Measurement of p0 – induced single leptons in the HADES RICH T. Eberl, T. Christ, L. Fabbietti, J. Friese, R. Gernhäuser, J. Homolka, H.-J. Körner, M. Münch, B. Sailer, and S. Winkler for the HADES collaboration Motivation: Study of close e+e- - pairs RICH ring properties Comb. backgr. in e+e- - spectroscopy Exp. Setup HADES with low B-Field Results Opening angle distributions of identified close e+e- - pairs Properties of Cherenkov Rings !!! T. Eberl, TU München

Dilepton Spectroscopy HADES : Low mass e+e- pairs from p, p, A + A collisions Comb. Backgr. Suppression Close pair recognition !!!! & rejection 2 close rings in RICH 2 close tracks in MDC full scale HGEANT simulation T. Eberl, TU München

Sources of Close Rings & Tracks Physics processes : p0 - Dalitz , p0 gg g - conversion, compt. scatt. Possible combinations P1 P2 RICH MDC e+ e- 1 1 e+ e- 1 2 e+ e- 2 2 e+ e- 2 1 fake ? e+ e- 2 3 ?? ........... e+/- 1 1 p 1 1 fake ! p+/- 1 1 fake ! rare cases : p e+/- 1 2 p+/- e+/- 1 2 < Q > ~ 13 0 e+ g e- p0 Dalitz e- g interesting pair ??? e+ e- < Q > ~ 2 0 Conversion in Target & Radiator T. Eberl, TU München

Experimental Setup @ GSI, Darmstadt HADES with RICH, MDC 1 / 2 , META , low B – field optimized for e+/- with low p full reconstr. of low mass pairs Deflection in B - field DQ = QMDC – QMETA ~ 1 / P DQ DQ standard T. Eberl, TU München

Acceptance for p0-Dalitz leptons Simulation: p0-Dalitz decay HADES acceptance 3g-threshold of RICH T. Eberl, TU München

C + C @ E = 1A GeV Experiment @ SIS - GSI Numerical Experiment Ibeam = 106 / s, Targ: stot = 5% Trig: MMeta > 3; s/stot ~ 0,7 Nexp = 9 * 106 evts. Numerical Experiment (Simulation) UrQMD + HGEANT Setup as in exp. Trig: MMeta > 3; included Nsim = 7 * 106 evts. Analysis conditions: Event selection: MDC I/II : multiplicity Mtrack > 3 RICH : min. 1 Ring corr. with MDC & META Hit matching conditions: before B- field : DQMDC-RICH < 2° ; DFMDC-RICH < 5° after B- field : DQMDC-META : no ; DFMDC-META < 5° no TOF cut !! T. Eberl, TU München

e+/e- cand. for C + C @ 1A GeV Rings in RICH Defl. angles DQ = QMDC – QMETA of reconstructed tracks in RICH & MDC & META Single ?? e- e+ Double T. Eberl, TU München

p0-Dalitz decay leptons ?! Measured RICH Rings C + C E = 1A GeV Ring recognition: Pattern Matrix Hough-Transf. p0-Dalitz decay leptons ?! T. Eberl, TU München

e+ e- pairs Opening angle distr. in MDC (before B – field) Reasonable agreement !!! C + C E = 1A GeV Exp Sim (total) Counts [a.u.] p0 – Dalitz new spektrum !! Conversion not visible Conversion Sim. normalised to Exp. Opening angle [deg] T. Eberl, TU München

Ring selection for single electrons e+e- pairs with 1 track / ring Opening angle distr. In MDC (before B - field) Single rings ! e+ e- 2 MDC tracks 2 RICH rings T. Eberl, TU München

e+e- pairs with 2 tracks / ring Rings for close tracks Opening angle distr. in MDC (before B - field) e+e- pairs with 2 tracks / ring Double rings ! e+ e- 2 MDC tracks e+ e- 1 MDC track 1 RICH ring T. Eberl, TU München

Ring properties: charge & pads Charge per ring Pads per ring double double e+ e- Counts [a.u.] Counts [a.u.] C + C E = 1A GeV single single e+ e- Counts [a.u.] Counts [a.u.] Sim Exp Charge/ring [a.u.] Pads/ring T. Eberl, TU München

Ring properties: cluster & quality Clusters per ring Pattern Matrix sum double double e+ e- Counts [a.u.] Counts [a.u.] Lower right is wrong color C + C E = 1A GeV single single e+ e- Counts [a.u.] Counts [a.u.] Sim Exp clusters/ring matrix sum [a.u.] T. Eberl, TU München

Lepton recognition p0 e- GEANT Simulation: p0 -Dalitz decay min 1 lepton required ! META MDC p0 RICH e+ Momentum [MeV/c] Polar angle [deg] RICH Stat box !! MDC META HADES reconstruction probability for a p0-Dalitz decay lepton C+C, E=1AGeV, low field setup, cons. param. set T. Eberl, TU München

Pair recognition p0 e- 2 leptons required ! GEANT Simulation: RICH MDC META e+ p0 2 leptons required ! GEANT Simulation: p0-Dalitz decay Polar angle [deg] Momentum [MeV/c] Explanation: probability to correctly recognize a lep with a certain mom and emission angle If also the other one was correctly recognized HADES reconstruction probability for a lepton from an identified p0-Dalitz lepton pair C+C, E=1AGeV, low field setup T. Eberl, TU München

Summary Dilepton spectroscopy with HADES : Results from prel. analysis Measurement of close pairs for low momentum e+e- Good statistics of p0 – Dalitz and conversion leptons Experimental data for comb. backgr. and eff. studies Results from prel. analysis Ident. of rings from fully reconstr. single leptons in RICH Full scale simulation vs. experiment : ok Simulation: single lepton reconstr. prob. for p0 – Dalitz Meas. of key ring prop. for singles and doubles (for each sector) Outlook: Quantitative comb. backgr. studies Efficiencies for single lepton identification T. Eberl, TU München