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

• Andy Buckley
• Jon Butterworth
• Mario Campanelli
• Marat Freytsis
• Peter Loch loch@physics.arizona.edu
• Deepak Kar
• Andrzej Siodmok
• Peter Skands peter.skands@monash.edu
• Dave Soper
• Gregory Soyez
• Frank Tackmann
• Jesse Thaler jthaler@mit.edu
• …

Link to GitHub repository: https://github.com/gsoyez/lh2015-qg

• 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.

• 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.

• 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.

• 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.

• 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 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 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

• ATLAS paper suggests that beta → 0 (i.e. hard core) is not as effective as NLL calculations suggest. (see http://arxiv.org/abs/1405.6583 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?

• 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.

• 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.

• 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

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

• 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)).

• 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.

• 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)

• 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

ga_10_20.pdf ga_10_10.pdf ga_10_05.pdf ga_00_00.pdf ga_20_00.pdf

• 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.

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

• 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?

* 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.]

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