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Stripping: where we are

B->hh stripping selection update Angelo Carbone CP measurements WG: gamma with loops 19 th March 2009. Stripping: where we are. A pre-selection algorithm has been tuned based on the online selection Minimum bias rate 22 ±2 Hz after L0 trigger

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Stripping: where we are

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  1. B->hh stripping selection updateAngelo CarboneCP measurements WG: gamma with loops 19th March 2009

  2. Stripping: where we are • A pre-selection algorithm has been tuned • based on the online selection • Minimum bias rate 22±2 Hz after L0 trigger • Minimum bias rate 5±1 Hz after L0 and HLT1 trigger • Assumed 1MHz L0 output

  3. HLT1 vs offline selection • HLT1 rejects minimum bias events better then offline stripping, why? • HLT1 efficiencies as function of cuts have been investigated • Used Bd->pp signal events • HLT1 (all lines) selects 77% of offline signal selected events passed L0 trigger • 85% of them are from DiHadron lines • Let’s investigate this line

  4. HLT1 DiHadron algorithm • From Jose’s private e-mail, DiHadron algorithm selects: • a track with IP>0.1 mm PT>2.5 GeV and associated to a L0 hadron cluster with ET>3.5 GeV… • … that forms a secondary vertex with a second track if DOCA<0.2 mm • the second track should have an IP>0.1 mm and PT>1 GeV • The vertex mast be separated from the PV in z, with Z(SV-PV)>1.5 mm • A pointing cut • the transverse momentum of the B candidate in the plane perpendicular to the flight B direction given by the SV and the PV divided by the sum of the pt of the tracks < 0.4

  5. HLT1 investigation DOCA Invariant mass HLT1DiHadron effi. HLT1DiHadron effi. GeV IP B mm HLT1 efficiency = (offline sel. & L0 && HLT1DiHadronDecision)/ (offline sel. & L0) HLT1DiHadron effi. Flat distributions: no differences between DiHadron line and offline selection mm

  6. Distance of flight Signal offline selected and L0 HLT1DiHadron effi. Counts mm mm • DiHadron The vertex separated from the PV in z, Z(SV-PV)>1.5 mm • Offline selected and L0 signal distribution start at 1mm • Slightly inefficiency due to different cut value • The cut Z(SV-PV)>1.5 mm seems to be smeared by online resolution

  7. IP IP max IP min HLT1DiHadron effi. HLT1DiHadron effi. Flat efficiency mm mm Signal offline selected and L0 • HLT1 apply a cut IP>0.1mm to both tracks • Signal offline selected events start at 0.06 mm IP min Counts mm

  8. Transverse momentum PT min PT max HLT1DiHadron effi. HLT1DiHadron effi. GeV GeV • Offline selection cut • pT min > 1 GeV • pT max > 3 GeV • pT max lower efficiency between 3-5 GeV • This is due to HLT1 DiHadron L0 confirmation and a cut ET>3,5 GeV on the associated tracks to L0 hadron cluster. • pT min shows a slightly inefficiency around 2 GeV: to be investigated, any idea?

  9. Update on stripping selection • Towards a unified stripping selection for B->hh and Bs->mm channels • relax the pT cut • use DOCA instead of Chi2 on secondary vertex • Reject ghosts using chi2 of tracks • Move the lower limit of invariant mass to 4.8 GeV in order to better study B3 body bkg and include Bpp • Tight some cuts, i.e. IP of pions and B • IP on B cut (IP<0.1) was inherited from online selection, but it can be tighter thanks to better offline resolution

  10. Example of stripping cuts soft pre-selected mbias event Chi2 of tracks IP B • IPB<0.06 and Chi2(track)<5 will ensure high signal efficiency Arbitrary units ghosts chi2 mm Offline sel. & L0 signal

  11. Stripping selection

  12. Stripping selection Efficiency are relative to the previous step Minimum bias starting sample is done with a soft pre-selection

  13. Results • Ghost rate means at least one of the two tracks is a not associated using the Link associator in DaVinci • Signal eff: pre-selected events over offline selected and passed L0 (&HLT1)trigger • Mbias rate calculated assuming an ouput of 1MHz after L0

  14. Conclusion (I) • The HLT1 signal efficiency has been investigated in order to understand the reason of higher power of HLT1 to reject mbias events with respect to pre-selection • With respect to offline selection HLT1 Di Hadron line has • a high IP min cut on tracks • a high pT max cut (i.e. ET>3.5 GeV) • high distance of flight cut • The stripping selection algorithm has been update in order to match Bsmm analysis requirements and to reduce minimum bias rate • Rate reduced from 22Hz to 8Hz with signal eff. moving from 98% to 95% • Ghost rate reduced from 73% to 56%

  15. Conclusion (II) • Applying the HLT1 there still a reduction of minimum bias rate from 8Hz to 3Hz with a reduction from 56% to 33% of ghost rate • Pre-selection selects 43 events and 24 are ghosts  19 events are not ghosts • applying HLT1 we got 15 events and 5 ghosts  10 events are not ghosts • There are still room for improvement • Reducing and understand high ghost rate after pre-selection • suggestion?

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