1. A Highly Selective Dilepton Trigger System Based on Ring Recognition Alberica Toia II Physikalisches Institut Justus-Liebig-Universität Gießen, Germany for the HADES Collaboration Overview HADES dilepton second level trigger: – The Matching Unit Performances of the Ring Recognition algorithm (J.Lehnert) – Identification of pair (investigation of close pairs) Opening angle distribution Properties of rings – Ratios online/offline Overview HADES dilepton second level trigger: – The Matching Unit Performances of the Ring Recognition algorithm (J.Lehnert) – Identification of pair (investigation of close pairs) Opening angle distribution Properties of rings – Ratios online/offline

2. Lepton Signatures The Matching Unit Combines electron signatures Determines p from  p (MeV/c) Momentum   Lepton selection The HADES magnet

3. Dilepton Selection Combines leptons into un-like sign pairs with a given opening angle and invariant mass range Invariant Mass m (MeV/c 2 ) Trigger condition% evts >= 1 online ring 10.5% >= 1 online lepton 2.9% >= 2 online rings 4.6% >= 2 online leptons 1.7% >= 1 online dilepton 0.3% Opening angle (°) Data: Nov01 C-C 1.5AGeV full field Opening Angle -> RICH is the most selective element of the second level trigger No Shower IPU included -> higher lepton reduction -> different acceptance for e + and e -

4. Analysis Strategy Which leptons are lost with a given trigger condition? -> need for a calibration reaction pp -> pp  Offline analysis RICH MDC TOF SHOWER Offline LEPTONS Found by online ??? Correlation of electron signatures from different detectors More complex algorithms than online are possible real leptons ??? Comparison of candidates found by online hardware algorithms with results from full offline analysis

5. Pairs of Rings x x x xx x x x x x Opening angle of online pairs Correlation with Mini Drift Chambers (MDC) fakes suppression position resolution x x IPU opening angle All rings found by online Fakes accompanying real rings Correlated with MDC local maximum needs to be extended to the 8 neighbouring pads ??? real rings main source of fake rings (~ 1 ring diameter) -> hint for smeared out rings (conversion pairs ???) unphysical distribution at large opening angle (phase space)

6. Matching Rings RICH MDC META high B field low B field  META - MDC p ~ 1/  META - MDC e-e- e+e+ positrons electrons MDC opening angle magnet coils Analysis steps: Before B field:-  RICH-MDC < 2° -   RICH-MDC < 5° After B field:-   MDC-META no cut -   MDC-META < 5° - 5 ns < tof < 8 ns -> best candidate selected Further coincidence with TOF / Shower („META“ - MultiplicityElectronTriggerArray )

7. „Singles“ still contaminated by unresolved close pairs Different behaviour of „singles“ and „doubles“ No clean discriminating criteria 1 ring matches 2 MDC („double“) 2 rings match 2 MDC („singles“) 1 ring matches 1 MDC cluster: unresolved close pair Properties of Rings # pads per ring doubles singles MDC opening angle all doubles singles

8. Comparison Online - Offline Online Ring correlated with MDC correlated with META Correlated Online Ring Online Pairs Online Ring correlated with MDC correlated with META Correlated Online Ring Offline Ring correlated with MDC correlated with META Correlated Offline Ring Offline Pairs Offline Ring correlated with MDC correlated with META Correlated Offline Ring PAIRS RATIO   (  PAIR ~  SINGLE LEPTON 2 ) : (Online AND Offline)/Online (Offline AND Online)/Offline as a function of opening angle in MDC

9. Singles found by offline Correlated with online 52% Doubles found by offline Correlated with online 72% Ratio of Pairs: online/offline Loss of pairs at ~ 4° <  < 10° (destructive interference of ring and veto region) Rings found by offline Correlated with online 66% opening angle Pair Ratio (  PAIR ~  SINGLE LEPTON 2 ) as a function of opening angle in MDC

10. Ratio of Pairs: offline/online Rings found by online Correlated with offline 66% Doubles found by online Correlated with offline 78% Singles found by online Correlated with offline 53%  online / offline ~  offline / online -> need for an absolute reference system, i.e.: pp -> pp  as a calibration reaction opening angle Pair Ratio (  PAIR ~  SINGLE LEPTON 2 ) as a function of opening angle in MDC

11. Online AND offline / offline Offline AND online / online Rings found by offline Correlated with online 66% Doubles found by offline Correlated with online 72% Singles found by offline Correlated with online 52% Singles found by online Correlated with offline 53% Doubles found by online Correlated with offline 78% Rings found by online Correlated with offline 66% opening angle

12. Simulation of Lepton Pairs 2 leptons per event shot from target 100 < p < 1000 MeV 25° <  < 40° 65° <  < 80° noise 1% opening angle  (deg) Single lepton efficiency  ASYMPTOTIC ~ 98%  ASYMPTOTIC ~ 90% --- stronger drop at 1° <  < 3° due to higher photon multiplicity (veto) --- lower saturation efficiency due to lower photon multiplicity

15. Summary and Outlook Alberica Toia Ingo Fröhlich, Adrian Gabriel, Daniel Kirschner, Wolfgang Kühn, Jörg Lehnert, Erik Lins, Markus Petri, Jim Ritman, Daniel Schäfer, Michael Traxler II. Physikalisches Institut Justus-Liebig-Universität Gießen, Germany Low magnetic field: possibility of resolving close pairs (conversion) better understanding of the ring properties Characterization of relevant figures of the online Ring Recognition Estimation and methods for: Single lepton efficiency Pair ratios online/offline and viceversa Reduction factor Characterization for different thresholds Full analysis of simulated data Lower limit for ring recognition efficiency Need for a calibration reaction Low magnetic field: possibility of resolving close pairs (conversion) better understanding of the ring properties Characterization of relevant figures of the online Ring Recognition Estimation and methods for: Single lepton efficiency Pair ratios online/offline and viceversa Reduction factor Characterization for different thresholds Full analysis of simulated data Lower limit for ring recognition efficiency Need for a calibration reaction