LPC meeting summary 03-07-2018 - final

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Minutes and Summary

Main purpose of the meeting: Discussion of 900 GeV test run results

 - Masaya, Sune, Hector, Patrick, Roderick, Mario, massimo, Helmut, Marco, Karlheinz, Isabel, Brian and Christoph

Inputs from Machine (Helmut and Hector)

Hector on collimation:
  - started in 2017 with 2016 flat top configuration (TCLAs only), TCTs added as well
  - for 3sigma TCP test on 22. tried also tungsten collimator - no difference seen with carbon collimator
   TCP(IR3) was response to fast growth of off-momentum
  - off-momentum repopulation tails happens very and unpratical to scrape every few minutes
  - do not expect to be able to gain much from optimization of collimation
    * max gain 10-50%
  - simulation doesn't show anything different between ALFA and TOTEM - not matching data
    did see background from beam-1 in narrow range in TOTEM, while little background in ALFA
   background from 1-stage simulation
  simulation only works betatron-cleaning, not off-momentum which could be dominating background
    simulation does show that putting in TCT in removes background.

 - could do simulation of off-momentum, but requires time from Hector


 - at high energy the bunch length is shrinking as IBS is very small than the damping by synchrotron radiation
 - Grows in horizontal and bunch length very fast at lower energy
 - Higher RF voltage gives less debunching, but faster IBS
 - good things: able to inject directly into 100m (maximum at injection). orbit corrected
 - beam-beam effect also high as seen in low lifetimes - best case 10-20 hours
 - concluding dominant effect is IBS
 - estimates on background rate very rough - need better numbers from ALFA
 - TOTEM had flat background in one case where they had a clean coincidence arm

 - check if brightness was lower in 2018 test or not since intensity was lowered, but emittance could be lower
 - RF voltage should be low as possible
 - check chromaticity and octupoles - could likely go much lower for test - octupoles might have been tried already

 - optics not quite same between ALFA and TOTEM as first have V4 with large beta* ratio

 - can we learn anything from ramping with 11m. Could we learn anything? POTs would be further out. Learn anything from collimator losses and repopulation time
 - can only see backgrounds. Would need to compare 900 and 1800 at 11m.

Inputs from ATLAS (Karlheinz)

Reminder most important criteria: background should be a few permille level for making a precision measurement
At higher energy achieved backgrounds betwween 0.11% and 0.7%. 0.7% is already on high side
In 2017 data did not have all stations included (not moved), so focus mostly on 2018 data
Also 2018 missing pots in first two data periods and the best configuration in last, worst period
After few minutes see domination by offline
beam shapes very different from beam1 and beam2
coincidence plots from DT1 show no evidence of elastic signal - "elastic rate" well described by background estimates apart first lb
DT3 period had very asymmetric backgrounds, gives worse background estimate from "anti-golden"
cutting to 5sigma acceptance in CNI region almost completely lost. Need to go 7sigma to see elastic events
in few configuration can get to 30% background by tight y-cut - far from the necessary level
? for non-colliding bunches, is backgrounds growing faster or slower  (single arm)

Inputs from TOTEM (Mario)

- clear sharp in background shape, but present to different extent.
- in DT1 had one reasonable arm with sharp shape dominant
- by increasing cut by 3mm (how much in sigma? ~3), increasing tmin from 3x10-4 to 6.5x10-4 (at 50% efficiency) gives much cleaner data analysis
  and acceptable background levels
  * should still be acceptable for columb region despite less acceptance as it still has more coulomb than in high energy run
  * cuts also helps in 2017, but different from period to period - worst in last region 3mm
- not yet addressed how much data needed for a 900 GeV
- for DS1 had rates in backgrounds.