Study of the Subjet Multiplicity at CMS

Manuk Zubin Mehta Prof. Manjit Kaur Panjab University Chandigarh

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Introduction

The jet shape study could provide the tool for quark-gluon jets separation:• The internal structure of a jet is expected to depend on the type of parton creating the jet (quark or gluon) and ET of the jet.• the study of jet shapes can help to test the models of underlying events .

Jet Jet StructureStructure

* Number of variables that has been proposed to study jet structure of jets. - Jet transverse momentum distribution measured as a function of distance from jet axis. - Jet mass . Etc.

* We have used Subjet analysis to study jet structure, which is useful for discriminating between quarks and gluon jets and for calculating ratio of color charges CA/CF = 9/4

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SubJets

*The Subjet multiplicity is a natural observable of a KT

algorithm.

*Subjets are defined by reapplying the KT algorithm, this time starting with a constituents of a jet.

*Number of Subjets within the jet is the Subjet multiplicity M.

*Jet initiated by gluons are significantly different than jets initiated by quarks. As compared to quark jets, gluon jets had more particles, a softer momentum distribution and are more spread out.

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Standard Event Selection For Data• Single Jet Trigger PT >30GeV• Technical Trigger Bits

– 0 AND (40 OR 41) AND NOT (36 OR 37 OR 38 OR 39)• Scraping veto• Primary vertex:

– >= 4 DOF– |z| < 15 cm

{ The Trigger 0 Bit and Beam Halo (36 OR 37 OR 38 OR 39) are dropped for MC}

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Jet Selection Criteria • Selecting Di Jet (Leading in ET)• Jet Quality Criteria :: Loose Jet ID criteria(EMF > 0.01, n 90

hits > 1, fHPD< 0.98)• |𝜟𝝓 – ℿ | < 1.0• PT > 50 GeV• |eta | < 3.0 • KT ,D=0.6 algorithm used

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Data Samples

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Runs < 135808/MinimumBias/Commissioning10-SD_JetMETTau-Jun14thSkim_v1/RECORuns >= 135808/JetMETTau/Run2010A-Jun14thReReco_v2/RECO

Runs > 137437 upto 139790/JetMETTau/Run2010A-PromptReco-v4/RECO

Integrated Lumi for Analyzed Data = 78.49 nb -1

MC Samples

/QCD_Pt30/Spring10-START3X_V26_S09-v1/GEN-SIM-RECO /QCD_Pt30-herwig/Spring10-START3X_V26_S09-v1/GEN-SIM-RECO

/QCD_Pt30-herwigjimmy/Spring10-START3X_V26_S09-v1/GEN-SIM-RECO

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Monte Carlo used: Herwig++, Pythia 6,Herwig6+Jimmy

• Herwig and Pythia use different hadronization model

•Also using different Underlying Event models(UE is used to tune MC generators to data and measured in transverse region)

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Plan of the near future• We will derive systematic uncertainties using Different MC, as different models of fragmentation lead to different shape and ratio of quark & gluon jets.

• We will also analyze the 2009+2010 900GeV Data and different tuned MC Sample .

Comments and suggestions are welcome!

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Back Up Slide

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The kt algorithm for hadron-hadron collision exists in two main forms, inclusive and

exclusive. They use the same interparticle (dij) and particle-beam (diB) distance

measures:

where kti, yi and φi are the transverse momentum, rapidity and azimuth of particle i and R

is a jet-radius parameter.

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Inclusive algorithm 1. In the inclusive kt > algorithm all particles end up in a jet. The recombination procedes as follows: 2. Find the minimum dmin of all the dij,diB. 3. If dmin is a dij merge particles i and j into a single particle, summing their four-momenta (this is E-scheme recombination); if it is a diB then declare particle i to be a final jet and remove it from the list. Repeat from step 1 until no particles are left.

Exclusive algorithm 1. The exclusive algorithm introduces a squared transverse momentum scale dcut, which acts as a threshold for separating out jets. 2. Find the minimum dmin of all the dij,diB. If dmin > dcut stop the clustering; all remaining particles are the final jets. Sometimes, instead of using an explicit dcut, the clustering is stopped when one reaches a given number