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Les Houches

2021 Session

Wikis of Previous sessions

Les Houches Themes



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Quark/Gluon Enrichment Studies

a.k.a. Hunting the White Whale of Jet Substructure

Link to GitHub repository:


Quark/gluon discrimination well-defined?

  • Of course, at the hadron level, you can't define a quark jet vs. a gluon jet unambiguously.
  • That said, one can talk about quark/gluon enriched samples, where restrictions are placed on the final state to preferentially select quark- or gluon-initiated jets (e.g. gluon enrichment in dijets, quark enrichment in vector boson plus jet).
  • In fixed-order QCD, there is an ambiguity from soft gluon splitting to wide-angle quark/anti-quark. However, in the eikonal limit, there is no ambiguity (up to power corrections), so quark/gluon calculations can be done at the parton level in the eikonal limit (relevant for resummed calculations).
  • If needed, we can use flavored jet algorithms to give an IRC safe definition of jet flavor at the parton level.

How to isolate quark vs. gluon samples?

  • Ultimately, we need an operational definition of quark- and gluon-enriched samples (e.g. event type, rapidity correlations, event shapes).
  • This will allow us to separate the measurement of jet properties from the interpretation of those properties in the context of discrimination/enrichment studies.
  • One has to be aware of process dependence, since a quark in one context may not look like a quark in another context (color correlations).
  • Ultimately, need MC studies to compare to behavior in data.

"Discrimination" really the right word?

  • Probably better to talk about “quark/gluon enrichment”.
  • For physics applications, we want to achieve S/sqrt{B} improvement, which isn't really the same as quark/gluon discrimination.
  • Similar issues arise in how to define a “hadronic W”.
  • Quark/gluon enrichment should be a piece of a more refined analysis.
  • We can provide general recipes, but should not aim for optimal analyses, which are only sensible in the context of specific physics goals.

What is the killer app of quark/gluon enrichment?

  • VBF tagging
  • Rejecting (stochastic) pileup jets (important for VBF)
  • SUSY multi-jet tends to be quark-enriched
  • Enhancing W/Z/t/H in moderately boosted regime (where we can quark-tag the subjets.

Physics Issues

Separating final state from initial state effects

  • Different jet shapes probe different phase space regions. For example, jet mass is more sensitive to wide angle physics while multiplicity is more sensitive to collinear physics.
  • Differences between MC programs appear in multiplicity-like observables, so most likely a final state effect.
  • We can probe different physics by looking at hard core (collinear, FSR, beta → 0) vs. wide angle (soft, ISR, beta → infty).

FSR effects

  • FSR effects should be dominant at small angles, yielding universal properties.
  • Tuning of gluon final state shower can affect jet shapes.
  • Examples: g → q qbar vs. g → gg, including spin-polarization information
  • Do beyond-LL effects help or hurt quark/gluon enrichment?
  • What about the impact of heavy flavor?

ISR effects

  • ISR effects should dominate at large angles
  • Highly process dependent, depends on color corrections of jet with ISR
  • We will attempt to deemphasize these in our study, if possible

Experimental Results

  • ATLAS paper suggests that beta → 0 (i.e. hard core) is not as effective as NLL calculations suggest. (see Appendix A.)
  • CMS finds ptD (an example of a beta → 0 observable) is quite effective.
  • ATLAS sees considerable process dependence, whereas CMS has not emphasized this issue. Is this connected to ISR in some way?
  • ATLAS A14 tune already uses jet shapes, and finds that alpha_s has to be tuned downward in Pythia 8. This, however, has a detrimental effect on LEP measurements, so one has to be cautious about this.
  • Is there a tuning flat direction?

Hemisphere quark/gluon definitions in e+e-

  • Consider the case of e+ e- → q qbar. Partition event into (thrust) hemisphere, define hemisphere flavor by summing over flavors of hemisphere constituents.
  • At LO, we can unambiguously define hemisphere flavors.
  • At NLO, we can also unambiguously define flavor via hemisphere, though there is now a small gluon fraction from gluon recoiling against q qbar pair.
  • At NNLO, things are more complicated.
    • Can have soft gluon splitting into q-qbar in different hemispheres, creates IRC safety issue.
    • One can use a flavored algorithm (BSZ) to define the flavour of two flavor-kt jets
  • Ultimately, want to give an operational definition of flavor based on the Born-level operator contributing to the process.
    • Claim: all subtleties are formally power suppressed.
    • Use case, VBF, two jets with a third jet veto, q/g well-defined in the exclusive limit.

Flavored Jet Algorithms

  • This is a topic worthy of its own Les Houches study.
  • For pp collisions, multiple possible uses of flavored jet algorithms.
  • One can just run flavor-kT
  • Or one can run flavor-kT to define flavor ghosts, and run standard anti-kT.
  • Or one can run flavor-kT for deflavoring constituents, and then run standard anti-kT.

Ultimate Goal for Les Houches Study

  • Recommendation to ATLAS and CMS for observables that should be measured which carry quark/gluon information.
  • These observables must be defined on the final state alone (i.e. fiducial cross section).
  • These observables should help enrich quarks over gluons (or vice versa).
  • Eventually, these observables should be useful for MC tuning, with controllable systematics.
  • Make recommendation about robustness vs. performance. We will likely emphasize robustness, since performance depends strongly on process dependence, pileup.
  • Question: How should we discuss low pT vs. high pT

Initial Les Houches Study

Key Question

  • Do we understand FSR modeling by workhorse parton showers?
  • Start with the clean case of e+e-, move to pp later.

Basic Plan

  • Take e+e- → q qbar, and e+e- → g g
  • Vary collision energy, jet radius
  • Choose a core set of jet shapes
  • Use as many MC options as possible.
  • Question: use ROC curves or mutual information (I(T;A)) to quantify discrimination power?
    • Answer: doesn't really matter, probably I(T;A) is easier to begin with.
    • Better answer: Use separation (S-B)^2 / (2 (S + B)).

Core Jet Shapes

  • Generalized angularities (kappa, beta)
    • (1,0.5)
    • (1, 1) – jet width
    • (1,2) – jet mass
    • (0,0) – multiplicity
    • (2,0) – ptD
  • Question: Apply on full events or just tracks
    • Answer: Apply on full events.
  • Question: Choice of axes? (issue of recoil)
    • Answer: WTA recombination axes from anti-kT
  • Question: Sum over particles (angularity-style) vs. sum over pairs (ECF-style)
    • Answer: Sum over particles (angularity-style)
  • Question: Plot linear or log scale?
    • Answer: Do both if it makes sense, better for angularities to have log scale.

Supplemental Jet Shapes

  • More generalized angularities (kappa, beta)
    • (0.5,0.5)
    • (0.5,1.5) – should give bad performance
    • limit (1+epsilon,0) / epsilon – should give good perfomance
  • Ellipticity/eccentricity
  • Covariance matrix observables
  • Pull
  • Psi® – the jet shape
  • Check Gallicchio and Schwarz catalog
  • tau21, or ECF(2,3)
  • Generalized angularities with soft-drop jets, varying beta_SD
  • Do sum over pairs version of angularities (i.e. ECF-style)

Analysis Workflow

  • Rivet analysis in place which computes from a HepMC event sample the various generalised angularity distributions.
  • Processes to consider:
    • mu+mu- → spin1 → q qbar take photons
    • mu+mu- → spin0 → g g take Higgs
    • for tests of universality: mu+mu- → spin0 → q qbar
  • Energies
    • Q=sqrt{s} = 50, 200, 800 GeV
    • Optionally: Q = 100, 400 GeV
  • Jet definition:
    • ee-antikt [genkt, p=-1], WTA_modp recomb scheme
    • R = 0.3, 0.6, 0.9
  • Add thrust from thrust hemispheres for anticipated analytic comparisons
  • Add multiplicity (event-wide) in bins of thrust:
    • T < 5 GeV/sqrt(S)
    • 5 GeV/sqrt(S) < T < 0.1
    • 0.1 < T < 0.2
    • 0.2 < T

Preliminary plots for meeting on Thursday


  • Is discrimination power (e.g. for width) coming from the hadronization regime?
    • Possibility: Isolate hadronization regime (thrust ~ LambdaQCD/Q) and shower regime (thrust ~ 0.1-0.2) and optionally hard jet regime (thrust >~ 0.25). Study scaling of, e.g., multiplicity as a function of Q in each of these regimes.
  • By testing pythia vs. herwig, can we test string vs. cluster hadronization?
  • Is there jet radius dependence?
  • Does matching help in controlling quark/gluon uncertainties?
  • Universality/process dependence of conclusions?
    • Related to whether the discrimination power comes from the core or the periphery of jet.

Next Les Houches Study (for after LH)

  • Above study at hadron colliders, using dijets, W/Z/gamma + j, and maybe t tbar samples

Analytic Les Houches Study?

  • Analytic predictions known/available/straightforward for:
    • Quark thrust: N^3LL' + N^3L0
    • Gluon thrust: N^2LL' + N^2L0
    • ang (kappa =1): NLL'
  • Can we do useful quark/gluon study from analytic results?

Notes from Tuesday Meeting

* 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 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)

        + 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

   . use groomed jets
        use soft-drop beta 

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


 - tool writing (jet shapes)
     use FJ, Rivet
   [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.]

Notes from Thursday Meeting

Meeting in Les-Houches

Presentation of the wiki notes: list of contributors, ...

Presentation of the status of the software: 
  start w e+e- and do pp later
  Rivet analysis in place which computes from a HepMC event sample the various generalised angularity distributions

Reminder: what we mean by a q and a g is e+e-\to qq and e+e-\to gg
  If we want to do something more refined:
    - at LO we can unambiguously sum flavours in hemispheres defined by thrust
    - at NLO we can unambiguously sum flavours in hemispheres defined by thrust
      we get a quark and a small gluon fraction
    - at NNLO things are more complicated. We can use a flavoured
      algorithm (BSZ) to define the flavour of each hemisphere
    - for pp collisions, we should use a flavoured algorithm to
      determine flavour, and then find a way (e.g. using ghosts) to
      run anti-kt jets. This would deserve a topic per se (a LH accord)?
    - Question: can we match to the Born and find an operatiroal
      definition up t power corrections?
      Use case: VBF, two jets with a third jet veto. q/g well-defined
                in the exclusive limit

Questions to look into:
 - is the discrimination power (e.g. for width) coming from the hadronisation regime?
 - plotting in log binninb?
 - pythia v. herwig important to test string v. cluster hadronisation
 - isolate hadronisation regime. Study the scaling in different bins
   of one angularity (e.g. thrust). Take a hadronisation region
   (T\propto LQCD/Q) and a shower region (T~0.1-0.2) plus optionally a
   "hard jet region" (T >~ 0.25)
 - does mathing help?
 - jet radius dependence (edit analysis and recompile)
 - analytic predictions?
    for thrust: ee->qq known at N^3LL' + N^3LO
                ee->qq known at N^2LL' + N^2LO
        ang(bkappa=1): NLL'
 - question of the universality/process dependence of the conclusions?
   Related to whether the power comes from the core or the periphery?

 - process to consider: 
     mu+mu- -> spin1 -> qq  take photons
     mu+mu- -> spin0 -> gg  take Higgs
   for tests of universality
     mu+mu- -> spin0 -> qq

 - Energies Q=sqrt = 50, 200, 800 GeV
   jetdef: ee-antikt [genkt, p=-1], WTA_modp recomb scheme
   radii: 0.3, 0.6, 0.9

 - add thrust from thrust hemispheres for analytic purpose

 - add multiplicity (event-wide) in bins of thrust:
     T < 5 GeV/sqrt(S)
     5 GeV/sqrt(S) < T < 0.1
     0.1 < T < 0.2
     0.2 < T
2015/groups/sm/qg.1433431094.txt.gz · Last modified: 2015/06/04 17:18 by jesse.thaler