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Michael Weber for the HADES collaboration. Inclusive e + e - pair production in p+p and p+Nb collisions at E = 3.5 GeV. Technische Universität München. Introduction HADES experiment Effects in cold nuclear matter Relative to p 0 Cross sections Comparison with transport models

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michael weber for the hades collaboration
Michael Weber

for the HADES collaboration

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

Technische Universität München

  • Introduction
  • HADES experiment
  • Effects in cold nuclear matter
    • Relative to p0
    • Cross sections
  • Comparison with transport models
  • Conclusions

WWND 2011

phase diagram
Phase diagram
  • Learn about properties of QCD matter at different (T,r)
    • EM structure of matter
    • hadrons in matter

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

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phase diagram1

o, 

w,r

o,

e+

e-

g*

e+

e-

w,r,f

g*

Phase diagram
  • Learn about properties of QCD matter at different (T,r)
    • EM structure of matter
    • hadrons in matter
  • Penetrating probe:
    • Dielectrons
    • Direct coupling of g*

to VM (JP=1-) w,r,f

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

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dense and hot matter

e+

e-

e+

e-

e+

e-

e+

e-

Dense and hot matter
  • Accessible in:

A+ A reactions

LHC

RHIC

SPS

FAIR

SIS

e+

e-

Dense matter at SIS:

J. Stroth

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

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cold nuclear matter

e+

e-

Cold nuclear matter
  • Accessible in:

p + A / g + A / p + A

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cold nuclear matter1
Cold nuclear matter

r meson

  • Accessible in:

p + A / g + A / p + A

  • Medium effects:

mass shift/ (collisional) broadening/ reabsorption/ regeneration

  • Measure: Spectral shape / Cross sections/ Attenuation
  • BUT: NON conlusive experimental results

P.Mühlich et al., NPA 780 (2006), 187

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hadesi
HADESI

HADES

[ EPJ A41 243]

Production of strangeness:

L. Fabbietti

21.08.2014

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hadesii
HADESII

HADES

[ EPJ A41 243]

High purity e+e- pair reconstruction

A = 1

A = 93

  • Beam: p
  • Ekin= 3.5 GeV
  • I ~ 6 · 106 1/s
  • Target : p
  • 4 cm LH2
  • ~1% reaction prob.
  • 3.5 · 109 collisions
  • 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

21.08.2014

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e e pair reconstruction
e+e- pair reconstruction
  • Signal to BG ratio
  • Comb. Background:
  • same event e-e- e+e+
  • correlated BG from p0 / h decays

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e e sources in p p
e+e- sources in p + p
  • particle production
    • π via resonance decays (D,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)
    • π: 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)

σω ~ 16 MeV/c2

Δ FF is fixed at the photon point

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Analysis by Anar Rustamov

e e sources in p p1
e+e- sources in p + p

D - N transition form factor

Space like: q2 < 0

Measured in electroproduction

N

D

g*

e-

e+

Two–component VDM type model:

Wan/Iachello, IJMP A20, 2005

only r relevant for D

exclusive analysis

NOT Measured

Time like: q2 > 0

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Analysis by Anar Rustamov

h e e
h e+e-

No direct decay (same for p + Nb):

 reduce BR (2.7 x 10-5) at least by factor of 3

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Analysis by Anar Rustamov

e e in p nb
e+e- in p + Nb

Scaling to same yield in p0 peak

Yield/p0 (p+Nb/p+p)

  • IM: 1.51
  • HM: 1.37
  • In w region 1.13

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

→ kinematic observables for diff. mass regions

LM

IM

HM

p0

h, D

r,w,f

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pnb vs pp p t and y
pNb vs. pp: pT and y

yCM, NN

LM

LM

Transverse momentum

pT2 = px2 + py2

Rapidity

y = ½ ln (E+pz)/ (E+pz )

yCM, NN = 1.12

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pnb vs pp p t and y1
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

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fast and slow e e sources
Fast and slow e+e- sources
  • Decays inside nucleus
  • In – Medium effects

p cut

HSD Simulation for HADES proposal

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fast and slow e e sources1
Fast and slow e+e- sources

HM

p cut

p+Nb Ekin= 3.5 GeV

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

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fast and slow sources in other exp
Fast and slow sources in other Exp.

KEK: p + A @ 12 GeV

JLAB: g + A @ 0.6 -3.8 GeV

S.Leupold, V.Metag and U.Mosel, nucl-th 0907.2388

R.Muto et al., PRL 98 (2007) 042501

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fast and slow e e sources2
Fast and slow e+e- sources

Low p

High p

  • High p: free p+p production
  • Low p: overshoot over p+p

different for r, w, and f

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cross sections
Cross sections
  • p+p:
  • X sect from spp, elastic
  • p+Nb:
  • X sect from sp-,acc comp to HARP
  • Bolshakova et al., EPJC (1997) 1865
  • Isospin:
  • spd = 2 spp ( DLS: Ekin > 2 GeV )
  • Wilson et al., PRC 57(1997) 1865
  • A Scaling:
  • Black disc: a = 2/3 Glauber model: a ~ 0.8
          • W.Cassing et al., PLB 238 (1990) 25

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r pnb vs p
RpNb vs. p

LM

HM

IM

a~0.7

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

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r pnb vs p1
RpNb vs. p

LM

HM

IM

a~0.7

  • High p: free NN prod. + same absorption for all sources
  • Low p : enhanced prod. (secondary processes)

and/or lower absorption in IM and HM

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a scaling for vector mesons
A scaling for vector mesons

Fit with Gaussians:

  • High p:

apNb,w = 0.69 ± 0.27

apNb,f = 1.04 ± 2.50

  • Low p:

apNb,w = 0.62 ± 0.36

Data : p+A @ 12. GeV KEK E325

 = 0.710 ± 0.021(stat) ± 0.037(syst)

 = 0.937 ± 0.049(stat) ± 0.018(syst)

  • w: momentum dependent absorption ?
  • f: production in whole nucleus volume (statistics!) ?

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

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transport models m
Transport models - M

Different D, h, r contributions !

GiBUU

UrQMD

HSD

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transport models p t
Transport models - pT

Different D, h, r contributions !

 Constrain models ?

GiBUU

UrQMD

HSD

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slow and fast sources gibuu
Slow and fast sources (GiBUU)

Collisional width in GiBUU for w

  • momentum dependent
  • 20 – 90 MeV ( 2 – 11 x Gvac)

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summary
Summary
  • e+e- - pair production p+p and p+Nb at E = 3.5 GeV ( M, pT and y)
  • clear w signal observed (sw/Mw ~ 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) ~ Aa
    • High p : all masses a ~ 0.7
    • Low p : p0 region: a ~ 0.7

h, Dregion: a ~ 0.82

r,w,f region: a ~ 0.86

BUT w a ~ 0.62

 in - medium omega width

  • Comparison with transport models

 Better understanding of sources in p + p

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outlook
Outlook
  • e+e- - pair production in HI reactions
    • Regeneration of VM
    • Higher densities
    • HADES upgrade
  • in p- induced reactions
    • Large X – sections
    • VM „at rest“
    • Beam particle tracking

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slide29

The HADES collaboration

Thank you

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slide30
EXTRA SLIDES

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collisional width of w in gibuu
Collisional width of w in GiBUU

free width

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Priv. Comm. Janus Weil

p p normalization
p + p normalization

Elastic scattering

Kinematic constraints:

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

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Analysis by Anar Rustamov

p nb normalization
p + Nb normalization

p- yield in HADES acceptance:

Systematic uncertainty

Fit to HARP data: pT > 300 MeV/c

WWND 2011

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