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Precision QCD Measurements with Proton–Proton Collisions at the Large Hadron Collider – Looking Inside Particle Jets with New Approaches to Detector Signal Definition and Calibration
Abstract: The Large Hadron Collider (LHC) at CERN, Geneva, Switzerland, produces the highest energy proton collisions at the highest intensity of any colliding beam experiment so far. The reconstruction of the final state of these collisions faces significant challenges in achieving the required precision in, for example, the measurements of the strong force. In particular, the measurement of the strong coupling constant α_s introduced in Quantum Chromo Dynamics (QCD), the principal theory for this force in the Standard Model of Particle Physics, is of paramount interest. Recent progress in QCD does not only extend the regime of perturbative QCD calculations to higher orders in terms of α_s for many processes but also includes new approaches to calculate the particle emission in a regime characterized by small angle (near collinear) and softer (lower energy) emissions. This theoretical progress imposes new precision requirements on experiments to confirm the theoretical findings and thus enhance our understanding of the strong force.
The internal particle flow pattern inside the particle jets emerging from the fragmentation of partons generated in the final state of the proton–proton collisions at the LHC is governed by QCD. Measuring this pattern provides access to the continuous running of α_s, a long-standing observation from other measurements based on typically accessing α_s at various selected fixed scales, within one coherently generated final state object. A recently re-discovered tool to analyze these flow patterns is the so-called Lund Jet Plane (LJP), a combined representation of both the emission angles and the emitted energies inside the jet. Its measurement requires highly precise signal reconstruction in a typically dense particle flow environment to compete with other α_s measurements at the LHC based on hadronic event shapes or gauge boson production.
In this talk we first discuss the major challenges of such measurements introduced by the LHC collision environment and the experimental limitations of the detectors in the ATLAS experiment, which is one of the two general purpose experiments operating at the LHC. This is followed by the presentation of present-day strategies to overcome some of these limitations by optimizing the composition and selection of the input signals to the jet reconstruction and some of the results at hand for QCD measurements so far. Lastly, expectations for further improvements of the precision that involve machine-learned approaches to signal calibration and classifications are discussed.
3:00 PM in PAS 201 / Zoom https://arizona.zoom.us/j/86395646910
Refreshments in PAS 236, 2:30PM