Run II Jet Physics: Proceedings of the Run II QCD and Weak Boson Physics Workshop

TL;DR

This work outlines a comprehensive plan to standardize Run II jet physics at hadron colliders by adopting a fully specified, infrared- and collinear-safe cone algorithm (ILCA with seeds/midpoints) and a robust Run II KT algorithm with preclustering and 4-vector recombination. It details practical implementation considerations, including seed handling, merging/splitting rules, preclustering schemes, and detailed calibration methods (offsets and Missing ET projections) to achieve precise jet momentum scales and resolutions. The paper presents theoretical and experimental justifications, Monte Carlo and data-based validation, and concrete specifications to bridge Run I results with Run II capabilities, enabling improved tests of QCD and parton distributions. The overall aim is to enable accurate jet cross-section measurements, substructure studies, and event-shape analyses with standardized tools across the CDF and DØ experiments, while preserving compatibility with past results.

Abstract

The Run II jet physics group includes the Jet Algorithms, Jet Shape/Energy Flow, and Jet Measurements/Correlations subgroups. The main goal of the jet algorithm subgroup was to explore and define standard Run II jet finding procedures for CDF and DO. The focus of the jet shape/energy flow group was the study of jets as objects and the energy flows around these objects. The jet measurements/correlations subgroup discussed measurements at different beam energies; strong coupling constant measurements; and LO, NLO, NNLO, and threshold jet calculations. As a practical matter the algorithm and shape/energy flow groups merged to concentrate on the development of Run II jet algorithms that are both free of theoretical and experimental difficulties and able to reproduce Run I measurements. Starting from a review of the experience gained during Run I, the group considered a variety of cone algorithms and KT algorithms. The current understanding of both types of algorithms, including calibration issues, are discussed in this report along with some preliminary experimental results. The jet algorithms group recommends that CDF and DO employ the same version of both a cone algorithm and a KT algorithm during Run II. Proposed versions of each type of algorithm are discussed. The group also recommends the use of full 4-vector kinematic variables whenever possible. The recommended algorithms attempt to minimize the impact of seeds in the case of the cone algorithm and preclustering in the case of the KT algorithm. Issues regarding precluster definitions and merge/split criteria require further study.

Figures (38)

• Figure 1: An illustration of infrared sensitivity in cone jet clustering. In this example, jet clustering begins around seed particles, shown here as arrows with length proportional to energy. We illustrate how the presence of soft radiation between two jets may cause a merging of the jets that would not occur in the absence of the soft radiation.
• Figure 2: An illustration of collinear sensitivity in jet reconstruction. In this example, the configuration on the left fails to produce a seed because its energy is split among several detector towers. The configuration on the right produces a seed because its energy is more narrowly distributed.
• Figure 3: Another collinear problem. In this case we illustrate possible sensitivity to $E_T$ ordering of the particles that act as seeds.
• Figure 4: A seedless clustering algorithm.
• Figure 5: A fully specified splitting and merging algorithm.