DESY’s CMS group has just completed the first physics measurement at the Large Hadron Collider (LHC) at CERN at the highest energies ever achieved: 13.6 teraelectronvolts (TeV). They studied pairs of top quarks and antiquarks produced in proton-proton collisions. The DESY team developed and implemented a new analysis strategy, which allowed them to efficiently verify the quality of new data collected by the CMS experiment. While this is essential information for the CMS collaboration at the beginning of a new data-taking period, it also allowed the DESY group to measure the production cross section, generating the first physics result at the new LHC energies. Their measurement was first reported in September 2022, and a corresponding paper has just been submitted to the Journal of High Energy Physics (JHEP). 

From 2019 until last year, the LHC had been under maintenance, with the collider and experiments undergoing numerous upgrades. In the summer of 2022, LHC collisions resumed at record energy, marking the beginning of LHC Run 3. Of course, scientists wanted to explore the latest data immediately, which proved an ideal opportunity to deploy a novel data analysis technique. 

Generally, when a detector records information about a collision, physicists convert the raw data into the reconstructed trajectories and energies of the detected particles. Such reconstruction is done both for data and for simulation. The reconstruction procedure typically requires minor corrections based on comparisons between data and simulations, derived by comparing to well-understood processes. That final calibration process can add substantial time, sometimes years, to the analysis. The DESY CMS group used a novel technique that measured some of these corrections as part of the measurement process, allowing them to calibrate their data almost immediately. 

Most analyses at the LHC use a combination of different experimental data comprising electrons, muons, and showers of particles called jets. In contrast to previous efforts, the new result examined a much more varied collection of different combinations of these particles. Including more data allowed the DESY team to determine the calibration corrections directly within the measurement procedure - thus shortening the analysis time significantly. This new method allowed the group to monitor the detector’s functionality and performance almost immediately. 

“This is not only the first measurement at the highest-ever collider energy – we’ve shown that we can also extract calibrations out of the same information,” says Laurids Jeppe, a PhD student at DESY.

“The key in this analysis is the use of this range of decay channels in combination – one gets ultimate precision,” says Alexander Grohsjean, a DESY scientist now at Universität Hamburg. “It is self-calibrating, meaning you don't have to do external calibrations of the data, and you can reduce uncertainties.”

“Self-calibrating” means that the method corrects itself for inconsistencies in the data that lengthier analyses usually catch at the end of data-taking. “With this strategy, we can perform a complete physics measurement on a very short timescale, allowing us to check the scientific viability of the data early on,” says Evan Ranken, a postdoc in DESY’s CMS group who was also deeply involved in the analysis. “The top quark pairs we study are produced in large numbers at the LHC and leave a distinct experimental signature, so it doesn’t take long to perform the measurement once the method is in place.”

Although many top quark pair events are present in the fresh data, the new method and the accumulated experience of the analysis team were crucial to obtaining such a fast and precise result. This measurement also helped verify that the detector is correctly calibrated for future high-precision analyses. By demonstrating the scientific viability of new collision data early on, the team can help ensure that the CMS collaboration can continue to use the same data, saving time and effort for future results. Moreover, the methods developed for this paper are not limited to the context of studying brand-new data at a record energy scale. In the coming years, we expect to see these techniques used in high-precision measurements at the LHC.