Stricter bounds on extra dimensions

What looks like a one-dimensional world to a large creature might actually be two-dimensional at smaller scales.

One of the goals of the LHC is to search for evidence of extra dimensions— spatial directions such as length, breadth and height, but curled or curved in a way that hides their existence in everyday life. In a recent paper, CMS scientists presented new bounds on such a scenario.

Extra dimensions could hide from our perception by curling into tiny loops. A particle that drifts a microscopic distance in this new direction would circumnavigate the loop and arrive at its starting point almost immediately. The universe might have several microscopic dimensions in addition to the three large dimensions that we know well.

Collisions in the LHC could reveal new dimensions by creating particles that are small enough to fit inside them. High-energy colliders are often called microscopes because the energetic particles they make fill less space than low-energy particles, which allows them to probe the smallest structures in nature. If extra dimensions exist, then some energy would seem to disappear from view as particles traverse the tiny loops. In the CMS detector, this would look like an energy imbalance in the highest-energy collisions.

In this analysis, the scientists studied collisions resulting in a large energy imbalance. Their observations are consistent with the conventional three dimensions, so if a fourth, fifth or sixth dimension does exist, then it must be curled up even smaller than previously hypothesized. For example, if there are six new dimensions, then they can be no larger than about 20 femtometers, which is as small as an atomic nucleus. This is two times smaller than what was previously considered.

In a nice feat of recycling, the same experimental measurement applies to a completely different theory. Hypothetical particles without a well-defined mass, called unparticles, would produce similar patterns in the collision debris. Wasting not, the authors tightened the world’s knowledge about unparticles as well.

Jim Pivarski

The above physicists played a major role in the search for extra dimensions and unparticles.
This analysis relies on an accurate measurement of energy imbalance, also known as missing energy. The physicists pictured above worked hard to make sure the missing energy measurement is accurate.