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2015:groups:sm:qg [2015/06/03 15:11] jesse.thaler created |
2015:groups:sm:qg [2015/06/06 17:33] philippe.gras |
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+ | ====== Quark/Gluon Enrichment Studies ====== | ||
+ | a.k.a. Hunting the White Whale of Jet Substructure | ||
+ | * Andy Buckley | ||
+ | * Jon Butterworth | ||
+ | * Mario Campanelli | ||
+ | * Marat Freytsis <freytsis@physics.harvard.edu> | ||
+ | * Peter Loch <loch@physics.arizona.edu> | ||
+ | * Deepak Kar <deepak.kar@cern.ch> | ||
+ | * Simon Plätzer | ||
+ | * Andrzej Siodmok <andrzej@cern.ch> | ||
+ | * Peter Skands <peter.skands@monash.edu> | ||
+ | * Dave Soper | ||
+ | * Gregory Soyez | ||
+ | * Frank Tackmann | ||
+ | * Jesse Thaler <jthaler@mit.edu> | ||
+ | * Philippe Gras <philippe.gras@cern.ch> | ||
+ | * ... | ||
- | ====== Original Notes from Gregory ====== | + | Link to GitHub repository: https://github.com/gsoyez/lh2015-qg |
+ | ===== Preliminaries ===== | ||
+ | |||
+ | ==== 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 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? | ||
+ | |||
+ | |||
+ | ==== 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(r) -- 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 ==== | ||
+ | |||
+ | {{:2015:groups:sm:ga_10_20.pdf|}} | ||
+ | {{:2015:groups:sm:ga_10_10.pdf|}} | ||
+ | {{:2015:groups:sm:ga_10_05.pdf|}} | ||
+ | {{:2015:groups:sm:ga_00_00.pdf|}} | ||
+ | {{:2015:groups:sm:ga_20_00.pdf|}} | ||
+ | |||
+ | ==== Questions ==== | ||
+ | |||
+ | * 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 ===== | ||
<code> | <code> | ||
Line 134: | Line 343: | ||
</code> | </code> | ||
+ | |||
+ | ===== Notes from Thursday Meeting ===== | ||
+ | |||
+ | <code> | ||
+ | |||
+ | |||
+ | 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 | ||
+ | |||
+ | |||
+ | </code> | ||
+ | |||
+ | ===== Notes for Jesse for Preparing Summary Talk ===== | ||
+ | |||
+ | * Quark is more of an adjective than a noun. | ||
+ | * Pseudo-quark? (That language doesn't go over very well.) | ||
+ | |||
+ | ==== What is a Quark Jet? ==== | ||
+ | |||
+ | (From ill-defined to well-defined) | ||
+ | |||
+ | * A quark parton | ||
+ | * A Born-level quark parton | ||
+ | * The initiating quark parton in a final state shower | ||
+ | * An eikonal line with baryon number 1/3 and carrying triplet color charge | ||
+ | * A quark operator that appears in a hard matrix element in the context of a factorization theorem. | ||
+ | * A parton-level jet object that has been tagged as a quark using a soft-safe flavored jet algorithm (automatically collinear safe if you sum constituent flavors). | ||
+ | * A phase space region (as defined by an unambiguous hadronic fiducial cross section measurement) that yields an enriched sample of quarks (as interpreted by some suitable, though fundamentally ambiguous, criterion). | ||
+ | | ||
+ | (Sometimes people think we care about the top of the list while we are really focused entirely on the bottom.) |