Dynamical approach to MPI four-jet production in Pythia

The European Physical Journal C, Jun 2015

We modify the treatment of multiple parton interactions (MPI) in Pythia by including the \(1\otimes 2\;\)mechanism and treating the \(2 \otimes 2\;\)mechanism in a model-independent way. The \(2 \otimes 2\;\)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 \(1\otimes 2\;\)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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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. - 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 a e-mail: b e-mail: 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)


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B. Blok, P. Gunnellini. Dynamical approach to MPI four-jet production in Pythia, The European Physical Journal C, 2015, pp. 282, Volume 75, Issue 6, DOI: 10.1140/epjc/s10052-015-3520-8