Calibration of the jet energy scale and resolution of small-radius jets using semileptonic $t\bar{t}$ events with the ATLAS detector

In the heart of the Large Hadron Collider, a machine built to smash protons together at nearly the speed of light, nature performs a chaotic dance of creation. When these tiny particles collide, they shatter into a spray of new matter, but the fundamental building blocks of this new matter—quarks and gluons—cannot exist alone. They are bound by a force so strong that they immediately clump together, forming tight, focused streams of particles called jets. These jets are the primary signatures scientists look for when hunting for new physics or studying the heaviest known particles, like the top quark. To understand what happened in a collision, researchers must measure the energy of these jets with extreme precision. If the measurement is even slightly off, the entire picture of the event can blur, making it impossible to distinguish between a known process and a discovery.

The ATLAS experiment, one of the massive detectors surrounding the collision point, is designed to catch these jets and measure their energy. However, the detector is a complex machine made of layers of sensors and materials, and it does not respond perfectly to every particle that hits it. Sometimes it absorbs a bit too much energy, sometimes too little. To fix this, physicists must apply a series of corrections to the raw data, essentially calibrating the instrument so that the energy they read matches the true energy of the particles. This process is known as calibrating the jet energy scale. While scientists have developed standard ways to do this using common collision events, they are always looking for new, independent ways to check their work, especially as the machine runs at higher energies and produces more complex data.

In a recent study, the ATLAS collaboration turned to a specific and rare type of event to refine these measurements: the decay of top quarks. The top quark is the heaviest known elementary particle, and it decays almost instantly into other particles, including a W boson. In about two-thirds of these cases, the W boson breaks apart into two jets of particles. Because the mass of the W boson is a known, fixed value, the energy of the two jets that come from it should add up to exactly that mass. This creates a natural "ruler" inside the data. If the measured mass of the W boson is too high or too low, it tells the scientists that their energy calibration for the jets is slightly off. By analyzing millions of these top quark events collected between 2015 and 2023, the team used the W boson as a precise reference point to tune their measurements.

The researchers focused on a specific subset of these events where one top quark decays into a charged particle like an electron or a muon, while the other decays into jets. This "semileptonic" signature makes the events easier to find and isolate from background noise. They selected jets that were moving with a momentum between 20 and 200 GeV and were located in the central part of the detector. Using a sophisticated statistical technique, they compared the distribution of the reconstructed W boson mass in their data against a vast library of computer simulations. These simulations allowed them to test different assumptions about how the detector responds to energy, effectively folding the data through various correction factors to see which one made the simulation match the real world best.

The results provided a new, independent set of correction factors for the jet energy scale and resolution. The study found that for the data collected in 2015 through 2018, the simulation needed to be adjusted downward by about 2 percent to match the observations, suggesting the computer models were slightly overestimating the energy. In contrast, the data from 2022 and 2023, collected after detector upgrades, matched the simulations much more closely, requiring corrections that were nearly zero. The uncertainty in these new measurements is impressively small, ranging from less than 1 percent to about 1.7 percent for the energy scale, and between 14 and 28 percent for the energy resolution. These uncertainties are comparable to, and in some cases better than, the standard techniques currently used by the experiment.

Crucially, this method relies on a different set of physics processes than the standard calibration techniques. While traditional methods often use jets produced by gluons, the jets in this study come from quarks, specifically those produced by the decay of the W boson. This difference provides a valuable cross-check, ensuring that the calibration works correctly regardless of the type of particle that created the jet. The dominant source of uncertainty in this new measurement comes from how well the computer simulations model the complex interactions of the top quark and its decay products, rather than from the detector itself. As the models for these interactions improve in the future, the precision of this technique is expected to increase even further, potentially reaching a precision of 0.5 percent for the energy scale.

By combining these new findings with the established calibration methods, the ATLAS collaboration can sharpen its view of the subatomic world. This improved precision is not just a technical victory; it is a prerequisite for future discoveries. When scientists search for rare new particles or subtle deviations from the standard model, they need to be certain that their measurements of energy are accurate to the last decimal place. This work, using the heavy top quark as a cosmic ruler, ensures that the ATLAS detector remains one of the most precise instruments in the world, ready to reveal the next layer of reality hidden within the collisions.