Dilepton production at SIS energies with the GiBUU ... · Dilepton production at SIS energies with the GiBUU transport model Janus Weil, ... /dm ee [m b/GeV] dilepton mass ... Dilepton

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


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)


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)


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


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


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


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


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

-


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]


dataGiBUU total

ρ → e+e

-

ω → e+e

-

φ → e+e

-

ω → π0e

+e

-

π0 → e

+e

-γ

η → e+e

-γ

∆ → Ne+e

-


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]


dataGiBUU total

ρ → e+e

-

ω → e+e

-

φ → e+e

-

ω → π0e

+e

-

π0 → e

+e

-γ

η → e+e

-γ

∆ → Ne+e

-


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

]


ρ → e+e

-

D13(1520)S11(1535)S11(1650)F15(1680)P13(1720)

all Res.S31(1620)D33(1700)F35(1905)


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

]


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)


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

]


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]


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)


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

]


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


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


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


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”?)


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


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


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


Documents

Dilepton production at SIS energies with the GiBUU ... · Dilepton production at SIS energies with the GiBUU transport model Janus Weil, ... /dm ee [m b/GeV] dilepton mass ... Dilepton