Higgs boson: ten years of the discovery of the century (so far)

Exactly 10 years ago, the European Laboratory for Particle Physics (CERN) announced the discovery of the Higgs boson.

The news had an unprecedented scientific and media impact on a subject of fundamental physics.

With the perspective of a decade, we can ask ourselves: Was all the fuss justified? What has the Higgs boson taught us about the universe?

And even more: what is it that we still do not understand?

What are the hottest questions in basic physics?

Going back to the beginning, the fuss of 10 years ago was fully justified.

Without a doubt, the discovery of the Higgs boson is the most important advance in recent decades in our in-depth knowledge of nature.

We could place it in line with the great scientific milestones of the last century, such as the understanding of the atomic structure of matter or the discovery of the traces of the Big Bang in the universe.

Why is the Higgs boson so important?

First of all, it sheds light on very basic and familiar concepts.

So familiar that we often don't think about them.

For example, we are all used to electric and magnetic forces.

Now, why are there electric forces? Why are they the way they are?

If we stop to think about it, it is a mysterious fact.

The Higgs boson helps us to understand (in part) this "fact of life", crucial for our existence.

The Higgs boson also sheds light on other concepts, such as the vacuum (is the vacuum really empty?) or weak interactions.

The latter are not as familiar as the electromagnetic ones, but they are just as fundamental, and the ones responsible for the Sun shining and for life to exist on Earth.

And our boson, now 10 years old, plays an absolutely crucial role in understanding them.

But above all, the Higgs sheds light on the notion of mass.

but they are just as fundamental, and the ones responsible for the Sun to shine and for life to exist on Earth.

And our boson, now 10 years old, plays an absolutely crucial role in understanding them.

But above all, the Higgs sheds light on the notion of mass.

but they are just as fundamental, and the ones responsible for the Sun to shine and for life to exist on Earth.

And our boson, now 10 years old, plays an absolutely crucial role in understanding them.

But above all, the Higgs sheds light on the notion of mass.

The mass is one of those concepts so everyday that we do not ask ourselves about its reason for being.

It just so happens that objects have mass.

But why do they have it?

Let us note at the outset that most of the mass of an object resides in its atomic nuclei, made up of protons and neutrons, whose mass comes almost entirely from the strong interactions that simmer inside them.

Here it is necessary to clarify that the protons and neutrons are not really elementary, but are composed of other particles called quarks.

The interior of a proton or neutron is a tiny world, very complex and full of energy, which is mainly responsible for its mass.

In contrast, the electrons in the atomic shell appear to be really elementary particles, with no internal structure.

Certainly its mass is small compared to that of the atomic nucleus,

but where does that mass come from?

As far as we know, the group of truly elementary matter particles is very small: 12 particles, among which are electrons, neutrinos and quarks.

Understanding the origin of its mass is an absolutely fundamental problem.

One might think that, after all, the mass of the electron contributes so little to the mass of things that it hardly matters.

Quite the contrary: if electrons had no mass they would travel at the speed of light (like photons) and could not be captured by atomic nuclei.

So there would be no atoms, no molecules, and of course no life.

But from a conceptual point of view, the mass of the electrons (and of the rest of the elementary particles) is an even more transcendental question.

To understand it, let's think about the following.

Everything we know about elementary particles is contained in a superb theory called the Standard Model.

So far, no observations have been found to contradict this theory, and there are thousands (even millions) endorsing it every day.

For its own internal consistency,

the Standard Model requires certain symmetries, that is, regularities of its equations.

At first glance, the mass of the particles disastrously breaks these symmetries, rendering the theory inconsistent and useless.

To be able to rescue it, it was necessary to imagine a mechanism capable of giving mass to the particles without breaking the symmetry.

This is the Higgs mechanism, formulated initially by Robert Brout and François Englert, and a few weeks later by Peter Higgs, in 1964.

Without going into technicalities, this mechanism postulates the existence of a field, H, called the Higgs field, which fills the entire universe homogeneously (perhaps it reminds you of the ether, and rightly so).

If the H field did not exist, the particles would have no mass, as the symmetries of the Standard Model seem to require, but it turns out that the particles interact with this omnipresent field, and that interaction, similar to the friction of a ball moving in a liquid , produces exactly the same effect as if the particle had mass “by itself”.

If the theory is correct, it would be possible to excite such an H field (the equivalent of stirring a liquid at rest).

And those elementary excitations are the Higgs bosons (yes, these were first predicted by Peter Higgs).

To verify this prediction, it is necessary to “stir the vacuum”, which was achieved 10 years ago by colliding protons at fabulous energies inside the LHC, the ring-shaped particle accelerator and collider located at CERN.

This involved the collaboration of thousands of scientists from many countries over two decades;

a truly global company with the romantic goal of gaining a better understanding of nature's “way of thinking”, its deepest ins and outs.

The existence of the Higgs boson is undoubtedly the most crucial prediction of the Standard Model, as well as giving us a new perspective on nature: the vacuum is not empty, it contains a mysterious field thanks to which we can exist.

It must be said that throughout these 10 years the properties of the Higgs boson have continued to be investigated,

that we know very well today.

In particular, it has been found that the boson interacts with the rest of the elementary particles with a force proportional to their mass, as predicted by the Higgs mechanism.

This finding in itself is of extraordinary importance: for the first time we are seeing new fundamental forces (beyond gravitational, electromagnetic, strong and weak interactions).

The origin of these new forces is totally unknown.

We know how to formulate them, but not their origin.

for the first time we are seeing new fundamental forces (beyond gravitational, electromagnetic, strong and weak interactions).

The origin of these new forces is totally unknown.

We know how to formulate them, but not their origin.

for the first time we are seeing new fundamental forces (beyond gravitational, electromagnetic, strong and weak interactions).

The origin of these new forces is totally unknown.

We know how to formulate them, but not their origin.

The Higgs boson has taught us many things, but the deepest mysteries remain to be understood and are the source of inspiration for physicists and particle physicists around the world.

Some of these mysteries are directly connected to the Higgs boson.

For example, why does the electron have the mass it does, and why are other elementary particles (such as the top quark) so much heavier?

No doubt it is because its interaction with the Higgs field is smaller, but why it is smaller, no one knows.

Furthermore, there are facts that clearly indicate to us that the Standard Model cannot be the last word for understanding the universe.

Perhaps the most important is the existence of dark matter, a mysterious substance that envelops galaxies and extends beyond them.

This matter is six times more abundant than ordinary matter;

that is, the one that is (magnificently) described by the Standard Model.

To understand it we must go beyond the Standard Model.

We also do not understand the origin of the matter-antimatter asymmetry in the universe or how to make gravity consistent with quantum mechanics.

These pending questions are investigated daily by theoretical and experimental researchers around the world.

Perhaps in the next few years the LHC at CERN (or perhaps another experiment) will give us a pleasant surprise and reveal this new physics.

That news will be even more important than the discovery that we are commemorating today, and that for the moment we can qualify as the discovery of the century.

To understand it we must go beyond the Standard Model.

We also do not understand the origin of the matter-antimatter asymmetry in the universe or how to make gravity consistent with quantum mechanics.

These pending questions are investigated daily by theoretical and experimental researchers around the world.

Perhaps in the next few years the LHC at CERN (or perhaps another experiment) will give us a pleasant surprise and reveal this new physics.

That news will be even more important than the discovery that we are commemorating today, and that for the moment we can qualify as the discovery of the century.

To understand it we must go beyond the Standard Model.

We also do not understand the origin of the matter-antimatter asymmetry in the universe or how to make gravity consistent with quantum mechanics.

These pending questions are investigated daily by theoretical and experimental researchers around the world.

Perhaps in the next few years the LHC at CERN (or perhaps another experiment) will give us a pleasant surprise and reveal this new physics.

That news will be even more important than the discovery that we are commemorating today, and that for the moment we can qualify as the discovery of the century.

These pending questions are investigated daily by theoretical and experimental researchers around the world.

Perhaps in the next few years the LHC at CERN (or perhaps another experiment) will give us a pleasant surprise and reveal this new physics.

That news will be even more important than the discovery that we are commemorating today, and that for the moment we can qualify as the discovery of the century.

These pending questions are investigated daily by theoretical and experimental researchers around the world.

Perhaps in the next few years the LHC at CERN (or perhaps another experiment) will give us a pleasant surprise and reveal this new physics.

That news will be even more important than the discovery that we are commemorating today, and that for the moment we can qualify as the discovery of the century.

I would like to end with a memory for Teresa Rodrigo, a great experimental physicist who recently passed away and chair of the scientific committee of CMS: one of the two LHC experiments that made this momentous discovery.

Alberto Casas

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