Thursday, September 11, 2008

Large Hadron Collider 101: The Higgs Boson

The Large Hadron Collider has been in the news a lot lately-mainly because some people are worried its going to destroy us by creating an earth swallowing black hole. Now that the collider has started operating-although it hasn't yet actually collided any particles together-the doomsday scenarios ought to be losing some credibility. In any case, all this publicity has lots of people wondering what the Large Hadron Collider is about. So, without getting bogged down by technical details, lets answer that question. In this post we're going to discuss the first item on the agenda at the LHC: the Higgs boson.

All objects you are familiar with have weight. Including yourself. You might get on the scale and find you weigh 140 pounds. But what does that mean? Your weight is determined by two things: the mass of your body and the force of gravity pulling on that mass.

Weight is not a fundamental quantity, because it depends on the force of gravity you are experiencing. Go to the moon, and your weight will be different. This is because the gravitational field of the moon is not the same as the gravitational field of the earth. But there is one thing that hasn't changed: the mass of your body. In a sense the mass of your body is the amount of "stuff" that makes it up. So while your weight might change, your mass does not, unless you go on a vigorous exercise program.

While the mass of your body might change, for fundamental particles mass is an intrinsic, fixed property that never changes. The value of the mass depends on the type of particle we are talking about. Scientists usually measure mass using metric units called Kilograms (kg). The electron, which is a fundamental particle, has a mass of 9.31 * 10^-31 kg. On the other hand, the photon has zero rest mass. Every particle has a characteristic mass that doesn't change.

So what is mass? You can think of mass as a type of charge that determines how a given particle will respond to a gravitational field. We saw that earlier, in that a person with more mass is going to have more weight. In addition, mass determines how a particle behaves as a source of gravitational field. More mass means that the object in question produces a larger gravitational field.

So why is it that particles have the masses that they do? Why does an electron have a certain mass, and a photon no mass?

To find the answer to this question, we can turn to the notion of a field. Think of a magnet and how it has a magnetic field around it. That field interacts with other magnets and changes their behavior. Might there be some kind of field that interacts with particles and gives them mass? Is it gravity?

It turns out gravity does not give particles their mass. Even worse, when physicists figured out how particles interacted with each other, their theories acted as if fundamental particles had no mass. Not just photons, but all particles. This was very perplexing. The theory that describes the known fundamental particles and their interactions is called the standard model.

Then Peter Higgs came along and developed a solution. He proposed that a kind of field existed which filled all of space. Particles interact with the field and acquire mass as a result. As an analogy, think of a swimming pool and waving your arm up and down. When you're not in the pool, you can wave your arm up and down very easily. When you get in the pool and go under water, waving your arm up and down is not nearly as easy. Your arm will move more slowly because of the resistance of the water. Imagine being out of the pool as the universe with no Higgs field. In that case all particles would be massless and would move at the speed of light. Getting under water is like "turning on" the Higgs field. The resistance of the water is akin to a particle interacting with the Higgs field and acquiring some mass.

Different particles interact with the Higgs field in different ways. This is because each type of particle has its own unique characteristics, so they all respond to the Higgs field individually. That is why electrons have mass, but photons don't.

Now bring in the quantum theory. To each field, there is a quantum, or particle manifestation of that field. You already know about quanta. The photon is the quantum of the electromagnetic field. The Higgs field also has a quantum, its called the Higgs Boson.

Interestingly, the Higgs boson interacts with the Higgs field too-so it also has mass. It has a rather large mass so large energies are needed to see it. Think of Einstein's famous equation: E=mc^2. This tells us that with energy E we can create a particle of mass m. But, the speed of light, c is a large number, and since m is multiplied by c^2 we need a very large energy to create a particle of mass m. To create the particle you need to concentrate that energy in a small volume of space.

The Higgs Boson actually has a pretty large mass, so lots of energy will be needed to create one. In fact until now the technology has not existed that would be able to see a Higgs boson. The Large Hadron Collider will be able to do it.

Something to keep in mind: the Higgs Boson is nothing more than a hypothesis at this point, even if its a very good one. We will have to wait until the experimental tests are done over the next year to find out whether or not the Higgs really exists. It might not. In that case other ideas are going to have to be explored. This is actually an exciting possiblity! That is one of the things that makes science interesting. While most physicists do expect the Higgs to be found because there is a lot of good theoretical reasoning to believe it, they'll be holding their breaths when the experiments are actually done.

Next time we'll talk about some other things the Large Hadron Collider will be used to investigate in the next decade.

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