Cole - e-A Event Generators Some profound thoughts 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. 12/3/2018 Cole - e-A Event Generators

Goals (What do we need it for?) 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 12/3/2018 Cole - e-A Event Generators

Physics (what goes in -- ideally) 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 12/3/2018 Cole - e-A Event Generators

Physics (what goes in -- ideally) (2) 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? 12/3/2018 Cole - e-A Event Generators

Cole - e-A Event Generators Physics -- Comments 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 … 12/3/2018 Cole - e-A Event Generators

What exists, is maintained, is relevant? 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. 12/3/2018 Cole - e-A Event Generators

Long-term Considerations 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. 12/3/2018 Cole - e-A Event Generators

Low-x physics: larger considerations 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. 12/3/2018 Cole - e-A Event Generators

Cole - e-A Event Generators How to proceed? One possible strategy 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. 12/3/2018 Cole - e-A Event Generators

Cole - e-A Event Generators Summary 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 12/3/2018 Cole - e-A Event Generators