Low-mass dileptons at HADES and CBM in a transport approach Janus - PowerPoint PPT Presentation

Low-mass dileptons at HADES and CBM in a transport approach Janus Weil FIAS with H. van Hees, U. Mosel, S. Endres, M. Bleicher CBM Collab. Meeting, 9. April 2014 Janus Weil Low-mass dileptons at HADES and CBM

Outline questions & topics to be addressed in this talk: what is so tough about dileptons at low energies? what is the current status w.r.t. the HADES results? how do transport models help to understand them? what are the limitations and challenges? what have we learned from HADES? what does it mean for CBM? Janus Weil Low-mass dileptons at HADES and CBM

Intro dileptons at high energies (NA60 etc): vacuum spectra dominated by mesons ρ gets broad in medium, coupling to baryons plays an important role at lower energies (HADES, DLS): baryons become more important (already in vacuum cocktail) bremsstrahlung, interference effects, ... how do baryons couple to em. sector? (how to describe R → e + e − N ?) baryon effects connect vacuum spectra at low energies to in-medium spectra at high energies! CBM bridges the energy region between SIS(18) and SPS, will be essential to create a consistent picture Janus Weil Low-mass dileptons at HADES and CBM

The GiBUU transport model hadronic transport model (microscopic, non-equilibrium) unified framework for various types of reactions ( γ A , eA , ν A , pA , π A , AA ) and observables BUU equ.: space-time evolution of phase-space density F (via gradient expansion from Kadanoff-Baym) ∂ ( p 0 − H ) ∂ F ( x , p ) − ∂ ( p 0 − H ) ∂ F ( x , p ) = C ( x , p ) ∂ x µ ∂ p µ ∂ p µ ∂ x µ Hamiltonian H : hadronic mean fields, Coulomb, “off-shell potential” collision term C ( x , p ): decays and collisions low energy: resonance model, high energy: string fragment. O. Buss et al., Phys. Rep. 512 (2012), http://gibuu.hepforge.org Janus Weil Low-mass dileptons at HADES and CBM

R → e + e − N : the ’traditional’ treatment R = ∆ , N ∗ , ∆ ∗ photon couplings ( R → γ N ) known from photoproduction experiments ( γ N → X ) extend to time-like region ( R → γ ∗ N ) via em. transition form factor (Wolf et al, Krivoruchenko et al.): ( m R + m N ) 2 d µ = 2 α d Γ α � ( m R − m N ) 2 − µ 2 � 3 / 2 | F ( µ, m R ) | 2 ( m R + m N ) 2 − µ 2 � m 3 R m 2 3 πµ 16 N problem: form factor basically unknown, often neglected but: surely contains some relevant physics! Janus Weil Low-mass dileptons at HADES and CBM

form factors electromagnetic N-∆ transition form factor only constrained by data in space-like region experimentally unknown in time-like region recent models: Wan/Iachello (red, IJMP A20, 2005), Ramalho/Pena (green, PRD85, 2012) no clear picture, large disagreements & uncertainties 3.029**2 10 2 W/I 1.23 W/I 1.43 W/I 1.63 W/I 1.83 W/I 2.03 R/P 1.23 10 1 R/P 1.43 |G M | 2 R/P 1.63 R/P 1.83 R/P 2.03 10 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 m ee [GeV] Janus Weil Low-mass dileptons at HADES and CBM

our approach first reasonable guess for form factor: vector-meson dominance! R → ρ N → e + e − N do decay in two steps: 1) R → ρ N , 2) ρ → e + e − transport-typical treatment: intermediate ρ is propagated, can rescatter etc assume strict VMD: R couples to γ ∗ only via ρ ! (probably not the final solution, but represents a simple hypothesis that can be tested by data) includes the full kinematics of the decay (important: m R dependence) Janus Weil Low-mass dileptons at HADES and CBM

our approach first reasonable guess for form factor: vector-meson dominance! R → ρ N → e + e − N do decay in two steps: 1) R → ρ N , 2) ρ → e + e − transport-typical treatment: intermediate ρ is propagated, can rescatter etc assume strict VMD: R couples to γ ∗ only via ρ ! (probably not the final solution, but represents a simple hypothesis that can be tested by data) includes the full kinematics of the decay (important: m R dependence) Janus Weil Low-mass dileptons at HADES and CBM

resonance model R → ρ N couplings taken from: Manley/Saleski, Phys. Rev. D 45 (1992) (just like all other resonance parameters and decay modes) PWA including π N → π N and π N → 2 π N data ρ ab ( m ) Γ R → ab ( m ) = Γ 0 R → ab ρ ab ( M 0 ) � b ) p ab d p 2 a d p 2 b A a ( p 2 a ) A b ( p 2 m B 2 L ab ( p ab R ) F 2 ρ ab ( m ) = ab ( m ) Janus Weil Low-mass dileptons at HADES and CBM

Delta ∆ → ρ N coupling can not be directly inferred from π N → 2 π N data ∆ is too light to decay into ρ N (on the mass shell) but: off-shell ∆ can decay into off-shell ρ this coupling can be important for dilepton spectra we introduce a p-wave decay with an (on-shell) BR of 5 · 10 − 5 ⇒ consistent model with (implicit) sVMD FF for all baryons p + p at 1.25 GeV off-shell ∆ decay width 10 0 data 10 1 GiBUU total π 0 → e + e - γ 10 -1 ∆ QED ∆ VMD N* VMD 10 0 10 -2 Brems. OBE d σ /dm ee [ µ b/GeV] 10 -3 Γ [GeV] 10 -1 10 -4 10 -5 10 -2 10 -6 total π N 10 -3 ( ρ N) P 10 -7 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 0 0.1 0.2 0.3 0.4 0.5 m [GeV] dilepton mass m ee [GeV] Janus Weil Low-mass dileptons at HADES and CBM

Resonances vs. Strings 60 data (tot.) N ∆ data (elast.) NR tot ∆∆ 50 NN ∆ R 40 σ pp [mb] 30 SIS18 SIS100 SIS300 20 10 0 2 3 4 5 6 7 8 sqrt(s) [GeV] resonance model can saturate total cross section up to √ s ≈ 3 . 4 GeV switch from resonance descr. to string model at that energy but: resonance effects might still be important above Janus Weil Low-mass dileptons at HADES and CBM

SIS18 is resonance land! highest HADES energy: pp at 3.5 GeV π N spectra show significant contributions of higher resonances ( N ∗ , ∆ ∗ ) arXiv:1403.3054 model 1 = “GiBUU-like”, model 2 = “UrQMD-like” Janus Weil Low-mass dileptons at HADES and CBM

HADES: elementary reactions 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 HADES data GiBUU total 10 1 10 1 ω → e + e - pp @ 1.25 GeV dp @ 1.25 GeV φ → e + e - ω → π 0 e + e - d σ /dm ee [ µ b/GeV] 10 0 10 0 π 0 → e + e - γ η → e + e - γ 10 -1 ∆ QED 10 -1 ∆ VMD N* VMD 10 -2 10 -2 ∆ * VMD Brems. OBE 10 -3 10 -3 10 -4 10 -4 10 1 10 1 pp @ 2.2 GeV pp @ 3.5 GeV d σ /dm ee [ µ b/GeV] 10 0 10 0 10 -1 10 -1 10 -2 10 -2 10 -3 10 -3 10 -4 10 -4 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 dilepton mass m ee [GeV] dilepton mass m ee [GeV] Janus Weil Low-mass dileptons at HADES and CBM

p T spectra at 3.5 GeV m < 150 MeV 150 MeV < m < 470 MeV 470 MeV < m < 700 MeV 700 MeV < m data 10 1 10 1 total ω → e + e - φ → e + e - ω → π 0 e + e - 10 0 10 0 d σ /dp T [ µ b/GeV] π 0 → e + e - γ η → e + e - γ ∆ QED 10 -1 10 -1 ∆ VMD N* VMD ∆ * VMD 10 -2 Brems. 10 -2 10 -3 10 -3 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 transverse momentum p T [GeV] VMD approach shifts p T spectra to lower values only way to obtain decent agreement with data solid confirmation of both VMD approach and importance of N ∗ contributions Janus Weil Low-mass dileptons at HADES and CBM

HADES: nucleus-nucleus collisions C + C @ 1.0 GeV C + C @ 2.0 GeV Ar + KCl @ 1.76 GeV 10 -2 10 -2 HADES data GiBUU total Res VMD 10 -3 10 -3 ω → e + e - φ → e + e - ω → π 0 e + e - π 0 → e + e - γ 10 -4 10 -4 η → e + e - γ 1/N π 0 dN/dm ee ∆ → Ne + e - pn Brems. 10 -5 10 -5 pp Brems. π N Brems. ππ 10 -6 10 -6 10 -7 10 -7 10 -8 10 -8 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 dilepton mass m ee [GeV] dilepton mass m ee [GeV] dilepton mass m ee [GeV] pure on-shell transport effects like rescattering, absorption and production kinematics included no explicit in-medium spectral functions Janus Weil Low-mass dileptons at HADES and CBM

lessons from HADES baryons play an important role at low energies! already in vacuum! VMD seems to be a good assumption for all baryons on-shell transport can capture a good part of the relevant physics in medium-sized systems also for Au+Au? remains to be seen ... Janus Weil Low-mass dileptons at HADES and CBM

moving to CBM central Au+Au: density evolution 10 25 GeV 8 central baryon density ρ/ρ 0 6 10 GeV 8 GeV 4 3.5 GeV 1.25 GeV 2 0 0 5 10 15 20 25 t [fm] density increases strongly but lifetime of dense phase decreases dileptons: vacuum should become simpler, but in-medium more interesting Janus Weil Low-mass dileptons at HADES and CBM

pp at 8/25 GeV p + p at 8.0 GeV p + p at 25 GeV 10 4 10 4 GiBUU total GiBUU total ρ → e + e - GiBUU total ρ → e + e - ω → e + e - 10 3 10 3 φ → e + e - Res ω → e + e - ω → π 0 e + e - φ → e + e - π 0 → e + e - γ 10 2 ω → π 0 e + e - 10 2 η → e + e - γ π 0 → e + e - γ ∆ → Ne + e - d σ /dm ee [ µ b/GeV] d σ /dm ee [ µ b/GeV] η → e + e - γ pp Brems ∆ QED 10 1 10 1 ∆ VMD pp Brems 10 0 10 0 10 -1 10 -1 10 -2 10 -2 10 -3 10 -3 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 0.2 0.4 0.6 0.8 1 1.2 1.4 dilepton mass m ee [GeV] dilepton mass m ee [GeV] at 8 GeV: some Res. effects in ρ production at higher energies probably negligible ∆: significant sensitivity to FF Janus Weil Low-mass dileptons at HADES and CBM