Boosted Higgs and jet substructure

Mrinal Dasgupta

The University of Manchester

Atlas-UK Higgs Workshop, Birmingham,

25 September, 2014

Overview• Quick introduction to jet substructure and

boosted particle searches/studies.• Examples of recent (and not so recent) Higgs

studies.• Introduction to substructure tools, challenges

and open questions.• Understanding substructure better.• The future: creating robust tools for future

applications (LHC run 2 and beyond)

Introduction

Period since 2008 has seen emergence of a vibrant research area dedicated to use of jet substructure for LHC discoveries.

Basic idea goes back to 1993 and work by Seymour.

It all started with the Higgs….

The famous 2008 “BDRS” paper. Discovery potential for Higgs ~ 120 GeV in VH production with H to bb.

Basic ideas

Signal vs background : tagging and grooming

How to tell signal (e.g. jet originating from a Higgs decay) from “boring” QCD jets i.e. tagging signal?• Exploit differences in “energy-sharing”. Cut on z discriminates against

background.• No large-angle soft radiation from colour singlet Higgs.

How to clean signal from contamination i.e. ISR, UE pile-up? Removal of soft large angle junk.

We have 10-20 different methods invented for this with > 100 papers over 5 years!

Since BDRS

• LHC Run 1 has shown that these methods actually work. Run 2 with higher energies will mean they become very important methods.

• And if the future holds 100 TeV colliders everything will be boosted!

Examples: ttH

Higgs production in association with tt was taken off list of promising discovery channels due to poor S/B. Important also for direct access to top Yukawa coupling.

Using fat jet methods and top+Higgs tagging Plehn, Spannowsky Salam turned S/B ~1/9 to S/B ~ 1/2 for same significance.

Examples: Boosted Higgs pair production

Presented by J.Rojo at BOOST 2014

Examples: hhvv couplingF

Requires a combination of resolved and boosted analysis. Also work on SM hh production with h to bb by Konstantinides et. al

Examples: CP properties

Uses jet substructure to reconstruct Higgs and study angular variables that discriminate between SM and BSM structure of HVV vertex. BSM contributions from higher dimensional operators enhanced by selection cuts in boosted analysis. Godbole, Miller, Mohan White 2013 and 2014

So what is left to do?The techniques are valuable in a variety of contexts and viable. However a number of open questions remain:• There are numerous tools (Taggers/groomers). Which one to

pick?• How do we compare tools? How do results obtained depend on

many parameters of the taggers? Optimal values?• How do we know we have made a discovery and have not

unearthed a feature of the taggers?• How to quantify theory uncertainties in substructure studies?

What tools to use? MC? Fixed-order? Combinations thereof?• Can results depend on shower models, tunes, jet algorithms?

What we need are formulae….

QCD jet mass

Comparison of taggers

Taggers look similar…..

Comparison of taggers

But only for a limited mass range. How to understand what we are seeing? Position of kinks etc? Calls for analysis and calculation.

Analytical vs MC : trimming

MD Fregoso Marzani and Salam 2013

Analytical vs MC: pruning

MD Fregoso Marzani and Salam 2013

Analytical vs MC mMDT (BDRS)

Non perturbative effects

Tagger performance

We now have a much better idea of the factors behind tagger performance and are using this to develop better tools

Outlook• Boosted object techniques have already been demonstrated to

be of great value for several Higgs studies. Many examples of previously unfavourable channels being converted to promising avenues.

• The focus now is on understanding the tools both in and beyond perturbation theory.

• The focus is on performance i.e signal significance but also on robustness and reliability.

• The understanding of substructure is still in its infancy.