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Indications for a structure near the 2 p threshold in the g p p gg reaction. David Trnka II. Physikalisches Institut, University Gießen for TAPS- and CB /ELSA -Collaboration. outline Motivation Experimental Setup A nalysis. Preliminary result from the WASA collaboration.

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Indications for a structure near the 2p threshold in the gp pgg reaction

David Trnka

II. Physikalisches Institut, University Gießen

for TAPS- and CB/ELSA-Collaboration

  • outline

  • Motivation

  • Experimental Setup

  • Analysis


Preliminary result from the WASA collaboration

  • • recently peaklike structure in

  • pp  ppgg near 2p threshold

  • reported i.e. @ DPG/Koeln

  • • angular distrubution consistant

  • with s-wave production

  • • BG: • pp  pppp (15%)

  • • double bremsstrahlung (rest)

  • • artifact peak due to

  • merged clusters (1 count)

  • (2 g‘s from 1 p decaying

  • in a similar way)

  • • pp  ppp+p- (0%)

  • Possible Interpretation:

  •  peak structure due to 2

  • photon radiation in the

  • mesonic s channel:

  • pp  pps  ppgg

Also: CB @ LEAR reanalysing data


Experimental Setup

accelerator facility [email protected]

side view of experimental area

tagging system:

taggingrange: 22 to 93 % of incident electron beam energy


4 p detector system

CB/TAPS @ ELSA

Crystal BarrelTAPS

1290 CsI 528 BaF2

side view

front view of TAPS

g

f = 360°

q = 30°to 168°

f = 360°

q = 5°to 30°


Analysis

• mesons identified via invariant

mass analysis of decay photons

• all beam energies allowed (ca. 500 MeV to 2.5 GeV)

• only tiny fraction of data analysed

• clean up spectra via exclusiv

measurent

require coplanarity

apply missing mass cut

(p treated as missing particle)


a quick look

comare to WASA

Tp = 1.2 GeV and 1.36 GeV in pp reactions corresponds to

Eg = 0.67 GeV and 0.77 GeV in g p reactions

low beam energies:

• peak structure around 2mp visible

• for solid target structure even below 2mp


a bit more sophisticated look

• scan through different beam energy bins

• remove/reduce merged cluster candidates

 cut on clustersize ( < 10 crystals)

 cut on opening angle of the photon pair (>20°)

• investigate pp background

bg not yet understood (FWHM = ?)

 slight shift of peak position with beam energy

 more detailed study would be great (CB/TAPS @ MAMI)


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