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Improving our understanding and modeling of the underlying event in high energy collider environment is important for more precise measurements at the LHC. CDF Run II data for the underlying event associated with Drell-Yan lepton pair production and early ATLAS data measuring underlying event activity with respect to the leading transverse momentum track are presented. The data are compared with several QCD Monte Carlo models. It is seen that no current standard Monte Carlo tune adequately describes all the early ATLAS data and CDF data simultaneously. The underlying event observables presented here are particularly important for constraining the energy evolution of multiple parton interaction models. One of the goals of these analyses is to provide data that can be used to test and improve MC models for current and future physics studies at the LHC. The underlying event observables presented here are particularly important for constraining the energy evolution of multiple partonic interaction models, since the plateau heights of the underlying event profiles are highly correlated to multiple parton interaction activity. The data at 7 TeV are crucial for MC tuning, since measurements are needed with at least two energies to constrain the energy evolution of MPI activity. PYTHIA tune A and tune AW do a good job in describing the CDF data on the underlying-event observables for leading jet and Drell-Yan events, respectively, although the agreement between predictions and data is not perfect. The leading-jet data show slightly more activity in the underlying event than PYTHIA Tune A, although they are very similar - which may indicate the universality of underlying event modeling. However, all pre-LHC MC models predict less activity in the transverse region (i.e in the underlying event) than is actually observed in ATLAS leading track data, for both center-of-mass energies. There is therefore no current standard MC tune which adequately describes all the early ATLAS data. However, using diffraction-limited minimum bias distributions and the plateau of the underlying event distributions presented here, ATLAS has developed a new PYTHIA tune AMBT1 (ATLAS Minimum Bias Tune 1) and a new HERWIG+ JIMMY tune AUET1 (ATLAS Underlying Event Tune 1) which model the p{sub T} and charged multiplicity spectra significantly better than the pre-LHC tunes of those generators. It is critical to have sensible underlying event models containing our best physical knowledge and intuition, tuned to all relevant available data.
A study of ''underlying event'' in Run 2 at CDF is presented. Several PYTHIA 6.2 tunes (with multiple parton interactions) are examined and compared with HERWIG (without multiple parton interactions) and with the ATLAS PYTHIA tune (with multiple parton interactions) and they are extrapolated to the LHC.
Many high-energy collider experiments (including the current Large Hadron Collider at CERN) involve the collision of hadrons. Hadrons are composite particles consisting of partons (quarks and gluons), and this means that in any hadron–hadron collision there will typically be multiple collisions of the constituents — i.e. multiple parton interactions (MPI). Understanding the nature of the MPI is important in terms of searching for new physics in the products of the scatters, and also in its own right to gain a greater understanding of hadron structure. This book aims at providing a pedagogical introduction and a comprehensive review of different research lines linked by an involvement of MPI phenomena. It is written by pioneers as well as young leading scientists, and reviews both experimental findings and theoretical developments, discussing also the remaining open issues.
This book describes research in two different areas of state-of-the-art hadron collider physics, both of which are of central importance in the field of particle physics. The first part of the book focuses on the search for supersymmetric particles called gluinos. The book subsequently presents a set of precision measurements of “multi-jet” collision events, which involve large numbers of newly created particles, and are among the dominant processes at the Large Hadron Collider (LHC). Now that a Higgs boson has been discovered at the LHC, the existence (or non-existence) of supersymmetric particles is of the utmost interest and significance, both theoretically and experimentally. In addition, multi-jet collision events are an important background process for a wide range of analyses, including searches for supersymmetry.
This book reviews the latest experimental results on jet physics from proton-proton collisons at the LHC. Jets allow to determine the strong coupling constant over a wide range of energies up the highest ones possible so far, and to constrain the gluon parton distribution of the proton, both of which are important uncertainties on theory predictions in general and for the Higgs boson in particular.A novel approach in this book is to categorize the examined quantities according to the types of absolute, ratio, or shape measurements and to explain in detail the advantages and differences. Including numerous illustrations and tables the physics message and impact of each observable is clearly elaborated.