2 pion photoproduction from G11A E. Golovach MSU Analysis Status for

G11A Data Analysis Energy loss corrections: eloss-package (E. Pasyuk/ CLAS-note...) Tagger corr. (gamma energy correction) (S. Stepanyan,...NIM A572...) Photon Flux: gflux-packages (E. Pasyuk) Beam current dependence of flux (M.Battaglieri, R.De Vita.) Trigger efficiency. (M.Battaglieri,...; D. Applegate..., CLAS-Note). Event Generator (Mokeev's model + preliminary cross section fit) Particle ID: Kin. Fit. + momentum correction (M. Williams. CMU) Fiducial cuts and particle detection efficiency (Essential source of errors\ for the absolute value of the cross section) Major steps done in data analysis:

Preliminary obtained cross section Major issues/problems with the obtained cross section:  Dead zone problem  JLAB cross section is essentially higher than the data from ELSA (W >~2 GeV)  The integrated cross section demonstrates too “vibrating” behaviour JLAB – black points ELSA – red and blue

Dead zone problem Dead zones must be accounted for! Attempts were taken to study dead zone problem and account for the lost cross section: 1) Exploit the symmetry of the x-section over the alpha-angle. It gives a little help. 2) Try to interpolate or extrapolate the cross section. It gives a little help. Only ~10% of the dead zones are eliminated. 3) The method proposed to fill dead zones (see next slides) 4) Compare the differential cross section over t with ELSA (see next slides)

Dead zone problem Accounting for the dead zones were made in the following way: In every bin of the 1-fold differential cross section the percentage of the dead zones from contributing multidimensional cells was evaluated as a ratio of the number of dead cells to the total number of contributing cells. Then the cross section was corrected by dividing by this factor. Obviously, this procedure works under assumption that in each bin of the 1-fold differential cross section the dead cells are uniformly distributed between all contributing cell. See next slide for figures.

Dead zone problem In this figures blue points present the initial cross section. Black points show the cross section corrected for dead zones.

Dead zone problem Results of the dead zone correction Blue points correspond to the not corrected cross section. Black points present corrected cross section. Red points are ELSA data.

Dead zone problem Dead zones are naturally concentrated mostly at low and high theta angles. It is useful to compare the obtained differential over theta cross sections with ELSA data but ELSA published only distribution over t( π + π – ) and t(p π + ). Next figures show the comparison of t-distribution. JLAB data are in black. ELSA data are in blue. The right figure presents the low t range of the right figure plotted in linear scale. (see next slides for more figures)

Dead zone problem

JLAB.vs. ELSA Conclusion at this point: One can see that the excess cross section from JLAB over the ELSA data occurs mainly in the region of low and high t-values. Thus, the increase in the JLAB data may be related to the accounting for dead zones in ELSA analysis. (low and hight t-values correspond in average to the low and high thetas)

Three pion admixture Another reason was studied that can give rise of JLAB cross section at high W. It is a 3 pion admixture, i.e. some events with ( π + π – π 0 ) in the final state may be reconstructed and misinterpreted as two pion events and could increase the cross section especially at high W. The influence of the 3-pion events to the 2-pion data was studied with the help of simulation. Two types of simulations data were made: -- Only 2 pion events in the EG. ( σ 2 ) -- Both 2 pion and 3 pion events ( σ 23 ) weighted according to the experimental cross section. 3 pion channel was simulated with a phase space approximation. In the plot below one can see the ratio of the cross sections obtained with two types of simulations.

3 pion admixture In the plot below you can see the ration of the cross sections obtained with two types of simulations. – The contribution of the 3 pion events, which are erroneously interpreted to be 2 pion events, is less than 2% for W<2.5 GeV. – Correction for 3 pion admixture is required for W > 2.5.

Tagger efficiency The next issue with the obtained cross section is that the integrated cross section demonstrates too “vibrating” behaviour, while the differential distributions (for any fixed W) are quite smooth. Tagger reconstruction efficiency was studied. Let's examine the integrated cross section as a function of E-counters. At first glance there are 20 to 30 E-counters showing unreasonable behaviour.

Tagger efficiency 20 to 30 E-counters showing unreasonable behaviour were cut off. On the plot you can see the cross section as a function of E-counters after cutting-off some counters. Apart for the single bad E-counters there are some bunches of counters revealing dips (or increases) in the cross section plot.

Tagger efficiency One can see a dip at Eid ~ The same plot as the previous one but for lowest and highest range in X-axis. Here is an excess at Eid ~ Missing x-sec at Eid = is a result of the cut-off of a bunch of the bad E counters

Tagger efficiency Left plot presents raw data. Right plot presents just unity divided by Photon Flux. The lowest plots shows (raw data)/(Flux) Two dips at Eid ~= 85 and 180 are obviously related with two T-counters. Certain T-counters are not stable. I corrected them manually by multiplying the cross section in the certain range.

Tagger efficiency Tagger efficiency correction – There are 20 to 30 E-counters of the tagger which should be eliminated. – There are 4 unstable T-counters that produce two dips, one rise and one hole In the cross section. – In case of 2 dips and 1 rise the cross section is corrected by shifting the cross Section in the corresponding range of W. In case of ''holes” the cross section is corrected just by interpolation.

Tagger efficiency Corrected cross section gets smoother. Jlab data with no Dead zone correction Jlab data with dead zone correction

Summary 1) The excess of the Jlab cross section compared to the ELSA data may be related to the way the dead zones are accounted for. Accounting for dead zones makes the cross section increase by ~20%. 2) After tagger efficiency correction the integrated cross section demonstrates reasonably smooth behaviour.