Simulation of multiple partonic interactions in Herwig++

Soft and diffractive scattering with the cluster model in Herwig

We present a new model for soft interactions in the event-generator Herwig. The model consists of two components. One to model diffractive final states on the basis of the cluster hadronization model and a second component that addresses soft multiple interactions as multiple particle production in multi- peripheral kinematics. We present much improved results for minimum-bias measurements at various LHC energies.

The Underlying Event in Herwig++

We review the modelling of multiple interactions in the event generator Herwig++ and study implications of recent tuning efforts to LHC data. A crucial ingredient to a successful description of minimum-bias and underlying-event observables is a model for colour reconnection. Improvements to this model, inspired by statistical physics, are presented.

A new model for soft interactions in Herwig

We present a new model for soft interactions in the Monte Carlo event-generator Herwig. The soft diffractive final states are modeled on the basis of the cluster hadronization model and interactions between soft particles are modeled as multiple particle production with multiperipheral kinematics. We further present much improved results of mininum-bias measurements at different energies.

Multiple partonic interactions in heavy-ion collisions

I discuss the role played by multiple partonic interactions (MPI) in the early stages of relativistic heavy-ion collisions, for which a weak-coupling QCD description is possible. From the Color Glass Condensate, through the Glasma and into the Quark-Gluon-Plasma phase, MPI are at the origin of interesting novel QCD phenomena.

Herwig++ Physics and Manual

In this paper we describe Herwig++ version 2.3, a general-purpose Monte Carlo event generator for the simulation of hard lepton-lepton, lepton-hadron and hadron-hadron collisions. A number of important hard scattering processes are available, together with an interface via the Les Houches Accord to specialized matrix element generators for additional processes. The simulation of Beyond the Standard Model (BSM) physics includes a range of models and allows new models to be added by encoding the Feynman rules of the model. The parton-shower approach is used to simulate initial- and final-state Q…

Constraining MPI models using sigma_effective and recent Tevatron and LHC Underlying Event data

We review the modelling of multiple interactions in the event generator Herwig++ and study implications of recent tuning efforts to Tevatron and LHC data. It is often said that measurements of the effective cross section for double-parton scattering, sigma_effective, are in contradiction with models of the final state of multi-parton interactions, but we show that the Herwig++ model is consistent with both and gives stable predictions for underlying event observables at 14 TeV.

Multiple Hard Partonic Collisions with Correlations in Proton-Proton Scattering

We propose a simple method for incorporating correlations into the impact parameter space description of multiple (semi-)hard partonic collisions in high energy hadron-hadron scattering. The perturbative QCD input is the standard factorization theorem for inclusive dijet production with a lower cutoff on transverse momentum. The width of the transverse distribution of hard partons is fixed by parameterizations of the two-gluon form factor. We then reconstruct the hard contribution to the total inelastic profile function and obtain corrections due to correlations to the more commonly used eikon…

在LEP和HERA的γ-γ和γ-P物理中的多重硬相互作用

Multiple Hard Interactions in Gamma-Gamma and Gamma-P Physics at LEP and HERA

在$e^+e^-$和$ep$对撞机上，伴随轻子束的几乎在壳层的光子的大通量导致喷流的光生。随着质心能量的增加，部分子密度中较小的$x$区域被探索，这些区域是高部分子密度的区域。结果，在单个$γγ$或$γp$碰撞中发生多于一个硬部分子散射的概率可能变得显著。这种效果已被模拟使用程函处方结合HERWIG蒙特卡罗程序。在这个框架内，在HERA和LEPII相关的能量范围内，研究了多个硬相互作用对事件形状和射流截面的可能影响。结果表明，影响可能是显着的。

利用带电粒子平坦度揭示质心系能量 $\sqrt{\textit{s}}=13.6$ TeV 的 pp 碰撞中多重软部分子相互作用的效应

Unveiling the effects of multiple soft partonic interactions in pp collisions at $\sqrt{\textit s}=13.6$ TeV using charged-particle flattenicity

本文提出前向多重数事件分类器 flattenicity，并用 PYTHIA 8 模拟 13.6 TeV pp 碰撞，评估其区分多部分子相互作用与碰撞硬度及减少硬过程偏差的能力。

Multiple Interactions in Pythia 8

Modelling multiple partonic interactions in hadronic events is vital for understanding minimum-bias physics, as well as the underlying event of hard processes. A brief overview of the current Pythia 8 multiple interactions (MI) model is given, before looking at two additional effects which can be included in the MI framework. With rescattering, a previously scattered parton is allowed to take part in another subsequent scattering, while with enhanced screening, the effects of varying initial-state fluctuations are modelled.

极高能核碰撞中的半硬部分子再散射

Semi Hard Parton Rescatterings in Nuclear Collisions at Very High Energies

用Lipatov微扰Pomeron的交换来表示半硬部分子相互作用，我们把半硬核截面表示为自遮蔽截面。借助于生成泛函技术，我们得到了多重半硬部分子碰撞的平均数，而不需要使用多部分子分布的显式表达式。定量地估计了给定抛射体部分子与靶核的半硬相互作用的平均数，并且表明随着厘米的增加，它非常迅速地增长到1以上。能源（数字可应要求提供）

Insight into Multiple Partonic Interactions and Production of Charmonia in p+p Collisions at the LHC energies

At the LHC energies, the underlying observables are of major topic of interest in high multiplicity $p+p$ collisions. Multiple Parton Interactions (MPI) is one of them, in which several interactions occur in a single $p+p$ collision. It is believed that MPI is the main reason behind the high multiplicity in $p+p$ collisions at the LHC. It was believed that MPI has only effect to the soft particle production, but recent ALICE result reveals that it can also affect the hard-particle production. In such case, the self normalized yield of heavy particle like $\rm J/ψ$ shows an increasing trend wit…

Soft multiple parton interactions as seen in multiplicity distributions at Tevatron and LHC

We analyse the multiplicity distributions of charged particles at Tevatron (p\bar p) and LHC (pp) energies in the framework of the independent pair parton interactions (IPPI) model. It is shown that the number of soft pair parton interactions (and therefore the density of the partonic medium) is large and increases with energy. The mean multiplicity at each parton interaction grows also with energy. This growth depends on the width of the rapidity window. The similar conclusions are obtained in the multiladder exchange model (QGSM).

伽玛碰撞中的多重相互作用

Multiple Interactions in gamma gamma Collisions

对于LEP能量以及500 GeV的光子对撞机，估计了具有（至少）两个独立部分子散射的双光子事件的截面。这导致事件与（至少）四个中央（迷你-）射流。此类事件可能在现有数据中发现，应在LEP第二阶段清楚地看到。

Proceedings of the Sixth International Workshop on Multiple Partonic Interactions at the Large Hadron Collider

Multiple Partonic Interactions are often crucial for interpreting results obtained at the Large Hadron Collider (LHC). The quest for a sound understanding of the dynamics behind MPI - particularly at this time when the LHC is due to start its "Run II" operations - has focused the aim of this workshop. MPI@LHC2014 concentrated mainly on the phenomenology of LHC measurements whilst keeping in perspective those results obtained at previous hadron colliders. The workshop has also debated some of the state-of-the-art theoretical considerations and the modeling of MPI in Monte Carlo event generators…

HERA上的多重硬部分子相互作用

Multiple Hard Parton Interactions at HERA

在HERA，伴随着电子束的几乎是真实的光子的大通量导致了大量的喷流。探索了入射粒子的小动量分数$x$区域，其中部分子密度较高。结果，在单个$γp$碰撞中发生多于一个硬部分子散射的概率可能变得显著。众所周知，这种效应改变了射流（微型射流）对总截面的贡献。我们讨论了最新的HERA数据的总$γp$截面在这种情况下。用蒙特卡罗程序研究了多种硬相互作用对HERA事件形状和射流截面的可能影响。我们回顾了一些现有的结果，这在一般情况下表明，多重相互作用的影响应该是显着的，可能已经表现在现有的HERA数据。

Tuning of MC generator MPI models

MC models of multiple partonic scattering inevitably introduce many free parameters, either fundamental to the models or from their integration with MC treatments of primary-scattering evolution. This non-perturbative and non-factorisable physics in particular cannot currently be constrained from theoretical principles, and hence parameter optimisation against experimental data is required. This process is commonly referred to as MC tuning. We summarise the principles, problems and history of MC tuning, and the still-evolving modern approach to both model optimisation and estimation of modelli…

Underlying event sensitive observables in Drell-Yan production using GENEVA

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 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 partonic N-jet cross sections in GENEVA as well as the s…