# Presentation: 944311

## From Nucleons and Mesonsto Quarks and Gluons

- Kees de Jager

- Science & Technology Peer Review

## JLab Scientific “Campaigns”

**The Structure of the Nuclear Building Blocks**
- How are the nucleons made from quarks and gluons?
- How does QCD work in the ‘strong’ (confinement) regime?
- How does the NN Force arise from the underlying quark and gluon structure of hadronic matter?
**The Structure of Nuclei**
- What is the structure of nuclear matter?
- At what distance and energy scale does the underlying quark and gluon structure of nuclear matter become evident?
**Symmetry Tests in Nuclear Physics**
- Is the “Standard Model” complete?  What are the values of its free parameters?

## From Nucleons and Mesons to Quarks and Gluons

- Introduction
- The Nucleon-Nucleon Interaction
- Proton Knock-out
- Nucleon-Nucleon Correlations
- Two-Nucleon Knock-out
- Inclusive Scattering at Large x
- Transition to the Quark-Gluon Description
- The Deuteron
- Elastic Scattering
- Photo-disintegration
- The Lambda-Nucleon Interaction
- The Nuclear Medium as Laboratory
- Medium Modifications of Nucleon Form Factors
- Color Transparency
- Summary

## The Nucleus as Composed of Nucleons and Mesons

- Single-Particle Description
- Mean-Field (Hartree-Fock) Approximation
- Electrons Interact with Single Nucleon Current
- Multi-Baryon Description
-  “Exact” Bound-State Wave Function or
- 	  Mean-Field Wave Function + Nucleon-Nucleon Correlations
- Two-Body Currents
- Meson-Exchange Currents (MEC)
- Isobar Configurations (IC)

- Describe basic properties of nuclear system in terms of a realistic N-N interaction

## History - Charge Distributions

- In ‘70s large data set was acquired on elastic electron scattering (mainly from Saclay) over large Q2-range and for variety of nuclei
- “Model-independent” analysis provided accurate results on charge distribution well described by mean-field Density-Dependent Hartree-Fock calculations

- Correlations??

## History - Proton Knock-out (NIKHEF)

## History - Proton Knock-out (cont.)

- Electron-induced proton knock-out has been studied systematically since high duty-factor electron beams became available, first at Saclay, then at NIKHEF with ~100 keV energy resolution.
- For complex (A>4) nuclei the spectroscopic strength S for valence protons was found to be 60-65% of the IPSM value

- Long-range correlations account for about 10%, but the rest was ascribed to short-range N-N correlations, by which strength was distributed at energies well above the Fermi edge
- These kinematics were not accessible at the accelerators of that era, but are at CEBAF

## E89-003 16O(e,e’p)

- First high-resolution (~0.8 MeV) (e,e’p) experiment at JLab
- Forward-backward asymmetry ALT at pm ≈ 300 MeV/c show need for fully relativistic calculations

- Constant (q,) kinematics to minimize variations in reaction kinematics

## E89-003 16O(e,e’p) (cont.)

- Cross-section data at Em up to 120 MeV over range of pm up to 340 MeV/c
- Broad peak at Em ≈ 40 MeV due to knock-out of 1s1/2-state protons
- Assuming a similar pm-behaviour as of the valence states, a large cross-section excess is observed at larger pm-values
- Calculations by Ryckebusch of (e,e’pp) and (e,e’pn) contributions are in reasonable agreement with the data

## Laget’s Calculations

- 3He Three-Body Disintegration

- 1Body

- 2 Body

- 3 Body

- n

- p

- p

- T=0

- T=0

- T=0

- +FSI

- p

- n

- p

- p

- n

- p

- Ground-State Faddeev WF (Paris potential)

## E89-044 3He(e,e’p)d

- Cross-section data both forward and backward of q up to over 0.6 and 1 GeV/c, resp.
- Well described by diagrammatical calculations by Laget (including correlations)

- color coding indicates different kinematics

## E89-044 3He(e,e’p)d (cont.)

- The left-right asymmetry ALT was also extracted from the data
- Udias’ calculations did not use a correct 3He wavefunction, so that his results for the cross section can not be compared to the data
- Both Laget’s and Udias’ relativistic calculations for ALT are in qualitative agreement with the data

- Ee = 4.8 GeV
- q = 1.5 GeV/c;  = 840 MeV

## E89-044 3He(e,e’p)pn (cont.)

- Earlier Saclay measurements of three-body break-up (PRL 60, 1703 (1988)) showed a bump in the cross section at Em-values increasing with pm, interpreted as indications of correlations
- However, the Saclay kinematics were in the dip region, increasing MEC contributions
- E89-044 repeated the Saclay measurements, but in x=1 kinematics
- A similar bump was observed, at smaller Em-values than predicted by Laget’s calculations
- In addition, FSI effects are shown to dominate in x=1 kinematics

## E97-111 4He(e,e’p)

- 4He(e,e’p) cross section in PWIA shows sharp minimum at ~450 MeV/c due to s-wave Short-Range Correlations
- How to Suppress FSI in (e,e'p)?
- higher momentum transfer
- momentum of outgoing proton increases
- FSI (elastic rescattering) decreases
- parallel kinematics
- always selects minimum pmiss
- in perpendicular kinematics: rescattered low pmiss nucleonscan contribute at larger pmiss
- Generalized Eikonal Approximation predicts minimal sensitivity to FSI for parallel kinematics

## E97-111 4He(e,e’p) (cont.)

- theory works at low missing momentum
- PWIA predicts minimum
- including FSI the minimum vanishes
- MEC and IC  give only minor contributions
- no minimum in experimental data

- calculations by J.-M. Laget

- Parallel Kinematics Q2 = 0.85 (GeV/c)2

## E97-111 4He(e,e’p) (cont.)

- PWIA (dashed):minimum at 470 MeV/c
- including FSI (dotted):minimum mostly filled in
- full calculation (solid):includes FSI + MEC + ICminimum mostly filled
- experiment:no sign of minimum at all

- calculations by J.-M. Laget

- Perpendicular Kinematics

## Proton Knock-out at JLab

- E94-004 2H            completed	cross section for x=1 up to pm ≈ 1 GeV/c
- E91-013  12C,Fe,Au  completed	 RL/RT for Tp 0.4,1.0 GeV,  for Tp 0.7,2.0 GeV
- E94-139 12C            completed	color transparency up to Q2 ≈ 8 (GeV/c)2
- E97-006 C,Al,Fe,Au analysis	S(pm,Em) for (pm,Em) up to (750 MeV/c, 75 MeV)
- E01-020 2H             analysis	RLT at Q2 ≈ 0.9-3.3 GeV2 and pm ≈ 0-400 MeV/c
- 		 in kins for x -0.5-1.5 and pm ≈ 0-400 MeV/c
- E00-102 16O                analysis	 RLT at constant (q,) pm to 0.5 GeV/c,Em to 150 MeV
- E01-108 4He             approved	 RLT at constant (q,) pm to 0.5 GeV/c
- 		 for pm to 1.2 GeV/c
- 		RL/RT for Q2 ≈ 0.8-4.1 GeV2

## Direct Observation of Short-Range Correlations

- Inclusive scattering at large |y|
- Multi-nucleon knock-out

## Inclusive scattering at large x

- E91-009

- Define y as the xB-value at which the minimum pmiss exceeds pFermi
- SRC model predicts:
- Scaling for xB > y and Q2 > 1.5 GeV2
- No scaling for Q2 < 1 GeV2
- In scaling regime ratio Q2-independent and only weakly A-dependent
- Glauber Approximation predicts:
- No scaling for xB < 2 and Q2 > 1 GeV2
- Nuclear ratios should vary with A and Q2

## Inclusive scattering at large x

- Cross-section ratios closely follow predictions of SRC-model
- Analogous results from E89-008 in Hall C

- a2 is Q2-independent and increases from ~4 (4He) to ~6 (56Fe)

## What Are Correlations?

- Observable: an NN-pair with
- large relative momentum
- small total momentum
- Distinguish between Correlations and Currents

-                  	          	         Correlations         Currents
- Two-Body Currents (MEC + IC)
- Not a Correlation
- Strongly enhance effect of correlation

- MEC

- IC

## Select proton/neutron with large 

- Two protons detected with p > 250 MeV/c > pFermi reconstruct neutron

- CLAS E89-027
- Preliminary results

- Select proton/neutron with almost all transferred energy (TN/ ≈ 1)
- Clear evidence of back-to-back excess over three-body absorption followed by phase-space decay simulation

## Select Quasi-Free Knock-out-> Isotropic Angular Distribution

## Summary of E89-027

- Select Quasifree Leading Nucleon in 3He(e,e’pp)n
- Fast NN Pair is:
-  **Back to Back **
** ****Isotropic with respect to q **
-  **Small Momentum along q**
**Fast NN Pair Not Involved in the Reaction**
- Similar Total and Relative Momentum Distributions for:
-  **pp and pn pairs**
** ****0.5 < Q****2**** < 1  and  1 < Q****2**** < 2  (GeV/c)****2**
-       WE ARE OBSERVING BOUND-STATE  CORRELATIONS!

**Notes:**
Thus, because when we select a quasifree leading nucleon the fast NN pairs are:
1 Back to Back,
2 Isotropic and
3 Have small average momentum along q
we conclude that the fast NN pair is not involved in absorbing the virtual photon.  
Because we measure similar total and relative momentum distributions for
pp and pn pairs and
0.5 < Q2 < 1 (Ebeam = 2.2 GeV) and 
1 < Q2 < 2 (GeV/c)^2  (Ebeam = 4.4 GeV) 
we conclude that we have measured bound state NN correlations.

## Transition from Nucleon-Meson to Quark-Gluon Description

- Deuteron
- Elastic Scattering
- Photo-Disintegration
- Polarization transfer

## Two Views of Deuteron Structure

- Two nucleons interacting via the (pion-mediated)NN force

- Two multi-quark systems interacting via the residue of the (gluon-mediated) QCD color force

## JLab Data Reveal Deuteron’s Size and Shape

- The nucleon-based description works down to < 0.5 fm

- Combined Data ->
- Deuteron’s Intrinsic Shape

## pQCD Counting Rules

- Data indicate that pQCD scaling is fulfilled for Q2 > 5 GeV2

- p

- q

- p+q

- Dimensional Scaling Quark Model

- Perturbative QCD

## Transition to the Quark-Gluon Description

- d/dt  f(cm)/sn-2
-   where   n = nA + nB + nC + nD
- 	s = (pA+pB)2, t=(pA-pC)2
- d  pn  n=13  d/dt  s-11

**Conventional**
**Nuclear Theory**

**p****A**

**p****B**

**p****C**

**p****D**

- Deuteron photo-disintegration probes momenta well beyond those accessible in (e,e’) (at 90°, E=1 GeV  Q2= 4 GeV2/c2)
- Conventional nuclear theory unable to reproduce the data above ~1 GeV
- Scaling behavior (d/dt  s-11) sets in at a consistent t   1.4 (GeV/c)2  (see  )
-  seeing underlying quark-gluon  description for scales below ~0.1 fm

## Exploring the Transition Region:  E99-008

## Exploring the Transition Region:  CLAS g2

- A microscopic theory for the Regge phenomenology.
- Non-perturbative approach (V.Grishina et al., EPJ A 10 (2001), 355)

- Production in the intermediate states of a color string leading to factorization of amplitudes

## Polarization Transfer in 2H Photo-disintegration

- In pQCD region hadron helicity should be conserved
- Appears valid in Py (induced polarization), not in Cx’ (polarization transfer) and marginally in Cz’
- Clearly the pQCD region has not been reached

**Notes:**
The PWIA calculation of Sargsian has Q2vsOmega, NN pair
opening angle, and relative momentum  distributions that
are consistent with the data . These simple PWIA calculation are surprisingly accurate. 
Npi + abs.  model fails in several key respects.
While it produce a large B2B peak in the $NN$ angular distribution (since a soft pion transfers a lot of energy but very little momentum), 
a) the average energy transfer is much larger than the data (typical of the
Delta(1232)) 
b) the relative momentum distribution is too large (since the minimum relative energy Erel = m_pi) and 
c) the ratio of the number of fast pn pairs to fast pp pairs is much lower than
the data (1 instead of 3).  Thus, while this mechanism might be very important for other three nucleon knockout experiments, it does not explain this data.

## The Lambda-Nucleon Interaction

- Electron-induced Hypernuclear Spectroscopy

## Hypernuclear Spectroscopy - Introduction

- central            spin-spin                          spin-orbit                                  tensor

- A hypernucleus AZ  is a hyperon impurity in the nuclear medium, hence without Pauli blocking
- Hypernuclear spectroscopy aims at a study of the -N interaction
- Practically all data so far obtained with secondary meson beams, significantly limiting the energy resolution (~2 MeV)

- Electroproduction offers the possibility
- to improve the energy resolution (~0.3 MeV)
- increase angular momentum transfer
- probe spin-flip amplitude
- to produce  throughout nucleus

## Hypernuclear Spectroscopy - E89-009

- First successful electroproduction of hypernuclei
- Using “zero degree” virtual photon tagging
- Good energy resolution (< 1 MeV)

- 12B

## Hypernuclear Spectroscopy - E94-107

- Next experiment in Hall A
- Symmetric set-up of HRS at 6° with two septum magnets (INFN/Rome)
- Energy resolution ~ 350 keV
- Improved particle ID with RICH
- Targets  7Li, 9Be, 12C, 16O, 52Cr
- Scheduled to run December 2003

## Hypernuclear Spectroscopy - E01-011

- Second-generation experiment in Hall C with specially designed HKS spectrometer
- (Tohoku University) with greatly improved production rates and energy resolution
- Expected to run in 2004

## Using the Nucleus as a Laboratory

- Medium Modifications
- Colour Transparency

## Polarization transfer in 4He(e,e’p)

- E93-049 (Ent, Ulmer)
- Measured 4He(e,e’p)3H in quasi-elastic kinematics
- 	for Q2 = 0.5, 1.0, 1.6 and 2.6 (GeV/c)2
- 	using Focal Plane Polarimeter
- Extracted Rexp = GEp/GMp

- Compared to calculations by Udias without and with inclusion of medium effects predicted by Thomas *et al*. (Quark Meson Coupling model)
- New proposal approved by PAC24

- Medium Modifications of Nucleon Form Factor

## Color Transparency

- At high Q2, the hadron involved fluctuates to a small transverse size  –  called the point-like configuration (PLC)
- The PLC experiences a reduced interaction with the  nucleus  –  it is color screened
- The PLC remains small as it propagates out of the nucleus
- So far, no hint of CT in (e,e’p) reactions

- CT refers to the vanishing of the hadron-nucleon interaction for a hadron produced in exclusive processes at high Q2

## Results from E94-104 ( n  p in 4He)

- Calculations use Glauber theory and correlations from Argonne v14 and Urbana VIII
- CT estimated from quantum diffusion model, normalization can be chosen arbitrarily
- Data show t-dependence seemingly at variance with traditional nuclear physics
- Clear need for extension to higher t-values

## Summary

- CEBAF has a broad and vibrant program of nuclear physics in all three halls
- The single-nucleon response is being studied through proton knock-out over a wide range of kinematics (in momentum transfer, missing momentum and missing energy) in few-body and many-body systems
- Nucleon-nucleon correlations have been clearly identified
- A variety of studies has established that the description of nuclei in terms of nucleons and mesons is valid down to a distance scale of less than 0.5 fm
- This description, however, is unable to reproduce photo-disintegration data above ~1 GeV, revealing the underlying quark-gluon description at scales below ~0.1 fm
- A ground-breaking experiment has established the feasibility of electron-induced hypernuclear spectroscopy with potentially excellent energy resolution
- The nucleus is being used as a laboratory to study the effect of the nuclear medium on nucleon properties and to search for the onset of color transparency