Underlying-event sensitive observables in Drell–Yan production using GENEVA

The European Physical Journal C, Nov 2016

We present an extension of the Geneva Monte Carlo framework to include multiple parton interactions (MPI) provided by Pythia8. This allows us to obtain predictions for underlying-event sensitive measurements in Drell–Yan production, in conjunction with Geneva ’s fully differential \(\mathrm {NNLO}\) calculation, \(\mathrm {NNLL}'\) resummation for the 0-jet resolution variable (beam thrust), and \(\mathrm {NLL}\) resummation for the 1-jet resolution variable. We describe the interface with the parton-shower algorithm and MPI model of Pythia8, which preserves both the precision of the partonic N-jet cross sections in Geneva as well as the shower accuracy and good description of soft hadronic physics of Pythia8. We present results for several underlying-event sensitive observables and compare to data from ATLAS and CMS as well as to standalone Pythia8 predictions. This includes a comparison with the recent ATLAS measurement of the beam thrust spectrum, which provides a potential avenue to fully disentangle the physical effects from the primary hard interaction, primary soft radiation, multiple parton interactions, and nonperturbative hadronization.

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Underlying-event sensitive observables in Drell–Yan production using GENEVA

Eur. Phys. J. C Underlying-event sensitive observables in Drell-Yan production using GENEVA Simone Alioli 0 Christian W. Bauer 1 Sam Guns 1 Frank J. Tackmann 2 0 CERN Theory Division , 1211 Geneva 23 , Switzerland 1 Ernest Orlando Lawrence Berkeley National Laboratory, University of California , Berkeley, CA 94720 , USA 2 Theory Group, Deutsches Elektronen-Synchrotron (DESY) , 22607 Hamburg , Germany We present an extension of the Geneva Monte Carlo framework to include multiple parton interactions (MPI) provided by Pythia8. This allows us to obtain predictions for underlying-event sensitive measurements in DrellYan production, in conjunction with Geneva's fully differential NNLO calculation, NNLL resummation for the 0-jet resolution variable (beam thrust), and NLL resummation for the 1-jet resolution variable. We describe the interface with the parton-shower algorithm and MPI model of Pythia8, which preserves both the precision of the partonic N -jet cross sections in Geneva as well as the shower accuracy and good description of soft hadronic physics of Pythia8. We present results for several underlying-event sensitive observables and compare to data from ATLAS and CMS as well as to standalone Pythia8 predictions. This includes a comparison with the recent ATLAS measurement of the beam thrust spectrum, which provides a potential avenue to fully disentangle the physical effects from the primary hard interaction, primary soft radiation, multiple parton interactions, and nonperturbative hadronization. 1 Introduction Exclusive Monte?Carlo event generators are an important tool to make theoretical predictions for collider observables. By including both perturbative and nonperturbative effects, exclusive generators are able to provide predictions for a wide range of observables, whether they are dominated by short-distance physics or not. They aim to correctly describe physical effects over a wide range of energy scales, including: For sufficiently inclusive observables, it is enough to include only the perturbative effects of the primary hard interaction to achieve precise predictions, and for such observables one can directly compare measured data against partonic calculations. For less inclusive, resummation-sensitive observables, the perturbative evolution of the primary interaction becomes important, while MPI and most of the nonperturbative effects still give a subdominant contribution. Finally, there is a large class of exclusive observables for which all physical effects mentioned above contribute to similar extent. The concept of the underlying event (UE) [ 1?4 ] was introduced as a means to describe the soft hadronic activity that underlies the primary hard interaction. Typically, the effects arising from MPI are directly associated with the UE, and the traditional approach taken to study the UE is to define observables that have strong sensitivity to MPI and for which the effect from the primary interaction is reduced. In the following, we call such observables UE-sensitive observables. However, in principle all of the above contributions can give rise to the effects that are experimentally associated with soft activity and the underlying event, which makes a precise theoretical definition of the UE challenging. For example, it is well known that including higher-order perturbative corrections to the primary interaction in parton-shower Monte Carlos can give a nontrivial contribution to traditional UE measurements [ 5, 6 ]. Similarly, interference effects due to perturbative soft initial-state radiation in the primary interaction can contribute to many observables in a similar way than MPI effects [ 7 ]. UE-sensitive observables are typically constructed by dividing each event into distinct angular regions [ 4 ]. The ?toward? and ?away? regions are defined to be aligned with the directions of the products of the primary hard 2 ? 2 interaction, while the ?transverse? region is the complementary coverage of the solid angle. While the toward and away regions are typically dominated by the primary interaction, those effects are reduced in the transverse region. This makes observables measured in the transverse region sensitive to MPI. However, the effects from the primary interaction, in particular soft radiation and hadronization effects, give a sizable contribution in the transverse regions and need to be included in a proper description. In recent years, there have been many efforts to combine precision calculations of the hard interaction with fully exclusive Monte Carlo generators, which aim to describe the perturbative evolution of the primary partons and add nonperturbative physics and MPI effects through physical models. This started with the combination of leading-order (LO) predictions for several multiplicities with parton showers [ 8,9 ], and shortly thereafter methods to combine next-to-leading order (NLO) calculations were developed [ 10?14 ]. By now there are several (...truncated)


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Simone Alioli, Christian W. Bauer, Sam Guns, Frank J. Tackmann. Underlying-event sensitive observables in Drell–Yan production using GENEVA, The European Physical Journal C, 2016, pp. 614, Volume 76, Issue 11, DOI: 10.1140/epjc/s10052-016-4458-1