LPC meeting summary 20-06-2016 - final

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Minutes and Summary

Main purpose of the meeting: Follow up on the apparent discrepency of the online luminosities of ATLAS and CMS. Summary of the general status.

Introduction (Jamie Boyd)

Jamie summarised the last weeks of LHC operation. In particular he noted that the peak luminosity could be increased by using a new SPS super cycle where the beam stays a shorter time at the flat bottom in the SPS and reaches the LHC with the lower emittance and higher intensity.
A new problem concering the MKI vacuum became apparent. During the injection the vacuum degrades in the MKI and approaches the interlock thresholds. So far it was always possible to inject the entire filling scheme, but with this could have been a lucky coincidence. The vacuum degradation is believed to be e-cloud induced. A flash-over at the kicker should be avoided under all circumstances to avoid damage which would require a long intervention and re-conditioning phase.

The mini-ramp up after TS1 was used to successfully complete a series of special activities:

Jamie reported also about a test to commission the controlled bunch-lengthening. The test was successull but the final parameters still need to be found and the procedure needs to be implemented in a way that it can be executed by the operators. Witold Kozanecki remarked in some fills some bunches had reached a pile-up of 40. Experiments are invited to look into this and come back to the LPC in case this poses problems. Stefano Redaelli asked if it is problematic for experiments to start with a shorter bunch length and hence a higher pile-up density at the start of a fill. Jamie commented that this is believed to be acceptable since we are still far from the acceptable pileup limits presented in Chamonix, but experiments should comment if necessary.

The techniques to potentially increase the luminosity have been mentioned. In addition it became clear in the discussion that the triplet movement in IP 1 is now reasoably well under control after the temperature of the beam screen is held constant by the cryo-team (Jörg Wenninger).

Finally Jamie announced a new tool to generate Filling Schemes available on the website of the LPC for those who are interested:

CMS 2016 luminosity status (David Stickland)

David Stickland presented the status of the luminosity analysis in CMS. He summarised the various techniques used in CMS to determine the luminosity. He compared the cross sections in all lumi devices to those found in 2015 and found that they consistently increased by 4%-5%. For the main luminometer the PLT, the cross sections of both years cannot be easily compared since the acceptance area of the PLT has been changed in 2016 to be able to measure luminosities up to a pileup of 40 in a linear PLT range. However, David concluded, that it is probable that the online Lumi 2016 which is based on the 2016 VDM calibration is ≈5% too low.

David pointed out that CMS is preparing Z->μμ rate measurements to check the scale and stability of the luminosity measurements.

David emphasised that CMS wants to make only one change of calibration during 2016 and therefore will wait with an adjustment of the online luminosity until final results will be available.

David also pointed out that when trying to integrate the ATLAS instantaneous luminosity values, the result would be 2% different from what is displayed by ATLAS as the integrated fill luminosity. Witold replied that the ATLAS fill lumi published via DIP is not adequate for a comparison at that precision since it is not actively maintained. For comparisons of integrate luminosities the values from the Massi files should be taken.

During the discussion Witold gave an updated of the ATLAS understanding of the online luminosity. A small bug was found in the VdM analysis leading to a lumi overestimation of 1.9%. Considering that in 2015 the transfer of the VdM calibration to the physics conditions resulted in a 2.5% overestimation of the online luminosity and knowing that this effect will be increased in 2016 due to a known mu-dependence Witold concluded that the currently published ATLAS online luminosity might be over-estimated by at least 5%.

During the discussion David underlined the usefullness of the μ-scan. He also remarked a problem with the FBCT measurements for each first bunch in a train.

ATLAS lumi validation with Z & W counting (Jamie Boyd)

Jamie summarised the status of the ATLAS lumi cross checks with Z and W counting. The aim of the anlysis is to check if the change in scale between the CMS and Atlas luminosity after the fill 4956 can be explained by discrepancies of ATLAS internal measurements comparing the Z and W counting rates to the measurements of the luminometers.

Jamie concludes from the results that this discrepancy cannot be explained by effects internal to ATLAS. (And effect of approximately 2% might be visible).

CMS is in the process of finalising a similar analysis which will be presented. Both collaborations will do investigations on time periods which are fractions of a fill.