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  1

Converted photons in LHCb 40% of the photons convert before the calorimeter Out of which: ~ 40% convert after the magnet (z>8m) 50% convert before the end of TT (z<3m)  we are trying to reconstruct those ones (MC truth) 2

Reconstruction of converted photons 1/ Bremstrahlung recovery: Start from StdAllLoosePhotons with PhotonID> 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: |Y e- -Y e+ |  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 3

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

Performances studies with B s  : 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 5 -New algo -Old algo d  =  rec -  MC

Performances studies with B s  : efficiency Reconstruction efficiency of converted photons drops at low Pt: the electrons don’t reach the ECAL 6 Pt  Efficiency Pt of photon Pt of electron from  Not a big problem for B s  since the photons are quite energetic -Efficiency for all events -Efficiency for events with 1 electron E>2GeV

Performance studies: D 0 *  D 0  The aim is to check the performances on data on a channel with high statistics  Try with prompt D 0 *  D 0  Problem: the photon has very low energy => efficiency drops: only few % at Pt ~500 MeV 7 -Efficiency for all events -Efficiency for events with 1 electron E>2GeV Pt photon (MeV)

First look at 2011 data: D 0 *  D 0  1/ Try to reconstruct D 0 *  D 0  with non converted photons (many thanks to Vincent for his code!!) 2/ with converted photons (10 times more data) 8 dM=M (D 0 *)-M (D 0 ) dM (MeV) dM=M (D 0 *)-M (D 0 ) Non converted  Converted  dM (MeV) -In both cases I used photons with Pt>600 MeV -Given the (low) efficiency the number of events with converted photons are as expected from the non-converted case =>Need to run on more data (I used only 1% of total sample)

First look at 2011 data: D 0 *  D 0  Can roughly measure the efficiency on data: ~ok with MC 9 Pt photon (MeV) -Efficiency from MC -Efficiency measured on data Other idea: look at B-  D 0 *  - as Vincent is doing Thought the photon would be more energetic but this is not the case....

Summary Converted photons reconstruction: –some improvements for tracks with double VELO charge (angular resolution /1.6) –Studies with D 0 *  D 0  : 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 10

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 B s  11 Minimum energy of the 2 electrons (MC)

END 12

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 - Converted photon not reconstructed - Converted photon reconstructed Minimum energy of the 2 electrons (MC) 13

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 14