About three months ago CERN has announced the discovery of the Higgs boson, a.k.a. the God Particle. Several of my friends have asked me if I, being a physicist, could explain what it was all about. I tried, but it was not easy.

Here is another attempt…:-)

Let me start with an overview.

In the 19th century it became increasingly clear that matter is composed of molecules, and that molecules themselves are composed of atoms. Only a limited number of different atoms exists, ninety occur in nature, quite a few more have been made in laboratories. Imagine the tremendous simplification, everything around us is composed of these building blocks!

The periodic table of elements. Uranium (92) is the heaviest element found in nature. Promethium (61) and Technetium (43) are radioactive and not found in nature, giving a total count of 90 elements occurring naturally.

A monumental breakthrough took place in the 20th century, when it was discovered that atoms themselves consisted of only three (!) elementary particles, protons, neutrons and electrons. Protons and neutrons in the nucleus of the atom with electrons orbiting around this nucleus. Just a matter of numbers. Carbon with 6 protons and 6 neutrons in its nucleus and 6 electrons around this nucleus. Add one of each, and you get Nitrogen, do this again and you get Oxygen.This amazing simplicity was one of the reasons I decided to become a physicist. Even a nuclear physicist..:-)

Actually two more particles had to be added to the list. Light also consists of particles, called photons. And some of the elements are not stable but radioactive, the nucleus can send out an electron and at the same time another particle, called neutrino. Everything controlled by four forces. The strong nuclear force, keeping the protons and neutrons in the nucleus together, the electromagnetic force, keeping the electrons in orbit, the weak nuclear force, responsible for the radioactivity and, finally, the force of gravitation.

But this is not the end of the story, soon it became more complicated again! In cosmic radiation, and also in laboratory experiments (using powerful accelerators to let elementary particles collide), new particles were discovered. Not stable, often only living for split seconds, before decaying in other elementary particles. They were named muons, pions, hyperons, a confusing multitude.

It was discovered that protons and neutrons were actually NOT elementary particles, but that they were composed of “quarks”. Not just one, but several families of quarks. Bound together by “gluons”. And the electron and the neutrino were accompanied by other particle families, the muon electron, the tau electron, with corresponding neutrino’s. The strong nuclear force is actually the force between the quarks, with three quarks forming a proton or a neutron.

It has also been discovered that the elementary forces are carried by “force particles” and that the photon is actually the force particle of the electromagnetic force. One of the big successes of the last decades, was the experimental observation of the “weak nuclear force” carriers, the W and Z bosons.

All these experiments lead to what is now called the Standard Model. Three families of quarks, three families of ‘electrons’, three elementary forces with their force particles. This leaves out until now gravitation. That is actually a big problem, but we will not discuss it here

The Standard Model. Gravity is not taken into account. There are three quark families (up-down), (charm-strange) and (top-bottom). And three “lepton” families, electron, muon and tau, with their corresponding neutrinos. Finally the force particles, photon, gluon and the (W,Z) bosons

All the particles in the picture above have been “observed”. Observed in quotes, because these particles are so short-lived that their existence must be concluded from the traces they leave behind when they die…

Much more complicated than the simple “proton-neutron-electron” model, but definitely one of the most impressive results of modern physics.

One problem remains. All these particles, the quarks, the leptons, the force particles, have mass. Some are heavy like the quarks, some are light like the leptons, the photon has no mass, the neutrinos almost nothing. Why?

In 1964 Higgs and a few others came with a theory. There might exist another force field, permeating the universe, acting as a kind of “syrup”, slowing down other elementary particles and in that way giving them inertia ( = mass!). But if that field existed, it should have its own force particle, the Higgs boson. Nicknamed the God particle, because it gave mass to all the other particles.

The theory was widely accepted, so the search for the Higgs boson was on. A fierce competition resulted between CERN and the Fermilab in USA.

It now looks like it has been found. With a mass about 130 times the mass of the proton. And so short-lived that I have not even been able to find an estimate on the Internet. So you have to look at the traces it leaves behind when it dies. Here is an artist impression.

And here is a picture of the experimental setup at CERN (the ATLAS experiment) Try to spot the human figure in the picture!

This is ATLAS, one of the four experiments at the Large Hadron Collider of CERN.

Is our understanding of the physical universe now complete? No way!

As mentioned before, there is still the problem left to combine the Standard Model with gravitation. Maybe string theory, but not everybody is convinced that this will be the solution.

There is a much bigger challenge. During the last decades it has become convincingly clear that there has to be more in our universe than quarks, electrons, photons, etc. Let’s call this “normal matter”. From what we know about the Big Bang and from the way our universe is expanding after the Big Bang, we now are sure that there are two more constituents of our Universe. There has to be “Dark Matter“, until now invisible. And there has to be “Dark Energy“, a repulsive force that actually accelerates the expansion of the Universe. That’s about all we know at the moment.

A small correction to the Standard Model? Absolutely not! Here is the present estimate. Normal matter takes only 4% (!). The rest is basically unknown at the moment!

We know a lot about the 4% normal matter and next to nothing about the rest!

A sobering thought. But also exciting. There is still a lot to discover and explore in our physical universe. And for the ambitious among you: a lot of Nobel Prizes to win!