Simultaneous photo-production measurement of the and mesons on the nucleons

at the range 680 – 1500 MeV

N.Rudnev, V.Nedorezov, A.Turinge for the GRAAL collaboration

Institute for Nuclear Research RAS, Moscow, 117312

New experimental data on partial and total photo-absorption cross sections for deuteron and carbon nuclei at E = 680 -1500 MeV are presented. Measurement has been done at the GRAAL facility. Two independent methods (subtraction of background and summing of partial channels) were applied to provide the high experimental accuracy. It is shown that the difference between total photo absorption cross sections for the quasi free proton and quasi-free neutron does not exceed 5% in absolute scale. The total photo-absorption cross section is compared with the literature data. A comparison of quasi free and free proton partial meson photo-production cross sections allows to conclude that nuclear media effects are caused not only by Fermi motion. The behavior and absolute values of the total and partial cross sections for the carbon nuclei is discussed.

Subtraction method

• Total yield from the full target (open circles), empty target (full triangles) and their difference (full squares) .

Scheme of the GRAAL experiment

Previous experiment on free proton data

Total photo-absorption cross section on the free proton obtained by the subtraction method. Full dots, triangles, and circles represent the GRAAL, Mainz,

and Armstrong data, respectively.

Total photo-absorption cross section for the deuteron, obtained by subtraction (full points) in comparison with literature Armstrong

data (full triangles)

Comparison of the total photo-production cross sections on proton and neutron

• Deuteron total photo-absorption cross section (triangles). Black points correspond to the free proton averaged over Fermi momentum distribution. Open points show their difference which would be considered as the neutron photo-absorption cross section.

Armstrong data of the total photo-absorption cross sections

on the proton and neutron

Selection of a proton and charged pion in BGO detector

Separation of the events for one charged pion photo-production on quasi-free nucleonRed – experiment, green – simulation

Angle between calculated and measured directions of the nucleon (reaction n=>p- )

Difference between calculated and measured energies of the forward nucleon (reaction n=>p-).

Here and later the black vertical lines specify the cuts for event selection

Separation of the events for one neutral pion photo-production on quasi-free nucleonRed – experiment, green – simulation

Invariant mass of two quanta in BGO detector (reaction p=>p0).

Missing mass of two g-quanta in BGO detector (reaction p=>p0).

Separation of the events for meson photo-production on quasi-free nucleonRed – experiment, green – simulation

Separation of the events for double 0 photo-production on quasi-free nucleon, Red – experiment, green – simulation

Invariant mass of two -quanta in BGO detector (reaction p=>p).

Invariant masses of two pairs of -quanta (reaction p=>p00). Rectangle marks area of the selected events.

Cross section evaluation method

Photon flux (a), yield (b), measurement efficiency (c) (reaction p=>p). Cross section (d) is obtained by division of the yield on the flux, and normalized on the measurement efficiency and thickness of the target.

Partial cross sections for one and double and meson photo-production

on free and quasi-free proton and quasi-free neutron

Red – free proton, green – quasi-free proton, blue – quasi-free neutron.

Specific media modification in different channels indicates that two nucleon correlations plays important role in addition to Fermi motion.

Total photo-absorption cross section for the deuteron, obtained by subtraction (open points) and summing method (full points) in comparison with literature Armstrong data (full triangles)

Results obtained by two different methods coincide within 5% error bars in the energy region up to 1.2 GeV. Above this energy, there is seen a noticeable disagreement, which is explained by the triple meson production channels contribution which was not included in the present evaluation of partial channels. We see the similar relative behavior, especially presence of the F15 (1680) resonance in the cross sections, obtained by subtraction and summing methods in contradiction with the Armstrong data seen.

Total photo-absorption cross sections on bound proton (green points) and neutron (red points) obtained by the summing method (deuteron target)

Slight (~5%) but systematic difference in the absolute scale above 1 GeV can be caused by the neutron efficiency uncertainty.

Comparison of the partial cross sections of the mesons photo-production.

Red, blue and green points are related to photo-production on free proton, carbon and deuteron, respectively.

Comparison of the obtained partial -meson photo-absorption cross sections with the literature data

Partial 0 and + photo-production cross section on the proton, measured with different wave length laser light. Green and red points correspond to 340 and 514 nm, respectively. Curve represents the literature averaged data from SAID.

,p -> n,+ ,p -> p,0

Comparison of the total photo-absorption cross sections for the proton (full triangles),

deuteron (open points) and carbon (full points), obtained by subtraction method.

The carbon cross section is in 30% less than the free proton and deuteron one.

Total photo-absorption cross section for the 12С.

Crosses correspond to GRAAL data, obtained by the subtraction method. Full and open points are taken from Bianchi e. a. and Mirazita e. a. data, respectively. “Universal curve” is marked by the full line. Good agreement with earlier published results and the “universal curve” is seen.

CONCLUSION

1. For neutron and proton we see the similar relative behavior and agreement in absolute scale, especially the presence of the F15 (1680) resonance in both cross sections in contradiction with the Armstrong (1972) results.

2. It is seen, that not only Fermi motion is responsible for the modification of

cross sections in the nuclear media.

3. Carbon partial photo-absorption cross sections are much more smoothed and the value of them on 20-30% less then the same cross sections on free proton and quasi-free proton and neutron in deuteron. The total photo-absorption cross section for the 12С nuclei obtained by the subtraction method has the same behavior and is in the good agreement with other experimental data and “universal curve”.