EXTRA SPACE NEEDED?

Scientists in the DØ (D-Zero) experiment at Fermilab have been using the world’s highest energy particle accelerator to see if any of the particles produced in collisions between protons and antiprotons, meeting head-on at almost the speed of light, can quite literally leave the known universe. They have been checking some revolutionary new suggestions from theoretical physicists who propose that some kinds of matter or forces could exist outside our familiar (three dimensional) space, travelling into “extra” invisible dimensions.

There is a relatively simple way to search for this. We have known for hundreds of years that momentum is balanced. This is still true at the particle level. When you fire a bullet from a gun, the gun recoils. Similarly, in the high energy proton-antiproton collisions at Fermilab, if something (like a jet of particles) shoots out to the left, we expect something else to come out to the right and balance its momentum. If we see nothing balancing in this way, there are three possibilities:

• The balancing momentum was carried off by a mundane normal particle, but our detection apparatus missed it somehow. For example, it could have passed through a crack between two detector pieces or through a support structure.
• The balancing momentum was carried off by a known particle called a neutrino, which interacts so weakly with normal matter that it can pass right through the detector. Neutrino properties are well-known and we can calculate how often they are produced.
• The balancing momentum was carried off by something new, for example something escaping from three dimensional space. This is what we want to look for.

In the late 1970’s, the first high energy collider was built at CERN in Switzerland, and achieved great success in discovering the W and Z particles that are responsible for radioactive decay. A year or two after that discovery, one of the teams working at CERN started to believe that they were seeing “monojets” ¾ a name given to the kind of unbalanced event where something seems to come out one side and nothing on they other. The excitement was so great that they even announced that they had seen unambiguous evidence for the production of a new kind of particle. Unfortunately, this “discovery” was not confirmed by a more cautious group working independently at the same accelerator, and it later turned out that they had underestimated the many ways in which well-known physics processes, both from missed particles and neutrinos, could give rise to what looked like a discovery. This bad experience showed how easy is to make a mistake in accounting for monojets, even with the best detector and the best team working on it.

Fast forward to 1998 and enter Nima Arkani-Hamed, Savas Dimopoulos and Georgi Dvali. These three theoretical physicists came up with one of the most exciting new ideas in recent decades. They proposed that the universe could be a much bigger place than we think, with our familiar three dimensions being only part of a five or more dimensional universe. These extra dimensions would be invisible to us because the particles that we are made of (and the world around us too) are trapped in only three dimensions. This new idea offered an explanation for why gravity is roughly 100,000,000,000,000,000,000,000,000,000,000,000,000 (a hundred billion billion billion billions) times weaker than other forces of Nature, like the electric and nuclear forces. In this new paradigm the answer is simple – gravity is just as strong as other forces, but appears very weak to us because gravity propagates through the entire space, including extra dimensions, while all other particles, including ourselves, are stuck to the three-dimensional world.

Figure 1. Artists view of a monojet event produced in a proton-antiproton collision. The jet is recoiling against the graviton that escapes in the extra space. Our entire three-dimensional world gains an extra momentum in extra space to conserve total momentum.

Now an idea like this is intriguing, but there needs to be some way that we can “test” it, by seeing if it makes a prediction for what we should see in experiments.

One of the exciting predictions of this theory of extra dimensions was that one should see plentiful monojets in proton-antiproton collisions. The “missing” momentum would be carried off into the extra space by a quantum particle of gravity, called a graviton.

Both big experiments at Fermilab, CDF and DØ, decided to go back and look for monojets in the huge amount of data that they had already recorded in 1992-1995. Given past experience at CERN it was obvious that this wasn’t going to be easy, and that it would be extremely important to carefully estimate all the ways in which standard physics could fake a monojet signal. These fakes are called “backgrounds”. The DØ team, who pioneered the search for extra dimensions at a hadron collider, has now reported results on their search for the monojet signature. (Also, see our Plain English Summary on a previous DØ search for extra dimensions in a different signature, which resulted in stringent limits on their existence.)

The DØ physicists sifted through 60 million proton-antiproton collisions and picked out about 300 interesting events that looked like monojets. The next, most important step, was to identify possible backgrounds and find ways to reduce some of them. As well as the known backgrounds from neutrinos and from mismeasurements, it turned out that there is a background not from the Fermilab accelerator at all but from very energetic cosmic rays (particles from space). These cosmic rays collide with the Earth’s upper atosphere. Very rarely, some of the produts of these collisions reach the surface of the Earth and leave energy in the dense material of our detector just at the same time as we expected a particle collision from the accelerator. If this energy deposition looks like a jet, it mimics a monojet. Careful analysis of the characteristics of these background events allowed the DØ team to come up with quality criteria that reduce these backgrounds significantly. As a result, the monojet-like sample was reduced to some 30 events, completely consistent with the predicted backgrounds (see Fig. 2).

This means we didn’t see any evidence for extra dimensions of space. That doesn’t mean they aren’t there ¾ just that if they do exist, they are either too small or there are too many of them to see with the data we have in hand.

Figure 2. "Missing" momentum in the monojet candidate events. Data (points) clearly agree with the predicted background (red bars), and not with an extra dimensions hypothesis (open bars).

This non-observation of an excess of monojet events is, of course, less exciting than a discovery. However, it is an important step towards the possibility of a discovery of extra dimensions at Fermilab. DØ is now taking data with a modified detector. Over the next five years we will accumulate more than 100 times the amount of data that was used for this search, meaning we will be able to look for much smaller effects. Watch this “space!”

If you have any questions about this subject, please contact Hai Zheng (hzheng@und.edu) or Greg Landsberg (landsberg@hep.brown.edu).