2015:groups:sm:qg
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2015:groups:sm:qg [2015/06/03 15:41] jesse.thaler |
2015:groups:sm:qg [2015/06/03 16:20] jesse.thaler |
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| a.k.a. Hunting the White Whale of Jet Substructure | a.k.a. Hunting the White Whale of Jet Substructure | ||
| + | |||
| + | * Jon Butterworth | ||
| + | * Marat Freytsis | ||
| + | * Peter Loch | ||
| + | * Deepak Kar | ||
| + | * Jesse Thaler | ||
| + | * Andrzej Siodmok | ||
| + | * Peter Skands | ||
| + | * Dave Soper | ||
| + | * Gregory Soyez | ||
| + | * who did I forget? | ||
| + | |||
| + | * Remotely: Andy Buckley, Mario Campanelli | ||
| ===== Preliminaries ===== | ===== Preliminaries ===== | ||
| Line 8: | Line 21: | ||
| * Of course, at the hadron level, you can't define a quark jet vs. a gluon jet unambiguously. | * 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 jet (e.g. gluon enrichment in dijets, quark enrichment in vector boson plus jet). | + | * 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). | * 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. | * If needed, we can use flavored jet algorithms to give an IRC safe definition of jet flavor at the parton level. | ||
| Line 14: | Line 27: | ||
| ==== How to isolate quark vs. gluon samples? ==== | ==== 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). | + | * 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. | + | * 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. | + | * 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? ==== | ==== "Discrimination" really the right word? ==== | ||
| Line 24: | Line 38: | ||
| * Similar issues arise in how to define a "hadronic W". | * Similar issues arise in how to define a "hadronic W". | ||
| * Quark/gluon enrichment should be a piece of a more refined analysis. | * Quark/gluon enrichment should be a piece of a more refined analysis. | ||
| - | * We can provide genial recipes, but should not aim for optimal analyses, which are only sensible in the context of a specific physics goal. | + | * 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? ==== | ==== What is the killer app of quark/gluon enrichment? ==== | ||
| Line 39: | Line 53: | ||
| * 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. | * 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. | * 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) vs. wide angle (soft, ISR). | + | * 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 ==== | ||
| Line 46: | Line 60: | ||
| * Tuning of gluon final state shower can affect jet shapes. | * Tuning of gluon final state shower can affect jet shapes. | ||
| * Examples: g -> q qbar vs. g -> gg, including spin-polarization information | * Examples: g -> q qbar vs. g -> gg, including spin-polarization information | ||
| - | * Do beyond-LL effects help or hurt quark gluon discrimination? | + | * Do beyond-LL effects help or hurt quark/gluon enrichment? |
| * What about the impact of heavy flavor? | * What about the impact of heavy flavor? | ||
| Line 53: | Line 67: | ||
| * ISR effects should dominate at large angles | * ISR effects should dominate at large angles | ||
| * Highly process dependent, depends on color corrections of jet with ISR | * Highly process dependent, depends on color corrections of jet with ISR | ||
| - | * We will attempt to deemphasize these in our study. | + | * We will attempt to deemphasize these in our study, if possible |
| ==== Experimental Results ==== | ==== Experimental Results ==== | ||
| Line 61: | Line 75: | ||
| * ATLAS sees considerable process dependence, whereas CMS has not emphasized this issue. Is this connected to ISR in some way? | * 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? | ||
| + | |||
| + | ===== 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? | ||
| + | |||
| + | ==== 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. | ||
| + | |||
| + | ==== 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) | ||
| + | |||
| + | ==== 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 | ||
| + | * 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 | ||
| + | |||
| + | ===== Next Les Houches Study (for after LH) ===== | ||
| + | |||
| + | * Above study at hadron colliders, using dijets, W/Z/gamma + j, and maybe t tbar samples | ||
| ===== Original Notes from Gregory ===== | ===== Original Notes from Gregory ===== | ||
2015/groups/sm/qg.txt · Last modified: 2015/07/14 08:27 by philippe.gras
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