Cong-Feng Qiao, Jian Wang, Yang-heng Zheng
DJpsiFDC is an event generator package for the process $gg\to J/ψJ/ψ$. It generates events for primary leading-order $2\to 2$ processes. The package could generate a LHE document and this document could easily be embedded into detector simulation software frameworks. The package is produced in Fortran codes.
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In this article we present an event generator based on the Monte Carlo program Powheg in combination with the matrix-element generator Recola. We apply it to compute NLO electroweak corrections to same-sign W-boson scattering, which have been shown to be large at the LHC. The event generator allows for the generation of unweighted events including the effect of the NLO electroweak corrections matched to a QED parton shower and interfaced to a QCD parton shower. In view of the expected experimental precision of future measurements, the use of such a tool will be indispensable.
The EtabFDC is a matrix-element event generator package for $η_b$ hadroproduction at LHC. It generates $pp\toη_b X$ events for all possible parton-level $2\to2$ leading-order processes with three commonly used $η_b$ decay channels being implemented. The \pythia interface is used for parton-shower and hadronization to obtain the hadronic events. The FORTRAN codes of this package are generated by FDC (Feynman Diagram Calculation) system automatically.
At the LHC era, the detector systems are operating at the harsh hadronic environment with the unprecedentedly high particle flux. Position sensitive silicon devices are usually positioned at the innermost regions of the experimental setups and must cope with highly non-uniform radiation fields. At the end of LHC Run II, fluence in silicon trackers reached 10$^{15}$ n$_{eq}$/cm$^2$. Initial simulation studies predict that the maximal fluence for the HL-LHC may be up to two orders of magnitude higher than the one seen in LHC Run I and Run II. In this paper, two general-purpose physics events gen…
We review the main software and computing challenges for the Monte Carlo physics event generators used by the LHC experiments, in view of the High-Luminosity LHC (HL-LHC) physics programme. This paper has been prepared by the HEP Software Foundation (HSF) Physics Event Generator Working Group as an input to the LHCC review of HL-LHC computing, which has started in May 2020.
In this article we describe Professor, a new program for tuning model parameters of Monte Carlo event generators to experimental data by parameterising the per-bin generator response to parameter variations and numerically optimising the parameterised behaviour. Simulated experimental analysis data is obtained using the Rivet analysis toolkit. This paper presents the Professor procedure and implementation, illustrated with the application of the method to tunes of the Pythia 6 event generator to data from the LEP/SLD and Tevatron experiments. These tunes are substantial improvements on existin…
At the Large Hadron Collider (LHC), the most abundant processes which take place in proton-proton collisions are the generation of multijet events. These final states rely heavily on phenomenological models and perturbative corrections which are not fully understood, and yet for many physics searches at the LHC, multijet processes are an important background to deal with. It is therefore imperative that the modelling of multijet processes is better understood and improved. For this reason, a study has been done with several state-of-the-art Monte Carlo event generators, and their predictions a…
This paper has been prepared by the HEP Software Foundation (HSF) Physics Event Generator Working Group (WG), as an input to the second phase of the LHCC review of High-Luminosity LHC (HL-LHC) computing, which is due to take place in November 2021. It complements previous documents prepared by the WG in the context of the first phase of the LHCC review in 2020, including in particular the WG paper on the specific challenges in Monte Carlo event generator software for HL-LHC, which has since been updated and published, and which we are also submitting to the November 2021 review as an integral …
I briefly discuss some theoretical aspects of top mass measurements at the LHC. In particular, I illustrate a recent theoretical study performed using next-to-leading order (NLO) calculations interfaced to shower generators (NLO+PS) of increasing accuracy, interfaced to both Pythia8 and Herwig7 Monte Carlo generators.
We review the physics basis, main features and use of general-purpose Monte Carlo event generators for the simulation of proton-proton collisions at the Large Hadron Collider. Topics included are: the generation of hard-scattering matrix elements for processes of interest, at both leading and next-to-leading QCD perturbative order; their matching to approximate treatments of higher orders based on the showering approximation; the parton and dipole shower formulations; parton distribution functions for event generators; non-perturbative aspects such as soft QCD collisions, the underlying event …
We investigate how a Generative Adversarial Network could be used to generate a list of particle four-momenta from LHC proton collisions, allowing one to define a generative model that could abstract from the irregularities of typical detector geometries. As an example of application, we show how such an architecture could be used as a generator of LHC parasitic collisions (pileup). We present two approaches to generate the events: unconditional generator and generator conditioned on missing transverse energy. We assess generation performances in a realistic LHC data-analysis environment, with…
A Monte Carlo event generator is constructed for a two-Higgs-doublet model with maximal CP symmetry, the MCPM. The model contains five physical Higgs bosons; the $ρ'$, behaving similarly to the standard-model Higgs boson, two extra neutral bosons $h'$ and $h"$, and a charged pair $H^\pm$. The special feature of the MCPM is that, concerning the Yukawa couplings, the bosons $h'$, $h"$ and $H^\pm$ couple directly only to the second generation fermions but with strengths given by the third-generation-fermion masses. Our event generator allows the simulation of the Drell-Yan-type production process…
Proton-Proton ($pp$) collisions at the Large Hadron Collider (LHC) are simulated in order to study events with a high local density of charged particles produced in narrow pseudorapidty windows of $Δη$ = 0.1, 0.2, and 0.5. The $pp$ collisions are generated at center of mass energies of $\sqrt{s} = 2.36$, $7$, $8$, and $13$ TeV, i.e. the energies at which the LHC has operated so far, using PYTHIA and HERWIG event generators. We have also studied the average of the maximum charged-particle density versus the event multiplicity for all events, using the different pseudorapidity windows. This stud…
State-of-the-art theoretical predictions accurate to next-to-leading order QCD interfaced with {\sc pythia} and {\sc herwig} are tested by comparing the unfolded $t\bar{t}$ differential data collected with the CMS detector at 8 TeV and 13 TeV. These predictions are also compared with the measurements of underlying event activity distributions accompanying ${\rm t\bar{t}}$ events. Furthermore, predictions of beyond NLO accuracy in QCD are compared with the data.
In this paper we study the theoretical uncertainties in the determination of the top-quark mass using next-to-leading-order (NLO) generators interfaced to parton showers (PS) that have different levels of accuracy. Specifically we consider three generators: one that implements NLO corrections in the production dynamics, one that includes also NLO corrections in top decay in the narrow width approximation, and one that implements NLO corrections for both production and decay including finite-width and interference effects. Since our aim is to provide an assessment of the uncertainties of purely…
A Portable Parton-Level Event Generator for the High-Luminosity LHC
针对高亮度LHC模拟的算力需求与硬件架构变化，提出可适配多类现代硬件的领头阶部分子级事例产生框架，验证了其在标准烛光物理过程中的性能，支撑可持续的对撞机模拟。
LO, NLO, and NNLO Parton Distributions for LHC Event Generators
本文提出适用于蒙特卡洛事件产生器的 NNPDF4.0MC 部分子分布集合，满足低尺度正定性、平滑外推和数值稳定等约束，并评估了其对 LHC 硬和软 QCD 过程截面的唯象影响。
Comparing particle multiplicity predictions: Insights from Pythia, Herwig with the LHCb use case
本文比较了Pythia和Herwig事件生成器在LHCb实验低能衍射和最小偏置质子-质子碰撞模拟中的表现，发现Pythia与实验数据更一致，仍是LHCb粒子物理分析的首选生成器。
We present an extension of the JHUGen and MELA framework, which includes an event generator and library for the matrix element analysis. It enables simulation, optimal discrimination, reweighting techniques, and analysis of a bosonic resonance and the triple and quartic gauge boson interactions with the most general anomalous couplings. The new features, which become especially relevant at the current stage of LHC data taking, are the simulation of gluon fusion and vector boson fusion in the off-shell region, associated $ZH$ production at NLO QCD including the $gg$ initial state, and the simul…
The determination of the primary energy and mass of ultra-high-energy cosmic-rays (UHECR) generating extensive air-showers in the Earth's atmosphere, relies on the detailed modeling of hadronic multiparticle production at center-of-mass (c.m.) collision energies up to two orders of magnitude higher than those studied at particle colliders. The first Large Hadron Collider (LHC) data have extended by more than a factor of three the c.m. energies in which we have direct proton-proton measurements available to compare to hadronic models. In this work we compare LHC results on inclusive particle pr…
Studies in the forward region of charged particle multiplicity and density, as well as energy flow, are presented. These measurements are performed using data from proton-proton collisions at a center-of-mass energy of 7 TeV, collected with the LHCb detector. The results are compared to predictions from a variety of Monte Carlo event generators and are used to test underlying event and hadronization models as well as the performance of event generator tunes in the forward region.
The PYTHIA 8 generator is used to estimate the percentage of non-diffractive and diffractive events at the LHC energies. It is shown that a simple condition of the absence of charged hadrons in the central pseudorapidity region is sufficient to remove almost all non-diffractive events. This opens the way to investigate diffraction without waiting for the future specialized detectors.
GRANIITTI: towards a deep learning-enhanced Monte Carlo event generator for high-energy diffraction
本文提出专为LHC胶球研究设计的蒙特卡洛事件生成器GRANIITTI，通过对比RHIC STAR数据验证模拟，并探索深度学习增强的衍射事件生成方向。
High-energy physics data analysis relies heavily on the comparison between experimental and simulated data as stressed lately by the Higgs search at LHC and the recent identification of a Higgs-like new boson. The first link in the full simulation chain is the event generation both for background and for expected signals. Nowadays event generators are based on the automatic computation of matrix element or amplitude for each process of interest. Moreover, recent analysis techniques based on the matrix element likelihood method assign probabilities for every event to belong to any of a given se…
We present results of the updated SuperChic 3 Monte Carlo event generator for central exclusive production. This extends the previous treatment of proton-proton collisions to include heavy ion (pA and AA) beams, for both photon and QCD-initiated production, the first time such a unified treatment of exclusive processes has been presented in a single generator. To achieve this we have developed a theory of the gap survival factor in heavy ion collisions, which allows us to derive some straightforward results about the $A$ scaling of the corresponding cross sections. We compare against the recen…
We improve the theoretical predictions for the production of extra neutral gauge bosons at hadron colliders by implementing the Z' bosons in the MC@NLO generator and by computing their differential and total cross sections in joint p_T and threshold resummation. The two improved predictions are found to be in excellent agreement with each other for mass spectra, p_T spectra, and total cross sections, while the PYTHIA parton and ``power'' shower predictions usually employed for experimental analyses show significant shortcomings both in normalization and shape. The theoretical uncertainties fro…
We present hadron-level predictions from the Monte Carlo generator Cascade and numerical level calculations of beauty quark and inclusive b-jet production in the framework of the kT -factorization QCD approach for CERN LHC energies. The unintegrated gluon densities in a proton are determined using the CCFM evolution equation and the Kimber- Martin-Ryskin (KMR) prescription. We study the theoretical uncertainties of our calcula- tions and investigate the effects coming from parton showers in initial and final states. Our predictions are compared with the recent data taken by the CMS collaborati…
The best limit on the strong FCNC anomalous couplings was obtained using the direct top production process at the Large Hadron Collider by the ATLAS collaboration. We perform a similar analysis but using a next-to-leading order generator, MEtop. We then show how the limits could be improved if the FCNC single top process $pp \to tj$ would be included as signal. Finally we discuss a slightly modified analysis with an extra hard jet in the final state.
Challenges for Monte Carlo generators
本文列举了未来轻子对撞机中蒙特卡洛事件生成器面临的挑战，描述了多用途生成器与专用代码现状，并概述了在数据采集前需解决的束流模拟与高阶修正等独特问题。
We introduce a new high dimensional algorithm for efficiency corrected, maximally Monte Carlo event generator independent fiducial measurements at the LHC and beyond. The approach is driven probabilistically using a Deep Neural Network on an event-by-event basis, trained using detector simulation and even only pure phase space distributed events. This approach gives also a glimpse into the future of high energy physics, where experiments publish new type of measurements in a radically multidimensional way.
Monte Carlo Event Generators are important tools for the understanding of physics at particle colliders like the LHC. In order to best predict a wide variety of observables, the optimization of parameters in the Event Generators based on precision data is crucial. However, the simultaneous optimization of many parameters is computationally challenging. We present an algorithm that allows to tune Monte Carlo Event Generators for high dimensional parameter spaces. To achieve this we first split the parameter space algorithmically in subspaces and perform a Professor tuning on the subspaces with …
A Generative-Adversarial Network (GAN) based on convolutional neural networks is used to simulate the production of pairs of jets at the LHC. The GAN is trained on events generated using MadGraph5 + Pythia8, and Delphes3 fast detector simulation. We demonstrate that a number of kinematic distributions both at Monte Carlo truth level and after the detector simulation can be reproduced by the generator network with a very good level of agreement. The code can be checked out or forked from the publicly accessible online repository https://gitlab.cern.ch/disipio/DiJetGAN .
A neural network technique is used to discriminate between quark and gluon jets produced in the qg->q+photon and q q->g+photon processes at the LHC. Considering the network as a trigger and using the PYTHIA event generator and the full event fast simulation package for the CMS detector CMSJET we obtain signal-to-background ratios.
Exploiting the large number of collected events and the symmetries in the W production and decay at LHC, it is possible to measure the W double differential cross section of transverse momentum and rapidity and the angular coefficients. With the proposed method a determination of the integrated spectrum of transverse momentum of W bosons is possible with an unprecedented granularity and increased precision with respect to the state-of-the-art generators. This enables the possibility to perform a theory-agnostic measurement of the W mass with competitive uncertainty by constraining the W produc…
This paper explores the physics reach of the High-Luminosity Large Hadron Collider (HL-LHC) for searches of new particles decaying to two jets. We discuss inclusive searches in dijets and b-jets, as well as searches in semi-inclusive events by requiring an additional lepton that increases sensitivity to different aspects of the underlying processes. We discuss the expected exclusion limits for generic models predicting new massive particles that result in resonant structures in the dijet mass. Prospects of the Higher-Energy LHC (HE-LHC) collider are also discussed. The study is based on the Py…
We present first measurements of charged and neutral particle-flow correlations in pp collisions using the ATLAS calorimeters. Data were collected in 2009 and 2010 at centre-of-mass energies of 900 GeV and 7 TeV. Events were selected using a minimum-bias trigger which required a charged particle in scintillation counters on either side of the interaction point. Particle flows, sensitive to the underlying event, are measured using clusters of energy in the ATLAS calorimeters, taking advantage of their fine granularity. No Monte Carlo generator used in this analysis can accurately describe the m…
The study of the sphericity of primary charged particles in minimum bias proton-proton collisions at 0.9, 2.76 and 7 TeV with the ALICE detector at the LHC is presented. The mean sphericity as a function of multiplicity is reported for events with different hardness. The ALICE results show a growing transverse sphericity with multiplicity for all collision energies, with a steeper rise at low multiplicity. The tendency of the MC generators is opposite at high multiplicity.
In this paper, we demonstrate the LHC discovery potential of new charged vector boson $W_1^{\pm}$ predicted by the Minimal Higgsless model in the process $pp\to W_1^{\pm}qq^{\prime}\to W^{\pm}Z^0qq^\prime\to \ell^{\pm}\ell^+\ell^-νqq^{\prime}(\ell=e,μ)$ by analyzing the generator level events of the signal and backgrounds. The generator for the signal $pp \to {W_1}^{\pm}qq^\prime\to W^{\pm}Z^0qq^\prime$ at tree level is developed with the Minimal Higgsless model and then interfaced with PYTHIA for the parton showers and hadronization. The backgrounds are produced with PYTHIA and ACERMC. We giv…
This document highlights a number of top-quark mass measurements carried out by the ATLAS and CMS collaborations based on the combined LHC Run 1 datasets at centre-of-mass energies of $\sqrt{s}=$7 and 8 TeV. A wide range of analysis strategies are employed for a number of final-state signatures. Measurements of both the pole mass as well as the value of the top-quark mass as defined by the Monte Carlo generator in simulated signal samples are discussed.
We address the question of understanding the production of $J/ψ$ particles regarding the global underlying event in proton-proton collisions. To do so, we propose to look at a new observable: the $J/ψ$ production as a function of the charged particles multiplicity of the event. We demonstrate the interest for an experimental measurement by varying the model of multiple interactions in the PYTHIA generator.
Events containing hadronically decaying heavy particles with large momentum, leading to so-called merged jets, are expected to play a significant role in both searches for new physics and measurements of Standard Model processes at the CERN Large Hadron Collider. In this article, a comparative study of the modeling of such merged jet topologies by different Monte-Carlo event generators is presented. The observed differences emphasize the need to refine such modeling based on the observation of Standard Model processes prior to claims of discovery.
Several observables sensitive to the fragmentation of $b$-quarks into $b$-hadrons are measured using 36 fb${}^{-1}$ of $\sqrt s = 13$ TeV proton-proton collision data collected with the ATLAS detector at the LHC. Jets containing $b$-hadrons are obtained from a sample of dileptonic $t\bar{t}$ events, and the associated set of charged-particle tracks is separated into those from the primary $pp$ interaction vertex and those from the displaced $b$-decay secondary vertex. This division is used to construct observables that characterize the longitudinal and transverse momentum distributions of the …
We present charged-particle distributions sensitive to the underlying event, measured by the ATLAS detector in proton-proton collisions at a centre-of-mass energy of 13 TeV, in low-luminosity Large Hadron Collider fills corresponding to an integrated luminosity of 1.6 nb$^{-1}$. The distributions were constructed using charged particles with absolute pseudorapidity less than 2.5 and with transverse momentum greater than 500 MeV, in events with at least one such charged particle with transverse momentum above 1 GeV. These distributions characterise the angular distribution of energy and particl…
The number of events suitable for a determination of the gluon distribution function g(x,Q^2) in a proton at the LHC for various intervals of x and Q^2 are estimated. The contributions of main background sources to ``direct photon+jet'' production processes are studied. The values of discrimination powers between quark and gluon jets as well as between a single photon and the products of pi^0, eta, K^0_s mesons decaying through the neutral channels are applied to estimate the final contributions of various event types to the "photon+jet" production in different intervals of x and Q^2. Monte Ca…
Event-shape observables measured using charged particles in inclusive $Z$-boson events are presented, using the electron and muon decay modes of the $Z$ bosons. The measurements are based on an integrated luminosity of $1.1 {\rm fb}^{-1}$ of proton--proton collisions recorded by the ATLAS detector at the LHC at a centre-of-mass energy $\sqrt{s}=7$ TeV. Charged-particle distributions, excluding the lepton--antilepton pair from the $Z$-boson decay, are measured in different ranges of transverse momentum of the $Z$ boson. Distributions include multiplicity, scalar sum of transverse momenta, beam …
Charged particle multiplicities in proton-proton collisions measured in the LHCb detector at a centre-of-mass energy of $\sqrt s$=7 TeV in different windows of pseudorapidity $η$, in the forward region of the vertex detector are studied by using different statistical distributions. Three distributions are compared with the data and the moments of the distributions are calculated. The data constituting two sets, one of minimum bias events and another of hard QCD events are analysed. The distributions considered derive from different functional forms based on underlying interaction dynamics. The…
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