Summary Jun 17 2009 Tools and Monte Carlos Session 1: SM issues.

Summary

Jun 17 2009

Tools and Monte Carlos

Session 1: SM issues

17 Jun 2009 P. Skands - WG MC Summary 2

Session 1: SM Issues

1. Tuning

2. Model (In)-dependence in Data/Theory Comparisons

3. Matching

4. Parton Densitites

5. Jet Physics


1. Tuning

• Automated Tuning– Rivet & Professor

• Important data sets for tuning

• Generator Uncertainties– Systematic Evaluation of Errors on tunes– Tuning in the presence of matching

• Underlying Event and Minimum-Bias Models– Energy scaling– MPI-induced X + 2j backgrounds

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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 HepMC 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 HepMC)– Interpolation in parameter space chi2 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, arXiv: 0906.0075 [hep-ph]

• A. Buckley


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 D0• Markus & Giacinto - got positive reply from Henrik Nilsen and he is willing to help

– Minimum bias data from UA5, E735 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)


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)


Generator Uncertainties

• Discussions highlighted need for systematic variations– 0th attempt: Perugia variations (Pythia)

• Central • + 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 v2.0)

• Energy Extrapolations– E-dependence of transverse mass dist?

• Bigger, Blacker, Edgier, …• (R. Godbole, S. Plaetzer, P. Skands)

arXiv: 0905.3418 [hep-ph]


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-pT end of the Z pT 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?

• P. Lenzi


MPI-Induced X + 2 jets

• Interested Parties: E. Maina, R. Chierici, P. Bartalini, S. Plaetzer, P. Skands, K. Mazumdar, S.Gieseke

• Contributions on:– W+4j vs (W+2j)x(2j)– Ttbar+2j vs (ttbar)x(2j) (background to ttH and b’ searches)

• E. Maina

• P. Bartalini


2. Model (in)dependence in Data/Theory Comparisons

• Fast Detector Simulation Specification and Usage

• Data Correction and Unfolding


Fast Detector Simulation Specification and Usage

• Continues in session 2. Midterm recommendations– Input format : HepMC. – 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


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?

– A. Buckley, G. Hesketh, P. Skands, J. Butterworth

• Better to coordinate with Drell-Yan in Matching Group? (see later)

Buckley et al, arXiv: 0906.0075 [hep-ph]


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


1- QCD 1.1- Existing tools and calculations

1.2- Effects of NLO corrections2-EW 2.1- NLO corrections 2.2- Multiples photons emissions 2.3- Sudakov Logs3- Combined effects Combination4- 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 +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,…)

S. Ferrag, K. Mazumdar

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From S. Ferrag


QED/EW Matching

• Working subgroup formed– 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

+ Wishlist on wiki page


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 + PYTHIA8 (Andersen, Skands)

– SHERPA• Start with ordinary shower soft-subtracted shower

(Andersen, Hoeche)

– ARIADNE, LDCMC• Similar (same?) logs already present, systematic comparisons (Lonnblad)

• J. Andersen


LHEF v2.0

• 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

• L. Lönnblad


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)

• S. Plätzer


4. Parton Densities

From F. Olness• F. Olness • R. Frederix• S. Forte• J. Rojo• J. Huston


PDF Uncertainties

From F. Olness


PDF’s for Monte Carlos

• 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

From J. Huston


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

• NB1: With Pythia8, Joey can use NLO pdf’s for the hard matrix element evaluation, while using LO pdf’s for the UE modeling/parton showering

• NB2: 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

Comparison of predictions for the W+ ydistribution at 10 TeV and 14 TeV

Project: (1) comparison of LHC predictions from NLO and LO (using mod LO pdf’s) (2) developing/comparing UE tunes using NLO pdf’s – Huston, Mrenna, …

From J. Huston


5. Jet Physics

• Jets and jet substructure– in QCD jets, ttH and high pT VH

• ttH discriminants – with NLO-ML

• Wbb in the high pT HW region – also involves ME/PS (N)LO matching

• Hard Multijet Radiation / Jet Vetos / High-y


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)

• M. Schwarz

• G. Piacquadio

• P. Francavilla

• P. Loch


Jets and Jet Substructure

• 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-kT, and then declustering with C/A works well.

M.SchwartzTop “monojets”Top Monojets


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 pT(third subjet) > pT(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.)

True Smeared


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-kT jets, use kT to get the y scale.

• Apply pile-up suppression cuts on particles at 0 -> 2 GeV.

• Change in jet pT as a function of the number of interactions in a bunch.

• Linear effect, flattened and at the 3.5% level after applying a 2GeV cut on particles.


Higgs & Jet Substructure• Shown to be promising in

HZ,HW when the Higgs has pT > 200 GeV. (G. Piacquadio, J.Butterworth et al)

• Can this technique help with ttH?

• 17% of ttH events contain a Higgs with pT>200 GeV, and ~50% have one with pT>100 GeV.

• Shows that subjet analyses could have a big impact on ttH.

• Plan to look at using the Higgs substructure analysis on ttH events from Sherpa (particle level)

M.SchwartzHiggs in ttH

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mH

“mono”-JetSee also

Guenther’s talk

ttH


Wbb in the high pT 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 pT of bb pairs and for extra jets, in the Wbb (and Zbb) process, will be important for the eventual high-pT H+W/Z analysis.

• 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)

Hb

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“mono”-Jet

• G. Piacquadio


Wbb in the high pT HW region• Wbb + jet

(ties into matching, PDFs, probes gbb)

• qg -> Wqbb opens up at order s2 : 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).


Plan to write one or more contributions to the proceedingsPlease contact [email protected] to join one of the activities (or suggest a new one!)

Radiation Between Jets and Rapidity Gaps Working Group

• 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)

• M. Campanelli

• P. Francavilla

mailto:[email protected]


Radiation between jets and rapidity gaps working group

Study the 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.

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?

P.Francavilla


Studentship Opportunities

• MCnet

– EU funded network comprised of five main general-purpose 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.)

http://www.montecarlonet.org

MCnet also organizes an annual school: This year’s school is in Lund Jul 1 – 4. Next summer in Karlsruhe


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

Summary Jun 17 2009 Tools and Monte Carlos Session 1: SM issues.

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