FAQ HIGGS BOSON

1. When has the theory about Higgs boson appeared ?

2. Why is this theory so important/interesting  ?

 Shortly after the birth of the Universe in the Big Bang, as the universe expanded the temperature fell below a critical value where a new type of field developed everywhere in the Universe (field, cmp. magnetic field around a magnet. Every point in space has a property: a measurable magnetic force and direction). We call this particular field the Higgs field. Some particles coupled to this field and the property they acquired is what we measure as mass. That is, particles are not solid in themselves but can be seen as a wave on a water surface. Although a wave moves no water from one side of a lake to another, it carries a lot of information: energy, momentum, amplitude, wavelength, etc. For particles mass is just another property acquired by interacting with the ever pervading Higgs field and that property we perceive as mass. Thus as the field developed below the critical temperature some particles acquired mass and some others not. The fact that the mass is "given" to the particles through an external mechanism solved the problems in the theory. You may find it strange that this field suddenly appeared in the Universe but there are several similar physics phenomena around us. A ferromagnet heated to a temperature higher than the "Curie temperature" will lose its magnetization. However, as it is cooled, it will again develop a magnetic field with a specific direction. We call this type of phenomena "spontaneous symmetry breaking". As you might know, interactions between particles is mediated by special types of particles. For instance, the electromagnetic field is carried by the photon. In the same way the Higgs field has its mediator, the Higgs boson.

3. Have we already seen the Higgs boson ?

The fascinating thing is that the theory mentioned above together with the Higgs mechanism is incredibly successful. It has been tested against experiments at extreme conditions for the last 30 years in particle physics experiments and it describes reality with an almost unbelievable precision. However, we have never seen a Higgs boson! You can imagine that confirming the Higgs mechanism is one of the most hot topics! How could we otherwise explain the success of the electroweak theory?

4. Whatís the mass of the Higgs boson ?

One problem is that we can't predict the mass of the Higgs boson itself. This means that we are obliged to scan off bit by bit the possible range for the Higgs boson. Today we have excluded that the Higgs boson is lighter than the total mass of 120 protons! With the new accelerator that we are building at CERN, the Large Hadron Collider (link), we should be able to cover the entire possible range and that's really exciting! If on the other hand we have to really exclude the Higgs mechanism completely, then there must be something entirely unexpected out there!

5. Why a particle should have a mass and what is the mechanism that assigns a mass ?

This is an interesting question for several reasons: first of all there are some particles whose mass is 0. So we want to understand why some particles have mass and others don't. The Higgs boson role in physics is to give a mass to elementary particles. That particles have a mass, is a well known experimental fact. In trying to understand all of these issues, we invented the so-called Higgs mechanism. This mechanism introduces a new particle: the Higgs boson, where "boson" indicates that the intrinsic spin of the particle is an integer; in fact, the spin turns out to be precisely 0, so that often the Higgs is also called the Higgs "scalar", instead of boson. This particle interacts with all particles which are required to have a mass. The interaction is exactly such that the particles start behaving as if they had a mass. You can think of this Higgs as a substance permeating the Universe (we would call this a "field"). When a particle moves through the Higgs field, it interacts with it. This interaction causes a sort of delay in the motion of the particle, as if it were moving through a viscous medium. It is not exactly friction, since friction would bring the particle to a halt, while here the particle does not loose energy. But the Higgs introduces some inertia, namely resistance to the motion. So although the particle itself is massless, interaction with the Higgs field makes it behave as if it had mass (this is, by the way, how the problem with the W bosons mentioned earlier gets solved).  

6. How can we detect the Higgs boson ?

We cannot really detect directly the presence of this underlying Higgs field, since it is uniformly distributed throughout the Universe. We can claim that it is there because we see that particles have masses. But we want a more direct proof to confirm the theory. We want to "see and touch" the Higgs. This could be achieved by concentrating enough energy in a point of space. This energy would disturb the continuous Higgs field, and would generate waves (it is a bit like throwing a stone in a lake, to turn the flat surface into a sequence of waves). These waves can then be detected, and the existence of the Higgs field proved. These waves are associated to the Higgs particle. Since the Higgs particle interacts with its underlying field, the Higgs itself has a mass. To produce a Higgs particle, the amount of energy we need to concentrate is at least equal to the Higgs mass. This number is large, and that's why we still have not seen it.
Going back to the example of the lake, the situation is similar to what follows: if we were immersed in an infinite body of still water, we could not know we are in a medium. We could easily think that we are in the perfect vacuum, the only difference being that if we try to move we experience some resistance (let's call it inertia). If we make a sudden movement, however, with energy large enough, we could generate some underwater wave. The wave is detected because it moves, and if it reaches some other observer imbedded in our lake-universe, he will feel the pressure of it. In the case of the real Higgs, we don't feel any pressure, but detect the decay products of the created Higgs particle. This, after having lived as a wave for a very short amount of time, decays to other particles, which will reach the detectors.  

7. How is Higgs boson likely to manifest itself within a particle collision?

It will not live very long and decay into energy (photons) and other particles, the total energy of which must of course add up to that of the Higgs.  What we will observe is a set of particles coming out of the point where the Higgs decays, and the pattern will be distinctive.  Such a distinctive decay pattern is sometimes referred to as the "signature" of the particle. The width of the Higgs particle should be of the order of GeVís so its lifetime should be of the order of 10^-22 s.