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)