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Jet substructures of boosted Higgs

Jet substructures of boosted Higgs. Hsiang-nan Li ( 李湘楠 ) Academia Sinica Presented at PPCHP Oct. 08, 2014 Collaborated with J. Isaacson, Z. Li, CP Yuan. Outlines. Motivation Jet factorization Higgs jet energy profile Summary. Introduction. Boosted heavy particles.

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Jet substructures of boosted Higgs

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  1. Jet substructures of boosted Higgs Hsiang-nan Li(李湘楠) Academia Sinica Presented at PPCHP Oct. 08, 2014 Collaborated with J. Isaacson, Z. Li, CP Yuan

  2. Outlines • Motivation • Jet factorization • Higgs jet energy profile • Summary

  3. Introduction

  4. Boosted heavy particles • Heavy particles (Higgs, W, Z, top, new particles) may be produced with large boost at LHC • Decaying heavy particle with sufficient boost gives rise to a single jet • If just measuring invariant mass, how to differentiate heavy-particle jets from ordinary QCD jets? • Use different jet substructures resulting from different weak and strong dynamics

  5. Fat high pT QCD jet fakes heavy-particle jet Thaler & Wang 0806.0023 Pythia 8.108 Jet invariant mass

  6. Planar flow • Make use of differences in jet internal structure in addition to standard event selection criteria • Example: planar flow • QCD jets: 1 to 2 linear flow, linear energy deposition in detector • Top jets: 1 to 3 planar flow Almeida et al, 0807.0234

  7. Trilinear Higgs coupling Higgs jets can be produced de Florian, Mazzitelli 2013

  8. Higgs jet • One of major Higgs decay modes H -> bb with Higgs mass ~ 125 GeV • Important background g -> bb • Both involve 1 -> 2 splitting • Analyzing substructure of Higgs jet improves its identification • For instance, color pull made of soft gluons, attributed to strong dynamics • Top jet substructure is attributed to weak dynamics Gallicchio, Schwartz, 2010

  9. Color pull • Higgs is colorless, bb forms a color dipole • Soft gluons exchanged between them • Gluon has color, b forms color dipole with other particles, such as beam particles

  10. Energy profile • We propose to measure energy profile • Energy fraction in cone size of r, • Quark jet is narrower than gluon jet due to smaller color factor (weaker radiations)

  11. Jet factorization Achieved by eikonalization and Ward identity

  12. Factorization at jet energy E • Factorize heavy Higgs jet first from collision process at jet energy scale E b Higgs jet ISR g FSR H H H

  13. Scale hierarchy E>>mH>>mb • The two lower scales mH and mb characterize different dynamics, which can be further factorized other gluons linking two b‘s go into soft function O(mH) O(mb) b g b-quark jet heavy-particle kernel

  14. Factorization into two sub-jets • Then factorize two b-jets from the Higgs jet at leading b b b b eikonalization = H H g

  15. Simpler factorization • Absorb soft radiation into one of b-jets to form a fat b-jet of radius R • Another is a thin b-jet of radius r • At small r, double counting is negligible test cone of radius r Higgs jet of radius R

  16. Merging criterion • As integrated over polar angle of thin b-jet, how distant can it be still merged into test cone? • If merged, whole energy of thin b-jet contributes to profile • d=r r < d < 2r d=2r • yes ? No • Calibrate it by gluon jet energy profile

  17. Gluon jet energy profile • LHS: an original gluon jet • RHS: Factorization into two sub-jets • Energy profiles in these two schemes equate Jg Jg Jg =

  18. d=1.7r • Energy profile from factorization into two sub-jets coincides with profile of gluon jet pT=500 GeV factorization into two sub-jets really works!

  19. Higgs jet energy profile

  20. Numerical result • Predictions insensitive to Higgs jet cone R • Without soft radiation, dead cone=0.2 pT=1 TeV

  21. Comparison with QCD jets • Dead cone effect, so Higgs jet profile is lower at small r. It increases faster with r due to energetic b- jets

  22. Summary • Jet substructures improve particle identification • QCD factorization and resummation provide reliable prediction, and independent check • Factorization of a fat jet into several sub-jets works well (checked via gluon jet profile) • Application to Higgs jet profile successful, showing moderated dead cone by soft gluons and fast increase due to pencil-like b jets • Will be extended to boosted hadronic top jet (u-jet + d-jet -> W-jet, W-jet + b-jet -> t-jet)

  23. Back-up slides

  24. Soft function • Soft radiation around two b jets plays important role • Feynman diagrams • Calculated as jet function soft radiation velocity vb S vb’

  25. Heavy-particle kernel • Adopt LO kernel from Higgs propagator • Due to soft gluon radiation, b- jet can be closer to Higgs jet axis, so dead cone effect is not obvious

  26. Comparison with CDF data quark, gluon jets, convoluted with LO hard scattering, PDFs NLO

  27. Compasion with CMS data

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