Next-to-next-to-leading order

Now that a new landscape has been opened up by the LHC, it’s time to survey every inch of it with precision measurements.

In the early months of the LHC, many of the studies performed were searches for striking phenomena: event patterns that could not be anything other than new physics. Before we had this window on the microscopic universe, all sorts of proposed new laws of nature were theoretically possible — they would have had negligible effects on low-energy experiments but would have been dramatically revealed in high-energy collisions. In its first two years, the LHC struck down hundreds of hypotheses such as these. In its third and fourth, it discovered and started measuring the properties of the long-sought Higgs boson, one prediction that turned out to be true.

While all of these activities are still going on, the tide is shifting toward thorough surveys of the new landscape. New laws of nature are probably still waiting to be discovered, which is to say that we probably don’t know everything there is to know just yet. But they might be subtle, lurking in the 10th decimal place. By measuring a wide variety of known processes with ever-increasing precision, scientists are casting a wide net. The surveyors might find what the conquerors missed.

One such analysis is the measurement of the production of a W boson accompanied by two b quarks. As with all experiments, there is a story behind it: Previous measurements of a W boson and at least one b quark disagreed with the theoretical calculations, especially in cases where multiple b quarks overlapped in the detector, masquerading as a single b quark. By requiring events with two distinct b quarks, this new analysis can help to resolve the issue. The discrepancy is probably not new physics, but an uncertainty in the structure of the proton at its smallest scales.

CMS scientists recently published a high-precision analysis of the Wbb signature. What they found is consistent with the theoretical prediction, and that narrows the range of possible reasons for the previous discrepancy. In fact, it is sensitive to “next-to-next-to-leading order,” two levels of refinement deeper than the basic calculation. The new results are already providing important feedback to the theoretical community, helping them to stamp out uncertainties in the computer simulations that are used to make predictions. It will likely have a subtle effect on scientific studies for years to come, the way that accurate maps improved upon the first crude sketches of the new world.

Jim Pivarski

The physicists pictured above performed a precision measurement of the production rate of W bosons accompanied by two b quarks.
Kevin Burkett was recently named acting head of the CMS Center. The CMS Center oversees Fermilab’s activities in the CMS experiment at the LHC.