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Reconstruction of converted photons E. Tournefier LAPP meeting Feb. 2,2012

Reconstruction of converted photons E. Tournefier LAPP meeting Feb. 2,2012. Algorithm for the reconstruction of converted photons Performances studies: B s  and D 0 *  D 0  MC First look at real data: D 0 *  D 0 . Converted photons in LHCb.

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Reconstruction of converted photons E. Tournefier LAPP meeting Feb. 2,2012

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  1. Reconstruction of converted photonsE. TournefierLAPP meetingFeb. 2,2012 • Algorithm for the reconstruction of converted photons • Performances studies: Bs and D0*  D0  MC • First look at real data: D0*  D0 

  2. Converted photons in LHCb • 40% of the photons convert before the calorimeter (MC truth) • Out of which: • ~ 40% convertafter the magnet (z>8m) • 50% convertbefore the end of TT (z<3m)  we are trying to reconstructthoseones

  3. Reconstruction of converted photons 1/ Bremstrahlung recovery: Start from StdAllLoosePhotons with PhotonID> -0.25 Add photons which satisfy: (X and Y refer to position measured or extrapolated to CALO) - |Ye-Y| 3Y (the magnetic field is oriented along Y) - |X - Xtr|  3X : the photon should match the track extrapolation - Ptr<Ee+ E or Ptr  Ee + E - 2E :to make sure the track momentum was measured before brem 2/ Converted photon reconstruction: - select electrons with CALO information and CombDLLe  0. - make electron pairs which satisfy: |Ye--Ye+ |3 Y - do brem recovery making sure each photon is added only to e- OR to e+ - define converted photon CL as CL= CombDLLe1 x CombDLLe2 - combine the 2 electrons to make the photon * if one electron track has double charge in VELO (>1.5): redefine the photon direction with this track - select pairs with Mee  200 MeV and CL>10

  4. Performances studies with Bs: energy resolution • Bs MC11a (100k evts used here) Look only at events with E(MC electron)>2GeV so that it has a chance to reach ECAL Reconstructed inv mass Long tracks Down tracks MeV Z of  decay: Conv photon reconstructed Conv photon not reconstructed • Converted photon reconstructedwithM<200MeV: 43% • + with|Erec-EMC|/EMC<0.1: 30%

  5. Performances studies with Bs: angular resolution • Angular resolution: • 25% of the events with converted photons have 1 electron track with double VELO charge (>1.5): define photon direction with this track => Angular resolution improved by a factor 1.6 for these events • New algo • Old algo d = rec- MC

  6. Performances studies with Bs: efficiency • Reconstruction efficiency of converted photons drops at low Pt: the electrons don’t reach the ECAL Efficiency • Efficiency for all events • Efficiency for events with 1 electron E>2GeV Pt of photon Pt of electronfrom Pt • Not a bigproblem for Bs since the photons are quite energetic

  7. Performance studies: D0*  D0  • The aim is to check the performances on data on a channel with high statistics • Try with prompt D0*  D0  Problem: the photon has very low energy => efficiency drops: only few % at Pt ~500 MeV • Efficiency for all events • Efficiency for events with 1 electron E>2GeV Pt photon (MeV)

  8. First look at 2011 data: D0*  D0  1/ Try to reconstruct D0*  D0  with non converted photons (many thanks to Vincent for his code!!) 2/ with converted photons (10 times more data) dM=M (D0*)-M (D0) dM=M (D0*)-M (D0) Converted  Non converted  dM (MeV) dM (MeV) • In both cases I used photons with Pt>600 MeV • Given the (low) efficiency the number of eventswithconverted photons are as expectedfrom the non-converted case • =>Need to run on more data (I usedonly 1% of total sample)

  9. First look at 2011 data: D0*  D0  • Can roughly measure the efficiency on data: ~ok with MC • Efficiency from MC • Efficiency measured on data Pt photon (MeV) • Otheridea: look atB- D0* - as Vincent is doing • Thought the photon would be more energetic but this is not the case....

  10. Summary • Converted photons reconstruction: • some improvements for tracks with double VELO charge (angular resolution /1.6) • Studies with D0*  D0  : • Very low efficiency due to low energy photons • No mass peak seen for converted photons… need to run on larger data sample • Efficiency measured on Data seems ok with MC • Next: try to use low energy electrons (with RICH ID) Pb: most of these tracks are not reconstructed

  11. Converted photons with ‘RICH electrons’ • Try to reconstruct converted photons with 1 CALO electron and 1 non-CALO electron • Stastitics increased by 15% if one includes these events for Bs Minimum energy of the 2 electrons (MC)

  12. END

  13. Non-reconstructed photons • Many of the non-reconstructed events have at least 1 low energy electron • Many have large brem => the electron does not reach the ECAL => Upstream track sharing a VELO segment with other electron: not reconstructed • Try to reconstruct the Upstream tracks sharing the VELO segment with another track? • Need to use tracks without calorimetric information but with RICH1 PID Minimum energy of the 2 electrons (MC) • Converted photon not reconstructed • Converted photon reconstructed

  14. Track types • Conversion into VELO (z<80cm): • 2 Downstream tracks: 53% of events • 2 Long tracks: 24% most (75%) share the same VELO segment • 1 Long + 1 Downstream: 23% • The VELO segment is often lost in the reconstruction (likely due to bremstrahlung) VELO charge: case of 2 Long tracks

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