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the hades spectrometer , which is described in more details in the contribution of ref .  , is a highly selective tool specially suited for the study of the high energy dielectron decay channels in nucleus - nucleus collisions around 1 - 2 a gev , as well as in proton and pion induced reactions . in order to provide absolute measurements of invariant mass distributions
a good knowledge of the dielectron reconstruction efficiency is mandatory , which can be achieved by means of well known calibration reactions , as the @xmath3 channel . +
a proton - proton run was done on january 2004 at 2.2gev , to measure the @xmath3 reaction through an exclusive reconstruction of the hadronic ( @xmath1 ) and the dielectron ( @xmath2 ) dalitz decays , whose branching ratios are known .
this fact allows to use the @xmath3 channel as a calibration reaction for the dielectron identification , in order to normalize the dielectron yields in theavy ions experiments .
the study of the @xmath0 meson is of particular interest for several reasons . in the vicinity of the threshold the production of the @xmath0 meson
was studied extensively and the inclusive cross sections are well known@xcite , while at higher energies the uncertainties are larger . moreover the observed electron pair invariant mass distribution for the @xmath0 dalitz decay branch can be used to check contributions from other sources ( such as @xmath4 dalitz , combinatorial background , etc . ) as well as the lepton trigger efficiency .
in addition , if both protons and both leptons are detected , the reconstruction of this decay mode is kinematically complete without requiring an additional photon detector , and a missing mass technique can be applied .
this would allow to separate the @xmath0 dalitz from all other sources and to map the effective acceptance of the spectrometer .
in hades , however , the main algorithm for particle identification requires a combination of momentum and time - of - flight measurements . in the pp experiment it was not possible to use a start detector as a time reference , because of the high number of secondaries produced .
this means that there is no common start time reference for tracks in the same event , but in general the data acquisition is started by the fastest particle which crosses the time - of - flight wall . in this case
we measure the difference in time with respect to the fastest particle , instead of the real time - of - flight .
thus it was necessary to develop an algorithm to reconstruct the start time of the reaction , and make particle identification possible .
+ the used algorithm relies on identifying one particle in the event and calculate its theoretical time - of - flight : from the assumed mass and the measured momentum we calculate the time - of - flight of the particle , thus the offset to the real start time of the reaction .
the first kind of identification comes from the cherenkov detector , which is hadron blind and can select electron / positron tracks . if a lepton candidate is not found , a negative charged track in the same event is assumed as a negative pion .
finally , we recalculate the time - of - flight of the other particles inside the same event using the extrapolated reaction time . in this way we obtained an average time resolution of 340 ps and an efficiency of about 92% for events with a lepton , and a resolution of 440 ps and an efficiency of about 93% for events with a negative pion .
+ after the start time reconstruction it is possible to use the recalculated time - of - flight in order to identify particles , for exclusive analysis of @xmath0 decay channels .
the main goal of the p+p experiment was to verify the dielectron reconstruction efficiency needed for a correct interpretation of the hi data , by means of the well known @xmath0 meson decays into hadronic@xcite and electromagnetic channels@xcite . in the simulation we assumed resonant production of the meson via @xmath5 , with the dalitz matrix element taken from ref .  .
for the eta hadronic decay the matrix element from ref .   has been implemented , and for the @xmath0 dalitz decay the vector dominance model ( vdm ) was assumed .
in addition a full cocktail of reaction channels based on measured proton - proton cross sections@xcite was generated .
simulated events were processed through the hades geant code and reconstructed with the same analysis program used for the experimental data .
reconstruction of the @xmath6 reaction started with charged particle identification ( pid ) based on momentum and reconstructed time of flight .
after pid a kinematical condition was used on events with two protons and two charged pions : by imposing a cut of @xmath7 around the @xmath4 region in the four - particle missing mass we selected the @xmath8 reaction , @xmath9 being determined from a fit to the missing @xmath4 peak .
the missing - mass resolution was further improved with a kinematical refit of the full event@xcite .
finally , the @xmath0 mesons were identified in the two - proton missing - mass distribution shown in the left plot of fig .
[ etahad_plot ] .
a prominent @xmath0 peak , centered at the expected position , is clearly visible on top of a non - resonant three - pion background . by fitting the distribution by means of a gaussian plus a background function ,
we obtain the value of the @xmath0 mass with a resolution of 2.5% , and evaluate the hadronic production yield .
moreover , by fitting the pp missing mass peak in various @xmath10 slices@xcite , the angular distribution on the @xmath0 meson emission has been evaluated and compared to simulation , by using the anisotropic angular model verified by the disto collaboration@xcite(right plot of fig .
[ etahad_plot ] ) .
the two distributions are in agreement within error bars , showing that the hadron efficiency is understood in the covered phase space region .
center - of - mass polar angle for @xmath1 events , without correction for acceptance end efficiency.,title="fig:",scaledwidth=42.0% ]   center - of - mass polar angle for @xmath1 events , without correction for acceptance end efficiency.,title="fig:",scaledwidth=42.0% ]    the reconstruction of the @xmath11 decay was done in a similar way . as a first step ,
events with two proton and two electron tracks were selected , thus the @xmath12 reaction channel was identified by imposing a @xmath7 selection on the four - particle missing mass around the missing photon peak . the left side of fig .
[ etadal_plot ] shows the proton - proton missing mass distribution , obtained after a kinematical refit was applied to the @xmath12 events with a constraint on a missing photon .
the prominent peaks centered at 140 and 546 @xmath13 correspond to the @xmath4 and @xmath0 mesons respectively .
the istribution is dominated by events @xmath14 ( branching ratio @xmath15 ) where one of the decay photons produces a conversion pair . however , as the simulation also shows , it can be completely removed by conditions on the quality of the track reconstruction and the pair opening angle . the right plot of fig .
[ etadal_plot ] shows the invariant mass distribution of dielectron pairs coming from the @xmath0 decay after the conversion rejection@xcite , in comparison with simulation by using qed ( point - like interaction ) and vdm ( @xmath0 form factor ) theoretical models .
the agreement demostrates our good understanding of the dielectron invariant mass spectrum , and that the form factor measurement is feasible with the hades spectrometer .
+ finally we compare the ratio r of the @xmath0 yields reconstructed in the hadronic and electromagnetic decays ( including conversion contribution ) to the predictions of the simulation with the following results : @xmath16 and @xmath17 .
the agreement in between the two values shows how the simulation well reproduces the experimental yields , and it confirms the global good understanding of the response of the hades spectrometer .
meson decay , after a kinematic refit procedure .
_ right _ : invariant mass distribution of dielectron pairs coming from @xmath0 decay , after the conversion rejection , in comparison with simulation by using qed or vdm @xmath0 form factor.,title="fig:",scaledwidth=42.0% ]   meson decay , after a kinematic refit procedure .
_ right _ : invariant mass distribution of dielectron pairs coming from @xmath0 decay , after the conversion rejection , in comparison with simulation by using qed or vdm @xmath0 form factor.,title="fig:",scaledwidth=42.0% ]
this work has been supported by ga cr 202/00/1668 and ga as cr iaa1048304 ( czech republic ) , kbn 5p03b 140 20 ( poland ) , bmbf ( germany ) , infn ( italy ) , cnrs / in2p3 ( france ) , mcyt fpa2000 - 2041-c02 - 02 and xuga pgidt02pxic20605pn ( spain ) and intas ref . nr .
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the hades spectrometer installed at gsi darmstadt is devoted to study the production of di - electron pairs from proton , pion and nucleus induced reactions at 1 - 2 agev . in pp collisions at 2.2 gev we have focused mainly on exclusive reconstruction of the @xmath0 meson decays in the hadronic ( @xmath1 ) and the electromagnetic channels ( @xmath2 ) . 
 we present analysis techniques and discuss first results on @xmath0 production , with the main focus on comparisons of reconstructed distributions to results obtained by other experiments and theoretical predictions .