Minutes and Summary

Main purpose of the meeting: Data taking progress First look towards PbPb Other news

LPC minutes 22 September 2025

Present (P = in person): Chris Young (P), Chiara Zampolli (P), Robert Münzer (P), Eric Torrence (P), Roderik Brice (P), Witold Kozanecki, Andrej Gorisek, Helga Timko, Peter Steinberg, Flavio Pisani (P), Michi Hostletter (P), Anna Sfyrla, Joanna Wanczyk (P), Georges Trad (P), Giulia Negro (P), Rosen Matev, Ivan Calì, Richard Hawikings, Matteo Solfaroli Camillocci, Andres Dellanoy, Filip Moortgat (P), Benoit Salvant, Stephane Fartouk, David Stickland, Gerardo Vasquez, Juan Esteban

Introduction (Chris Young)

Roderik Brice: concerning the background from fragments, we should also check LHCb since they will have much tighter TCTs to fit the 1m b*. It would be good to check that everything is fine before we go into the intensity ramp up. This test would happen in ADJUST. 

Flavio Pisani: Would you need the BGI? In ADJUST we cannot close the VELO.

Chris Young: you should check the background, even if I don’t know what you use to monitor this.

Flavio Pisani: usually we check injecting SMOG, e.g. for the ghost charge measurement. With VELO open, I am not sure it is accurate. I need to check. 

Roderik Bruce: you can think about it offline.

Robert Muenzer: we’d like to test also the impact of the solenoid field. Now in pp we see that reducing the field, enhanced the background in our inner tracker. So it would be useful to add a test with low B field. It should not be a big deal to ramp it down, as far as I have seen. 

Roderik Bruce: how much time does it take to ramp it down? Can you do with the beam in the machine?

Chris Young: yes, and it is ~20 minutes.  

Michi Hostletter: in the past, it could only be ramped with no beam or stable beams. But of course we’ll have neither of that. 

Robert Muenzer: it does not depend on the beam state itself, it is that the TPC needs to be ready. We need, maybe in ADJUST we cannot turn it on. We need to discuss the plan in detail.

Roderik Bruce: will you change the field during the ion run?

Robert Muenzer: no, but we plan at some point to take data at low field. So it would be good to get prepared for this.

Robert Muenzer: the 1 TeV special run for LHCb/SMOG, how long should it be?

Flavio Pisani: it should be around 1 fill.

Michi Hostletter: it would be good to have a number in b-1, because for a full machine, we’d need to do a ramp up. Probably the best compromise in terms of time is to stop somewhere at the ramp up and then take this one instead of doing the full ramp up. But it depends on the target.

Chris Young: the target should be for a fixed-target lumi rather than a collision lumi, which complicates our lives even further since we’d need to know the gas pressure and other parameters.

Flavio Pisani: the experts should be able to get this info.

Eric Torrence: could it be combined with the HL-LHC tests?

Chris Young: we won’t be running at 1 TeV during the tests. We’ll use 3 TeV.

Michi Hostletter: otherwise the bunch length grows over time. If we go too low in energy, then you don’t have enough sync radiation damping and the bunch length grows over time, and the heating that we want to probe goes down over time. 

Eric Torrence: can LHCb use higher energy?

Flavio Pisani: no. There was a request for 450 GeV, but this is complicated for the machine. So 1 TeV is ok. I can ask, but probably 3 TeV  is a bit too much. 

Georges Trad: during MD3 we will most likely do commissioning for a new cycle up to 3 TeV and we will have ramped up for intensities higher than safe beam. So if LHCb could use this, it would be good.

Chris Young: we ran at 2.5 for the pp ref, but this is not what LHCb wants. They want as low energy as possible, but high enough to close the VELO. Which is why we got to 1.

Michi Hostletter: there was some more discussion with aperture and collimation experts, and apparently to fully close the VELO at 1 TeV, we need to squeeze a little bit because at 10 m we don’t have otherwise enough aperture, but we cannot squeeze to the full 2m that you have at the end because then we have another aperture limitation at the triplet. The present working hypothesis is 3.1 m, which is a partial squeeze; since the ramp is so short, it cannot fit in the ramp, and it will have to be done at the 1 TeV flat top. You’d add something like 5-10 minutes to the cycle, but this is not a big deal. It would have to be commissioned, though.

Chris Young: the b* does not matter much for the gas program.

Flavio Pisani: no but we need to close the VELO. From the detector point of view, we could close partially, but the SMOG cell itself is spring-mounted to the VELO, so if you don’t fully close it, the limiting factor for the aperture is not the VELO itself but the SMOG cell which is spring loaded. This requires more studies. By opening the VELO a bit, you might not gain much in aperture.

Roderik Bruce: for the slides on the topics for 2026: at some point we should add whether we do pPb or PbPb, like setting criteria. 

Chris Young: yes. Experiments should think about what is the sufficient amount of lumi collected in PbPb in 2025 to consider pPb. We should all use the same numbers.  

Filip Moortgat: we gave these numbers, the problem is more if we don’t agree. CMS said 7 nb-1.

Roderik Bruce: 7 was the total for ATLAS and CMS in Run 3, ALICE gave a bit less. 

Filip Moortgat: yes, then somebody needs to decide on that number, probably the Reserach Board.

Chris Young: we’ll not have finished PbPb by the time of the Research Board this year. We should have the criteria and make sure that the numbers are delivered or recorded for everybody, if there are other quality criteria etc. We’re also looking at updating some projections for pPb in 2026, like done last year. We’ll look at the intensities of the lead beam and the emittances. Then we can narrow down some of the uncertainties. We also have some filling schemes, but not yet accurate.

Chris Young: one more comment: MD4 is now before the ion. There is the possibility that the Ion commissioning will be done before the MD4 so that if there is a quench test in the MD4, then the TS is used to recover the quench. People should be aware for the background test in ALICE and LHCb.

Georges Trad: another possibility is to shift down the last ½ day of the MD, when protons are still available. Then the disruption becomes only half a day.

Chris Young: yes, we should discuss since it would put all the commissioning for ions by Roderik in the night.

 

ATLAS (Eric Torrence)

 

No comments. 

ALICE (Robert Muenzer)

Robert Muenzer: the next polarity will be the same as now, just higher (nominal) field.

Roderik Bruce: do I understand correctly that with a longer readout frame you are not worried about the data quality anymore. This mitigates the problem?

Robert Muenzer: yes. These two layers are essential for the tracking since they connect the inner and the outer. If you reduce the deadtime to a value to 1%, the quality is fine. We know from full field that these deadtimes are enough for physics quality.

Chris Young: how much of the low field data are after the fix?

Robert Muenzer: the numbers on the slides are after the fix. The issue affects a little portion of the data. We lost 0.2 pb-1.

Chiara Zampolli: do you see this on both sides? Do you know where it could come from?

Robert Muenzer: no, we are still waiting for these results. The plot of the occupancy is integrated on both sides. 

Chiara Zampolli: and the MFT does not have the acceptance of the affected layers?

Robert Muenzer: no, this is different from PbPb when the inner part was affected. But it is probably not from the C-side, or it would hit the MFT also, but it is not clear. It must be some source that is affected by the lower field, that makes it move to higher layers. Such features we don’t see in any other detectors. We also don’t have the ZDC that could give other information. And with this xing angle, they cannot run.

Chris Young: it cannot be high energy particles if they are affected by the solenoid field. They seem to come almost parallel.  

Chiara Zampolli: could you check the other layers in the nominal field? Maybe the source of background is bent differently and hits somewhere else.

Robert Muenzer: with the full field, we don’t see anything even in the other layers. There is also quite a large gap between the two layers. 

Chris Young: what if they are moving through phi and hitting different staves?

Robert Muenzer: this is also what I thought when we checked reverting the B field, but nothing changed. 

Michi Hostletter: it could be interesting to get in touch with the ecloud experts, to see if this could be potentially a shift of where the ecloud hits the chamber. That is one effect that you would expect changing the solenoid field. But I don’t have the feeling of how strong the effect could be. We saw when we had the magnet fully off but the dipole on, so it is hard to assess.

Robert Muenzer: for the ecloud with solenoid off, there was indeed an increase in the background in our lumimeter, but in this case it is not. 

Roderik Bruce: if it is ecloud, would it not be expected to hit the beam pipe? It is then puzzling that it is so much out, and not in the innermost layers.

Michi Hostletter: even if it hits the pipe, the question would be where it would then be detected. It would be interesting to understand it to see whether it could be a problem for ions. If it is ecloud related, it should be there only with high intensity proton beams.

Roderik Bruce: it is 200 mm radius, and we don’t see anything inside 200 mm. I have a hard time to understand how electrons could make it there without being seen somewhere else. Even the electrons are pretty low energy. 

Robert Muenzer/Michi Hostletter: yes, we should see something also in the innermost layers.

Michi Hostletter: this is true for almost anything that comes from the beam. 

Roderik Bruce: for other backgrounds, there are high energy muons that can go out from the center of the beam pipe and then they can go through the magnet and everything and hit the detectors at larger radius. From these studies, we have things at higher radius. 

Robert Muenzer: we’ll provide more details when we have more findings.

 

LHCb (Flavio Pisani)

Chiara Zampolli: Tandem 3 was the one that was already having issues, and was then fixed, right?

Flavio Pisani: yes. They replaced the inner valves of the compressor at that time and when they inspected them, they did not find any problem. With the new valves things were working fine, and now they suspect that there might be something with the cooling fluid. At the next MD they: replace a valve in Tandem 1 (each tandem has 2 compressors, and we’re using one now, which is why we’re running in degraded mode). This should fix the issue with Tandem 1. Then they will replace the cooling fluid for Tandem 3, maybe even for all of them. But they don’t know what the problem is exactly. 

Chris Young: the valve is a long intervention, so we wait for the MD, right?

Flavio Pisani: yes. For this there is also an issue with spare availability, they might not have all the needed ones.

Chris Young: the smoke sensors: are they especially close to the IP? Since it was mentioned that maybe the issue is related to radiation.

Flavio Pisani: not really. It should be few mSv/h during collision. But I would need to double check. For LS3 the plan is to change all the sensors. But it is something long, it cannot be done in the YETS. Maybe the other experiments have better hermeticity to radiation. But I think that CMS had a similar issue this year with a smoke sensor.

Giulia Pisano: yes. 

 

CMS (Giulia Negro)

Chris Young: is the request for the low mu fixed?

Giulia Negro: it is more to be considered as a working assumption. 

Chris Young: as for where we’ll put it, we’ll have to see, we’ll do some optimization depending on whether both or only one detector will do it, if we can use it as cool down for the VdM period, which would also help the MD team removing the constraint on them to have some cool down for the VdM… And then the TS needs to be added.