Minutes and Summary

Main purpose of the meeting: Current data taking and LHCb filling scheme change for PbPb

LPC minutes 19th August

Introduction (Chris Young)

(Catrin Bernius, Jorg Wenninger) It was asked what parameters could be pushed. The only thing is really the intensity. For the extra squeeze in beta* the machine is not keen due to the hierarchy issues seen earlier and the commissioning overhead would not make this worth it. It was asked if there is margin for this. The RF finger vacuum group are the most concerned and want to wait until Jeune Genevois. It is also not clear from the conditioning how much overhead we have. It is likely any increase would only be to 1.65e11 ppb or similar rather than a large change.

(Jorg Wenninger) The vertical emmitance increase at the start of the train has now gone again without explaination. It was only present for a couple of fills.

(Andrea Ferrero, Jorg Wenninger) Will there be draft filling schemes and optics for the pp ref VdM by the 2nd September. In particular it is interesting if the ZDC can be calibrated during this fill if the crossing angles are appropriate. This is on the to do list of the machine and it will be discussed on the 2nd September.

(Andrea Ferrero, Federico Alessio) Do you plan to have a new document from the previous one for the MASSI files? Yes, a new EDMS entry will be made and the old information removed and the experiments asked to fill in their relevant section.

(Catrin Bernius, Federico Alessio, Jorg Wenninger, Matteo Solfaroli Camillocci) From the FASER/SND simulation it looks like the horizontal crossing doesn't look great and this is also bad for AFP. If it is found to be great for FASER/SND then we would like to discuss this as it would mean the end of the AFP program.  If we stay with the current configuration then it is also important to know early as then AFP will look at installing additional shielding. The final decision is usually made in Chamonix but this is clearly too late for the installation of any shielding. The decision is also dependent on the outcome of the decision of whether to extend the run. LPC and OP hope that discussions would happen earlier than this and a full plan/recommendation is put together for JAPW for discussion.

(Jamie Boyd) The beta* will actually be 60 cm in this MD rather than 30 cm as written in the slide as this is a limitation of horizontal optics. FASER will also be pushing to reverting to non-RP optics as this is known to perform well. Everyone also want to make the that we study all the configurations in simulation and/or in the machine well in advance to avoid another surprise.

CMS (Giulia Negro)

(Federico Alessio) For 1% precision will you prepare how much time you need as this is not far from the precision of the high-mu data. Yes, this is under discussion, and this is the level of precision that has been asked for by analyses.

(Michi Hostettler, Catrin Bernius, Witold Kozanecki) Should we expect similar adjustments to those from CMS from ATLAS to account for the detector aging? This is accounted for each run in ATLAS so we don't expect additional corrections during the year.

ATLAS (Catrin Bernius)

(Chris Young) The sign of the crossing angle in the pp reference will be positive and this will be corrected in the slides.

(Chris Young, Marko Milovanovic) Are the AFP team ready 24h or only during the day? They are available 24h. If there is a cryo intervention this would work, but the cryo intervention is not currently planned. The required time was clarified to be 2h + RP time. The coolest time is clearly at the end of the MD block.

ALICE (Andrea Ferrero)

(Jorg Wenninger, Chris Young) It was clarified that an adjustment of the z-position can be done at any time. The current drift would be moving back so it is not a cumulative effect of these multiple corrections.

(Jorg Wenninger) Cryo has informed OP that they are looking to do an intervention for 12 hours to fix the issue at point 8. They are interested in doing this in the coming week but not Friday afternoon which would suit AFP. It was pointed out that matching this with an SND emulsion exchange and the PHOS intervention would be efficient.

LHCb (Elena Dall'Occo)

(Jorg Wenninger, Chris Young) There wasn't an instruction to the shifter to do anything different to normal operation, and the intervention couldn't have been done in the night. It was acknowledged that keeping the RCs in the loop would have been better than them learning it from page 1. Everyone agreed that the decisions made were the correct/optimal ones but the communication of the reasons for the changes could have been more efficient.

LHCb physics considerations in PbPb (Tom Boettcher)

(Fred Blanc) There is a strong effort within LHCb and it is also increasing significantly. Additionally the new detector increases the reach to more central PbPb collisions in LHCb which is the primary reason for the new request from LHCb.

(Alexander Kalweit) A lot of the reasons that you need the extra luminosity is for the more periferal collisions and for low pT b-hadrons. However, even then your periferal bin is very wide - 50-100% and will include the photon dominated region. In most HI results we like fine bins such that the Glauber model can be used. We have also seen very smooth transitions between the different sizes of system using this approach. A point at 30-40% and then extrapolating to pPb without a measurement in periferal collisions should work well without the need for the higher lumi for the periferal measurement. It was responded that higher luminosity would improve the ability to measure in finer bins

(Alexander Kalweit) Similarly for low pT it isn't so interesting - it is the high pT region that is interesting, which is easier to measure. It was responded that the increase in luminosity improves the precision across the spectrum and with a downward fluctuation no measurement in this most interesting bin, and this is why it was presented this way on the following slide. It was acknowledged that lower pT is not the focus of the measurement.

(Chris Young, Alexander Kalweit) Is the primary motivation for the measurement to show suppression of R_AA -- ie. numbers below 1, or a difference in R_AA between B and D hadrons -- ie. solid and dotted lines are different, or a non-0 measurement of R_AA? The primary motivation is the difference between B and D mesons. CMS already has a very nice meausurement of this using non-prompt D-mesons. The easiest way to do this is in the 2-6 GeV region.

(Fabio Cerutti) A 40% increase in luminosity will only give a 18% decrease in the uncertainty in statistically limited cases, although the processes you pick jump over the possibility of measurement with this increase. For the assumptions used in the estimates, how reliable are they? They are based on previous semi-leptonic decay measurements at LHCb. The cross-section scalings are pretty sound theoretically. For the signal to background this is harder to determine. For the different channels it is likely to be different, but the guess is that this is a pessimistic assumption. Does this imply that you wouldn't need so much luminosity for 3 sigma.

(Peter Steinberg, Elena Dall'Occo) Is there any operational downside to running with more bunches in LHCb. Not really. There will also be Pb-SMOG data taken at the same time that has a higher rate which is the main operational challenge. An additional 150 bunches wouldn't impact operations.

(Alexander Kalweit) Do you have mass plots from the PbPb run last year. Yes, the plot shown of the D0 peak is from last year and there are public versions for different centrality. However, the detector conditions last year were very different to what is expected this year.

(Chris Young, Alexander Kalweit, Fabio Cerutti) To the other experiments are there measurements of R_AA in heavy hadrons. There is one on B0 from CMS. From ALICE there are many non-prompt measurements, and measuring the same quantities as shown here in the central rapidity is a core part of the ALICE program. LHCb brings a measurement at more forward rapidity. In ATLAS there is an inclusive semi-leptonic charm and beauty hadrons of R_AA.

(Alexander Kalweit) The disagreement shown in R_lambdaC/D has existed for many years without any solution. It is unclear if a new measurement would help, but doing a measurement is always interesting. Particularly for more periferal collisions it is probably less important to make a new measurement. Similar dynamics should appear in the b and c hadron sectors from the non-prompt measurements.

(Reyes Alemany Fernandez, Federico Alessio, Alexander Kalweit) How much would a 5% reduction is luminosity hurt the physics program of ALICE, ATLAS and CMS? Should they also have to justify this? The question is phrased this way as it is LHCb which is asking for the change to the agreed baseline. It is obviously the case that 5% will not make a big difference but every piece of luminosity is important and many changes/improvements that are discussed are similar to this magnitude. From ALICE they would happily exchange the 5% which is about 1 day of running for a day of pp running but this would not be considered by the other experiments. It is obviously harder to get to the targets that have been discussed in the past as we are behind what was expected so far if there is less given to the other three experiments.

(Aaron Angerami) To connect this to the underlying physics, is there a qualitative or quantitative estimate of the improvements in eg. transport coefficients or similar quantities that the higher luminosity would give? This is what drives the target luminosities for the other experiments. There aren't predictions in the precision of transport coefficients. However, most measurements are in central rapidity and LHCb would give measurements in more forward rapidity. But would we expect the QGP dynamics to be different in more forward regions? The current models appear to struggle to simultaneously describe central and forward rapidity data that currently has been measured and LHCb would add significantly to the forward rapidity data.

(Peter Steinberg) Is the curves for the D mesons for 30-40% centrality or 0-10% centrality. It is for 0-10% centrality as this is what was available. This makes it significantly harder to tell the D meson curve from the B meson one if the curve was updated to the 30-40% prediction. This means LHCb would need more data to tell them about further motivating the change. It is thought that the B curve is also for 20-40%. It would be good to update the plot to have consitent centrallity such that we can tell if it will be possible to tell the two types of meson apart.

(Federico Alessio, Catrin Bernius) It would be good to show the projection for 2024+2025 as the change would also be adopted for the rest of Run 3 such that the total improvement can be shown. The plan would be to measure double-differentially with the full Run 3 dataset. It was clarified that the request is to change the baseline rather than just for this year. It is implied that it wouldn't be discussed again next year again, but this would be up to the Research Director for an ultimate decision.
 
(Fabio Cerutti, Federico Alessio, Chris Young) It is difficult to judge a change in this kind of meeting. There is an LHC working group on heavy ions where it would have been good to discuss this with a wider audience. However, as the change is relatively small, although reaching the target is important to ATLAS, maybe it is fine to just discuss here. For a larger change it would be good to discuss in a wider group. As it applies to the operation of the machine this is why the Research Director suggested that it was discussed here. But feedback that a wider discussion is required is also valuable feedback to send back to the Research Director. The brief history of the request was outlined and further discussions will happen between the spokespeople. All the experiments are encouraged to send further feedback to their spokespeople.

(Chris Young, Alexander Kalweit) In the bulk of the presentation you discuss the heavy flavour production, is there a broader program beyond this. For example, how many people are working in this area. The working group has 40-50 people actively participating. Several areas, including jet quenching are expanding. Within the working group this is also the fixed target, SMOG, measurements which these people are working on. The items listed on the slide are those that have been done in the past so will certainly be covered and others will be expanded on. For collective flow do you have measurements? We have charged particle measurements that would be expanded to lower centrality, and additionally these will be expanded to heavy flavour collective flow. The other experiments do make measurements in the forward region but LHCb will bring more open charm and beauty measurements.

(Federico Alessio) Will having more PbPb reduced the SMOG data? No this will not be significantly affected.

(Federico Alessio) Everyone is welcome to send feedback to LPC or directly to their spokespeople.