07.02.2011 WWND 2011

Michael Weber

for the HADES collaboration

Inclusive e+e- pair production in p+p and p+Nb collisions at E = 3.5 GeV

Introduction

HADES experiment

Effects in cold nuclear matter Relative to 0

Cross sections

Comparison with transport models

Conclusions

Technische Universität München

07.02.2011 WWND 2011

Phase diagram

Learn about properties of QCD matter at different (T,)EM structure of matter

hadrons in matter

http://www.ice.csic.es/en/graphics/phase.jpg

07.02.2011 WWND 2011

Phase diagram

Learn about properties of QCD matter at different (T,)EM structure of matter

hadrons in matter

Penetrating probe:Dielectrons

Direct coupling of *

to VM (JP=1-)

e+

e-

e+

e-

http://www.ice.csic.es/en/graphics/phase.jpg

07.02.2011 WWND 2011

Dense and hot matter

LHC

SPS

FAIR

SIS

e+

e-

e+

e-

e+

e-

e+

e-

e+

e-

RHIC

http://www.ice.csic.es/en/graphics/phase.jpg

Accessible in:

A+ A reactions

Dense matter at SIS:J. Stroth

07.02.2011 WWND 2011

Cold nuclear matter

Accessible in:

p + A / + A / + A

e+

e-

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Cold nuclear matter

Accessible in:

p + A / + A / + A

Medium effects:

mass shift/ (collisional) broadening/ reabsorption/ regeneration

Measure: Spectral shape / Cross sections/ Attenuation

BUT: NON conlusive experimental results

meson

P.Mühlich et al., NPA 780 (2006), 187

07.02.2011 WWND 2011

HADESI

04/22/23

observable detector

p MDC (Magnet)

β TOF(ino)

dE/dx MDC

TOF(ino)

e+/e- RICH

HADES[ EPJ A41 243]

Production of strangeness:L. Fabbietti

07.02.2011 WWND 2011

HADESII

22.04.23

HADES[ EPJ A41 243]

Beam: p• Ekin= 3.5 GeV• I ~ 6 · 106 1/s

Target : p• 4 cm LH2 • ~1% reaction prob.

3.5 · 109 collisions

A = 1 A = 93

High purity e+e- pair reconstruction

Beam: p• Ekin= 3.5 GeV• I ~ 2 · 106 1/s

Target : 93Nb • 12 x 0.45 mm• ~3% reaction prob.

9.2 · 109 collisions

observable detector

p MDC (Magnet)

β TOF(ino)

dE/dx MDC

TOF(ino)

e+/e- RICH

07.02.2011 WWND 2011

e+e- pair reconstruction

Comb. Background:• same event e-e- e+e+

• correlated BG from 0 / decays

Signal to BG ratio

07.02.2011 WWND 2011

e+e- sources in p + p

• particle production• π via resonance decays (,N*)• η, ω, ρ via phase space• Δ through 1 π exchange

• particle decays• form factors• mass dep. Width

PLUTO, Fröhlich et al, arxiv:0708.2382

• cross sections in 4π (mb)• : 16 ± 2.6 (from data)• Δ: 7.5 PYTHIA • η: 0.93 ± 0.2 (fit to data)• ω: 0.25 ± 0.05 (fit to data)• ρ: 0.38 ± 0.07 (fit to data)

Δ FF is fixed at the photon point

σω ~ 16 MeV/c2

Analysis by Anar Rustamov

07.02.2011 WWND 2011

e+e- sources in p + p

- N transition form factor

Time like: q2 > 0

Space like: q2 < 0

Two–component VDM type model:Wan/Iachello, IJMP A20, 2005

only relevant for exclusive analysis

Measured in electroproduction

NOT Measured

N

e-

e+

Analysis by Anar Rustamov

07.02.2011 WWND 2011

e+e-

No direct decay (same for p + Nb): reduce BR (2.7 x 10-5) at least by factor of 3

Analysis by Anar Rustamov

07.02.2011 WWND 2011

e+e- in p + Nb

Scaling to same yield in 0 peak

0

Yield/0 (p+Nb/p+p)• IM: 1.51

• HM: 1.37

• In region 1.13

→ diff. production and/or absorption mechansims for diff. sources

→ kinematic observables for diff. mass regions

LM

IM

HM

07.02.2011 WWND 2011

pNb vs. pp: pT and y

Transverse momentum

pT2 = px

2 + py2

Rapidity

y = ½ ln (E+pz)/ (E+pz )

yCM, NN = 1.12

LM LM

yCM, NN

07.02.2011 WWND 2011

pNb vs. pp: pT and y

For both mass bins:

• Higher yield

• At high pt

• Shift to target rap.

add. slow source

IM IM

HM

HM

07.02.2011 WWND 2011

Fast and slow e+e- sources

Decays inside nucleus In – Medium effects

HSD Simulation for HADES proposal

p cut

07.02.2011 WWND 2011

Fast and slow e+e- sources

HADES: Significant e+e- yield with low pair momenta ( ~ 35 % in HM)

p+Nb Ekin= 3.5 GeV

p cut

HM

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Fast and slow sources in other Exp.

KEK: p + A @ 12 GeV JLAB: + A @ 0.6 -3.8 GeV

R.Muto et al., PRL 98 (2007) 042501 S.Leupold, V.Metag and U.Mosel, nucl-th 0907.2388

07.02.2011 WWND 2011

Fast and slow e+e- sources

High p: free p+p production

Low p: overshoot over p+p

different for , , and

High p Low p

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Cross sections

p+p:X sect from pp, elastic

p+Nb:X sect from -,acc comp to HARP

Bolshakova et al., EPJC (1997) 1865

Isospin:pd = 2 pp ( DLS: Ekin > 2 GeV )

Wilson et al., PRC 57(1997) 1865

A Scaling:

• Black disc:= 2/3 Glauber model: ~ 0.8W.Cassing et al., PLB 238 (1990) 25

AdMd

dMdR

pp

pApA

)/(

)/(

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RpNb vs. p

RpNb = 23.9 ± 1.1 RpNb = 27.4 ± 1.4 RpNb = 28.8 ± 2.4

pNb = 0.70 ± 0.10 pNb = 0.73 ± 0.13 pNb = 0.74 ± 0.21

• High p: free NN prod. + same absorption for all sources

LM IM HM

~0.7

07.02.2011 WWND 2011

RpNb vs. p

RpNb = 23.7 ± 0.2 RpNb = 40.9 ± 1.5 RpNb = 48.4 ± 5.3

pNb = 0.70 ± 0.02 pNb = 0.82 ± 0.11 pNb = 0.86 ± 0.36

• High p: free NN prod. + same absorption for all sources

• Low p : enhanced prod. (secondary processes)

and/or lower absorption in IM and HM

LM IM HM

~0.7

07.02.2011 WWND 2011

A scaling for vector mesons

Fit with Gaussians:

• High p:

pNb, = 0.69 ± 0.27

pNb, = 1.04 ± 2.50

• Low p:

pNb, = 0.62 ± 0.36

• : momentum dependent absorption ?

• : production in whole nucleus volume (statistics!) ?

T. Tabaru et al.. Phys.Rev. C 74(2006)

Data : p+A @ 12. GeV KEK E325

= 0.710 ± 0.021(stat) ± 0.037(syst)

= 0.937 ± 0.049(stat) ± 0.018(syst)

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Transport models - M

UrQMD

HSD

GiBUU

Different , , contributions !

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Transport models - pT

UrQMD

HSD

GiBUU

Different , , contributions !

Constrain models ?

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Slow and fast sources (GiBUU)

Collisional width in GiBUU for • momentum dependent

• 20 – 90 MeV ( 2 – 11 x vac)

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Summary

• e+e- - pair production p+p and p+Nb at E = 3.5 GeV ( M, pT and y)• clear signal observed (/M ~ 2%)• significant e+e- - yield with low pair momenta (p < 800 MeV/c)

• Additional prod. w.r.t p+p at intermediate masses ( hard pT and target rapidity )

• A scaling of differential cross sections: (p+Nb/p+p) ~ A

– High p : all masses ~ 0.7 – Low p : 0 region: ~ 0.7

region: ~ 0.82 region: ~ 0.86

BUT ~ 0.62 in - medium omega width

• Comparison with transport models Better understanding of sources in p + p

07.02.2011 WWND 2011

Outlook

• e+e- - pair production in HI reactions

– Regeneration of VM– Higher densities

– HADES upgrade

• in - induced reactions

– Large X – sections– VM „at rest“

– Beam particle tracking

07.02.2011 WWND 2011

The HADES collaboration

07.02.2011 WWND 2011

EXTRA SLIDES

07.02.2011 WWND 2011

e+e- sources in p + p

07.02.2011 WWND 2011

Collisional width of in GiBUU

free width

Priv. Comm. Janus Weil

07.02.2011 WWND 2011

accelN

accel

accel

Norm

NF

mbddd

dacc 16.0)sin(

125

55

Elastic scattering

Kinematic constraints:0

21 180||

865.2)cot()cot( 221 cm

Kammerud et al. Phys. Rev. D 4 (1971), 5

p + p normalization

Analysis by Anar Rustamov

07.02.2011 WWND 2011

p + Nb normalization

)3()(min 1,, LVLHADEStrigHADES MNFbiasN

- yield in HADES acceptance:

Systematic uncertainty

ddp

NdF

ddp

d

T

HARPnorm

T

HARP ,2

,2

Fit to HARP data: pT > 300 MeV/c