Introduction to Jet Substructure

Shuo Yang （杨硕）Physics Department, Dalian University/ CHEP ， Peking University

First two years at the LHC 2012.06.13

Outline

• Large Hadron collider• What is jet.• Jet cone size and fat jet.• Jet substructure Top taggers and Higgs taggers Grooming Hybrid-R—heavy charged Higgs

• Summary

LHC

Large Hadron Collider

p p

1. Incoming proton Parton distribution function

2. Hard process Matrix element calculation , LO or NLO

3. Radiation (Parton Shower) Initial state radiation (ISR) and Final state radiation (FSR)

4. Hadronization Universal models

5. Underlying event

I. What is jet?

quark quark

quarkquark

gluon

hadr

onis

atio

n

Why do we see jets? Parton fragmentation

Jet Definition

• Which? • How?

Jet algorithms provide a set of rules for grouping particles into jets. + Parameters

Recombination schemes define how to combine their momenta . (mostly E-scheme)

Jet Algorithms

Cone Algorithm Recombination AlgorithmUAI,JetCLU,SISCone…. Jade,Kt, Cambrige-Aachen, Anti-Kt….

Based on fixed geometryIdea: Put cone along dominatant direction of flow direction

Idea: Undo branching

1. Calculate the distance dij between all pairs of pseudo-jets and the beam distance diB for each pseudo-jet.

2. Find the minimum in the list of dij and diB .

3. If the minimum is dij, recombine i and j into a new particle and then return to step 1.

4. If it is diB, define it as a final-state jet and remove it from the jet list. Return to step 1.

5. Iterate this process until the original list is empty, i.e., all peudo-jets have been clustered to jets.

The dij and diB in most popular sequential recombine algorithms for use at the LHC can be parameterized as:

P is a parameter, 1 for kt, -1 for anti-kt, 0 for CA

Cartoon for the Kt algorithm

Cartoon for the Kt algorithm

Cartoon for the Kt algorithm

Cartoon for the Kt algorithm

Cartoon for the Kt algorithm

Cartoon for the Kt algorithm

Cartoon for the Kt algorithm

Cartoon for the Kt algorithm

Cartoon for the Kt algorithm

A 2 jets event

Borrowed from Salam’s talk

Remarks• There is no standard jet algorithms. The jet algorithm and the

parameters could be chosen flexibly.• All these recombination algorithms are infrared-safe and

collinear safe.• The anti-kt algorithms gets circular jets area which act much like

an idealized cone-based algorithm, while the kt and CA get irregular jets area.

• The kt and CA algorithms incline to combine the harder pseudo-jets or pseudo-jets with a large separation in the later, so it is expected that the combination in the last step is the reverse of the main QCD branching or the 2-body decay of a massive particle. So decluster can be used in CA and kt in substructure study.

Rest frameLab frame

R=0.4 R=0.7

Hadronically decaying EW massive particles at high Pt = 2 jets

Boosted Massive Particles

Jet cone size and fat jet

R=0.5

R=1.5

R=1.0

small cone vs. large cone

perturbative fragmentation

non-perturbative fragmentation

underlying event & pile up

multi-hard-parton

Fat Jet• Cone size should be chosen flexibly for

physical intention.• Boosted massive hadronically decaying objects

is common at the LHC. M_Z/TeV• Fat jet is useful to capture hadronical decay of

the highly boosted massive particle. W, H, t • Generally, mass discriminator is not enough!

In order to separate signal, one have to anatomize the Jet Substructure and reconstruct delicate kinematical variables on subjets !

• 今天您吃了吗？ 今天您收集事例了吗 吃了！ 收集了！• 您吃的什么？ 收集到什么事例了？ 爪子 ( 蹄子 ) jets

什么爪子 ? 几个指头 什么样的 jets ，几个subjets

Brief history of Jet Substructure• Michael H. Seymour Searches for new particles using cone and cluster jet

algorithms: A Comparative study, Jun 1993, Z. Phys. C62(1994) 127-138• Butterworth, Cox, Forshaw, Jan 2002 WW scattering at the CERN LHC,

Phys. Rev. D65 (2002) 096014• Butterworth, Davison, Rubin, Salam, Feb 2008, Jet substructure as new

Higgs search channel at the LHC, Phys.Rev.Lett.100(2008) 242001.• Kaplan, Rehermann, Schwartz and Tweedie, ``Top Tagging: A Method for

Identifying Boosted Hadronically Decaying Top Quarks,'' Phys. Rev. Lett.101(2008) 142001 (“JH top-tagger”)

• Filtering ,Trimming, Pruning , Variable R, NSubjettiness…….• Higgs –tagger, top-tagger, W-tagger ….. • H.N.Li, Z. Li, C.-P. Yuan, Phys.Rev.Lett.107(2011)152001• Jet shape ……………………

• Higgs taggers• Top taggers• Grooming• Searching for heavy charged Higgs. Hybrid-R

ppVH Hbb

Go to high transverse momentum !?

Large background gg tt lvbbjj Only 5% of the total events.(Pt >200GeV)Kill top pair background? Gain significance?

Borrowed from G.P. Salam’s talk

Borrowed from G.P. Salam’s talk

JH Top Tagger PRL101 (2008)142001

GroomingResearch by Bionsen, a natural deodorant company, found that the average woman's daily grooming and make-up routine means she 'hosts' a staggering 515 different synthetic chemicals on her body every single day.

Dog grooming

If grooming can be used for human and dogs, one is led to ask whether it can be used for jets.

Jet Grooming• Filtering • Trimming• Pruning

1. Grooming narrows the mass distributions of jet and the reconstructed massive particles. 2. It improves both signal purity and the signal significance. 3. It can be used to refine any jet. 4. Grooming is time consuming but is promising.

Figures are borrowed from G.P. Salam’s talk

Filtering

Refine the jet resolution

• Filtering: break jet into subjets on angular scale Rfilt , take n filtered hardest subjets.

• Trimming: break jet into subjets on angular scale Rtrim, take all subjets with Pt,sub>εPt,jet

• Pruning: If the subjets about to be recombined have ΔR>Rprune and min(Pt1,Pt2)< ε(Pt1+Pt2), discard the softer one.

taggered top

Searching for heavy charged Higgs with jet substructure

Shuo Yang, Qi-Shu Yan JHEP02(2012)074 arXiv: 1111.4530

Hybrid-R Reconstruction Method

1. CA jet algorithm with a large cone and top tagger to capture the highly boosted top t1. Form a jet list J0 .

2. Recluster the pseudo-jets in the event with a small cone (anti-kt R=0.4) and then get a new jet list L0 .

3. If a small-size jet is within the larger cone of the direction of the tagged top jet, remove it and then form a new jet list L1 .

4. We identify the most energetic jet in L1 as the b3 jet. The unused jets in the jet list form L2 .

5. We require there are at lest 3 jets in the list L2. Reconstruct the second top and W by +

1 2

3 4

R size vs. mass distribution

One typical signal event

Hybird-R method• Jet Substructure is helpful to identify highly boosted massive

particles or to tell the jet origin.• We proposed a hybrid-R method with top-tagger. By studying the full hadronic mode of , we find that this

method can greatly reduce the combinatorics and can successfully reduce background down to a controlled level.

• Our method can be generalized to case where some jets are decay products of one highly boosted massive particle while some of others come from a massive particle near the threshold. For examples, and .

Hybird-R method• Jet Substructure is helpful to identify highly boosted massive

particles or to tell the jet origin.• We proposed a hybrid-R method with top-tagger. By studying the full hadronic mode of , we find that this

method can greatly reduce the combinatorics and can successfully reduce background down to a controlled level.

• Our method can be generalized to case where some jets are decay products of one highly boosted massive particle while some of others come from a massive particle near the threshold. For examples, and .

1. The jet substructure technique anatomizes the inside characters of the jet.

2. It can tell jets from different physical origins.

3. Jet substructure methods can improve mass resolution, suppress background, identify heavy particle, improve the signal significance.

4. It is a new technique mainly developed after 2008.

5. Jet substructure methods are interesting, developing and promising.

Summary about Jet Substructure

What is the status?

Higgs New Physics ？

千呼万唤始出来 犹抱琵琶半遮面Higgs

Jet substructure----Sharpen the Sword

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