The Madgraph/MadEvent generator: an update.

Transcription

1 The Madgraph/MadEvent generator: an update. The Web Generation Fabio Maltoni UCL Tim Stelzer UIUC

2 Yuri Dokshitzer, Lyon, 5 hours ago

3 Outline Motivation Current Status Standard Model Matrix Element and Feynman Diagram Protons, Jets, Les Houches and user specified Models Integration / Event Generation Examples Outlook

4 Motivation A large amount of time and energies are devoted to produce tree-level Monte Carlo data sets These are the key to many foreseeable measurements Most of it is a solved problem => no fame! It involves both theorists and experimentalists Automatize the full procedure => free us!

5 Big Picture User requests any process (leťs say in the SM): process ppp ppp > e+ >b b~ ve h jjjjjj jj j QCD order 0 36 QED order 0 23 MadGraph/MadEvent Returns: List all contributing sub processes Feynman diagrams for each sub process Self-Contained Fortran Package Integrates to get the cross section Generates unit-weighted events Les Houches ready

6 How can I generate events? 1. Log on to the web page and input the process 2. Download the code and run it 3. Check your results with a browser It seems easy, but is it really? Leťs see!!

7 How can I generate events? 1. Log on to the web page and input the process 2. Download the code and run it 3. Check your results with a browser It seems easy, but is it really? Leťs see!!

8 Flexibility User defined models: - Standard Model - MSSM (Plehn and Rainwater) - Higgs EFT NEW!

9 Flexibility II Given a model the user can choose the order of the QCD and QED couplings Include multi-particle labels for protons and Jets: pp > W+ W- b b~ j Require / Exclude particles in a process: pp > tt~ >W+ W- b b~ j //Z Example: Spin correlations can be included exactly for any process!

10 Example: pp>ttbb pp t th t tb b is an important discovery mode for a light LHC Backgrounds are large and difficult to curb!

11 Example: pp>ttbb In collaboration with CompHEP (Boos) Amp 2 = A QCD 2 α 4 S + A EM 2 α 2 Sα 2 EM But what about interference? +2α 3 Sα EM Re(A QCD A EM ) Iťs negative and large (~20%)!

12 MadGraph Capabilities Good news: Matrix elements generated minutes All sub processes identified and calculated Extendable to new models Large number of particles Color information for showers (see later!) Limitations: Must integrate over phase space. Maximum 9999 diagrams/sub process (W+5j)

13 MadEvent Integration and Event Generation Automatization of integration of all sub processes Hard problem because matrix elements are sharply peaked All previous Monte Carlo methods cannot be adapted to the general-purpose phylosophy

14 Single-Diagram-Enhanced summary Key Idea Any single diagram is easy to integrate Divide integration into pieces, based on diagrams Get N independent integrals Errors add in quadrature so no extra cost No need to calculate weight function from other channels. Can optimize # of points for each one independently Parallel in nature What about interference? Never creates new peaks, so we re OK!

15 Single diagram information The integration diagram-by-diagram offers unforseen insights on the process Each diagram is not a gauge-invariant quantity => the corresponding integral is not physical by itself. Anyway, its topology is invariant and can give useful information on the dominant kinematic configuration.

16 Single diagram information Example: gg b bh w/o cuts on the b-jets Q: What are the dominant contributions? 8 diagrams in total Hints to the use of the right scale and/or to b-pdf to resum possibly large log m b m h

17 Integrates cross section and generates unweighted events for any tree-level processes Events are stored in the Les Houches format: Interfaces for PYTHIA and HERWIG are available! Advantages Can handle general problem Parallel in nature => use a PC cluster! Limitations: MadEvent Capabilities - Factorial growth of the Feynman diagrams: Example: W+5 jets (7,000 diagrams w/ hundreds sub processes) Yuri said: It grows like MAD...

18 W+jets: MC4LHC comparison

19 Just a few recent applications Samples for ME-PS matching studies of Mrenna- Richardson: e+ e- > n jets and pp>w + n jets Single top at CDF and D0 (signal and background) ATLAS: WBF studies (B. Mellado) CMS PTDR, Backgrounds for: - Charged Higgs - Radion searches - gg H WW

20 Just a few recent applications Z+(1,2) jets are known at NLO. (Campbell & Ellis, MCFM) Z+ n jets is known at LO+PS matching. (Alpgen or Madgraph + Pythia, Sherpa) PATRIOT=MADGRAPH+PYTHIA The (N)NLO calculations are necessary to fix the normalization and test the distributions. LO+PS works very well in (Z,W)+jets both relative rates.

21 The Decay routine A new routine has been added to MadEvent It allows to decay unstable particles off-line Implemented decays for Higgs,W, Z, top, tau in various channels. Example: pp t th with t bjj, t bl ν l, h b b

22 Latest developments Web based generation: User inputs cuts/parameters Code runs in parallel on farms (US and IT). Returns cross section, plots, root file, and events! Advantages Reduces overhead to getting results Reduce computing cost by load leveling Events can easily be shared/stored Centralize maintenance Come tomorrow to the tutorial and see this working!!

23 To Do: 1. Update the documentation and officially release the latest version. 2. Release of SMADGRAPH and its embedding in MadEvent. 3. Open up the web production to the experimental groups. 4. Produce fully showered and hadronized events directly from the web. 5. Create fully inclusive samples from the web...

24 Summary Madgraph/MadEvent is stable (V3.80) Written in the good old Fortran 77 + sch + Perl Generates unweighted parton-level events in the Les Houches Accord (and also root format). Codes for specific processes are generated on air upon user request and stored in DB Pythia and Herwig interfaces are available Code is maintained and under continuos improvement (SUSY, new models...) Web generator and event repository available ~400 registrations and 10k requestes fullfilled

25 Conclusions MadGraph/MadEvent are available Generate events for ANY tree-level process Multi-purpose philosophy entails: flexibility => huge range of applications immediate response to new questions/needs one framework for all => no overhead/no errors more time to do real analsys! Present and Near future: The original Available Now! Available Soon!