Gravity and the Standard Model

The idea of heavy gravitons is not a traditional one, but if extra spatial dimensions exist, it is completely credible. Figure modified from

In the 1960s, physicists were working out the details of how the electromagnetic force and the weak nuclear force were related. After considerable effort, they showed that the forces’ differences disappeared at higher energy. The design parameters of the LHC, most importantly the beam energy, were chosen to explore the transition from the unified electroweak regime to the point where the two forces acted differently.

Physicists think that a similar unification of the electroweak, strong nuclear and gravitational forces will occur at much higher energy. The problem is that the energy at which this grand unification should occur is the Planck scale, which is about 1019 GeV. To give a sense of how much bigger this is than the energy of electroweak unification, if we equate the electroweak scale to the height of a person, the Planck scale is about halfway to Alpha Centauri. This enormous difference in scale is simply not understood and is called the hierarchy problem.

One possible answer to this mystery invokes additional spatial dimensions. If there are more dimensions beyond the familiar three, the traditional method of estimating the grand unification scale is simply wrong and could occur at much lower energy, perhaps low enough to be accessible to the LHC. In order to conform with existing data, these extra dimensions must be “curled up.” In one particular theory, called the Randall-Sundrum model, a new class of gravitons is predicted. Because of the extra dimensions, these are not traditional, massless gravitons. These gravitons have mass. In a specific version of the theory, the gravitons are predicted to preferentially decay into pairs of Z bosons.

CMS searched for massive particles decaying into a pair of Z bosons. In order to enhance the sensitivity of the measurement, they restricted their search to particles with a subatomic spin of 2, which is the spin expected to be carried by gravitons. The spin of a particle governs the angles at which its daughter particles can be created, so the decay angles were a central part of the analysis. The measurement was consistent with Standard Model expectations, so no massive gravitons were observed. However, physicists will continue to look at the data in even more creative ways.

—Don Lincoln

These physicists contributed to this analysis.
In order to select interesting events from the onslaught of collisions that occur at the center of CMS, physicists must teach the detector to “trigger” itself when a collision occurs that is an example of the phenomenon that physicists want to study. This group works in the JETMET group, which specializes in selecting events that have jets or events with missing energy. Events with missing energy can be the signature of dark matter, supersymmetry and other exotic phenomena.