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Discoveries Are Hard to Predict
One of the most maddening questions I'm asked when I give public lectures about the LHC is “So, what are you guys going to find?” Think about that for a minute. The point of the LHC and the experiments based there is to make discoveries. By definition, you can't predict a discovery because, after all, if you knew you would find it, it wouldn't be a discovery, now would it? Just ask Columbus how that trade route to the Indies worked out...
In fact, before we launch into the ideas about things we might discover, let's spend just a few lines talking about the more pedestrian measurements that are planned. We currently have an excellent set of theories that we believe explain the world. These theories, taken together, are called the standard model of particle physics, or just the standard model for short. We know about quarks, which not only make up the familiar proton and neutron, but also make up hundreds of types of particles discovered since the 1940s in earlier accelerator experiments and in cosmic ray studies. There are six types of quarks, with whimsical names: down, up, strange, charm, bottom, and top.[3] Of the six, only up and down quarks are found in ordinary matter and the other four are made in particle accelerators. In addition, we know of a class of particles called leptons, not found inside the nucleus of atoms. The most familiar of the leptons is the electron, but there are cousin particles called muons and taus, as well as three different types of lepton particles called neutrinos. The neutrinos are elusive, interacting so rarely with matter that they could traverse many light years of solid lead (tens or hundreds of trillions of miles), with only a 50% chance of interacting.
[FOOTNOTE 3: I was a member of one of the two teams that co-discovered the top quark.]
We also know of four forces that govern our world: familiar gravity; the electromagnetic force, which holds atoms together; the weak force, which governs some kinds of radioactive decay; and the strong force, which holds the quarks inside protons and neutrons. Of these four forces, all but gravity are well understood at the subatomic level, while an understanding of gravity remains elusive in the quantum realm. Each of these four forces comes with a particle or particles that make it work. These particles are: gravitons (gravity, thus far unobserved), photons (electromagnetism), the gluon (strong force, named because it “glues” the quarks inside the proton), and the W & Z particles (weak).
While all of these particles and forces have been studied at lower-energy particle accelerators, we need to confirm that we see what we expect when we make predictions at the higher LHC energies. If our predictions are not confirmed, this will be very interesting in that it would indicate that we are seeing evidence for something unexpected or, as we physicists call it, “new physics.” If our predictions are confirmed, this will also be welcome news in that it shows that our understanding of the universe is really quite good.
However, for all the successes of the standard model, there are mysteries. We don't know why there are the numbers of quarks there are. We don't understand why particles have mass and why the different particles have such disparate masses. (The top quark has a mass of about 170 times that of a proton, while the photon has no mass at all.) We don't understand why the various forces have such different strengths and ranges. We definitely don't understand how gravity works in the quantum realm. In fact, while we understand a lot (being able to put a man on a moon and communicate almost instantaneously across the globe), we don't really know why the universe follows the rules it does. We hope that the LHC will give us some clues to help figure this out.
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The Higgs Boson
The question that physicists are most optimistic that the LHC will answer is the origin of subatomic particle mass. This sounds like a fairly esoteric subject, but it is something we've been working to clarify for over 40 years. While we talk about there being four forces, physicists in the late 1960s were able to show deep linkages between the electromagnetic and weak forces. They even went so far as to say that the two forces were one and the same and could be called an “electroweak” force.
There was only one teensy-weensy problem. The two forces didn't really look much like one another. The electromagnetic force, which governs electricity, magnetism, light, and the atomic structure of matter, is about a thousand times stronger than the weak force, which governs some forms of radioactive decay. Further, the photon, the particle associated with the electromagnetic force, was measured to have no mass at all, while the particles of the weak force, called the W & Z particles, were extremely massive—about a hundred times heavier than a proton. These two phenomena seemed to be irreconcilably different.
The theoretical physicists of the 1960s made a simplifying assumption and just assigned all particles zero mass. They knew this was wrong, as the particles obviously did have mass, but they also knew that the higher the energy the collision, the less that mass mattered. If at high energy, the mass could be ignored, perhaps at lower energy the effect of mass might be a small perturbation. In any case, it was a good starting point. If they did this, they would be able to generate a new electroweak theory that combined everything from what originally appeared to be two separate phenomena.
On the other hand, no matter what the theoretical physicists assumed, the fact remained that the two forces (electromagnetism and weak) still looked quite different. So either they were just building theoretical castles in the air, or there was something else going on that would reconcile all the conflicting ideas.
In 1964, a Scottish physicist by the name of Peter Higgs brought together a number of ideas floating around at the time and proposed an answer. Suppose that there was an energy field in the universe that interacted differently with different particles. The particles that interacted more with this new hypothetical field would be more massive. If this field interacted more with the weak force particles (the W & Z particles), this would explain their large mass, and if they did not interact at all with the photons of electromagnetism, then the photons would be massless. It's a lot like how a dolphin cuts gracefully through water as compared to a sumo wrestler. The wrestler is held back more by the water—in the field analogy, the wrestler and dolphin interact differently with the “water field.” With the Higgs field, different particles would interact differently with it and, in doing so, get a different mass.
This idea was quite wonderful as it allowed for there to be only a single electroweak force and it allowed for the electromagnetic and weak forces to look quite different. So the question then became “Well, that's a great idea and all, but is it true?” or, more precisely, “Does Higgs’ idea make a prediction that can be validated?” The answer turned out to be yes. The new theory predicted the existence a new particle, technically called the Higgs boson, but often called “The God Particle” in the popular press, after Lederman and Teresi's 1993 book of the same name.
You might be wondering how a new energy field would mean a new particle. In some respects, this is just an argument by analogy. We know a lot about electric fields. Take a latex balloon and rub it on your arm. Then hold the balloon above your arm and you can feel your arm hairs being lifted by the electric field that was created. The electric field appears to be everywhere in the region of your arm and yet we know the field is actually generated by photons jumping back and forth between the balloon and your arm. The individual photon particles “add up” to make the electric field.
This concept is not so obvious to even a scientifically sophisticated reader, so let's think about another familiar example of where something appears to be everywhere and yet consists of tiny particles. Think about a swimming pool.
If you jump into a filled swimming pool, you're going to get wet. It's not possible to somehow jump into the pool and stay dry. While you're in there, scoop out a handful of water and take a close look at it. Everywhere there is water, with no places where there is none. The water is an analogy for an energy field.
While the description of water given above is accurate, you also
know full well that water is made of individual molecules. Take two hydrogen atoms and one of oxygen, combine them in the right environment, and voila!—instant H2O. A cup of water consists of countless individual water molecules that together make up the reality that is a pool full of water.
Similarly, if there is an energy field of the sort postulated by Higgs, it must be made of Higgs particles. The trick is to find one. It is this search for which the LHC was predominantly designed. If Peter Higgs’ idea has any merit at all, the LHC will find a Higgs particle. If it fails to find this particle, then the Higgs idea is dead and it's back to the drawing board. However if such a particle is found, another piece of the tapestry of the standard model will have been unveiled.
Physicists will search for the Higgs boson by searching for all the possible ways by which it might decay. Unfortunately, for many of these decay modes (e.g. the Higgs boson decaying into a pair of bottom quarks), the backgrounds are very high. Backgrounds in a physics context are things that look like the thing you want, but aren't. Think about rooting around in a bowl of cubic zirconia for the lone diamond and you get the idea.
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Figure 2: Expected Higgs boson signal after one year of running with beams at design intensity. The actual signal is the dark bump at 130 GeV, while the lighter gray are events that could be Higgs bosons, but aren't. [Figure courtesy CERN and CMS collaboration.]
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We expect that one of the best ways to find the Higgs boson is to search for its decay into two photons. Figure 2 shows what we expect to see after a year of running at full beam intensity. We will look at selected pairs of photons, make the assumption that they come from the decay of a single particle, and determine the mass of the potential parent particle that made them. If the Higgs boson exists, we expect to see a couple of thousand events that stand out from the much larger background. Of course, the early running of the LHC will be at only 10% of the design luminosity, so it will be a number of years before we know if the Higgs boson exists.
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The Inexplicable Weakness of Gravity
While the question of the origins of particle mass is an interesting one, there are many others. Another very interesting question is “Why is gravity so weak compared to the other forces?” Now you may wonder about this question. Gravity sure seems pretty strong to me. If I trip and fall, the impact with the ground might well be gravity saying, “What do you mean I'm not strong?” Toss in the fact that it is gravity that governs the motion of the heavens and you have a statement that needs defending. How can I claim that gravity is not very big?
So do the following experiment. Find a paperclip and a magnet. Let the magnet pick up the paperclip and hold it, paperclip dangling from the magnet. This is proof that electromagnetism is vastly stronger than gravity. Upwards, you have a modest little magnet lifting the paperclip, while the entire mass of the Earth is pulling the magnet downwards. Little magnet up, entire planet down, and the magnet wins. The magnetic force is much, much stronger than gravity. And this leads us to some interesting questions.
It turns out that the strengths of the three stronger forces differ a lot. The strongest (strong) is about 100,000 times stronger than the weakest (weak); however, gravity is much weaker, about 1040 times weaker. Why this should be so is simply not understood. There are many ideas about this, but one of the most popular involves a term sure to excite science fiction fans: extra dimensions.
In a physics context, we don't mean parallel dimensions. There is no “You mean like the Star Trek episode where Spock had a beard?[4]” involved. The idea is that maybe there are additional dimensions that gravity can enter, but the other forces can't. If that were the case, then perhaps we could explain gravity's relative weakness.
[FOOTNOTE 4: “Mirror, Mirror,” first broadcast October 6, 1967.]
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Figure 3: If the extra dimensions are small, gravity can initially spread into the extra dimension until it is filled up. Here when the small second dimension is filled, the space looks one-dimensional thereafter.
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Let's describe a familiar situation that gets across the most important ideas. Think about playing billiards. Ordinarily, the balls are confined to roll around on the surface of the table. We say that the balls are confined to two dimensions. However, the sound we hear when two balls hit can be heard everywhere, above the table, below it or from any angle. The sound caused by balls moving in two dimensions can be heard in three dimensions. In a similar way, the idea is that the three stronger forces (strong, electromagnetic, and weak) are constrained to three dimensions, while gravity may be able to enter more.
These additional dimensions are not the same as our familiar left/right, up/down, and in/out. These familiar dimensions are infinite and large, meaning that if you jumped in a rocket ship going left, you could go forever. The extra dimensions of gravity would be relatively small—fractions of a millimeter or even smaller. But even these small extra dimensions would be enough to explain why gravity is so weak.
We know that these extra dimensions must be small because of a seventeenth-century concept. Isaac Newton showed that the force between two objects was proportional to the inverse of the distance between them raised to the second power. We have subsequently realized that that exponent (2) must always be one less than the number of dimensions in which we live. Three dimensions means a two in Newton's equation.
We have measured the behavior of gravity down to a distance much smaller than a millimeter and Newton's law holds. So if there is another dimension accessible to gravity, it must be smaller than that. If such small dimensions exist, we can explain gravity's relative weakness. Gravity simply has “more places to go” than the other forces.
Figure 3 shows how the gravitational field would spread out in a universe with two dimensions, one smaller than the other. Suppose that the universe was restricted to the surface of a soda straw. Add a mass to the universe and look at the gravitational field. If you look at the field at distances smaller than the radius of the straw, you see it looks basically like it would in an ordinary plane. But, as the lines of gravitational force diverge, they wrap around the straw and run into one another. To spread out further, they can only travel in the 1D dimension along the straw axis. Thus viewed from small distances, the gravitational field would look two-dimensional, but viewed from larger ones, it would look only one-dimensional. Of course the universe isn't a soda straw, but the essential ideas hold. If the other forces couldn't see the small dimensions, but gravity could, these small extra dimensions could explain gravity's relative weakness.
The idea that not all dimensions are the same is a curious one, but one that isn't so difficult to imagine. Think about the tightrope walker shown in figure 4. She can walk in only one dimension, forward and backwards. However, an ant on the rope can walk in two dimensions. The ant can walk in the (relatively) infinite forward and backward dimension, but it can also walk around the surface of the rope. This second dimension is clearly much smaller than the long one, but it is definitely there and accessible to a small enough observer. The core idea is that gravity can sneak into these smaller additional dimensions. The experimental signature would be that a graviton, the hypothetical particle that causes gravity, would escape into the smaller dimension(s) and be invisible to our detector (which exists in the ordinary three dimensions). Thus events in which energy seemed to disappear would be of special interest and studied carefully to see if they are consistent with the hypothesis of extra dimensions.
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Figure 4: A tightrope walker is constrained to move in one dimension, while the smaller ant can move in two. [Figure courtesy Lawrence Berkeley Laboratory.]
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This gets us back to our black hole and safety question. If it turns out that there really are extra dimensions of the right size, it is possible that the LHC will be able to concentrate enough energy into the right size volume and itty-bitty black holes will be ma
de at the LHC. In fact, if the extra dimension idea is right, there are theories that suggest that lots of little black holes will be made and we will finally be able to get an understanding of gravity at super-small sizes. Of course, the earlier argument about there being no danger still applies (in fact, we'll be able to identify the black holes by the Hawking radiation that evaporates them), but there's just something fascinating about studying microscopic black holes in a laboratory here on Earth.
There are literally thousands of physics topics that will be studied at the LHC and only the briefest sketch of some of the more interesting ones are described here. With over 5,000 physicists sifting through the data the LHC will generate, it is clear that there will be a rich and exciting research program indeed. And of course, the most exciting prospect is that my colleagues and I will discover something entirely unexpected, something that completely changes how we think about the universe. The LHC is, after all, a discovery machine. We will be a bit disappointed if we don't encounter a few head-scratching moments along the way.
The future of particle physics is bright indeed. For the first time in a quarter century, an entire new vista is opening up, promising new knowledge. At the LHC, we will be able to recreate the conditions of the early universe, approaching a scant 10-13 seconds after the Big Bang. It is at facilities like this and others that the mysteries of the universe will be further revealed, explaining the deep linkages between matter and energy, space and time. Until a newer accelerator[5] comes online, the LHC will be the place to do particle physics research for the foreseeable future, making it the only place to explore the quantum frontier.
[FOOTNOTE 5: The planning for such a new accelerator, called the International Linear Collider or ILC, is underway. It will collide beams of electrons and antimatter electrons in a manner similar to the LHC. The turn on date for this facility is at least two decades in the future.]
Analog SFF, July-August 2009 Page 10