CompHEP: Present and Future

Alexandre Kryukovon behalf of CompHEP collaboration

(E. Boos, V. Bunichev, M. Dubinin, L. Dudko, V. Ilyin, A. Kryukov, V. EdneralV. Savrin, A. Semenov, A. Sh.)

General motivation and goals How CompHEP works: symbolic and numerical parts Physics Models Flavour combinatorics simplification Large-scale calculations: distributive calculation, batch

scripts Interface to PYTHIA MCDB – MC Database for particle event samples ->

L.Dudko, next report. Concluding remarks

Outline

The increase of the collider energies requires simulation of processes for more and more complex processes with better and better precision (NLO, NNLO, NLL resummation)

1.LEP I – basically 2 fermion physics;

2.LEP II – basically 4 fermion physics;

3.TEVATRON, LHC and LC – 4,5,6 and even 8 fermion physics with additional hard photons and/or gluons (jets);

● Single top in the t-channel mode – 5 fermions;

● Top pair production with decays – 6 fermions;

● Strongly interacting Higgs sector in hadron collisions – 6 fermions

● Yukawa coupling – 8 fermions

Physical motivation

A number of automatic (may be partly) programs can be found on the market: CompHEP, GRACE, MadGraph, AlpGen, Omega/WHIZARD, Amegic, ...

Goals: Automation of tree level diagram calculations A full computational chain from Lagrangian until event

flow. Interfacing to other generators (for showering and

hadronization) for full simulation. Interfacing to NLO cross section calculators (programs

calculating full NLO or higher “number”)

Technical motivation

Large number of diagrams and large number of subprocesses (Tevatron, LHC)

Very incomplete list of processes computed by means of CompHEP in the past:

CompHEP(Computation in High Energy Physics)

CompHEP generates Tree Level Feynman diagrams for a given

parton process

Symbolically calculates squared Feynman diagrams. User (mostly for theoretical investigation) can output precise symbolic formular for squared matrix elements.

MC algorithm to obtain total cross section, different distribution and generation of event flow.

Rich set of model: CompHEP can work with 0,1/2,1-spin particles,

Majorana and Dirac spinors, ghosts fields, 3- and 4-leg vertices.

User-friendly interface: GUI interfaces for symbolic and numerical

CompHEP parts, case sensitive build-in help (F1), simple batch scripts.

CompHEP ideas

Choose physics model: SM, MSSM, your own. User can change

model parameters (masses, couplings, etc.), add/remove/change vertices and composite particles in existing models or in his(her) own models.

Set initial beams or decay particle and set a final state. CompHEP generates corresponding Feynman diagrams. User can look at and remove some particular diagrams or subprocesses.

Prepare a numerical MC generator. CompHEP squares and

symbolically calculates the diagrams. After that it can keep them as a C/REDUCE/FORM code. The most applicable case – C code. By standard make CompHEP compiles and links a program for numerical calculations for the process (numerical Monte-Carlo generator)

How CompHEP works (symbolic).

Set necessary kinematic cuts, Q2, PDF set, etc.

Customize numerical MC generator. The most complicated part is selection of right phase space parametrization and regularizations of singularities.

Calculate full cross section and distributions. CompHEP uses adaptive VEGAS algorithm for MC calculations. User may set different variables (PT, inv. mass, rapidity, etc.) to draw corresponding distributions.

Generates events. After cross section calculation CompHEP can generate events for the given process. User set a number of the events.

If the process consists of some subprocesses, the procedure applies to the each subprocess.

How CompHEP works (numerical).

There are several models implemented in CompHEP:Simple “teaching” models: QED and Fermi ModelStandard Model both in unitary and t'Hooft-Feynman gaugesComplete MSSM both in unitary and t'Hooft-Feynman gauges with

the Higgs sector . mSUGRA and GMSB models.

●The FeynHiggs and ISASUSY library are necessary. Some models are available by request:

● Top quark Lagrangian with anomalous couplings as follows from the dimension 6 effective operators●Excited fermion Model●Complete two-Higgs-doublet Model with conserved or broken CP invariance

LanHEP program (the part of the CompHEP project) allows to generate Fynmann rules for new models from (effective) Lagrangian.

CompHEP Models

A serious computational problem is the large number of partonic subprocesses at hadron machines (for example, pp-> W+jj consist of 472 subprocesses).

Reasons:

Many quark partons with different flavors

Many additional diagrams for each subprocess because of CKM quark mixing

Basic ideas:

Rotation of down quarks: thus, transfering the mixing matrix elements from vertices of subprocess Feynman diagrams to PDFs

Diagrams are divided into gauge invariant classes which are convoluted with different combinations of PDFs

Simplification of Flavour Combinatorics

* E.Boos, et al. JHEP 0005 (2000) 052

“Hash” models in CompHEP. Transferring of CKM elements to PDFs allows to unify two generations of light quarks to one – “hash” generation – (u#, d#)

Two approximations:

1) Mu = Md = Ms = Mc = 0

Advantage:

1. SM, pp-->W+jj: 472 processes, 6160 diagrams

2. Hash SM: 42 subprocesses, 532 diagrams

Simplification of Flavour Combinatorics

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VV

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sincos,

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Symbolic batch (symb_batch.pl)

All parameters are set in a file (process.dat) and scipts launches CompHEP in a non-GIU mode with these parameters

Numerical batch (num_batch.pl)

MC generators parameters are kept in one file (batch.dat) for all subprocesses

The batch script starts numerical calculations including all (or some) usual steps in CompHEP – cross section calculation, event generation

MC generator customization is realized by hands or in GUI mode.

num_batch.pl has detailed help (run ./num_batch.pl –help)

Larga-scale calculations: batch regime

CompHEP have to use external generators for hadronization and parton showering:

Problems: Interfacing Squared Matrix Element generators with

Showering and Hadronizating generators): Les Houches Accord 1, event file formats;

Les Houches Accord 2: uniform interface to different PDF sets.

Les Houches Accord 3: Interfacing SUSY codes to MC generators for parameters, spectrum, decays;

Generator environment

The CompHEP-PYTHIA interface allows to use processes 2-->2..6 computed by CompHEP as new processes for PYTHIA

Main goal: provide ISR/FSR, hadronization (including jet fragmentation) and decays by PYTHIA.

CompHEP generates unweighted events and writes to event files.

Special mix_flows utility mixes several event files in one event file according to their relative contributions to cross section. The file can is used by PYTHIA as input.

We provide the interface library and an example of program (main.f).

CompHEP 4.4 – PYTHIA 6.2 interface

CompHEP with the interface to PYTHIA is a powerful tool for a simulation of different physical processes at hadron and lepton colliders.

CompHEP allows to study problem for wide set of physics models: SM, MSSM (two gauges, some SUSY violation scenarios). Physicist can create and use own model.

Comphep calculates cross section, prepares different distributions, generates unweighted event flow and more.

CompHEP is compatible with Les Houches Accord I. The interface with Pythia allows to generate event flow that is ready for further investigations by phenomenologiests and/or experimentalists.

Symbolic and numerical batch modes simplify large-scale calculations The CompHEP is the LO program. However it allows to include

(partly) some NLO corrections: NLO Tree Level 2-->N+1 corrections to the process 2->N can be computed. One can include NLO structure functions, loop relations between parameters (K-factors), and existing from papers loop contributions as effective vertices (functions).

Concluding Remarks

The nearest plans: Development of distributed Monte-Carlo calculation and event generation

on computer clusters as well as GRID capabilities. Implementation of the FORM computer algebra program for symbolic

calculations: form-factors, new symbolic algorithms, models with extra dimensions, dimensional regularization, spin density matrices for external lines of squared diagrams;

Les Houches Accord I based interface to HERWIG; Les Houches Accord II based interface to PDFs; SUSY Les Houches Accord (III) based interface to various SUSY

parameter, mass (etc.) calculation codes.

The long term plans: Amplitude techniques for symbolical and numerical calculations including

the 1-loop case. Automation of regularization singularities. Incorporation of the gauge invariant classes of diagrams.

Future plans

CompHEP collaboration: E. Boos, V. Bunichev, M. Dubinin, L. Dudko, V. Ilyin, A. Kryukov, V. Edneral, V. Savrin, A. Semenov, A. Sh.

CompHEP homepage: http://theory.sinp.msu.ru/comphep There are CompHEP itself, LanHEP, cpyth (CompHEP-PYTHIA interface)

References:

early CompHEP versions (3.**, 41.10): A. Pukhov et al., Preprint INP MSU 98-41/542, hep-ph/9908288.

recent CompHEP versions (4.2p1, 4.4.0): E. Boos et all., CompHEP 4.4.0, hep-ph/0403123, will be published in Proceedings of ACAT’03

General information

CompHEP Collaboration(incomplete list)