Exercises linked to DaVinci session 1

The purpose of these exercises is to allow you to write a complete though simple selection algorithms for a typical decay: Bs->J/psiPhi. We start with a very simple algorithm that looks at muons and fills some histograms.

1. Setting up the environment

... You should already have done that all

Choose the DaVinci environment

cd $HOME/cmtuser/
DaVinciEnv v17r5 ;  getpack Tutorial/Analysis v6r1 ; getpack Phys/DaVinci v17r5 cd Phys/DaVinci/v17r5/cmt
emacs requirements

add the following line

use Analysis v* Tutorial

Then go ahead

cmt config
cmt br make
source setup.csh
cd $ANALYSISROOT

2. Start to write the options

Open a new file in the options directory and write

#include "$DAVINCIROOT/options/DaVinciCommon.opts"
#include "$ANALYSISROOT/options/BolognaOptions.opts"
ApplicationMgr.DLLs += { "Analysis" };// Don't forget the DLL
ApplicationMgr.TopAlg += { "GaudiSequencer/TutorialSeq" };
TutorialSeq.Members += { "TutorialAlgorithm" };

2. Start to write the algorithm

We will make a small algorithm that loops over muons and plots some variables.

Open a file

emacs src/TutorialAlgorithm.{cpp,h} &

Answer "D" for DVAlgorithm twice

Before you forget it, declare it to Analysis_load.cpp:

DECLARE_ALGORITHM(TutorialAlgorithm)

Now let's add something to the execute method:

StatusCode TutorialAlgorithm::execute() {
  debug() << "==> Execute" << endmsg;
  StatusCode sc = StatusCode::SUCCESS ;

  // code goes here  
  LHCb::Particle::ConstVector muons = desktop()->particles(); 
  sc = loopOnMuons(muons);
  if (!sc) return sc;

  setFilterPassed(true);   // Set to true if event is accepted.
  return StatusCode::SUCCESS;
}

Here we get the particles from the hysDesktop tool and added a method that will do the actual work.

Declare the method in the header file:

private:
  StatusCode loopOnMuons(const LHCb::Particle::ConstVector&)const ;

Create it in the cpp file:

//====================================================
// loop on muons
//====================================================
StatusCode TutorialAlgorithm::loopOnMuons(
      const LHCb::Particle::ConstVector& muons)const {

  StatusCode sc = StatusCode::SUCCESS ;
  
  // code goes here
  
  return sc ;
}

In the method add:

  for ( LHCb::Particle::ConstVector::const_iterator im =  muons.begin() ;
        im != muons.end() ; ++im ){
    plot((*im)->p(),  "Muon P",  0., 50.*Gaudi::Units::GeV);  // momentum
    plot((*im)->pt(), "Muon Pt", 0., 5.*Gaudi::Units::GeV );  // Pt
    debug() << "Mu Momentum: " << (*im)->momentum() << endmsg ;
  }

Now get the primary vertices (outside the muons loop):

LHCb::RecVertex::ConstVector pvs = desktop()->primaryVertices();

And loop over them (inside...):

    for ( LHCb::RecVertex::ConstVector::const_iterator ipv = 
          pvs.begin() ; ipv != pvs.end() ; ++ipv ){
      double IP, IPE;
      debug() << (*ipv)->position() << endmsg ;
      sc = geomDispCalculator()->calcImpactPar(*(*im), *(*ipv), IP, IPE);
      if (sc){
        plot(IP, "Muon IP", 0., 10.*Gaudi::Units::mm);
        if (IPE>0.) plot(IP/IPE, "Muon IP/error", 0., 10.);
      } 
    }

3. Run!

First we need some data

We need to tell the algorithm what particles to use:

#include "$DAVINCITUTORIALDATA"

Define the rules for creation of "Standard Particles" #include "$COMMONPARTICLESROOT/options/StandardMuons.opts"
We need to tell the algorithm what particles to use:

TutorialAlgorithm.PhysDesktop.InputLocations = { "Phys/StdLooseMuons" } ;

The PhysDesktop will automatically add the /Event/.

We want to save the histograms, so add the following line to your options:

HistogramPersistencySvc.OutputFile = "DVHistos.root";

Now run:

DaVinci $ANAYSISROOT/options/MyOptions.opts

4. Look at the histograms

Open root

root.exe

This will get you the version of root used by DaVinci.

Note that

root

sometimes also works, but sometimes doesn't...

The simplest way to look at histograms is to use the TBrowser

TBrowser B

Which has a windows-like look and feel.

But if you just want to save the histograms, you could as well just paste the following in the root prompt

TCanvas* c1 = new TCanvas("c1","Bologna",800,800);
c1->SetLogy();

TFile* F = new TFile("DVHistos.root") 
F->ls()
F->cd("TutorialAlgorithm")               
F->ls() 
TH1D* H1 = F->Get("TutorialAlgorithm/1")
TH1D* H2 = F->Get("TutorialAlgorithm/2")
TH1D* H3 = F->Get("TutorialAlgorithm/3")
TH1D* H4 = F->Get("TutorialAlgorithm/4")

H1->SetLineColor(2)
H1->SetLineWidth(3)
H1->Draw()
gPad->SaveAs("H1.eps");

H2->SetLineColor(2)
H2->SetLineWidth(3)
H2->Draw() gPad->SaveAs("H2.eps");

H3->SetLineColor(2)
H3->SetLineWidth(3)
H3->Draw() gPad->SaveAs("H3.eps");

H4->SetLineColor(2)
H4->SetLineWidth(3)
H4->Draw() gPad->SaveAs("H4.eps");

For sure one could do this shorter.

To quit root:

.q