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

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Presentation Transcript

Michael Weber

for the HADES collaboration

Inclusive e+e- pair production in p+p and p+Nb collisions at E = 3.5 GeVTechnische Universität München

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

WWND 2011

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

WWND 2011

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

WWND 2011

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

WWND 2011

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

WWND 2011

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

- 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

- 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

WWND 2011

Analysis by Anar Rustamov

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

WWND 2011

Analysis by Anar Rustamov

h e+e-

No direct decay (same for p + Nb):

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

WWND 2011

Analysis by Anar Rustamov

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

WWND 2011

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

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

WWND 2011

Fast and slow e+e- sources

- Decays inside nucleus
- In – Medium effects

p cut

HSD Simulation for HADES proposal

WWND 2011

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)

WWND 2011

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

WWND 2011

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

WWND 2011

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

WWND 2011

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

WWND 2011

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)

WWND 2011

Slow and fast sources (GiBUU)

Collisional width in GiBUU for w

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

WWND 2011

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

WWND 2011

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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EXTRA SLIDES

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e+e- sources in p + p

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p + p normalization

Elastic scattering

Kinematic constraints:

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

WWND 2011

Analysis by Anar Rustamov

p + Nb normalization

p- yield in HADES acceptance:

Systematic uncertainty

Fit to HARP data: pT > 300 MeV/c

WWND 2011

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