ELSA and MAMI Program Advisory Committee
Bonn, December 15-16, 2003
Report
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At its December 15-16, 2003 meeting in Bonn, the Program Advisory Committee has
reviewed proposals and letters-of-intent for the electron accelerators ELSA at Bonn and
MAMI at Mainz. Both laboratories provide unique and complementary experimental facilities
for the study of the electromagnetic structure of nucleons and nuclei.
After finishing the GDH experiment, the operation of ELSA has concentrated on running the
first round of experiments that were approved for the Crystal Barrel detector. This detector,
consisting of 1380 CsI crystals, has been transferred from CERN to ELSA and has been recommissioned in the tagged photon beam line. Its large solid angle coverage and high
granularity make it a unique instrument for the detection of multi-photon final states,
especially in combination with (the travelling) TAPS detector covering the forward region.
First results for  photoproduction off the proton, which have been submitted for publication,
extend the present world data to higher energy and show the ability of the detector to do
precision measurements. In addition, analysis of the data obtained with the SAPHIR detector
(which has been dismantled) is producing high quality results and publications on strange
particle and vector meson photoproduction.
One of the recent exciting developments in our field was the mounting evidence for the
existence of a penta-quark resonance, the +. ELSA has made a major contribution to this
development by publishing a signal in the p Kos+ K+n) channel obtained from the
re-analysis of existing SAPHIR data. In addition, the Crystal Barrel/TAPS combination has
recently seen a preliminary exotic signal in the p Kos+ Kolp) channel. A major
effort was required to re-arrange the ELSA run schedule to make it possible for the Crystal
Barrel/TAPS to take additional data. The PAC would like to commend the ELSA
management and the experimenters for their fast response to an exciting new development.
Most of 2004 will be devoted to move the Crystal Barrel from its present location (the former
SAPHIR beam line) to the new location that has been freed up by the GDH experiment. The
new Crystal Barrel location provides better spin transfer characteristics and more space
downstream of the detector. This space can be used to complement the Crystal Barrel by
additional forward detectors, e.g. a forward plug to replace TAPS, a forward spectrometer for
charged particle detection, or an electron spectrometer at 0 degrees for very low Q2 electron
scattering.
The MAMI facility is continuing to make excellent use of the existing 880 MeV electron
accelerator while pursuing aggressively the construction of the MAMI-C project which will
increase the available energy to 1.5 GeV. Major recent achievements include the
measurement of polarization observables in virtual Compton scattering, the measurement of
the electric form factor of the neutron, completing the data taking for the GDH integrand on
deuterium (using a novel high polarization target substance), and the first result from the
parity violation experiment A4. 
The construction of MAMI-C is making good progress. All four large magnets have been
installed and field mapped; they show excellent magnetic field homogeneity exceeding the
expectations. Problems with the vendors responsible for the fabrication of the accelerating
structures and RF generators have now been solved. Final component delivery will be in early
2005, commissioning is expected to start later in 2005.
A new capability for the detection of neutral events with large acceptance and good resolution
will be provided by the Crystal Ball detector which has been transferred recently from
Brookhaven National Laboratory to MAMI. The detector has been refurbished and has been
equipped with new electronics; commissioning in the tagged photon beam has been started.
After adding the TAPS detector, the first experiment, the measurement of the magnetic dipole
moment of the (1232) resonance, will be carried out making use of the present MAMI-B
accelerator.
New technical developments are necessary to make effective use of the higher energy energy
of MAMI-C. These include upgrading the bremsstrahlung tagging system to the higher
energy, and the construction of a new magnetic spectrometer for charged kaon detection at
small angles, the KaoS spectrometer. The short flight path of the KaoS spectrometer will give
access to the investigation of kaon electroproduction close to threshold (to test predictions of
chiral perturbation theory), and ultimately to the high-resolution spectroscopy of hypernuclear
levels. The close proximity of the spectrometer to the primary beam and the high-rate
requirements for the detectors make the KaoS spectrometer a challenging and interesting
hardware project.
General Comments on the Future ELSA Program
The first round of experiments approved for the Crystal Barrel at ELSA has been completed.
Most of the experiments used tagged photons on an unpolarized hydrogen target. Some data
were taken on deuterium and nuclear targets. The strength of the Crystal Barrel detector is the
efficient detection of multi-photon final states, e.g. in the reaction channels p p
and p p. These final states are of crucial importance for the identification of the
complete spectrum of excited states of the nucleon, especially for the 'missing resonance'
search.
The future Crystal Barrel program will pursue similar physics goals but with an emphasis on
getting the extensive experimental information, especially single and double polarization
observables, necessary to perform Partial Wave Analyses for multi-particle final states. This
requires performing experiments that combine tagged polarized photons and a polarized
target. ELSA and its user community have developed these capabilities: linearly and
circularly polarized photons, and a longitudinally polarized target have already been used; a
transversely polarized target is presently under development. In addition, the community is
developing the analysis tools required to extract the relevant information from the data.
Five letters-of-intent were presented to the PAC to outline major components of the future
program. The PAC considers baryon spectroscopy with double polarization observables to be
the core program which is expected to have the largest impact on the field. The combination
of high energy tagged photons from ELSA, and the detection capabilities of the Crystal Barrel
provide a unique facility that is complementary to other facilities in the world (especially to
Jefferson Lab in the U.S.A.). The planned additions to the Crystal Barrel will provide
additional capabilities, or will reduce the dependence on the availability of TAPS. Detailed
comments on the letters-of-intent are listed below.
ELSA/1-2003
Magnetic Moment of the S11(1535) Resonance
Contact person: M. Kotulla
The physics motivation for a determination of the magnetic dipole moment (MDM) of the
S11(1535) via measurement of the reaction p’p is judged to be strong. The dominance
of the S11 resonance in the  photoproduction channel compared to contributions of other
resonances or background is the key. Further, there are dramatically different predictions
(opposite signs) for this quantity depending on whether it has a (qqq) or K nature.
In the letter considered by the PAC, the projected sensitivity in the energy differential cross
section for the measurement on the proton was not convincing. It is clear that many other
effects could mask the signal for the MDM in the theoretical model used to analyze the data.
In addition, a measurement on the neutron using a deuterium target was discussed. This
eliminates the bremsstrahlung contribution present in the proton but requires the detection of
the recoiling neutron.
A future proposal should make the case that the necessary sensitivity required to isolate the
MDM is demonstrated. This will require consideration of many effects in the theoretical
model.
ELSA/2-2003
Photoproduction of ’-mesons from the nucleon
Contact person: B. Krusche
This letter-of-intent proposes sound and doable physics. The chief objective is to investigate
the contribution of s-channel resonances to ’ photoproduction. Only N* states are involved
which will facilitate the interpretation of the results. There are three established, five 1-2 star,
and many missing N* resonances, usually fairly broad (several hundred MeV). The complete
description of ’ photoproduction requires 4 amplitudes, thus we need 7 measurements,
including the ones with a transversely polarized target which is not yet available for the
Crystal Barrel. The PAC encourages a full proposal in due time which should address the
problem of the uniqueness of the multipole determination, and the ability of this experiment
to untangle the 8 or more N* states. Attention should be given to handling the expected
background from 4o and o production. Other facilities are also actively pursuing ’
photoproduction; therefore, it would be useful to have a comparison of the capabilities of the
competing approaches. Overall, the PAC is favorably inclined to this program.
ELSA/3-2003
Baryon spectroscopy in the high-mass region with double polarization observables
Contact persons: Ch. Weinheimer, U. Thoma, R. Novotny
The letter-of-intent outlines a program to investigate the baryon spectrum up to mass 2.6 GeV
at ELSA by using circularly and linearly polarized photon beams and polarized hydrogen and
deuterium targets. The PAC agrees that the measurement of single and double polarization
variables will provide crucial input for the partial wave analysis of the reactions proposed.
The program is timely, and the PAC looks forward to the submission of a full proposal.
ELSA/4-2003
Photoproduction of the (1405)
Contact person: H. Schmieden
This letter-of-intent outlines a measurement of the t-dependence of (1405) photoproduction
using the Crystal Barrel and various added forward-angle detectors. In addition, the LOI
mentions possible measurements of the + baryon, polarization observables in the decay of
the P11(1710) into K+, and polarization observables for the D13(1900). Each measurement
incorporates detection of a charged track in the forward direction. To this end, two types of
forward-angle detectors are envisaged: (i) a non-magnetic time-of-flight system for lower
momenta, and (ii) a forward magnetic spectrometer with tracking chambers similar in style to
LEPS at SPring-8 for higher momenta. The PAC took note that the common thread in these
measurements was the need to have a forward-angle triggering and tracking device to
complement the Crystal Barrel in the post-TAPS era.
The PAC feels that there is interesting and worthwhile physics to explore in the areas
mentioned and encourages submission of a full proposal. The proposal needs to make a
convincing physics case for each of the mentioned measurements. In the main example, when
determining the (1405) t-dependence, one would need to show on the basis of theoretical
models what kind of sensitivity to the internal structure can be expected from this
measurement. Depending on the reaction mechanism, there may not be much size sensitivity
in the t-slope. To justify the design and implementation of a magnetic spectrometer system
requires an experimental program broader than only one (potential) experiment. A set of key
measurements needs to be identified to serve as the focus for a design. The PAC recommends
that a set of proposals be developed which collectively could form the program for using a
magnetic spectrometer.
ELSA/5-2003
Meson and Baryon Photoproduction from Nuclei
Contact person: S. Schadmand
The letter-of-intent discusses a broad spectrum of possible baryon and meson in-medium
modification experiments at MAMI and ELSA. Part of the program was submitted as separate
proposals (A2/3-03 and A2/7-03). The PAC acknowledges that the physics is highly topical
and recommends that future, more detailed proposals focus on specific aspects of the
program.
Evaluation of the MAMI Proposals
MAMI A1/1-03
Separation of transverse and longitudinal structure functions in (e,e’K+) at W = 1660
MeV, Q² = 0.2 (GeV/c)² in parallel kinematics
Contact person: P. Achenbach
Proposal A1/1-03 concerns the first commissioning experiment for a new experimental
program in strangeness physics using virtual photons. The centerpiece of the set-up is the
KaoS spectrometer that was recently obtained from GSI. Also used are some of the existing
spectrometers in Hall A1. KaoS is a very compact magnetic spectrometer which makes it
especially suitable for the detection of charged kaons. The combination of MAMI upgraded to
1.5 GeV with this detector allows an extensive exploration of the characteristics of threshold
 and  production. Encouraged by the success of threshold o photoproduction
measurements at MAMI for showcasing the predictive power of Chiral Perturbation Theory, it
is hoped that a similar success will emerge for threshold kaon photoproduction.
In the presentation of the proposal, the PAC missed the discussion of the choice of the
kinematics as well as a reasonable evaluation of the background and a more elaborate Monte
Carlo calculation of the expected detector performance. The PAC is looking forward to the
next round of proposals for this special detector, and expects that sufficient attention will be
given to presenting the physics program in much greater detail than given now. The PAC
would also like to see a report on the background and the detector performance.
The PAC recommends the approval of the requested 500 hours of beam time to get the
instrument commissioned. We wish the group the best of luck with the upcoming engineering
run.
MAMI A1/2-03
Progress Report on Measurements of the Low Q² Behavior of the  * p   Reaction
Contact person: M. Distler
A polarized electron beam was used at the three-spectrometer facility in 2003 to determine the
ratios E2/M1 and C2/M1 in the -resonance region via the reaction p(e,e’p)o at low fourmomentum transfers. The PAC appreciates the physics goals of the experiment and is
impressed by the quality of the data.
MAMI A1/3-03
Investigation of the Reaction 12C (e, e' p  )11C 
Contact persons: D. Bosnar, U. Mueller
A previous experiment using the 3-spectrometer facility found sharp structures in the
invariant mass spectrum of the reaction 12C(e, e’p -)11Cg.s. at large recoil momenta, which
were interpreted as evidence for bound -resonance states. A subsequent experiment could
not confirm this evidence due to the lack of statistics. The present proposal aims at
investigating the possibility of increasing the statistical accuracy of the experiment by
replacing the proton spectrometer with a large solid angle silicon detector telescope.
The PAC recommends this pilot study to proceed with the requested 100 hours of beam time.
MAMI A2/1-03
Measurement of the Magnetic Dipole Moment of the (1232)-Resonance via the
 + p n + + +  reaction
Contact person: D.P. Watts
This proposal aims at determining the magnetic moment of the +(1232) resonance by
measuring both the differential cross section and the beam polarization asymmetry for the
reaction  + p n + + + . Measurements of magnetic moments are very important to test a
variety of models, from constituent quark models to lattice QCD, that describe the physical
properties of baryons. The method proposed would be complementary to the already approved
experiment exploiting the similar reaction  + p -> p + 0 + . However, it is clear that due to
the reduced bremsstrahlung from the proton with respect to the pion, the latter reaction will
have a higher sensitivity to the magnetic moment of the +(1232). Therefore, the opinion of
the PAC is that the measurement of the reaction considered in the present proposal is to be
regarded as a check of the reaction model specifically developed by the theorists for the
extraction of the +(1232) magnetic moment, rather than as an actual way to evaluate the
magnetic moment itself. This point of view is also partly supported by the estimate of the
statistical error on the beam asymmetry presented in the proposal. The quoted error does not
appear to be small enough to discriminate between the curves corresponding to different
values of the magnetic moment with high confidence.
From the experimental point of view, + detection with the Crystal Ball does not appear to be
obvious, but the simulations presented in the proposal seem to indicate that it may be feasible.
Rejection of background from competing reactions is an important point as the cross section
to be measured is relatively small. Although the PAC tends to believe that the data analysis
can provide sufficient constraints to identify the reaction of interest, it would have been
appreciated to have some explicit simulations showing that this is actually the case.
In summary, considering in particular that the experiment would be using already approved
beam time, the PAC recommends approval and encourages the proponents to pursue this
measurement as additional information to check the reaction model used for the extraction of
the +(1232) magnetic moment.
MAMI A2/2-03
Measurement of the 20 Photoproduction off the Proton at Threshold and in the second
Resonance Region
Contact person: M. Kotulla
The aim of this experiment is to obtain data on the double 0 production off the proton with
linearly polarized photons from threshold up to about 450 MeV photon energy. The first
physics goal is to compare the data in the threshold region to calculations based either on
chiral perturbation theory or on other effective lagrangians. Since double 0 production in
chiral perturbation theory originates entirely from chiral loops, the measurement would
provide an important test of this approach that is considered to be a good representation of
low energy QCD. The second physics goal is to obtain information from the data on the role
in this reaction of the P11(1440) (Roper resonance) to check the interpretation of recent data
on the same channel obtained by the GRAAL collaboration at higher energies. Information on
the Roper resonance would also be valuable to improve the accuracy of calculations in chiral
perturbation theory.
The PAC believes that the goal of testing chiral perturbation theory in the threshold region
with accurate data on the total cross section and the beam asymmetry is important; the same
holds for obtaining more information on the role of the Roper resonance. With the beam time
specifically requested for the threshold region, the experiment proposed would certainly
improve significantly the statistics on the total cross section for beam energies below 350
MeV, compared to the available data, and would provide interesting data on the beam
asymmetry in this energy region. The PAC would have liked to see in the proposal a
theoretical estimate of the expected asymmetry. Instead, the upper limit on the energy of 450
MeV is not sufficient for investigating the Roper resonance over its full width, but will allow
to measure only the leading edge of this excited state. On the other hand, this part of the
proposal relies on already approved beam time; therefore, it is certainly worthwhile to analyse
the data at the higher energy to verify the effect of the Roper. The experimental set-up and the
method proposed for all the measurements appear to be adequate.
In summary, the PAC recommends approval of this experiment and, in particular, approval of
the 200 hours of beam time with linearly polarized photons (coherent peak set at 350 MeV).
MAMI A2/3-03
Threshold Double Pion Photoproduction from Nuclei
Contact persons: S. Schadmand
The authors of this proposal suggest to study systematically double-pion photoproduction
from protons, light, medium and heavy nuclei. The two-pion invariant mass spectra will be
used to gain information on the in-medium  interaction. Possible differences in  from
+0 spectra might be interpreted as a signature of the long searched for -meson and its
peculiar behavior (in-medium mass shift) due to chiral symmetry restoration effects. The
variation of the nuclear size allows access to different reaction volumes and hence to probe
the anticipated effects as a function of nuclear matter density, although in a very limited
regime.
In a previous experiment the proponents have taken data of sufficient quality for light nuclei
up to carbon, and of very poor statistics for Pb. Since the data for the lead target show already
some hints for a downward shift in the invariant mass distribution an updated data
sample with improved statistics is highly desirable.
The committee is very much in favor of the proposed study and the underlying physics goals,
but it views the extraction of chiral symmetry restoration effects as very difficult and
recommends more intense theoretical efforts. For the experiment the PAC suggests to
concentrate on the verification of the effect and to measure two heavy nuclei (Ca and Pb) with
good statistics.
In view of the large acceptance and high data rate of the CB TAPS setup, and assuming that
photon energy bins of 20 MeV are sufficient, the PAC recommends allocating 200h of beam
time for this proposal.
MAMI A2/4-03
Test of Chiral Perturbation Theory and C and CP Invariance in Eta Meson Decay
Contact person: B. Nefkens
High precision measurements of rare decays of -mesons are very helpful for studying the
symmetries underlying fundamental interactions. Data on the kinematical distributions in
these decays are particularly stringent tests of the theoretical descriptions.
This proposal has two components. Phase I would be devoted to a high accuracy determination of the branching ratio and decay energy spectrum of the  process. In the same
experiment, the slope parameter of the Dalitz plot distribution of the  decay would
be measured with improved accuracy. Phase II of the proposal deals with C and CP violating
rare  decays (, , , and ). Both phases use -mesons
produced by a beam of tagged photons incident on a liquid hydrogen target. The -mesons are
tagged by the detection of the recoiling proton. The final state photons are detected by the
Crystal Ball combined with TAPS as forward wall.
The PAC is very favorably inclined towards Phase I of the proposed experiment. The accurate
measurement of the  decay is a strong test of higher order Chiral Perturbation
Theory (with resonance saturation for the counter-terms). The spectrum of the photon
energies reflects directly the interplay between loop corrections and counter-terms. Its
experimental determination would, therefore, put strong constraints on the theoretical understandding of the  decay.
The physics outlined for the second phase of the experiment is considered to be very
interesting. The PAC feels, however, that more detailed studies of the proposed measurements, based in particular on the experience gained by realizing the first phase of the program,
are needed in order to show quantitatively the gain in accuracy expected from taking new data
on these rare  decays at MAMI.
The PAC recommends allocating the 30 hours of beam time requested for testing the set-up
and the 240 hours needed for data taking on Phase I.
MAMI A2/5-03
Coherent pion photoproduction from nuclei from threshold to 350 MeV
Contact persons: D.P. Watts
This proposal seeks to determine the matter form factor of light, medium, and heavy nuclei
using the coherent 0 photoproduction. The experiment is an extension of previous measurements using TAPS which showed that the coherent component of (,0) production can be
isolated reasonably well, and that the diffractive peaks can be detected. The PAC was
convinced that the combination of Crystal Ball and TAPS will allow greatly improved
measurements of this process to be made, particularly through better control of unwanted
effects related to energy leakage at the edges of the photon detectors.
The PAC was initially not satisfied with the discussion of the physics motivation and the
estimated experimental sensitivity as given in the proposal. However, the proposers
submitted an addendum to their proposal entitled "RE: CB@MAMI Exp.-Nr. A2/5-03:
Coherent pion photoproduction from nuclei from threshold to 350 MeV" dated January 14,
2004. In this addendum they addressed the issues raised by the PAC. First, past results from
elastic proton scattering were reviewed, showing why the proposed measurement will be
significant. The sensitivity issue was addressed with new calculations of the form factor
change given a 0.1 fm change in the r.m.s. matter radius, and a reasonable effort was made to
relate this to the experimental statistical accuracy. The range of uncertainty that can be
expected due to pion wave distortions was also satisfactorily demonstrated.
The PAC recommends approval of this experiment.
MAMI A2/6-03
Measurement of the Photon Asymmetry in Neutral Pion Photoproduction from the
Proton near Threshold
Contact person: D. Hornidge
Measurement of threshold pion photoproduction from the proton has been a fertile area for
understanding hadron structure, especially in the context of chiral perturbation theory.
Experiments at Mainz have been particularly significant. This proposal aims to extend this
important work through use of linearly polarized photons. The main consequence of this
experiment will be to greatly reduce the uncertainty in the parameter  which determines the
imaginary part of the amplitude E0+.
The PAC recommends approval of this experiment.
MAMI A2/7-03
Coherent Photoproduction of -mesons from light nuclei - search for -mesic nuclei
Contact person: B. Krusche
The goal of the present proposal is to search for a bound state of -mesons in a light nucleus.
The search is related to a detailed study of the interaction of -mesons with nucleons and
nuclei which, according to theoretical predictions, is expected to be attractive. The broad
interest in a better understanding of this interaction has initiated several ongoing
investigations in various laboratories which utilize near-threshold  production with both
hadronic and electromagnetic probes. First results have already been obtained, and more
detailed information is expected in the near future.
In a preceding experiment the authors have already observed coherent  production off 3He
nuclei, and have found hints for a possible signature of a bound -mesic state. However, the
existing data set suffers from low statistical significance. The PAC views the search for such a
state as very important and suggests to the proponents that once CB TAPS is available all
measures should be taken to verify the 3He case as soon as possible, in particular also with
respect to the international competition. Although 7Li is an interesting case too, it is regarded
as much more complicated and less important than 3He. Therefore, an explorative study
should only be started after the 3He case has been verified.
The committee recommends allocating 300h of beam time with the 3He target with high
priority, and 200h of beam time with the 7Li target later.