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February 14th 2018 - Charge mult. 2 - ATLAS
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Emily chairs. Small attendance so more dicsussion == less detailed minutes: sorry!

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Topics:

   - Method differences between ATLAS and CMS
   - Discussion of diffractive processes using Emily's (great) slides
   - Plot d

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Details:

Fake tracks are not considered? just don’t do anything

 - material description most significant source of systematics

 - In each multiplicity interval, tracks are randomly removed (p.g 12), this 
    is just to assess the significance of worngly estimating them! Not done in 
    actual measurement!

 - non-closur in unfolding. A discussion of what that means. Patrick recommends this 
   paper: https://arxiv.org/pdf/1505.04768.pdf

 - MPI? Message Passing Interface (jk jk),  Multiple Parton Interaction

 - colour coherence? plot d is crazy
 
 - coherence length == longtikudina; distance between

 - Emily made slides! Diffractive is where there si energy transfer. 

 - The general scattering uses LHC information in both EPOS and Pythia, so same information but different tune

 - What is the follow-up after these measurements? Marjorie (we asked later) says they go back and try and turn
    teh knobs to get teh tune to fit the data. It's an iterative tuning. 

 - KNO scaling should roughly scale the same way. All was violated, 
     so they also try to tune ti understand why this violation happens: even in a mathematical way let alone 
     (NB: this is discussed in the ALICE paper)


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NEXT WEEK: Charge multiplicity 3 - ALICE
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CliffNotes:

- Min bias: can we discuss more about what that means:
    - ideal trigger would be random but at low lumi that’s too inefficient
    - so instead used a minimally biased trigger which:
        - uses the MBTS = Min Bias Trigger Scintillator, polystyrene octagon slice
        - can trigger on single scintillator hits or require multiple hits
        - sits outside of atlas detector https://indico.cern.ch/event/64893/attachments/1012375/1440522/6-Camacho.pdf (slide 7)
- Trigger selection: one counter above threshold from either side of detector (OR): single-arm trigger
- Control trigger: selects random events at L1, filtered by HLT and require 1 track with pT > 200 MeV.
- Prim charged particles required to have pT > 500 MeV, eta < 2.5 (tracking efficiency?)

Event selection:

- PV from ≥ 2 tracks w/ pT ≥ 100 MeV
- To remove contamination from events with > 1 interaction per BC, events w/ SV w/ ≥ 4 tracks are removed
- At least ONE track must:
    - pT > 500 MeV
    - |eta| < 2.5
    - 1 pixel hit
    - 6 SCT hits
    - IBL hit if expected (ie. if module not dead) or B-layer
    - |d0_beamLine| < 1.5 mm
    - |z0. sin theta| < 1.5 mm