Minutes and Summary

Main purpose of the meeting: Schedule, beam spot size, miscellanea

LPC minutes 31st March 2025

Present (P = in person): Chris Young (P), Chiara Zampolli (P), Robert Muenzer (P), Silvia Pisano (P), Michi Hostettler (P), Andrej Gorisek (P), Filip Moortgat (P), Giulia Negro (P), Paula Collins (P), Eric Torrence (P), Rosen Matv (P), Jorg Wenninger (P), Andrea Massironi, Anna Sfyrla, Flavio Pisani, Jamie Boyd, Lorenzo Bonechi, Maciej Trzebinski, Rende Steerenberg, Reyes Alemany, Roderik Bruce, Stephane Willocq, Witold Kozanecki, David Stickland (P), Joanna Wanczyk (P), Mirko Pojer (P), Juan Esteban Muller, Matteo Solfaroli Camillocci, Benoit Salvant, Stephane Fartouk

Introduction (Chris Young)

Andrej Gorisek: Please CCC call us when you need that we have the BCM masked.

Jorg: you could also just leave it masked.

Michi: note that when we do the first setup, we will probably also disable the post-mortem generation to avoid that when we do threading and we dump on a detector BCM or on our own BLM, also due to losses, we have 5 minutes delay for the analysis of the post mortem events. The flags whether they are enabled or not is on Page1 (bottom right, for each beam). It is something we do every year.

Jorg Wenninger: in the estimates for the beam size, the bunch length of 1.3 ns is a bit pessimistic, this can explain why the beam size in z in the results from the LPC tool is a bit overestimated compared to what was seen by ATLAS.

Chris: for sigma_z one which beta* to use when the b* is different in H and V as we’re having in 2025?

Jorg Wenninger/Witold Kozanecki: the one in the crossing plane, which is the one that stays at 60, so yes, the one in x in this example for ATLAS (which crosses in H).

Witold Kozanecki: the units for sigma_z, should they be mm?

Chris Young: yes, sorry, a mistake in the slides.

Witold Kozanecki: do you know why LHCf needs non-colliding bunches that do not collide anywhere?

Chris Young: the motivation was that they did not want to have bunches affected by collisions at other IPs so that they don’t look different from those that collide at IP1. But I made the point that bunches that collide at IP1 collide also somewhere else, in ALICE ot LHCb. So it is not clear why this is needed.

Witold Kozanecki: and the beam beam parameters are very small so there should be no beam-beam distortions, they should see more differences from the fact that the tune is slightly wrong.

Lorenzo Bonechi: when the request of non-colliding was made,  did not realize that there are already bunches non colliding in IP1. We are checking internally if it is really necessary that they do not collide anywhere. If it is possible to have, it is maybe better to have non-colliding anywhere to monitor beam-gas effects without effect on the bunch, even if we do not expect effects on the bunches. But this is being checked.

 

ATLAS (Eric Torrence)

 

Eric Torrence: splashes on 14.04: will ATLAS get them?

Jorg Wenninger: there was no official request, so they are not on the spreadsheet.

Eric Torrence: we’d be happy to have some too.

CMS (Filip Moortgat)

Filip Moortgat: new sensors in BRIL may be more sensitive compared to last year and may assert an abort during the splashes.

Michi Hostettler: if it happens and you dump, we’ll then disable the post mortem, so we won’t have an inhibit from anyone else, and you should make sure that you don’t remove the injection permit for n-minutes, which would be inefficient. But as long as the injection permit is in, we’ll be armed, so in 1 minute we can reinject.

Chris Young: if they dump the beam, then you’ll need to mask them, right?

Michi Hostettler: yes.

David Stickland: the problem won’t be there for injection, but for the 14-15 April for the splashes.

Jorg Wenninger: we can also disable the post-mortem on the 14th.

Michi Hostettler: yes, for the splashes, we may just disable them. There is indeed also a small probability that on the splashes we trigger the BLM. 

Jorg Wenninger: it depends on the probe intensity and other details.

Filip Moortgat: low mu can be taken the last 14 - 3 days (the last 3, really no collisions in CMS) at an intensity which is a factor 10 less than during physics.

Jorg Wenninger: at some point we need to understand how much data would be needed. How many hours of beam… 

Filip Moortgat: for now, we present this as an option. Then it depends on what the machine wants to do, if there is some time of stable condition, this is a possibility.

Chris Young: PPS not included in the OO run at the current request.

Andrej Gorisek: ATLAS might like to have AFP on both sides in pO. We will come back with more details. 

Chris Young: doing a dedicated alignment in OO would be a bigger overhead since it is a new separate alignment than adding two stations. 

Roderik Bruce: what sigma settings the experiments would like to go with the roman pots in pO and OO? To see if we can use predefined settings without dedicated alignment.

Andrej Gorisek: let’s revert the question: if you tell us how close we can go, we can check. But we can ask.

Maciej Trzebinski: in case of pO, when we do the BBA (Beam-Based Alignment) for the O-side, which I believe can be done in parallel while PPS does its p-side, in this case we could target the closest possible approach; the question is whether it should be the limit of TCT plus 3 sigma plus 0.3 mm and then we need to know how TCT will be closed. And in case of OO, it will be defined according to what you allow us from the collimation point of view since it will go without BBA. So we’re waiting for feedback.

Roderik Bruce: we haven’t fully identified the strategy for the pO in that sense. It is low intensity, so we could of course reconsider the values of the TCT plus 3 sigma, but this needs and offline discussion, and we will come back to you.

Chris Young: it’d be interesting to know what you want to do on the O side, and whether you need to be close for it.

Filip Moortgat: the same discussion is happening in CMS. In the case of PPS, there is no acceptance for O, but if O breaks in a lighter ion, that could have acceptance, but then we don’t know what they want to do with it.

Chris Young: it can be at all sorts of different angles, so do you need to be close at all? That is the question.

Maciej Trzebinski: the question is tricky, it is an uncharted area, so we don’t know which fragments will survive, if they will be even produced, the cross-sections from the generators are uncertain… which is also why this measurement could be interesting, because the MC predictions differ by orders of magnitude between generator and generator. It is a good question whether we’d like to be as close as possible or a bit far away because our detectors are limited in size. About the TCLs, will they be fully opened for pO and OO? 

Roderik Bruce: we have not yet defined the strategy but I think there is no need of the TCLs, because the luminosity is extremely low compared to protons, so most likely we’ll leave them out since they’re complicated to align them.

Maciej Trzebinski: so the only limit comes from aperture.

Roderik Bruce: yes. And potential background that you could be hit by if you go too far in.

Chris Young: we need a motivation to go close which needs beam alignment and adds time. If you can take data being far away, this saves us time.

Filip Moortgat: CMS is discussing 18 mm from the beam, ou, to measure cross-section O16 → O15 + X, but there is only one person interested. This is why it is not approved yet, and we don’t have an official request.

Maciej Trzebinski: the BBA, for P1 and P5, on a given beam, is not done at the same time. Usually we’re doing B1 and B2, but now we’ll probably do one experiment p beam and the other O beam, then viceversa, so there is no overhead. This should happen even if we align only the p side, since one experiment should then wait for the other.

Chris Young: yes, if the O alignment is simple.

ALICE (Robert Muenzer)

Jorg Wenninger: next week, the access should be tentatively on Tue or Wed.

Chris Young: LHCb to provide smog during your run?

Robert Muenzer: yes. 

Chris Young: polarity is +/+ in O? 

Robert Muenzer: yes.

LHCb (Paula Collins)

Paula Collins: should the low mu ATLAS request be together with the one for magnet off for LHCb?

Chris Young: better in different fills, so that the fill is not long, otherwise the fill with magnet off will be too long because it will take long to burn off the beam with low mu in ATLAS.

Michi Hostettler: [about SMOG commissioning] the only thing with the gas to be careful with is the pressure with which you inject. Once, probably during the ion run, it was high and losses went to warning. Start slowly please.

Jorg Wenninger: also don’t do it during loss maps, setup of collisions.

Michi Hostettler: do you want SB for this?

Paula Collins: no.

Michi Hostettler: do you need collisions?

Paula Collins: no, it is independent.

Jorg Wenninger: in most of the cases it is transparent. Just call the control room when you do it and we can give green light.

Michi Hostettler: normally it is not a problem, it is only when we’re sensitive to losses like in loss maps, or when we’re sensitive to your lumi measurement which normally should be compensated for the extra rate, but I remember in the past it was not always a 100% the case, and an optimization was needed.

Paula Collins: when does the machine need lumi from PLUME?

Jorg Wenninger: a bit earlier than we thought initially, since it was requested to have SB before Easter. On the slide, you mention “flat top” for the injection os the SMOG for PLUME, can you clarify?

Pauls: before Plume can reliably provide lumi, they need a few minutes of data taking at flat top so that they can inject smog and see the signal. So this should happen after the SMOG commissioning, and before PLUME should provide lumi at LHC. 

Jorg Wenninger: there are no many occasions at flat top before we go to SB.

Michi Hostettler: what we need for the collision setup to observe a peak as they come into collision is “a” lumi signal, but like a rate. We don’t care about offsets, calibrations, whatsoever, just the peak should be in the right position. So if you’re not playing with the signal while we optimize, it is not crucial that it is perfect. We need to see if we’re colliding head-on or not. Afterwards you can have a few fills where you can calibrate on.

Chris Young: Can this be done at the end of the optics measurements at flat top?

Michi Hostettler: you’d have only the pilot. If you want to see the interaction with the gas, you should probably have the nominal in the machine: with the pilot you might not see significant rate. But like said, if the signal is not well calibrated, it is not an issue for us. 

Rosen Matev Matev: in any case we cannot calibrate that well.

Chris Young: you need this after you setup the first item [SMOG new system commissioning]?

Paula Collins: yes, we cannot inject the SMOG till the first test have been completed. This could take 4-5 days.

Chris Young: then when this is done, we can bring this up in the morning meetings, and see if there is a slot. 

Paula Collins: in case of complete desperation, would the BRAN signal be enough?

Michi Hostettler: for flat top probably yes. In the past, with the high sensitivity BRANs, we could get a reasonable signal in P8. The BRAN-B. 

Jorg Wenninger: at flat top we’ll mostly have probe bunches.

Michi Hostettler: what I mean is to find collisions later on, when we have nominal, and then with the high sensitivity BRAN that we would turn on, we will see something in P8. Of course, your signal is much appreciated. 

Jorg Wenninger: at injection, because of your crossing angle, if you don’t provide anything, we have no idea where we are.

Chris Young: so you need it for the lumi at injection before Easter, basically.

Jorg Wenninger: exactly, or actually well before. We need a fill to setup, adjust only, then do loss maps, then stable beam. So counting back, as soon as possible next week. To have a safety margin, if the first SB is on Thu, we should start on Tue. 

Chris Young: you should let us know as soon as you’re done with the SMOG stuff so that you’re ready to go, and then we can try to fit it in. You cannot do it before the SMOG stuff is setup even for lumi at injection, you cannot do the timing for the detector, so…

Jorg Wenninger: but you don’t need 10 days to do all the tests, or it would not work, right?

Paula Collins: we’ll cut down the tests to the minimum needed. This is work for the vacuum group.

Filip Moortgat: so first 900 GeV should come on Thu in the Easter week?

Jorg Wenninger: Thu is the latest day to have something before Easter. Maybe first collisions before Wed, but no stable beams. If we try to have setup collisions like at 5 am or late in the evening, with some prewarning?

Chris Young: you’re asking since you need lumi from experiments?

Jorg Wenninger: yes, to see if everybody is aligned. Let’s see.