TutorialonROOTandDataAnalysis

ArunNayakIns5tuteofPhysics

Bhubaneswar

Objec5ve•  ThistutorialisaboutdoingdataanalysisusingROOTTree

•  Wediscusswri5ngandreadingeventsto/fromtree,wheretreesaredefinedinvariousformats

•  Westartfromsimplerexamplesandslowlyaddmorefeaturesthatmakesdatastorageandanalysiseasier.

•  Theprogramsfortheexamplesdiscussedhereareavailableinthetutorialdirectory.Youneedtolookatthoseprograms,whenwediscussthem,trytounderstandandusethemforprac5ce.

•  Pleasedon’thesitatetoaskanyques5onsorreportyourproblemsanddifficul5es

•  Oncewegothroughtheexamples,therearesomeexercisesforyoutoworkon.

•  Reference:hNps://root.cern.ch/root/htmldoc/guides/users-guide/ROOTUsersGuide.html

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Tree

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ROOTTree(TTreeclass)isdesignedtostorelargequan55esofsame-classobjects,andisop5mizedtoreducediskspaceandenhanceaccessspeed.Itcanholdallkindofdata,suchasobjectsorarrays,inaddi5ontosimpletypes.Anexampleforcrea5ngasimpleTree

Runthemacro:root-ltree_example1.C

MoreonTBranch

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•  TheclassforabranchiscalledTBranch

•  AvariableinaTBranchiscalledaleaf(TLeaf)

•  Iftwovariablesareindependenttheyshouldbeplacedonseparatebranches.However,iftheyarerelateditisefficienttoputthemonsamebranch

•  TTree::Branch()methodisusedtoaddaTBranchtoTTree

•  Thebranchtypeisdecidedbytheobjectstoredinit

•  Abranchcanholdanen5reobject,alistofsimplevariables,contentsofafolder,contentsofaTList,oranarrayofobjects.

Forexample:tree->Branch("Ev_Branch",&event,"temp/F:ntrack/I:nseg:nvtex:flag/i");

Where“event”isstructurewithonefloat,threeintegers,andoneunsignedinteger.

AccessingTTree

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Showanentry:root[]TFilef("myfile.root")root[]T->Show(10)======>EVENT:10px=0.680243py=0.198578pt=0.708635random=0.586894

PrinttheTreestructure:root[]T->Print()

**************************************************************Tree:T:simpletree**Entries:10000:Total=202834bytesFileSize=169658**::Treecompressionfactor=1.19***************************************************************Br0:px:px/F**Entries:10000:TotalSize=40594bytesFileSize=37262**Baskets:2:BasketSize=32000bytesCompression=1.08**............................................................................**Br1:py:py/F**Entries:10000:TotalSize=40594bytesFileSize=37265**Baskets:2:BasketSize=32000bytesCompression=1.08**............................................................................**Br2:pt:pt/F**Entries:10000:TotalSize=40594bytesFileSize=35874**Baskets:2:BasketSize=32000bytesCompression=1.12**............................................................................**Br3:random:random/D**Entries:10000:TotalSize=80696bytesFileSize=58600**Baskets:3:BasketSize=32000bytesCompression=1.37**............................................................................*

Scanafewvariables:

root[]T->Scan("px:py:pt")****************************************Row*px*py*pt*****************************************0*0.8966467*-1.712815*1.9333161**1*1.5702210*0.5797516*1.6738297**2*0.6975117*0.1442547*0.7122724**3*0.0616207*-1.009907*1.0117853**4*-0.054552*1.3832200*1.3842953**5*-2.017178*1.4682819*2.4949667**6*0.8903368*2.5101616*2.6633834**7*-1.098390*-0.318103*1.1435256**8*0.3865155*0.0235152*0.3872301**9*1.8970719*1.9546536*2.7238855**10*0.6802427*0.1985776*0.7086347

AccessingTTree

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OpenandDrawbranchesusingTBrowser:

UsingTTree::Draw()T->Draw("pt")

2Dplot:T->Draw(“py:px”)

SimpleAnalysisusingTTree::Draw()

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Applyingcuts:T->Draw(“pt”,“px>0&&py>0”)Forapplying“weights”tothedistribu5on:Selec5on=“weight*(booleanexpression)”Where“weight”isaevent-by-eventweightstoredasabranchinthentuple(e.g.crosssec5on*luminosityforthenormaliza5on)

UsingTCuts:TCutc1=“px>0”TCutc2=“py<0”TCutc3=c1&&c2T->Draw(“pt”,c3)T->Draw(“pt”,c1&&c2)T->Draw(“pt”,c1&&“py>0”)TCutc4=c1||c2T->Draw(“pt”,c4)T->Draw(“pt”,c1||c2)

FillingHistograms:TH1F*hist=newTH1F("hist","",100,0.,2.);T->Draw("pt>>hist");hist->SetLineColor(2);hist->Draw("same")

ASimpleAnalyzer

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Runthemacro:root-ltree_example2.C

Task:AddhistogramaNributes(axis5tle,linecoloretc..)

AddingBranchwithArrays

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RunMacro:root-ltree_example3.C

Seethecontents:

ReadingBranchwithArrays

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BranchwithVectors

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YougetonlypointerstovectorsinTTree::Show()methodTTree::Draw()worksonlyforbasictypevectors

Advantage:NoneedtodefinearrayswithMAXSIZE

Exercises

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•  Therearetworootfilesinyourtutorialdirectory:“ntuple_array.root”and“ntuple_vector.root”

•  TheycontaineventssimulatedwiththeprocessppàZàe+e-orµ+µ-

•  Thetreecontainsbranchesformomentum,par5cleIDetc..forgeneratedelectron/muon(branchnameslikegenPar5clePxetc..),momentum,iden5fica5on,isola5onvariablesetc..forreconstructedelectronsandmuons(branchnameslikemuonPx,electronPxetc..)

•  File“ntuple_array.root”containstreewithbranchesasvariablearrays•  File“ntuple_vector.root”containstreewithbranchesasvectors

•  Writeyourownmacro/code(twoseparatemacrofortwoinputfiles)toreadthetreesfromthefiles

•  Selecteventswhereyouhavetwogeneratedmuons(orelectrons)ofoppositecharge.Combinethemomentaofthegeneratedµ+µ-(e+e-)pairtogetthemomentumofZboson.PlotthemassofthegeneratedZboson

•  Selecteventswhereyouhavetworeconstructedmuons(orelectrons)ofoppositecharge.Combinethemomentaofthereconstructedµ+µ-(e+e-)pairtogetthemomentumofZboson.PlotthemassofthereconstructedZboson

BranchwithEventClass–I

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FirstwriteyourownEventclassinaseparateheaderfile.Anexample“MyEvent.h”:

Createasharedobjectfile,usingthecommandinrootprompt:.LMyEvent.h+Itwillcreatea.sofile,likeMyEvent_h.so,inthesamedirectory.Note:ItusesACLiCtogeneratedic5onariesOR,youcanusetherootcint.Forthat,usetheMakefilegiveninthetutorialdirectory.Justusethecommand“make”

BranchwithEventClass-II

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Torunthisprogram,youneedtofirstloadthelibrarycontainingtheclassdefini5on,asfollows:root–l.LlibMyEvent.so.Ltree_example8.Ctree_example8()

SeethetreecontentusingTTree::Show()andinTBrowser

BranchwithEventClass-III

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Runthisprogram,asfollows:root–l.LlibMyEvent.so.Ltree_example9.Ctree_example9()

Lookattheproducedplots.Doyougetthesamedistribu5onsasexample2?

BranchwithEventClass-IV

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Inthepreviousexample,youhaveoneobject(par5cle)perentry.However,younormallyhavemanypar5clesperevent.Inthiscase,youcanusevector<MyEvent>asthebranch.

Runthisprogram,asfollows:root–l.LlibMyEvent.so.Ltree_example10.Ctree_example10()

BranchwithEventClass-V

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Runthisprogram,asfollows:root–l.LlibMyEvent.so.Ltree_example11.Ctree_example11()

Lookattheproducedplots.Doyougetthesamedistribu5onsasexample7?

Exercises

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•  ReadtheTreefrom“ntuple_array.root”•  Writeyourownclassforeachpar5cletype,likeMyGenPar5cle.h,MyElectron.h,

MyMuon.h•  Createanewtreewithbrancheslikevector<MyGenPar5cle>,vector<myMuon>,

vector<MyElectron>•  Writetheeventsfrom“ntuple_array.root”tonewtreeandwritethenewtreeto

anewfile.

•  Readyournewtreefromthenewlycreatedfileanddothefollowingexercisesasbefore.

•  Selecteventswhereyouhavetwogeneratedmuons(orelectrons)ofoppositecharge.Combinethemomentaofthegeneratedµ+µ-(e+e-)pairtogetthemomentumofZboson.PlotthemassofthegeneratedZboson

•  Selecteventswhereyouhavetworeconstructedmuons(orelectrons)ofoppositecharge.Combinethemomentaofthereconstructedµ+µ-(e+e-)pairtogetthemomentumofZboson.PlotthemassofthereconstructedZboson

Analysisusing“MakeClass”method

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ItisausefulmethodprovidedbyROOTtododataanalysisusingTree.(Youcanalsolookat“MakeSelector”method,whichisnotdiscussedhere)

Forexample:produceatreeusingthetree_example8.Cprogram,tostartfromasimpleroneLoadthefileinrootandusefollowingcommand:root-lTFilef("myfile.root").lsT->MakeClass("MyClass")

ItcreatestwofilesMyClass.handMyClass.CEditMyClass.C(modifyingMyClass::Loop()func5on)toaddhistograms,doeventselec5ons,savethehistogramstofileThen,runtheprogramasfollowing:root–l.LMyClass.CMyClassmm.Loop()

Analysisusing“MakeClass”method

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voidMyClass::Loop(){//Add/defineoutputfile,histogramshere,like:TH2F*h_pxpy=newTH2F("h_pxpy","pyVspx",100,-2.0,2.0,100,-2.0,2.0);TH1F*h_pt=newTH1F("h_pt","pt",100,0.,5.0);if(fChain==0)return;Long64_tnentries=fChain->GetEntriesFast();Long64_tnbytes=0,nb=0;for(Long64_tjentry=0;jentry<nentries;jentry++){Long64_tientry=LoadTree(jentry);if(ientry<0)break;nb=fChain->GetEntry(jentry);nbytes+=nb;//if(Cut(ientry)<0)con5nue;//Addeventselec5onandhistogramfillinghereFloat_thPt=0;Int_th_index=-1;if(pt->size()>0){for(Int_tj=0;j<pt->size();j++){if((*pt)[j]>hPt){hPt=(*pt)[j];h_index=j;}}h_pxpy->Fill((*px)[h_index],(*py)[h_index]);h_pt->Fill((*pt)[h_index]);}}//Plotthehistogramshereh_pxpy->Draw("colz");}

LorentzVector

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ROOTprovidesgeneralfour-vectorclassesthatcanbeusedeitherforthedescrip5onofposi5onand5me(x,y,z,t)ormomentumandenergy(px,py,pz,E)

TLorentzVector•  Itprovidesageneralfourvectorclassthathasvariousmethodsto

ini5alize,setandgetvariousquan55esofafourvector(e.g.,pt,eta,phi,massetc..)

•  Performarithme5copera5ons,Lorentztransforma5ons•  Computeangularsepara5onbetweenfourvectors•  AsmallexampleofusingTLorentzVectorisgiveninnextslide•  Moredetails:hNps://root.cern.ch/doc/master/classTLorentzVector.html

Moreadvancedfour-vectorclassesareprovidedbyROOTMathCorelibrary:hNps://root.cern.ch/root/html518/MATHCORE_Index.htmlForexample:ROOT::Math::LorentzVector<ROOT::Math::PxPyPzE4D<double>>

ExampleforTLorentzVector

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ExampleforTLorentzVector

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Runthemacro:root-ltree_example12.C