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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - MCnetITN3 (Innovative Network for Monte Carlo Event Generators for LHC Physics)

Teaser

Summary

MCnetITN3 is an Innovative Training Network dedicated to developing and supporting general-purpose Monte Carlo event generators throughout the LHC era and beyond, training the next generation of event generator developers and providing training of a wide selection of its user base, particularly through funded short-term \'residencies\' and Annual Schools.

Monte Carlo event generators are computer programs that simulate the final states of high energy particle collisions, and are central to particle physics. They are used by almost all experimental collaborations to plan their experiments and analyze their data, and by theorists to simulate the complex final states of the fundamental interactions that may signal new physics.

The network consists of the Universities of Manchester, Durham, Glasgow, GÃ¶ttingen, Louvain and Lund, University College London and the Karlsruhe Institute of Technology, joined by academic partners CERN, Heidelberg, Monash (Australia) and Vienna and non-academic partners B12, Blue Yonder, d-fine and Ion Beam Applications. Discussions continue with other potential partners.

Our training programme is implemented through the recruitment of 11 long-term Early Stage Researchers who will complete PhDs in Monte Carlo event generator physics, techniques and related disciplines, and of 144 ESR-months of short-term positions, whereby students registered for a PhD elsewhere can receive complementary training by working in one of our groups for three to six months, and through Annual Schools in Europe both for our own ESRs and for the external community, as well as up to two schools outside Europe, annual training events for our ESRs and bi-annual network meetings. The long-term ESRs are all offered non-academic secondments in one of our partners, as well as academic secondments as part of their ongoing research.

Our research programme is divided into six sub-projects, three of which develop fully-fledged general purpose event generators (Pythia, Herwig and Sherpa) capable of simulating all the stages of a high-energy particle collision, two of which develop more specialised simulations of specific event generator steps (Madgraph and Plugin, which includes sub-projects Ariadne and HEJ) and one of which (CEDAR) develops a suite of programs related to the deployment and optimisation of these generators for particle physics experiments and analysis.

Work performed

\"All 11 of our long-term ESR positions have been filled. After the short-term studentship programme has been running for 18 months, 31 of the 144 ESR-months have been committed. We have held 2 Annual Schools and are planning 2 for 2019, in the UK and Vietnam. We have held an employability training event, a scientific computing school, 4 Network Meetings, and our Mid-Term Review meeting. Our secondments have started, with one ESR taking a non-academic secondment with B12 and another an academic secondment with Vienna.

All of our event generator development projects have released versions with significant new features, aimed at improving the precision and scope of simulation for collider physics experiments, particularly the LHC.

The Pythia sub-project has extended its successful simulation of proton-proton collisions to the collisions of nuclei (\"\"heavy ions\"\") either with protons or with each other. It has also made important improvements in the accuracy of the parton shower process and the non-perturbative processes that convert partons into the hadrons seen in experiments.

The Herwig sub-project has implemented an automated algorithm for merging higher order matrix elements with its parton shower algorithms, as well as making key improvements to these shower algorithms. It has also experimented with a simple model to extend its simulation to heavy ion collisions, and with non-perturbative improvements.

The Sherpa sub-project has been the first to demonstrate next-to-next-to-leading order accuracy for a key process; generalisations to other processes are ongoing. It has also generalised its QCD machinery to electroweak corrections, which become increasingly important as the energy probed by the LHC increases, and for future very high energy machines.

The Madgraph sub-project has made important progress in the simulation of physics Beyond the Standard Model, particularly in parametrising such physics through effective field theories, rather than constructing specific models, and also in the simulation of Dark Matter. A module has also been developed to simulate a future generation of experiments that search for new physics in high intensity \"\"beam dump\"\" collisions.

The Plugin sub-project consists of several projects. In Ariadne/DIPSY, a new model has been developed to account for collective phenomena in heavy ion and proton collisions. HEJ is aimed at describing processes in LHC collisions where many jets are produced well separated from each other with similar momentum scales. Its formal accuracy has been extended to next-to-leading order and it has been merged consistently with a parton shower.

The CEDAR sub-project is an umbrella for an array of projects focussed on optimal use of event generators for particle physics experiments. Most active have been Rivet (which provides tools to validate event generators against existing data) and Contur (which similarly evaluates new physics models against existing data), with progress also on Professor (for tuning event generator parameters) and LHAPDF (providing parton distribution functions). All of these are important enabling projects for event generators. Important developments have also been made here for heavy ion collisions.\"

Final results

All of the event generator projects already represent the state of the art in the simulation of high energy particle collisions. By the end of the network period they will have completed their current revolutionary progress to higher precision, particularly in the merging of the parton shower approach with matrix element calculations performed at ever higher accuracy. They are also the only tools to simulate non-perturbative transitions between partonic and hadronic degrees of freedom in particle collisions and important progress is expected here, particularly in the further extension of current treatments to heavy ion collisions and high-multiplicity proton collisions, in which collective phenomena appear to play a role.

The primary impacts are through the better understanding and exploitation of LHC data, and through the deeper training of ESRs, many of whom will go on to careers outside our immediate field. The LHC is one of the highest-profile scientific experiments in history, and highly influential in the public\'s perception of science, and in attracting young people to study science, many of whom go on to careers that apply their scientific knowledge and approach to societal problems (three of our non-academic partners were founded by particle physics PhDs, and all four recruit heavily from our field). The LHC\'s headline achievements would have been impossible without the simulations provided by MCnetITN3\'s projects and as such we have a major impact. The accuracy of many LHC analyses is now outstripping the theory predictions with which they are being compared and the chief goal of the MCnetITN3 funding period is to address this balance by a significant upgrade to the accuracy of the simulations. This will leave a legacy of impact beyond our field and well beyond the period of our current network.