Minutes and Summary

Main purpose of the meeting: - PbPb data-taking progress - Planning MD block and polarity reversal - News from the RB - 2026 questions

LPC minutes 01 December 2025

Present (P = in person): Chris Young (P), Chiara Zampolli (P), Martijn Mulders (P), Andrej Gorisek (P), Eric Torrence (P), Robert Muenzer (P), Filip Moortgat (P), Ivan Calì (P), Archie Sharma (P), Silvia Pisano (P), Matteo Solfaroli Camillocci (P), Anna Sfyrla, Paula Collins, Flavio Pisani, Giulia Negro, John Jowett, Krystian Roslon, Riccardo Longo, Joanna Wanczyk (P), Roderik Bruce (P), Gerardo Vasquez, David Stickland, Andres Delannoy, Dragoslav Lazic, Geroges Trad, Giulia Ripellino

Introduction (Chris Young)

Chris Young: ATLAS should check whether the luminosity that they report for PbPb needs to be corrected for background, since it is higher than CMS, even accounting for the period in which CMS was leveled at 10% their usual luminosity.

Chris Young: from the way the xing angles are configured, as last year you would expect a tiny advantage for CMS, so this is a bit surprising. 

Roderik Bruce: there was also a discussion that when you numerically integrate the instantaneous luminosity in Timber, you don’t get the integrated one, which is some percent higher. This happens only for ATLAS. 

Matteo Solfaroli: [concerning the low mu run] trying to make ATLAS and CMS run at low mu at the same time might have a cost. It is not a yes/no necessarily, we might get unstable once and not the second time and vice versa. So it is clear that trying to do it at the same time will have a cost which won’t be there if we do it separately.

Filip Moortgat: we have a lot of issues when we have full blast intensity beams, a lot of beam in the machine, while if we are at 1.4 or similar we are more relaxed. Of course if a power supply breaks, it breaks, but the beam intensity induced problems you don’t have. I would like a relaxed three weeks.

Matteo Solfaroli: you risk to pay the price to dump once in a while. We can have an LBOC dedicated and make studies, but there are risks since we’d run at the limit. 

Filip Moortgat: Michi said that if the separation is not crazy, there should be no problem. 

Chris Young: the separation is not at 1.5 sigma which is the minimum of the Landau damping, but it is at 3 sigma, which is not crazy. You could even make the separation bigger by making more beta* steps at the beginning. But it is something that we never tested before. It is going in the unknown. 

David Stickland: when you talk about alternating, a complete counterproductive thing would be to do one day ATLAS separated, one day CMS etc. If we interpose high intensity and low intensity in CMS, in our luminometers in particular, we will lose the advantage of having a high precision VdM for these data. 

Chris Young: what is meant in the slides is to do one experiment for 3 weeks, and other three weeks the other. Or you might do it week by week. Not fill by fill. 

Filip Moorgat: you still want to go to 1.8 in both beams, next year in the first two weeks. Is this enough?

Matteo Solfaroli: yes, it is a good test. In general, I don’t mind either way, I only want to say that even if it is not the worst condition that we ever had, still it is something that we never tried. If the simulation says that we should be safe, we don’t really know how it will be. 

Filip Moorgat: yes, what we want all is the minimum downtime.

Chris Young: in the scenario shown on the slide [s10], we go to 1.8 almost as soon as we have the machine filled, and then we could go back to 1.8 after the low pileup. 

David Stickland: alternating week by week is not better than fill by fill, since it is not a contiguous period of low intensity where we do not expect the luminometer to have any damage.

Chris Young: we can make first all 3 and then all the other 3. 

Andrej Gorisek: MD17143, MD17164 do they have collisions? We would need no collisions. 

Chris Young: there might be losses, but no collisions. 

Andrej Gorisek: if you change something in the planning, could you let us know?

Eric Torrence: in the reported luminosity we might be 5% higher than it should be, and this might explain the difference to CMS.

ATLAS (Andrej Gorisek)

Chris Young: [about the low mu run] the statement that might come out of the LBOC may be “possible”, “not possible”, but it will be with significant uncertainties. 

Eric Torrence: we could also separate early in the year to test, like for 1 hour.

Andrej Gorisek: maybe we could try one fill with both separate and then see what the outcome is.

Chris Young: we could probably do something like that during the ramp up.  

Filip Moorgat: maybe it could be done with lower intensity, like 1.2e11. Or 1.3e11, it should still be many hours of leveling for LHCb. 

Andrej Gorisek: we could do a fill as a test during the ramp up and see what the outcome is. 

Filip Moortgat: would the instability be something occasional, or is it a different concern?

Michi Hostletter: too low intensity is also not good because then you have issues with the electron cloud, which becomes worse at about 1.1e11 or so.

Matteo Solfaroli: the thing is quite systematic, but if we’re running at the edge, it may happen once and not all the time. That is why it is difficult to say. It may never happen. It may be systematic, or maybe neither one nor the other, and it shows up once or twice. 

Andrej Gorisek: then maybe it makes more sense to run separately.

Filip Moortgat: but don’t forget that running with 1.8e11 in the machine will give intensity induced problems for sure and downtime attached to that which we wanted to avoid.

Chris Young: the argument against that is that the low mu run that we did in the pp ref, is still with full intensity in the bunches and we had good availability but in none of these runs including that one we managed to go regularly beyond 24h. We always tripped on something. 

Filip Moortgat: there is surely still the random stuff happening.

 

CMS (Archie Sharma)

No comments. 

 

ALICE (Robert Muenzer)

Chris Young: were the loss maps of the off momentum looked at?

Roderik Bruce: we still see high losses in the TCTs in IR2, Bjorn is looking at the offline analysis now, it should be available soon.

Robert: we can decide after the 400b.

 

LHCb (Paula Collins)

Chris Young: what is the normalization used for the bottom plots of slide 5?

Paula Collins: this is just a total occupancy per unit area where the unit is am extremely small fraction of a mm^2. It is more reflecting the length of the run than anything else, not normalized per mu.

Chris Young: ok, I was wondering why the middle plot was not a factor 10 lower or so.

Michi Hostletter: this is beam gas, so it should be mu-independent. 

Paula Collins: yes, this is correct, it is beam1-empty, so it should not depend on the mu.

Chris Young: the test seems a good idea, we should make sure to not go too head on in LHCb. 

Michi Hostletter: I would propose, to be on the safe side, to do it at the end of the leveling when we can go head on. If we start from a head on position we avoid any jump or transients when moving. Then basically we move the usual knob that we use for leveling which is beam 1 and beam 2 independent. Normally we move them with opposite signs but for this test we’ll move them with the same sign by the same amount. It is not likely that this is the cause but we can try it. 

Roderik Bruce: you could do it with the fill that we’re having now with 400b.

Chris Young: yes, either with the 400b fill or at the end of a fill, avoiding to go head on. 

Paula Collins: how many hours into the 400b fill should we do this test?

Chris Young: you could do it straight away in the 400b since in this case, even if head on, you will not go beyond the allowed lumi limit. You can contact the CCC and Michi will be around. 

Filip Moortgat: to be clear, there were no other issues, after moving the VELO out, right?

Paula Collins: yes, after we moved the VELO out, and lowered the voltage too.