e-A Event Generator

1. e-A Event Generator • 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. Cole - e-A Event Generators

2. 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 Cole - e-A Event Generators

3. 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 Cole - e-A Event Generators

4. 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? Cole - e-A Event Generators

5. 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 … Cole - e-A Event Generators

6. 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. Cole - e-A Event Generators

7. 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. Cole - e-A Event Generators

8. 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. Cole - e-A Event Generators

9. 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. Cole - e-A Event Generators

10. 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 Cole - e-A Event Generators