Presentation on theme: "Impulse Approximation limitations to the (e,ep) reaction on 208 Pb Identifying correlations and relativistic effects in the nuclear medium E06-007, March."— Presentation transcript:
Impulse Approximation limitations to the (e,ep) reaction on 208 Pb Identifying correlations and relativistic effects in the nuclear medium E06-007, March 3 – March 25, 2007 K. Aniol, A. Saha, J. M. Udías, G. Urciuoli Spokespersons Students: Juan Carlos Cornejo, Joaquin Lopez, + ?
Outline 1)What will be done? 2)Physics Motivation 3)Experimental Preparations 4)Invitation for students and Post Docs 5)Call for shift workers and run coordinators
What will be done? 208 Pb and 209 Bi(Kin04) Kinematics q [GeV/c] E o [GeV] ω [GeV] E e [GeV] θ e [degrees] P p [GeV] θ p [degrees] p m [GeV/c] Kin Kin Kin Kin Kin Kin Kin Kin Kin Kin Kin Kin Kin
What will be done? 208 Pb and 209 Bi(Kin04) Investigate states near the Fermi surface Based on s-FACTORs, the 4 lowest levels are predominantly single-proton hole states 3s½, 2d3/2, 1h11/2, and 2d5/2, and the 3474 level is predominantly the 1g7/2 single-proton hole state. 207 Tl Ex J config 0. 1/2+ 3s1/ /2+ 2d3/ /2- 1h11/ /2+ 2d5/ /2+ 1g7/2 209 Bi Ex J config 0 9/2- 1h9/2
Physics Motivation How well do we understand nuclear structure ? The underlying physical picture Dense system of fermions whose motions to first order can be treated as independent particles moving in a mean field Electromagnetic interactions Best probes for investigating the validity of the independent particle picture because they are sensitive to a large fraction of the nuclear volume Realistic microscopic calculations of the spectroscopic factors are not possible for medium-heavy (A>20) nuclei. Approximations are made that build on top of the shell model (mean field or independent particle picture) that is still the basic building block upon which most calculations of complex nuclei rely
Deviations from independent particle motion for orbits near the Fermi surface are attributed to effects beyond mean field (correlations) which reveal their presence in two ways: (i) Changes in the occupation and spectroscopic factors with respect to mean-field predictions (kin01-kin07, 0 to 300 MeV/c in p miss ) (ii) Changes in the momentum distribution of particles, especially at high momentum and binding energies (kin08-kin11, 400 to 500 MeV/c in p miss ) V.R. Pandharipande, I. Sick and P.K.A. deWitt Huberts, Independent particle motion and correlations in fermions systems, Reviews of Modern Physics 69 (981) 1997 Physics Motivation
The (e,ep) reaction at quasielastic kinematics and in exclusive conditions, for the outermost shells, becomes one of the most powerful and cleanest test of the mean field and the correlations needed to supplement it 208 Pb is the most suitable candidate to employ the mean field prediction, and thus it has been measured in the past in order to measure spectroscopic factors The reaction has been measured in the past, mainly in parallel kinematics and for moderate values of Q 2. Spectroscopic factors have been measured Physics Motivation
Still open issues: (i) possible dependence on Q 2 of the spectroscopic factors? 12 C(e,ep) data over wide range of Q 2. There appears to be a Q 2 dependence to the spectroscopic factors observed in this reaction. This interpretation has been disputed and the Q 2 dependence attributed to analysis methods of SRC 208 Pb(e,e'p ) has been studied in the past at low momentum transfers and spectroscopic factors in the range of 0.6 to 0.7 have been reliably extracted from parallel kinematics [4,5,6] at low Q 2 A measurement at several high values of Q 2 will directly address the question of momentum transfer dependence of the spectroscopic factors data , theory . Kin01, kin12, kin13
Shell model (mean field) calculations: the shape of the cross-section is well described, but the measured spectroscopic factors are below the mean field prediction. How large/small must be the spectrocopic factors? About 30% depletion is observed for states near the Fermi level. This cannot be explained only with short-range correlations Shape at moderate p m and parallel kinematics is well understood Long range correlations are predicted to be visible at large p m
Open issues: (ii) Long range correlations and cross-sections at high p m (> 300 MeV/c) If long range correlations are the reason for the small spectroscopic factors, then they should produce a large effect at high missing momentum. An experiment was performed at NIKHEF-K to measure the large momentum region, but the kinematics was far from X B =1. Additional strength was indeed found, but this can be explained either via long-range correlations [Bobeldijk,6] or by relativistic effects in the mean field model [Udias,7]. I. Bobeldijk et al., PRL 73 (2684)1994 x B = 0.18 E. Quint, thesis, 1988, NIKHEF J. M. Udias et al. PRC 48(2731) 1994 J.M. Udias et al. PRC 51(3246) 1996 Excess strength at high p miss
open issues: (iii) Are there any other signatures of relativistic dynamics in nuclei besides the spin orbit force? It seems that TL-observables (that change sign when the cross-section is measured when the knocked out proton is at both sides of the transferred momentum) are sensitive to dynamical relativistic effects A measurement at both sides of q will directly measure A TL and address the possible dynamical relativisitc effects. Yet such a measurement for 208 Pb has never been attempted kin01-kin07, for 0
Pb + C spectra, Kin01, p miss = 0 MeV/c Ee = GeV GEANT simulation Experimental Preparations
E miss for Pb + C, P miss = 200 MeV/c kin04 GEANT simulation Experimental Preparations
E miss, Pb+C P miss = -400 MeV/c kin09 GEANT simulation Experimental Preparations
Determine spectroscopic factors from the low p m kinematics (kin01 to kin07). Expected data were extracted with the peak fitting procedure after averaging the theory over the acceptance with a detailed GEANT simulation Experimental Preparations
A TL for all the shells added up, compared with the acceptance averaged simulations Non relativistic dynamics Relativistic dynamics Experimental Preparations
208 Pb(e,ep) 207 Tl(all 5 shells) Long range correlations are needed to explain the 30% depletion. If correlations are excluded, neither Nonrel. or Rel. theory can produce large strength at high p miss. This is in contrast to the ambiguous signature at x B << 1 of former experiment at NIKHEF-K With correlations Without correlations Look at large p miss for correlations. x B = 1 (II) Mahaux and Sartor (S F =0,6) (I) Ma and Wambach (S F =0,7) We compare to theoretical predictions after acceptance averaging. We do not attempt to separate the five shells in this region nor to compute A TL. If cross-sections are higher than the mean field prediction, the improved statistics might eventually allow for shell separation and A TL measurement
Target Issues Cold Lead in diamond sandwich E has demonstrated that a 0.5 mm lead foil sandwiched between two 0.15 mm diamond foils at cryogenic temperatures can withstand currents up to 80 uA and calculations indicate that they can operate at 100 μ A. Thinner (0.2mm) lead foil is not a problem Experimental Preparations Direct cooling of target block using cryogen flow from loop2. Target block contains 2 Pb targets and a Bi target. Will the diamond structure survive weeks of bombardment? High thermal conductivity is due to diamond structure.
Call for shift workers, run coordinators Requesting a 5 shift minimum or comparable contribution to be listed as an author on publications. Cryotarget operators will be needed because cryogen will be drawn from the ESR.