1. 1 Lepton identification in CBM Tetyana Galatyuk for the CBM Collaboration Goethe-Universität, Frankfurt Outline: ✗ Dileptons as a probe for extreme matter ✗ Efficiency, purity and background rejection ✗ Performance studies ✗ Résumé

2. 2 D Searching for the landmarks of the phase diagram of matter SHM: P. Braun-Munzinger, K. Redlich,J. Stachel, nucl-th/0304013 J. Cleymans, K. Redlich, PRC 60 054908 lQCD: Z. Fodor et al., hep-lat/0402006, F. Karsch, QM04 : Schäfer, Wambach (priv. communication) Quark-Gluon Plasma Hadron gas Critical point? (Lattice QCD) Cross over LHC RHIC Early Universe Nuclei SIS AGS FAIR SPS Neutron Stars What do we know experimentally? ✗ chemical „freeze-out“ points obtained from a fit of the SHM to data. What does theory says? ✗ LQCD explores unknown regions along the temperature axis. ✗ QCD inspired effective models predict rich structure of phase diagram at finite  B : o Substantial depletion of the chiral over almost the full lifetime of the fireball. o Separation of the chiral from the deconfinement phase transition. o 1st-order transition with a critical end point Introduction

3. 3 D The SIS100/300 heavy-ion energy regime ✗ Probing nuclear matter at: o densities:  max /  0  10 o moderate temperature ✗ System stays above ground state density for ~5 fm/c  What are the best probes? I prefer penetrating probes. Evolution of net baryon density (  system ) Energy density (net baryon density) [CBM Physics Book (2009)] Introduction

4. 4 Electromagnetic structure of dense/hot matter ✗ Lepton pairs couple to hadrons through time-like virtual photons. ✗ The reconstruction of virtual photons via dileptons gives access to the electromagnetic properties of nuclear matter under extreme conditions. l+l+ l-l- Decays of (long-lived) neutral mesons (  ) Resonance decay Bremsstrahlung ** ** l+l+ l-l- ** l+l+ l-l- π+π+  π-π- e-e- e+e+   J P = 1 - for both  * and VM  Strong coupling of  * to VM (VMD model) Introduction

5. 5 The experimental challenge… ✗ Rare probes (BR < 10 -4 ) ✗ Large physical background in o e + e - from: - Dalitz decays (  0 ) - Conversion pairs      from: - Weak , K decays SIS RHIC/LHC SPS © BNL Introduction

6. 6 D e + e - pair yield after subtraction of the "hadronic cocktail“, Pb+Au, 158 AGeV Dilepton pair excess at SPS [D. Adamova et al. nucl-ex/0611022] [Rapp, Wambach EPJA 6 (1999) 415]  Main source:      e + e -  Strength of dilepton yield at low masses is due to coupling to baryons! π N -1 Δ > > N* N -1       pair yield after subtraction of the "hadronic cocktail“, In+In, 158 AGeV R. Rapp, CPOD’09, to be published on PoS Introduction

7. 7 D  Dalitz decays of baryonic resonances - dominant source at low beam energies.  Medium effects at moderate energies are closely linked to the effects at high beam energies through VMD.  Understanding these contributions is important for low-mass lepton pair program at FAIR.  Will be investigated with HADES.  Theoretical treatment: o Off-shell transport approach (HSD) o Hybrid approaches (?) Dilepton pair excess: from SPS to SIS… N*N*  N ** l+l+ l-l- e + e - from transport e + e - pair yield, Ar+KCl, 1.76 AGeV Preliminary Introduction

8. 8 D Searching for the landmarks of the phase diagram of matter   Highly interesting results from RHIC to SIS  importance of baryons!  No measurement for beam energies of 2-40 AGeV  Experimental focus on rare diagnostic probes: o Charm: J/ψ, ψ‘, D,  c for a complete picture)  how are the produced charm quarks propagating in the dense phase (J/ψ, ψ‘, D,  c for a complete picture) o Low-mass lepton pairs:  electromagnetic structure of hadrons,  emissivity of dense matter,  thermal radiation? Introduction

9. 9 Excitation function of excess yield from 8 to 45 GeV/u ✗ Accuracy ✗ Significance ✗ Systematic The CBM mission: D

10. 10 D D Strategy of CBM ✗ Fastest HI detector ever: more than one million reactions / second ✗ Fast high resolution tracking in a compact dipol field directly after the target ✗ High speed DAQ and trigger ✗ Excellent particle identification ✗ Flexible arrangement of PID detectors and calorimeters: o Aim: optimize setup to include both, electron and muon ID muons electrons The CBM mission

11. 11 D Di-eletron reconstruction in CBM: the challenge Electron option  Without hadron-blind detector before the tracknig  Background due to material budget of the STS  Sufficient  discrimination (misidentification <10 -4 ) ~350  0  98.8%   e + e -  1.2%  ~3  target  e + e - ~700  +/- could be identified as an electron zero impact parameter UrQMD: Au+Au collision at beam energy 25AGeV, zero impact parameter

12. 12 Electron identification Electron option RICH TRD TOF

13. 13 D D Electron identification in RICH ✗ RICH: strategy and R&D o Conventional design based on commercial products o Float glass mirror (carbon as backup) o Multi-anode PMT photodetector e +/-   Ring radius vs. momentum photo detector mirror Electron option

14. 14 D Electron identification using Time-Of-Flight information σ T =80 ps, σ x =5 mm RICH identified electrons in TOF Electron option ToF nicely works for p > 1 GeV/c! m 2 vs momentum distribution All tracks  supression factor

15. 15 D  TRD: strategy and R&D o Thin gap design based on ALICE TRD  3 TRD detectors, each consist of 4 layers Electron identification using TRD Use statistical analysis of the energy loss spectra to further discriminate  Electron option F  supr. = 9500  ~ 50%

16. 16 D The muon option ✗ Goal: o Clean dilepton signal for charm measurement and low-mass pairs ✗ Challenge:   at low energies! - Large energy loss and substantial multiple scattering of muons in the absorber o High areal particle rates in first detector. - smallest pad 2.8  2.8 mm 2 - 0.7 hit/cm 2  0.4  A/cm 2 (full intensity) ✗ Strategy: o Identification after hadron absorber with intermediate tracking layers o Detector technology still under discussion, probably combination of several depending on rates o Triple GEM detectors with pad read-out 20 20 20 30 35 100 cm 20 20 20 30 35 100 cm Fe Muon option

17. 17 D Detector performance: efficiency and purity Production vertex in z-direction of secondary muons reconstructed in the STS STS statistic: o 0.4% of all reconstructed tracks –  from weak decays (94% - , 6% - K weak decay) Muon option ✗ Reconstruction efficiency for tracks passing the absorber (225 cm Fe, “hard  ”) ~90% ✗ Only tracks with p > 3 GeV/c can be reconstructed! Track reconstruction efficiency as a function of momentum p ~ 3 GeV/c

18. 18 The Charm of CBM D

19. 19 D J/   m = 38 MeV/c 2 electrons Au+Au, 25 AGeV Reconstruction of J/  Invariant Mass Spectra ✗ Signal efficiency ~10% ✗ Signal-to-background ratio > 10 can be further improved if time-of-flight … and can be further improved if time-of-flight information is obtained for each track ■ with mass cut ■ no mass cut Au+Au, 35 AGeV muons Invariant mass spectra of tracks ID as electrons in RICH and TRD, p t >1.2 GeV/c Invariant mass spectra of tracks ID as muons in MuCh The Charm of CBM

20. 20 D Reconstruction of J/  Phase space coverage Full phase space very well covered! y CM muons electrons The Charm of CBM

21. 21 D The low-mass Signal in CBM MesonN/eventDecay modeBR  36  e + e -  5.×10 -3  23  e + e - 4.7×10 -5  38  e + e -  0  e + e - 7.7×10 -4 7.18×10 -5  1.28  e + e - 2.97×10 -4  0 mass distribution generated including: o Breit – Wigner shape around the pole mass; o 1/M 3, to account for vector dominance in the decay to l + l - ; o Thermal phase space factor; o Ansatz:  is governed by the  phase space. Invariant mass spectrum in 25 AGeV Au+Au collisions (full phase space, b=0) The Low-mass signal

22. 22 D Reconstruction of the Low-mass Signal : e + e - ✗ Reduction of physical background by reconstructing pairs from  -conversion (~3/event) and   -Dalitz decays (~8/event) by means of their track topology ✗ Transverse momentum cut of single electron – powerful, but has to be taken with special care! ✗ Pair cuts, i.e. opening angle cut  medium  e + e - eeee Track Segment Identified e +/- Track Fragment Fake pair π 0  γ e + e -   π 0 e + e - η  γ e + e - ρ  e+e-  e+e-φ  e+e-ρ  e+e-  e+e-φ  e+e-ρ  e+e-  e+e-φ  e+e-ρ  e+e-  e+e-φ  e+e- All e + e - CB Invariant mass spectra Before cuts After cuts Central Au+Au@25AGeV Electron option

23. 23 D Efficiency of cuts, S/B ratio : e + e -  0 Dalitz region Enhancement region  /  region ε S/B ratio  [%]  M [MeV]  0.46.713  0.329.413  -4.7 The Low-mass signal

24. 24 D Reconstruction of the Low-mass Signal :     signals  ρ  ω  φ  η  η Dalitz  background S/B ratio  [%]  M [MeV]  0.083.710  0.03612  0.0012.7 Major background from:  ,  decays into   punch through of hadrons  track mismatches Can performance be improve? Use TOF! Muon option

25. 25 Background rejection using TOF information MuCh TOF layer Nr.12 ToF RPC The Low-mass signal

26. 26 D Muon identification using TOF information  p S/B ratio  [%]  0.17 (0.08 * )1.5 (3.7 * )  0.06 (0.03 * )3. (6 * ) * - TOF information not used The Low-mass signal Work in progress

27. 27 D Reconstruction of Low-mass Sigal  Phase space coverage        0  e + e – y CM  Muons: use so called "hard * -hard„ and "soft ** -hard" pairs: the latter improve the acceptance towards midrapidity, however on account of a much higher background * - “hard ” – after 125 cm Fe ** - “soft ” – after 90 cm Fe  Electrons: no phase space limitation The “sweet spot” is at midrapidity and low pt! The Low-mass signal

28. 28 D Pair detection, with p t cut on single e +/- Self-consistent avarage spectral function of the  meson for  N =    N = 2     N = 5   Coverage in pair p t -m inv plane no p t -cut M2M2 muons electrons The Low-mass signal

29. 29 D Overview of existing dilepton experiments (summary) ExperimentSystem√s dN ch /dη ES/B Sys error (%) CERESPb+Au8.862165.91/620 CERES (σ/σ tot = 28% ) Pb+Au17.22452.311/1324 CERES (σ/σ tot = 7% ) Pb+Au17.23502.581/2116 NA60(central)In+In17.219331/1125 NA60(semi-central)In+In17.213321/825 NA60(semi-peripheral)In+In17.26321/312 NA60(peripheral)In+In17.2171.523 CERESS+Au19.512551/4.325 PHENIX(0-10% centrality)Au+Au200650?= 31/500?= 50 SIMULATION CBM (real) (b=0fm)Au+Au8300?1/16 * - ree cocktail only (without medium contribution) * - free cocktail only (without medium contribution) The Low-mass signal

30. 30 D Signal-to-Background ratio, Enhancement NA60 In+In @ 158 AGeV CERES Pb+Au @ 40 AGeV CERES Pb+Au @ 158 AGeV (σ/σ tot = 28%) CERES Pb+Au @ 158 AGeV (σ/σ tot = 7%) CERES Pb+Au @ 158 AGeV PHENIX Au+Au @ √s = 200 AGeV The Low-mass signal

31. 31 D Signal-to-Background ratio for CBM safety factor ;) The Low-mass signal

32. 32 D Résumé ✗ Both electron and muon option give access to low-mass vector mesons and charmonium They are still subject to further optimization! ✗ Feasibility studies are based on full event reconstruction and electron/muon identification. They are still subject to further optimization! ✗ Performance on low-mass vector mesons (at E kin = 25 GeV/u) is comparable, mid-rapidity coverage is more difficult for muons ✗ Performance on charmonium is similar for electrons and muons ✗ If we could achieve such results in reality – would be nice ! ✗ Electron measurements rely on established detector technology (RICH, TRD) ✗ Detector issues for muon measurements not yet solved ✗ CBM will not be limited by statistics, systematic uncertainties might be limiting in the end A measurement of both, muons and electrons will be the best systematic study we can ever do!

33. 33 China: Tsinghua Univ., Beijing USTC, Hefei CCNU, Wuhan Croatia: University of Split RBI, Zagreb Czech Republic: Techn. Univ., Prague CAS, Rez France: IPHC Strasbourg Germany: GSI, Darmstadt FZ Dresden-Rossendorf Univ. Heidelberg, Phys. Inst. Univ. HD, Kirchhoff Inst. Univ. Heidelberg, ZITI The CBM Collaboration Univ. of Kashmir, Srinagar Banaras Hindu Univ., Varanasi Korea: Korea Univ. Seoul Pusan National Univ. Norway: University of Bergen Poland: Silesia Univ. Katowice AGH Univ. Krakow Jagiellonian Univ., Krakow Warsaw Univ. Portugal: LIP Coimbra Romania: NIPNE, Bucharest Bucharest University Univ. Frankfurt, IKF Univ. Frankfurt, Inst.Comp.Sc. Univ. Münster Univ. Wuppertal Hungaria: KFKI, Budapest Eötvös Univ. Budapest India: Aligarh Muslim Univ., Aligarh IOP, Bhubaneswar Panjab Univ., Chandigarh Gauhati Univ., Guwahati Univ. of Rajasthan, Jaipur Univ. of Jammu, Jammu IIT, Kharagpur SAHA, Kolkata Univ. of Calcutta, Kolkata VECC, Kolkata Russia: VBLHE, JINR, Dubna LIT, JINR, Dubna LPP, JINR, Dubna PNPI, Gatchina ITEP, Moscow MEPhI, Moscow Kurchatov Inst. Moscow SINP, Moscow State Univ. Obninsk State Univ. IHEP, Protvino KRI, St. Petersburg St. Petersburg Polytec. U. INR Troitzk Ukraine: INR, Kiev Shevchenko Univ., Kiev Split, 2009 56 institutions > 400 members

34. 34 Thank you D