Report of the QCD Tools Working Group

TL;DR

The report surveys the status and progress of QCD tools for heavy flavors and new-physics searches, focusing on improving parton-shower accuracy, matching to matrix elements, and integrating resummation with MC approaches. It highlights advances in PYTHIA, including matrix-element matching, heavy-flavor hadronization, multiple interactions, and plans for a C++ transition, as well as comparisons between MC predictions and transverse-momentum resummation formalisms (CSS/ResBos) across gauge-boson, diphoton, and Higgs processes. The document also covers MCFM, a fixed-order NLO parton-level Monte Carlo, and experimental strategies to estimate backgrounds for new-physics channels, including heavy-flavor and missing-energy signatures. Finally, it analyzes the underlying event in hard scattering, using data-driven comparisons to multiple MCs to dissect beam remnants, initial-state radiation, and multi-parton interactions, with implications for tuning models and interpreting Run II data.

Abstract

We report on the activities of the ``QCD Tools for heavy flavors and new physics searches'' working group of the Run II Workshop on QCD and Weak Bosons. The contributions cover the topics of improved parton showering and comparisons of Monte Carlo programs and resummation calculations, recent developments in Pythia, the methodology of measuring backgrounds to new physics searches, variable flavor number schemes for heavy quark electro-production, the underlying event in hard scattering processes, and the Monte Carlo MCFM for NLO processes.

Figures (41)

• Figure 1: $(a)$ The prediction of the $W$ boson transverse momentum distribution in Run I at the Tevatron (solid line) compared to the DØ data. The prediction includes the effects of the modified parton distribution functions, the correction to the hard scattering process, and a primordial $k_T$ of 2.0 GeV; (b) The prediction of the $Z$ boson transverse momentum distribution in Run I at the Tevatron (solid line) compared to the CDF data. The prediction includes the effects of the modified parton distribution functions, the correction to the hard scattering process, and a primordial $k_T$ of 2.0 GeV.
• Figure 2: The $Z^0$$p_T$ distribution (at low $p_T$) from CDF for Run 1 compared to predictions from ResBos and from PYTHIA. The two PYTHIA predictions use the default (rms) value for the non-perturbative $k_T$ (0.44 GeV) and the value that gives the best agreement with the shape of the data (2.15 GeV).
• Figure 3: A comparison of the PYTHIA predictions for di-photon production at the Tevatron for the two different subprocesses, $q\overline{q}$ and $gg$. The same cuts are applied to PYTHIA as in the CDF di-photon analysis.
• Figure 4: A comparison of predictions for the Higgs $p_T$ distribution at the Tevatron from ResBos and from two recent versions of PYTHIA. The ResBos and PYTHIA predictions have been normalized to the same area.
• Figure 5: A comparison of predictions for the Higgs $p_T$ distribution at the LHC from ResBos, two recent versions of PYTHIA and HERWIG. The ResBos, PYTHIA and HERWIG predictions have been normalized to the same area.