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Generators at the LHCconcerns for the startup

Albert De RoeckCERN

MC4BSM Workshop

Thanks to: P. Bartalini, I. Hinchlife, J. Huston, B. Kersevan, F. Moortgat, A. Nikitenko, M. Spiropulu

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The LHC: a proton proton collider

The LHC will be completed in 2007

7 TeV + 7 TeV

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BSM Physics at the LHC: pp @ 14 TeV

Supersymmetry?

Extra Dimensions? Black Holes???

Impressive variety of proposals for physics that can be expected @ LHC !!!Tools for cross sections calculation, event generation etc. indispensable!

Little Higgs?

New Gauge Bosons? ZZ/WW resonances?Technicolor?

Split Susy?

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Staged Commissioning for 2008

Present schedule:First 14 TeV collisions June/JulyCollect 0.1-1 fb-1 in 2008 ?Collect 10 fb-1 in 2009?

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Start-up Physics 2008

0.1-1 fb-1

In 2008 we have to rediscoverthe Standard Model at 14 TeVand compare to calculationsand generators.…And tune generators

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Early discoveries? E.g. Di-lepton Resonance

If we are lucky:a signal could be seen very early on

First months of operation

CMS PTDR

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Supersymmetry

SUSY could be at the rendez-vous very early on!

Main signal: lots of activity (jets, leptons, taus, missing ET)Needs however good understanding of the detector & SM processes!!

Msp(GeV) (pb) Evts/yr500 100 106-107

1000 1 104-105

2000 0.01 102-103

10fb-1

Therefore:SUSY one of the priorities of the“search” program

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Inclusive SUSY Searches

Searches based on different signatures

Low mass SUSY(mgluino~500 GeV) shows excess for O(100) pb-1

Time for discovery determined by:Time to understand the detector performance, Etmiss tails, jet scale,lepton idTime collect SM control samples such as W+jets, Z+jets, top..

CMS PTDR

More in Maria’s talk

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BSM and SM backgrounds

M. PeskinSUSYtoolsAnnecy 06

There are more signals (top, …) that will be used, but it illustrates the point

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Can we observe an early top signal with limited detector performance ?And use it to understand detector and physics ?

tt 250 pb for tt bW bW bl bjj

Top signal observable in early days with no b-tagging and simple analysis measure tt to 20%, m to 10 GeV with ~100 pb-1 ?• commission b-tagging, set jet E-scale using W jj peak • understand detector performance for e, , jets, b-jets, missing ET, …• understand / constrain theory and MC generators using e.g. pT spectra

Isolated lepton pT> 20 GeV

ETmiss > 20 GeV

4 jets pT> 40 GeV

NO b-tag !!

2 jets M(jj) ~ M(W)

ATLAS preliminary

50 pb-1

W+n jets (Alpgen) + combinatorial background

3 jets with largest ∑ pT

Early Top-quark events

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Generators in Experiments

• No one generator adequately reproduces the physics for the complete program– Use parton shower MCs, Matrix Element MCs, Matching MC’s,

NLO MCs, Cross section calculators.• Essential that experimentalists understand which techniques

are applicable to which kinematic regimes Les Houches’03 write-up of used/available generators.

(“old”) Several workshops have been organized/increased attention • Uniform interface necessary (Generators Experimental

software)• Generator tools should be accessible to whole collaboration and

easy to use.• Event Generation can become very time consuming

(efficiencies?) An event data base: MCDB?

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Generators in use (ATLAS/CMS)

Workhorses so far PYTHIA and HERWIG Moving to PYTHIA4 and Herwig6.5xx for next data challenges Also include ALPGEN 2.11 at large scale (CMS)

Other MC’s that are used (CMS) HERWIG, CompHep, HDECAY, HNMDECAY, FeynHiggs, ISASUGRA,

ISASUSY, Prospino2, SDECAY,ISAWIG,TAUOLA, PHOTOS,TopReX, SIMUB, ALPGEN, POMWIG, PHOJET, MadGraph, MadEvent, PYQUEN, HYDJET, ExHume, EDDE, DPEMC MC@NLO, MCFM, Charybdis, Truenoir, SHERPA, and private codes for BSM…

Next: EVTGEN, SOFTSUSY, SPHENO, SUSPECT, Cascade, CalcHep

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30 % of total WW background30 % of total WW backgroundafter all cuts ! “signal like behaviorafter all cuts ! “signal like behavior

Signal

region

Predictions for Background

Gluon induced WW background to Higgs boson searches at the LHC

Gluon induced background will have to be taken from MC

Example

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Experiment startup exercisesaka data challenges

• Simulate data taking startup (rates/events)– CSC06 (CMS) 50 M events PYTHIA– CSC (ATLAS) >20M events PYTHIA, HERWIG, ALPGEN,+– CSC07 (CMS) >100M events PYTHIA, ALPGEN,+ summer

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• Preparing for the first data– Are we using the right/best generator for the right part of phase

space?– Do we have the right SM cocktail?

• Note: software cycles in large experiments are long– Need to freeze validated generators well before time (end 07/start

08 for the 2008 run)

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Next: Startup Concerns

• Prime concern now is to get ready for the LHC startup (2007) 2008• Jets, W-Z-t(t)+ njets, WW-ZZ+njets,W-Zbb,ttbb,W,Z, min bias…

• Strategy– Measure min-bias, underlying event, QCD jet, W, Z, top with first

data.• Tune MC’s to the data

– Measure W, Z, top + njets in data in available control regions• Tune/Normalize MC’s and extrapolate in new regions (tails)

Remember: early discoveries are possible!

• MC Production for startup physics for 2008– Choice of models and model versions (PYTHIA/HERWIG/Alpgen/…)– What settings/parameters? PDFs, underlying events, PS, UE…– What processes are still missing?– LO/NLO importance? Alternative showerign (SCET…)– Do we understand QCD sufficiently in the new LHC kinematic regime?– How to normalize the MC’s

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Double Parton interactions

Not well know what to expect… (see HERA-LHC workshop this March)

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PDFs: E.g. MRST/CTEQ differences

YES: FL can referee the gluon distribution! FL is like F2: little theoretical ambiguity (compared to e.g. F2

C)

H. Stenzel R. Thorne

),()(),(])1(1/[ 222

222 QxFyfQxFFyyF LLr

Need to lower the energy of proton or electron beam for this measurement

Can we know more by the time of the LHC startup?

HERA starts a low energy run this week !

Strong case madeduring HERA-LHCWorkshops 2004/2005

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Using NLO PDFs for (LO) MC’s?

Proposal by J. Huston et al

Still matter of debate…CurrentlyATLAS LOCMS discussing

New: R. Thorne:“special” PDFs forMC generatorsMore soon!

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Higher QCD corrections/K factors

Effect on Higgs ‘discovery’Many cross sections now calculated to NLOK factors? Not always sufficient/can be huge in some phase space partsReweighting Monte Carlo? Select key weighting variables Complete NLO Monte Carlo! Quite some progress in the last years.More processes wanted !!

Priority wish list from the experiments hep-ph/0604120 (Les Houches 05)

+ Zbb, Hbb

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NLO/LO

Example HWW studyNot just K-factors

NOTE: MC@NLO with PYTHIA?

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But even at LO

• On the other hand there are relevant backgrounds that are still missing even at LO, for example the case of gg->ZZ through the box diagram which should account for ~ 30% of the total ZZ cross section. This process was calculated long time ago for on shell Zs and as of today there's no calculation for off shell Zs.

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From LHC theory initiative white paper (2005)

Uli Baur Fermilab W&C Aug 18

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Tools for MC Tunning

• RunMC• HZTool• Rivet• Vista

Discussion during the HERA-LHC workshop

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Underlying Event: waiting for data

Strategy in place in ATLAS and CMS

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ALPGEN

• Getting the shapes right for njets final states ALPGEN ME+matched PS for a number of channels, usually as a background study to Higgs or BSM searches

• ALPGEN version 211• Used parton shower matching CKKW

with PYTHIA 6.25/6.35

Examplett+njets

Big effort to simulate all channelsEg bb + 4j sample : 3 min on P4 2.8 GHz. per event (unweighted)

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Normalization procedure

Normalize to data or to NLO total cross section?

Annecy 06

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Comparisons between MCs

ALPGEN MC@NLO Top + jet(s) production

Differencesin jet rapidity!

M. Mangano et al.

hep-ph/0611129

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Spin correlations

Spin effects matter Top + jet(s) production

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Is it SUSY? Spin in the generators

Look for variables sensitive to the particle spin eg. lepton charge asymmetriesin squark/KKquark decay chains Barr hep-ph/0405052; Smillie & Webber hep-ph/0507170 Allanach & Mahmoudi hep-ph/0602198

KK like spectrum (small masssplitting)

SPS1a benchmarktype spectrum

Method works better or worse depending on (s)particles spectrum

Can the LHC distinguish UED SUSY?

e.g. Cheng, Matchev, Schmaltz hep-ph/0205314

need spin correlations in decay chains of generators

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Comparison between MC’s

Eg. ATLAS

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Monte Carlo Generators

BSM GeneratorsWeb repository (September 05 P Skands et al.) Workshops FNAL: MC4BSM 20-21 March 06 LAP-LAPTH: Tools for SUSY and BSM 26-28 June 06CERN: MC4LHC 17-26 July 06Princeton: MC4BSM 21-24 March 07MCnet European network April 07HERA-LHC workshops March07

Les Houches 2007, MC4BSM series?Needs organization SPA-like project?A combination of the above?

Future A toolkit/framework for HEP event generators

T. Sjostrand

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MC Activities

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Tools Repository

http://www.ippp.dur.ac.uk/montecarlo/BSM/

After the Les Houches 05 workshop

FERMILAB-CONF-05-521

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BSM Tools

M. Muehlleitner

Plenty of toolsNeed Toolbox?

Collection of programs on http://www.ippp.dur.ac.uk/montecarlo/BSM/

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SUSY Tools/SLHA Accord

M. Muehlleitner

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LHC Experimentalist Generator Wish List

Have more processes available: compiled list from CMS/ATLAS– Universal extra dimensions with/wo KK number conservation– Bulk scalars with Higgs interference– Radions and interference with the Higgs (now hack in HDECAY)– RS generator with SM fields in the bulk (G. Moreau et al.)– Implementation of different running couplings (see hep-ph9807522)– More flexible/complete generation of KK resonances in TeV-1 and RS models,

which include many resonances, effects of brane kinematic terms (hep-ph0212279). TeV-1 matrix elements in ‘standard toolkit’

– Branons– More sophisticated black hole generators…? (remnant, radiation phases,

spin)– String ball effects (black hole-like but different radiation/lower mass)– Transplankian effects, especially high ET 2 jet production – SUSY+ ED scenarios (modify the phenomenology as in )– Thick branes, brain tension, rigid and soft branes– Valley physics (M. Strassler et al)– Black holes with very soft decays (low scale gravity/ G. Dvali et al)– Even more recent scenarios (such as intersecting branes, Higgsless EWSB)

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A Propos: New Models

• We need the MC for possible new signals and models. The signal efficiency trigger/analysis for the objects is model dependent! At startup you have to rely on that the signal MC can map to our measurements for the objects.

• Partial models are of limited use. A complete description of all the production rates and decay channels is needed. Every particle must be included if its production rate, either directly or in the decay of other particles is not negligible.

• The Les Houches format is the way to go. Hacked versions of other generators such as PYTHIA and HERWIG are not acceptable; the possibility of a an accidental interface with some other component is too large.

• Note: not only generators: some models have long lived particles or new massive particle which interact with the detector. This needs action in the SIMULATION. Geant4 allows for this, but so far mostly available in private code (e.g R-hadrons, long lived staus,…)

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SUSY+Njets

T = 600 GeV topsps1a

1) Extra 100 GeV jets are there ~ 25%-50% of the time!

2) Extra 50 GeV jets - ??? No control We only know ~ a lot!

(S)MadGraph Numbers

Additional jet contribution will be important

Skands, TeV4LHC 05

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Recent Studies: New Signatures

Split Supersymmetry• Assumes nature is fine tuned and SUSY is

broken at some high scale• The only light particles are the Higgs and

the gauginos

- Gluino can live long: sec, min, years!

- R-hadron formation: slow, heavy particles containing a heavy gluino.

Unusual interactions with material

eg. with the calorimeters of the experiments!

Gravitino Dark Matter and GMSB • In some models/phase space the gravitino

is the LSP• Then the NLSP (neutralino, stau lepton) can

live ‘long’

Challenge to the experiments!Sparticles stopped in the detector,walls of the cavern, or dense ‘stopper’ detector. They decay after hours---months…

K. Hamaguchi,M Nojiri,ADR hep-ph/0612060ADR, J. Ellis et al. hep-ph/0508198

A KraanArkani-Hamed, Dimopoulos hep-th/0405159

Energy fract.deposited in calorimeter

~ 1000 eventsper year

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Hidden Valleys?

Example H 4 b

The Fear Factor: A real challenge for the triggers at the LHC

String Theory inspired (M. Strassler)Eg. Strassler & Zurek hep-ph/0604261

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The Inverse Problem

Lester et al., hep-ph/0508143

MARMOSET

Other

hep-ph/0703088, N. Arkani-Ahmed et al

Looks like an interesting communication tool with theorist

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Warning: Inverse problemsM. Schmalz The SUSY hidden sector strikes back!

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LCG Generator Serviceshttp://lcgapp.cern.ch/project/simu/generator/

• mandate of the project:– "...to prepare validated LCG compliant code for both the

theoretical and experimental communities at the LHC..."• present tasks:

– 'technical'• provide a centralized installation of all the MC generators

used by LHC experiments on all the LCG supported platforms• perform testing of the generators on different platforms in

order to identify any potential bugs– 'physics-related'

• perform basic physics validation (for ex. _tot (pp->Z+jets, W + X, t + X) )

– 'user-related'• provide 'first-level' support on MC generators for the LHC

usersGENSER: used by ATLAS and CMS

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Generator Services Status and Plans

• present status– 'technical work' done

• generators available for the LHC experiments• a number of tests/validations (at least one for each

generator) implemented• future plans and extensions

– direct involvement in the tuning of the generators– participation in the integration/testing of specific generators

within the LHC experiments– close collaboration with the Rivet (replacement of HZTool)

project on the validation of the generators

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Summary

• SM background processes:– Prime concern now is to get ready for the LHC startup (2007) 2008

– Jets, W-Z-t(t)+ njets, WW-ZZ+njets,W-Zbb,ttbb,W,Z, min bias…

• LHC will measure these processes and generators will be tuned to the first LHC data – PDF’s, underlying event, multiple interactions,… – Higher order QCD (EW) corrections to processes

• BSM MC process tool box/repository– Presently repository but still (many) missing processes– Models which give potentially new signatures are wanted asap!– Include details in the MC, eg. spin correlations. These correlations

likely to become very important when LHC will discover a new object.

– In principle easy to contribute with present generator developments (Les Houches accord/…)

• Good Generators will be extremely important from day 1

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References

• Also online at ROP

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.QuickTime™ and a

TIFF (Uncompressed) decompressorare needed to see this picture.

Standard Model benchmarks

See www.pa.msu.edu/~huston/ Les_Houches_2005/Les_Houches_SM.html

http://stacks.iop.org/0034-4885/70/89

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Further Information

• http://www.ippp.dur.ac.uk/montecarlo/BSM/– BSM tools repository

• http://lss.fnal.gov/archive/2006/conf/fermilab-conf-06-082-t.pdf– Summary of the MC4BSM workshop, FNAL, March 03

• http://hepforge.cedar.ac.uk/– A web based development platform for event

generators and tools• http://www-theory.lbl.gov/tools/

– A discussion forum for LHC tools

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SCET

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Low-x issues

HERA data at low-x (x ~10-3-10-4)• More pt in the event than predicted by DGLAP evolution

based parton showers• Good description using CCFM evolution (H. Jung)

CCFM: Catani, Ciafaloni, Fiorani, Marchesini

Relevant for LHC generators?NLO low-x: J. Andersen, A. Sabio VeraStrikman: lots of mini-jets at the LHC

H. Jung

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Eg. Pt Spectrum of the Higgs

H. Jung

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Proton AntiProton

Multiple Parton Interactions

PT(hard)

Outgoing Parton

Outgoing Parton

Underlying Event Underlying Event

Outgoing Parton

Outgoing Parton

from R. Field

Underlying event/multiple interactions

Studies and tunes made on Tevatron/lower energy data New models on the market that should be tested (new Pythia, Jimmy, Sherpa)

CMS so far used the ATLAS tune for the PTDR (A. Moraes et al/LH03) Propose to move to a new tune (R. Field) Discussion this Friday AM

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Requirements: tools and calculations

• Les Houches 05: What are the real uncertainty bands (from higher orders,scale uncertainties, PDFs). This has not yet been done even for some simple cases!

We will obviously try to use our own data as much as possible (sidebands, independent measurements) but tuned Monte Carlos will play an important role in the analysis. Hence Standard Model processes will be important particulalry W,Z + jets, tt +jets , bb + jets, n-jets…

Calculations and generators; NLO, NNLO; ME+PS matching• Upgrade MC@NLO for

– WW, WZ with spin correlations, DY, W+jets• MC@NLO+PYTHIA• Event generator including EW effects• Event generator based on Ariadne QCD treatement• PYTHIA6.3 tuned version• Underlying event/minimum bias event decriptions • Toolkits for SM and BSM processes (via LH-accords)

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• Specfic examples for Higgs Study – NLO predictions for ttbb and ttj(j)– NLO, NLLO for ggH; bbH; H/A– WbWb background to HWW– Gluon induce WW backgrounds to Higgs searches– pp ZZ 4l with Z width and spin correlations– tt WbWb with spin correlations

Time to repeat the MC@LHC workshop in summer 2006?

http://mlm.home.cern.ch/mlm/mcwshop03/mcwshop.html

Requirements: tools and calculations

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From LHC theory initiative white paper

Uli Baur Fermilab W&C Aug 18