Sunday, 8 July 2012

The Higgs: What is it, what does it do and have we found it?

Lets start with a joke! A Higgs boson walks into a church, the priest taking the service quickly runs over and says: "We don't allow Higgs bosons in here" and the Higgs says "But without me, how can you have mass?"

Ok, the joke isn't very funny, in fact, it was almost as bad as the Faster-Than-Light-Neutrino joke made back in November.

Now, recently there has been a lot of news to do with the Higgs boson, or the frightfully named "God particle". Sadly, despite the importance of both the idea and the data so far collected, articles seem to be giving a full, complicated explanation or dumbing it down so far as to be nearly fiction.

What I am going to try to do here is to give you a full, easy description of one of the greatest scientific discoveries to happen in decades.

The most simple definition of the Higgs boson is both general and unhelpful, it is the particle that shows the existence of the Higgs field and gives particles mass. Many questions arise from this statement though. What is the Higgs field, what is mass and it does very little to actually tell us what the Higgs really is. To actually understand the whole phenomenon, we are going to have to work through this definition slowly. Firstly, what is mass?

There are a couple of ways to think of mass. The one you will be most familiar with, in a practical setting, is that it defines your weight. Put more scientifically, the larger the mass of an object, the more it is pulled towards the earth. We can tell that a car has a much higher mass than a person, as it is a lot easier to lift a person than to lift a car, or that it is a lot more difficult to stop a car that is rolling down a hill. This leads us onto another, more general and more complete definition for mass. Mass is simply the resistance of an object to a change its movement. That can be shown with an equation that has come up before:
Where F is the force, m is the mass and a is the acceleration.
The equation shows that the higher the mass of an object, the larger the force (a push or pull) that has to  be applied to it to make it accelerate even a little. This can be shown again if you try to push a car or if you try to push a person on a bike. The Higgs does not give mass to macro systems like a car, instead it is only relevant at tiny levels, giving mass to the tiny particles that make up the big object. This eventually adds up to the total mass.

Now we get to the crux of the matter. How does the Higgs give mass to the particles? Well I have heard a few good analogies for this one, I think I am going to use one I read a while back in a newspaper. Lets imagine a party. The party is quite big, there are a lot of people in one room, but they aren't squashed together:
Some of them are wearing party hats. It isn't relevant to the analogy, but what is a party without party hats?
As well as the people, there is a drinks table at the opposite end of the room from the door. Every person who enters the room really wants a drink and is trying to get to the drinks table.
Now, lets say a person that knows none of those attending comes into the room. They can walk all the way across the room as fast as they want. In fact, the only thing limiting their speed is their own ability.

Now, what about if someone who is friends with a few of the people in the room comes in?


A few of their friends clump around them, making it quite difficult for them to move across the room. They are slowed by the sheer number of people around them. They can still move fast, but it takes a lot more effort to do so.

Ok, lets have one more situation. What if a celebrity walks into the room?


They quickly get crowded by nearly all of the people in the room and it is very difficult for them to move across the room without exerting a large amount of effort.

Now, lets put this into actual physics terms. We can think of the party as the Higgs field. Put simply, a field is a place where a certain thing can take effect. In this case, the Higgs field, or the party is through the whole of the universe. The people originally in the room are the Higgs bosons. They are part of the Higgs field, and are the particles that actually give everything else mass. The first person that walked in is a particle like a photon. It doesn't react with any of the Higgs bosons (or the Higgs field) and therefore can move at the fastest speed possible. In the universe, this is the speed of light (about 300 million meters per second). Its mass is zero. The second particle could be an electron. It interacts quite a bit, but not too much. It can move quite fast, and its mass is quite small. Now the celebrity could be a quark. Quarks are some of the heaviest particles. They find it very difficult to move fast. The difference in mass of an electron and the heaviest quark is like comparing the weight of a 10 year old boy to that of a fully loaded Boeing 747 jet.

Now we know what the Higgs does, what about how they found it? Well last Wednesday, CERN, the European Institute for Nuclear Research, put out a press release after an announcement they made, saying that they had discovered a fluctuation in their data:

This fluctuation seemed to suggest, with little uncertainty, that there was a new particle they had discovered. Before we get to that though, lets first delve into how CERN got these readings.
It was one particular facility that led to this discovery. A huge network of tunnels underneath Geneva, called the Large Hadron Collider (LHD) for reasons we will discuss in a minute:

The whole group of experiments relies on one small, beautiful and very famous formula.
Where E is energy, m is mass and c is the speed of light.
When we look at mass in this formula, we now have to stop thinking about the Higgs, otherwise it just gets too complicated. Instead we are going to look at the basic implication of this equation. All it says is that mass and energy are interchangeable. We see mass being converted to energy in stars. It also implies that the opposite is true. Energy can become mass. In fact, this fact is so important that we don't measure the mass of a particle like we normally do, but instead we measure it in units of energy, electron volts, usually shortened to eV. However, this isn't too important.

Lets go back to converting energy to mass. It is theoretically possible, however, due to the very large square of the speed of light you need a very large amount of energy to create a small amount of mass. However, because particles like the Higgs (which does itself have mass) don't form in an observable situation normally, sometimes there is only one way to nudge it into a detector. Groups of protons, a type of particle, are shot around the LHC (the large circular tube we discussed a minute ago), one group clockwise and the other anti-clockwise. Eventually, when they are moving fast enough, they are brought together to collide in detectors.

The two main detectors are called ATLAS and CMS. Suddenly, there is a huge amount of energy in this small space and a large amount of strange particles are created. The detectors look to see if some energy may be missing, suggesting an undetectable particle has been created or there has been another strange occurrence. The detectors also look for the abundance or lack of some particles. You see, a Higgs, even when formed in this manner, is not around for long. In fact, it disappears before it is detected. Instead, the detectors look for the particles that could be created when a Higgs disappears and something formed in its place. The particles that come out of a collision can be changed by changing the energy of the colliding particles.

And here is where it starts to come together. The CMS and ATLAS detectors have, over the course of around 4 years, found that at the energy level of 125 Gev (giga-electron volts), there is particularly large amounts of certain particles produced. In fact, they have given it a 5-sigma rating. The sigma rating shows how certain they are about the discovery, 1-sigma is just expected deviation, 3-sigma is an observation and 5-sigma is a discovery. However, we can't be too rushed here. They have found something, and it seems to act like they expect the Higgs to act. However, we cannot confirm whether it is the Higgs or actually something else. Still, we live in an incredible age and no matter what this particle is, it will bring on even more brilliant and fascinating physics. I am personally on the edge of my seat for any more announcements that will be made in the immediate future and beyond!

This subject is huge. Here are a few links to help you if you want more information:
Another simple guide to the Higgs
A great, in depth and detail article on the Higgs, the implications of the find (which I could not go into) and the standard model
An article about the methods and maths behind finding the Higgs
More information on the actual experiment
A brief summery of everything to do with the Higgs
An article on the experiments finding the Higgs and similar experiments in the past
A CERN video about the search itself
A CERN video about what the Higgs is

We hope you enjoyed this post. The current situation we are in is fantastic and I hope you will stick with us to learn more every week!

Ned Summers

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Check out our last two posts:
The Physics of Field Athletics: Hammer Throw, Angular Momentum and what if everyone in the world spun around at the same time? - Why does a hammer-thrower spin before they throw?
Maths of the Heptathlon: Why the scoring system is flawed? - How is the Heptathlon biased towards athletes that are good at throwing? And how can we fix the system?

What are we posting about next:
The Higgs: Why do we need it, why were we looking for it in the first place and what do we do now? - Now you know what the Higgs does, now you can find out why we wanted to find it!

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