1. Recent GRAAL Results on Nucleon Spectroscopy at 750 -1500 MeV
N.V.Rudnev*, A.Lapik, V.G.Nedorezov, A.A.Turinge for the GRAAL Collaboration
Institute for Nuclear Research RAS, Moscow, Russia

2. GRAAL collaboration GRenoble Accelerateur Anneau Laser ESRF – European Synchrotron Radiation Facility , Grenoble
a Universit`a di Roma 2 ”Tor Vergata”, I Roma, Italy
b INFN Sezione di Roma 2 ”Tor Vergata”, I Roma, Italy
c Universit`a di Catania, I Catania, Italy
d INFN Laboratori Nazionali del Sud, I Catania, Italy
e IN2P3, Laboratoire de Physique Subatomique et de Cosmologie, Grenoble,France
f INFN Sezione di Genova, I Genova, Italy
g IN2P3, Institut de Physique Nucl´eaire d’Orsay, Orsay, France
h INFN Sezione di Torino, I Torino, Italy
i INFN Sezione di Roma I, I Roma, Italy
j INFN Sezione di Catania, I Catania, Italy
k Institute for Nuclear Research, Moscow, Russia
l INFN Laboratori Nazionali di Frascati, I Frascati, Italy

3. content
1) Introduction
2) GRAAL facility – high quality of the gamma beam and detectors.
3) New experimental data on total photoabsorption in the nucleon resonance energy region for free and bound nucleons.
4) Interpretation of partial cross sections in frame of the MAID – 2007.
5) New methods and perspectives.

4. Introduction
Amplitude for the photon Compton forward scattering on quasi-free nucleon :
f = ’ f1() + i  ’* x  f2(),
Where  – invariant operator of the EM field,
 – spin operator of the nucleon,
 – photon energy.
At  = 0 (low energy theorem): ff0) = (k 2 / 2M 2 ),
Where M – mass,  = e2 /4qhc = 1/137, eZ – electric charge,
k - nucleon anomalous magnetic moment .

5. Free proton above  - resonance (Armstrong – 1972)

6. “Free” neutron above  - resonance (Armstrong – 1972) Subtraction of the proton contribution from the deuteron yield

7. Armstrong – Fermi correction

9. Total photoabsorption on quasi-free nucleons (Mainz , Frascati 1990-th) “Universal curve”

10. Actinide nuclei (Novosibirsk VEPP-4 , CEBAF - 2000)
Free proton - dotted line
Actinide nuclei - solid line
Nuclei with A = 7 – 238
(universal curve) – experimental points

11. GRAAL
E=600÷1500 МeV
E=16 МeV
P100%
NOT IN SCALE

12. Energy spectrum of gamma beam measured by tagging system (trigger — spaghetti or thin monitor)

13. Identification in the BGO ball

14. Identification in the forward direction E – TOF

15. Identification in the forward direction E – TOF

16. Stability: proton yield vs file number

17. Stability: proton yield vs file number spaghetti & thin monitor

18. Stability: proton yield vs file number spaghetti & thin monitor

19. Random coincidences trigger - spaghetti
No cuts
Coincidences
betweeen plastics of the
tagger

20. BACKGROUNDS (LH target) for the full (rhombs)and
Angular  -distribution
(LH target) for the
full (rhombs)and
empty (squares) target.
Difference between full and
empty target yields.
Rhombs - experiment
Triangles - simulation

21. Subtraction Method The total hadron yield Total yield - open points
N is the number of nucleons target;
Nis the gamma flux,
is the total photoabsorption cross section;
is the measurement efficiency (near 90%)
evaluated by simulations.
Total yield - open points
Empty target yield - stars
Difference - full points
12-C target

22. Subtraction method Simulation of experimental efficiency

23. Total photoabsorption for for p, d, C (GRAAL data)

24. Free proton cross section
Armstrong
GRAAL

25. Experimental results Free proton
Experimental data from
GRAAL (black points – subtraction method) ,
Armstrong (open points) ,
and Mainz (triangles).

26. Deuteron cross section
GRAAL
Armstrong

27. Difference between deuteron and averaged proton
P averaged
difference

28. New result : proton and neutron are represented by solid line : there is no difference

29. Total photo-absorption cross section for 12С.
Crosses - GRAAL data,
full points – Bianchi e.a.
open points - Mirazita e.a.
“Universal curve” - solid line.

30. Summing method Simulation of experimental efficiency
N - number of nucleons;
Ngamma flux,
partpartial cross section;
- measurement efficiency evaluated by simulations.
In brackets the geometry
efficiencies are shown

31. Simulation of efficiency
Computer program chain -
LAGGEN (LAGrange GENerator) - event generator to evaluate angular distributions for reaction products basing on existing experimental data.
Geometrical efficiency – probability of particle to touch the detector.
LAGDIG (LAGrange DIGitation) – GEANT code for definite experimental conditions (thresholds, cluster size, cuts etc).
Instrumental efficiency - probability for the particle to be measured in accordance with the detector response function
PREAN (PRE-ANalysis).
Total efficiency - ratio of simulated events (obtained in accordance with the described above algorithm) to the total number of events simulated for selected reaction using the event generator.

32. Separation of the events for one charged pion photo-production on quasi-free nucleon Red – 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

33. Separation of the events for one neutral pion photo-production on quasi-free nucleon Red – experiment, green – simulation
Missing mass of two g-quanta in BGO detector (reaction p=>p0).
Invariant mass of two quanta in BGO detector (reaction p=>p0).

34. Separation of the events for meson photo-production on quasi-free nucleon Red – 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=>p00). Rectangle marks area of the selected events.

35. Cross section evaluation
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.

36. Systematic accuracy for p => p

LASER :
Green points нм
red popints нм
Curve - MAID-2001

37. Systematic accuracy for p => n

LASER :
Green points нм
red popints нм
Curve - MAID-2001

38. Systematic accuracy for p => p
LASER :
Green points нм
red popints нм

39. Systematic accuracy for p => 0+n
LASER :
GREEN -UV 340 нм
RED нм

40. Total photoabsorption cross section for free proton (subtraction and summing methods) GRAAL-2008

41. Experimental results Bound proton (deuteron target)

42. Experimental results Bound neutron (deuteron target)

43. Green points – proton Red points - neutron Equality of p and n
Total photo-absorption cross section for the bound nucleon (deuteron target) GRAAL data (summing of partial channels)
Green points – proton
Red points - neutron
Equality of p and n
cross sections

44. Free and bound proton (deuteron target) GRAAL data (summing of partial channels)
Blue points – free proton
Red points – bound proton
Expanded scale

45. Ratio of free and bound proton photo absorption cross sections

46. Partial channels for the deuteron (MAID-2007 for the free nucleon)
p > + n
n > - p

47. Partial channels for the deuteron (MAID-2007 for free nucleon)
p > 0 p

48. Partial channels for the deuteron (MAID-2007 for free nucleon)
p >  p

49. Partial channels for the deuteron (MAID-2007 for free nucleon)
p >  p

50. Partial channels for the deuteron (MAID-2007 for free nucleon)
p > +0n
n > -0p

51. Partial channels for the deuteron (MAID-2007 for free nucleon)
p > -+ p

52. Partial cross sections for one and double pion and  meson photo-production on free and quasi-free proton and quasi-free neutron Deutron target Red – free proton, green – quasi-free proton, blue – quasi-free neutron.

53. Nuclear medium effects
small In deuteron (<5%)
strong in carbon ( 30%)
- final interactions of products,
- absorption of pions,
- coherent interaction.
New experimental methods are required

54. New methods are following
Tagged mesons
Elastic and inelastic interactions of short living mesons with nuclear medium

55. Meson Photoproduction in Light Nucleus Tagging of mesons by recoil nucleon
M, MeV    
Threshold E = GeV GeV
 = ,19 keV ,43 MeV
1/2 = h/ = ,5 * c ,8 * c
R =  c = ,6 105 Fm Fm
(slow mesons (v/c =10-2) ?? ??

56. For each event cal Pexp
Signatures for elastic scattering of meson (, ) in nuclear medium
For each event cal Pexp
Restricted conditions: Fermi motion
Experimental resolution
Spectator ?

57. E = 0.71 - 0.76 GeV (near threshold for )
Deuteron ( PFmax = 0.05 GeV/c) Simulation and experiment for  distribution
Experimental GRAAL data
Simulation of Fermi motion
E = GeV (near threshold for )

58. Signatures for inelastic scattering Simulation for ideal case (no backgrounds, ideal resolution) ( INC = Inelastic Nuclear Cascade)
Nucleus N-14
p = 20 – 100
Multiple (n <= 4) meson production is included
[I.Pshenichnov e.a. Moscow, EMIN-2001, p.170]
Correlation between kinematics variables

59. Another kinematics variables for the same conditions
(as previous slide) [LAGGEN + INC]
Correlation between recoil momentum and angle of emission (E g = 1,4 - 1.5 GeV);

60. Model independent correction on Fermi motion Eγ and θcm correction
Коррекция Eγ и θcm с учётом ферми-импульса нуклона мишени:
Еsp, ГэВ
Photoproduction of -mesons on the deuteron
── bound proton free proton
Θcm correction
Effective Eγ evaluation

61. Tagging of mesons production by recoil nucleons N > 
Experiment
Kinematics is not included
simulation  
Number of the charged tracks in forward = 1
Number of the neutral clusters in BGO = 2
20<theta<100
GRAAL facility allows to study interaction of unstable mesons with nuclear medium

62. CONCLUSION
Total cross sections for proton and neutron are equal to each other within 5% of experimental accuracy (deuteron target). F15 (1680) resonance is seen in both cross sections.
This means, probably that
- Charge invariance in photoabsorption is valid not only in the
asymptotic region but in the resonance region as well.
- the excited door-way states as universal nucleon matter can exist ?
2. Carbon cross section is in 30% less than free nucleon one above resonance region
Only Fermi motion can not explain modification of cross section in
nuclear medium, even for light nuclei.
To study intranuclear interactions a new method (Tagged mesons)
can be developed.