LPC meeting summary 24-10-2016 - final
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LPC meeting summary 24-10-2016 - final |
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Main purpose of the meeting: Follow up on special fills taken in the last weeks.
Jamie summarised the data taking period since the last LPC meeting. He stressed that in spite of the large number of special activities data taking for physics was very efficient with 2.7/fb-2.9/fb in IP 1&5. He listed the special activities. For the leveling test (Fill 5439) he showed the narrow pileup distribution over the fill comparing it to a "normal" fill as an interesting feature. This leveling test was performed with help of a new tool by Michi Hostettler, which automatically kept the luminosity at the desired level.
Jamie pointed out a feature in the plot of the ratio of the peak luminosities of ATLAS/CMS (shown on the LPC-website). Where the first fills after the change of the crossing angle as expected show a peak lumi ratio closer to 1 compared to previous fills, the most recent fills (from fill 5418 on) have a smaller ratio (around 0.9). In the discussion it was pointed out by David Stickland CMS is currently investigating if this feature is related to some effects in their luminometers.
For the upcoming MD5 it was pointed out that experiments should deliver luminosity during MD1434 (Thursday 0h00 - 7h00), MD1814 (Thursday 9h00-15h00), MD1878 (Thursday 17h00 - Friday 1h00), MD1257 (Saturday 10h00-20h00) and MD979 (Saturday 22h00 - Sunday 6h00). In MD1814 Stable Beams will be declared allowing detectors to switch on their Tracking detectors. The first 2 MDs will be executed in "Proton Physics" mode so that the CMS Tracker does not need to be switched off completely before the second MD where it is needed to be switched on.
Concerning the luminosity measurement during the 5 TeV part of the HI programme Witold Kozanecki proposed "coarse scans" of about 30min per experiments to at least have some kind of luminosity accounting in this period. The LPC will follow up with John Jowett if these scans can be taken parasitically without risk or disturbance to the ALICE data taking. ALICE mentioned that they will concentrate on data taking without a lumi scan. The LPC should also follow up on the desired units for publishing luminosity values during the Heavy Ion run.
Finally Jamie reminded of the upcoming common LPCC/LPC meeting on the planning of the future forward physics programme on the 31st of October in the Theory Auditorium, and of the LMC meeting to discuss the activities in the EYETS on the 2nd of November.
Greg commented on the various special fills in the last weeks.
Witold asked if the scan in September was already analysed to get further inside into the lumi ratio of ATLAS/CMS. Greg answered that this is work in progress.
ATLAS found that the beam spot position during the leveling fill did not change in a different way than during a "normal" fill. Witold confirmed that there should be no effect on the beam spot during leveling since the applied beam separation is achieved by symmetrical movement of both beams.
For the high pileup fill ATLAS remarked a long processing time for the busy events. The peak pileup was measured to be around 90. The DAQ handled this run smoothly. The beamspot plots do not show unusual features. A thorough analysis is ongoing.
The tighter TCT collimator settings had no effect on backgrounds in ATLAS. Also during the RF modulations no visible effects on Beamspot or Timing could be established. Witold remarked that the machine should be provided with beamspot information per bunch.
Witold summarised the motivation and the detailed execution of the Crossing Angle Scan performed in Fill 5422. He remarked that the emittance scans shown in his slides were done at injection and therefore do not necessarily match the specific luminosities shown later in the talk. The evolution of the emittance during the fill has been measured, and will have be taken into account in the future bunch-by-bunch analysis.
On slide 8 Witold pointed out a 5% difference of the Cap Sigma ratio at 140µrad and the specific luminosity ratio L140µrad / L0 rad. This difference may be due to the fact that these comparisons average over bunches of very different emittance; it will have to be analyzed on a bunch-by-bunch basis.
Witold summarised that the ATLAS/CMS luminosity ratio increases by 5% - 6% when reducing the crossing angle from 140µrad to 0. Therefore the crossing angle effect can likely explain a part of the apparent luminosity difference between ATLAS and CMS (currently O(10%) at the start of a fill).
Referring to the unexplained drop in the ratio of the peak luminosities in ATLAS and CMS as of fill 5416 (15 Oct 2016), Witold pointed out that the internal ATLAS ratios, between different luminometers at the peak of the fill, have been stable within a 1% envelope over a 10-day period (10-19 Oct 2016) bracketing this jump.
ALICE did an analysis of the RF modulation fill. The longitudinal vertex position difference between sets of bunches (grouped into sets of 4 bunches) is up to 3mm in z in ALICE. No displacement in x-y is observed. The collision time shows a saw-tooth like modulation. The analysis has been sent to the RF experts for feedback.