**Special PAC 18 Review of the Science Driving the 12 GeV Upgrade**

Executive Summary

The Jefferson Laboratory experimental program is producing important new
insights into the properties of hadrons and nuclei, especially at the
smallest distance scales. The future experimental program is being
shaped by these insights, along with recent advances in theoretical
understanding of the nuclear many body system and non-perturbative
aspects of QCD, to take maximum advantage of new opportunities. This has
led the electron scattering community to conclude that qualitatively new
discoveries, unforeseen at the time of conception of CEBAF, lie just
beyond the scope of the present accelerator. The combination of
continued progress in the development of the superconducting
radio-frequency accelerator technology, and foresight in the original
implementation of the accelerator makes an energy upgrade of the
facility to 12 GeV well matched to the science challenges and very
cost-effective.

The laboratory and the user community have developed an impressive
scientific case that demands this new capability. The Jefferson Lab
Program Advisory Committee was charged by the laboratory to review this
science, and to review the plans for the associated experimental
equipment.

The committee concludes that an outstanding scientific case has been
identified which requires the unique capabilities of the JLAB 12 GeV
upgrade. The results of these experiments are likely to significantly
change the way we think about nuclear physics and the strong
(non-perturbative) limit of QCD. Two major new thrusts can produce
definitive results: the experimental verification of the origin of quark
confinement by QCD flux tubes as predicted by lattice gauge
calculations, and the determination of the quark and gluon wave
functions of the nuclear building blocks. The full technical
capabilities of the upgrade are required for this progress. New research
domains are also opened up that show great promise in leading existing
research efforts to new levels of understanding.

The proposed experimental equipment are well suited to addressing these
new physics opportunities. The choices capitalize on the powerful
existing equipment at the laboratory without compromising the physics
goals.

The Program Advisory Committee was excited by the research potential
that the 12 GeV upgrade makes possible. The scope of the upgrade is very
well matched to the problems we see driving the field for the next
decade. The time has come to bring these opportunities to nuclear
physics.

Introduction

As the scientific program at Jefferson Lab blossoms, the Jefferson Lab
User community and the laboratory have become convinced that a
compelling scientific case exists for a significant upgrade of the
facility to 12 GeV beams. Progress in accelerator technology and
foresight in the initial planning of the laboratory have combined to
make this upgrade very cost-effective. The JLAB community has, over the
past six years, extensively evaluated their options for the future and
concluded that this is the best major step forward for studies of
hadrons and nuclei with electromagnetic probes. In the past year, a
series of workshops and working groups have made detailed studies of the
science issues and the instrumentation required. A list of these
activities is given in Appendix 1.

The laboratory has asked PAC 18 to review the science driving the 12 GeV
upgrade with the following charge:

*Jefferson Lab requests the PAC18:*

*1) Comment on the intellectual framework presented for the 12 GeV
"white paper." Is this the best way to present the science case to NSAC
and the larger nuclear physics community? Are there flaws or omissions
in the framework?*

*2) Review the experiments that are under consideration for being
highlighted in the white paper. Do they represent compelling science
that must be done to advance our understanding of nuclear physics? Have
we omitted any key science initiatives that could be supported by a 12
GeV electron beam?*

*3) Is the experimental equipment proposed well matched to the key
physics experiments motivating the upgrade?*

The membership of the PAC is given in Appendix 2 and the agenda of
presentations for the special review is given in Appendix 3.

In December 1999, a PAC subcommittee, chaired by David Cassel, reviewed
one principle motivation for the upgrade, a definitive search for meson
states with exotic quantum numbers using real photon beams. The
subcommittee concluded this was very high priority physics that could
uniquely be done at JLAB and that a technically sound design of the
experimental equipment existed to demonstrate that the scientific
potential could be realized. With this report in hand, this PAC did not
review the meson spectroscopy program in detail, but did consider how it
would fit into the presentation of the scientific case for the upgrade.

## Technical Scope of the Upgrade

The science case discussed below requires electron beams of 12 GeV to
produce the 9 GeV polarized photon beams required for the meson
spectroscopy program and electron beams of near this energy to make a
major extension of the kinematic regime accessible at the laboratory.
The performance of superconducting r.f. cavities has continued to
improve and with cavities of the present capability, the linac sections
can be upgraded with limited additions and replacements to achieve 1.1
GeV acceleration in each linac. With modest improvements in the
recirculation arcs, 11 GeV beams (with a total beam power of 1MW) could
be delivered to the present experimental halls. The addition of one new
set of arc magnets allows the beam to be recirculated for one further
pass through a linac section and a low power beam would be extracted
into the photon tagger system for a new experimental hall (Hall D) at 12
GeV, a hall dedicated to meson spectroscopy with polarized photon beams.

This concept for the Jefferson Lab upgrade provided the technical basis
for the PAC consideration of the scientific issues. In each case, it was
found that the scope of the upgrade was well matched to the breakthrough
scientific thrusts.

## Response to First Charge 

1\) Comment on the intellectual framework presented for the 12 GeV
"white paper." Is this the best way to present the science case to NSAC
and the larger nuclear physics community? Are there flaws or omissions
in the framework?

The white paper draft presented three primary areas where a 12 GeV
upgrade would make a major impact on our understanding of hadronic
physics and its QCD substructure. The PAC reviewed each of these areas
in turn and found all of them sound. The following comments summarize
these deliberations

-   *The experimental verification of the origin of quark confinement by
    QCD flux tubes.*

For the first time in three millennia of reductionism, science has
encountered a degree of freedom that cannot be isolated. The properties
of strong QCD and the nature of confinement are among the outstanding
unresolved problems in physics. At the turn of the 21st century there is
strong evidence from theory and computation that confinement of quarks
is intimately due to the development of flux tubes between them. This
seminal insight awaits experimental confirmation; its wider implications
are yet unknown.

As matter is heated, theory predicts that the gluonic degrees of freedom
in the flux tubes must be excited, the first manifestations of which
will be the appearance of \"hybrid\" states in the hadron spectrum.
Predictions for the energy scale of their production have now converged,
and estimates of their characteristics and production rates in
electromagnetic processes are now rather precisely predicted. In
particular the existence of exotic combinations of spin-parity-charge
conjugation (J^PC^) quantum numbers among the hybrid mesons will aid
their identification. These particular exotic states have their valence
quark-antiquark content coupled to spin-1 and are thereby especially
suited to production by photons, both real and virtual, in diffractive
processes. A 12 GeV upgrade at CEBAF will provide unique means to
produce these states, separate them from background and establish a new
spectroscopy in QCD.

-   *The determination of the quark and gluon wave functions of the
    nuclear building blocks.*

Historically, information on the parton structure of hadrons has
predominantly been restricted to knowledge of probability distributions
for gluon and quark flavors and spins. Recently, theoretical advances
have shown that \"off forward\" (or generalized) parton distributions
may be accessed in deep exclusive processes under suitable kinematic
conditions. The beam energy and quality of the proposed upgrade would
offer an extensive program of investigation in this kinematic regime and
reveal details of hadron structure that go far beyond what has hitherto
been available. Exclusive reactions such as deeply virtual Compton
scattering and exclusive meson production provide new and unique
information on the quark-gluon wave functions of the hadrons.

One limit of the generalized parton distributions is the electromagnetic
form factors of the hadrons. The pion is the lightest hadron and of
fundamental importance in its connection to chiral dynamics. While
significant measurements of the charged-pion form factor are underway at
JLab, it is only at the momentum transfers available with the upgrade
that these measurements will cleanly test the applicability of
perturbative QCD in exclusive reactions and the quark-antiquark
distributions of the pion.

Measurements with the upgrade can give unique information on the spin
and angular momentum structure within nucleons. This will touch on one
of the great enigmas discovered in recent years, namely the nucleon spin
problem, which in turn has focussed attention on the general question of
spin and flavor correlations within hadrons. These questions have become
particularly sharp in the extreme valence limit, x→1 where the role of
symmetry breaking, gluon dynamics and other aspects of QCD can be
uniquely determined with data in this hitherto poorly explored kinematic
region. A 12 GeV electron upgrade at JLab will for the first time
provide high precision deep inelastic scattering data for x\>0.65.

-   *Open important new research domains in areas already under
    investigation.*

The PAC encourages the community to be extremely cautious in including
many topics in this area in the highest level synopsis of the physics
case. All of the research thrusts broken out in this bullet were
regarded as excellent research programs for an existing facility. All
would greatly benefit from 12 GeV beams and should be included in the 12
GeV white paper and executive summary. However the laboratory must be
wary of being viewed as proposing to do "more of the same", or of having
promised "more than could be delivered" in the past. When these
experiments are prime foci of work at the present machine, the concerns
are that they might not appear sufficiently fresh or the ongoing work
might resolve the issues. In other instances the interest is very high
but the scientific underpinnings would need more work to be highlighted.
Two experiments reviewed by the committee definitely require higher
energy beams and did resonate with the PAC members: the x\>1 study of
short-range nuclear correlations and the coupling of the η and η\'
mesons to photons. These would be the examples the PAC would pick to
support the third bullet in a one page synopsis of the physics case. We
encourage the community to continue to sharpen the case for other
physics programs under this broad thrust.

The increased energy and unique beam qualities at CEBAF would allow a
substantially new study of nucleon correlations by measurements in the
x\>1 region. The parton structure that could be generated by such
correlations would become accessible at sufficiently large momentum
transfers. This could thereby provide important insights into high
density, multi-nucleon configurations in nuclei and more general
insights into their short distance structure. In addition, the newly
accessible kinematic regions will provide opportunities for
investigating the transition from the nucleonic picture of nuclei to one
based on quark degrees of freedom.

The η\' has been an enduring mystery that can be probed with the new
facility. This touches on a number of fundamental issues such as
anomalies in QCD, the nucleon spin problem and the role of glue in
mesons. By means of the Primakoff mechanism, the 12 GeV facility would
open investigations of both η and η\' coupling to photons as the
existing facility did for the π^0^. The ability to vary the virtuality
of one of the photons (from the scattered electron) can provide a
qualitatively new class of information on the substructure of the η and
η\' mesons.

## Response to the Second Charge

*2) Review the experiments that are under consideration for being
highlighted in the white paper. Do they represent compelling science
that must be done to advance our understanding of nuclear physics? Have
we omitted any key science initiatives that could be supported by a 12
GeV electron beam?*

In this section the PAC will review the individual experiments that were
presented as the basis of the 12 GeV program. The order of the
discussion simply follows the order of the presentations. The PAC has
tried to provide constructive criticism in each case to help guide the
users to strengthen the scientific and technical issues.

# 

# Several experiments highlight the unique contribution that can be made to studies of the valence quark structure of the nucleon. Indeed, the great discovery of three decades ago was the existence of the quark substructure of the hadrons. The quark-parton model emerged from the early inelastic electron scattering experiments as the physical picture of nucleon structure. In those early days, studies of the scaling features of the data predominated, with studies of the Q^2^ dependence made mostly at the highest Q^2^ values available. Bjorken scaling was the predominant feature of the landscape, down to surprisingly low values of Q^2^. Logarithmic scale breaking soon superceded the concept of Bjorken scaling, and experiments pushed to the highest energies to study these logarithmic deviations. Because of the history of ever increasing energies and Q^2^, the high-x region of kinematics we associate with the valence quarks in the nucleon, x\>0.5, was hardly covered. The high-energy beams available lacked the intensity and resolution needed to investigate the quark-parton model at high x. The 12 GeV upgrade at Jefferson Lab will be ideally suited to study the valence quark distributions in the nucleon in detail and with considerable precision. The following two sections discuss several "must do" measurements that could substantially strengthen our understanding of the nucleon in the valence quark region of the quark-parton picture.

### 

### The d/u Quark Ratio in the High-x Range

Raising the energy to 12 GeV opens up the high-x region for inelastic
electron scattering studies and a unique opportunity for new information
on nucleon structure. Measurements of the deep inelastic scattering
(DIS) structure functions F~2~^n^ and F~2~^p^ at high x can be readily
interpreted in terms of the ratio of the quark distribution functions,
d(x)/u(x). Textbook discussions of the quark-parton model often contain
plots of the ratio, F~2~^n^(x)/F~2~^p^(x) up to x of 0.8 obtained from
comparisons of deuteron and proton data. However, the binding energy and
Fermi motion effects must be correctly taken into account to extract the
value of F~2~^n^ for a free neutron. These corrections can be important,
since they can significantly modify the uncorrected values of F~2~^n^ .
In addition, the discovery of the so-called "EMC Effect" in DIS (for A
\> 3) has shown that the quark distributions are modified by the nuclear
medium and led to the consideration of other nuclear effects. This has
brought into question the validity of the wave functions used for the
deuteron as well. If there are uncertainties in the deuteron wave
function, theoretical errors associated with the uncertainties in the
deuteron are large for the ratio F~2~^n^/F~2~^p^ at high x, (x\>0.6.)

Jefferson Lab, with a 12 GeV Upgrade, can significantly improve on the
uncertainties in F~2~^n^(x)/F~2~^p^(x), and therefore on d(x)/u(x) at
high x, by a comparison of deep inelastic scattering from ^3^H and ^3^He
targets. Since the binding energies and density distributions are nearly
the same for these two nuclei, the nuclear effects should be quite
similar allowing a more model independent extraction of F~2~^n^
/F~2~^p^.

# 

# A~1~^n^, the asymmetry in inelastic scattering of longitudinally polarized electrons from a polarized neutron target 

QCD makes a clear prediction that the asymmetry of polarized electron
scattering on a polarized neutron target, A~1~^n^, must approach 1 as x
approaches 1.0. However the existing data, of reasonable quality only
below x of 0.4, find A~1~^n^ to remain negative or consistent with zero.
Measurements of A~1~^n^ must be extended to high enough x to determine
the x=1 value. More generally, analysis of A**~1~^n^**(x) and the
corresponding asymmetry data for the proton, A~1~^p^(x), have resulted
in the "spin crisis" (as seen by the failure of the Ellis-Jaffe Sum
Rules) and in the confirmation of the Bjorken Sum Rule, a test of QCD.
Currently models for A**~1~^n^** are being used in the high-x region to
evaluate the sum rules. High-x data on A**~1~^n^** (x) are desirable to
refine the *experimental* evaluation of the sum rules, to refine the
models used, and primarily to study the relationship between current
quarks and constituent quark models of the nucleon. If A**~1~^n\ ^** is
not found to approach 1 as x approaches one, this will be a dramatic
refutation of our current understanding of the quark model of nucleon
structure.

The extraction of A~1~^n^ requires that corrections for the binding of
the nucleons be made. This is a straightforward matter for most values
of x, but other admixtures in the ^3^He wave function, such as
low-probability ∆ components could become relatively more important at
the highest x values. The collaboration needs to investigate this point
in more detail.

Jefferson Lab has the ideal experimental conditions to contribute to the
large x regime. An experiment for 6 GeV and x \< 0.5 is being planned
for Hall A. The 12 GeV upgrade would allow for much improved
measurements in the deep inelastic scattering region that cover up to x
= 0.75, and with significantly improved errors over those of a 6 GeV
run. A 12 GeV upgrade would make a definitive measurement of
A**~1~^n^**(x) possible and would provide tests of quark structure which
could not be done anywhere else with comparable quality.

The range of kinematics from a 12 GeV Upgrade covers resonances of the
nucleon in the high-x range. Considerable interest in the behavior of
the resonances exists, including the possibility for better
understanding of duality and the possibility of using these data to
extrapolate to the x = 1 limit. The application of duality will be
discussed in more detail in a subsequent section.

While not presented to the PAC, the situation is somewhat similar for
the corresponding asymmetry for the proton, A~1~^p^(x). The data extend
out to x = 0.7, but with rather large errors for x \> 0.5. A program to
study the valence quark distributions should include measurements of
A~1~^p^(x) at large x for the same purposes mentioned for the neutron
asymmetry. In the case of A~1~^p^(x), the data are in general agreement
with quark model calculations, but improvement in these data would be
useful for refinement of the spin structure functions, for studies of
the nucleon resonances, and for the behavior as x approaches 1. An
improved measurement of A~1~^p^ with the 12 GeV Upgrade at Jefferson Lab
would support the unique opportunity to greatly improve our confidence
in the constituent quark model understanding of the nucleon.

### Deep exclusive scattering

Recent theoretical advances suggest that there are new approaches to
probe the structure of the nucleon. The generalized parton distributions
for the proton and the neutron can be experimentally determined by
studying deep exclusive scattering processes in various final states.
The measured cross sections are, in general, dependent on four universal
(for a given target) structure functions for each quark flavor. They are
functions of momentum fraction of the parton before the interaction, the
momentum fraction of the parton after the interaction, and the square of
the momentum imparted to the final state baryon. Accurate measurements
of these structure functions in the scaling region and over a broad
range in the kinematic variables can isolate the individual parton
distribution functions. The generalized parton distributions contain a
wealth of information about the transverse momentum and angular momentum
carried by the quarks in the proton which is not available in the quark
distribution functions measured in deep inelastic scattering. For
example, an important sum rule connects the integral over two of the
structure functions with the total angular momentum (spin plus orbital)
carried by the quarks.

The application of perturbative techniques is expected to be most
straightforward for deeply virtual Compton scattering where the
elementary verticies are purely electromagnetic and this process will be
the benchmark for deep exclusive scattering studies. Exclusive virtual
photon-induced meson (π,ρ \...) production will test the universality of
the generalized parton distributions expected from factorization and
permit the separation of the individual structure functions. It is
believed that at least 10 GeV is essential for beginning such an
exploratory program. It would represent a new basic approach for
studying QCD and parton distributions in particular and shows great
promise for providing fundamental **new** information on the parton wave
functions. Only the data can verify how extensively one can apply these
techniques. The upgrades proposed for the CLAS detector have been
motivated in part by the requirements for this class of experiments.

-   **Parton-Hadron Duality**

Recent experimental results from JLAB have revitalized the subject of
duality between quark and hadron descriptions of inelastic electron
scattering. This significant progress has encouraged the community to
consider new applications of duality concepts in hadronic structure. The
PAC believes that in order to study duality and its manifestations in a
quantitative way, the two following items have to be addressed:

1\) A precise averaging procedure of the resonances has to be defined,
both on experimental and theoretical grounds. This procedure has to be
first applied in the kinematic region in Q^2^ and W where scaling is
established.

2\) This procedure can then be extended to the lower Q^2^ and W regions
and to neutron and polarization observables.

The issue of the W lower limit commonly used as a frontier for the
scaling domain is an important one, and the somewhat arbitrary values in
common use will be better defined when duality is understood, and the
transition between the "resonances" and the scaling region can be
explained.

The primary advantage of a 12 GeV beam for studying duality itself is
the possibility of greater overlap between the resonance region and the
scaling region at this laboratory. Sufficient data can be taken with 6
GeV beams to address the proper averaging procedure in F~2~ or g~1~
relative to higher Q^2^ data from other laboratories. The extension of
the studies to semi-inclusive data or to σ~L~/σ~T~ will profit much
more, in a second step, from the 12 GeV upgrade. If duality were
demonstrated to be understood, it will provide a very appealing tool for
high-x studies at 12 GeV. The resonance region will become very useful
in obtaining information hard to get elsewhere.

-   **Short Range Nuclear Correlations Using DIS**

The suggestion to use DIS at x\>1 to search for super-fast quarks in
nuclei as a signature of short range correlations (SRC) is a very
important one. Certainly the observation of SRC has long been a goal of
traditional nuclear physics, but quantitative evidence for correlations
beyond the mean field has been difficult to obtain. Understanding the
reaction mechanism of knockout reactions in the kinematic regime where
correlations are expected to be visible has proved difficult. Two
nucleon knockout experiments are now possible, but it remains to be seen
whether they will prove easier to interpret. In any case, a clear
experimental signature of SRC in the the parton sector would be highly
desirable.

Inclusive DIS appears to provide just such a method. In the scaling
regime, which is expected to be achievable at 12 GeV even for moderately
heavy nuclei, the reaction should be straightforward to interpret in
terms of parton distributions. At x\>1, these are \"super-fast\"
partons. In the calculations shown, these predicted distributions are
interpreted as evidence for two-body and many-body correlations. The
signatures in the calculations are dramatic and unmistakable, almost too
good to be true. It is important that the theoretical interpretation of
the predicted super-fast partons in terms of the long sought short-range
correlations be reviewed as rigorously as possible. While the PAC finds
this interpretation eminently plausible, this subject has been fraught
with problems of interpretation for so long that the initial reaction of
outsiders could well be skeptical. Critical theoretical commentary
should be sought.

Deep inelastic scattering directly measures the distribution of the
light front momentum fractions, ξ, of the partons. The physical effect
here is that the many-body correlations result in significant
enhancements of the light front distributions of heavy nuclei for ξ\>1.
The light front distribution should not have a strong Q^2^ dependence
and an observation of this weak dependence will demonstrate one is in
the scaling region. The physics case would be much clearer if it were
recast in terms of the ξ dependence of the ratio of nuclear
distributions to separate the Q^2^ dependence from the quark momentum
dependence. The Nachtmann variable, ξ, rather than x, is the appropriate
experimental measure of the light front momentum fraction. It is the
large Q^2^ range available at 12 GeV that gives us confidence that the
scaling region will be reached in these measurements. As was previously
discussed in extracting nucleon structure functions from three body
systems, the existence of other objects such as ∆'s or more exotic
components can cloud the interpretation, but these would also signal
fascinating physics. It is important to directly relate the scale of the
predicted correlations to the scale of correlations sought in knockout
reactions at lower energies. A plot of n(p) for the mean field alone,
then with two-body and then many-body correlations would be helpful.

-   **Short Distance Nuclear Structure \--Form Factors of Light Nuclei
    at Large Q^2^**

The main features of presently measured form factors of the few nucleon
systems (A=2,3,4) are fairly well described in the standard framework of
interacting nucleons and mesons. Several models based on the
relativistic approach of the impulse approximation including meson
exchange currents and isobar configurations have proven to be relatively
successful.

Extending the measurement of the form factors to much larger Q^2^ would
provide fundamental nuclear physics data that would test our present
knowledge of the NN interaction at short distances. It is of high
interest to investigate a much larger Q^2^ domain to search for the
diffractive behavior predicted by traditional models that would be
absent in QCD-inspired models. The parallel investigation of ^3^H and
^3^He elastic form factors would also permit disentangling the isoscalar
and isovector contributions.

From an experimental point of view, the rapid decrease of the elastic
cross section with increasing Q^2^ will require high beam energy and
large acceptance spectrometers with energy resolution good enough to
separate the reactions of interest from competing channels. CEBAF at 12
GeV with the proposed instrumentation would be able to accomplish this
program, increasing the Q^2^ range by about 50% relative to the plans
for experiments at 6 GeV.

The PAC agrees that this is an important investigation to push to the
highest possible momentum transfers. However, a broad kinematic range
remains to be examined at 6 GeV and there is real potential for new
discoveries in those experiments.

-   **The electromagnetic form factor of the pion, F~π~**

![](media/image1.wmf)

The pion is the lightest hadron. Its relation to broken chiral symmetry
is a matter of considerable theoretical interest. Understanding its
structure has fundamental significance and therefore the electric form
factor of the charged pion is a quantity of prime importance to our
understanding of hadronic structure. A 12 GeV upgrade to CEBAF will
offer the possibility to measure this form factor to good precision out
to Q![](media/image2.wmf)= 6 (GeV/c)![](media/image3.wmf) for the first
time. This in turn offers the best possibility to study the transition
between the dominance of \`soft' and \`hard' processes in the dynamics,
and to learn where the perturbative QCD limit may be reached.

The pion form factor cannot be measured directly -- for practical
purposes it must be deduced from a careful study of the longitudinal and
transverse cross sections for p(e,e'![](media/image4.wmf))n in
kinematics selected to enhance the sensitive t-channel process and to
minimize background contributions. The t-channel longitudinal cross
section dependence is then fitted to the best available theoretical
model to determine the value of F![](media/image5.wmf). The F~π~
collaboration has applied this technique at CEBAF to determine the pion
form factor out to 1.6 (GeV/c)![](media/image6.wmf) and assesses the
theoretical uncertainty in extracting F![](media/image7.wmf) from the
data to be of order 6% in this range. A 12 GeV machine will open up a
new kinematic range to the experiments, allowing them to be performed at
higher W and low \|t\| at higher Q![](media/image8.wmf) which should
minimize the theoretical uncertainty in the results. It will not be
possible to fully assess the theoretical systematic error until the
program of longitudinal and transverse measurements has been carried
out, but current estimates by the collaboration indicate that
F![](media/image9.wmf) can be measured to ![](media/image10.wmf)7%, with
the theoretical uncertainty dominating the error bar. The committee
cautions that additional theoretical work needs to be done to fully
assess the model uncertainties with the aim of extracting
F![](media/image11.wmf) from the data with the best possible precision.
This measurement will be an important contribution to our knowledge of
hadronic structure and is ideally and uniquely suited to the CEBAF
physics program with the 12 GeV energy upgrade.

-   **Production of J/ψ mesons near threshold.**

The 12 GeV upgrade reaches one major production threshold compared to
the existing facility, the charm quark threshold at 8 GeV. The large
mass of the charmed quark immediately provides a short distance scale
for the QCD dynamics. The J/ψ meson is an inherently small object and
high momentum scales are involved in its production at these energies.
With the interest in the interaction of compact objects aroused by color
transparency studies and the need to understand J/ψ-nucleon interactions
in heavy ion collisions, it is unfortunate that the J/ψ-nucleon cross
section is not well determined by existing data.

The existing data on low energy electroproduction of the J/ψ's are
extremely sparse and are our only source of information on J/ψ-nucleon
interactions. Since the nucleon contains precious little c-cbar content,
quark exchange is strongly suppressed in J/ψ-nucleon interactions and
one has the opportunity to directly examine the role of gluon exchange.
The existing data hint that the energy dependence of J/ψ
photo-production changes dramatically near threshold and this has been
suggested to be a signal of a three gluon exchange mechanism taking over
from the two-gluon exchange mechanism that dominates in higher energy
exclusive channels.

Exploratory experiments addressing both the photoproduction mechanism on
the proton target and the nuclear dependence of J/ψ production can be
performed at the JLAB upgrade with the proposed experimental equipment.
Measurements of the energy dependence of the nuclear effects are
essential. A dedicated electromagnetic calorimeter electron detector
would allow significant increases in the statistical precision and
kinematic range of these experiments. Other more speculative programs
such as the search for hidden color states using J/ψ production could
also become possible.

While the current ideas in this program are quite focused, we expect
that the underlying physical domains are sufficiently different than
those of light quark systems that a number of new research initiatives
are likely to emerge here in the future.

-   **Precision Measurements of the electromagnetic properties of
    pseudoscalar mesons at 11 GeV using the Primakoff effect**

The physics questions addressed by the proposed measurements are very
important and appealing and follow two main axes:

1\) Precision measurements of the η and η' widths for 2-γ decay.

2\) The transition form factors for π^0^, η and η' decays into γγ\*.

Both these lines address the low energy manifestations of QCD in the
non-perturbative regime for which chiral theory is appropriate and the
π's and η's are the Goldstone bosons. The study of the π^0^, η and η'
mesons therefore provides a powerful tool to better understand the low
energy behavior of QCD.

A precise measure of point 1) will provide a clean means of getting the
mixing angle θ between the η~8~ and η~0~ flavor states, and of
extracting the f~η0~ parameter. A comparison of these data with other
approaches could also lead to establishing the possible gluonic content
of the η'.

The form factor measurements might be even more important, although they
are quite difficult. The existing π^0^ data do not even agree on the
sign of the form factor slope, and η and η' data do not exist. The
intercomparison of these 3 form factors will lead to very valuable
information on the quark-gluon content of these mesons. The PAC
recognizes the peculiar nature of the η' meson which motivates the
interest in its study, but has some concerns on the applicability of
chiral perturbation theory for such a heavy object.

This experimental program requires very challenging techniques,
especially in the performance of the recoil detector. The PAC strongly
encourages work to assure the feasibility of this promising program.

-   **Color transparency**

Color transparency is the vanishing of final or initial state
interactions at high Q^2^ in nuclear quasi-elastic scattering, or more
generally in hard color coherent processes. This vanishing is predicted
by QCD for such reactions that proceed by the formation of color
neutral, point-like configurations (PLC). The effects of gluons emitted
from a PLC cancel in coherent processes. But the necessary values of
energy and momentum transfer must be determined experimentally because
non-perturbative effects become relevant as the PLC expands as it moves
through the nucleus. There is recent very strong evidence for color
transparency at Fermilab in the observations of coherent di-jet
production in pion induced nuclear scattering, and there have been
indications observed in ρ meson production experiments. There is also
evidence from (p,pp) reactions at Brookhaven, in which the transparency
(relative cross section) is seen to rise rapidly and then fall as energy
increases. The fall is believed to be due to an interference effect that
would not occur in electron proton interactions. The discovery of color
transparency at the kinematics available to Jefferson Lab at 12 GeV
would demonstrate the existence of PLC as an important feature of the
nucleon form factors. Furthermore, there would be significant
implications for the structure of nucleons bound in nuclei. The PLC in
the nucleon wave function would not interact with the attractive nuclear
potential and therefore their probability amplitude would be reduced.
This would correspond to a suppression of high momentum components of
the nucleon wave function that would have consequences for diverse
measurements including deep inelastic scattering from nuclei and
measurements of the form factors of bound nucleons.

The program proposed at the JLAB upgrade involves three processes:
quasielastic (e,e\'p) reactions on nuclei, ed→e'pn reactions in
perpendicular kinematics and ed→e\'dρ^0^ reactions. The committee was
told that carrying out these experiments would not be difficult.

A. The quasielastic (e,e\'p) reaction is the simplest process. Previous
measurements can be extended to Q^2^=17 (GeV/c)^2^. The effects of color
transparency, manifested as a rise of the relative cross section as Q^2^
increases, have been predicted using models consistent with the BNL
(p,pp) experiment. The increase would be 20-30%, depending on the
nucleus over the available range of Q^2^. This is a simple reaction, but
the predicted effects are not huge.

B. The quasielastic ^2^H(e,e\'p)n process (detecting the proton and
neutron in perpendicular kinematics) offers the potential of large
experimental signals of color transparency. Rescattering of the struck
proton would lower the cross section for producing a final state in
which the spectator neutron has a small (\~200 MeV/c) momentum p~⊥~, but
it would greatly enhance the cross section in cases when the neutrons
have higher p~⊥~ (\~400 MeV/c). At large values of Q^2^, the effects of
color transparency would severely suppress rescattering thus drastically
changing the ratio of cross sections for large to small p~⊥~. Large
factors, 2 or more, are predicted. The interpretation does require that
the momentum of the final neutron not be too large.

C. The process ed→e\'dρ^0^ requires more and more rescattering of the
vector meson as the momentum transferred to the deuteron, \|t\|,
increases from 0.4 to 0.8 (GeV/c)^2^. At large values of Q^2^ the
effects of color transparency are predicted to suppress the
rescattering. The predicted suppression factors are typically about a
factor of 5.

The availability of 11-12 GeV electrons is absolutely necessary for each
of the experiments put forth. These experiments offer the opportunity to
definitively demonstrate or rule out the influence of color transparency
effects for the physics of nuclei.

-   **Possible Omissions in the program**

It is our conclusion that the community has done an excellent job in
identifying the principal scientific issues that can be addressed with
the JLAB 12 GeV upgrade. We would like to see further study in two
areas, but we cannot immediately identify critical experiments that we
are confident will lead to major progress.

1\) Jefferson Lab is a world leader in using parity-violating electron
scattering to study hadron and nuclear structure. A future experiment is
under consideration to extract the weak charge of the proton. We would
encourage further study in this sector of the electroweak interaction to
determine if any new possibilities arise with the higher energy beams.

2\) The nuclear dependence of deep inelastic scattering remains an
important unsolved problem. The study of short-range correlation effects
in the parton distributions at x\>1 was identified above as an important
research thrust. We see the possibility of other interesting new
measurements of the nuclear EMC effect, both in inclusive and
semi-inclusive scattering as the kinematic reach of the facility expands
in the traditional scaling region. The recent HERMES measurements of the
nuclear dependence of the ratio of longitudinal and transverse structure
functions is one example of a measurement that could be well suited for
the upgrade. Others are the use of hadron tagging of the flavor
dependence of nuclear effects, the search for coincident correlated
particles in the high-x region or the possibility of more complete
determination of nuclear final states resulting from deep inelastic
reactions.

## Response to the Third Charge

*3) Is the experimental equipment proposed well matched to the key
physics experiments motivating the upgrade?*

### Hall A

The 12 GeV program in Hall A includes a variety of reactions that
require in addition to the existing High Resolution Spectrometers, a
spectrometer featuring higher momentum capability and larger solid
angle, because of the high momentum of the reaction products and the low
cross sections of the reactions of interest. The design of the proposed
6 GeV/c (maximum momentum) MAD spectrometer meets these requirements by
using two identical dipole/quadrupole magnets with independently tunable
superconducting coils.

The overall momentum and angle resolutions are sufficient for the
reactions considered. The detector system was only indicated globally at
this stage but the laboratory has considerable experience in this
momentum range.

### Hall B CLAS-Upgrade

A major issue for the CLAS is to provide more complete kinematic
coverage to uniquely identify the final state. One important example is
the study of deep exclusive scattering to probe the generalized parton
distributions, with typical reactions like ep→e'p*m* in which the meson,
*m,* and electron are produced with high energies in the forward
direction (high W and Q^2^, low t). As an example: γ and π^0^ final
states need to be distinguished. The CLAS detector has to be modified
for that purpose. However, major parts will be kept. Since the reactions
of interest have low multiplicity they need to be discriminated against
high multiplicity backgrounds. This will be achieved by adding
additional detector parts: a central detector (superconducting solenoid
with tracker chamber and barrel calorimeter inside) to detect particles
emitted at large angles (θ\> 35^o^), and a segmented calorimeter
(scintillators with short radiation length, choice of material and
construction not yet decided) to cover the inside torus walls. In order
to cover the forward angles (θ\<35^o^) with good resolution the present
Region 1-3 chambers will be replaced by chambers with higher wire
density and lower polar angle (θ) acceptance. In order to be able to
distinguish between single photons (Compton) and the two decay photons
from π^0^ mesons (at minimum 10 cm apart at the forward calorimeter) a
preradiator calorimeter will be added, with good spatial resolution at
small θ.

There are still open questions that are being worked on. The concept is
solid, the open questions will certainly be answered.

**Hall A/C Electromagnetic calorimeter:**

The instrument will serve to detect hard photons and needs to provide
distinction between Compton photons and photons from π^0^ decay to
optimize the study of deep virtual Compton scattering. Therefore, it
needs high granularity for good position resolution, and good energy
resolution. Furthermore it needs to be fast and radiation hard
(positioned in forward direction). The choice made is to use PbF~2~
crystals (radiation length 0.98 cm, Cerenkov threshold for electrons ≈20
MeV) in an array of 1600 items, 3 by 3 by 15 cm^3^, and 3.5 m from the
target (for which there are existing examples). This will just be enough
to detect the two photons from the highest energy π^0^ meson decay
separately (≈7 cm apart at the calorimeter). For pile-up and background
reduction 1 GHz sampling ADC\'s and fast phototubes will be used for the
crystal readout. The design is adapted to the problem, it cannot be
built simpler and will fulfill the requirements.

**Hall C SHMS Spectrometer:**

The 12 GeV program in Hall C focuses on (e,e\'x) coincidence reactions,
where in addition to the scattered electron, which will have typical
energies of up to about 7 GeV, a hadron x, which can have a large
momentum, has to be detected in or close to the direction of the
three-momentum transfer, **q**. Since the existing SOS spectrometer has
too low maximum momentum and the most forward angle of the HMS
spectrometer is only about 10 degrees, a new spectrometer, the Super
High Momentum Spectrometer (SHMS), is proposed, which will accommodate
momenta up to 11 GeV/c and (central) scattering angles as small as 5.5
degrees. These values are well matched to what is needed. Because of the
forward angle capability the solid angle of the spectrometer will be
relatively small, but since most reaction products of interest will be
close to the direction of **q** this is not a major drawback. Also the
aspect ratio with its larger vertical than horizontal acceptance
alleviates the problem, since by taking data at more scattering angles
the θ and ϕ dependence of the cross section around **q** can still be
mapped out over a sizeable area.

The anticipated momentum and angle resolutions will yield missing mass
resolutions of typically 20-30 MeV/c^2^ when the \'struck\' hadron is
detected, which is more than sufficient. The angle resolution (and
accuracy) is especially needed in structure function separations and in
\'inverse detection\' reactions like ^1^H(e,e\'p)π^0^. At the highest
values of Q^2^ the missing mass resolution in the latter reaction will
be on the edge of what is needed and detailed simulations are required.

The wire chambers, scintillators and Pb-glass calorimeter of the
detection system are standard and have proven their reliability.

With the higher hadron momenta involved, the particle identification
techniques used in the HMS spectrometer are no longer sufficient. One
possibility is to use an additional Cerenkov detector at low pressure,
but other options like a transition radiation detector or aerogel, are
being considered.

The laboratory needs to have one spectrometer that can cover the entire
momentum range of the incident beam after the upgrade. The SHMS appears
well suited to this need.

#### Summary

The Program Advisory Committee concludes that an outstanding scientific
case has been identified which requires the unique capabilities of the
JLAB 12 GeV upgrade. The results of these experiments are likely to
significantly change the way we think about nuclear physics and the
strong limit of quantum chromodynamics. Major thrusts to identify the
properties of low lying QCD states beyond the well-known quark-antiquark
states and to measure the single quark wave functions of the hadrons are
expected to produce definitive results. If nature does not match our
current expectations, the quality of the data will be such that our
field will likely be forced to undergo a significant rethinking of
strong QCD.

We find that this experimental program can be carried out with the
Jefferson Lab upgrade. The proposed experimental equipment is well
suited to the research program and appears to involve proven
technological choices. While the major research thrusts are well served
by these choices, in some instances, the initial program will be
exploratory and concepts for new equipment will naturally evolve.

The PAC would like to acknowledge the major effort the user community
and laboratory have devoted to coming to grips with the best science
that can be addressed with electromagnetic probes in the 12 GeV regime
and thoroughly exploring the physics alternatives. The few additions
suggested by the PAC are more speculative in nature and involve
legitimate open issues about their usefulness at this time.

Appendix 1

12 GeV Workshops in 2000

The user community has been studying the scientific potential of higher
energy beams at Jefferson Lab for several years with major workshops in
1994 and 1998. Over the past year these efforts crystalized with a
series of workshops to develop the scientific case for the upgrade that
was presented to the Program Advisory Committee. A partial list of the
workshops held this year is given below.

Physics Opportunities with 12-GeV Beams

Hadrons in the Nuclear Medium

Workshop on the Nucleon Structure in High

x-Bjorken Region

Charm Studies at 12 GeV at JLab

Exclusive Reactions and Skewed Parton

Distributions

Workshop on Quark-Hadron Transition in Structure

And Fragmentation Functions

Exclusive Reactions and Skewed Parton Distributions

The Road to 12 GeV

January 13-15, 2000

March 7-10, 2000

March 30-April 1, 2000

April 3, 2000

April 13-15, 2000

April 17-18, 2000

May 26, 2000

June 21-23, 2000

Appendix 2

## **PAC 18 Membership**

DON GEESAMAN (Chair)

Argonne National Laboratory

Physics Division, BLDG 203

9700 South Cass Avenue

Argonne, IL 60439

Phone/Fax: (708)252-4059/3903

<Geesaman@anl.gov>

JUERGEN AHRENS

Institut fuer Kernphysik

Universitaet Mainz

D-55099, Mainz, Germany

Phone/Fax: 49 (0) 6131 39 25195/22964

<Ahrens@kph.uni-mainz.de>

HENK BLOK

Dept. of Physics and Astronomy

Vrije Universiteit

DeBoelelaan 1081

1081 HV Amsterdam

Phone/Fax: 31 20 444 7901/7992

<Henkb@nat.vu.nl>

FRANCIS CLOSE

Rutherford Appleton Lab

Didcot

0X110QX England

Phone/Fax: 44 1235 445605/523302

<F.E.Cose@RL.AC.UK>

JAMES FRIAR

Los Alamos National Lab

Theory Division, MS B283

P.O. Box 1663

Los Alamos, NM 87545

Phone/Fax: (505)667-6184/4055

<Friar@sue.lanl.gov>

CHARLES GLASHAUSSER

Department of Physics

Rutgers University

136 Frelinghuysen Rd.

Piscataway, NJ 08854-8019

Phone/Fax: (732)445-2526/4343

<Glashaus@physics.rutgers.edu>

MAURO TAIUTI

Dipartimento di Fisica

Universita' di Genova

Via Dodecanneso, 33

1-16146 Genova, Italy

Phone/Fax: 39 010 353 6458

<Mauro.Taiuti@ge.infn.it>

BARBARA JACAK

Dept of Physics & Astronomy

SUNY at Stony Brook

Stony Brook, NY 11794-3800

Phone/Fax: (516)632-6041

<Jacak@skipper.physics.SUNYSB.edu>

STANLEY KOWALSKI

Massachusetts Institute of Technology

Department of Physics, MS 26-427

77 Massachusetts Avenue

Cambridge, MA 02139

Phone: (617)253-4288

<Sk@MITLNS.MIT.EDU>

JACQUES MARTINO

DAPHNIA/SPhN

CEA Saclay

91191 Gif-sur-Yvette Cedex, France

Phone/Fax: 33 1 6908 7455/7584

[Jmartino@cea.fr](mailto:Jamrtino@cea.fr)

Gerald Miller

Physics. Box 35-1560

University of Washington

Seattle, WA 98195-1560

Phone/Fax: (206)543-2995/685-9829

<MILLER@nucthy.phys.washington.edu>

Shelley Page

Department of Physics and Astronomy

University of Manitoba

301 Allen Building

Winnipeg, Manitoba Canada R3T 2N2

Phone/Fax: (204)474-6202/7622

<PAGE@Physics.Umanitoba.CA>

CHARLES PRESCOTT

SLAC MS78

P.O. Box 4349

Stanford, CA 94309

Phone: (650) 926-2856

<Prescott@SLAC.Stanford.EDU>

Appendix 3

##### Agenda

### Friday, July 14

8:00 Executive Session and Charge to Committee (L102/104)

9:00 Welcome & Background (Auditorium) C. Leeman

9:15 Science Driving the 12 GeV Upgrade (30 min) N. Isgur

10:15 Coffee

10:45 Detector for the 12 GeV Upgrade (30 min) L. Cardman

11:45 A\^n_1 for Valence Quarks (30 min) Z-E. Meziani

2:45 Executive Session/Working Lunch

1:45 d/u as x -\> 1 (30 min) M. Petratos

2:45 Coffee

3:15 Deep Exclusive Scattering (45 min) M. Guidal

4:45 Committee Executive Session (L102/104)

6:30-7:30 Reception (Arc Auditorium)

### Saturday, July 15

8:00 Executive Session (L102/104)

8:30 Duality (15 min) C. Keppel

9:00 Short Range Correlations with DIS (15 min) W. Boeglin

9:30 High Q2 few body form factors (15 min) M. Petratos

10:00 Coffee

10:30 F\_{/pi} (15 min) G. Huber

11:00 The Threshold /psi N Interaction (25 min) E. Chudakov

11:45 Primakoff Production of \\eta and \\eta\' (15 min) L. Gan

12:15 Executive Session/Working Lunch

1:15 Color Transparency (25 min) R. Ent

2:00 Executive Session (L102/104)

Note: The closeout for the 12 GeV reviews will be included in the normal
PAC closeout

4:30 Wednesday afternoon.