Tools and Monte Carlos Session 1 SM issues

Tools and Monte Carlos Session 1: SM issues Summary Jun 17 2009 17 Jun 2009 P. Skands - WG MC Summary

Session 1: SM Issues 1. Tuning 2. Model (In)-dependence in Data/Theory Comparisons 3. Matching 4. Parton Densitites 5. Jet Physics 17 Jun 2009 P. Skands - WG MC Summary 2

Constraints 1. Tuning • Automated Tuning – Rivet & Professor • Important data sets for tuning Calculations • Generator Uncertainties 17 Jun 2009 – Systematic Evaluation of Errors on tunes – Tuning in the presence of matching • Underlying Event and Minimum-Bias Models – Energy scaling – MPI-induced X + 2 j backgrounds P. Skands - WG MC Summary 3

• A. Buckley Automated Tuning: Rivet and Professor • Rivet http: //projects. hepforge. org/rivet/ – Set of experimental analyses model constraints – Convenient infrastructure for adding your own analyses • Can in principle also be used for theory-to-theory comparisons – Uses Hep. MC records generator independent – Thursday we had a Rivet tutorial (Andy Buckley) • Professor http: //projects. hepforge. org/professor/ – MC tuning tool: set of command line programs and an underlying library – Rivet used to generate MC data and retrieve experimental ref data • (So can be used by you, if your generator Hep. MC) – Interpolation in parameter space chi 2 minimization Set of optimal parameters, but beware: still both art and science – First generation of (central) Professor tunes now ready • A second generation explore ‘tuning uncertainties’? Buckley et al, ar. Xiv: 0906. 0075 [hep-ph] 17 Jun 2009 P. Skands - WG MC Summary 4

Important Data Sets for Tuning • Identify important data sets (for inclusion in Rivet) – CDF inclusive jet shapes • in already & validated – CDF b-jet shapes. • Available in rivet but needs to be validated. – ZEUS jet shapes paper (quark/gluon jets) • ? – Differential Cross sections in Z/gamma events by D 0 • Markus & Giacinto - got positive reply from Henrik Nilsen and he is willing to help – Minimum bias data from UA 5, E 735 which is in HZTOOL • Should be moved to rivet. (Jon B) – Run I CDF data on Kaons and Lambdas • ? – Theory: NLL event shape data • • Could be used as pseudo-data for MC comparisons Matt S – Fragmentation functions from LEP, CDF • • ? Additional volunteers, additional data sets, let us know! (use LH wiki) – “Writing a rivet class is fun, useful and not too hard!” (Jon) 17 Jun 2009 P. Skands - WG MC Summary 5

Important Data Sets for Tuning • Note: CDF max/min cone analysis seems to have been fixed/validated in rivet during the meeting – (Andy Buckley, Markus Warsinsky). – (Was a problem with using full in stead of charged particle jets) Pythia 6. 420 (Tune 325: Perugia LO*) Track jets Calo jets M. Warsinsky (using Rivet) 17 Jun 2009 P. Skands - WG MC Summary 6

Generator Uncertainties • Discussions highlighted need for systematic variations – 0 th attempt: Perugia variations (Pythia) • Central ar. Xiv: 0905. 3418 [hep-ph] • + vars: Hard, Soft, CR, Energy-Scaling, PDF – Enter the Professor … (? ) • Add vars for data set weighting? (e. g. , tunes dedicated to B physics, etc. ) – Events with uncertainties? ( LHEF v 2. 0) • Energy Extrapolations – E-dependence of transverse mass dist? • Bigger, Blacker, Edgier, … • (R. Godbole, S. Plaetzer, P. Skands) 17 Jun 2009 P. Skands - WG MC Summary 7

• P. Lenzi Tuning in the Presence of Matching • Interested parties: – Piergiulio Lenzi, Vitaliano Ciulli, Peter Skands, Leif Lonnblad, Simon Platzer, Jon Butterworth, Jeppe Andersen, Mario Campanelli, Markus Warsinsky, Matthew Schwartz, Peter Loch, Dieter Zeppenfeld, Giacinto Piacquadio, Paulo Francavilla +. . . • Chart the problem. Determine some Classic Examples – Effects of ME Corrections on low-p. T end of the Z p. T distribution compared for PYTHIA, HERWIG, Sherpa, Alpgen. – Preliminary conclusion: matching strategies that preserve total normalization (reweighting), in particular Pythia soft region affected by hard matching! • How would low-x logs/high-energy limit affect tunes? – Expect extra events at with high rapidity jets – Contains logs not necessarily present in the umatched MC – Presumably distort min bias tunes etc. The first effect actually can be modelled by HERWIG+JIMMY, to some extent at least. – Jeppe needs a shower (see later) • (Not addressed: ) – How dependent on matching strategy? – Are there ways of revamping pre-matched tunes? 17 Jun 2009 P. Skands - WG MC Summary 8

• E. Maina • P. Bartalini MPI-Induced X + 2 jets • Interested Parties: E. Maina, R. Chierici, P. Bartalini, S. Plaetzer, P. Skands, K. Mazumdar, S. Gieseke • Contributions on: – W+4 j vs (W+2 j)x(2 j) – Ttbar+2 j vs (ttbar)x(2 j) (background to tt. H and b’ searches) 17 Jun 2009 P. Skands - WG MC Summary 9

2. Model (in)dependence in Data/Theory Comparisons • Fast Detector Simulation Specification and Usage • Data Correction and Unfolding 17 Jun 2009 P. Skands - WG MC Summary 10

Fast Detector Simulation Specification and Usage • Continues in session 2. Midterm recommendations – Input format : Hep. MC. – Detector simulation : generator independent restrict itself to looking at final state (status code 1) particles. – Standardized output to Rivet and/or user code • suggested ideas based on "Reconstructed Objects" (4 -vector with optional list of numbers for efficiency, isolation etc). • How specified should these things be? Do jets point to constituents? Strong preference for keeping it simple. • Useful to define some key plots for checking external simulation packages against the detector in-house versions. • Short write-up of current internal experiment simulations, and – next session – of external products (e. g. Delphes) Simon Dean, Jon Butterworth, Peter Loch, Samir Ferrag, Frank-Peter Schilling, Fabio Maltoni, Matthew Schwartz, Steve Mrenna, Andy Buckley, Joanna Weng 17 Jun 2009 P. Skands - WG MC Summary 11

Data Correction and Unfolding • Some interesting discussions (Steve M, Jon B, +…) but no specific project to take forward… • Not the critical mass/expertise here to really follow up/agree on treatment of EW corrections to final state leptons (recall intro talks from both Peter & Jon) • Contribution outlining / highlighting the issue? Buckley et al, ar. Xiv: 0906. 0075 [hep-ph] – A. Buckley, G. Hesketh, P. Skands, J. Butterworth • Better to coordinate with Drell. Yan in Matching Group? (see later) 17 Jun 2009 P. Skands - WG MC Summary 12

3. Matching • Matching Benchmarks – F. Maltoni, P. Skands, S. Hoeche, K. Hamilton, S. Plaetzer, S. Mrenna, L. Lonnblad, P. Francavilla, J. Winter, M. Schwarz, P. Lenzi, J. Huston, J. Andersen, J. Weng, F. -P. Schilling, P. Uwer, R. Chierici, P. Bartalini, L. Reina – Little work done AT Les Houches • Projects will need attentive babysitters! – – – Comprehensive gg->H study (Babysitters: Maltoni + Andersen) W + >= 2 jets & Wbb (see Jet Physics) ttbar + jets (Babysitter: F. -P. Schilling; with P. Bartalini on MPI) Radiation in WBF (Babysitter: ? ? ? No names on wiki) QCD n-jets (Babysitters: P. Francavilla, ? ? ? ) “Pathological Observables”: (Babysitters: M. Schwarz, P. Skands) • To interact with matching studies, observables that: – Test Higher-Order / Higher-Log properties of schemes – Are sensitive / insensitive to remaining higher-order ambiguities 17 Jun 2009 P. Skands - WG MC Summary 13

From S. Ferrag Focus Here Previous LH Observables: Drell-Yan (2 -e) 1 - QCD 1. 1 - Existing tools and calculations 1. 2 - Effects of NLO corrections 2 -EW 2. 1 - NLO corrections 2. 2 - Multiples photons emissions 2. 3 - Sudakov Logs 3 - Combined effects Combination 4 - Uncertainties theory 4. 1 PDFs 4. 2 Energy scale (QCD, QED) 4. 3 Input Scheme 4. 4 Others a-QED+QCD b-NNLO… c-Shower… d-Underlying events e- … 5 - Uncertainties from experimentation (Acceptance: pt, eta. . (trigger det performance), ) AP PRIORI 100% Experimentalist business but… 6 - Experiment vs theory agree on what to be compared 6. 1 -Drell Yan definition: lep +X, lep+lep + vetoes (jets, MET), …. impact on backgrounds 6. 2 -Cuts: Pt, eta, Isolation 6. 3 - observables: choice of observables to emphasize corrections (pair pt for NLO, …) 17 Jun 2009 P. Skands - WG MC Summary S. Ferrag, K. Mazumdar 14

QED/EW Matching • Working subgroup formed + Wishlist on wiki page – Tests + manual matching • • So far based on ALPGEN + PYTHIA Z + 0 gamma + 0 jets + Pythia QED shower Z + 1 gamma + 0 jets m gamma + n jets – Automated matching procedure being validated • m gamma + n jets S. Gascon, F. Piccinini, C. Baty, K. Mazumdar, R. Pittau 17 Jun 2009 P. Skands - WG MC Summary 15

• J. Andersen Hard Multijets / High-y • Several people thought Jeppe needs a shower – Avoid log double counting • Could envision several different routes, start with simplest – In PS region populated by BFKL, do pure-collinear showering avoid double-counting of soft singularities – In PS region not populated by BFKL, do full shower – HYPERJET • BFKL LHEF VINCIA + PYTHIA 8 (Andersen, Skands) – SHERPA • Start with ordinary shower soft-subtracted shower (Andersen, Hoeche) – ARIADNE, LDCMC • Similar (same? ) logs already present, systematic comparisons (Lonnblad) 17 Jun 2009 P. Skands - WG MC Summary 16

• L. Lönnblad LHEF v 2. 0 • Comprehensive update of Les Houches Event Files designed for matching applications (including NLO) • Preliminary proposal presented yesterday – Leif Lönnblad, see wiki page • To be discussed with other MC generator authors at CERN TH institute in August produce writeup 17 Jun 2009 P. Skands - WG MC Summary 17

• S. Plätzer Automating LO/NLO Matching • Several discussions acquainted both sides (fixed-order + shower-MC communities) with the issues relevant on each side – Important first step • Now awaiting NLO standardization and preparing pilot projects for stress tests possible proceedings contributions – S. Plaetzer, S. Mrenna, P. Skands, … • Summary of the issues and preliminary proposal available on wiki (S. Plaetzer) 17 Jun 2009 P. Skands - WG MC Summary 18

• F. Olness • R. Frederix • S. Forte • J. Rojo • J. Huston 17 Jun 2009 From F. Olness 4. Parton Densities P. Skands - WG MC Summary 19

From F. Olness PDF Uncertainties 17 Jun 2009 P. Skands - WG MC Summary 20

PDF’s for Monte Carlos From J. Huston • LO pdf’s (in LO Monte Carlo programs) can lead to predictions with not only the wrong normalization but the wrong shape; see the W+ y distribution • Better shapes for cross section predictions can be provided by NLO pdf’s, but the underlying event tunes with these pdf’s are problematic • Modified LO pdf’s can be designed to look like LO pdf’s at low x and NLO pdf’s at high x 17 Jun 2009 P. Skands - WG MC Summary 21

Modified LO pdf’s • • • Modified LO pdf’s, first from MSTW and now from CTEQ, attempt to provide better shape and normalization for the LO predictions by relaxing the momentum sum rule, and in the case of CTEQ, adding NLO LHC pseudo-data from benchmark processes into the fit NB 1: With Pythia 8, Joey can use NLO pdf’s for the hard matrix element evaluation, while using LO pdf’s for the UE modeling/parton showering NB 2: it is necessary to use NLO pdf’s for the UE modeling for NLO programs such as Powheg or MC@NLO, so good UE tunes with NLO pdf’s are still necessary 17 Jun 2009 From J. Huston Comparison of predictions for the W+ y distribution at 10 Te. V and 14 Te. V Project: (1) comparison of LHC predictions from NLO and LO (using mod LO pdf’s) (2) developing/comparing UE tunes using NLO P. Skands - WG MC Summary 22 pdf’s – Huston, Mrenna, …

5. Jet Physics • Jets and jet substructure – in QCD jets, tt. H and high p. T VH • tt. H discriminants – with NLO-ML • Wbb in the high p. T HW region – also involves ME/PS (N)LO matching • Hard Multijet Radiation / Jet Vetos / High-y 17 Jun 2009 P. Skands - WG MC Summary 23

• M. Schwarz • G. Piacquadio • P. Francavilla • P. Loch Jet Substructure • Boosted hadronic decays of massive particles (W, Z, Top, H, BSM. . . , with session 2) and jet mass/shape studies with QCD jets in early data. • Interested parties (Session 1) : – Matt Schwartz, Giacinto Piacquadio, Mario Campanelli, Paulo Francvilla, Jon Butterworth, Peter Loch, Ezio Maina, Leif Lonnblad, Keith Hamilton, Simon Dean, Rohini Godbole, Jan Winter… • Types of object – QCD jets (quark gluon separation) • SUSY cascades rich in quark jets. Use q ID to simplify decay chains? – Colour singlet heavy objects, two body decay (W, Z, H. . . ) • For Higgs searches, SM, or SUSY/Exotics – Colour singlet heavy objects, three body decay (neutralino…) – Coloured heavy objects, three body decay (top) 17 Jun 2009 P. Skands - WG MC Summary 24

Jets and Jet Substructure M. Schwartz Top “monojets” Top Monojets • Different jet algorithms: – Smallest invariant mass of the sum of any 2 subjets of the top jet (which can contain 4 subjets) • Variable is sensitive to the QCD singularity in the background (solid, red) and has a W mass peak in the signal (blue). – Note that anti-KT declustering does not provide strong discrimination, but finding the jets with anti-k. T, and then declustering with C/A works well. 17 Jun 2009 P. Skands - WG MC Summary 25

MC Issues in Jet Substructure • Differences in heavy object decays; different parton showers, matrix element corrections in some MC, not in others; spin correlations; – Keith Hamilton, Giacinto Piacquadio, Matt Schwartz, Leif Lonnblad, Jan Winter to produce a short summary of the effects implemented in different MCs. • How sensitive are the various subjet methods to the differences? – Examples from G. Piacquadio – First plot shows that Herwig fills almost no events with p. T(third subjet) > p. T(second b-subjet), while PYTHIA does (due to ME corrections). – Rad spectrum therefore softer in HERWIG with respect to PYTHIA. – Is this the reason one sees a degradation in the mass resolution at hadron level in HERWIG vs PYTHIA? – (detector smearing difference smeared out and mass distributions are again quite comparable. ) 17 Jun 2009 True P. Skands - WG MC Summary Smeared 26

Detector Issues in Jet Substructure • • • Pile-up, calorimeter noise, granularity, acceptance: The reconstruction quality for the various jet shape variables need to be understood. Study (by P. Francavilla, P. Loch) of the effect of pile-up (average 4 events per bunch, poisson distribution) on various jet/subjet variables. Anti-k. T jets, use k. T to get the y scale. Apply pile-up suppression cuts on particles at 0 -> 2 Ge. V. Change in jet p. T as a function of the number of interactions in a bunch. Linear effect, flattened and at the 3. 5% level after applying a 2 Ge. V cut on particles. 17 Jun 2009 P. Skands - WG MC Summary 27

Higgs & Jet Substructure • Shown to be promising in HZ, HW when the Higgs has p. T > 200 Ge. V. (G. Piacquadio, J. Butterworth et al) • Can this technique help with tt. H? • 17% of tt. H events contain a Higgs with p. T>200 Ge. V, and ~50% have one with p. T>100 Ge. V. • Shows that subjet analyses could have a big impact on tt. H. • Plan to look at using the Higgs substructure analysis on tt. H events from Sherpa (particle level) 17 Jun 2009 See also Guenther’s talk tt. H P. Skands - WG MC Summary m. H “mono”-Jet b H W M. Schwartz Higgs in tt. H u l 28

• G. Piacquadio Wbb in the high p. T HW region • Interested parties: Laura Reina, Giacinto Piacquadio, Sally Dawson, Jon Butterworth, Ketavi Assamagan, Steve Mrenna, Matthew Schwartz, Rohini Godbole, +. . . • Reliable predictions for mass and p. T of bb pairs and for extra jets, in the Wbb (and Zbb) process, will be important for the eventual high-p. T H+W/Z analysis. m. H b “mono”-Jet H W u l • Having a better idea of the rate is also interesting now, to estimate the sensitivity. • After cuts, parton showers/LL seem to be doing a reasonable job but still need to get fixed-HO and mass effects under control (see distributions from G. Piacquadio available on wiki) 17 Jun 2009 P. Skands - WG MC Summary 29

Wbb in the high p. T HW region • Wbb + jet (ties into matching, PDFs, probes g bb) • qg -> Wqbb opens up at order as 2 : large scale dependence. • Enhanced by gluon PDF. • Vetos on extra jet reduce K-factor from around 3 to probably below 1. 5. • Need to examine LO matched calculations (do they exist? ) and the possibility of doing a NLO matched calculation. • Plan to make NLO distributions in exact kinematic region of the analysis (L. Reina). 17 Jun 2009 P. Skands - WG MC Summary 30

Radiation Between Jets and Rapidity • M. Campanelli Gaps Working Group • P. Francavilla • Activities: – Study of the density of jets using the area method, to discriminate between jets mainly coming from UE and from radiation (P. Francavilla, M. Campanelli) – Study of long-distance over short-distance fluctuations of the UE jets (see previous references) (P. Loch) – Study of rapidity gaps, after subtraction of underlying event. Comparison of activity in the various gap regions in eta and phi, to region between gap and beam line. Collective vs local effects (M. Campanelli, J. Weng) Plan to write one or more contributions to the proceedings Please contact Mario. Campanelli@cern. ch to join one of the activities (or suggest a new one!) 17 Jun 2009 P. Skands - WG MC Summary 31

Radiation between jets and rapidity gaps working group Study the shape of radiation off jets, possibly separate jets from hard scattering and underlying event. Separation should improve our ability to distinguish colour singlet and octet exchange processes, and define events with rapidity gaps for diffractive and VBF Higgs studies. P. Francavilla Example: jet density for mainly UE jets (red) and hard scattering (black) at parton level. Can we do an event-by-event UE density determination? What happens with hadronisation/detector? 17 Jun 2009 P. Skands - WG MC Summary 32

Studentship Opportunities • MCnet http: //www. montecarlonet. org – EU funded network comprised of five main generalpurpose Monte Carlo nodes • CERN, Durham, Karlsruhe, Lund, UCL – Extensive funds available for studentships • Experimental students, working on a particular MC issue together with generator experts (or Jon) • Theory students, working on modeling of SM/BSM physics or pheno studies • 3 – 6 months, includes travel and “pocket” money (for books, etc. ) MCnet also organizes an annual school: This year’s school is in Lund Jul 1 – 4. Next summer in Karlsruhe 17 Jun 2009 P. Skands - WG MC Summary 33

Summary • Complaint heard most often: – “There are too many sessions scheduled this year” • But kind of looked about the same as previous times… – Conclusion? • More people must have wanted to go to more sessions • More interest across fields, groups, subgroups, … • So actually a sign of success? • Monte Carlo Tools – Still moving at the first few orders in expansions in multiple parameters (of which the coupling order is just one) • Writing a good generator is still not an exact science • However, with increasing exact-science input, there is hope for better constraints on the arts-and-humanities aspects use science to improve art – Increasing emphasis on “reliable” uncertainty estimates 17 Jun 2009 P. Skands - WG MC Summary 34