Dilepton measurements in heavy ion collisions: fixed-target versus collider experiments 1. Experimental setups 2. Multiplicities 3. Luminosities 4. Rates

Dilepton sources in Heavy-ion Collisions

Single electron spectra central Au+Au collisions 25 AGeV Background sources 1. external pair conversion:   e + e - 2.Dalitz-decays:  0   e + e - (BR = 1.2·10 -2 )    e + e - (BR = 4.9·10 -3 ) 3. Bremsstrahlung: pn  pn e + e - 4. misidentified pions Background in muon measurements: π→μν, K→μν    μ + μ - (can be determined by   μ + μ - )

Acceptance for e + e - pairs: 0.3% Massresolution:  m/m = 10% G. Roche et al., Phys. Lett. B 226 (1989) 228 The pioneering experiment: DLS at the Bevalac

Bestimmung der Teilchen- Geschwindigkeit durch Messung von θ (Ringradius des Lichtkegels) Ring Imaging Cherenkov detector (RICH) cosθ α = 1/(βn)

DLS-data: R.J. Porter et al.: Phys. Rev. Lett. 79 (1997) 1229 BUU calculation: E.L. Bratkovskaya et al.: Nucl. Phys. A634 (1998) 168 DLS data

HADES at GSI

HADES

CERES/NA45 at SPS

Electron-positron pairs from CERES CERES 2000: 159 AGeV Pb+Au beam intensity: 10 6 ions / spill 1 spill = 4 s beam and 15 s pause targets: 13 x 25 μm Au ( ~ 1 % interaction) trigger: 8% most central Event rate = 470 / spill (~ 25 Hz = 15 Mio events/week)

Low mass vector mesons (CERES/CERN) D.Adamova et al., PRL 91 (2003) Calculations by R. Rapp: thick dashed line: unmodified rho thick dashed-dotted line: in-medium dropping rho mass thick solid line: in-medium spread rho width Data: ~ 180 signal pairs

hadron absorber and trackingmuon trigger magnetic field iron wall muon other tracks Muon identification: NA38/50/60 targets Concept of NA60: place a silicon tracking telescope in the vertex region to measure the muons before they suffer multiple scattering in the absorber and match them to the tracks measured in the muon spectrometer  Improved kinematics; dimuon mass resolution at the  : ~20 MeV/c 2 (instead of 80 MeV/c 2 in NA50) Origin of muons can be accurately determined 2.5 T dipole magnet beam tracker vertex tracker

Dimuon pairs measured by NA60 (CERN) 5-week-long run in Oct.–Nov ~ 4 × ions delivered in total signal pairs In+In 158 AGeV

s NN = (E 1 + E 2 ) 2 – (p 1 + p 2 ) 2 collider: p 1 + p 2 = 0 →  s NN = E 1 + E 2 fixed target: E 2 = m, p 2 = 0 s NN = (E kin + 2m) 2 – p 1 2 s NN = 2m·(E kin + 2m) for E kin >> m :  s NN = 1.4·  E kin

PHENIX Physics Capabilities 2 central arms: electrons, photons, hadrons –charmonium J/ ,  ’  e  e  –vector meson   e  e  –high p T       –direct photons –open charm –hadron physics 2 muon arms: muons –“onium” J/ ,  ’,      –vector meson      –open charm combined central and muon arms: charm production DD  e  global detectors forward energy and multiplicity –event characterization designed to measure rare probes: + high rate capability & granularity + good mass resolution and particle ID - limited acceptance Au-Au & p-p spin PC1 PC3 DC magnetic field & tracking detectors e+e+ ee  

PHENIX data Data absolutely normalized Cocktail filtered in PHENIX acceptance Charm from –PYTHIA –Single electron non photonic spectrum w/o angular correlations  c = N coll x 567±57±193  b submitted to Phys. Rev. Lett arXiv: Low-Mass Continuum: enhancement 150 <m ee <750 MeV: 3.4±0.2(stat.) ±1.3(syst.)±0.7(model) Intermediate-Mass Continuum: Single-e  pt suppression & non-zero v2: charm thermalized? PYTHIA single-e p T spectra softer than p+p but coincide with Au+Au Angular correlations unknown Room for thermal contribution?

CERN and the Large Hadron Collider (LHC)

The ALICE experiment at CERN

Transition radiation Total energy  γ Θ = 1 /γ

Transition Radiation Detectors (TRD) p = 1 GeV/c γ e = 2000 γ  = 7.1

storage and cooler rings beams of rare isotopes e – A Collider stored and cooled antiprotons GeV primary beams 5x10 11 /s; GeV/u; 238 U 28+ factor increased intensity 4x10 13 /s 90 GeV protons /s 238 U 35 GeV/u ( Ni 45 GeV/u) secondary beams rare isotopes GeV/u; factor increased intensity antiprotons 3(0) - 30 GeV accelerator technical challenges Rapidly cycling superconducting magnets high energy electron cooling dynamical vacuum, beam losses Facility for Antiproton and Ion Research (FAIR)

Dipol magnet The Compressed Baryonic Matter Experiment Ring Imaging Cherenkov Detector Transition Radiation Detectors Resistive Plate Chambers (TOF) ECAL Silicon Tracking Station Tracking Detector Muon detection System

Electron identification with RICH and TRD Cherenkov ring radius (cm) RICH TRD

Critical endpoint: Z. Fodor, S. Katz, hep-lat/ S. Ejiri et al., hep-lat/ crossover at small μ B ε =0.5 GeV/fm 3 first order phase transition baryon density:  B  4 ( mT/2  ) 3/2 x [exp((  B -m)/T) - exp((-  B -m)/T)] baryons - antibaryons FAIR/NICA GSI SPS RHIC LHC Mapping the QCD phase diagram with heavy-ion collisions lattice QCD ? Recent L QCD calculations: T C = MeV

GSI Meson production in central Au+Au collisions W. Cassing, E. Bratkovskaya, A. Sibirtsev, Nucl. Phys. A 691 (2001) 745

J/ψρωφ multiplicity2· BR (→μμ) · · ·10 -5 μμ multiplicity 1.2· · · ·10 -4 μμ min bias 3· · · ·10 -5 Vector meson yields for central Au+Au collisions at  s NN = 7.1 GeV (25 AGeV)

N 1, N 2 = beam particles per bunch B = number of bunch crossings per sec F = beam size in cm 2 Typical numbers: N 1 = N 2 = 10 9 B = 10 6 → L = cm -2 s -1 F = cm 2 Reaktion rate R = L · σ σ = reaction cross section σ =  · (2 ·R) 2 = 4  ·(r 0 ·A 1/3 ) 2 with r 0 =1.2 fm Au+Au collisions: A=197  σ = 6 barn, 1 barn = cm 2 Collider reaction rates for Au+Au: R = cm -2 s -1 · 6· cm 2 = 6000 s -1 Collider Luminosity: L = N 1 ·N 2 ·B / F [cm -2 s -1 ]

N B = beam particles/sec N T /F = target atoms/cm 2 = N A ·  ·d/A with Avogadros Number N A = 6.02·10 23 · mol -1, material density  [g/cm 3 ], target thickness d [cm] atomic number A Typical numbers: N B = 10 9 s -1 Au target:  = 19.3 g/cm 3, A = 197 d = 0.3 mm (1% interaction rate) L = 1.8·10 30 cm -2 s -1 Fixed target reaction rates for Au+Au: R = L · σ = 1.8·10 30 cm -2 s -1 · 6· cm 2 = 10 7 s -1 Fixed target Luminosity: L = N B ·N T / F [cm -2 s -1 ]

Acceptances and Efficiencies  =   ·   p ·  Det ·  Trigg ·  DAQ ·  analysis with   = angular acceptance   p = momentum acceptance  Det = detector efficiencies  Trigg = trigger efficiencies  DAQ = dead time correction of DAQ  analysis = efficiency of analysis (track finding, cuts for background suppression,...) Typical values:    0.5,   p  0.8,  Det  0.9,  Trigg  0.9,  DAQ  0.5,  analysis  0.3,   0.05

Low-energy RHIC run at  s NN = 9 GeV peak luminosity ~ 2 ·10 23 cm -2 s -1 Reaction rate Au+Au ~ 1 Hz further reduction: average luminosity, large diamond improvement by upgrades incl. e - cooling NICA collider luminosity design value ~ 1 ·10 27 cm -2 s -1

Expected dilepton yields for minimum bias Au+Au collisions at  s NN = 7.1 GeV (25 AGeV) Assumption: experimental efficiency ε = 10 % Multiplicity of J/ψ: M·ε = 3·10 -8 Multiplicity of ω: M·ε = 8·10 -5 Collider reaction rate 100 s -1 Yield of J/ψ: 3·10 -8 ·100 s -1 = 3·10 -6 s -1 = 1.1·10 -2 h -1 = 19 in 10 weeks Yield of ω: 8·10 -5 ·100 s -1 = 8·10 -3 s -1 = 29 h -1 = in 10 weeks Fixed target reaction rates: 10 7 s -1 with J/ψ trigger: 1.9 ·10 6 J/ψ in 10 weeks 10 5 s -1 without trigger: Yield of ω: 5·10 7 in 10 weeks