PiENu

The PIENU experiment at TRIUMF (Vancouver, Canada) aims at precisely measuring the pion decay branching ratio (π+ → eν)/(π →μν). The current experimental value for this ratio is 20 times less precise than the theoretical calculation done within the SM, leaving a large window in which non-standard physics - new interactions or exotic particles - can be involved. Or, on the other hand, improved precision that would confirm the SM would set tighter constraints on new physics scenarios.

PPE People

Currently, David Britton, DanProtopopescu and Ian Skillicorn are contributing to this experiment.

Shift Schedule

Available here: http://pienu02.triumf.ca/pienu_shifts/index.php

Documentation

• Official website at Triumf: http://pienu.triumf.ca/
• INPC 2010 talk by Chloé Malbrunot
• PIENU Proposal

Code compilation

The MC code was obtained from svn (pienu02.triumf.ca/pienumc/trunk) and unpacked on the UI. Specific versions of CLHEP (2.0.4.2) and Geant4 (4.9.0.3.p02) were installed in the user directory in ~/packages/. Detailed download and compilation instructions for CLHEP, Geant4 and the main pienu code were provided by Aleksey Sher via email (May 2011).

Our MC code version is slightly modified to record the angles and energies of the Bhabha-scattered e+ and e-. During our analysis we have discovered that a spurious production cut was forgotten in the code, and this was introducing, among others, incorrectly high (2 MeV) energy thresholds for the Bhabha-scattered electrons. This is now corrected everywhere.

Simulation and Reconstruction

Pienu or Pimunu events are simulated with pienu, and a Root output is produced. This output is subsequently processed with MC2Data, which does the reconstruction and produces a Root file with additional information. The reconstructed data is then analysed with various macros that were custom-written for our study.

Bhabha events tagging in MC

Bhabha scattering events are flagged in SteppingAction, by assigning to E_Bh the energy of the positron if the process involved at a certain step is "eIoni" and the volume where this occurs is "Target":

   if (theParticleName == "e+" 
     && thePostVolume == "/pienu/Target" 
     && theProcessName == "eIoni") {
    runAction->TgtBhabha(postEnergy);
    }

The energies and momenta of the initial positron and the outgoing positron and electron are recorded with the following conditionals:

    if(theParticleName == "e+"
       && thePostVolume == "/pienu/Target"
       && theProcessName == "eIoni") {
      runAction->PositronFromBhabha(postEnergy, postMomentum);
      runAction->PositronPreBhabha(preEnergy, preMomentum);  
    }

    if(theParticleName == "e-" 
       && thePostVolume == "/pienu/Target"
       && theCreatorProcessName == "eIoni"
       && theTrack->GetCurrentStepNumber()==1) {
      runAction->ElectronFromBhabha(prePosition, preEnergy, preMomentum);        
    }

The "eIoni" physics processes are implemented in module G4MollerBhabhaModel.cc from the Geant4 MC simulation package.

Events such tagged were studied to understand energy and angular distributions of the Bhabha-scattered e- and e+ and compare them with 'non-Bhabha' events.

Analysis

The first step was to validate our software by reproducing all plots done with MC from Chloe's thesis. The second step was to try to reproduce data results from Chloe's thesis with our MC (adding radiative effects, pileup etc. if necessary). We have satisfactorily achieved these steps.

In terms of understanding the energy and angular distributions of the Bhabha-scattered e- and e+, we have prepared a report (attached to this wiki) for the PiENu collaboration. Since the Bhabha correction to the tail correction is not directly measurable, the plan is to estimate it from the MC and then validate the MC by comparing a measurable Bhabha effect with data.

Some of the ROOT macros used are attached here.