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Di-lepton spectroscopy in CBM

Di-lepton spectroscopy in CBM. Claudia Höhne, GSI Darmstadt CBM collaboration. Outline. Introduction & motivation physics case of CBM dileptons at maximum baryon densities Detector concept of CBM overall concept dilepton measurement: electrons - muons Simulations

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Di-lepton spectroscopy in CBM

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  1. Di-lepton spectroscopy in CBM Claudia Höhne, GSI Darmstadt CBM collaboration

  2. Outline • Introduction & motivation • physics case of CBM • dileptons at maximum baryon densities • Detector concept of CBM • overall concept • dilepton measurement: electrons - muons • Simulations • detector performance & challenges • feasibility studies • Summary • CBM posters on Di-leptons • T. Galatyuk Di-electron spectroscopy in CBM • K. Antipin Systematic study of the optimization potential for di-lepton measurements in the CBM experiment • A. Kiseleva, P. Bhaduri Muon measurement with the CBM experiment at FAIR

  3. Physics case • with the CBM energy range we will reach • net baryon densities of (6-12) r0 • excitation energy densities e* of (0.8-6) GeV/fm3 for time spans of ~6 fm/c (e*=e-mNr) • → get access to the electromagnetic radiation from the fireball by the study of dileptons! • Compressed Baryonic Matter @ FAIR – high mB, moderate T: • searching for the landmarks of the QCD phase diagram • first order deconfinement phase transition • chiral phase transition (high baryon densities!) • QCD critical endpoint • in A+A collisions from 2-45 AGeV starting • in 2015 (CBM + HADES) • physics program complementary to RHIC, LHC • rare probes! (charm, dileptons) • (interaction rates up to 10 MHz!) [Andronic et al. Nucl. Phys. A 772, 167 (2006).

  4. r-meson spectral function  r  e+, μ+ e-, μ- "SPS" "FAIR" • r-meson couples to the medium: "melts" close to Tc and at high mB • vacuum lifetime t0 = 1.3 fm/c • dileptons = penetrating probe • connection to chiral symmetry restoration? • particular sensitive to baryon density n  p p ++ • illustrate sensitivity to modifications caused by the baryonic component of the medium: • r-meson spectral function weighted by 1/M to resemble the dilepton rate, Bose-factor will further amplify the low-mass part • m < 0.4 GeV/c2 of special interest! no measurement between 2-40 AGeV beam energy yet! [R. Rapp, priv. com. (CBM physics book)]

  5. Charm production at threshold • CBM will measure charm production at threshold • → after primordial production, the survival and momentum of the charm quarks depends on the interactions with the dense and hot medium! • → direct probe of the medium! • charmonium in hot and dense matter? • relation to deconfinement? • relation to open charm? [W. Cassing et al., Nucl. Phys. A 691 (2001) 753] HSD simulations no measurement of charmonium below 160 AGeV beam energy yet!

  6. Physics topics and Observables • The equation-of-state at high B • collective flow of hadrons • particle production at threshold energies (open charm) • Deconfinement phase transition at high B • excitation function and flow of strangeness (K, , , , ) • excitation function and flow of charm (J/ψ, ψ', D0, D, c) • charmonium suppression, sequential for J/ψ and ψ' ? • QCD critical endpoint • excitation function of event-by-event fluctuations (K/π,...) • Onset of chiral symmetry restoration at high B • in-medium modifications of hadrons (,, e+e-(μ+μ-), D) • mostly new measurements • CBM Physics Book (theory) in preparation

  7. The CBM experiment • tracking, momentum determination, vertex reconstruction: radiation hard silicon pixel/strip detectors (STS) in a magnetic dipole field • hadron ID: TOF (& RICH) • photons, p0, h: ECAL • PSD for event characterization • high speed DAQ and trigger → rare probes! • electron ID: RICH & TRD •  p suppression  104 • muon ID: absorber + detector layer sandwich •  move out absorbers for hadron runs ECAL TOF TRD RICH absorber + detectors magnet aim: optimize setup to include both, electron and muon ID STS + MVD

  8. STS tracking – heart of CBM Challenge: high track density  600 charged particles in  25o • Task • track reconstruction: • 0.1 GeV/c < p  10-12 GeV/c • Dp/p ~ 1% (p=1 GeV/c) • primary and secondary vertex reconstruction (resolution  50 mm) • V0 track pattern recognition silicon pixel and strip detectors add detectors for particle identification behind the STS → challenge for di-leptons! D+→ p+p+K- (ct = 312 mm) D0 → K-p+ (ct = 123 mm)

  9. Challenges of the di-electron measurement • clean electron identification (p suppression ≥ 104) • large background from physical sources • g-conversions in target and STS, p0 Dalitz decays • → use excellent tracking and two hit resolution (≤ 100 mm) in first pixel • detectors in order to reject this background: • → optimize detector setup (STS, B-field), use 1‰ interaction target RICH high ring densities and interaction rates → MAPMTs + fast self triggered read out electronics TRD high rates! → reduce gas gap prototype:double-sided pad plane

  10. Challenges of the di-muon measurement • major background from p,K decays into mn, punch through of hadrons and track mismatches • → use TOF information to reject punch through K,p • → compact layout to minimize K,p decays • → use excellent tracking to reject p,K decays in the STS by kink detection • → absorber-detector sandwich for continous tracking • low momentum m! 125 cm Fe ≡ 7.5 lI → p > 1.5 GeV/c 225 cm Fe ≡ 13.5 lI → p > 2.8 GeV/c

  11. Muon detector R&D • up to 1 hit/cm2 in first muon chambers! • high rate capability required! • detector technology still under discussion: Si-pad (first plane), Micromega, GEMs, ... first TGEM production and test at PNPI, St. Petersburg first double GEM under test at VECC, Kolkatta

  12. CbmRoot simulation framework • investigation of both options in detailed simulations: • detector simulation (GEANT3 implemented through VMC) • full event reconstruction: - track reconstruction, add RICH, TRD and TOF info • - tracking through the muon absorber • result from feasibility studies in the following: central Au+Au collisions at 25 AGeV beam energy (UrQMD)

  13. Low mass vector mesons 25 AGeV central AuAu All e+e- Comb. bg ρe+e-  e+e-φe+e- π0 γe+e-  π0e+e-ηγe+e- • invariant mass spectra • electrons: pt > 0.2 GeV/c • background dominated by physical sources (75%), 1‰ int. target • muons: intrinsic p>1.5 GeV cut (125 cm Fe absorber), • background dominated by misidentified muons, 1% int. target electrons: 200k events muons: 4 ∙108 events w,fsm = 14 MeV/c2 w,fsm = 11 MeV/c2

  14. Phase space coverage 25 AGeV central AuAu • r-meson • 25 AGeV beam energy: midrapidity = 2 • electrons: full coverage • muons: acceptance forward shifted, weak for low-pt • intrinsic p>1.5 GeV cut (125 cm Fe absorber) electrons muons

  15. Coverage in pt and minv 25 AGeV central AuAu • Dilepton pair coverage in pt and minv (signal pairs): • electrons: acceptance also for low pt and lowest masses (no pt-cut) • muons: cutoff at 2m threshold electrons muons

  16. J/y and y' 25 AGeV central AuAu • invariant mass spectra • electrons: p < 13 GeV/c, pt > 1.2 GeV, 1‰ interaction target (25 mm Au) • muons: 225 cm Fe absorber, pt > 1 GeV/c, 1% int. target muons: 3.8 ∙1010 events electrons: 4 ∙1010 events J/ysm = 27 MeV/c2 y' sm = 29 MeV/c2 J/ysm = 22 MeV/c2 y' sm = 23 MeV/c2

  17. Phase space coverage 25 AGeV central AuAu • J/y meson • 25 AGeV beam energy: midrapidity = 2 • full phase space well covered electrons muons

  18. Yields and S/B 25 AGeV central AuAu • S/B ratio in a 2s region around the peak, for r from 0.2-0.9 GeV/c2 • no trigger for low-mass vector mesons, a factor 10 maybe achievable for muons • trigger for J/y, rate in dielectron channel depends on interaction length of target (segmented target?) overall similar performance of electron and muon channel! minbias = 1/5 central central Au+Au, 25 AGeV

  19. Summary: Dileptons in CBM • dileptons are only one of several very interesting physics topics of CBM • CBM: comprehensive measurement of A+A interactions from 10-45 AGeV • including rare probes (charm, dileptons), flow, correlations, fluctuations • measurement of dileptons (low and high masses) very interesting at FAIR: • CBM: 10-45 AGeV, HADES 2-10 AGeV • highest baryon densities reached, phase border to partonic phase • restoration of chiral symmetry? critical point? • charm production at threshold? charm propagation in-medium? • dileptons from r to y' measurable in electron and muon channel • similar performance – although background is of very different origin • good phase-space coverage • low-mass dielectrons even down to lowest masses and pt • detector development started • CBM will (hopefully) 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!

  20. CBM collaboration China: CCNU Wuhan USTC Hefei Croatia: University of Split RBI, Zagreb Univ. Münster FZ Rossendorf GSI Darmstadt Korea: Korea Univ. Seoul Pusan National Univ. Russia: IHEP Protvino INR Troitzk ITEP Moscow KRI, St. Petersburg Hungaria: KFKI Budapest Eötvös Univ. Budapest Norway: Univ. Bergen Kurchatov Inst. Moscow LHE, JINR Dubna LPP, JINR Dubna Poland: Krakow Univ. Warsaw Univ. Silesia Univ. Katowice Nucl. Phys. Inst. Krakow Cyprus: Nikosia Univ. India: Aligarh Muslim Univ., Aligarh IOP Bhubaneswar Panjab Univ., Chandigarh Univ. Rajasthan, Jaipur Univ. Jammu, Jammu IIT Kharagpur SAHA Kolkata Univ Calcutta, Kolkata VECC Kolkata Univ. Kashmir, Srinagar Banaras Hindu Univ., Varanasi LIT, JINR Dubna MEPHI Moscow Obninsk State Univ. PNPI Gatchina SINP, Moscow State Univ. St. Petersburg Polytec. U. Czech Republic: CAS, Rez Techn. Univ. Prague France: IPHC Strasbourg Portugal: LIP Coimbra Romania: NIPNE Bucharest Germany: Univ. Heidelberg, Phys. Inst. Univ. HD, Kirchhoff Inst. Univ. Frankfurt Univ. Mannheim Ukraine: Shevchenko Univ. , Kiev 51 institutions, > 400 members Dresden, September 2007

  21. Backup

  22. Acceptance in pt and minv 25 AGeV central AuAu • pair detection probablitity/ efficiency versus invariant mass for different pt-bins: • electrons: acceptance also for low pt and lowest masses (no pt-cut) • muons: cutoff at 2m threshold (plot "hard-hard" and "hard-soft" pairs) electrons muons 0.2 GeV/c2

  23. Detector performance - muons • background tracks • punch through and track mismatches (p.K decay!)

  24. STS tracking - simulation 25 AGeV central AuAu • excellent track reconstruction, momentum resolution achieved • optimization of layout ongoing, material budget ≥ 3.2 mm Si equivalent • (x/X0≥ 3.4%) tracking efficiency momentum resolution

  25. Detector concept: electrons • RICH: • gaseous RICH detector, size still to be optimized • aim: "simple, compact and robust" • → N2 radiator, glass mirrors, MAPMT as photodetector • Nhits/ring = 22, N0 ~ 150 cm-1, <R>e = 6.2 cm, sR = 2.5% • ~ 90 rings per central collision, 25 AGeV Au+Au (occupancy ~2-4%) • TRD: • 3 x 4 TRD layers appr. at 4, 6, 8 m behind the target, fast gas detectors • also used as intermediate tracking detectors towards TOF • detector development builds upon knowledge gained from ALICE • TOF: • RPCs, 80ps time resolution

  26. Target – electrons • <p0> ~ 350 for 25 AGeV, central Au+Au collisions • p0→ gg (98.8%) • rejection only by opening angle • (difficult with magnetic field) • can be dominant background even for J/y • → use high quality, high intensity • beam from FAIR and work with • 1‰ interaction target! number of g-conversions versus target thickness • low-mass vector mesons: • no trigger possible • → higher rate in order to saturate DAQ • J/y: trigger required! • max. beam intensity 109 ions/s • → 1% target max. interaction rate 10 MHz • 1‰ 1MHz • or use segmented target 25 mm → ~3 e± per event 25 mm ≡ 1‰ interaction length

  27. Performance of combined e-ID • use TRD and TOF detectors for further electron identification • combined purity of identified electrons ~96% p [GeV/c] p [GeV/c]

  28. Detector performance - muons • suppression of low momentum muons! • → low-mass vector mesons: 125 cm Fe absorber: cutoff at 1.5 GeV/c "hard m" • 90 cm 1 GeV/c "soft m" • → charmonium: 225 cm Fe absorber 2.8 GeV/c • mismatches → include TOF information (distorted for background) • depending on detector layout, • tracking cuts: • ~ 0.4 identified m±/event • (125 cm Fe) • reconstruction efficiency for tracks • passing the absorber ~70% • (125 cm Fe) J/y w p p absorption of muons from different sources in dependence on absorber thickness (Fe)

  29. r-meson spectral function (II) "SPS" "FAIR" • illustrate sensitivity to modifications caused by the baryonic component of the medium: • r-meson spectral function weighted by a factor 1/M to resemble the dilepton rate, Bose-factor will further amplify the low-mass part • region with m < 0.4 GeV/c2 of special interest! [R. Rapp, priv. com. (CBM physics book)]

  30. r-meson spectral function  r  e+, μ+ e-, μ- • r-meson couples to the medium: "melts" close to Tc and at high mB • vacuum lifetime t0 = 1.3 fm/c • dileptons = penetrating probe • r-meson spectral function particular sensitive to baryon density • connection to chiral symmetry restoration? n  p p ++ [Rapp, Wambach, Adv. Nucl. Phys. 25 (2000) 1, hep-ph/9909229]

  31. Simulation of vector mesons • input: vector mesons generated with Pluto, embedded into central Au+Au collisions, 25 AGeV beam energy from UrQMD • full event reconstruction and particle identification, appr. realistic detectors descriptions: always work in progress! • 0 mass distribution generated including: • Breit – Wigner shape around the pole mass; • 1/M3, to account for vector dominance in the decay to e+e-; • Thermal phase space factor

  32. First test beam data → hit density? • crucial issue for Muon detectors • hit densities after absorbers? • (reliability of simulation?) • first results from p test beam (6 GeV/c) at CERN, PS on high granularity gas detectors (ALICE prototypes): ADC counts → increase hit density in GEANT 3 appr. by factor 2 without Pb converter with Pb converter (1.5 cm ~ 3X0)

  33. STS Hit Producers Hit Producers STT Dipole Map Dipole Map Pluto DPM TOF TOF Solenoid Map Active Map ECAL EMC Oracle Conf, Par, Geo const. field digitizers digitizers Urqmd EVT TRD MUO const. field MVD MVD Track finding Track finding ASCII ASCII ZDC TPC MUCH DIRC CBM Code Panda Code RICH DCH Geant3 ROOT G3VMC Geant4 G4VMC Geometry Virtual MC Close contact FlukaVMC FLUKA Root files Hits, Digits, Tracks some features Cuts, processes Application IO Manager Track propagation Run Manager RTDataBase Event Display Root files Conf, Par, Geo Event Generator Magnetic Field Always in close contact Detector base Tasks common developments

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