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

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

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 – ) New methods and perspectives.

Introduction Amplitude for the photon Compton forward scattering on quasi-free nucleon : f =  ’  f 1 (  ) + i   ’* x  f 2 (  ), 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,  = e 2 /4  qhc = 1/137, eZ – electric charge, k - nucleon anomalous magnetic moment.

Free proton above  - resonance (Armstrong – 1972)

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

Armstrong – Fermi correction

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

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

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

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

Identification in the BGO ball

Identification in the forward direction  E – TOF

Stability: proton yield vs file number

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

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

Subtraction Method The total hadron yield 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

Subtraction method Simulation of experimental efficiency

Total photoabsorption for for p, d, C (GRAAL data) p d !2-C

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

Deuteron cross section GRAAL Armstrong

Difference between deuteron and averaged proton D P averaged difference

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

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

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

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.

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

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

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.

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.

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

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

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

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

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

Experimental results Bound proton (deuteron target)

Experimental results Bound neutron (deuteron target)

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

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

Ratio of free and bound proton photo absorption cross sections

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

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

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

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

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

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

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.

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

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

Meson Photoproduction in Light Nucleus Tagging of mesons by recoil nucleon M, MeV   Threshold E  = GeV 1.1 GeV  = 1,19 keV 8,43 MeV  1/2 = h /  = 5,5 * c 7,8 * c R =  c = 1, Fm 23 Fm (slow mesons (v/c =10 -2 ) ?? ??

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

Deuteron ( P Fmax = 0.05 GeV/c) Simulation and experiment for   distribution E  = GeV (near threshold for  ) Experimental GRAAL data Simulation of Fermi motion

Signatures for inelastic scattering Simulation for ideal case (no backgrounds, ideal resolution) ( INC = Inelastic Nuclear Cascade) Nucleus N-14  p = 2 0 – 10 0  Multiple (n <= 4) meson production is included [I.Pshenichnov e.a. Moscow, EMIN-2001, p.170] Correlation between kinematics variables

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

Model independent correction on Fermi motion E γ and θ cm correction Коррекция E γ и θ cm с учётом ферми-импульса нуклона мишени: Е sp, ГэВ Photoproduction of  -mesons on the deuteron ── bound proton free proton Θ cm correctionEffective E γ evaluation

simulation Experiment Kinematics is not included Number of the charged tracks in forward = 1 Number of the neutral clusters in BGO = <theta<10 0 Tagging of mesons production by recoil nucleons  N >  GRAAL facility allows to study interaction of unstable mesons with nuclear medium  

CONCLUSION 1.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.