Jet Substructure

From Wiki Les Houches 09

Revision as of 14:24, 15 June 2009

Back to Tools_and_Monte_Carlo_Session_1_(SM)#Jet_Physics.

Boosted hadronic decays of massive particles (W,Z,Top,H,BSM..., with session 2) and jet mass/shape studies with QCD jets in early data.

Interested parties (Session 1)

Matt Schwartz, Giacinto Piacquadio, Mario Campanelli, Paulo Francvilla, Jon Butterworth, Peter Loch, Ezio Maina, Leif Lonnblad, Keith Hamilton, Simon Dean, Rohini Godbole, ...

Examine/discuss the different regions of validity for calculations of variables like jet mass, jet width, subjet multiplicity, using e.g. (N)LO ME, PS, matched and resummed calculations etc... (see also A Collection of Matching Benchmarks). Review, compare, critique literature, think about future ideas.

Types of object one might use substructure on

• Substructure tagging minireview. (Schwartz...)

QCD jets (quark gluon separation)

• Wjj provides a sample of quark jets at LHC?

• SUSY cascades are rich in quark jets. Could use quark ID to simplify decay chains.

• Experimental papers using substructure to separate q.g jets
  • LEP papers.
  • CDF conference proceedings (2004).
  • CDF inclusive jet shapes
  • CDF b-jet shape paper. Available in rivet.
  • ZEUS paper. Fig.13 p27 of this review shows a comparison of the angular distributions for quark/gluon jets identified using subjet techniques and using charm tagging.

Colour singlet heavy objects, two body decay (W,Z,H...)

• Butterworth+ WW (kT y scales)
• The above analysis was carried out with a full detector simulation in the WW scattering chapter (p1769) of this big ATLAS paper.
• Butterworth+ W,Z,H in SUSY cascades (kT y scale again)
• Butterworth+ W/Z+higgs (Cambridge/Aachen). For some related discussion on the Wbb background, see Wbb in the high pT HW region.* ttH CMS study No substructure used yet...
  • CMS PhD thesis
• ttH Atlas CSC results Also no substructure yet.
• See also TtH developments.

Plot to come here from Matt showing that 17% of ttH events contain a Higgs with pT>200 GeV, and ~50% have one with pT>100 GeV. Shows that subjet analyses could have a big impact on ttH.

Colour singlet heavy objects, three body decay (Neutralino,...)

• Butterworth+ neutralinos (kT yscales and Cambridge/Aachen)
• ATLAS public note (SUSY09) on neutralinos from subjet structure

Coloured heavy objects (top,...)

• Kaplan+ top tagging
• Schwartz talk on top-tagging(May 18, 2009)
• Almeida+ and Almeida+ top event shapes
• Top ID using kT y scales this Les Houches report (p106)

• Comparison of tagging for different jet algorithms on top (Matt Schwartz):

The mass of the W candidate subjets in a top, as identified by various algorithms, is shown. Main point is that anti-KT declustering does not do well (no W mass peak) but taking an anti-kT jet and declustering with C/A is fine.

MC issues

Differences in heavy object decays; different parton showers, matrix element corrections in some MC, not in others; in Herwig, Herwig++, pythia, sherpa. POWHEG improvement soon. See talk from Carlo Oleari Friday afternoon for some discussion of MC@NLO and POWHEG in Higgs decays. Dead cones etc. Keith Hamilton, Giacinto Piacquadio, Matt Schwartz, Leif Lonnblad to produce a short summary of the effects implemented in different MCs.

• Keith's slides on Dead Zones to link here.
• Hamilton+ Showers for top decay

How sensitive are the various subjet methods to the above differences?

Would be nice to have some truth level comparisons, of simple variables (jet mass etc) and also of some of the various boosted decay methods (esp top?) if possible.

Detector issues

Pile-up, calorimeter noise, granularity, acceptance: The reconstruction quality for the various jet shape variables need to be understood. A first look at jet masses and y-scale in ATLAS can be found at (transparencies), S.D.Ellis+, and at ATLAS Collaboration.

Studies from GP.

Proposed cuts for studying this (highest priority ones in bold):

• pT cut on constituents (MeV) 0,100,500,1000,2000.
• QCD jets in pT bins between 17,35,70,140,280,560,1120 GeV. Look at the leading jet. Radipity +/- 5.
• Pile-up. 0,4,8,16-20 interactions per signal event (Poisson). (Think about doing this as a function of number vertices in full sim).
• Jet algorithms. Use Anti-kT R=0.4, R=0.6. And also C/A, kT etc where appropriate, and also studies of using e.g. C/A to recluster anti-kT jets

• Sarah Allwood's thesis can be found in the Manchester thesis archive.

This includes studies of the effect of pile-up on W reconstruction using the y-scale method (e.g. p103).

Find the jet first, then cut on constituents, or cut on constituents first then find the jet?

Constituents = final state particles, or pre-clustered final state particles, or towers, or topoclusters...

Miscellaneous References

* Seymour+ clustering (kT algorithm)
* Dokshitzer+ clustering
* S. Ellis+ jet pruning, using top as an example
* Krohn+ variable R
* Bahr+ Herwig++ manual (includes heavy particle decays)