Dynamical approach to MPI four-jet production in Pythia
Eur. Phys. J. C
Dynamical approach to MPI four-jet production in Pythia
B. Blok 1
P. Gunnellini 0
0 Deutsches Elektronen-Synchrotron (DESY) , Notkestraße 85, 22761 Hamburg , Germany
1 Department of Physics, Technion-Israel Institute of Technology , Haifa , Israel
We modify the treatment of multiple parton interactions (MPI) in Pythia by including the 1 ⊗ 2 mechanism and treating the 2 ⊗ 2 mechanism in a model-independent way. The 2⊗2 mechanism is calculated within the mean field approximation, and its parameters are expressed through generalized parton distributions extracted from HERA data. The parameters related to the transverse parton distribution inside the proton are thus independent of the performed fit. The 1 ⊗ 2 mechanism is included along the lines of the recently developed formalism in perturbative QCD. A unified description of MPI at moderate and hard transverse momenta is obtained within a consistent framework, in good agreement with experimental data measured at 7 TeV. Predictions are also shown for the considered observables at 14 TeV. The corresponding code implementing the new MPI approach is made available.
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It is widely realized now that hard multiple parton
interactions (MPI) play an important role in the description of
inelastic proton–proton ( pp) collisions at high
center-ofmass energies. Starting from the 1980s [1–5] until the last
decade [6–32], extensive theoretical studies have been
carried out. Attempts have been made to incorporate multiparton
collisions in Monte Carlo (MC) event generators [33–37].
Multiple parton interactions can serve as a probe for
nonperturbative correlations between partons in the nucleon wave
function and are crucial for determining the structure of the
underlying event (UE) at Large Hadron Collider (LHC)
energies. Moreover, they constitute an important background for
new physics searches at the LHC. A large number of
experimental measurements have been performed at the Tevatron
[38–40] and at the LHC [41–44], showing evidence for MPI
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at both soft and hard scales. This latter case is usually referred
to as “double parton scattering” (DPS), which involves two
hard scatterings within the same hadronic collision. The cross
section of such an event is generally expressed in terms of
the σeff. In the mean field approximation σeff [1–27,32], is
the effective area which measures the transverse distribution
of partons inside the colliding hadrons and their overlap in a
collision.
Recently, a new approach based on perturbative quantum
chromodynamics (pQCD) has been developed [22–25] for
describing the MPI and its main ingredients are:
• The MPI cross sections are expressed through new
objects, namely double generalized parton distributions
(GPD2);
• besides the conventional mean field parton model
approach to MPI, represented by the so-called 2 ⊗
2 mechanism (see Fig. 1 left), an additional 1 ⊗
2 mechanism (Fig. 1 right) is included. In this
mechanism, which can be described in pQCD, the parton from
one of the nucleons splits at some hard scale and creates
two hard partons that may participate in MPI. This
mechanism leads to a significant transverse-scale dependence
of MPI cross sections.
• The contribution of the 2 ⊗ 2 mechanism to GPD2 is
calculated in a mean field approximation with
modelindependent parameters.
The use of this new formalism at LHC experiments needs its
implementation in MC event generators, which has not been
performed yet. The purpose of the present paper is to make
a step ahead toward the implementation of this formalism
into MC generators. We use the standard simulation of the
MPI implemented in Pythia [35], but with values of σeff
calculated by using the QCD-based approach of [22–25], i.e.
including 1 ⊗ 2 processes.
The current approach used for the description of the MPI
in Pythia is based on [34,35]. The Pythia code uses parton
distribution functions, dependent on the impact parameter
of the collision. From the theoretical point of view these
are just one-particle generalized parton distributions GPD1
(see e.g. [45,46] for a review). The parameters set in the
Pythia simulation relative to the transverse parton density
are extracted from fits to experimental data on UE, sensitive
to the contribution of the MPI. This procedure is closely
related to mean field-based schemes; see e.g. [22].
Such an approach has, however, a number of difficulties,
both conceptual and practical. First of all, a problem arises at
the level of mean field approximation. The transverse parton
distributions have been extracted from J/ photoproduction
measurements at the HERA collider, using QCD
factorization theorems [19–21,45,46]. Hence they cannot be treated as
free parameters of the model. Secondly, it has been observed
that different Pythia parameters are obtained when data
sensitive to a different region of the MPI spectrum are used for
the fits. For example, it has been shown [47] that different
parameters result for fits to UE o (...truncated)