Minutes and Summary

Main purpose of the meeting: To discuss the 2026 running, and look back at the van der Meer session

LPC 20 April 2026

Present (P = in person): Chiara Zampolli (P), Martijn Mulders (P), Eric Torrence (P), Witold Kozanecki (P), Krystian Roslon (P), Silvia Pisano (P), Flavio Pisani (P), Archie Sharma (P), Julia Negro (P), Filip Moortgat (P), David Stickland (P), Sune Jakobsen (P), Andres Delannoy (P), Michi Hostettler (P), Jorg Wenninger (P), Cedric Hernalsteens (P), Roderik Bruce (P), Joanna Wanczyk (P), Sofia Kostoglou (P), Fedrica Oliva, Stephane Willocq, Ivan Amos Cali, Riccardo Longo, Sergy Barsuk, Richard Hawkings, Gabriella Pasztor, Giulia Ripellino, Christina Dimitriadi Peter Steinberg, Maciej Trzebinski, Juan Esteban, Tomasz Bold, Anna Sfyrla, Daniele Mirarchi, Benoit Salvant, Peter Major, Wei Li

 

LPC intro (Chiara Zampolli)

About ending the low-mu fills after 40-44h to avoid instabilities, Cedric Hernalsteens asks whether thre are well-defined criteria for ending the fill. Chiara Zampolli clarifies that if there are no instabilities, we should keep the fill going. 

[after the meeting it was agreed to stop the fill when 10 or more bunches become unstable] 

Witold Kozanecki needs to have information about the separation during the 1.2 TeV run. Michi Hostettler agrees to provide that information. 

Discussion about how long the experiments, in particular CMS, would like to continue the low-mu running. General agreement that it would be simplest, and best, to run with low mu until the start of MD1. 

 

MD18223: collisions of HL-LHC baseline with 1.8e11ppb and high pile-up (Sofia Kostoglou)

David Stickland: [about the request for bunch-by-bunch luminosities] … do you need this for the MD in April, or for the high-intensity test in June? 

Sofia: Ideally, this would be for both periods. However, I understand your constraints, and we can look at alternative options in June. 

Witold Kozanecki: for the bbb luminosity, is relative lumi good enough, or does the absolute scale matter?

Sofia Kostoglou: absolute.

Flavio Pisani: do you need the values for off-line analysis, or live? 

Sofia Kostoglou: live is better. We have online analysis, so we are doing it online.

Filip Moortgat: Do you want the Roman Pots in? 

Sofia Kostoglou: not really. But if you want to insert them there is no problem.

 

Mini lumi MD – non-factorization study with beam-beam & coupling (Joanna Wanczyk)

Chiara: Okay, then the next step for this is to have a presentation, probably, and the LLCMWG meeting to discuss if we have the follow-up MD

Joanna Wanczyk: Yes, I fully agree. The full results take a lot of time, similar to the analysis of the vdMeer scans, but we hope to have some preliminary results soon, so we can then discuss it with all the Lumi experts to see if we want to continue.

 

CMS (Filip Moortgat)

 

Interim report on fill 11614 vdM (David Stickland)

Discussion about the differences between bunches from the different Booster Rings. And question how a small bias (in CMS) in crossing angle could enhance the effect (tails) for bunches from the ‘bad’ Booster rings. 

Jorg Wenninger: it is puzzling… a 5 microrad crossing angle means that at 1 meter you are at 10 micrometer difference of the beams, right? So you have a tenth of a sigma effect.

David Stickland: yeah if you are colliding on the main bunch you are ok, but if you collide with the satellite bunch you are now colliding one side of the tail with the opposite side of the tail… 

Witold Kozanecki: ALICE has seen that very clearly in their PbPb vdM scans, where they really have to pay attention to contamination from satellites. Now, in their case, they can use this DDT to TDC (?) timing to get that under control.

David Stickland: [...] I think that what we're seeing is that our effect is some convolution of the fact that there are satellites, and that we have a crossing angle.

[...]

Witold Kozanecki: have you checked that the amount of additional luminosity you would get in the case of an upward-going tail is consistent with the charge fraction of the satellites?

Witold Kozanecki: and a question for Michi: would it be possible for you to remake the plot on the right? adding the BCID plus 1 or BCID -1, because it could be collisions between the main and the ghost charge in plus 1, minus 1, or even in plus 1 minus 2, etc. 
Michi Hostettler: Yes, this could be done, actually, could be done even for any bucket in between, because, of course, for the LDM, we get even finer than bucket-by-bucket information. So, here I just did the one for the bucket at minus 2, but if you are also suspecting that the others might be contributing, then, of course, this could also explain a bigger asymmetry, because then they are on the much more open part of the crossing angle. 

[...]

Cedric Hernalsteens: if we change the filling scheme, to something that is better in terms of ghosts? If we reduce the total number of bunches, would you just scale up the duration of each step in the scans? 

David Stickland: No, we work on a per-bunch analysis anyway, and the statistical disagreement between bunches is not a dominant effect in our systematics, okay? So, if we really have to run with half the bunches, and they're good enough, we would just do that.

Martijn Mulders: so, ideally, if  you wanted to ask an additional vdM scan, it would have to be this week right? 

David Stickland: IF we would ask, it would have to be this week. But we have NOT asked to do one, ok? 

Discussion follows about how 4 injections from 2 rings could be better than 2 injections from 4 rings. But nothing is certain, of course. 

 

ATLAS (Eric Torrence)

 

ALICE (Silvia Pisano)

Discussion about the TCLIA collimators which are close to the end of their estimated lifetime. We may not need to close them for  heavy ion runs, because the injected intensity is so much lower. To be confirmed.

 

LHCb (Flavio Pisani)

Flavio explains that the large clusters seen in the VELO at higher rates in 2026 than in 2025 look similar in shape in proton-proton as it was with lead-lead last year. But the mechanism is probably completely different. Clusters of these size can either be caused by a highly charged particle, or a very collimated shower of particles.

One mechanism believed to be maybe credible by the experts, is that you have, for example, a high-energy neutron that hits the material, and then knocks out nuclei from the material, or something like this [...] you can correlate with the crossing angle, like you can see in the Vandermeer, for example. The spots are angled differently, and they follow more or less the crossing angle.