Minutes and Summary

Main purpose of the meeting: PbPb feedback, pPb projections and timetable for 2025/6

LPC minutes 11th November

Introduction (Federico Alessio)

(Catrin Bernius) Would the MD still start on Thursday morning? Yes, but then it would stop at 11pm, then physics overnight and MD on the Friday morning shift to complete the 24h. ATLAS Tile calorimeter would like to do a calibration that takes over 5 hours. It should still fit but if there are problems we will let you know.

(Catrin Bernius, Andrea Ferrero, Michi Hostettler) Does the increase in ions per bunch increase the chance of problems? Yes, but more dumps from losses rather than cryo conditions problems which take a long time to recover. Is this higher charge in the injectors and aggressive scraping so we get the same intensity in the LHC or more ions in the LHC. No, there will be some scraping to reduce the chances of problems with losses, but the idea is to try to get higher bunch intensity in the LHC. Think >20% increase in intensity and 1-2% loss due to scraping.

(Catrin Bernius) What exact feedback do you need for Montreux. This will become a bit clearer after Chris' presentation. We would like to present scenario B as a baseline and different splittings between the years will be shown later in the meeting to see what people's opinions are.

ALICE (Andrea Ferrero)

(Witold Kozanecki) If the two beams have different length scales then you can get this effect as you aren't moving them by the same amount. Can your expert estimate the difference in the length scale of the two beams to see how different they would need to be. If they are only a few % different then you will need to correct them separately in the vdM scan but a difference this size would not be very surprising.

(Federico Alessio) Do you see deadtime from the beam background? This seems to be under control but more detail will be given next time.

(Federico Alessio, Catrin Bernius, Witold Kozanecki, Michi Hostettler) For the cogging I think this 3mm shift can be done if there is no objections. It is towards CMS. Is it seen in the 850b fill? Yes, the points that are between are the vdM scan in CMS. For the very first fills we have different types of beams so things could be different. The other experiments don't see a significant shifts but they aren't particularly sensitive. If it is a real shift then it should be visible by all experiments, but if it is a crossing angle effect then they won't see it. All experiments should see the shift when the correction are made. In pp when we did this it was more evident in ALICE than the others, and there was one which had the wrong sign. In the next fill all experiments saw some change though. All the experiments will check offline.

LHCb (Elena Dall'Occo)

(Federico Alessio) What does eb mean? Beam in beam 2, no beam in beam 1 (e is from "empty", b is from "beam").

(Chris Young, Federico Alessio) What happens if it isn't possible to subtract this effect? We would have to re-do it. Would it just need to be the SMOG-off part? Yes. This also means that we need to know very soon -- before we finish data-taking!

CMS (Filip Moortgat)

(Filip Moortgat) One of the SMP (safe machine parameter) crates has been off due to a broken power supply. After fixing it, to test it works it was used to dump the beam before the access at the agreed time. How the crate is used internally in CMS is still being investigated as it doesn't appear to have been being used by BCM, but this is an internal CMS issue to work out how it is being used. The information is propagated but not from that crate apparently.

ATLAS (Catrin Bernius, Brian Cole)

(Filip Moortgat) Was the beam 2 background plot completely flat last year? It was similar to the blue line.

(Michi Hostettler, Federico Alessio, Witold Kozanecki) In the 1200b fills we did not normally clean the abort gap at flat top as it was below the threshold where this is required to be done. Would the amount observed in the abort gap be similar to the amount of beam in other empty bunches. Yes, in principle. In LHCb about 1% of a full bunch is seen in empty bunches so there could be collisions. BSRL could also be used to look at this across the whole beam, or integrated across the abort gap. We can try cleaning the abort gap in the next fill to see if this improves for ATLAS ZDC.

p-Pb Beam Parameters & Luminosity Projections/Estimates (Roderik Bruce)

(Riccardo Longo, Chris Young) Do you know the rough mu for the peak luminosities? I can add a table. Would they be much higher than 2016? They should be higher, particular in the optimistic scenarios. ALICE was leveled lower in the past, also in 2016 we got about 14nb-1/day and now we are looking at 25nb-1/day. We are looking at luminosities

(Witold Kozanecki) If you flip the species then for precision luminosity measurements we will want vdM programs for both directions as the luminometers are not guaranteed to be symmetric. In 2016 there was a short vdM for each direction. This is not taken into account in the numbers presented in Chris's presentation as it assumes a single vdM subtracted from the total number of HI days.

(Chris Young) Roderik has presented quite a few scenarios and also some you can interpolate between for example intermediate bunch intensities. They are designed to give a ball-park estimate of what could be done per day and were used for the numbers in the next presentation which involves how many days we will run.

(John Jowett) I would be caution about assuming we will use any of the 25ns configurations, as Roderik said, as this could have significant complications in terms of beam dynamics.

Timetable HI options (Chris Young)

(Riccardo Longo) There is certainly discussion to be had over the energy if there won't be another pp reference run at the energy we run at.

(John Jowett, Brian Cole) For pp reference there was 8 TeV pp data taken in 2012 - would this work as a reference? This is difficult, even a non-starter, as the data was taken before some detector upgrades and also the software has moved on a lot since 2012. This makes it not possible. The 2016 dataset was great but we have not got the physics out of it that we should due to the lack of a pp reference dataset. The physics program has really been hamstrung by this so we are keen that this doesn't occur again.

(Filip Moortgat, Chris Young) For the max 2025 scenario you do have the 4 weeks that was requested but as this is p-Pb rather than Pb-Pb so we will need to wait for the studies from RP. When we discussed with the RP experts they described that after PbPb they are really still seeing the tail of the pp running such that the intensity of PbPb doesn't matter in the regime we are discussing. Obviously p-Pb is different but if it is near the same order of magnitude then this might still hopefully be the case. Additionally, the ALICE RP contact found that p-Pb was not particularly different from Pb-Pb. So far RP experts have indicated that p-Pb will work well for cool-down but studies are still on-going.

(Federico Alessio) For ALICE & LHCb we will need to know the preferred choice of p-Pb or Pb-p in the case that only one of them is doing.

(Roderik Bruce) Firstly, as you mentioned, the luminosity numbers are pretty optimistic for p-Pb.

(Roderik Bruce, Chris Young) Secondly, there is obviously the risk of doing anything at the end of the Run, such that we should be aware of this when discussing what physics program we want to put at the end. All 4 experiments have expressed that the PbPb program is more important than the p-Pb program which is why it is assumed that the p-Pb program is put at the end of the run, as something is needed there for cooldown.

(Federico Alessio) All experiments have expressed that they are only interested in p-Pb once the Pb-Pb targets are met such that we don't need to make a decision on p-Pb at this time, but for the timetable which will be approved by the RB we need a baseline of when the PbPb/pPb running will occur as this also affects the injectors schedule.

(Andrea Ferrero, Federico Alessio) What is the implication of the 7 nb-1 statement from CMS, as this is a new target? This is not a target but a request to take more data before switching to p-Pb. Year-by-year targets can be made when we know the full performance this year which will be in the coming weeks. This is beyond what is wanted from the yellow-report. Also the statement from ATLAS is even stronger. In all cases we can wait and see how much data is taken both in 2024 and 2025 before deciding if we do p-Pb in 2026 (in option b), but we can end up constrained if we have too many/few days assigned to 2026, and the middle scenario minimizes this risk.

(Federico Alessio) We will show all of the splits in option b and also option c to the LHCC to see what their opinions are.

(Filip Moortgat, Brian Cole, Ricardo Longo) Would the projected luminosities need to be adjusted in the case of an additional pp reference run. If we run at a lower energy then the 2024 pp reference run can be used, and this has plenty of luminosity. The higher energy is very similar to 2016 and we don't have a reference run so this scenario is disfavoured without an additional pp reference run.

(Andrea Ferrero) We will come back on the question of the desired energy from ALICE in the next LPC meetings. It might be we assumed that we would run at the lower energy.

(Chris Young) Arguments against any of these sub-scenarios are certainly welcomed as this would simplify what we need to present/choose between.

(Roderik Bruce, Ricardo Longo, Filip Moortgat) How long a pp reference run would be needed? It would be either up to how long we did this year ie. 1 week, or maybe as short of a couple of days (after intensity ramp-up) at higher pile-up if not driven by ALICE.

(Roderik Bruce, Chris Young) For the lower energy the luminosity numbers might also change as it is unclear what parameters could be achieved and there is the fundamental relativistic gamma-factor in the luminosity formula.