Parton Shower Monte Carlo Event Generators Mike Seymour University - PowerPoint PPT Presentation

Parton Shower Monte Carlo Event Generators Mike Seymour University of Manchester & CERN MC4LHC EU Networks’ Training Event May 4 th – 8 th 2009 http://www.montecarlonet.org/

Overview and Motivation Babar’s observation of η b Parton Shower MCs 1 Mike Seymour

Overview and Motivation D0’s observation of ZZ production Parton Shower MCs 1 Mike Seymour

Overview and Motivation ATLAS’s observation of H →γγ ? Parton Shower MCs 1 Mike Seymour

MCnet • Marie Curie Research Training Network • for Monte Carlo event generator – development – validation and tuning • Approved for four years from 1 st Jan 2007 Parton Shower MCs 1 Mike Seymour

CERN Durham Mike Seymour Lars Sonnenschein Alberto Ribon Alex Flossdorf (incl. Cambridge) Peter Skands Michal Deak Userlink Andrzej Siodmok Stefan Hoeche Peter Richardson Steffen Schumann Frank Krauss Jan Winter Bryan Webber Andy Buckley Martyn Gigg David Grellscheid MCnet James Monk Jon Butterworth Seyi Latunde-Dada Keith Hamilton Mark Sutton Emily Nurse Sasha Sherstnev Frank Siegert Manuel Baehr Robert Thorne Jonathan Tully Marek Schoenherr Noam Hod Ben Waugh Tanju Gleisberg Jonathan Ferland Piergiulio Lenzi Mike Whalley Jennifer Archibald Alex Richards Matthew Wing Andreas Beate Heinemann Karlsruhe Papaefstathiou Lund Filip Moortgat (incl. Krakow) Steve Mrenna Paolo Nason Leif Lönnblad Stefan Gieseke Simon Plaetzer Torbjörn Sjöstrand External Advisors Hendrik Hoeth Manuel Baehr Mark Gibbs Nils Lavesson Luca D’Errico Paolo Nason Richard Corke Christoph Beate Heinemann Christopher Flensburg Hackstein Filip Moortgat Florian Bechtel Andreas Morsch Parton Shower MCs 1 Mike Seymour Gloria Corti

Jon Butterworth Frank Siegert Emily Nurse Andy Buckley Ben Waugh James Monk Mike Whalley Piergiulio Lenzi CEDAR Lars Sonnenschein Alex Richards Hendrik Hoeth PYTHIA Herwig Torbjörn Sjöstrand Peter Richardson Bryan Webber Steve Mrenna MCnet Mike Seymour Peter Skands Richard Corke Alberto Ribon Hendrik Hoeth Stefan Gieseke Andrzej Siodmok Florian Bechtel Martyn Gigg Noam Hod David Grellscheid ThePEG SHERPA Keith Hamilton Seyi Latunde-Dada Ariadne Sasha Sherstnev Jonathan Tully Frank Krauss Leif Lönnblad Simon Plaetzer Tanju Gleisberg Nils Lavesson Manuel Baehr Christopher Flensburg Stefan Hoeche Luca D’Errico Steffen Schumann Michal Deak Jan Winter Andreas Papaefstathiou Frank Siegert Parton Shower MCs 1 Mike Seymour Marek Schoenherr

MCnet objectives Training: • To train a large section of the user base in the physics and techniques of event generators • To train the next generation of event generator developers Through Research: • To develop the next generation of event generators intended for use throughout the lifetimes of the LHC and ILC experiments • To play a central role in the analysis of early LHC data and the discovery of new particles and interactions there • To extract the maximum potential from existing data to constrain the modeling of the data from the LHC and other future experiments. Parton Shower MCs 1 Mike Seymour

MCnet main activities • Four postdoc positions • Two joint studentships (Karlsruhe–Durham, Durham–UCL) • Annual School • Two annual meetings (Januarys @ CERN, summer with school) • Annual programme of ‘residentships’: short term studentships held January by experimental and theoretical users Usually 3–4 months, 2007 maximum 6 months – Mechanism for enhanced user support in 2008–2010 with CTEQ • Networking – Travel and visitor money – Web site (www.montecarlonet.org) Parton Shower MCs 1 Mike Seymour

MCnet opportunities 2009: • MC4LHC (with Heptools and Artemis) • MCnet school (  May 1 st ) • Short-term studentships: for th. and exptl. students to spend 3-6 months with MC authors in: - CERN - Durham/Cambridge - Karlsruhe - Lund - UCL on a project of their choice - Next closing dates: - May 4 th - August 3 rd Parton Shower MCs 1 Mike Seymour

Introduction to Parton Shower Monte Carlo Event Generators • Basic principles • LHC event generation • Parton showers • Hadronization • Underlying Events • Practicalities Parton Shower MCs 1 Mike Seymour

Structure of LHC Event Simulations 1. Hard process 2. Parton shower 3. Hadronization 4. Underlying event Parton Shower MCs 1 Mike Seymour

Hard Process Simulation Typically use fixed-order perturbative matrix elements Leading order can be largely automated… • MADGRAPH/MADEVENT • GRACE • COMPHEP Matrix elements squared • AMAGIC++ (SHERPA) positive definite  simple • ALPGEN Monte Carlo implementation • HELAC Next-to-leading order getting there… • MCFM • NLOJET++ Real and virtual contributions have • MC@NLO equal and opposite divergences  • … … naïve Monte Carlo fails Parton Shower MCs 1 Mike Seymour

Hard Process Simulation Typically use fixed-order perturbative matrix elements Leading order can be largely automated… • MADGRAPH/MADEVENT • GRACE • COMPHEP Matrix elements squared • AMAGIC++ (SHERPA) positive definite  simple • ALPGEN Monte Carlo implementation • HELAC But… • Fixed parton/jet multiplicity • No control of large logs  Need hadron level event generators • Parton level Parton Shower MCs 1 Mike Seymour

Intro to Monte Carlo Generators 1. Basic principles 2. Parton showers 3. Hadronization 4. Introduction to the MCnet Monte Carlo Event Generator projects Parton Shower MCs 1 Mike Seymour

Parton Showers: Introduction QED: accelerated charges 1. annihilation to jets. radiate. 2. Universality of collinear QCD identical: accelerated emission. colours radiate. 3. Sudakov form factors. gluons also charged. 4. Universality of soft  cascade of partons. emission. = parton shower. 5. Angular ordering. 6. Initial-state radiation. 7. Hard scattering. 8. Heavy quarks. 9. Dipole cascades. 10. Matrix element matching Parton Shower MCs 1 Mike Seymour

annihilation to jets Three-jet cross section: singular as Rewrite in terms of quark-gluon opening angle and gluon energy fraction : Singular as and . Parton Shower MCs 1 Mike Seymour

can separate into two independent jets: jets evolve independently Exactly same form for anything eg transverse momentum: invariant mass: Parton Shower MCs 1 Mike Seymour

Collinear Limit Universal: Dokshitzer-Gribov-Lipatov- Altarelli-Parisi splitting kernel: dependent on flavour and spin Parton Shower MCs 1 Mike Seymour

Resolvable partons What is a parton? Collinear parton pair single parton Introduce resolution criterion, eg Virtual corrections must be combined with unresolvable real emission Resolvable emission Finite Virtual + Unresolvable emission Finite Unitarity: P(resolved) + P(unresolved) = 1 Parton Shower MCs 1 Mike Seymour

Sudakov form factor Probability(emission between and ) Define probability(no emission between and ) to be . Gives evolution equation c.f. radioactive decay atom has probability per unit time to decay. Probability(no decay after time T) = Parton Shower MCs 1 Mike Seymour

Sudakov form factor Probability(emission between and ) Define probability(no emission between and ) to be . Gives evolution equation Sudakov form factor =Probability(emitting no resolvable radiation) Parton Shower MCs 1 Mike Seymour

Multiple emission But initial condition? Process dependent Parton Shower MCs 1 Mike Seymour

Monte Carlo implementation Can generate branching according to By choosing uniformly: If no resolvable radiation, evolution stops. Otherwise, solve for =emission scale Considerable freedom: All formally free choices, Evolution scale: but can be very important numerically z: Energy? Light-cone momentum? Massless partons become massive. How? Upper limit for ? Parton Shower MCs 1 Mike Seymour

Running coupling Effect of summing up higher orders: absorbed by replacing by Much faster parton multiplication – phase space fills with soft gluons. Must then avoid Landau pole: now becomes physical parameter! Parton Shower MCs 1 Mike Seymour

Soft limit Also universal. But at amplitude level… soft gluon comes from everywhere in event.  Quantum interference. Spoils independent evolution picture? Parton Shower MCs 1 Mike Seymour

Angular ordering NO: outside angular ordered cones, soft gluons sum coherently: only see colour charge of whole jet. Soft gluon effects fully incorporated by using as evolution variable: angular ordering First gluon not necessarily hardest! Parton Shower MCs 1 Mike Seymour