* original discussion from 2015-06-02

Generic commentes:
------------------

- ill-defined but OK in the eikonal limit
  one speaks of quark or gluon-enriched samples

- we need an operational definition defined by final-state.
  e.g. rapidity correlation or shape
  separate measurement v. interpretation

- be worried about the process dependence and the detector effects (MC
  study v. behaviour in data)

- try to speak of a S/\sqrt{B} improvements rather than q/g discrimination
  [this is also related to the definition of a hadronic W]

- can we find a better naming convention?
  Suggestion: quark-gluon enrichment

- Why?
  Is there a killer app?
  use q/g as a piece of an analysis
  general recipe v. optimal analysis

- can we use flavoured jet algorithms?

- FSR issues (pointed out by Jesse in the introductory talk)
  related to tuning gluon jets in MC
  related to colour correlations with ISR
  [ISR should be sensitive to large angle => see some process-dependence]
  [FSR should be fimen at small angle => see universality]

- Look at hard core v. wide angle.

- Experimental results
   . ATLAS v. CMS: 
      ATLAS suggests beta->0 is not effective
      CMS says pTD is good
   . analytics suggest low beta works best
   . ATLAS sees a large process dependence
   . is there a connection with ISR?

     see http://arxiv.org/abs/1405.6583 Appendix A.

- Spin information.
   . is it in the MC? [yes]
   . does it help or hurt?

- g \to qqbar v. g \to gg
   does it help or hurt

- quark v. heavy flavour?


Use cases:
----------

- VBF tagging
- SUSY multijet is q-enriched
- pileup jet rejection (stochastic?)
- enhance W/Z/t/H in boosted regime

Concrete study(ies) for Les Houches?
------------------------------------

Ideal result: recommendation for experiment for observables that carry
  info, defined in the final state and define q/g and eventually
  useful for MC tuning (check the systematics)

- A14 tune uses jet shape 
  this brings alphas down
  Q: what happens to LEP?

- 1st study: do we understand FSR modelling?

   . take e+e- to qqbar and gg
   . vary energy and jet radius
   . vary shapes
   . ROC v. mutual info I(T;A)
   . use as many MC options as possible

shapes: (kappa,beta)=(1  ,0.5)
                     (1  ,1)
                     (1  ,2)
                     (0  ,0)
                     (2  ,0)
                     (0.5,0.5)
                     (0.5,1.5)

        + ellipticity
       (+ pull)?
        + Psi(r)
        + check Gallicchio and Schwarz
        . tau21 of ECF(2,3)
        + ??
      on full event or only on tracks?

     questions: choice of axes (recoil)
                sum or sum over pairs?

   . for hadron colliders: 
        look at dijets
                W/Z/gamma+j
                ttbar

   . use groomed jets
        use soft-drop beta 

   . question of robustness v. performance 
     (including process dependence, pileup, low pt v. high pt)

Tasks:
------

 - tool writing (jet shapes)
     use FJ, Rivet
     professor?
   [Jesse, Gregory, Deepak]  [+Andy B??]
 - MC for e+e-
     HERWIG, Pythia, SHERPA [hook Franck, Mark?], VINCIA, ...
     + options
     [Peter S, Andrzej]
 - MC for pp
     above, +PU, VBF
     [Peter L.]