Do extra dimensions hide under our toes?

A man on a high wire can only move in one dimension, but an ant on the same wire can move either along or around the wire. Does such a compact extra dimension exist as a fourth dimension of space?

A man on a high wire can only choose to move along the wire towards either end. For all his acrobatic prowess, he could not imagine moving along the wire in more than that single dimension. On a smaller scale, perhaps from the point of view of an ant sharing the wire with an acrobat, the second dimension of movement around the wire becomes apparent. Though this extra dimension is compact, or both finite and looped back upon itself, the surface of the wire is certainly two dimensional and noticing that extra dimension is simply a question of the scale of your probe.

The universe appears to have three spatial dimensions, but compact extra spatial dimensions might exist. It’s possible that we simply haven’t noticed them yet due to the limitations of our probes. The smaller a compact dimension is, the higher the energy of the probe must be in order to notice that dimension. Using the Tevatron as a probe, physicists at DZero have recently completed a search for a universal extra dimension that all particles could propagate through. Motion in this finite extra dimension would allow a set of resonant modes, or new excited states, for every particle produced in a proton-antiproton collision. The lightest excitation among all particles would be stable and a candidate for describing the dark matter in the universe.

Each time an excited quark state is created it will undergo a cascading decay that could produce many charged leptons. By focusing their analysis on events with two muons that have the same charge sign, the DZero team was able to perform the search in a channel that has a low background. The team further enhanced their sensitivity to a universal extra dimension by creating a signal discriminant using multivariate analysis techniques. Seeing no evidence for new physics in the DZero data, the analysis team set direct limits on the size of a universal extra dimension.

—Mike Cooke

These physicists made major contributions to this analysis.
This team monitored, maintained and serviced the Silicon Microstrip Tracker, the innermost and highest resolution subdetector of the DZero experiment critical for many DZero studies, including the analysis described above.