Minutes and Summary

Main purpose of the meeting: - The end of the year! - 2026 pp configuration - Preparing for Chamonix

LPC minutes 15 December 2025

Present (P = in person): Chris Young (P), Chiara Zampolli (P), Martijn Mulders (P), Eric Torrence (P), Archie Sharma (P), Robert Muenzer (P), Juan Esteban (P), Flavio Pisani (P), Stefano Redaelli (P), Giulia Negro, Anna Sfyrla, Dragoslav Lazic, Federica Oliva, Georges Trad, Johanna Wanczyk, Mika Huhtinen, Riccardo Longo, Richard Hawkings, Tomasz Bold

Introduction (Chris Young, Chiara Zampolli)

Flavio Pisani: for LHCb it is not a problem to change the filling scheme. 

Robert Muenzer: for ALICE, the change in filling scheme won’t make any difference since we anyway level on luminosity.

Nobody against changing the filling scheme → most likely it will be changed with the new proposed one.

Chiara Zampolli: it is important to know how strong the requirement to have the low mu run separately for ATLAS and CMS is. What is the lumi target for CMS?

Filip Moortgat: for CMS it is 2 fb-1.

Robert Muenzer: for the preference PbPb vs pPb, ALICE will prefer PbPb. 

Stefano Redaelli: can we then assume that this is a final decision?

Chiara Zampolli: since all the experiments agree, it should be that we will have PbPb in 2026. [note that during the week a mail was sent to the current and new management, in case of any comment.]

Eric Torrence: in fact there are no optional things between MD1 and PbPb. We have to do the VdM, the 1 TeV run… 

Chiara Zampolli: yes, one proposal that we started discussing was to have the 2 days in MD1 for the high intensity test after the 1 TeV run and the VdM, to split MD1 in practice, to protect the physics program. This should still be discussed in detail, though, so the schedule does not reflect that. 

Robert Muenzer: so the low-mu is all scheduled before the MD1?

Chiara Zampolli: we don’t know since we need to understand whether the experiments really want to do it together. The machine (Xavier) is in favour of splitting them. 

Georges Trad: what is considered as “Jan’s proposal” [s.9] was now endorsed by operation and MD coordination. In addition, if a downtime occurs, one of the scenario that we should discuss is also the case in which then the high intensity test cannot take place at all. What would we do in those two weeks?

Chiara Zampolli: yes, we are considering it. 

Chris Young: to make it clear for the experiments, if the high intensity test has issues and it cannot be done in this second period [the last two weeks], then there is potential to do more PbPb. So you might need your magnets on during the high intensity test. It is important eg for CMS who wanted to switch off their magnet. Maybe the same for ATLAS.

Robert Muenzer: could we run pp in terms of radiation?

Chris Young: probably not collisions with high intensity, so it is more that what can run after is ions. But they might also want to study what the problem is. Still you should keep it in mind, so to not make it impossible to have more ions, in case something happens.

ATLAS (Eric Torrence)

Chris Young: in the 1 TeV run, the assumed b* in ATLAS, CMS, and LHCb is 3.1m, as we squeeze all three together. Eric Torrence: for the 12b non-colliding: we use them but we need to understand how important they are, and if the small gain in having them colliding would be more beneficial then having them non-colliding for studies. We’ll give a final statement in January.

Eric Torrence: will there be another 25/50 ns test in 2026? If so, we could try some more things out with our trigger strategy to give then a better answer. 

Chiara Zampolli: it is not yet defined, but there is some interest. 

Chris Young: it might be suggested in the LBOC that both ATLAS and CMS level at 3 and 5 for a few hours in one of the ramp up steps to test if it works or not, since the beam-beam effects should be the same with fewer bunches.

Chris Young: For the request by AFP to be moved in 2026, on wed they will do a survey of the tension on the vacuum modules, and then it will be discussed what to do.

 

LHCb (Flavio Pisani)

Stefano: does this [results shown on s3]  rule out that the cause of the bkg is some machine configuration changes?

Flavio Pisani: the comparison is unfortunately not apple to apple, but we can say that there was some bkg also last year, maybe we did not see it due to a combination of effects like the fact that we were a bit further away and this year the VELO increased a bit the voltage to compensate for aging, and with highly ionizing particles you can have some effects on the silicon: you can create some plasma in the silicon that then kills the ASIC. The monitoring was also different, so it is more difficult to see in the plot from last year than in the one from this year. Now we’ll have some online plots to monitor. People are trying to find some data in which the background was there, looking at the unbiased data (randomly sampled, and quite small though) because in none of the physics events that we select the background is present. For the data when we separated all the IPs, even if I don’t have numbers now, it seems that the losses might come from IP7 because there is no major difference when you separate the different IPs. This is a quite rare effect, and we are working on a dedicated trigger line to isolate this type of events. When we have a better idea of what to look for, we can put in place a better trigger strategy. 

[discussing the event display on s4] What one sees is the interaction in the VELO, one particle from the beam halo hitting the silicon. It is a low rate event, but it produces a lot of charge. What creates the damage is the fact that the particle hits the VELO. Increasing the voltage to compensate for detector aging as they did made the sensor more susceptible to this type of effects. With a very high ionizing particle you can create a kind of plasma inside the silicon and this creates charges that can damage the ASIC. We don’t know the details of what is broken, they can do some tests, but it is difficult since it is when they apply HV, so you cannot simply irradiate. 

We can partially read out some of the broken modules because the reading is organized in columns and as soon as you reach a broken section, you cannot go further since there is too much noise, but they managed to re-equalize. They decided to not change anything during the YETS because even if the intervention would be 1 week, there are some things that if you break them, e.g. vacuum and cooling connections, then it is very bad, while the current situation is not too bad for physics, just a few percent of loss in some channels. 

Stefano Redaelli: what do you expect to see if there is such event?

Flavio Pisani: a long straight bunch of tracks. This would not be used to assess if the VELO would be damaged but it would allow you to try machine settings to reduce the effect, and could be a metrics to study the background without having the VELO closed. 

Stefano Redaelli: pity we don’t have reference results with the 2024 b* configuration. 

Flavio Pisani: for PbPb we don’t need the same resolution on the impact parameter as in pp, so we can keep the VELO a bit open. The only limit we have is that to inject the SMOG we have to close the cell, but I think we can do it without fully closing the VELO. We have to see, maybe we can also just limit the HV to 400 instead of 600 kV. 

Flavio Pisani: for the 25ns Pb: we did not have any issue from the no data acquisition side, but VELO had some problems (unrelated to background), so we could not take physics data, so we don’t know if HLT or detector wise the 25 ns is a problem, but for sure we will have some spill over in the next event of some subdetectors, even though with low mu, the chances to have two consecutive collisions are pretty low. They should do some studies.. It would be nice to have more MD, but maybe to use it for 2026 is a bit risky.

 

ALICE (Robert Muenzer)

Robert Muenzer: decided to run PbPb, one motivation is also that we don’t know if we can go to full IR 50 kHz, and we’d lose less in PbPb. Plus there are also other internal discussions that went in favour of PbPb. For the 25 ns, our ZDC electronics might have some limitations for next year. Finally, if there is any chance that there is more PbPb in the last 2 weeks, we will need to be prepared with the appropriate shift crew.

 

CMS (Archie Sharma)

Archie Sharma: for the 12 non-colliding bunches, they are important to improve the background studies but not critical. We’ll provide a final answer soon.