Higgs and top quarks together

The Higgs boson and the top quark are connected in a deep and fundamental way. If they were people, they’d vacation together. The CMS experiment continues studying the connections between them to try to get a better understanding of the manner in which mass is given to fundamental particles.

In 2012, CMS and ATLAS scientists discovered a new boson. While the particle couldn’t be explained by a version of the Standard Model that lacked the Higgs boson, it could be explained by a version of the Standard Model that contained it. This doesn’t mean that what was found was the Higgs boson, but the fact that the data was consistent with the predictions of Higgs theory caused some incautious reporters to trumpet “Higgs boson found.” Scientists continue to try to verify that what they found was the Higgs boson. While the data continues to strengthen the idea, it will be a long time before that claim can be made. For instance, while it is likely that we have found a Higgs boson, it could be just one of many, as predicted by some extensions of the Standard Model.

One way in which CMS attempted to establish that the new boson is the Higgs boson was to measure properties of the new particle, such as electrical charge and spin, to see how well they match with the predictions of Higgs theory. A second method for vetting the particle was to measure more precisely the decay of the observed boson to see if these decays continue to support the Higgs boson hypothesis.

But today’s article focuses on the various ways in which Higgs bosons can be produced in LHC collisions. The most common ways in which they can be produced are (a) alone, (b) at the same time as a W or Z boson, (c) at the same time as two ordinary quarks, and (d) at the same time as two top quarks. Case (a) is the most likely while (d) is least likely—case (a) is predicted to happen about 250 times more often than (d).

Production mode (d) is nevertheless supposed to happen, and scientists went looking for it. (This is true of (b) and (c) as well.) CMS physicists looked for events in which a Higgs boson is produced at the same time as a top and an antimatter top quark. This is a particularly interesting class of events. The LHC collides two protons, each of which emits a top quark-antiquark pair. The top quark from one proton annihilates the antiquark from the other proton and forms a Higgs boson. This type of production allows scientists to directly study the interaction of the Higgs boson with top quarks, which are the heaviest fundamental particles.

Because this kind of interaction is very rare, not enough data has been collected to claim that this process has been observed, though the limits we set were consistent with those predicted by Higgs theory. However when the LHC resumes operations with much higher energy and collision rates, it is imperative that researchers see this kind of collision. If we see it at the predicted rate, we will be even more confident that we’ve found the Higgs boson and it will allow a unique glimpse into how the Higgs boson interacts with top quarks.

—Don Lincoln

These U.S. physicists contributed to this analysis.
The U.S. CMS Tier-2 computing facilities team held their annual meeting as part of the Open Science Grid All-Hands Meeting in Indianapolis on March 11. There are eight Tier-2 sites in the United States, at Caltech, U of Florida, MIT, U of Nebraska-Lincoln, Purdue, U of Wisconsin-Madison, UC San Diego and Vanderbilt. These computing sites perform large-scale simulations of LHC collisions and run thousands of data analysis jobs each day for CMS physicists. These computations are needed for every single physics result that comes from CMS. The U.S. sites are among the largest and most productive in CMS.