The increasing statistics in the HERA experiments provides an improved
basis for detailed physics studies. Monte Carlo (MC) generators for different
processes play an important, sometimes crucial, role in these analyses.
Although many observed phenomena can be well described by existing generators,
there are also severe discrepancies that demonstrate problems in generators
or underlying models. The problems are both of technical and conceptual
nature and their resolution will improve our understanding of the physics
involved. This will require improvements in existing MC's or development
of new ones. There are always some developments of the MC's, but a major
coordinated effort has not been made since the HERA 1991 workshop (e.g.
not within the 95/96 workshop on 'Future physics at HERA'). The coming
luminosity upgrade of HERA will lead to even higher precision which calls
for refinements of the Monte Carlo generators. In view of the interesting
present and future analyses of HERA data, the time is now right to make
an effort on Monte Carlo generators for HERA physics.
Topics to be studied:
The interest will be focused on various hard and soft QCD processes/issues,
but other topics in and beyond the Standard Model are also of interest.
A first discussion has given the following list of topics:
the transition from DIS to photoproduction
2+1-jet rate: problems in next-to-leading order (NLO) calculations, resolved
NLO matrix element (ME) programs and their possible transformation to complete
generators with parton showers (PS) and hadronization
small-x parton dynamics; parton emission a la DGLAP, BFKL, CCFM, LDC
linked dipole chain (LDC) model and MC
MC event generators for CCFM, forward/backward evolution, hadronisation
MC for processes described by 'semihard' theory
matching of matrix elements with parton showers
parameterisation of parton densities and their use in MC's
PS generator with splitting functions in NLO
soft gluon treatment in PS, cutoff, coherence, angular ordering
hadronization issues: proton and photon remnant, soft color interactions,
clusters, strings etc.
higher order QED radiative effects
MC's for special processes: W-production, lepton pair production, etc.
MC's for new physics processes: SUSY, leptoquark, instantons, etc.
upgrade of tools for comparison with data, e.g. HZTOOL
Time plan and organization:
Real progress in terms of improved or new programs cannot be achieved in
a single meeting, but require substantial work and thereby time. Therefore,
we plan a plenary starting meeting April 27-30, 1998 to discuss the problems,
set priorities and form study groups. These groups will then have informal
meetings as necessary over a rather long period. We will avoid having a
big organization with many formal groups and conveners, but some coordination
will be needed for different topics. A mid-term plenary meeting is planned
for late September or early October -98 and the final meeting at DESY in
January -99. Those interested are welcome to spend some time at DESY, not
only for these meetings, but also for more extensive collaborative work.
Presentation of results:
Final meeting at DESY with presentation of results for the HERA community.
The main outcome should be improved and easily accessible generators together
with associated manuals, studies and physics papers. These will be collected
in proceedings on the www. In particular, all generators should be briefly
described and have links to their home pages, where code, manual, sample
programs and news/updates can be obtained. Also a collection of relevant
papers, in particular resulting from the workshop will be collected on