Differences

This shows you the differences between two versions of the page.

--- 2015:groups:sm:qg [2015/06/03 15:11]
jesse.thaler created
+++ 2015:groups:sm:qg [2015/06/03 15:41]
jesse.thaler
@@ Line 1: / Line 1: @@
+====== Quark/Gluon Enrichment Studies ======
+a.k.a. Hunting the White Whale of Jet Substructure
+===== Preliminaries =====
-====== Original Notes from Gregory ======
+==== 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 jet (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 genial recipes, but should not aim for optimal analyses, which are only sensible in the context of a specific physics goal.
+==== 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) vs. wide angle (soft, ISR).
+==== 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 discrimination?
+  * 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.
+==== 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?
+===== Original Notes from Gregory =====
 <code>

Les Houches

User Tools

Site Tools

Differences

Page Tools