Prof. Dr. Peter Loch, University of Arizona Pushing the Precision Limit for QCD Measurements with Proton–Proton
Collisions at the Large Hadron Collider – Looking Inside Particle Jets with the
ATLAS Detector with New Approaches to Signal Definition and Calibration
Peter Loch
Department of Physics
University of Arizona
Tucson, Arizona, USA
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 𝛼𝑠 introduced in Quantum Chromo Dynamics (QCD), the principal theoretical
model describing this force in the Standard Model of Particle Physics, is of paramount interest. In
addition, recent progress in QCD does not only extend the regime of perturbative QCD calculations
to higher orders in terms of 𝛼𝑠 for many processes but also includes new approaches to calculate
the particle emission in the regime of lower scales characterized by small angle (near collinear) and
softer (lower energy) emissions. This imposes new precision requirements on experiments to
confirm the theoretical findings and thus enhance our understanding of the strong force.
The formation of particle jets emerging from the fragmentation of partons or the decay of shortlived particles generated in the final state of the proton–proton collisions at the LHC is governed by
QCD. The internal particle flow pattern of these jets, if reconstructed with sufficient precision in the
experiment, can provide access to a large spectrum of emission characteristics within one
coherently generated composite final state object, up to the limit of the experimental sensitivity.
This makes the running of 𝛼𝑠 , a long-standing observation in other measurements, accessible in
these flow patterns. A modern tool useful for the extraction of 𝛼𝑠 is the so-called Lund Jet Plane
(LJP) that represents a simultaneous measurement of both the emission angles and the emitted
energies. The LJP is not only employed in comparisons of perturbative QCD calculations with
experimental data in the kinematic domain of the jet fragmentation that is covered by such
calculations but also for the corresponding comparisons to the phenomenological modeling in nonperturbative domains. Its measurement requires highly precise signal reconstruction in the
typically dense particle flow environment inside jets to (1) compete with recent other 𝛼𝑠
measurements at the LHC involving gauge boson production and hadronic event shapes, and to (2)
serve as a reference for new or re-tuned non-perturbative emission models and as a target of such
tuning.
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 potentially involve machine-learned approaches to signal calibration and classifications are
discussed.
