outline

• Motivation

• Experimental Setup

• Analysis

David Trnka

II. Physikalisches Institut, University Gießenfor TAPS- and CB/ELSA-Collaboration

Indications for a structure near the 2threshold in the p p reaction

Preliminary result from the WASA collaboration

Preliminary result from the WASA collaboration

• recently peaklike structure in

pp pp near 2 threshold

reported i.e. @ DPG/Koeln

• angular distrubution consistant

with s-wave production

• BG: • pp pp • double bremsstrahlung (rest)

• artifact peak due to

merged clusters (1 count)

‘s from 1 decaying

in a similar way)

• pp pp

Possible Interpretation: peak structure due to 2

photon radiation in the

mesonic channel:

pp pp pp

Also: CB @ LEAR reanalysing data

accelerator facility ELSA@Bonn

side view of experimental area

ebeam EEE

tagging system:taggtagginging range:range: 22 to 93 % of 22 to 93 % of incident electron beam energyincident electron beam energy

Experimental SetupExperimental Setup

front view of TAPSside view

to

detector systemCB/TAPS @ ELSA

detector systemCB/TAPS @ ELSA

Crystal Barrel TAPS 1290 CsI 528 BaF2

to

• 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

)cos1(E2E

EE

2121

221

221

ppm

AnalysisAnalysis

apply missing mass cut

(p treated as missing particle)

require coplanarity

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

E = 0.67 GeV and 0.77 GeV in p reactions

a quick looka quick look

low beam energies:

• peak structure around 2m visible

• for solid target structure even below 2m

a bit more sophisticated looka 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 background

bg not yet understood (FWHM = ?)

slight shift of peak position with beam energy

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