a.k.a. Hunting the White Whale of Jet Substructure
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Link to GitHub repository: https://github.com/gsoyez/lh2015-qg
* 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
(From ill-defined to well-defined)
(Sometimes people think we care about the top of the list while we are really focused entirely on the bottom.)