Minutes and Summary

Main purpose of the meeting: Post TS1 planning and data taking feedback

LPC minutes 10th June

Introduction (Federico Alessio)

(Catrin Bernius, Michi Hostettler, Matteo Solfaroli Camillocci) Could you clarify what you mean by reduced tails. The current BCMS is like a Q-Gaussian with larger tails than a true Gaussian, the aim is to change this to a more Gaussian shape in the transverse plane.

(Chris Young, Federico Alessio) The 3.5 hours that SND aims for is after RP have given the all clear to enter, so the full length of the stop is longer than this. It therefore requires quite a long stop.

(Jorg Wenninger, Roderik Bruce, Matteo Solfaroli Camillocci) When the LHCb magnet is off we cannot setup LHCb, therefore it doesn't count if their magnet is off. This would mean that we would need to re-do the setup again once the magnet is on and it has to be in the correct polarity that it will have at nominal intensity. Doing a fill with the magnet is a very special case as the crossing plane becomes completely vertical. This is not easy and it takes some manipulation. There will also be e-cloud problems such that we wouldn't be able do this with the full machine and would need to do this with 400b or 800b maximum. LHCb will therefore start with down (+) configuration for the setup. If there is to be a dedicated fill with the magnet off then this would need to be during the day. The differences shouldn't impact the aperature or orbit as it should only be local effects.

(Andrea Massironi) Just to check, the 4b/12b will be removed. Yes, this is the case.

(Andrea Massironi, Federico Alessio, Jorg Wenninger, Michi Hostettler, Chris Young) In Run 2 there also was a difference in the luminous region size in Z of a similar magnitude. Therefore I would trust the luminosity derived by the experiments rather than the luminous region. One possible way to address the difference is to reduce the crossing angle in CMS but this could run into problems to go far below 150, otherwise we could increase it in ATLAS. Increasing it in ATLAS might not be good as it is specifically small to give more margin in the aperature. This would also screw up the symmetry of the machine and you would be introducing a fundamental difference. The statement is that the experiments want to have similar geometric factors. The horizontal emmitance should grow quicker than the vertical emmitance such that we should see the difference reduce over time. Perhaps the vertical dispersion correction that is meant to be introduced after TS1 might resolve some of the difference. It was suggested to measure the emmitance several times during a fill. For lower intensities the scan itself can introduce instabilities but it should be possible at the end of leveling and at the very end of the fill where we don't mind losing the beam.

Combined beta* and separation levelling (Michi Hostettler)

(Federico Alessio) Why after we go head-on are the virtual and measured not the same? This is because it is assumed that the emmitance is constant as this avoids spurious measurements of the emmitance that can occationally occur. The burn-off is considered as this comes from the fast-BCT.

(Chris Young) Could the orbit corrector trip issue happen in LHCb? It can but as the beams are very separate it would cause an increase but it wouldn't put the beams fully head-on.

(Chris Young, Andrea Massironi, Catrin Bernius) What would be the effect on ATLAS/CMS of this happening. Presumably some dead-time, trips, busies? Yes, and it won't damage either detector.

(Catrin Bernius) We are very happy with the stability that we have at the moment.

(Federico Alessio) Can the lumi in IP2/8 be affected by the steps in IP1/5. Yes, this can happen and the feedforward attempts to correct these orbit drifts that happen in each leveling step. Otherwise you get 20-30% variations which also happened after the last polarity flip as the feedforwards had to be re-setup the feedforwards.

CMS (Andrea Massironi)

(Chris Young) Did you check that the roman pot alignment run is in Jorg's list. Yes it is.

(Chris Young) The idea is that AFP and the CMS pots would be aligned in the same fill, not necessarily at the same time.

(Jorg Wenninger) I don't think we should do anything at the moment to compensate for this ~3% difference. If we did want to change something then we would need to re-do the loss maps. If the new BCMS changes something then we would then have to undo what we did and then do another re-validation. A 5urad change is fine for a test but it would need loss maps for physics running.

(Andrea Massironi, Matteo Solfaroli Camillocci) If we don't change things now to wait to see what the new BCMS beams are like then would there be another opportunity to change things. There isn't another opportunistic time but some time would need to be dedicated to making this change.

(David Stickland, Catrin Bernius) Is it planned that ATLAS will update their luminosity in the coming months? This will be covered in the ATLAS slides. This would be based on last years data.

(David Stickland) As the emmitance measurements are independent then this lends a lot of credance to the difference being real.

ATLAS (Catrin Bernius)

(Federico Alessio) If we lowered the ATLAS crossing angle then while the beamspot length would have better agreement, but this would make the luminosity difference larger.

(Andrea Massironi) I feel there will be some subtle differences in the z-width definition as this differences have been present for a long time. Particularly as this is not a true Gaussian.

(Federico Alessio) This test proved that when we change the crossing angle we do get the shifts in luminous regions and luminosity we expect. It is therefore a tool which could be used if we want to adjust the calibration.

(Federico Alessio) If you make the change will you update all the MASSI files of the previous runs. Yes, if the change is done then this will be updated.

(Matteo Solfaroli Camillocci) Could the OO/pO be switched with the VdM program. Yes, there is some shared person-power but this would be possible.

(Roderik Bruce) From the TCL6 test there was negligible difference such that it appears that TCL6 is not the solution for this problem. The RP measurement was for the safety of the personnel rather than the R2E effects on the electronics. There is discussions on installing some more sheilding which could help this issue. For now there is nothing requested from ATLAS for any further studies. It would be surprising if there were large differences in radiation to electronics and not to people.

(Roderik Bruce) Do any other experiments want to take data during the loss maps. The experiments will give feedback on whether they want to do this.

(Matteo Solfaroli Camillocci, Jorg Wenninger) Is the request to go to stable beams in one of the 3-bunch fills. However, this cannot be done as the loss maps won't have been done before this fill. Therefore a fill would need to be added. We will see if the other experiments also want a 3 bunch fill in stable beams.

ALICE (Andrea Ferrero)

(Chris Young) Could the 2h with NO BEAM be done after the first fill (which doesn't have stable beams). This will be followed up.

(Matteo Solfaroli Camillocci) This is an issue with the lumi server rather than the BLM system.

(Matteo Solfaroli Camillocci) Do you need the mode to be NO BEAM or can it be the usual ones we have in the pre-cycle.

(Federico Alessio) Could this be 12b rather than 4b? The request is to have a very low rate, and this is problematic if there are too many colliding bunches. We can make a scheme with less bunches colliding in ALICE.

(Chris Young) Is it useful to the machine if LHCb and ALICE sync their polarity flips? As the ALICE one is very transparent this doesn't matter to the optics and no re-validation is required so there is no need to sync them.

LHCb (Giovanni Cavallero)

(Federico Alessio, Jorg Wenninger) Is there an urgency to do the magnet off fill? This will be followed up. It might be possible to do one of the steps after starting with the magnet on for the validation. STABLE BEAMs are needed for this fill. So we can do 4/12b with magnet on for validation, then 75b fill with magnet off, then 400b, etc. with magnet on. This will only happen if the 75b fill happens during the day. If the 75b fill is in the night then we will need to discuss whether the 400b can be an option of if we should do a dedicated fill.

(Chris Young) Is it ok that the ATLAS Solonoid is off for the validtion if it starts on Friday. This is ok, and it should be back on Friday.