Measuring extra dimensions

A branch that is one-dimensional to a chameleon is two-dimensional to an ant, and particles or waves that travel along the short dimension can loop around and even resonate.

Perhaps the most surprising thing about the LHC is that it has the potential to discover new dimensions. This is strange because dimensions are mutually perpendicular directions, like length, width and height — a new dimension would be a direction that is perpendicular to all three. Not only is it hard to believe that such a thing could have gone unnoticed until now, but how could colliding protons reveal it?

If a fourth dimension (not counting time) were exactly like length, width and height, we would have always known about it. We would describe the size of a box with four numbers, rather than three. When physicists speak of “extra dimensions,” they mean one or more dimensions that do not affect our macroscopic world, either because we’re stuck to a three-dimensional slice of the larger-dimensional space or because the extra dimension loops back on itself: If you travel far enough along it, you end up where you started, and “far enough” is a fraction of a proton’s width.

In one popular theory, both effects are responsible for hiding extra dimensions. The dimensions are small, and all particles are stuck to our three-dimensional slice except gravitons. This theory could explain why gravity is so weak compared to electromagnetism and nuclear forces — most gravitons would be lost in the extra dimensions.

In such a scenario, colliding protons would reveal the extra dimensions by creating a resonance of gravitons spinning around the extra dimensions. That is, the collision would create gravitons that go into the extra dimensions, loop around them, and arrive where they started. At the right energy, the gravitons would resonate like a ringing bell. The final result of this resonance would be to produce more particles, which can be observed by a detector like CMS.

The problem is that ordinary collisions also produce lots of particles: How would ordinary particle production be distinguished from extra dimensions? A group of CMS scientists approached the problem by measuring angular distributions of the observed particles, since extra dimensions would produce a different angular distribution than ordinary collisions. In fact, these scientists also used the angular distribution to determine if quarks, the constituents of protons, are themselves made of smaller particles.

The result was that no extra dimensions or quark substructure was seen, at scales that are 10 thousand times smaller than a proton’s radius.

Jim Pivarski

These physicists contributed to this paper.
Patricia McBride is leaving her position as U.S. CMS project manager to run the Fermilab Particle Physics Division. She will be replaced by Fermilab scientist Lothar Bauerdick. Thanks, Patty, and good luck in your new position. Welcome, Lothar!