• Yes, we need one. ( when?
)
• No, nothing suitable currently exists.
• Yes, writing and maintaining one will require substantial effort.
• No, we can’t ask theorist(s) to sacrifice their career simply to provide us one.
 And even if we did ask they w(sh)ould say no.
• Writing/assembling an e-A event generator will require experimental/theory partnership.
Cole - e-A Event Generators Slide #1 4/13/2020

• Acceptance/efficiency corrections for physics.
• Detailed detector design/performance studies.
• High-level detector design (?), rate estimates
• Physics studies of not-so-obvious final states
 e.g. forward b jet  B (?)  semi-leptonic e.
• Test-bed for evaluating experimental sensitivity to different theoretical input.
• “Codifying” a self-consistent reference version of saturation, final-state interactions, diffraction, …
• Tool for developing intuition re: e-p/eA physics
Cole - e-A Event Generators Slide #2 4/13/2020

• Full e-parton physics, including IS/FS radiation
 How to not reinvent the wheel?
 i.e. can we easily splice w/ existing e-p generators?
• Implemented with Nuclear PDFs ?
 Obvious next step, but is it really the right approach?
 Issues:
– b dependence of PDFs
– Shadowing/saturation  scattering off multiple nucleons, so coupled with description of nuclear fragmentation
 Target rest frame/dipole picture might be better for describing/implementing low-x interactions
 But then still need not-low-x: EMC, anti-shadowing
Cole - e-A Event Generators Slide #3 4/13/2020

• pQCD final state: parton shower + fragmentation
• Outgoing parton interaction w/ nucleus
 Tricky: separating coherent higher-twist corrections from incoherent FSI.
 Hadron cascade for hadronization inside nucleus
 Nuclear modification of hadronization
• Diffraction (hard + soft)
 What is best physical picture and can we easily combine with non-diffractive EG – or separate?
• Nucleon fragmentation, nuclear cascade
 Simulate nuclear fragment distributions? How?
Cole - e-A Event Generators Slide #4 4/13/2020

• The combination of physics listed above would result in most sophisticated event generator ever.
 Daunting challenge given that much of that physics has never been implemented in an event generator.
 And some of it is not completely (!) understood.
 And even what we do understand evolves with time.
• But, we’re planning on building an entirely new facility where e-A is critical part of the program.
 If we can’t take the time to put our best understanding of the relevant physics into an event generator then …
 Yes, but consider RHIC …
Cole - e-A Event Generators Slide #5 4/13/2020

• CASCADE (Jung) – low-x e-p, p-p (maintained)?
• RAPGAP (Jung) – Inclusive, diffractive DIS (p)
 With EM radiative processes
• LEPTO (Ingleman et al ) – Inclusive, diffractive DIS.
 Is it still maintained? (web page updated 2004)
• DPMJET (III) (Engel, Ranft) – Does e-A, intra-nuclear cascade, evaporation, fission
 Uses LEPTO as start for e-A, CASCADE instead?
 Good starting point but no specific low-x treatment
 Nuclear PDFs? – probably not.
• DPMJET is only viable e-A option as far as I know.
Cole - e-A Event Generators Slide #6 4/13/2020

• We’re considering a physics program that will likely be still running 20 years from now.
 Lots of time for improving & evolving event generator.
 But many of existing event generators are already 10-
20 years old. And look it …
 How many students can read Fortran any more?
• We should look towards the future:
 Pythia 8 written in C++ (almost ready for prime time)
 Herwig++
 Presumably others will follow.
• With some strategic thinking/planning we could take advantage of this revolution.
Cole - e-A Event Generators Slide #7 4/13/2020

• RHIC and LHC also need a low-x event generator.
 (semi) hard scattering processes at low x in p/d-A
– With BFKL decorrelation, Cronin, forward shadowing
 Inclusive particle production, multiplicity, …
• Would be extremely helpful to have a “complete” and self-consistent saturation-based generator.
 Would force us to collect, assimilate, make consistent everything we know about low-x physics in one place.
• Not an EIC Collaboration issue per se but would provide valuable foundation for an e-A generator that incorporates saturation.
Cole - e-A Event Generators Slide #8 4/13/2020

• Adopt DPMJET as temporary solution
• Decide that EIC collab. is going to catalyze the creation of new e-A generator.
• Set up working group/task force/??? to:
 Talk to experienced event generator authors
 Get ideas from low-x gurus
 Organize a workshop to get experts together
 Solicit participation from non-EIC interests @ RHIC, LHC
 Define the scope of event generator, basic approach
 Start working.
Cole - e-A Event Generators Slide #9 4/13/2020

• We will need an event generator for e-A that includes the most important physics.
• Such a generator doesn’t exist.
• Experience at RHIC indicates that we cannot count on spontaneous generation of one.
• So, we have to be proactive and make it happen.
 It’s too big a job for 1 or 2 people.
 We need a joint effort of experimentalists & theorists
 It should be well thought out and well organized
 It should use modern computing tools, libraries.
• While the above is going on use DPMJET
Cole - e-A Event Generators Slide #10 4/13/2020