Minutes and Summary

Main purpose of the meeting: Start of 2024 data taking, experiment feedback and plans for upcoming data-taking.

LPC minutes 22nd April

Introduction (Chris Young)

(Jorg Wenninger) The power values shown in the plot are the online values and should be reduced by about 10 W/hc with offline calibration.

(Catrin Bernius) Before the cryo re-configuration were we limited by the same sector (S78) as we are now? Yes, it is the same sector that is limiting us, although S81 is also not a good sector.

(Catrin Bernius, Jorg Wenninger) In the last fills the imbalance appears to grow over time rather than being an offset during the decay. If the H and V emmitance grows differently then this would naturally introduce a slope. From physics reasons we might expect H to grow quicker than V.

(Witold Kozanecki, Chris Young, Jorg Wenninger, Michi Hostettler) What effects can introduce a difference in the luminous size in x,y as the crossing angle doesn't contribute to the size in x,y. It is very difficult to measure the size in x,y compared to z. It would be useful to go back to the fundamental equations to check what the luminous size in x,y can depend on as it shouldn't depend on the geometrical factor. The experiments will follow-up with the tracking experts on the reliability of the values. Both experiments believe that the values shown are the offline values already. The width in z is crossing angle and b* dependent so there is no reason that they should not be different. The ratio of the luminous lengths gives the ratio of the geometric factors such that this would explain a difference in luminousity. A non-roundness of the beams could also introduce a difference a the experiments cross in different planes. Therefore it could be either crossing angle or a non-roundness of the beam. Reducing the ATLAS crossing angle would increase their luminosity but this would increase the beam-beam effects, such that increasing the angle in CMS might be preferable. In direction the ratio agrees with the different reported luminosities. At the moment there are other things that require investigation such that any change is likely to only occur at TS1. There was an end-of-fill test that showed the decreasing the crossing angle in ATLAS increased the collimator hierarchy problem such that it is unclear if this can be done.

(Jorg Wenninger) The single beam test will have 12 bunches in beam 1 as well and these need to be colliding so a custom filling scheme will be used for this.

(Catrin Bernius) Please make sure that outside working hours the control rooms are called instead of

(Gianni Masetti) Is it considered to move to the hybrid scheme that was running last year? It is hoped that this isn't necessary and that we can get to similar numbers of collisions in ATLAS/CMS with the better setup for

(Mirko Pojer) This morning at the TC meeting the cryo group were very strong that they will not be able to fill the machine with 48b trains. Already the conditioning was not getting much better, and they had managed to make improvements tuning the system before the 1950b step but they were not confident that much further improvement was possible. They therefore suggest to move to 36b trains. This is for QURCA, with QURCB the situation is much worse.

(Chris Young, Jorg Wenninger, Michi Hostettler) 3x36b BCMS would be possible but 5x36b would not be sensible. It is usually better to be at injection longer in the LHC rather than at flat bottom longer in the SPS so maybe 3x36b with BCMS is better than 5x36b. This will be discussed at LBOC.

(Catrin Bernius) Have MPP given the Ok for not re-doing a ramp-up. Not officially but they will ask people to look as we go up slowly for the cryo in any case. We will do a 48b fill with 1200b first and switch to 36b the next day and will prepare all the filling schemes. It is likely to start with 1200b or 1800b to check the cryo case. LPC will distribute the filling schemes along with which ones they think are most likely to be used first.

(Jorg Wenninger, Jamie Boyd) We hope to move the access from the 2nd May to the 6th May as there is a VIP access to ATLAS. Therefore the hope is that the FASER/SND exchanges can happen that day instead. It is thought that they will only visit the ATLAS cavern and not also the LHC tunnel so the FASER transport will not interfere with the visit.

ATLAS (Catrin Bernius)

(Chris Young, David Stickland) It was asked and confirmed that the fills shown are not the ones when the different luminometer was used. Additionally the different luminometers might have different calibrations but they shouldn't have non-linearities.

(Michi Hostettler) The fill shown is when there was an additional squeeze at the end of the fill to 30cm. This decreases the difference between ATLAS and CMS which is consistent with the difference in the geometric factor as this is changed, but the numerical size would need to be checked. When we do this change we also change the crossing angle but during the rest of the non-leveling part the crossing angle is kept constant.

CMS (Giulia Negro)

No questions.

ALICE (Andrea Ferrero)

(Jorg Wenninger, Michi Hostettler) The shift can be due to non-optimized crossing plane. The effects seen are in the first half hour. In the next fill we can check the feed-forward by looking at the optimization of the crossing plane before and after the beta* leveling. This will be followed up in the next physics fill by doing the optimzation at 60cm and corrections can be put in if it is seen to be due to this effect. ALICE will try to provide the plots in close to real time to follow this up.

(Chris Young) Previously we saw a wave over many hours, now things seem much more stable. There were some improvements in one RF line but it is unknown if this could explain this.

(Chris Young) It was asked how long the polarity changing takes and this is around 1-1.5 hours and are usually done between in inter-fill ie. during the LHC ramp-down.

LHCb (Elena Dall'Occo)

No questions.