Do tops go bump?

Scientists on a recent analysis looked for an unexpected number of top quark-antiquark pairs. The data (black dots) clearly favor known physics (red and green areas) and not a prediction of new physics (dashed line).

In particle physics things develop pretty rapidly. The top quark was discovered in 1995 at the Fermilab Tevatron using just a few of tens of events taken over a couple of years. Nowadays, top quarks are a lot easier to come by. For instance, at the LHC, they are produced at the rate of about one per second.

The most common way in which top quarks are made at the LHC is by way of a pair of gluons, which fuse to make a short-lived intermediary particle called a massive virtual gluon, which then decays into a top quark-antiquark pair. This is pretty straightforward and well-understood physics.

While this process is a fairly ordinary example of how the strong force works, the top quarks remain special. After all, they are enormously massive — the heaviest subatomic particle ever discovered, about 170 times heavier than a proton. This is because the top quark preferentially interacts with the Higgs boson. Why that should be is still not known. But it does point to something unusual about the top quark.

Given that the top quark is unusual, physicists have wondered if ongoing studies of top quarks might lead to other discoveries. For instance, there is no shortage of new theories of new physics that preferentially decay into top quark-antiquark pairs. We generically call this particle a Z’, in analogy with the well-known Z boson, but there are many ideas, including a proposed new heavy Higgs boson and heavy gravitons that appear in theories that invoke extra dimensions of space.

Given the plethora of ideas, CMS physicists did a general search for events in which top quark-antiquark pairs are made. The idea is to look for unexpected “bumps” in the data. As we can see in the figure at the top, the data basically looks like the predictions of expected physics, with no evidence for a bump. This means that CMS did not find any new physics, but the data did allow them to rule out some proposed theories. This is an update of an earlier measurement. Scientists will pursue an analysis approach vigorously using the new data now being recorded using the refurbished LHC accelerator and CMS experiment.

Don Lincoln

These physicists contributed to this analysis.

See other science results from Fermilab.