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With the tentative discovery of this new particle, the Higgs boson, physicists are excited and the global media are helping to push the buzz of this important discovery. There is a great deal of news and information that states what the Higgs boson is but they do not give much of an explanation of what it actually is. Given the level of abstraction that much of modern physics works in, it is natural for there to be a gap between cutting edge research and simple explanations for the layman. The possible discovery of the Higgs boson is, as Prof Brian Cox says, "so important." So here is my attempt to explain the Higgs in a simple, intuitive way that assumes very little prior knowledge and hence bridge the gap between cutting-edge researchers and the layman.

 

Introduction

There are two lines of thought needed to understand what the Higgs is and how it works: (1) what elementary particles are (with some understanding of how they are detected) and (2) the concept of symmetry breaking. The latter is only necessary for a fuller understanding of how the Higgs particle was discovered from a theoretical point of view and how it plays a role in modern theories of particle physics. That said, it is not completely necessary to understand what symmetry breaking is or how it works in order to see what the Higgs particle is.

I have divided this articles into sections. Each section assumes knowledge of the previous section, but each section can be skipped if the reader comfortably has knowledge that relates to the section title. The first section provides an introduction to atomic matter. Section two looks into the proton.

 

Atomic matter

A intuitive way to understand the elementary particles is to consider a simple glass of water. Most people have heard that water is made up of molecules, they are small "lumps" of matter that are so small that they can't be seen with the naked eye or with ordinary microscopes. From our experience we know that water is wet while sand is dry. Furthermore, sand actually looks like it is made up of small lumps of matter while water does not. The experience of wetness, or dryness, is not a fundamental property of the underlying molecules but is rather a collective effect from many molecules. Wetness is caused by the interaction forces between the water molecules when they move around, it is like a stickiness that comes from an attractive force between the molecules which pulls them together.

Water molecules are made up of 3 particles called atoms, two of these atoms are the same (Hydrogen) while the third one is different (Oxygen). This can be expressed as the chemical formula H20 or as spoken in the vernacular "H-two-O". These three atoms all have an attraction towards each other which is what holds them tightly together to form a molecule. This is precisely what a molecule is, a collection of atoms that are held together. Atoms and molecules are what makes up all of the matter around us from Steel to Coca-Cola to Air. All of the items we are familiar with in the household are made up of different types of atoms and molecules that behave in different ways. In the following diagram I zoom in on a section of water and show what the water molecule looks like. The red particle is the Oxygen atom, while the two white atoms are Hydrogen.

 

While the concept of matter being made up of small particles is an old idea the discovery/ proof of this occurred around the start of the 20th century. Naturally, scientists asked whether atoms were the most fundamental particles or not. Were they the smallest particles that we would ever find, or is there something smaller hiding inside the centre of an atom? Soon after the discovery of matter being composed of atoms came the discovery that atoms have a structure. This structure shows that the atoms have set of particles at their centre called nucleons and that these particles are surrounded by a cloud of other particles called electrons. The nucleons come in two different types: protons and neutrons.

 

Protons

Hydrogen is the simplest atom of all, it is made up of only one nucleon (a proton) and a single electron. The simplest model of Hydrogen depicts the electron orbiting the proton, this line of thinking leads to the orbital model of atoms which is easy to picture but not entirely correct. This model works well, at least, for Hydrogen. Let's ignore the electron and concern ourselves with the proton. One of the first questions to ask is whether the proton is a fundamental particle or whether it is made of even smaller particles. In this diagram of the Hydrogen atom I show its inner structure of a proton at the centre which is orbited by an electron.

Quarks

It turns out that the proton is composed of even smaller particles called quarks. Protons are made up of 3 particular types of quarks: 2 up quarks and 1 down quark. In total there are 6 types of quarks which have been given the names up, down, top, bottom, strange and charm. Sometimes top and bottom are known as truth and beauty. There is also a further 6 anti-quark which are the anti-matter equivalent of the quarks, this means that their properties are a mirror image of each other. A quark might have positive electric charge while an anti-quark would have negative electric charge. Interestingly, the anti-quarks don't have negative mass. In fact, no particle or anti-particle has negative mass: that is to say that gravity is always attractive because mass "charges" are always positive . A digram of a proton showing the 3 quarks (blue, red and green) plus the gluons which are shown as squiggly lines:

Force-carrying Bosons

Just as the atoms in a water molecule are bound together under the electric force, quarks in a proton are also held together by a force. This force is known as the strong nuclear force (or simply strong force). In the electric force there is a mediating, or propagating, particle that is the force carrier. That is to say that there is a particle that "carries" the force from one particle to the other. In the electric force the charge carrying particle is the well known photon. Likewise, in the strong force there is also a particle that acts as a force carrier. Without a force-carrying particle then one particle would not feel the attraction (or repulsion) of any other particle. The force carrier in the strong force is known as the gluon.

You can visualise this by playing with two magnets: each magnet is attracted to the other because particles "jump" from one magnet to the other carrying with them the (electro-)magnetic force. A small caveat here is that the force carriers are not actual photons but virtual photons; unfortunately, I'll have to leave that explanation for another day (... or Wikipedia).

Gluons come in a few different types which have been colour coded as red, blue and green. Naturally, these particles don't have a real colour but this is just a memorable way of differentiating them. As the gluon (should remind you of glue) is force carrier of the strong force then it is what holds quarks together in order to form protons. A pattern should be forming here between force carriers having an associated particle which helps to keep things together (or push things apart). These force carrying particles are known as bosons.

While all force-carrying particles are bosons, not all bosons are force carriers. The term boson is the classification of all particle with a particular type of spin. This property spin is somewhat like a measurement of a spinning top but not exactly the same. The important point is that it is another fundamental property of all particles. Bosons in particular have integer spin, that is to say that their spin is the same as the counting numbers, e.g 0,1,2.

At present we don't know if the force of gravity has a propagating particle, if gravity is like all our other fundamental forces then it should surely have a force-carrying particle. This particle is called the graviton and it belongs to the family of particles known as bosons. Confusingly, the relationship between mass and gravity is not entirely clear. If mass is responsible for gravity, and the Higgs is responsible for mass, then there is surely a relationship between the Higgs boson and gravity. Unfortunately, the graviton is thought to have a spin of 2, while the Higgs has a spin of 0. This suggests that the force of gravity and its relationship to mass may be more complicated than expected.

 

Collisions

A fair question to ask is how do we know that quarks exist? Atoms can be 'seen' using electron microscopes and protons can be detected using cloud chambers but quarks are something else. The discovery of quarks relied upon a different type of machine that essentially collided protons and electrons (technically: hadrons and leptons). This was done at the Stanford Linear Accelerator Center (SLAC) and is somewhat similar nature to the current experiments in the LHC project at CERN. At CERN the current experiment is known as the Large Hadron Collider which is essentially colliding protons into other protons. While the SLAC was a linear accelerator that propelled particles in straight lines the particle accelerator at CERN sends the particles around in a giant ring.

The giant ring uses magnets to keep the particles following the curved path instead of letting them accelerating them in straight lines. The benefit is that this type of accelerator can send the particles around the ring many times and on each pass the particle's speed can be increased. Another way, a particle travelling in a straight line can only go so far while a particle travelling in a circle can be kept going indefinitely. At the LHC, two protons are accelerated in different directions (one clockwise, the other anti-clockwise) and then forced to collide into each other. When the protons collide into each other they reveal what's inside: quarks, gluons and other things come piling out.

This multitude of "things" (particles) is colloquially known as the particle zoo. Whenever they started to collide particles together, such as electrons and protons, they began to find many more particles than they could have ever thought possible. According to wiki there are 61 elementary particles in the standard model of particle physics, all of which are smaller than atoms and would have completely unexpected at the start of the 20th century. This number of 61 includes the Higgs boson but does not include the graviton (if such a particle exists) nor the proposed super-symmetric particles.

So whenever protons are thrown together they release even smaller particles but the exact type of particles released is not necessarily the same every time. In fact, the particles released have a probability of occurring. Some particles are more likely to be released than others due to the energy required to produce some of the rare particles. One of the possibilities is the hypothesized (and hopefully now discovered) Higgs particle. It has been postulated as the particle that provides mass to all other particles.

 

Higgs

Like the force carrying particles mentioned above the Higgs particle is also a boson and, as mentioned, is the proposed particle for providing all other particles with mass. At least that's the idea in simple terms. It is not quite technically true but a close approximation.  A good overview is shown in the video created by Jorge Cham, the creator or PhD webcomics: Higgs video. Provided you have enough energy during a collision then you can make anything you like, this is the way that the Higgs particle was eventually discovered at the LHC.

The energy requirements to detect the Higgs particle has only been possible with the current experiment at CERN: the LHC. It is able to accelerate the protons to a high enough velocity (hence energy) that makes it possible to see the Higgs be "released" from the collisions of protons (recall that protons are composed of quarks). In the diagram below I show what happens when two quarks (q) decay into W or Z bosons (so far unmentioned) which then produces the Higgs boson (H) [picture from wiki].

 

The Higgs particle has no electric charge (see electrons), colour charge (see quarks and gluons) and no spin (zero intrinsic angular momentum). The Higgs boson is also its own anti-particle (aka a majorana particle), it can also interact itself most of the other elementary particles (excluding photons and gluons as they are suspected to be massless). It is the interaction between the Higgs boson and the other particles that give them mass. The following diagram shows the interaction relationships between all of the elementary particles (note that anti-particles follow very similar interactions but are omitted from this diagram) [credit: wiki].

 

The Higgs mechanism

I'm going to couple and paste some lines from wiki here, they have a pretty concise statement of what the mechanism is: " The Higgs mechanism is a kind of mass generation mechanism, a process that gives mass to elementary particles. According to this theory, particles gain mass by interacting with the Higgs field that permeates all space."

That's the simple version. The more advanced, but still concise version is this: "More precisely, the Higgs mechanism endows gauge bosons in a gauge theory with mass through absorption of Nambu–Goldstone bosons arising in spontaneous symmetry breaking."

While wikipedia is concise it doesn't do a great job of explaining how the mechanism works, nor how to picture what it "looks" like. The simple English version is not much better. The big phrase that you will have heard is "symmetry breaking" but you probably have no idea what it means, and for a long time I have to admit I had no real understanding of what it was either. This is deep into the realms of esoteric / abstract physics that only lightly touches upon everyday understanding. The notion of broken symmetry suggests that someone was once symmetric but not it isn't. That's the basic concept but what does it actually mean?

I will start with an analogy and try to create a solid picture of what is going on. This analogy shouldn't be taken entirely literally but rather to provide a better platform for understanding. Imagine a football (a sphere, not the American version). It is highly symmetric: it looks the same all over. You can draw a line down the middle and see that two halves are indeed identical. You can do this with any line that completely circumnavigates the ball. This is intuitively obvious. A cube also have many lines of symmetry where you can cut it in half and have two identical halves. There are of course differences between the two objectives, in terms of symmetry, but that isn't important for this article.

Now let's cut a hole in our football, like a small but randomly cut hole on the surface of the ball. We could say that the symmetry is now broken, the ball can no longer be simply cut in half and for us to easily find mirror symmetry. If you cut the ball in half then you are very unlikely to have two identical pieces. Not impossible, of course, but the main point is not whether you can cut the ball into perfectly symmetric halves but to understand that the number of possibilities has decreased (NB: decreased from infinity, don't dwell on that too long). Perhaps instead of cutting the ball, you could imagine drawing a spot with a pen. The result is pretty much the same.

Ok, so we will have to be a little bit more abstract and consider our football emitting a small "football-particle", where the end result is that our original football has lost some of its symmetry. Almost as if though a small black pen spot has been left on the original football after the smaller one has been emitted. The exact picture is not worth worrying about. The main idea is that symmetry is lost after a particle is emitted. This emitted particles can then interact with other particles. This interaction is what gives mass to the ''other particles'', the mass was "in" the original football but is now transferred to another football.

A key point is that mass is not created but merely transferred. The original football is a Higgs football which we can imagine the entire Universe covered in these footballs. This is the Higgs field. The later footballs that gain mass are a different type of football. An important point to understand is that the Higgs field is not made up of tiny footballs (as you probably guessed), the field is a continuous entity without start or end. It isn't made up of little discrete particles that you can count. The field is essentially still a model and even if the new LHC results confirm its existence it doesn't mean that the picture is complete. The Higgs field is still really a phenomenological fix: it was postulated a mathematical way of fixing a problem that existed in the standard theory of particle physics. It is easier to invent a mathematical field that is infinite in extent and has other special properties, but the reality can be a little bit different. The tiny football that are emitted are Higgs bosons, as noted, they interact with other particles to give them mass.