Big Ideas, Small Particles

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The Compact Muon Solenoid, one of the Large Hadron Collider's enormous detectors. It will be utilized in the search for new particles, including the Higgs boson. (image credit: CERN)

Have you ever wondered what you are truly made of? At first glance, the answer is pretty obvious—skin, muscles, bones, and so on. But you can keep going—what is your skin made of? Then what are your cells made of? Eventually you’ll get down to the most fundamental components—things that can’t be broken down any further.

Thanks to particle physics, knowledge about these components has come a long way in the past 100 years. As you may know, all of Earth’s matter—from you, to the food you eat, to the air you breath—is made of molecules, which are in turn made of groups of atoms bonded together. For many years, people believed that atoms were the fundamental particles. Then, physicists discovered that atoms themselves were made of three smaller particles—protons, neutrons, and electrons. We now know that even neutrons and protons have smaller components, called quarks.

So how do we identify and study these particles? And how might new particles be discovered? Put simply, one way is to take some of the smallest particles we can get our hands on and smash them together, breaking them open, so we can study what comes out. The Large Hadron Collider (LHC), which spans the border between France and Switzerland, is the most recent, most extreme version of these smashers. LHC, which officially launched this fall, will be fully operational in early 2009.

At LHC, two beams of protons, moving in opposite directions, will whiz around a race track more than sixteen miles long at speeds of 99.999999% the speed of light, guided by a series of very powerful magnets. When the particles collide, an extreme amount of energy will be released. This energy will quickly condense into particles. Detectors the size of a small building will record a wealth of complicated data from these explosions and the particles that result, which thousands of physicists, including those at Northwestern, will analyze. In a way, the LHC is like a microscope that allows us to observe nature at an almost unimaginably small scale.

Thus far, scientists have identified at least 16 fundamental particles that make up our universe—ones that can’t be broken down, as far as we can tell, into smaller pieces. Only a handful of them make up the matter we knowingly interact with on Earth. Some particles, like neutrinos, are constantly moving through our atmosphere but simply pass right through, not reacting with anything or serving any purpose (that we’re aware of). Others can only be produced as a result of high-energy collisions (like those at LHC) and are very unstable, existing for only an instant before they decay.

We are able to witness and study these particles because the LHC gives us a glimpse at what our universe was like when it was just beginning. According to the “Big Bang” theory, a very, very small—smaller than an atom—and very, very dense mass exploded about 14 billion years ago, releasing all of the energy that makes up our universe. The energy reached by the colliding protons at the LHC will recreate what the universe was like at a very young age, a trillionth of a trillionth of a second after the big bang! This will give researchers incredible insight into the particles and forces that created the universe we live in today.

Physicists predict the incredible capabilities of the LHC will shine some light on a number of big questions. One in particular is just how these particles came to have mass in the first place. We’ve already noted that, from the absolute bottom up, everything on earth is made of these fundamental particles. And, in order to make you (and the food you eat, the ground you walk on, etc.), these particles must have mass—a quantity of matter. For most practical, earthly purposes, mass can be likened to weight.

However, as Einstein famously told us, mass is directly proportional to energy—they are interchangeable, or two sides of the same coin. Specifically, Einstein revealed that the total potential energy of an object is equal to the mass of that object multiplied by the speed of light squared, or E= mc2. Under the right conditions, mass can be converted to energy, and energy can be converted to mass. Pure energy, by definition, travels at the speed of light (light is one form of energy). So, if all of a particle’s mass were converted to energy, the particle would be traveling at the speed of light.

Here’s where things get a little tricky. The standing theory of fundamental particles—quarks, electrons, and so on—predicts that all particles started with zero mass. But, as Einstein showed us, without any mass at all, these particles would be pure energy, constantly traveling at the speed of light. That can’t be right—we know they must have mass. Otherwise, the world as we know it wouldn’t exist.

One theory, proposed by physicist Peter Higgs, offers an idea of how these particles came to have mass. Imagine for a moment that the universe is a swimming pool these particles travel through, and consider this pool to be the “Higgs field.” If you have ever tried to walk through a body of water, you’ll know that it is a lot more difficult than just walking through air. That is because the water provides resistance. Now, instead of this pool being full of water molecules, it is full of fundamental particles called Higgs bosons. These bosons provide resistance, just like the water would, slowing the particles down.

But how do the particles acquire mass? Well, one way mass can be defined is “a resistance to change in motion.” Objects that have mass and are in motion resist a change in that motion (i.e., slowing down). It’s like trying to stop a car rolling down a hill—that car would much prefer to keep rolling. The more the objects resist to slowing down, the more mass they have. For example, it would be much easier to stop a child’s wagon than a car. So, by slowing the particles down, the Higgs bosons “give” the particles mass. Some particles, such as quarks, resist slowing down more than others, like electrons, accounting for their differences in size. Particles that don’t have any mass, like photons (light particles) won’t be slowed by the Higgs bosons at all.

The problem with the Higgs theory is, of all the fundamental particles that have been discovered, the Higgs boson isn’t one of them. Researchers predict that the high-energy particle collisions at LHC will produce evidence that they exist.

Many questions stand to be answered by LHC experiments. We have ideas for how these answers might look—answers involving new particles, and new forces. We need experiments like the LHC to help us choose from among our many ideas, and to tell us which ones are consistent with the experimental results. Or perhaps, nature will even reveal patterns we have never thought of!

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