1. 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

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
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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
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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?
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Cole - e-A Event Generators

5. 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 …
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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.
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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 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.
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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.
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Cole - e-A Event Generators

9. 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 RHIC, LHC
Define the scope of event generator, basic approach
Start working.
12/3/2018
Cole - e-A Event Generators

10. 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