Electron scattering from nuclei in the quasielastic region and beyond

Transcription

1 Electron scattering from nuclei in the quasielastic region and beyond Donal Day University of Virginia Part 1 HUGS 2007 June 2007 Newport News, VA

2 Nuclear Response Function Q 2 = q 2 ν 2 ν = (E E ) (ν ω) I II III IV "QUARKS" PROTON x-l xlzmit 2

3 Structure of the nucleus nucleons are bound energy (E) distribution shell structure nucleons are not static momentum (k) distribution short-range determined by N-N potential repulsive core scan/test/nn_pot.agr long-range attractive part on average: binding energy: 8 MeV distance: 2 fm r d r

4 How well do we understand nuclear structure? The shell model Basis upon which most model calculations of nuclear structure rely. 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 probe for investigating the validity of the independent particle picture because they are sensitive to a much larger fraction of the nuclear volume 4

5 Early hint on shell structure in the nucleus particular stable nuclei with Z,N = 2, 8, 20, 28, 50, 82, 126 (magic numbers) large separation energy E S E S - W.F average (Weizsäcker Formula, no shell structure, bulk properties) neutron number N shell closure Weizsäcker Formula == Semi Empirical Mass Formula

6 Large deviations from the SEMF curve at small mass number, e.g. A = 4. Systematic pattern of deviations occurs, with maxima in B occurring for certain magic values of N and Z, given by: N/Z = 2, 8, 20, 28, 50, 82, 126. These values of neutron and proton number are anomalously stable with respect to the average the pattern must therefore reflect something important about the average nuclear potential V(r) that the neutrons and protons are bound in... (Binding energy)/a [MeV] Atomic number 6

7 Shell structure (Maria Goeppert-Mayer, Jensen, 1949) Nobel Prize, 1963 nuclear density kg/m 3 With the enormous strong force acting between them and with so many nucleons to collide with, how can nucleons possibly complete whole orbits without interacting? But: there is experimental evidence for shell structure Pauli Exclusion Principle: nucleons can not scatter into occupied levels: Suppression of collisions between nucleons

8 Independent Particle Shell model (IPSM) single particle approximation: nucleons move independently from each other in an average potential created by the surrounded nucleons (mean field) spectral function S(E, k): probability of finding a proton with initial momentum k and energy E in the nucleus factorizes into energy & momentum part nuclear matter: Z(E) Z(k) occupied empty E F = k F 2 2M occupied empty E F E k F k

9 IPSM Simple model yet excellent first approximation to structure of the nucleus The single-particle energies ξ α and wave function Φ α are the basic quantities in IPSM In high energy knockout reaction we can directly measure ξ α and Φ α Observed first in Uppsala in 1957 in (p,2p) reactions on 12 C(p,2p) 11 B S( p, E) = i Φ α (p) 2 δ(e + ɛ α ) The spectral function should exhibit a structure at fixed energies with momentum distributions characteristic of the shell (orbit). 9

10 Quasi free Knockout Reactions m Initial State (k o,e o ) M A Final State m M A-1! 2 (k 2,E 2 ) m! 1 (k A-1,E A-1 ) (k 1,E 1 ) k A 1 = k o k 1 k 2 = k E s = T 1 + T 2 + T A 1 T o Momentum of particle in target nucleus that is knocked out Separation energy (missing energy) Energy required for separation of the nucleon from the target nucleus [Includes possible excitation of residual nucleus] 10

11 Tyren, Hillman & Maris (p+c 12 -> p + p + B 11 ) Uppsala in 1957,c v ~ ~ o BE('~*' ~,, ~'~'o I I I I Exc. E. (Met/) Fig. 4 and Fig. 5. Absolute cross sections for t he (p, 2p) reaction a t 185 MeV versus b i n d i n g energy of the removed proton. Separate energy scales for each t a r g e t also show the corresponding excitation energy of the residual nuclei. R e l a t i v e errors should not be m u c h larger t h a n t h e statistical errors shown on each point; absolute errors sho'lld be less t h a n 40 %, and t he error in comparing two spectra s o m e w h a t less. (p,2p) experiments provided information on the binding energies of the inner shells of nuclei and their momentum. These experiments suffered from distortion of the proton (strongly interacting): Jacob and Maris (1966) suggested using high energy electrons - nucleus is almost transparent to them 11

12 (e,e p)-reaction: coincidence experiment measured values: momentum, angles M A electron energy: E e proton: p p electron: k e E e = k e e reconstructed quantites: missing energy: e q k initial momentum E m = E e E e T p T A 1 missing momentum: p m = q p p p p = k + q in PWIA: direct relation between measured quantities and theory: E E m k p m

13 IA and IPSM The QFS reaction cross section dσ fi = KS( k, E) dσfree de 1 dω 1 de 2 dω 2 dω factorized Other reaction proportional S(p,E) are single nucleon pickup [(p,d), (d, 3 He), (ϒ,p)] Provides complimentary information but... strong absorption in nucleus hinders mapping out the spectral function. IA and IPSM is a considerable simplification Assumption that asymptotic p m and E m are equal to values just before knockout Elementary reaction = free No FSI Factorized form is preserved when strong interaction effects are considered - DWIA 13

14 The first (e,e'p) measurement: identification of different orbits Frascati Synchrotron, Italy U. Amaldi, Jr. et al., Phys. Rev. Lett. 13, 341 (1964). 12 C(e,e'p) 27 Al(e,e'p) moderate resolution: FWHM: 20 MeV

15 5(p,E) Saclay 9 9& e'p) Be(e,e p) 8 Li P (MdY~ Fig. 10. Proton separation energy spectra for the 9Be(e,e'p) reaction, urithin di f - ferent recoi l momentum bins. The energy resozutian of ti 0.8 MeV renders visible acme d i f ferent exc i ted states of BLi at Zo~u separat i on energy. Azta have been corrected for radiative effects, but the overal l absolute aoaze is arbitrary. Characteristic momentum behavior of the s and p shells can be clearly seen. J. Mougey ``The (e.e'p) reaction Nuclear Physics A Volume 335, (1980) S( p, E) = i Φ α (p) 2 δ(e + ɛ α ) 15

16 NIKHEF: resolution 150 kev shape described by Lorentz function with central energy E α + width Γ α J π 3/2 5/2 1/2 g.s. 11 B 3/2 g.s. 12 C 0 + Steenhoven et al., PRC 32, 1787 (1985) ==> electrons are a suitable probe to examine the nucleus

17 Shell Model: describes basic properties like spin, parity, magic numbers... momentum distribution [(GeV/c) -3 /sr] NIKHEF results p m [MeV/c] Momentum distribution: - characteristic for shell (l, j) - Fourier transformation of Ψ lj (r) ==> info about radial shape

18 Theory on previous slide (solid line): Distorted wave impulse approximation (DWIA) solves the Schrödinger equation using an optical potential (fixed by p- 12 C) (Hartree-Fock, self-consistent) real part: Wood-Saxon potential imaginary part: accounts for absorption in the nucleus Correction for Coulomb distortion NIKHEF well reproduced shape strength of the transition smaller! IPSM 1.0 Number of nucleons in each shell (IPSM): = 2j + 1 Spectroscopic factor Z α Z/IPSM 65% Z α = 4π kf de dk k 2 S(k, E) single particle state α = number of nucleons in shell

19 CBF IPSM c12_spectheocomp_nice.agr k F k < k F : single-particle contribution dominates k k F : SRC already dominates for E > 50 MeV k > k F : single-particle negligible consequence: search for SRC at large E, k method: (e,e p)-experiment

20 spectral/c12_momtheocomp_nice1.agr signature of SRC: additional strength at high momentum Modern many-body theories: Correlated Basis Function theory (CBF) O. Benhar, A. Fabrocini, S. Fantoni, Nucl. Phys. A505, 267 (1989) Green s function approach (2nd order) H. Müther, G. Knehr, A. Polls, Phys. Rev. C52, 2955 (1995) Self--consistent Green s function (T = 2 MeV) T. Frick, H. Müther, Phys. Rev. C68 (2003)

21 Missing strength already at moderate p m compared to IPSM 200 MeV/c < p m < 300 MeV/c k4que01_ipsmben250.eps Q 2 = 1.5 (GeV/c) 2 data on 12 C IPSM*0.85 CBF radiation tail Spectral function containing SRC: good agreement with data

22 Setup in Hall C: SOS GeV/c D iron magnets D p detector system HMS GeV/c target e GeV collimator Q Q Q D e detector system superconducting magnets, quadrupole: focussing/defocussing Performance HMS SOS momentum range acceptance δ (%) ±10 ±15 p = p 0 (1 + δ) solid angle (msr) target acceptance (cm) ± 7 ±1.5

23 Data at high p m, E m measured in Hall C at Jlab: targets: C, Al, Fe, Au kinematics: 3 parallel p q P' P m P' q q P m To map out S(E m,p m ) vary q keeping p (T p ) constant so that FSI are constant kinematics: 2 perpendicular p q e P m e'! e q! p P' Fix e, θ e, p Vary E m thru e Vary p m with proton angle θ p

24 Data at high p m, E m measured in Hall C at Jlab: targets: C, Al, Fe, Au Covered E m -p m range: perpendicular kinematics parallel kinematics kinp1 EmPm_allkins.eps kin4 kin3,θ qpi <45 kin3,θ ο qpi >45 ο kinp2 kin5 high 24 E m - region: dominated by Δ resonance

25 Extraction of the spectral function: only in PWIA possible, care for corrections later exp. c.s.: dσ fi de e dω e de p dω p = K σ free } } e p cs S(p m, E m )T A ( Binning of the data (E m,p m ) ij : dσ de e dω e de p dω p ) ij ΔE m = MeV, Δp m = 40 MeV/c = K σ free S( p m, E m ) ij T p = (N ij N bg ij )/ɛ L P i,j FWHM: 0.5ns N ij bg e & p: N ij N ij bg Luminosity phase space from M.C. Efficiency, dead time... 2ns

26 ck5k4k3_e01trec5n_cc1on_nice.agr

27 Integrated strength in the covered E m -p m region: 670 Z C = 4π dp m p 2 m de m S(E m,p m ) 230MeV/c Strength distribution in % (from CBF) 800 region used for integral p m (MeV/c) contains half of the total strength E m (MeV)

28 correlated strength in the chosen E m -p m region: 12 C experimental area in total (correlated part) exp. CBF theory G.F. 2.order selfconsistent G.F % % 12% 20% contribution from FSI: -4 % 10% of the protons in 12 C at high p m, E m found first time directly measured comparing to theory leads to conclusion that 20% of the protons in Carbon are beyond the IPSM region Rohe et al., Phys. Rev. Lett. 93, (2004)

29 e "QUARKS" e q k PROTON x-l xlzmit X Q1 Q2 Q3 Dipole 27m 29

30 Inclusive Electron Scattering from Nuclei Two distinct processes Quasielastic from the nucleons in the nucleus e k e k + q, W 2 = M 2 e k e W 2 (M n + m π ) 2 M A M A 1, k M A M A 1, k x > 1 x < 1 Inelastic and DIS from the quark constituents of the nucleon. Inclusive final state means no separation of two dominant processes x = Q 2 /(2mυ) 30 υ,ω=energy loss

31 Formalism L µν = 2 [ k µ ek ν e + kν ek µ e g µν (k e k e ) ] dσ 2 dω e de e W µν = X = α2 Q 4 E e E e L µν W µν 0 J µ X X J ν 0 δ (4) (p 0 + q p X ) Currents can be written as sum of one-body currents which (eventually) allows (See O. Benhar) W µν (q, ω) = where d 3 k de ( m E k ) [ ZS p (k, E)w µν p ( q) + (A Z)S n(k, E)w µν n ( q) ] w µν describes the e/m response of a bound nucleon with momentum k which consists of an elastic and inelastic component. QES in IA DIS d 2 σ dωdν d 2 σ dωdν d k d k deσ ei S i (k, E) } } δ() Spectral function de W (p,n) 1,2 S i (k, E) } } Spectral function 31 G p,n E (Q2 ) and G p,n M (Q2 ) W p,n 1,2 (Q2, ν) W p,n 1,2 (x) + log(q 2 ) corrections

32 k 1 k 2 p q p X There is a rich, if complicated, blend of nuclear and fundamental QCD interactions available for study from these types of experiments. P A P A - 1 The two processes share the same initial state d 2 σ QES in IA dωdν d k deσ ei S i (k, E) δ() } } DIS d 2 σ dωdν d k Spectral function de W (p,n) 1,2 S i (k, E) } } Spectral function The limits on the integrals are determined by the kinematics. Specific (x, Q 2 ) select specific pieces of the spectral function. n(k) = de S(k, E) However they have very different Q 2 dependencies σ ei elastic (form factor) 2 W 1,2 scale with ln Q 2 dependence Exploit this dissimilar Q 2 dependence 32

33 Relation to charged current neutrino-nucleus scattering e + A e + X dσ 2 dω e de e = α2 Q 4 E e E e L µν W µν ν l + A l + X dσ 2 dω l de l = G2 k 32π 2 k L µνw µν Both can be cast in the same form d 2 σ dωdν d k deσ ei S i (k, E) } } δ() Spectral function σ ei σ νi weak charged current interaction with a nucleon 33

34 Early 1970 s Quasielastic Data -> getting the bulk features 500 MeV, 60 degrees q 500MeV/c R.R. Whitney et al., Phys. Rev. C 9, 2230 (1974). Li C Pb Nucleus k F ɛ 6 Li C Mg Ca nat Ni Y nat Sn Ta Pb compared to Fermi model:fit parameter k F and ε

35 The quasielastic peak (QE) is broadened by the Fermi-motion of the struck nucleon. 3 He SLAC (1979) The quasielastic contribution dominates the cross section at low energy loss (ν) even at moderate to high Q 2. The shape of the low ν cross section is determined by the momentum distribution of the nucleons. As Q 2 >> inelastic scattering from the nucleons begins to dominate We can use x and Q 2 as knobs to dial the relative contribution of QES and DIS. 35

36 A dependence: higher internal momenta broadens the peak 36

37 Correlations and Inclusive Electron Scattering High energy loss side of qe peak Shaded domain where scattering is restricted solely to correlations. Czyz and Gottfried (1963) ω c = (k + q)2 2m + q2 2m ω c = q2 2m qk f 2m Czyz and Gottfried proposed to replace the Fermi n(k) with that of an actual nucleus. (a) hard core gas; (b) finite system of noninteracting fermions; (c) actual large nucleus. Low energy loss side of qe peak 37

38 Studying Superfast Quarks In the nucleus we can have 0<x<A In the Bjorken limit, x > 1 DIS tells us the virtual photon scatters incoherently from quarks Quarks can obtain momenta x>1 by abandoning confines of the nucleon deconfinement, color conductivity, parton recombination multiquark configurations correlations with a nucleon of high momentum (short range interaction) DIS at x > 1 is a filter that selects out those nuclear configurations in which the nucleon wave functions overlap. We are studying the dynamics of partons that have abandoned the confines of the nucleon. < r NN > 1.7 fm 2 r n = 1.6 fm The probability that nucleons overlap is large and at x > 1 we are kinematically selecting those configurations. 38

39 Short Range Correlations (SRCs) Mean field contributions: k < k F Well understood, Spectroscopic Factors 0.65 High momentum tails: k > k F Calculable for few-body nuclei, nuclear matter. Dominated by two-nucleon short range correlations Isolate short range interactions (and SRC s) by probing at high p m : (e,e p) and (e,e ) Poorly understood part of nuclear structure Sign. fraction have k > k F Uncertainty in SR interaction leads to uncertainty at k>>, even for simplest systems 39 Deuteron k < 250 MeV/c 85% of nucleons 40% of KE V(r) Carbon 0 ~1 fm NM N-N potential r [fm] k > 250 MeV/c 15% of nucleons 60% of KE

40 3 2 1 % 4 H e X I I I q (frfi I) -1 "_,~. ~ - 2 '. "<. -3 "'. \ \"\ \ - i -' i -7 o 3J i1 ;60 1] ~ ' \ i i. I I I. ~ q (f rill ) :"~>,. - 4 " - 5. :: \ "N i ") \ X, HJ -6- SSCB'~" k RSC Calculations of SRC Show up at large momentum -8-9 iunc Fig. 2. Momentum distributions for 4He, H J: Hamada- Johnston potential, RSC: Reid soft core potential, SSCB: de Tourreil-Sprung super soft core potential B, UNC: uncorrelated, for the RSC potential. The other uncorrelated distributions do not differ appreciably for q > 2 fm-1. ] Zabolitzky and Ey, PLB 76, " ^. : UNC Fig. 3. Same as fig. 2, for 160. Van Orden et al., PRC21,

41 Q 2 = 2 Correlations are accessible in QES and DIS at large x (small energy loss) Rozynek & Birse, PRC, 38 (2201) 1988 ω (GeV) CdA, Day, Liuti, PRC 46 (1045) 1992 L. Conci and M. Traini, UTF 261/92. Q 2 = 50 41