Notes from the Q&A: 

• Exploration of physics cases for forward physics (mainly in Run 5, also through Run 4) is ongoing in the LPCC Forward Physics WG - there will be some overlap because they will also explore e.g. UPCs from small to large systems.  We will stay in contact through the LPCC so that the two white paper efforts are complementary.
• What is the lead time necessary for the accelerator folks to prepare for the various runs?
  • There will be no ion runs until 2031, so the white paper deadline of 2029 gives us two years to prepare.  It also depends on the ion species, some have been run before and some are more feasible than others.  This is also why there are accelerator experts involved in the white paper, so the physics cases for the different species will not come as a surprise in 2029; the main ones will likely by known by September 2026.   
• While it’s good to consider whether it’s possible to run a given ion species in the LHC, many things are feasible - so be creative!  
  • However, in Run 4 we will not run any radioactive nuclei
  • Detailed study of several species was already done, see: https://arxiv.org/abs/2508.19653
  • Aim at ~1-week pilot/opportunistic runs
• Send motivation letters to Giuliano/Reyes, but if anyone sends them to the HIWG convenors we will forward them
• While the white paper will be coordinated by the LPCC HIWG convenors and a steering group, it is not exclusive to the LHC - for example the community of interested people will include low-energy experiments, high- and low-energy theorists, etc
• Can we submit motivations to study ion species that have already been collided (e.g. O+O)?  
  • Yes, for sure. But we should consider what cannot be done with the datasets that we already have (e.g. motivate why we need more statistics in O+O, and how much?)
• It’s important to figure out what luminosity is needed for the physics cases — threshold values for the amount of data that is necessary for certain physics conclusions
• Boron-10 has been requested by NA61 for Run 4; it’s important to see that there are synergies between the different experiments — by doing one test, the machine serves several communities
• Some ideas are not tied to a specific species - e.g. energy loss studies could be done in O+O or Ne+Ne or some other light ion collisions - so it’s fine to include a mass range in the motivation letter, and then we can group physics cases and find a common species that addresses multiple physics interests
  • But in addition to considering the nuclear size, we should also consider the nuclear shape (e.g. clusterisation pattern of nucleons)
• Can we have theoretical guidance on what light-ion systems can help resolve outstanding puzzles?  Specifically, it would be useful to have an idea of which systems can help distinguish between models.  
  • Example: If there is a jet quenching model that agrees with the data in Pb+Pb, O+O, p+Pb, can it point us towards which system will help us understand the origin of quenching in A+A and the lack of quenching in p+A?  
• Strong interest in Pb+O collisions for UPC studies, but this would require a second ion source.  The funding for purchasing the hardware will only be made available if there is an approved experiment expressing the need for such a source (e.g. LHC collaborations request a run of Pb+O), and it will also take >3 years to procure the hardware.  Therefore we plan to focus the white paper mainly on physics cases that do not require the second source, but discussions on how to make the argument for a second ion source should also continue - maybe this is something which the HIWG could also follow up