CompHEP: Present and Future

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CompHEP Present and Future

• Alexandre Kryukov
• on behalf of CompHEP collaboration
• (E. Boos, V. Bunichev, M. Dubinin, L. Dudko, V.
  Ilyin, A. Kryukov, V. Edneral
• V. Savrin, A. Semenov, A. Sh.)

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• 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 -gt
  L.Dudko, next report.
• Concluding remarks

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• 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)
• LEP I basically 2 fermion physics
• LEP II basically 4 fermion physics
• 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

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

• 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)

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• 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.

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• 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)

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• 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.

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• There are several models implemented in CompHEP
• Simple teaching models QED and Fermi Model
• Standard Model both in unitary and
  t'Hooft-Feynman gauges
• Complete 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.
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• A serious computational problem is the large
  number of partonic subprocesses at hadron
  machines (for example, pp-gt Wjj 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

E.Boos, et al. JHEP 0005 (2000) 052
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• 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
• SM, pp--gtWjj 472 processes, 6160 diagrams
• Hash SM 42 subprocesses, 532 diagrams

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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)

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• The CompHEP-PYTHIA interface allows to use
  processes 2--gt2..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).

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• 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--gtN1 corrections to the process
  2-gtN 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).

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• 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.

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• 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/comphe
  p 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 ACAT03

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CompHEP Collaboration(incomplete list)