Multipurpose Event Generators and ep Physics Simon Plätzer IPPP, Department of Physics, Durham University & PPT, School of Physics and Astronomy, University of Manchester & Particle Physics, University of Vienna at the VHEeP Workshop | München, 2 June 2017
Multipurpose Event Generators Indispensable tools for experiments & phenomenology. Realistic, fully detailed simulation spanning orders of magnitude in relevant energy scales. Factorization dictates work fmow: Hard process calculation Parton shower algorithms Multiple interaction models Hadronization models
Event generators overview Multipurpose Herwig 7 Pythia 8 Sherpa 2 Hard amplitudes some internal, some internal, general LO internal, general via libraries general via ev. fjles loops via libraries Shower options QTilde, pt ordered, CSShower, Dipoles DIRE, VINCIA DIRE NLO Matching internal automated, external internal automated, sub. & mult. S-MC@NLO NLO Merging yes yes yes Hadronization Cluster String Cluster Specialized (ep context): Ariadne (dipoles), Cascade (CCFM), DIPSY (IS evolution), ...
Multipurpose Event Generators: State of the Art Hard process calculation One loop, many legs. Automated. Parton shower algorithms Hard to claim controllable uncertainty. → Multiple interaction models Eikonal or Interleaved, Difgraction (?) Hadronization models New insights into colour reconnection. Hot stufg Pushing showers to higher orders.
The Toolbox LHEF & PDF Hooks LHAPDF (T*L) |T|^2 Showers, BLHA MPI, Merging Hadronization, σ Ti.Tj Decays Extra T Native L HepMC S &M Ф
ep Status All LHC-age multipurpose event generators can simulate DIS. Just showers can't work beyond inclusive stufg – desperately need a good description of hard jets: Matching & merging mandatory. Thanks to general frameworks and automation, ep simulation is possible including all of the state-of-the-art enhancements. Dedicated ep, eA studies by specialized efgorts, e.g. DIPSY [Gustafson, Lönnblad et al. '07 – ] LHC physics sets the priority for multipurpose event generators. ep studies highly demanded to cross check assumptions and new development in all (also LHC relevant) physics domains. Little to present though – also high energy studies mostly done for FCC-hh/ee. Remarks and suggestions later, focus on recent (perturbative) development.
Parton Shower Variations [Bellm, Nail, Plätzer, Schichtel, Siodmok – Eur.Phys.J. C76 (2016) 665] [Bellm, Plätzer, Richardson, Siodmok, Webster – Phys.Rev. D94 (2016) no.3, 034028] [Mrenna, Skands – Phys.Rev. D94 (2016) no.7, 074005] [Bothmann, Schönherr, Schumann –Eur.Phys.J. C76 (2016) no.11, 590] Aim at evaluating event generator uncertainties in a global prescription Need to evaluate uncertainties of building blocks one at a time. → Then pin down cross feed, making minimal assumptions. → Start with the perturbative part: Parton showers and matching/merging. On-the fmy reweighting available in all multipurpose event generators. Constrain by demanding controllable uncertainties: Small/large where showers are expected to be reliable/unreliable. → Consistent between systematically difgerent algorithms. →
Logarithmic structure [Bellm, Nail, Plätzer, Schichtel, Siodmok – Eur.Phys.J. C76 (2016) 665] Look at generic Sudakov exponent: AlphaS running on top, also PDF arguments.
Hard Shower Scales [Bellm, Nail, Plätzer, Schichtel, Siodmok – Eur.Phys.J. C76 (2016) 665] Resummation needs to be cut ofg at a typical hard scale veto on hard emissions, region → to be fjlled by matching. Resummation properties are heavily infmuenced by the way resummation is being switched ofg. Study scale variations in angular ordered and Dipole showers at a benchmark setting where we observe absolutely comparable resummation properties: Hard veto scales, factorization/renormalization scales in the shower and hard process.
Controllable uncertainties – LO [Bellm, Nail, Plätzer, Schichtel, Siodmok – Eur.Phys.J. C76 (2016) 665] Choice of the hard veto scale is crucial to reproduce hard process input: typically average transverse momenta of hard objects. Controllable uncertainties can only be established by narrow, smeared versions of a theta function, confjrming simple LL arguments. We can now check the impact of higher order improvements. Still “qualitative” procedure unless showers get higher order corrections.
NLO Matching logarithmic structure “leading” contribution LL NLO LO NLL coupling order “accuracy” inclusive cross section exclusivity/resolution difgerential “jet bin” cross section
Showers in a nutshell Showers have virtual and real emission contributions: Showers preserve the total inclusive cross section: Unitarity. . Showers approximate tree level matrix elements: In the collinear limits, and in the soft limit for large number of colours N.
The Matching Condition .
Solving the Matching Condition Infrared cutofg prevents fjnite weights. Add power correction (IR safe observables!) to fjx divergences.
Solving the Matching Condition dσ matched = + – Infrared cutofg prevents fjnite weights. Add power correction (IR safe observables!) to fjx divergences.
Solving the Matching Condition dσ matched = + – Infrared cutofg prevents fjnite weights. Add power correction (IR safe observables!) to fjx divergences.
Solving the Matching Condition dσ matched = + – Infrared cutofg prevents fjnite weights. Add power correction (IR safe observables!) to fjx divergences.
NLO Matching Highly automated – uncertainties and scale setting currently addressed in detail, but no other ambiguities left. This is default for LHC simulation, including complex VBF processes. [Rauch, Plätzer – Eur.Phys.J. C77 (2017) no.5, 293] DIS input needed for understanding And constraining systematics in matching for VBF processes, which show several interesting features. First implementation of DIS matching inside Herwig within this context. [D'Errico, Richardson – Eur.Phys.J. C72 (2012) 2042]
(N)LO Multijet Merging logarithmic structure “leading” contribution LL NLO LO NLL coupling order “accuracy” inclusive cross section exclusivity/resolution difgerential “jet bin” cross section
(N)LO Multijet Merging logarithmic structure “leading” contribution LL NLO LO NLL coupling order “accuracy” inclusive cross section exclusivity/resolution difgerential “jet bin” cross section
(N)LO Multijet Merging logarithmic structure “leading” contribution LL NLO LO NLL coupling order “accuracy” inclusive cross section exclusivity/resolution difgerential “jet bin” cross section
(N)LO Multijet Merging logarithmic structure “leading” contribution LL NLO LO NLL coupling order “accuracy” inclusive cross section exclusivity/resolution difgerential “jet bin” cross section
Multijet Merging – How?
Multijet Merging – How? “It's complicated.”
Motivation: Multiple Shower Emissions . Basic idea: replace approximate matrix elements with exact ones, but keep Sudakov factors which regularize divergences.
LO Merging – Phase Space Considerations Cut phase space into matrix element and parton shower populated regions. .
LO Merging – Phase Space Considerations Cut phase space into matrix element and parton shower populated regions. .
LO Merging – Phase Space Considerations Cut phase space into matrix element and parton shower populated regions. .
LO Merging – Phase Space Considerations Cut phase space into matrix element and parton shower populated regions. .
LO Merging – Phase Space Considerations Cut phase space into matrix element and parton shower populated regions. .
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