The Higgs Boson: a revolution with respect to common thinking

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1. The discovery

The physicists at CERN are certain that they have seen a "new particle" and have measured several of its properties, albeit with large experimental uncertainties. Within these uncertainties, the new particle appears to be the long-sought after "Higgs boson". What is it, what does it imply and why is it so important? What changes in our vision, not just of the physical world but also as from a personal viewpoint?

We will try to answer these questions and to make a comment on the experimental situation.


2. Evolution and revolution in the concept of "mass": a brief history

The concept of "mass" belongs in our daily lives yet for those studying Physics it has always been very intriguing. The fact is that it requires us,as scientists, to go very deep.

In everyday life the concept of mass has for centuries been confused with that of "weight". Newton, with “F = ma”, told us that the mass “m” is something more fundamental and general: it is the constant of proportionality between the force “F” and the acceleration “a” that it communicates to a body: the greater the mass, the greater its inertia to motion is. It is better to be logically rigorous and call it the "inertial mass".

Then we realized that the confusion with the weight comes (and remains for the unacquainted) from the fact that the force that we observe at all times and measure, with a scale, is the attraction “F_g ” from the Earth, linked to the mass by “F_g = GmM/R² ”, where M is the mass of Earth, R the radius of the Earth and G is the "gravitational constant". No wonder there is confusion if one does not know Physics well: only if we were on a spaceship, free from gravity, would we not see it in action.

By the way, stating that G is a constant valid for all bodies (including those heavenly), Newton allowed us to talk about the Law of Universal Gravitation, or, in more current terms, Gravitational Interaction.

To be logically rigorous, a mass defined as above is not conceptually connected to the inertia of the motion: let's use the same symbol m, but let's give it the name of "gravitational mass".

Logical rigor rewards in the end, but also creates deep conceptual dilemmas. After Newton, it took more than two centuries and it was Einstein's Theory of Relativity that solved the dilemma by stating that inertial mass and gravitational mass are manifestations in different phenomena of the same mass m.

With E = mc² , the Theory of Relativity tells us more. It tells us that energy and mass are conceptually the same thing. They are linked only by a constant, c² . Let's leave aside the usual systems of measurement units (cgs or MKS that is), born of practical considerations (still love the British measure length in inches or feet ) and completely arbitrary. We choose, and as it is possible and is done in Elementary Particle Physics, a system where the speed is one of the fundamental physical quantities and take the speed of light c as unit of measurement. The numerical value of c[/i] is 1 and numerically the expression E = mc² becomes E = m. This suggests that E and m can be considered as two different ways of expressing quantitatively the same physical concept.

Please note that, since the mass is an energy dependent quantity, when we now speak of mass as an intrinsic property (the one that you find in books on elementary particles) we mean the so-called "rest mass", ie the value it has when it is not moving.

In a famous tahitian painting by Paul Gauguin (now in the Museum of Fine Arts, Boston) the following sentence appears as the title: "D'où venons-nous? Que sommes-nous? Où allons-nous?”, that is "Where we come from? Who are we? Where are we going?".



Paraphrasing those words, we have seen what the mass is. Now let us turn to the question: where does it come from? To understand this kind of key issue, physicists know what to do: study it in the most simple situation: to see what happens at the level of "elementary particles".

At the level of elementary particle a new revolution begins. You stop seeing the mass as a "static" thing, ie as a property attributed, a priori, regardless of everything else. One begins rather to see it as a "dynamical effect", that is, one begins to relate the mass to the particle's "interactions".

In this framework, the existence of a mass for the elementary particles is not, however, theoretically framed in a simple way, in the absence of "other". The mass should in fact be the result of a "self-interaction" of the same particles, but the laws that govern the interactions between elementary particles do not allow for these self-interactions.

Let us look at how the so-called Standard Theory of Elementary Particles "theoretically" solves this problem by introducing the Higgs boson which, until its discovery at CERN, did not have any direct experimental evidence. This crucial piece was missing. Yet the predictions of the theory had already been verified in detail at the experiments.

In the Standard Theory the particle mass is the result of a continuous interaction with a vacuum that is not really empty but is filled with this "other" (by Higgs bosons).

In Physics the vacuum is defined as the state of minimum energy. The Higgs boson has a peculiar characteristic: its minimum energy state does not correspond to its complete absence. The "emptiness" of the Higgs boson, in fact, corresponds to its slight omnipresence. So the particles can find something to interact with even in the vacuum!

The effect of this interaction turns out to be very deep: a mass is generated for almost all the particles (except for the mediator of the electromagnetic force, the photon, and the mediators of the strong force, the gluons). All this is made possible by the quantum and relativistic nature of the fundamental interactions among elementary particles.



We can understand how the interactions can lead to this effect by following an analogy. It is inspired by how physics phenomena are described in condensed matter physics.

Imagine the following scene: a party, with a number of people that illustrate the "emptiness" of a room. Let us call them Higgs bosons. A stranger does not interact with anyone and crosses the room without having contact with anyone. An important person however, has many contacts and proceeds surrounded by others.

Now let’s express the same effect in terms of Physics. The important person interacts and proceeds in a group with other people and makes "mass" with them. Therefore they are more difficult to stop like a bus compared to a car. So we can describe the observation by saying that as a result of its interactions, the important person acquires a "mass". We did not add anything, we only described what we observed in the language of Physics. As an analogy, it's not bad. For more information, see www.exploratorium.edu/origins/cern/ideas/higgs.html .


3. The Higgs boson

Unavoidably, the picture above involves the existence of the Higgs boson, a new real particle, which can be created in collisions at high energy and observed through the analysis of the products of its decay. Its observation was predicted for the first time by Peter Higgs, hence the name. Incidentally, the bosons are all the particles that have spin angular momentum equal to zero, or an integer times the unit with which it is measured.

As a consequence, the observation of the Higgs boson is necessary to witness the mechanism for generating the masses of the elementary particles as predicted in detail by the Standard Theory.

When we know, in more detail, the properties of the new particle found at CERN we will be able to definitely identify it as the Higgs boson of the current Standard Theory. Otherwise we will need to conceive of an extension to the Standard Theory which describes the observed particle as a novel feature. The discovery at CERN is, in either case, crucial.


4. How the new particle was obseveed?

In the proton-proton collider LHC (Large Hadron Collider) at CERN, protons, which are launched at high energy, are required to rotate in opposite directions around a ring of 27 km in circumference. High intensity magnetic fields produced by an extraordinary system of superconducting magnets, keep them in orbit. The protons are grouped in bunches with about 100 billion protons in a few inches, separated by a 7.5 m wide interval. In some places in the ring the bunches collide. In practice, they cross each other, since they are packets of rarefied gas.

For each bunch crossing about 20 interactions between protons takes place, but this happens in such a short time that the traces of all the numerous particles produced in these interactions are imprinted on to the same image. It seems like an impossible task to disentangle the traces of the particles produced in each interaction as the images are so crowded like those shown. Great precision in the reconstruction of the individual tracks, combined with refined methods of analysis, allows for this miracle.

The energy released in the collisions of two protons is made available for the creation of new particles, by virtue of the equivalence of mass and energy as dictated by the Theory of Relativity. The high energy of the colliding protons can therefore be converted into "new" particles with such a large mass that they have never been seen before experimentally. For many years we have tried to produce the Higgs boson by raising the energy of the collisions higher and higher, but until July 2012 there was no direct indication of its existence.

Let us now try to answer the question: how have the scientists at CERN identified the production of a new particle according to the traces of the usual particles produced in the interactions between protons? For simplicity, we refer to it as the Higgs boson.

The tracks that we see in the images are those made by particles (or anti-particles) that are electrically charged and stable - such as electrons, muons and protons - or those that live long enough - typically pions – to cross a good part of the experimental apparatus before they “decay”, or in other words, disintegrate, giving birth to other particles. The electrically neutral particles - such as photons - do not leave any trace: their production is highlighted by the deposit of their energy in special elements ("calorimeters") of the experimental apparatus which are able to absorb them.

The Higgs boson, due to its very high mass, is highly unstable and decays very rapidly: its very short path may not be detected. Likewise for the new observed particle. But how can we then claim to have "seen it" in the sense of having identified its presence by observing the products of the interactions? In more general terms, the issue moves to the following: how are the unstable particles identified or "seen"?

An unstable particle is identified by searching among the many particles detected and putting together the "daughter" particles generated in the decay and rebuilding it. By measuring the directions and energies of the “daughter particles” it is even possible to determine the mass of the "mother" particle, in our case the Higgs boson.

An unstable particle generally has different "channels" open for its decay even at the first stage of the decay chain (the daughter particles may in turn generate other particles, for decay or interaction). The theory predicts that at the first stage the Higgs boson typically decays to a quark-antiquark, gluon-gluon, tau⁺ tau^-, photon-photon, W⁺ W^-, Z⁰ -Z⁰ or Z⁰ -photon pair. Remember that the W and Z particles are the mediators of the Weak Interactions and that the tau is similar to the electron but has a much larger mass.

In the final stage of the chain, quarks and gluons create "jets" of particles barely distinguishable from those produced in normal interactions. The final states that contain electrons, muons, or photons are rarer but turn out to be more reliably identified.

At the LHC there are two experiments dedicated to the observation of the Higgs boson: ATLAS and CMS. Although their experimental apparatus are almost completely different, a fact which reinforces the discovery, both observed a new particle that decays into two photons. In practice, they have found a number of interactions that are characterized by the presence of two photons coming from the decays in which the mother particle has mass equal to about 125 proton masses, within the experimental uncertainties. Due to the low probability that such events could have come from known processes, this is considered sufficient evidence to declare the existence of a new particle.


5. Higgs boson or unexpected scenarios?

The research into the Higgs boson is just at the beginning. The theoretical and experimental physicists are focusing their efforts to analyze and interpret the vast amount of data collected by ATLAS and CMS. If the new particle is the Higgs boson they must “see” the other decay channels, as well as the observed decay channel into two photons, in the proportions provided by the Standard Theory.

They are now beginning to "see" the new particle also by analyzing the interactions with four and only four charged leptons (electrons or muons) in the final state: in a number of these interactions the four leptons have energies and directions strongly consistent with the hypothesis that they come from the decay of a single particle with the same properties as the one discovered which decays into two photons.

New results are coming. Will they be in line with the discovery of the Higgs boson? Will they open Physics up to unexpected scenarios? Again we paraphrase Gauguin: Where are we going?