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 (γA, eA, νA, pA, πA, AA) and observables BUU equ.: space-time evolution of phase-space density F (via gradient expansion from Kadanoff-Baym)

∂(p0−H) ∂pµ ∂F(x,p) ∂xµ

− ∂(p0−H)

∂xµ ∂F(x,p) ∂pµ

= C(x, p) 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 → γN) known from photoproduction experiments (γN → X) extend to time-like region (R → γ∗N) via em. transition form factor (Wolf et al, Krivoruchenko et al.):

dΓ dµ = 2α 3πµ α 16 (mR + mN)2 m3

Rm2 N

• (mR + mN)2 − µ2

(mR − mN)2 − µ23/2 |F(µ, mR)|2

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

100 101 102 0.2 0.4 0.6 0.8 1 1.2 1.4 |GM|2 mee [GeV] 3.029**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 R/P 1.43 R/P 1.63 R/P 1.83 R/P 2.03

Janus Weil Low-mass dileptons at HADES and CBM

• ur approach

first reasonable guess for form factor: vector-meson dominance! R → ρN → e+e−N do decay in two steps: 1) R → ρN, 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: mR dependence)

Janus Weil Low-mass dileptons at HADES and CBM

• ur approach

first reasonable guess for form factor: vector-meson dominance! R → ρN → e+e−N do decay in two steps: 1) R → ρN, 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: mR dependence)

Janus Weil Low-mass dileptons at HADES and CBM

resonance model

R → ρN couplings taken from: Manley/Saleski, Phys. Rev. D 45 (1992) (just like all other resonance parameters and decay modes) PWA including πN → πN and πN → 2πN data

ΓR→ab(m) = Γ0

R→ab

ρab(m) ρab(M0) ρab(m) =

• dp2

adp2 bAa(p2 a)Ab(p2 b)pab

m B2

Lab(pabR)F2 ab(m)

Janus Weil Low-mass dileptons at HADES and CBM

Delta

∆ → ρN coupling can not be directly inferred from πN → 2πN data ∆ is too light to decay into ρN (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

10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 Γ [GeV] m [GeV]

• ff-shell ∆ decay width

total πN (ρN)P

10-3 10-2 10-1 100 101 0.1 0.2 0.3 0.4 0.5 dσ/dmee [µb/GeV] dilepton mass mee [GeV] p + p at 1.25 GeV data GiBUU total π0 → e+e-γ ∆ QED ∆ VMD N* VMD

• Brems. OBE

Janus Weil Low-mass dileptons at HADES and CBM

Resonances vs. Strings

10 20 30 40 50 60 2 3 4 5 6 7 8 σpp [mb] sqrt(s) [GeV] SIS18 SIS100 SIS300 data (tot.) data (elast.) tot NN N∆ NR ∆∆ ∆R

resonance model can saturate total cross section up to √s ≈ 3.4GeV 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 πN 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

10-4 10-3 10-2 10-1 100 101 0.2 0.4 0.6 0.8 1 dσ/dmee [µb/GeV] pp @ 1.25 GeV HADES data GiBUU total ω → e+e- φ → e+e- ω → π0e+e- π0 → e+e-γ η → e+e-γ ∆ QED ∆ VMD N* VMD ∆* VMD

• Brems. OBE

10-4 10-3 10-2 10-1 100 101 0.2 0.4 0.6 0.8 1 dp @ 1.25 GeV 10-4 10-3 10-2 10-1 100 101 0.2 0.4 0.6 0.8 1 dσ/dmee [µb/GeV] dilepton mass mee [GeV] pp @ 2.2 GeV 0.2 0.4 0.6 0.8 1 10-4 10-3 10-2 10-1 100 101 dilepton mass mee [GeV] pp @ 3.5 GeV Janus Weil Low-mass dileptons at HADES and CBM

pT spectra at 3.5 GeV

10-3 10-2 10-1 100 101 0.2 0.4 0.6 0.8 1 dσ/dpT [µb/GeV] m < 150 MeV 0.2 0.4 0.6 0.8 1 transverse momentum pT [GeV] 150 MeV < m < 470 MeV 0.2 0.4 0.6 0.8 1 470 MeV < m < 700 MeV 0.2 0.4 0.6 0.8 1 10-3 10-2 10-1 100 101 700 MeV < m data total ω → e+e- φ → e+e- ω → π0e+e- π0 → e+e-γ η → e+e-γ ∆ QED ∆ VMD N* VMD ∆* VMD Brems.

VMD approach shifts pT spectra to lower values

• nly 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

10-8 10-7 10-6 10-5 10-4 10-3 10-2 0.2 0.4 0.6 0.8 1 1/Nπ0 dN/dmee dilepton mass mee [GeV] C + C @ 1.0 GeV HADES data GiBUU total Res VMD ω → e+e- φ → e+e- ω → π0e+e- π0 → e+e-γ η → e+e-γ ∆ → Ne+e- pn Brems. pp Brems. πN Brems. ππ 0.2 0.4 0.6 0.8 1 dilepton mass mee [GeV] C + C @ 2.0 GeV 0.2 0.4 0.6 0.8 1 10-8 10-7 10-6 10-5 10-4 10-3 10-2 dilepton mass mee [GeV] Ar + KCl @ 1.76 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

• n-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

2 4 6 8 10 5 10 15 20 25 central baryon density ρ/ρ0 t [fm] central Au+Au: density evolution 25 GeV 10 GeV 8 GeV 3.5 GeV 1.25 GeV

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

10-3 10-2 10-1 100 101 102 103 104 0.2 0.4 0.6 0.8 1 1.2 1.4 dσ/dmee [µb/GeV] dilepton mass mee [GeV] p + p at 8.0 GeV GiBUU total GiBUU total ρ → e+e- Res ω → e+e- φ → e+e- ω → π0e+e- π0 → e+e-γ η → e+e-γ ∆ QED ∆ VMD pp Brems 10-3 10-2 10-1 100 101 102 103 104 0.2 0.4 0.6 0.8 1 1.2 1.4 dσ/dmee [µb/GeV] dilepton mass mee [GeV] p + p at 25 GeV GiBUU total ρ → e+e- ω → e+e- φ → e+e- ω → π0e+e- π0 → e+e-γ η → e+e-γ ∆ → Ne+e- pp Brems

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

Au+Au at 8 AGeV

10-5 10-4 10-3 10-2 10-1 100 101 0.2 0.4 0.6 0.8 1 1.2 dN/dmee dilepton mass mee [GeV] GiBUU total π0 → e+e-γ η → e+e-γ ∆ → Ne+e- ρ → e+e- ππ → ρ R → ρN ω → e+e- ω → π0e+e- φ → e+e- πρ → φ

ρ mostly from ππ minor resonance effects some nontrivial effect in the φ (to be checked!)

Janus Weil Low-mass dileptons at HADES and CBM

Au+Au at 25 AGeV

10-5 10-4 10-3 10-2 10-1 100 101 0.2 0.4 0.6 0.8 1 1.2 dN/dmee dilepton mass mee [GeV] GiBUU total ρ → e+e- R → ρN ππ → ρ ω → e+e- πρ → φ φ → e+e- ω → π0e+e- π0 → e+e-γ η → e+e-γ ∆ → Ne+e-

somewhat similar to 8 GeV case again: just on-shell transport significant in-medium effects expected (see next talk)

Janus Weil Low-mass dileptons at HADES and CBM

Summary / Conclusions for CBM

baryonic resonances should play a lesser role in vacuum (still significant at SIS100?) but: expected to be more important in medium

• n-shell transport probably not sufficient to describe dileptons

consistent off-shell transport is very hard! coarse graining might be a good compromise (see next talk) ultimate goal: understand dilepton data in a wide energy range within one consistent framework CBM will provide crucial input we need open and transparent models for FAIR!

Janus Weil Low-mass dileptons at HADES and CBM