High pT jet analysis for the study of BSM jet event

TadaAki Isobe, Shoji Asai,Koji Terashi, and Michiru Kaneda

ICEPP, Univ. of TokyoAugust 25th, 2008

ATLAS Exotics Jet+X subgroup meeting1

Simple dijet event topology could have rich structure in BSM physics

• Many BSM scenarios predict various dijet signatures.– Z’ ,Graviton, Kaluza-Klein gauge bosons and so on.

• Model-independent search for dijet excess has been studied in ATLAS.

• Even if dijet signatures are seen, nobody knows what cause them and we have to understand what’s going on.– Similar with the large missing-ET event.

• Flavor analysis gives one of the important hints to understand such event.– It’s best to measure the branching ratio for such exotic dijet event.

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How can we identify the source of high-pT jets?

• High-pT jets of BSM events come through top, bottom, W and light quarks.– But the pT of jets from such events is quite high and

standard method can not be used for ID.

• There are some information for the ID.– Jet substructure

• See the composite of single objects

– Non-isolated muon in jet• W →ln, semi-leptonic decays

– Displaced track in jet• long life time of bottom

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Jet substructure analysis

• Search the associate subjets in the leading jet which covers wide R range (i.e. Cone7).– Subjets are reconstructed also with narrow R

range.– So far, we try to use kt and Durham algorithm for

the subjet reconstruction.• Analysis with splitting Y-scale is different from

the exclusive reconstructed subjet analysis.

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Sub-jet analysis with kt/Durham algorithm

kt Durhamr=0.05r=0.1r=0.2r=0.3

Ecm*sqrt(ycut)=5GeVEcm*sqrt(ycut)=10GeVEcm*sqrt(ycut)=20GeVEcm*sqrt(ycut)=30GeV

‣Profile of number of subjets in a leading jet as a function of primary top pT for each jet reconstruction parameters.‣Truthjet is used in order to check the algorithm itself. ‣Use privately produced KK gluon(M=6TeV)->ttbar sample.

‣ Top quarks decay into hadrons only.

Sub-jet multiplicity depends on top pT. Sub-jet multiplicity well stable over a wide pT range.Ecm=200 GeV seems best.

Durham Sub-jets for each jet samples

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KKgluon->ttbarGraviton->WWKKgluon->bbarKKgluon->jj

Ecm*sqrt(ycut)=20GeV fixed

Splitting Y-Scale (kT algorithm)

Graviton->WWKKgluon->jjKKgluon->ttbat

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‣Use analysis tool announced by Gustaaf at July.‣TruthJet is used for checking its characteristics.‣Each events are produced privately with atlfast.

‣ v14.2.10.1, 100K events‣pT dependence of splitting y-scale is stable for wide pT range.‣The results seem to be similar with ATLAS-PHYS-INT-2007-015.

Jet-pT(GeV/c)Jet-Mass(GeV/cc)

Ysc

ale1

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Ysc

ale1

2

ID using non-isolated muon

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‣While the efficiency is low, requiring non-isolated muon can enhance the top and bottom contribution.

Leading 2Jet invariant mass(GeV/cc)

Eve

nts/

10fb

-1

ALL Events #{non-isolated muon (pT>10GeV/c)}>0MET>50GeV

Leading 2Jet invariant mass(GeV/cc)

Displaced track

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‣There are many large impact parameter tracks in top and bottom jets.‣Requiring the jets that include tracks with large IP, top and bottom jets can be identified.

Signed d0 significance

information for high-pT Jet ID • top

– JetMass– #subjet– Y-scale– Non-isolated muon from W or b– Displaced track

• W– JetMass– #subjet– Y-scale

• bottom– #subjet– Non-isolated muon from semi-leptonic decay– Displaced track

• light quarks– #subjet

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Information for high-pT jet ID is prepared and it is confirmed that each information is useful for high-pT jet ID.

Next step: We are going to study the best cut to identify each jet source, and to establish the ID strategy.(Efficiency and fake rate should be estimated.)

Red : SignalBlue : QCD-BGBlack : Total

With top ID

Mjj(GeV/c2)

KKgluon search with identified top

‣KK gluon→ttbar full simulation samples are generated privately.‣Reconstruct two jets with standard jet algorithm and perform subjet analysis with Durham algorithm (H1TowerParticleJet).‣Details of analysis are presented at previous meeting. ‣Significant improvement in S/N with top ID (=100GeV<Mjjj<250GeV)

Mjj(GeV/c2)

No top ID

Invariant mass spectra of jets identified as top

Scaled to 1fm-1 luminosityM=4TeV/cc

Summary and Outlook

• Substructure of jets, non-isolated muons and displaced tracks are useful information to identify the high-pT jets.

• We will estimate the performance of each cut more quantitatively and establish the ID strategy for high-pT jets.

• Compare the high-pT jet performance of exclusive reconstructed subjet analysis and splitting Y-scale analysis.

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backup

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Durham Jet reconstruction algorithm

• In addition to the kt algorithm, jet reconstruction with durham algorithm is tried in order to divide the jets from high-pT top.

• The durham algorithm is used for e+e- collider experiment, and known as good algorithm in terms of fine jet splitting.

• For the fine jet reconstruction, durham jet reconstruction algorithm is implemented into athena analysis framework and tried to reconstruct the jets.– while it is not at public cvs

area.

• Reconstruction algorithm itself is similar with Kt clustering algorithm, the metric (Distance in Kt) of the durham jet reconstruction algorithm is

• One have to define the yij to limit the scale for jet reconstruction.– Set to 0.01.

• Ecm value is fixed, since Ecm cannot be measured at p+p collider experiment.– Ecm in yij intends the mass

scale for multi-jet production.14

2

22 )cos1)(,min(2

cm

ijjiij

E

EEy

Subjet reconstruction with kt/Durham

• Played with 6TeV KK gluon samples.

– Produced with atlfast privately

• TruthParticleJet is used for performance check.

• Top ID scheme– Search a leading jet

reconstructed with Cone7 supposed to be from top.

– In the leading jet, search highest-pT 3jets reconstructed with kt (or durham) algorithm.

• Reconstruct the 3 jet mass and see top peak.

15Reco w/ Cone7

Reco w/ kt/Durham

Number of Subjets in a leading Jet

Kt Durham

Ecm=200GeVr=0.2

Number of Durham SubjetsGraviton->WWKKgluon->ttbar

KKgluon->jjbarKKgluon->bbbar