Minutes and Summary

Main purpose of the meeting: Data taking, discussions on PbPb/ppRef and 2025+2026 schedule

LPC minutes 7th October

Introduction (Federico Alessio)

(Catrin Bernius) Will the 2.5 days of PbPb setup that is being advanced be with ions or protons. This will be ions but unlikely to be with trains.

(Chris Young, Catrin Bernius, David Stickland) The key is that the BBA AFP experts needs to be available either before these 2.5 days or after, and the luminosity experts available 2.5 days later if this change is made. AFP have indicated that they are flexible. Both ATLAS and CMS indicated that their luminosity experts would also be available either timeslot.

(Chris Young, Andrea Ferrero) The ALICE background would also be tested during this 2.5 day period. This will take some preparation and coordination as the inner tracker will need to be in the ready state. ALICE are keen to do this test early and will coordinate with Roderik.

(Enrico Bravin) Note that there is also a request to prepare the crystal collimators before the TS. This will be mentioned later in the slides.

(Michi Hostettler, Andrea Ferrero, Catrin Bernius) From the machine side we need to be sure that the setup is safe for these collision tests (in all IPs) for the background even if the full collimation system is not validated. There are reasons we don't usually declare stable beams at this stage. From ALICE, ideally both the ITS and ZDC will be used for this test. ATLAS ZDC would also benefit from collisions in IP1.

(Maciej Trzebinski) Following the BBA there will need to be loss maps. Yes, we are aware and this is the other part of the ppRef setup in the timeline.

(Chris Young) For the emulsions we should also be aware that we want to be close to the 13 fb-1 limit, but we believe with the crystal collimation shift added we will not be far short of this limit/target.

(Michi Hostettler) Still only LHCb have sent the vdM files. If it is the same as in the past an email is sufficient and they can be transferred to the proper place.

(Catrin Bernius, Michi Hostettler) In the current fill the leveling in ATLAS was 60 rather than 64 at the beginning of the fill. This has now been resolved - it was an OP configuration problem.

(Andrea Ferrero, Chris Young, Robert Munzer) Are the days marked dark red the ppRef. Yes. What is the total additional ion days? We are still waiting on an official estimate and we are likely conservative such that you get additional days when this is done properly. Would the additional days be added at the end of the TS moved? The TS has to occur during the week, it can therefore be moved to accommodate additional days and there is some flexibility at the 1-2 day level that LPC will utilize to make sure the TS doesn't go over a weekend. One or two days of PbPb commissioning could be before the TS as the ZDCs aren't required for this, and it would also allow the BLM thresholds to be calculated over the TS. When will there be a better estimate of the number of HI days? This depends on several things like injectors and also the YETS length and activities so we don't know exactly.

(Witold Kozanecki) Where is the vdM in 2025. It is currently before the pO/OO runs but it can easily be swapped with this run. This timetable is not meant to be exact for small features like this.

(Andrea Ferrero, Michi Hostettler, Chris Young) What would be the overhead of switching to pPb from PbPb? This requires new commissioning so at least 4 days. The injectors will still have protons so this is not an issue but there would also be RF things in the LHC that would need commissioning so it could be longer. This hasn't been done since Run 2 so we would need to ask Jorg or Matteo for a more accurate estimate of the total commissioning time.

(Andrea Ferrero) The goal was 13 nb-1 for Run 3+4 but then this has been modified several times and the ratio between the runs has changed.

(Catrin Bernius) Could the TS start on a weekday other than Monday? Yes, we believe this is the case. LS3 should also start on a weekday.

(Chris Young) In the previous pPb runs we didn't do any pp reference runs, perhaps because it was close to 8 TeV where we had taken data. This is therefore another question to the experiments whether a reference run would also be required.

(Catrin Bernius) For the end of 2026 is the 1 week of MD sufficient? Or will low-mu also be required? At the last LMC 7 days of MD was proposed so they will see if this is sufficient for the first activities in LS3. Otherwise they will come back to us. The experiments will also need to provide input on what their first activities will be at the start of LS3.

(Federico Alessio) There is also the possibility, even though we disfavoured it, of having all the HI at the end of 2026, and the experiments are also open to suggest other configurations to us, but we will need to go to the next stages with a maximum of two ordered by preference options.

(Michi Hostettler, Chris Young) Is there a physics motivation to do p-Pb/Pb-p? If there is then it should definitely be done in a separate block to the PbPb as there is no motivation to put them together.

(Reyes Alemany Fernandez) Would it be possible to do Ne-Ne collisions in 2026 as this would add to the physics of O-O as they are similar mass but different geometry nuclei? Formally the light ion collisions are special runs so are different from the PbPb and p-Pb programs. This would therefore need to be proposed and then presented to LHCC and then the Research Board to see if they would want to assign time to this as a "special run".

(Chris Young) As the O-O/p-O run is very short and involves multiple configurations LPC don't think it is appropriate to use for cool down, as it also gives the contingency of over-running by a day where it currently is in the schedule. This also matches the injectors schedule. Finally putting it at the end of 2026 would be an extremely long delay.

(Andrea Ferrero) There is presumably the same number of days as it is the same setup time if we did p-Pb and PbPb all together as doing it separately. Yes, but the motivation to put it together is efficiency which is lost in this case. Putting it together would also mean more difficulty for the experts getting everything ready for a single period so we would like to avoid this. Therefore if we are doing p-Pb we would split the HI period between the two years rather than try to do both types in one year. It was pointed out that an advantage to having them together would be to tune the switch over depending on how much data is taken, but this would also need all the experiments to agree when to switch which would be complicated.

(Chris Young, Roderik Bruce) In future schedules we might move two of the ion commissioning days to the other side of the TS such that we automatically do the advancing of the commissioning and don't need to discuss it like we have done this day. But this wouldn't affect the overall number of ion days. It is not only BLM thresholds, it is also collimation settings and ALICE background checks that are very useful to do in advance so this is certainly supported from the machine side. As the ALICE ZDC is movable remotely it can be easily inserted for such a test even if it is before the TS.

LHCb (Elena Dall'Occo)

(Chris Young) There is quite a strong reason for pO before OO so it is nice you don't have a preference.

(Catrin Bernius) What you you mean than the pledges are lower? We made requests for further storage but they were not granted yet.

ALICE (Andrea Ferrero)

(Federico Alessio) We are all now agreed that the ALICE/LHCb vdM can go before ATLAS/CMS.

(Chris Young) Regarding the disk space; is the reco ready to run when the data is taken? Yes, but it takes months. So a slightly longer YETS between 25/26 wouldn't help. Not significantly.

CMS (Guilia Negro)

(Catrin Bernius) The sign of the crossing angle is independent between the experiments? Yes, but for horizontal only positive is possible, while in vertical the other experiment can choose. The planes need to be opposite for the two experiments for pp running. For PbPb we have more flexibility. It is unclear for low-mu if we have flexibility and ZDC might be interested in any low-mu run in ATLAS.

(Chris Young) Changing the crossing angle changes the dispersion and hence the ideal rotation. You see that for different crossing angles it doesn't matter too much. I think Stephane wants to know how far from optimal you can be without losing too much performance.

ATLAS (Catrin Bernius)

(Filip Moortgat) 70 is very high is the cryo ok with this? It was discussed with Benjamin this morning and they need notice to adjust the settings and it needs to be done slowly. It should be done by phone and the CCC will do the adjustments rather than using the automatic tools.

(Roderik Bruce, Brian Cole) For the early PbPb commissioning will there be collisions. Yes, there will be collisions with single bunches in this commissioning. This is ideal for ATLAS ZDC.

(Roderik Bruce) There is a chance that the cryo will be ready early so if AFP could be ready on Thursday night that would be good -- they should be. It should also be possible to do AFP alignment, then PbPb commissioning and then loss maps for ppRef if that ends up being desired.

(All) Everyone expressed desires to also follow the clock switch test.

(Filip Moortgat) Is it really worth changing the cross-section just for the ppRef run? You also need to remember to change it back and you could just use lumi. Is it not error prone. In ATLAS it is not thought that this will be more error-prone. For CMS they will not level if the peak is 6-7, if it is 8-9 then there will be some leveling. CMS will manually calculate the lumi to level on rather than updating the hard-coded cross-section.

(Reyes Alemany Fernandez, Roderik Bruce, Andrea Ferrero) Does it matter for the ppRef/PbPb setup which might be interleaved what the polarity for ALICE is as we would like to start with positive polarity (-183) for ppRef. Would for PbPb setup it needs to be different. From ALICE it doesn't matter but it depends a bit on balancing the datasets. For the PbPb setup it doesn't matter which polarity is used.