Dilepton production at SIS energieswith the GiBUU transport model
Janus Weil, Ulrich Mosel
Institut fur Theoretische Physik, JLU Giessen
DPG FruhjahrstagungMainz, 19.03.2012
J. Weil (JLU Giessen) Dilepton production at SIS energies
Outline
1 motivation: in-medium physics
2 the GiBUU transport model
3 dileptons from elementary reactionsp + p, d + pp + Nb
see arXiv:1203.3557 [nucl-th]
4 outlook: heavy-ion collisionsC + CAr + KCl
5 conclusions
J. Weil (JLU Giessen) Dilepton production at SIS energies
Motivation: Hadrons in Medium
how do vector mesons behave inside ahadronic medium?
Brown/Rho, Hatsuda/Lee:mass shift (restoration of chiral sym.)m∗
V (ρ)/mV ≈ 1− α(ρ/ρ0) ,α ≈ 0.16± 0.06
collisional broadening (LDA):Γcoll = ρ < vrelσVN >
QCD sum rules (Leupold, NPA 628, 1998)
coupling to resonances can introduceadditional structures in the spectralfunction (Post, 2003)
J. Weil (JLU Giessen) Dilepton production at SIS energies
Why Dileptons? No FSI!
drawback of hadronic decay modes: strong final-stateinteraction of the decay products
example: ω → π0γ (measured by CBELSA/TAPS inphotoproduction experiments)
0
0.5
1
1.5
2
0.55 0.6 0.65 0.7 0.75 0.8 0.85
dσ/d
mπ0 γ
mπ0γ [GeV]
γ + 93Nb --- Eγ = 0.9 - 1.3 GeV
-75%
no π0 FSI
with π0 FSI
vacCB
CB+shiftshift more on ω photoprod.:
S. Friedrich,HK 30.4 (Mi)
F. Dietz,HK 38.3 (Do)
J. Weil (JLU Giessen) Dilepton production at SIS energies
The GiBUU Transport Model
BUU-type hadronic transport modelunified framework for various types of reactions(γA, eA, νA, pA, πA, AA) and observablesBUU equ.: space-time evolution of phase space density(∂t + (∇~pHi )∇~r − (∇~rHi )∇~p
)fi (~r , t, ~p) = Icoll [fi , fj , ...]
Hamiltonian Hi :hadronic mean fields, Coulomb, “off-shell potential”
collision term Icoll :decays and scattering processes (2- and 3-body)low energy: resonance model, high energy: PYTHIA
O. Buss et al., Phys. Rept. 512 (2012),http://gibuu.physik.uni-giessen.de
J. Weil (JLU Giessen) Dilepton production at SIS energies
Neutrinos in GiBUUνA → ℓ X
O. Lalakulich, arXiv:1203.2935
e.g. 1-pion production on Fe
Strangeness production in pA reactions
A. Larionov, HK 39.2 (Do)
Predictions for PANDA
J. Weil (JLU Giessen) Dilepton production at SIS energies
Resonance Model
NN → NR,∆R (R : ∆, 7 N∗ and 6 ∆∗ states)
based on Teis RM [Z. Phys. A 356, 1997] with several extensions
resonance parameters and decays taken from Manley/Saleski
all π, η and ρ mesons produced via R decays (ω, φ: non-res.)
good descr. of total NN cross sections up to√s ≈ 3.5GeV
0
5
10
15
20
25
30
35
40
2 2.5 3 3.5 4
σin
el [
mb
]
sqrt(s) [GeV]
pp → X
2 2.5 3 3.5 4 0
5
10
15
20
25
30
35
40
sqrt(s) [GeV]
np → X
datainel.N∆
NN*N∆*∆∆
∆N*∆∆*
NNπω/φ
BYK
J. Weil (JLU Giessen) Dilepton production at SIS energies
elementary reactions
if we want to investigate in-medium effects, we better makesure we understand the dilepton signal from elementarycollisions (in the vacuum)
this is not a trivial task and represents the most importantprerequisite for understanding the heavy-ion collisions
HADES has measured:p+p at 1.25, 2.2 and 3.5 GeVd+p at 1.25 GeV
J. Weil (JLU Giessen) Dilepton production at SIS energies
p+p at 3.5 GeV
first try: using PYTHIAstring model
some tuning needed
’gap’ at intermediate masses
question: which processcould fill this gap?
10-3
10-2
10-1
100
101
0 0.2 0.4 0.6 0.8 1
dσ
/dm
ee [
µb
/Ge
V]
dilepton mass mee [GeV]
dataGiBUU total
ρ → e+e
-
ω → e+e
-
φ → e+e
-
ω → π0e
+e
-
π0 → e
+e
-γ
η → e+e
-γ
∆ → Ne+e
-
J. Weil (JLU Giessen) Dilepton production at SIS energies
p+p at 3.5 GeV
use em. transition formfactor for ∆ Dalitz decay
naive VMD form factorwould overshoot the data
two-component quark modelby Wan/Iachello seems togive a good fit[Int. J. Mod. Phys. A 20 (2005)]
but: is this really a propermodel of the ∆ FF?
10-3
10-2
10-1
100
101
0 0.2 0.4 0.6 0.8 1
dσ
/dm
ee [
µb
/Ge
V]
dilepton mass mee [GeV]
dataGiBUU total
ρ → e+e
-
ω → e+e
-
φ → e+e
-
ω → π0e
+e
-
π0 → e
+e
-γ
η → e+e
-γ
∆ → Ne+e
-
J. Weil (JLU Giessen) Dilepton production at SIS energies
p+p at 3.5 GeV
now: use resonance model
ρ production via baryonicresonances
R → ρN → e+e−N
low-mass part enhanced bylight resonances (and 1/m3
factor from dilepton decaywidth) 10
-3
10-2
10-1
100
101
0 0.2 0.4 0.6 0.8 1
dσ
/dm
ee [
µb
/Ge
V]
dilepton mass mee [GeV]
dataGiBUU total
ρ → e+e
-
ω → e+e
-
φ → e+e
-
ω → π0e
+e
-
π0 → e
+e
-γ
η → e+e
-γ
∆ → Ne+e
-
J. Weil (JLU Giessen) Dilepton production at SIS energies
p+p at 3.5 GeV: resonance contributions
ρ channel is given by a mixof several resonancecontributions
shape depends on the massof the resonance and angularmomentum of the decay
contributions of singleresonances not wellconstrained, but goodagreement in total
cross check from πN massspectra needed!
10-3
10-2
10-1
100
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1
dσ
/dm
ee [
µb/G
eV
]
dilepton mass mee [GeV]
ρ → e+e
-
D13(1520)S11(1535)S11(1650)F15(1680)P13(1720)
all Res.S31(1620)D33(1700)F35(1905)
J. Weil (JLU Giessen) Dilepton production at SIS energies
p+p at 2.2 GeV
again: resonance productiongives improvement overphase-space ρ
but: data suggest thatrelative strengths ofresonance contributions arenot quite right
D13(1520) underestimated,P13(1720) overestimated?
10-3
10-2
10-1
100
101
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
dσ
/dm
ee [
µb/G
eV
]
dilepton mass mee [GeV]
dataGiBUU total
ρ → e+e
-
ω → e+e
-
ω → π0e
+e
-
π0 → e
+e
-γ
η → e+e
-γ
∆ → Ne+e
-
D13(1520)P13(1720)S31(1620)
J. Weil (JLU Giessen) Dilepton production at SIS energies
P+P / D+P at 1.25 GeV
10-3
10-2
10-1
100
101
0 0.1 0.2 0.3 0.4 0.5
dσ
/dm
ee [
µb/G
eV
]
dilepton mass mee [GeV]
p + p at 1.25 GeV
dataGiBUU total
π0 → e
+e
-γ
∆ → Ne+e
-
ρ → e+e
-
10-4
10-3
10-2
10-1
100
101
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7d
σ/d
mee [
µb
/Ge
V]
dilepton mass mee [GeV]
d + p at 1.25 GeV
dataGiBUU total
π0 → e
+e
-γ
η → e+e
-γ
∆ → Ne+e
-
ρ → e+e
-
slight deviation in pion channel (ang. distr.? accept. effects?)
p+p overall well described, d+p misses factor 2-10
reason not completely clear, OBE models might help (Shyam)
J. Weil (JLU Giessen) Dilepton production at SIS energies
p+Nb at 3.5 GeV
10-1
100
101
102
103
0 0.2 0.4 0.6 0.8 1
dσ
/dm
ee [
µb/G
eV
]
dilepton mass mee [GeV]
preliminary dataGiBUU total
ρ → e+e
-
ω → e+e
-
φ → e+e
-
ω → π0e
+e
-
π0 → e
+e
-γ
η → e+e
-γ
∆ → Ne+e
-
10-1
100
0.55 0.6 0.65 0.7 0.75 0.8 0.85
dσ
/dm
ee [
µb/G
eV
]dilepton mass mee [GeV]
preliminary datavacCB
CB+shiftshift
after [email protected] is fixed: good overall agreement in p+Nb(without ∆ FF!)
moderate medium modifications (VM spectral functions)
note: data still preliminary
J. Weil (JLU Giessen) Dilepton production at SIS energies
12C + 12C at 1.0 and 2.0 GeV
10-5
10-4
10-3
10-2
10-1
100
101
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
dσ
/dm
ee
dilepton mass mee [GeV]
C + C @ 1.0 GeV
subthresholdρ missing!
dataGiBUU total
ρ → e+e
-
ω → e+e
-
φ → e+e
-
ω → π0e
+e
-
π0 → e
+e
-γ
η → e+e
-γ
∆ → Ne+e
-
D13(1520)S31(1620)
10-4
10-3
10-2
10-1
100
101
0 0.2 0.4 0.6 0.8 1
dσ
/dm
ee
dilepton mass mee [GeV]
C + C @ 2.0 GeV
dataGiBUU total
ρ → e+e
-
ω → e+e
-
φ → e+e
-
ω → π0e
+e
-
π0 → e
+e
-γ
η → e+e
-γ
∆ → Ne+e
-
D13(1520)S31(1620)F35(1905)
C+C is a light system, can be described roughly by asuperposition of NN collisions
2 GeV data well described by GiBUU
some discrepancies at 1 GeV (“deuteron problem”?)
J. Weil (JLU Giessen) Dilepton production at SIS energies
40Ar + 39K 35Cl at 1.76 GeV
10-5
10-4
10-3
10-2
10-1
100
101
0 0.2 0.4 0.6 0.8 1
dσ
/dm
ee
dilepton mass mee [GeV]
in-mediumeffects?
dataGiBUU total
ρ → e+e
-
ω → e+e
-
φ → e+e
-
ω → π0e
+e
-
π0 → e
+e
-γ
η → e+e
-γ
∆ → Ne+e
-
D13(1520)S31(1620)F35(1905)
]2 [MeV/ceeM
0 0.2 0.4 0.6
)ee
/dM
NN
)/(d
Nee
/dM
AA
) (d
N0
πAA
/N0
πNN
(N 0
1
2
3
4
5
6
7
8
(b)Ar+KCl 1.76A GeV
C+C 1A GeV
C+C 2A GeV
Ar+KCl seems to show some excess over NN (∼ factor 3)
GiBUU with vacuum SF: similar discrepancy⇒ room for in-medium effects
J. Weil (JLU Giessen) Dilepton production at SIS energies
Conclusions
1 elementary p+p reactions well understoodwith resonance model description(higher resonances are important!)
2 some problems remaining in d+p (at low energies)
3 p+Nb: good agreement with data(moderate medium modifications)
4 to do for heavy-ion reactions:comparison with pT , y spectra and pion datainvestigation of in-medium effects (VM + Res)197Au + 197Au at 1.25 GeV
J. Weil (JLU Giessen) Dilepton production at SIS energies