Fermi National Laboratory

Volume 25  |  Friday, June 28, 2002  |  Number 11
In This Issue  |  FermiNews Main Page

Particle Physics Made Painless

When nothing means something

by Greg Landsberg

An experimental null-result is not the end of the trail in the search for extra dimensions

Greg Landsberg When you’re searching for something, you can usually count on finding it in the last place you look.

The search might take you through countless nooks and crannies, but each one that comes up empty serves to reduce the number of nooks and crannies remaining to look.

Physics works the same way—in fact, all of science works the same way. If we don’t find something in one place, that doesn’t necessarily mean the “something” doesn’t exist. We might not have looked in enough places. But at the same time, we’ve scratched one more place off the list, reducing the number of nooks and crannies that lie ahead.

Here’s a case in point: the monojet—a single quark or gluon spotted in a particle detector, appearing to recoil against nothing. Could the last place we look for it turn out to be an extra dimension? A recent result from Fermilab’s DZero detector tells us we haven’t yet reached that last place to look— but we have trimmed the list.

Since the late 1970s, when the first proton-antiproton collider was built at CERN, physicists have been intrigued by the idea of finding a monojet, the simplest particle signature imaginable. On the surface, this scenario would seem to violate the conservation of momentum, but in fact the lack of a second object in the detector might also imply that another undetected particle (or particles) was produced in the collision along with the jet, and balances the momentum of the jet.

For a perfectly sealed particle detector, there is only one known type of particle that can be missed: the neutrino, which can easily pass through the entire Earth without a single interaction.

DZero collaboration: Still on the trail of extra dimensions. Since no detector is perfectly sealed, some other processes could appear to be a monojet because the balancing particle has escaped or eluded us. For example, a dijet event (two quarks scattering off each other) could look like a monojet if one of the pair escapes detection by literally falling through the cracks in the detector. Since dijets are produced in proton-antiproton collisions all the time, they might represent an important background, even if the number of cracks is very small and the cracks themselves are tiny.

What if the particles produced along with the jet are of an unknown kind—something we can’t yet identify? Then, if we see an excess of monojet events above the prediction from the known physics processes, we might have a hint of new physics beyond the Standard Model, our current picture of the universe.

The theory of supersymmetry spurred a monojet search as early as the 1970s. In supersymmetry, every integer spin particle (a boson or forcecarrier) has a semi-integer partner (a fermion) and vice versa. In addition to doubling the total number of fundamental particles, this theory predicts massive neutral particles barely interacting with other matter, which would be produced in violent proton-antiproton collisions along with the other ingredients in the particle soup.

Carlo Rubbia, co-winner of the 1984 Nobel Prize with Simon Van der Meer for the discovery of the W and Z bosons, thought he and his colleagues had seen evidence for supersymmetry in an excess of monojet events in the UA1 detector at CERN’s proton-antiproton collider in the early 1980s. However, the evidence was not confirmed by the CERN UA2 experiment, and Rubbia’s monojets turned out to be background from underestimated physics processes. The monojet lost some of its allure.

Fast forward to 1998, and enter Nima Arkani- Hamed of UC Berkeley, Savas Dimopoulos of Stanford, and Georgi Dvali of New York University. These three theorists proposed that the universe could be a much bigger place than we think. Our three spatial dimensions could be extended to five, or perhaps even more, by the existence of extra dimensions that are curled up with a microscopic radius—although that radius might be as large as a millimeter.

Hai Zheng of Notre Dame University presented the DZero results in a June 14 seminar. This new hypothesis offered a possible solution to the disturbing “hierarchy problem” of the Standard Model:

Why is gravity about 1038 times weaker than the other three forces of nature?
That’s 100,000,000,000,000,000,000,000,000,000,000,000,000
(a hundred billion billion billion billions) times weaker.

Extra dimensions could offer a simple answer: Gravity is just as strong as other forces, but it appears weak to us because it propagates through the entire space, including extra dimensions, and thus spends only very little time in the threedimensional world of our everyday experience, where all other particles and forces are stuck.

This theory of extra dimensions predicted large numbers of monojets in proton-antiproton collisions, with the “missing” particle being a quantum of the gravitational field, the graviton, escaping into the extra dimensions (see Fig. 1). Monojets regained their allure.

Both Fermilab collider detectors, CDF and DZero, have searched for monojet signatures in the huge amount of data recorded in Collider Run I of the Tevatron (1992-1996). Given the UA1 experience, it was extremely important to estimate all possible backgrounds very carefully, distinguishing them from an excess of monojet events that might indicate the “echo” from extra dimensions.

The DZero team, the first to carry out a search for extra dimensions at a hadron collider, reported results on its search for the monojets in a June 14 seminar at Fermilab, offered by experimenter Hai Zheng of Notre Dame University. DZero physicists sifted through 60 million protonantiproton collisions and identified a sample of about 300 monojet-like events.

The next, most important step was to identify possible backgrounds and filter them out.

It turned out that one additional background, not quite envisioned in the Standard Model, is the signature of very energetic cosmic rays passing through the Earth (including the DZero detector). Muons account for most of the cosmic ray flux reaching the surface of the Earth; very rarely, these muons interact in the dense material of the detector, depositing some of their energy there. In some cases, this energy deposit looks like a single jet, mimicking the monojet signature.

Other backgrounds include W and Z particles recoiling against a jet, with the products of the W or Z decay missed or mismeasured by the detector. Careful comparison of the characteristics of the background and expected signal events allowed The DZero team to design special criteria, or “cuts,” to reduce these backgrounds significantly. As a result, the monojet sample was reduced to some 30 events, consistent with the predicted backgrounds (see Fig. 2).

Figure 2. 'Missing' momentum in the monojet candidate events. Data (points) clearly agree with the predicted background (colored bars), and not with an extra dimensions hypothesis (open bars). This result was used to put stringent limits on the size of extra dimensions. For example, if there are only two of them, they should be curled up tighter than about 0.2 mm each. Analogous limits were set on other numbers of extra dimensions. These limits are comparable with, and complementary to those obtained in the sister channel: production of monophotons in electron-positron collisions, recently studied at CERN’s LEP machine. Especially for more than four extra dimensions, the Tevatron sensitivity exceeds that at LEP.

It’s interesting to note that in string theory, the preferred number of extra dimensions is six or seven; any number of those can be as large as predicted by Arkani-Hamed, Dimopoulos, and Dvali. However, the sensitivity to extra dimensions in the monophoton channel at the Tevatron is not as high as at LEP, as the very recent CDF analysis has demonstrated.

The non-observation of an excess of monojet events is, of course, less exciting than a discovery. However, it is an important step toward a possible discovery in this channel. Collider Run II of the Tevatron will increase the previously recorded data sample by two orders of magnitude. DZero has performed the first search for extra dimensions in the monojet channel, resulting in stringent limits on their size, and demonstrated that accurate accounting for the backgrounds is possible in this very challenging and exciting signature for new physics.

We’re trimming the list of places to look.

On the Web:
Searching for Extra Dimensions
DZero Plain English Summary

Gravity in Large Extra Dimensions
Berkeley Lab Research Review

last modified 6/28/2002   email Fermilab