# What exactly is the Higgs boson theory

## The search for the Higgs boson and the origin of mass

In 1964, the British physicist Peter Higgs and colleagues postulated the Higgs boson. Since then, particle physicists worldwide have been on a feverish search for a particle that should explain the existence of mass in our universe.

Ever since experiments at particle accelerators in Europe and the USA (LEP and Tevatron) indirectly found indications for the existence of the Higgs particle, the Higgs search has been a central task of modern particle physics. But why do many particle physicists long for this new particle? What can the Higgs mechanism explain? And how can you finally find what you are looking for at the Large Hadron Collider LHC after long years of unsuccessful searches?

### What actually is “mass”?

In physics we distinguish two types of mass: heavy and inert mass. We recognize heavy masses by the fact that they attract each other. For example, bodies with a heavy mass fall to the ground as they are attracted by the heavy mass of the earth. Inertial mass, on the other hand, expresses itself in the fact that you have to use force to set it in motion or to slow it down. According to Einstein's principle of equivalence, however, the heavy mass and the inertial mass of a body are identical.

The particles in the Standard Model

The observation shows that almost all known elementary particles have an inert mass. For the description of the elementary particles and the forces acting between them, however, it turns out that the consideration of their mass in the equations of motion poses a massive problem: The property mass disturbs the principle of symmetry (the so-called calibration principle) from which these equations of motion can be derived . This principle of symmetry works so wonderfully that all measurements of the forces between elementary particles can be explained with it - so it is difficult to completely give up this concept that works so well. And even if one were willing to do so, taking into account the mass in the equations of motion of the particles at high energies leads to pointless results: The probability that two massive particles interact with one another then increases beyond all limits, i.e. at some point it becomes greater than one - an impossible one Result.

### Way out Higgs mechanism

At first glance, the rescue plan for the calibration principle appears bizarre: The hypothesis is set up that the elementary particles do not have the property “mass” at all. The calibration principle is then saved and all forces between the particles are correctly and consistently described - even at high energies. Now we “only” have to explain why our measurements show that most of the particles behave as if they had mass, although we assumed they were massless.

At this point it is worth looking at an alternative definition of the property “mass”: Since only massless particles move at the speed of light in a vacuum, a particle with mass could be defined as a particle that moves slower in a vacuum than the speed of light.

The trick lies in the restriction “in a vacuum”: The central hypothesis of the Higgs mechanism is that what we experience as “empty space”, that is, as a vacuum, is not empty at all. The entire “empty space”, it is postulated, is filled with a field, the Higgs field. All particles that feel this field change their free movement so that the particles no longer move at the speed of light, but more slowly. Particles that feel the Higgs field behave as if they had the property "mass" although they are actually massless (that is, in a really empty space).

Weight and buoyancy as an analog

In this model, “mass” is no longer a property of a particle, but the result of its interaction with the Higgs field. The stronger the interaction of a particle with the Higgs field, the greater its apparent mass. This can be compared to a stone that appears lighter under water than in air due to its buoyancy. However, the Higgs field does not cause an apparent decrease in weight, but rather an apparent increase in inertial mass. If one now postulates that the different particles have different strengths of interaction with the Higgs field, the different (apparent) masses of the particles can be "explained".

### The price: the Higgs particle

It is not difficult to introduce a field in the theoretical model that changes the empty space in such a way that the particles acquire an apparent mass. However, new contradictions quickly arise from the most naive approaches. The trick was to insert the Higgs field into the theory in such a way that itself the basic calibration principle is sufficient. Because one wants to save this principle.

Constructing the properties of such a field was the accomplishment of Peter Higgs (and others): The Higgs field has a so-called vacuum expectation value. It interacts with the particles in empty space, but still obeys the calibration principle. The price for this Higgs field: It is inevitably linked to the existence of a new elementary particle, the Higgs boson.

The discovery of the Higgs boson would explain the phenomenon “mass” and reveal it as an interaction with the (filled) vacuum. So far, nobody has observed such a Higgs boson - but this does not yet disprove the Higgs mechanism hypothesis. Because the Higgs boson also has a mass that it receives from the interaction with its own vacuum field. Therefore, the Higgs boson could simply be too massive to be detected in previous particle physics experiments.

### First signs?

Unfortunately, the mass of the Higgs boson cannot be accurately predicted. However, from the experiments at the LEP accelerator - one of the predecessors of the LHC - we know that its mass must be greater than 114 GeV / c² (about the mass of a silver atom). Otherwise it should have shown itself there.

But it is possible that the Higgs boson has already left its first traces at LEP. This is because the measurements of particle reactions observed in the LEP experiments were so precise that contributions from a possible Higgs boson had to be taken into account. The measurements are compatible with a Higgs boson lighter than about 200 GeV / c². They represent a convincing first indication of the Higgs boson. Because models that try to get along without the Higgs mechanism have great difficulty in explaining these precision measurements, even though the measurements do of course not yet provide evidence of the Higgs mechanism.

However, we also know that the Higgs boson cannot be heavier than about 700 to 1000 GeV / c², since otherwise it would not solve the problems with the conservation of probability that were already mentioned and thus lose its meaning. At the Large Hadron Collider LHC we have the opportunity to discover the Higgs boson. In contrast to all other previous accelerators, the LHC can search the entire theoretically possible mass range for the Higgs particle. This will either confirm the Higgs mechanism for explaining the phenomenon “mass” or refute it once and for all over the next few years.

### How do you recognize a Higgs boson?

At the LHC, protons are brought to collision at an energy of 7000 GeV (7 TeV). However, since protons are composite objects, the protons are not available as a whole for the creation of new particles. Rather, it is the components of the proton, the quarks and gluons, that represent the actual collision partners. In the vast majority of all cases, known processes will take place during these collisions of quarks and gluons.

If the Higgs boson exists, it is very rare for two gluons to briefly merge to form a Higgs boson. This process happens only once in around ten billion proton-proton collisions. The problem here is not that too few Higgs bosons are being produced - the expected generation rate is one to ten Higgs bosons per second. Rather, the difficulty lies in fishing a Higgs event out of the ten billion non-Higgs events.

So how do you recognize a Higgs boson? Higgs bosons decay practically immediately (after about 10-22 Seconds) back into known particles - in the range preferred by the LEP experiments for the Higgs mass, around eighty percent into the heavy b-quarks. Although b-quarks can be easily identified at the LHC, they are so omnipresent in other processes that b-quarks cannot be isolated from Higgs decays.

Simulation: Higgs decay

The search therefore concentrates on easily identifiable Higgs decays, which, unfortunately, are much rarer. For light Higgs bosons, the decay into a pair of high-energy photons is very promising. Photons can be identified quite well and, above all, both their energy and their direction can be precisely measured. The mass of the hypothetical mother particle can be calculated from this information. If one finds an excess of such photon pairs, which all point to the same mass of a mother particle, then this is a good indication of the production of a new particle. In a similar way, the Z boson was discovered at the CERN SppS accelerator in 1984 when it decays into an electron-positron pair. Unfortunately, such a decay into a photon pair only happens in about one of a thousand Higgs decays, so that one has to collect correspondingly long data.

Further searches exploit the decay of the Higgs boson into a pair of tau leptons. If the Higgs boson is heavier than about 140 GeV / c², the decay into a pair of Z bosons, which in turn can each decay a pair of electrons or muons, offers a reliable signature for the Higgs discovery. The discovery of the Higgs boson will not be quick. It is expected that it will take at least two to three years for the data to be taken until the necessary calibration of the detectors has been completed and sufficient data has been collected. Only then can the Higgs boson be discovered or excluded.

### Alternatives?

Should the Higgs boson not be found, as predicted by the standard model in its simplest form, there is initially the possibility that the “mass-donating” property is distributed over several new Higgs bosons. So a whole family of Higgs bosons could be discovered right away. Many such scenarios have been investigated for the LHC, with the result that in most cases at least one of these new particles can be detected. However, there is no one hundred percent guarantee of discovery in such models. Speculatively, for example, a whole continuum of new Higgs-like states was postulated, a scenario that could only be ruled out (or confirmed) with a new electron-positron linear collider.

If there are no indications at all of Higgs bosons at the LHC, the problem addressed above with the conservation of probability remains. After several years of data collection, the LHC can also investigate how nature regulates this problem.

Even if we do not have any very convincing solutions for a higgsless nature, one thing is clear: The scattering of elementary particles at the highest energies accessible at the LHC will give us important insights into this. In this sense, the "discovery" of the absence of the Higgs mechanism is certainly not only a certain disappointment, but also the exciting opportunity to learn fundamentally new things about microphysics - even if the theory would then certainly have a lot of catching up to do.