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2015:groups:sm:qg [2015/06/03 15:11] jesse.thaler created |
2015:groups:sm:qg [2015/06/04 14:40] gregory.soyez [Original Notes from Gregory] |
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+ | ====== Quark/Gluon Enrichment Studies ====== | ||
+ | a.k.a. Hunting the White Whale of Jet Substructure | ||
+ | * Jon Butterworth | ||
+ | * Marat Freytsis | ||
+ | * Peter Loch <loch@physics.arizona.edu> | ||
+ | * Deepak Kar | ||
+ | * Andrzej Siodmok | ||
+ | * Peter Skands | ||
+ | * Dave Soper | ||
+ | * Gregory Soyez | ||
+ | * Jesse Thaler <jthaler@mit.edu> | ||
+ | * who did I forget? | ||
- | ====== Original Notes from Gregory ====== | + | * Remotely: Andy Buckley, Mario Campanelli |
+ | |||
+ | 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? | ||
+ | |||
+ | ===== 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/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) | ||
+ | ===== Next Les Houches Study (for after LH) ===== | ||
+ | |||
+ | * Above study at hadron colliders, using dijets, W/Z/gamma + j, and maybe t tbar samples | ||
+ | |||
+ | ===== 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|}} | ||
+ | |||
+ | ===== Original Notes from Gregory ===== | ||
<code> | <code> |