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January 24th 2017 - Luminosity 2: Detectors and formulae
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Second session of Luminosity paper, Patrick chairs. Starting from section 4.4.
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Topics:
- Brief review of the difference between lumi measurements and the vanDerMeer scans
- Beam conditions: contamination, afterglow, ghosts
- Timescale of observable variations in beam conditions
- Discussion of Figures 1, 3, 10
- Bunch-population measurements
- Uncertainties and sources of error in luminometers
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Details:
- we talk about why the plots in figure 1 have different absolute magnitudes
- we talk about the difference between the lumi measurements and vanDerMeer scans
- Patrick showed some plots of the beam spot position variation in time: https://twiki.cern.ch/twiki/bin/view/AtlasPublic/BeamSpotPublicResults
- Afterglow is highly non-linear. Greg asked about how we separate its effects from real lumi measurements. Patrick explains that for track counting it’s a problem.
- Beam conditions: matched, unmatched, empty. When the beam is unmatched is when you observe UFO’s which can then contaminate the next filled crossing.
- PV reco resolution: 30 um in longitudinal and 20 um in transverse. Sets a limit as to the resolution of the beam spot.
- Section 4.7: beam-beam corrections: patrick says this is obvious.
- We talk about figure 3. Greg guessed that they assumed some factorization proportional to the vertical offset assuming the beam shape and that this plot showed the deviation from it. Otherwise why would deviation ever be anything other than expected?
- We talk about the fact that most deviations and effects aren’t visible on the BCID scale but on the lumi block scale.
- In 4.9 the paper conclude that they don’t make any corrections to the beam size explicitly but instead take it into account with the uncertainties
- We talk a little about the intuitive nature of the bunch-population measurement and the detectors
- We talk about how the beam-beam interaction is also very intuitive and sets a time limit on the scans
- ghosts! woooooo
- The luminometers don’t depend on the beam corrections.
- 3.5% is not a negligible amount! LUCID efficiency, pg. 19, dropping due to PMT degradation over the course of 3 months
- Final bullet-point on page 19 says: there is a 2% discrepancy over 3 months in the BCM that ‘cannot be primarily instrumental in nature’. We all find this very puzzling.
- Discussion of Figure 10b: there is a 2% difference (max) between LUCID and BCM in lumi measurements
- Lucid is still the standard l u m i n o m e t e r
- Greg asked about an absolute variation in one of the methods, we discuss figure 10a by way of answer.
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Next time: Lumi part 3 w/ Patrick
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