Everything you always wanted to know about the Higgs boson but were afraid to ask…

The God particle, the particle that gives mass to all other particles, the main reason for building the LHC, the missing piece of the Standard Model... Although all this is enough to generate reams of headlines and long articles in the world’s press, it doesn’t explain the true nature of the famous particle at all. The Bulletin discovers the Higgs boson in theory before the LHC does it in practice (if it exists!).

Let’s get the picture straight: assuming that the best accepted physics theories are correct, a yet undiscovered type of ‘ether’ (in physics terms, a ‘field’) permeates the Universe, the vacuum is not empty and the particles acquire their mass by interacting with this field. You may have guessed by now – and you would be right – that the field in question is produced by the star of the particle zoo: the Higgs boson.

Named after one of the scientists who originally proposed the theory, the Higgs boson is thought to have no electric charge, and no spin. As for its mass, unsuccessful searches at LEP and precise measurements of the weak interactions put it in the range of 114 to ~200 GeV. This is true if we stay strictly within the Standard Model: the collection of theories embodying our current understanding of the behaviour of particles. In the presence of new physics beyond the Standard Model, its mass could be as great as ~1000 GeV, but no greater as this limit is set by the fundamental laws of Nature. The Higgs mass range has defined the technical parameters of the LHC, in particular the collision energy and the luminosity. “The LHC parameters and the performance of the detectors are optimized to find the Higgs boson of the Standard Model, as well as whatever may replace it in the task of giving mass to particles”, says Michelangelo Mangano from the Theory group. That is to say, a yet undiscovered particle must exist out there and the LHC will see it. Whether or not this is the Higgs boson with the properties predicted by the corresponding theory remains to be seen.

But why does Nature seem to need such a new particle so much? In Nature there are two fundamental types of particles: left-handed and right-handed (see box for explanation). “Left-handed particles feel the weak interaction – responsible for the observed radioactive phenomena – whereas right-handed particles don’t. Handedness acts as a sort of charge for the weak force, 1 for left-handed, 0 for right-handed”, explains Mangano. Particles that have a mass, however, will be observed as left-handed or right-handed, depending on the reference frame chosen by the observer (see box for details). On the other hand, natural phenomena cannot depend on the specific reference system in which scientists do their calculations, and whether or not particles feel the weak force cannot just be such an ill-defined property. Now you see the problem: where does the weak charge of a left-handed particle (1) disappear to while it is observed as right-handed (0)? Furthermore, how does Nature decide whether or not to assign a mass to particles (therefore causing the particle quandary)?

According to the Higgs mechanism, the property that we measure macroscopically as ‘mass’ is the result, , in more microscopic terms, of a dynamic exchange of quanta between a permanently mass-less particle and the field that permeates the Universe. “A massive particle keeps ‘flipping’ from a left-handed state to a right-handed state, exchanging a weakly-charged Higgs quantum with the ‘ether’. The ‘ether’ stores the weak charge of the left-handed state while it becomes right-handed. In this way the weak charge is always conserved, and no paradox is encountered”, says Mangano.

Whether the Higgs field is an elementary particle as postulated in the Standard Model or some more complex object and whether there is only one or there are more of them, the problem of understanding the existence of massive particles in a left-handed world requires the existence of a new phenomenon. The LHC experiments are expected to provide the final word and solve this enigma. However, be patient… given the complexity of the problem and the rarity of the phenomena that can experimentally reveal the boson’s existence, several years of data-taking and analysis will be needed to sort out the puzzle.

Further reading: The Higgs particle: a useful analogy for physics classrooms.

Box – Right-handed and left-handed particles

In particle physics, the spin (S in the picture below) is a fundamental property of particles, which is represented by a quantum number. The allowed values of S are: 0, 1/2, 1, 3/2, 2, etc. Particles with half-integer spin are known as fermions. Examples of fermions include: electrons, positrons, quarks that make up the protons and neutrons, and neutrinos. Particles with integer spin are known as bosons. Examples include the Higgs boson, the gluon, the photon, etc. Most of the known elementary bosons have spin=1. The exceptions are the Higgs boson, expected to have S=0, and the graviton, expected to have S=2.

The spin of a particle is used to define its handedness: a particle is right-handed if the direction of its spin is the same as the direction of its motion. The particle is left-handed if the directions of spin and motion are opposite.

However, because the direction of motion depends on the reference system, if we take a reference system moving faster than the particle (something that is always possible for massive particles that cannot move at the speed of light), the particle will appear left-handed in this reference frame even if it was right-handed in another system.

by CERN Bulletin