechanics, any field has a corresponding particle. For electromagnetic fields, for example, that particle is the photon. That particle carries the individual quanta of the field. If the Higgs field existed, it would also have a corresponding particle, the Higgs boson. Theoretical modeling gave the Higgs boson a potential mass of between 114 and 500 GeV. In particle physics, particle masses are typically given as energy. An electron-Volt (eV) is the amount of energy that an electron gains when accelerated in an electric field of one Volt. For comparison, the rest mass of a proton is about 938 MeV (million eV), of just under 1 GeV (billion eV). The most massive particle discovered, the top quark, has a mass of about 171 GeV.

In the time that the LHC has been running, it has been slowly narrowing the range of potential mass, until, in late June, the teams at two of the detectors has a 99.9999% confidence that they had detected a hitherto unknown boson with a mass of about 126 GeV.

The discovery, if it is further confirmed, will be a triumph for the Standard Model. It will help further our understanding of matter. Furthermore, if super symmetric versions of the Standard Model are true, there are possibly four more Higgs bosons out there. The discovery of another one would be a significant step into learning the nature of dark matter.

Next time, the case of the missing galaxies.