Hartmuth Arenho¨vel [email protected] Hartmut Backe [email protected] Dieter Drechsel [email protected] J¨org Friedrich [email protected] Karl-Heinz Kaiser [email protected] Thomas Walcher [email protected] University of Mainz Institute for Nuclear Physics Johann-Joachim-Becher-Weg 45 55128 Mainz, Germany The articles in this book originally appeared on the internet (www.eurphysj.org) as open access publication of the journal The European Physical Journal A – Hadrons and Nuclei Volume 28, Supplement 1 ISSN 1434-601X (cid:2)c SIF and Springer-Verlag Berlin Heidelberg 2006 ISBN-10 3-540-36753-5 Springer Berlin Heidelberg New York ISBN-13 978-3-540-36753-6 Springer Berlin Heidelberg New York Library of Congress Control Number: 2006929544 Thisworkissubjecttocopyright.Allrightsreserved,whetherthewholeorpartofthematerialisconcerned, specificallytherightsoftranslation,reprinting,reuseofillustrations,recitation,broadcasting,reproduction onmicrofilmorinanyotherway,andstorageindatabanks.Duplicationofthispublicationorpartsthereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from SIF and Springer. Violations are liable for prosecution under the German Copyright Law. Springer is a part of Springer Science+Business Media springer.com (cid:2)c SIF and Springer-Verlag Berlin Heidelberg 2006 Printed in Italy Theuseofgeneraldescriptivenames,registerednames,trademarks,etc.inthispublicationdoesnotimply, evenintheabsenceofaspecificstatement,thatsuchnamesareexemptfromtherelevantprotectivelaws and regulations and therefore free for general use. Typesetting and Cover design: SIF Production Office, Bologna, Italy Printing and Binding: Tipografia Compositori, Bologna, Italy Printed on acid-free paper SPIN: 11804239 – 5 4 3 2 1 0 Sponsors Deutsche Forschungsgemeinschaft, Bonn Institut fu¨r Kernphysik, Universita¨t Mainz The European Physical Journal, www.eurphysj.org ACCEL Instruments GmbH, Bergisch Gladbach BRUKER, Wissembourg, France DANFYSIK A/S, Jyllinge, Denmark SIGMAPHI, Vannes, France SFAR STEEL, Creusot, France THALES Electron Devices, V´elizy, France V The European Physical Journal A Volume 28 • Supplement 1 • 2006 (cid:2)Foreword 71 M. Vanderhaeghen Two-photon physics 81 M. Ostrick (cid:2)Many Body Structure of Strongly Electromagnetic form factors of the nucleon Interacting Systems ExperimentsatMAMI 1 R.G. Milner 91 H. Schmieden The beauty of the electromagnetic probe Photo- and electro-excitation of the Δ-resonance at MAMI 7 L.S. Cardman PhysicsattheThomasJeffersonNationalAccelerator 101 S. Kowalski Facility Parity violation in electron scattering 19 W.U. Boeglin Few-nucleon systems at MAMI and beyond 107 F.E. Maas Parity-violating electron scattering at the MAMI facility in Mainz 29 D. Rohe Thestrangenesscontributiontotheformfactors A1 and A3 Collaboration ofthenucleon Experiments with polarized 3He at MAMI 117 N. d’Hose 39 M. Schwamb Virtual Compton Scattering at MAMI Few-nucleon systems (theory) 129 H. Merkel 49 H.-W. Hammer Experimental tests of Chiral Perturbation Theory Nucleon form factors in dispersion theory 59 S. Scherer 139 W. Hillert Chiral perturbation theory The Bonn Electron Stretcher Accelerator ELSA: Successandchallenge Past and future VI 149 A. Jankowiak 197 M. El-Ghazaly et al. The Mainz Microtron MAMI —Past and future X-ray phase contrast imaging at MAMI 161 A. Thomas 209 B.A. Mecking The Gerasimov-Drell-Hearn sum rule at MAMI Twenty years of physics at MAMI —What did it mean? 173 R. Beck Experiments with photons at MAMI 185 W. Lauth et al. (cid:2)Author index Coherent X-rays at MAMI Eur. Phys. J. A 28, s01, VII VIII (2006) EPJ A direct DOI: 10.1140/epja/i2006-09-022-5 electronic only Foreword This volume contains the proceedings of the Symposium on Twenty Years of Physics at the Mainz Microtron (MAMI), which was held at the Johannes Gutenberg-Universit˜at Mainz, October 19-22, 2005. The Symposium marks theretirementofseveralmembersoftheInstitutfu˜rKernphysikwhoseworkhasbeendevotedprimarylytooscientiflc researchatMAMIover manyyears.ItwastheprimaryaimoftheSymposiumtoreviewpastandcurrentactivities in the fleld of hadronic structure investigations with the electroweak interaction. However, the Symposium also gave an outlook on the physics with the MAMI upgrade, a double-sided mictrotron that is expected to provide a high-quality beam of up to 1.5 GeV later this year. The Institut fu˜r Kernphysik was founded in the early 1960s by the late Hans Ehrenberg who served as its director formorethantwodecades.HeprovidedtheInstitutewitha350MeVpulsedlinearelectronaccelerator,whichbecame available in 1966 for studies of charge and magnetization distributions in nuclei and nucleons as well as photonuclear investigations in collaboration with the Max-Planck-Institut fu˜r Chemie. Hans Ehrenberg knew about the importance of having excellent facilities for performing outstanding physics from his earlier studies at Bonn and Stanford, with the later Noble Prize winners Paul and Hofstadter, respectively. There- fore,hededicatedgreatefiortinI)buildingupaperfectinfrastructureofmechanics,electronics,vacuumandcomputer workshops, and II) attracting a young accelerator physicist, Helmut Herminghaus, to the Institute. In the late 1960s it became common wisdom that the next accelerator generation had to provide a high duty- factor in order to perform coincidence experiments for detailed studies of hadronic physics. Helmut Herminghaus had conceived a blueprint for such a device in 1975, a three-stage racetrack microtron (RTM). Shortly after a physics programaroundthisRTMwasworkedoutandtheproposalwassenttothesponsoringagencies.Theprojectreceived the support of the University and the State of Rheinland-Pfalz and sometime later also of the federal agencies. In the fall of 1978, the state minister was informed by the federal minister of research and technology (BMFT) that the projecthadbeendiscussedwiththeGermanScienceCouncil,theDeutscheForschungsgemeinschaft(DFG),theMax- Planck-Gesellschaft, and members of the scientiflc community. As a result these representatives agreed to support the proposalinordertoI)demonstratethatalsoalarge-scaleresearchfacilitycanberealizedatauniversity,II)withstand a further emigration of such research from the universities, and III) flnd a constructive solution that could serve as a model for university research. As a matter of fact such a solution was found in the following years. However, it has to be said that the full flnancial support would never have arrived if the RTM had not been designed stage by stage, and each time delivered in perfect shape (often to the surprise of outside experts) by Helmut Herminghaus and his crewofphysicistsandtechnicians.TheflrststageoftheRTM(14MeV)wentintooperationalreadyinMay1979,the second stage (183 MeV) followed in 1983, and the last stage was ready for the experiments in the fall of 1990. Atpresentthemicrotrondeliversacontinuousbeamofanintensityofabout100μAforunpolarizedand40μAfor polarized electrons with a polarization degree of about 80 %. Its energy close to 1 GeV provides the perfect resolution to study the distributions of charge, magnetization, and strangeness inside the nucleon and light nuclei, the threshold production of the Goldstone bosons pion and eta, the polarizabilities of nucleons and pions, and the excitation of the most prominent nucleon resonance, the Δ(1232). Since the physics with the flrst two stages of the RTM was summarized already at an earlier workshop ( Physics with MAMI A ), the present Symposium concentrates on the achievements of the years with the 855 MeV stage (MAMI B). The organizers also decided to invite as speakers, with a few exceptions, young colleagues who have made a career with their work at MAMI. It remains to say thank you to many people and institutions for continuing support. We are grateful to all the colleagues from the Institute, the postdocs, Ph.D. and younger students who contributed to the MAMI project. VIII Special thanks go to the people in the workshops and in the administration without whose efiorts the project could never have succeeded. We are grateful to the colleagues from the neighbouring Institut fu˜r Physik for their work on polarized beams and targets, for the TAPS detector brought to Mainz by the Gie en group, to the Bonn/Bochum groupforthepolarizedH2-target,andtomanyotherGermaninstitutionsforactiveengagementandvariousdetection devices, notably Darmstadt, Erlangen, Go˜ttingen, and Tu˜bingen. Our thanks go to the foreign colleagues who have participated in the project from the very beginning, notably to our Scottish colleagues who built the photon tagger with the support of their SERC, the groups from Pavia sponsored by the INFN, from Saclay supported by the CEA/DAPNIA and from Orsay supported by the CNRS. We appreciate common experimental and theoretical work with physicists from various other places in Europe, e.g. Amsterdam (NIKHEF), Basel, Genova, Gent, Lljubljana, Trento and several Russian universities and institutions, and from overseas, e.g., Jefierson Lab, MIT, Florida State University, University of Nagoya, George Washington University, and TRIUMF. Finally, in view of the upgrade two more collaborations have developed in recent years. The Crystal Ball Collaboration has shipped its detector from the Brookhaven National Lab to Mainz, and the KAOS detector is being installed in Mainz with the help of the GSI Darmstadt. Last but not least we are grateful to the members of the international Program Advisory Committee and of numerous evaluation and expert committees for their invaluable scientiflc advice and moral uphold. Concerning the institutions we flrst and foremost thank our Physics Faculty, the Johannes Gutenberg-Universita˜t and the State of Rhineland-Palatinate for continued and coherent support. We are extremely grateful to the state and to the federal ministries (BMFT, BMBW, BMBF) who flnanced the construction of the new accelerator and experimental halls as well as the large spectrometers via the university construction program (HBFG). Our special thanks go to the Deutsche Forschungsgemeinschaft that backed up the project by means of Collaborative Research Centers(SFB201,CRC443)whoseresourceswereoftheutmostimportancetosustainourpostdocandPhDprogram. Finally,wereceivedrecentsupportbytheEuropeannetworkingactivitiesviatheI3HP/TransnationalAccessprogram. Last but not least the organizers are grateful to the speakers of this Symposium for summarizing the various achievements with MAMI and related research, and for bringing back memories of the past. Though retirees enjoy the latter aspects very much, there is no reason to engage in retrospection: The double-sided microtron is expected to yield its 1.5 GeV electron beam later this year, and we wish our colleagues and their students all the success in the years to come! Mainz, April 1, 2006 Hartmuth Arenho˜vel Hartmut Backe Dieter Drechsel J˜org Friedrich Karl-Heinz Kaiser Thomas Walcher The Editors Eur. Phys. J. A 28, s01, 1 5 (2006) EPJ A direct DOI: 10.1140/epja/i2006-09-001-x electronic only The beauty of the electromagnetic probe R.G. Milnera MIT-BatesLinearAcceleratorCenter,LaboratoryforNuclearScience,MassachusettsInstituteforTechnology,Cambridge,MA 02139, USA / Published online: 15 May 2006 (cid:2)c Societa Italiana di Fisica / Springer-Verlag 2006 Abstract. Precision experiments using the electromagnetic probe have recently produced important new data on fundamental properties of the nucleon, e.g. charge, magnetism, shape, polarizability, spin and sea quark structure. These experiments have been made possible by a new generation of high duty factor electron accelerators, advances in spin polarization technology (beams, targets and recoil polarimeters), and the development of unique, optimized detector systems. In this contribution, the role of multiple photonexchangeinelectronscatteringfromtheprotonandtheroleofseaquarksinnucleonstructureare highlighted. PACS. 13.40.Gp Electromagnetic form factors 13.60.-r Photon and charged-lepton interactions with hadrons 13.60.Fz Elastic and Compton scattering 14.20.Dh Protons and neutrons 1 Introduction form-factors have been carried out. In particular, the relatively small neutron electric form-factor has Understanding the structure of the nucleon in terms of been determined to better than 7% over the range the fundamental constituents of the Standard Model, the 0.1<Q2 <2 (GeV/c)2. quarks and gluons of Quantum Chromodynamics (QCD), is a major research area in Physics. The ultimate goal – The shape of the proton through study of electroex- is to test QCD with precision measurements and ab ini- citation of the π0 at the Δ(1232)-resonance at low tio calculations. Over the last decade, experimentalists Q2 ∼ 0.1 (GeV/c)2 using out-of-plane detection at have made substantial progress in determination of the Bates and Mainz [6]. It has been established that quark and gluon distributions at high energies (ECM ∼ the proton shape is slightly non-spherical. A chiral 100GeV) and measurement of fundamental properties of extrapolation [7] of lattice QCD calculations [8] is in thenucleonatlowenergies(ECM ∼1GeV).Theoristsare good agreement with the data. startingtoproducefullQCDMonteCarlosimulations(al- beit with heavy pion masses) of nucleon structure using – The electric and magnetic polarizabilities of the advanced computers [1]. proton through measurement of Virtual Compton The experimental study of the structure of the proton Scattering from the proton at Mainz [9] and JLab [10] and of atomic nuclei is best carried out using the point- and using out-of-plane detection at Bates [11]. like electroweak probe, the best understood interaction in Nature. Intense beams of highly polarized electrons have – The quark and gluon contributions to the spin become available at energies of 0.5 to 6GeV at high duty structure of the proton using deep inelastic scat- factor. Highly polarized proton, deuteron and 3He tar- tering at HERMES/DESY [12], JLab [13], COM- gets have been developed as well as e– cient polarimeters PASS/CERN [14] and RHIC-spin [15]. fordetectionofrecoilpolarization.Optimizedexperiments utilizing uniquely designed detectors have been carried – The role of strange quarks in the long distance mag- out. New data and insights have been obtained in mea- netic and electric charge distribution of the proton at surement of the following properties of the nucleon: Bates, Mainz and JLab [16,17]. There are hints of a non-zero strange quark magnetic moment of the pro- – The proton and neutron charge and magnetism ton but these need to be conflrmed by more precise through spin-dependent elastic electron scattering experiments. at Mainz [2], Bates [3], NIKHEF [4] and JLab [5]. Precise measurements of all four of the nucleon elastic Here I concentrate on two areas of research where im- a e-mail: [email protected] portant results have recently been obtained. 2 The European Physical Journal A Fig. 2. The quark and gluon momentum distributions at Q2 =10(GeV/c)2 asafunctionofpartonmomentumxasde- Fig.1.TheJefiersonLabdata[18]ontheratioGpE/GpM show- terminedbytheZEUSexperiment[23]attheHERAelectron- ing the discrepancy between the recoil polarization (solid cir- protoncollider.NotethattheseaquarkmomentumxS andthe cles) and the Rosenbluth (other symbols) techniques. gluonmomentumxg distributionsaredividedbyafactorof20. 2 Evidence for multiple photon effects in of multiple photon exchange and so give an incorrect de- elastic electron scattering from the proton termination at higher Q2, i.e. above about 1 (GeV/c)2. This multiple photon exchange contribution to elas- tic electron-proton scattering can be conflrmed by precise Essentially all electron scattering experiments to study comparison of electron-proton with positron proton elas- proton and nuclear structure to date have been analyzed intermsofsinglephotonexchange.Theflnestructurecou- tic scattering or by measurement of the asymmetry Ay in scatteringofunpolarizedelectronsfromaverticallypolar- pling constant α∼1/137 is small enough that leading or- ized proton target [21]. If conflrmed, this is a very signifl- der has been adequate. There are a few speciflc examples cant result. where multiple photon exchange is known to be signif- icant, e.g. in comparison of electron and positron scat- tering in kinematics where the single photon exchange 3 Role of sea quarks in nucleon structure cross-section is small, or in radiative processes. Thus, it came as a surprise when the Jefierson Lab Hall A recoil polarizationmeasurementsofelectron-protonelasticscat- QCD tells us that the nucleon comprises three valence tering at momentum transfers of about 2 (GeV/c)2 [18] quarks and a sea of quark-antiquark pairs. From the ear- showed a substantial deviation from the data obtained liest days of nuclear physics, these sea quarks in the form over several decades with the Rosenbluth technique [19], of mesons, have been viewed as playing an important role which is based on precise cross-section measurements. inthelongdistancestructureofthenucleone.g.themag- Thisdiscrepancyhasbeeninterpretedastheefiectofmul- nitude and sign of the proton and neutron magnetic mo- tiplephotonexchangeintheelasticelectron-protoncross- ments. In addition, the most successful hadronic theoreti- section [20]. The cross section for elastic electron-proton caldescriptionsoflightnucleiincorporatemesonexchange scattering in the one-photon exchange approximation can between nucleons as an essential element of nuclear bind- bewrittenintermsofthepointlikeMottcross-section,the ing.This mesoncloud structuretothenucleonhasgen- SachsformfactorsGp andGp andtheelectronscattering erally been accepted but has lacked both a rigorous the- E M angle θ as oretical underpinning and a deflnitive quantitative basis from experiment. (cid:2) (cid:3) (cid:4) (cid:5) p2 p2 The role of valence quarks in nucleon structure has dσ dσ G +τG θ = · E M +2τGp2tan2 , been studied extensively. The efiects of sea quarks and dΩ dΩ 1+τ M 2 Mott gluons are relatively poorly determined, in large part be- cause they require high center-of-mass energy, and are a whereτ =Q2/4M2.Figure1showstherecoilpolarization major focus of interest for the future [22]. One of the im- p p determination of G /G (solid circles) as a function of portant contributions over the last decade has been the E M momentum transfer Q2. The Rosenbluth data (all other experimental measurement of deep inelastic scattering at data points) are believed to be uncorrected for the efiects highenergiestodeterminetheefiectsoftheseaquarksand R.G. Milner: The beauty of the electromagnetic probe 3 Fig. 4. Theprotonchargeelasticform-factorwiththesmooth contribution subtracted in the parameterization of Friedrich and Walcher [24]. A 2% dip in the parameterization is obvious at Q2 ∼0.1 0.2 (GeV/c)2, which coincides with the location of the peak in the neutron charge elastic form-factor Gn. In the E absence of realistic QCD calculations, it is hard to deflni- tively state that this structure at low Q2 is due to the meson cloud structure of the nucleon. However, it is a physically plausible explanation. 4 BLAST Experiment at MIT-Bates AnewsetofprecisionmeasurementsofthelowQ2 elastic form factors of the proton and neutron have been carried out using the South Hall Ring (SHR) at the MIT-Bates Linear Accelerator Center. The Bates Large Acceptance SpectrometerToroid(BLAST)wasconstructed[25]tode- tectscatteredelectrons,protons,neutronsandpionsinthe Fig.3.Comparisonofthegluonandseadistributionsfromthe scatteringoflongitudinallypolarizedelectronswithanen- ZEUS-S NLO QCD flt for various Q2 values [23] as measured ergy of 850MeV from polarized targets of hydrogen and at the HERA electron-proton collider. deuterium.Thepolarizedinternalgastargettechniqueof- fersminimalsystematicuncertaintiesandahighstatistics sampleofdataweretakenbytheBLASTexperimentover gluons. In particular, data taken by experiments at the an eighteen month period from late 2003 to mid 2005. HERA electron-proton collider [23] have for the flrst time The BLAST data are under analysis and will be able allowedadeterminationofthegluonmomentumdistribu- to provide new and independent experimental constraints tion in the proton, as shown in flg. 2. The QCD evolution of the Friedrich-Walcher ansatz. of HERA data [23] shows a signiflcant sea contribution at Thepolarizedprotonsanddeuterons(bothvectorand low Q2, in contrast to the gluon contribution which van- tensor) were produced using an Atomic Beam Source ishes, as seen in flg. 3. This supports the point of view of (ABS) [26], which was located in the substantial and spa- a strong role for sea quarks at low Q2. tially varying magnetic fleld of the BLAST toroid. The Atlowenergies,electronscatteringexperimentsdeter- target spin state was alternated every flve minutes by mine the elastic electric and magnetic form factors of the switching the flnal RF transition immediately before the proton and neutron. Friedrich and Walcher have postu- targettoensureequaltargetdensitiesforeachofthethree lated that the Q2 dependence of the elastic form factors states (vector +, vector −, tensor −). The electrons scat- in the region 0.1 to 0.5 (GeV/c)2 may be sensitive to the teredfromthepolarizedprotonsanddeuteronsinacylin- meson cloud structure of the nucleon and have produced drical, windowless aluminum target tube 600mm long, parameterizations of world data which suggest that there 15mm in diameter and with a wall thickness of 50μm. may be experimental support for this ansatz [24]. They The polarized target was tuned and monitored using a flt the measured four form factors with a parameteriza- Breit-Rabisystemwhichcontinuouslysampledtheatomic tion which consists of a smooth contribution and a bump polarizationofasmallfractionoftheincomingbeamfrom contribution. Figure 4 shows the world’s data for the pro- theABS.Thevectorpolarizationsofboththeprotonand tonelasticformfactorplottedasafunctionofmomentum deuteron was typically 0.75. Data were taken with stored transferQ2,wherethesmoothcontributionissubtracted. electron beam intensities up to 225mA.