Conveners:

Fabio Maltoni (UC Louvain)

Jon Butterworth (UC London)

Peter Skands (Fermilab)

Tools and Monte Carlos

Session 1: SM issues

Session 1: SM Issues

1. Tuning

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

3. Matching

4. Parton Densitites

5. Jet Physics

6. ?

General Procedure

• Kick-Offs: brainstorm sessions today + tomorrow– IMPORTANT to attend– Later, further storms may be scheduled as needed

• Conveners have a list of possible interesting topics to use as a baseline. Not exhaustive nor exclusive.

– Declare your interests– We’ll collect a list of possible projects with at least one

name attached to each one – We’ll put these on the wiki and add/remove as

necessary. Each will have its own wiki page proceedings contribution

Schedule on Wiki

14:00

16:00

17:00

09:00

11:00

14:00

16:00

After first few days, organization will be more dynamical up to you

Think something needs to be discussed? Book a room!

Tuning – The IssueUA5 @ 540 GeV, single pp, charged multiplicity in minimum-bias

events

Simple physics models ~ Poisson

Can ‘tune’ to get average right, but

much too small fluctuations

inadequate physics model

More Physics:

Multiple interactions +

impact-parameter

dependenceMoral:

1) It is not possible to ‘tune’ anything better than the underlying physics model allows

2) Failure of a physically motivated model usually points to more physics (interesting)

3) Failure of a fit not as interesting

Tuning – Current Issues

• Automation• Validation• Uncertainties (of tunes, and of MCs in general)• Impact of Tunes on Exp. Calibrations

– Cf Model (in)-dependence of TH/EX comparisons

• Tuning in the presence of matching– Cf Matching

• Extrapolations to the LHC– What to do with first data and what we get from it

Tuning – Organization

• Tuning Brainstorm Session

• Anything connected with precision of Monte Carlos, uncertainties, tuning, systematics, fragmentation/hadronization, automation, validation, programme of LHC measurements for SM/MC validation, etc.

WEDNESDAY 9:00, AUDITORIUM

Model Independence

• Covered by Jon• Example: Monte Carlo Truth

– What is it?– Can we even define it?– Can we define it better?– Can we define it

independently of MC? – Cf Tevatron Drell-Yan pT

distribution• Crucial for tuning

PS, arXiv: 0905.3418 [hep-ph]

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

Matching – The Issue

• A (Complete Idiot’s) Solution – Combine

1. [X]ME + showering

2. [X + 1 jet]ME + showering

3. …

• Doesn’t work– [X] + shower is inclusive– [X+1] + shower is also inclusive

X inclusiveX inclusive

X+1 inclusiveX+1 inclusive

X+2 inclusiveX+2 inclusive ≠X exclusiveX exclusive

X+1 exclusiveX+1 exclusive

X+2 inclusiveX+2 inclusive

Run generator for X (+ shower)

Run generator for X+1 (+ shower)

Run generator for … (+ shower)

Combine everything into one sample

What you get

What you want

Overlapping “bins” One sample

NLM + MC Overlap: Ensure consistent concepts and language

The Matching Problem

• [X]ME + shower already contains singLL{ [X + n jet]ME }

– Adding full [X + n jet]ME is overkill

LL singular terms are double-counted

• Solution 1: work out the difference and correct by that amount add “shower-subtracted” matrix elements – Correction events with weights

wn = [X + n jet]ME – Shower{wn-1,2,3,..}

– I call these matching approaches “additive”

• Solution 2: work out the ratio between PS and ME multiply shower kernels by that ratio (< 1 if shower is an overestimate) – Correction factor on n’th emission

Pn = [X + n jet]ME / Shower{[X+n-1 jet]ME}

– I call these matching approaches “multiplicative”

Matching – Tree-Level

• Benchmarks– Important Processes (Pheno)

• W + jets• QCD jets, Top, Bottom, Higgs, …• BSM benchmarks? (with session 2)

– Pathological Observables (Theory Tests)• Obs that are explicitly sensitive to subleading logs• Changing matching different subleading logs

tuning affected. This problem needs to be charted.

• Automation (tree-level)

Matching – with NLM

• Matching at NLO – State of the field: comparisons, tests

• Analytical tests in simpler theories?

– Multileg matching at NLO• Keep eye on Standardization discussions…• Matching at NNLO?

– Showers as phase space generators

• Matching beyond fixed order?– BFKL×DGLAP, MC@NLL, …

Matching – Organization

• Tree-level brainstorm

• + joint brainstorm with NLM

WG MCTuesday, 14:00, LIBRARY

WG MC + NLMTuesday, 17:00, AUDITORIUM

+ Spillover: Wednesday, 16:00, AUDITORIUM

PDFs

• Heavy-quark PDFs• NNPDFs, MC PDFs, CTEQ, MRST, …• Special Effects in PDFs

– QED (MRST2004QED)

– Unintegrated PDFs– PDFs for Monte Carlos (LO* and Beyond)– Correlations

• Brainstorm Joint with NLM

WG MC + NLMTuesday, 16:00, AUDITORIUM

Jets

• Covered by Jon– Jet algorithms, substructure, subtractions, calibrations, …

• Jet Calculations (of jets and jet stucture)– Jet-jet correlations, jet masses, ΔRjj

• Matching benchmarks?

• Connects to substructure

– Large rapidity, definition of rapidity gaps– Jet definitions for matching– New Showers (Catani-Seymour, Antenna, Sector, …)

• Comparisons, Energy and color flows, coherence

WG MCWednesday, 14:00, AUDITORIUM

Summary

• QCD Phenomenology is in a state of impressive activity– Increasing move from educated guesses to precision science

– Better matrix element calculators+integrators (+ more user-friendly)

– Improved parton showers and improved matching to matrix elements

– Improved models for underlying events / minimum bias

– Upgrades of hadronization and decays

• Early LHC Physics: theory– At 14 TeV, everything is interesting

– Even if not a dinner Chez Maxim, rediscovering the Standard Model is much more than bread and butter

– Real possibilities for real surprises

– Timely discussions on “non-classified” data, such as min-bias, dijets, Drell-Yan, etc allow rapid improvements in QCD modeling (beyond simple retunes) after startup

Summary

• Need for good tools already demonstrated

Solution provided by GudrunMore conventional, but got the job done