Nucleon Structure
Overview: The study of nucleon structure is vitally important for completing our understanding of the transition between the hadronicand quark/gluon descriptions of nuclei. We will specifically investigate the internal landscape of the nucleon by addressing the question “What is the structure of the proton and neutron and how do hadrons get their mass and spin?”as posed in the 2009 report of the STFC Nuclear Physics AdvisoryPanel . This is at the core of the recent Long Range Plans developed by the European and North-American nuclear physicscommunities . The Glasgow group are prominent both in Europe and the USA, playing a major role in shaping the hadron physics programmes of the leading electron accelerator facilities.
Work carried out in recent years.
The work carried out over a period of several years was concentrated at three laboratories, JLab, DESY and MAMI. We also contributed significantly to the preparation of the physics programme of PANDA at FAIR (Darmstadt, Germany). We have led, and will be leading, a large number of activities within EU FP6 and FP7 Integrated Infrastructure Initiatives/Integrating Activities (HadronPhysics, HadronPhysics2 and HadronPhysics3), and have a major presence in Halls A and B of Jefferson Laboratory (JLab), with Glasgow spokespersons on eight experimental proposals in the field of nucleon structure. We are also major contributors to the HERMES collaboration at DESY and the Crystal Ball and A1 collaborations at MAMI. Our work has resulted in the publication of 33 papers (including 7 PRLs) in this theme.
Measurement of the elastic form-factors of proton and neutron: The elastic electric (GE) and magnetic (GM) form-factors are related to the distributions of charge and magnetism within the nucleon, and remain a central topic of hadron physics. Precision measurements of both the proton and neutron form factors, over a large range in the four-momentum transfer (or distance scale) Q2, exercise powerful constraints on theoretical models of the nucleon and also provide a “base line” to constrain Generalised Parton Distribution fits. The Q2 scaling of the proton form factor ratio GEp/GMp was early evidence of the importance of quark orbital angular momentum. We took a leading role in the pioneering experiments at Mainz to extract the electric form factor of the neutron, GEn, from double-polarised asymmetries and have extended the Q2 range with BigBite in Hall-A to 3.4 (GeV/c)2, using a polarised 3He target and neutron detectors originally employed in the Mainz experiments and developed by Dr Annand. Double-polarised techniques to extract the electric form factor are superior to Rosenbluth separation when GE<< GM, and we have made an important contribution at JLab to proton GEp/GMpmeasurements up to Q2 = 8.5 (GeV/c)2. This produced the revelation that the oldRosenbluth results for GEp not only lacked precision, but also were wrong at high Q2. This is now thought to arise from the sensitivity of Rosenbluth separation of GE to inadequacies of the one-photon-exchange approximation of electron scattering. These start to become significant at Q2 > 1 (GeV/c)2. By contrast double-polarisation observables are inherently insensitive to these effects, as has been examined in further work at JLab .
Real Compton Scattering: Real Compton scattering at low-to-intermediate photon energy (50 MeV – 1 GeV) probes the global response of the nucleon’s constituents to an external electromagnetic field. The optical theorem links the forward, Compton scattering amplitude to the total photoabsorption cross section, and the Glasgow Tagger at Mainz has enabled not only proton Compton scattering experiments , but also measurements of the Gerasimov Drell Hearn (GDH) sum rule . The former have provided scalar proton polarisabilities, parameterising the rigidity of the composite system to applied electromagnetic fields, while the latter links the helicity-dependent, total cross section to the nucleon's magnetic moment. The latter is also related to the forward spin polarisability, which is a combination of the four spin polarisabilities that we propose for future Mainz measurements. More recently, GDH measurements have been extended to “polarised neutron” targets (deuterated butanol and 3He) measurements so that the neutron may also be explored. At MAX-lab we have made a significant contribution to the coherent, deuteron Compton scattering experiment . Using Chiral Effective Field Theory, one can extract the scalar neutron polarisabilities from the coherent Compton cross section, but the model dependence of this procedure must be checked. This can be performed effectively using 3He, which is proposed for the future . At higher momenta we have been heavily involved in proton Compton scattering in Hall-A. Initial studies have attempted to establish the poorly known reaction mechanism ,but the eventual goal in high transverse momentum scattering would be to access the vector, axial and tensor form factors, RV, RA, RT, which are functions of Mandelstam t. As with elastic electron scattering form factors, these are moments of the underlying GPDs.
Measurement of quark transversity in semi-inclusive Deep Inelastic Scattering: The quark transversity distribution was the last leading twist quark distribution function to be accessed experimentally. It is measured in Semi-Inclusive Deep Inelastic Scattering (SIDIS), where at least one final-state hadron is detected in addition to the scattered electron. This requires a large acceptance spectrometer with excellent particle identification and ideally one measures a single-spin asymmetry resulting from a transversely polarised nucleon target. We have made a major contribution to pioneering measurements at HERMES which led to the discovery of a wealth of new phenomena linked to quark distribution and fragmentation functions (Boer-Mulders, Sivers, Collins) and allowed the first ever extraction of quark transversity, and the separation of the Sivers and Collins fragmentation functions. These results, confirming a non-zero Sivers function, indicate a non-zero quark orbital angular momentum inside the nucleon and have spawned a number of experiments to explore these effects worldwide. We have participated in neutron transversity measurements with BigBite in Hall-A , where a polarised 3He target provided the transversely polarised neutron, and both final state pions and kaons were detected. Unlike the HERMES experiment, the Hall-Aexperiment explored the region of relatively high xB, where current quark (as opposed to sea quark) effects should dominate. This work continues with two approved experiments in CLAS , a conditionally approved experiment with SuperBigBite in Hall-A and several additional Hall-A proposals in the pipe line.
Measurement of hard exclusive reactions and access to Generalised Parton Distributions: Generalised Parton Distributions (GPDs) are presently the only known framework capable of providing a detailed “3D” understanding of thepartonic structure of the proton. Such an understanding would link together the various projections (form factors, parton distribution function etc.) that we currently measure, but that goal is some way off. The extraction of GPDs will require the measurement of many observables. Deeply Virtual Compton Scattering (DVCS) and hard exclusive meson production have the best theoretical basis for interpretation of the data in term of GPDs. We are engaged at the two world-leading experiments in the area, HERMES at DESY and CLAS at JLab. At HERMES, polarised and unpolarised nucleon targets, together with electron and positron beams were available, allowing the determination of both spin and charge asymmetry measurements in DVCS. CLAS complements the HERMES measurements by covering a slightly different kinematic range and, given the higher integrated luminosity, provides improved precision. Both experiments also study hard exclusive meson production, providing world’s first data on hard exclusive pion and rho electroproduction . These topics were the focus of the final phase of HERMES measurements and are the core of our activities in this area. Four PhDs have been awarded to students working on DVCS measurements at HERMES ,leading to six publications , with a further two in an advanced drafting stage. Three more papers were published on HERMES data on hard exclusive mesonproduction and four from CLAS data with several more in production. At Glasgow, two PhD projects on DVCS are ongoing, one using HERMES data with the recoil detector, the other using data from CLAS. The latter work will be continued in two accepted proposals for DVCS measurements on the proton at CLAS and a further proposal on the neutron DVCS at CLAS, which was recently submitted.
Current activities and future plans
Our future work builds on the successes of the old programme and seeks to expand our understanding of the open questions uncovered by our ongoingresearch. Our understanding of the nucleon in terms of its partonic degrees of freedom is far from complete and we wish to examine how global properties emerge from the microscopic degrees of freedom (quarks and gluons). These challenging questions will be the focus of our research activities in nucleon structure and we plan to build on the tremendous opportunities presented by experiments at JLab (Halls A and B), MAMI (tagged-photons) and DESY (HERMES and OLYMPUS).
Measurement of the elastic form-factors of proton and neutron: Recent progress in experimental techniques and apparatus allow measurements on the proton and neutron over a large momentum range (and hence distance scale), employing the double-polarisation technique. We aim to measure all four Sachs form factors over a large range in Q2 which will allow a u/d flavour separation and provide a rich testing ground for nucleon models ranging from Regge inspired approaches to Generalised Parton Distributions, from light-cone wave functions to lattice QCD. These form factors are also the zeroth moments of leading order GPDs and will thus provide important constraints on their extraction. The measurements will be performed in Hall-A at JLab, exploiting the Super BigBitespectrometer, where Drs Annand and Hamilton have made major contributions to the design. Four experiments are already approved and a fifth, addressing neutron form factors has been submitted by Dr Annand . A PhD project associated with the latter started in 2010. Drs Annand and Hamilton will direct the experimental programme, coordinating data taking and data analysis, while Mr Lumsden, will assist in the maintenance of Super BigBite.
Search for the two photon exchange contribution to electron-nucleon scattering: Recent measurements of the high-Q2 scaling behaviour of the ratioGEp/GMp of the proton highlight a striking difference between results obtained from Rosenbluth separation and from polarisation-transfer asymmetries. A plausible explanation is the interference between the one-photon and two-photon exchange amplitudes, to which Rosenbluth separation is relatively sensitive compared to polarisation transfer. Though a contribution from two photonexchange is not surprising, it cannot explain the full effect observed and the quality of the current data is insufficient to differentiate between different model predictions. We are involved in several experiments to quantify these effects. HERMES have recently published on this topic , a high-precision JLabmeasurement of GEp/GMp and a CLAS measurement using secondary e-, e+beams is underway. We plan to continue this research at the OLYMPUS experiment in DESY . Dr Lehmann will be responsible for the running and analysis of the OLYMPUS ToF system. Dr Seitz and Prof Kaiser will provide the academic leadership and supervision. These complementary, high precision experiments promise to quantify two-photon effects, and will have a major influence on the interpretation of lepton scattering experiments worldwide.
Measurement of quark distributions using semi-inclusive and exclusive reactions: We plan to continue our research into the quark-structure of the nucleon by measuring inclusive, semi-inclusive and hard exclusive reactions. Drs.Murray and Seitz will continue to lead the DVCS effort with the HERMES collaboration, completing the final analysis of HERMES data. For new experiments, our efforts are concentrated in Halls A and B at JLab. In Hall-B, Dr Seitz has two proposals to study non-kt integrated quark distributions , while Prof Ireland and Prof. Kaiser lead two proposals on proton DVCS . Drs Murray and Lehmann will run these experiments and lead the analysis of the data. In Hall-A, DrAnnand and Prof Rosner lead a proposal to measure the neutron spin asymmetry in the valence quark region and are involved in polarised SIDIS measurements using transversely polarised targets and Super BigBite. Dr Hamilton will lead this analysis to publication and also investigate the possibility with Super BigBite to extend high-momentum Compton scattering measurements, which provide alternative information on nucleon structure. One PhD project on CLAS DVCS measurements and one on HERMES data are ongoing, supervised by Prof Ireland, Dr Murray and Dr Seitz. The topic is part of two European Joint Research Activities (3D-MOMand HardEx2, Seitz spokesperson). Mr Lumsden provides the necessary technical support, both in Glasgow and at the experimental facilities.
Measurements of the electromagnetic response of nucleons using Compton scattering: Compton scattering at low-to-intermediate momentum measures the electric and magnetic rigidity of the nucleon, parameterised in terms of thepolarisabilities. These provide not only testing grounds for nucleon models, but can also reveal global properties, such as the pion cloud surrounding the nucleon. Following a long standing involvement at MAMI and MAX-lab , we will measure for the first time (Drs Annand and MacGregor spokespersons) the proton spinpolarisabilities, exploiting the Glasgow Tagger, the Crystal Ball and polarized targets at MAMI. At MAX-lab we plan to adapt the active target (Dr Annandspokesperson) to extract neutron scalar polarisabilities from coherent Compton scattering on 3He. These experiments will provide a rich testing ground for predictions of chiral effective field theories. We will collaborate with the theory group at the University of Manchester to interpret the results. Dr Livingston is in charge of the linearly polarised photon beams at MAX-lab and MAMI. Dr Annandbuilt, and is responsible for, the DAQ and data analysis systems in Mainz and designed the active target for MAX-lab . Dr Hamilton will develop simulations and data analysis tools. Mr Lumsden is responsible for the care and maintenance of the Glasgow Tagger in Mainz and the MAX-lab active target.
Hadron Spectroscopy
Overview: Hadron spectroscopy is a major part of the worldwide programme to investigate the nature of strongly interacting matter. Several questions from the 2009 Nuclear Physics Advisory Panel report will be addressed in this theme: What is the mechanism for confining quarks and gluons in strongly interacting particles (hadrons)? Can we understand the excitation spectra of hadrons from the quark-quark interaction? Do exotic hadrons (multiquark states, hybrid mesons andglueballs) exist? As with Theme 1, it is highlighted in the recent long range plans of both NuPECC and NSAC . All these questions address the need to understand the strong nuclear interaction at the energy scale of most of the visible mass in the universe. To do this, one needs to map out completely the spectrum of baryonic and mesonic states, and understand their properties.
A. Work carried out in recent years
Baryon resonance spectroscopy: An understanding of the nucleon and its excited states relies on the use of QCD-inspired models. Measurements of cross sections and spin observables provide crucial input to these models. Constituent quark models, based on SU(6)´O(3) symmetry, do a good job of explaining experimental results but overpredict the number of nucleon resonances – the so-called missing resonances, and it is clear that an extraction of amplitudes from several reaction channels will be required for the unambiguous determination of the baryon spectrum. In pseudo-scalar meson photoproduction, a complete measurement of the amplitudes is possible provided one measures with sufficient accuracy a minimum of seven polarization observables, in addition to differential cross sections . This was proposed as early as 1975 and has been a holy grail of hadron physics for several decades. These observables include measurements with combinations of polarized beam, target and recoil, which pose a major experimental challenge and, until recently, have not been possible. However, with access to polarized photon beams and polarized targets, combined with detector systems capable of capturing multi-particle final states over a large portion of 4π, we are in the process of making these measurements. We carry out this programme at the world’s two leading facilities, CLAS at JLab, and MAMI in Mainz. The most significant indication of our recent achievements is the publication of 36 hadron spectroscopy papers in refereed journals (including 6 PRLs) since 2008, many describing world first measurements.
At JLab we are deeply involved in the N* programme , which uses CLAS 4π charged particle spectrometer in Hall B. Prof Ireland has recently been elected Chair of the CLAS Collaboration Hadron Spectroscopy Physics Working Group, which has over 100 members, and coordinates the entire hadron spectroscopy programme at CLAS. We have made use of the new frozen spin target (FROST ),and have led the series of experiments with linearly polarized photons - a field in which Dr Livingston is one of the world’s leading specialists . Specifically we are focused on the complete measurement of the helicity amplitudes in strangeness photo-production, which comprises several experimental run periods, including linearly and circularly polarized photons, polarized andunpolarized proton and neutron (deuterium) targets. The strangeness channels are very attractive because the polarization of the recoiling hyperon (L or S) can be measured directly from the angular distribution of the decay products, rather than with a polarimeter. Prof Ireland and Dr Livingston are spokespersons on three proposals in this programme . Our main experimental effort at JLab since 2008 was the preparation and running of the second part of the g9Frost experiment using a transversely polarized proton target. In addition to being responsible for the polarized photon beam, the Glasgow group participated in the development of the target and coordinated run periods over the 6 months of beam time from Jan-Jul 2010. We provided expertise in several aspects of the analysis phase. Dr Livingston is the analysis coordinator for two run periods, and our PhD students contributed to the international effort of calibration and data reduction before undertaking their final physics analysis. Since 2008, three out of six PhDs awarded to students working on this program have been from the Glasgow group . Two more PhD students from the group are currently working on the N* program at CLAS, and on general techniques for the extraction of polarization observables from such data – a topic in which Prof Ireland has significant expertise . Prof Ireland has also worked extensively with the Gent theoretical group in developing an isobar model of kaon photoproduction and the extraction of resonance parameters from fitting to data.
At MAMI in Mainz we have been carrying out a complimentary program using the4π Crystal Ball detector, which is optimized for the detection of neutral particles (as opposed to CLAS, which is a charged particle spectrometer). We requested funding to make a complete measurement of the helicity amplitudes in π0 and η photo-production via a series of experiments using linearly and circularly polarized photons, a longitudinally and transversely polarized frozen spin target and, crucially, a ~4π recoil polarimeter developed specifically for these measurements . Dr Annand was responsible for the trigger hardware, DAQ and analysis software for the Crystal Ball collaboration and plays the most essential role in the success of the experimental programme. As part of this programme we also proposed the first measurement of the G double-polarization observable forπ0 to investigate the basic parameters of the P11(1440) [Roper] resonance, which are still very poorly known. The programme is already well underway. The recoilpolarimeter was successfully commissioned in 2008 and the frozen spin target has been operating in transverse mode since early 2010. Several beam times have been allocated to π0 and η production, allowing the measurement of cross-sections and single and double polarization observables. The first results of cross section and single polarization observables have already been published . The groups in Glasgow and Edinburgh (our close collaborator) have both recently awarded PhDs on this work , and we each have a PhD student currently working on the programme. We also measured the magnetic moments of baryon resonances . Here, the radiative decay of a nucleon resonance to itself, before a strong decay to a nucleon and a pion takes place, may be used to infer the magnetic moments of the resonance by comparison with calculations of polarized and unpolarizedobservables. The results of the radiative π0 measurement has recently beenpublished , and this Glasgow-led programme of measurements is being extended to the S11(1535) via radiative η photo-production . The η production was also extended to include a search for a narrow nucleon resonance, N*(1680), for which some experiments claim evidence . The Glasgow tagging spectrometer and microscope offered the ideal opportunity to investigate with much better energy resolution than previous measurements. This data is currently being analysed by a Glasgow PhD student under Dr MacGregor’s supervision.
Meson spectroscopy: The small resources available for work in this area allowed us to participate in two run periods with CLAS that concerned meson spectroscopy. Photoproduction of the f0(980) was carried out for the first time, showing that CLAS was ideal for detecting mesonic states. Additionally, theHyCLAS experiment to search for exotic mesons was performed, and the analysis is in progress .
Current activities and future plans
Baryon resonance spectroscopy: At JLab the data taking phase using a 6 GeVbeam will come to an end in 2012. This includes the HDIce experiment to measure polarization observables on the neutron . The goal is to complete the analysis of the strangeness channels from this and other recent experiments, to obtain the world’s first complete measurement for several channels in pseudoscalar meson production on both the neutron and proton. The analysis of the data from CLAS experiments is highly complex; experiments can accumulate up to 1TB of data per day and the calibration of the detector with several thousand instrumentation channels needs a lot of manpower and computing power. Typical timescales in CLAS experiments, from run period to publication, are five years, including roughly a year for the collaboration approval process. We anticipate that many of the results will be published within three years, but analysis of the more recent experiments is likely to continue beyond for several years. The Glasgow group will play a leading role in all aspects of that process. Specifically, we will continue to work on the measurement of the single polarization observables S, P, T and double polarization observables Ox and Oz for all reaction channels relating to recoiling hyperons (L or S) produced with linearly polarized photons on polarized and unpolarized nucleon targets. Dr Livingston will work on the event identification, the separation of the signal from the very high pion background, and will extract the beam polarization and systematics for all recent linearly polarized photon experiments within the N* programme. Dr Protopopescu will develop an event generator and extend the CLAS detector simulation to deal with polarized beams and targets, and Prof Ireland will oversee the development of likelihood analysis techniques for the extraction of polarization observables and helicity amplitudes. There is already a PhD student working on that topic, aided by Drs Lehmann and Murray who developed similar techniques at HERMES. We anticipate roughly six publications with lead Glasgow authors, and in the region of 30 publications from the N* programme within the next 5 years.
In Mainz we are already reaping the benefits from the upgrade of the Glasgow Tagger (recently funded by EPSRC), which is now capable of operating with electron beams of energies (E0) from 180 to 1600 MeV and can analyse momenta in the range 5 – 95% of E0. Currently there are ten accepted proposals with baryon spectroscopy aims , including three with Glasgow spokespersons , and the programme is in full production mode with frozen spin 1H and 2H targets. In contrast to JLab’s N* programme, where the CLAS detector is optimized for multi charged particle final states and recoil polarization is only accessible for hyperons, the Mainz Crystal Ball + TAPS apparatus has very high efficiency for the multi-photon final states produced by the electromagnetic decays of neutral mesons (p0, h, h`, K0). Furthermore, the recent development of a nucleon recoil polarimeter has opened up the possibility of measuring a range of single and double polarization observables at Mainz. Specifically, over the next few years we will participate in a range of experiments to measure cross sections and polarization observables with combinations of linearly and circularly polarized photons, transverse and longitudinally polarized proton and neutron targets and recoil nucleon polarimetry. One of the major goals of this work will be to make the firstcomplete measurement of helicity amplitudes for p0 and h production on the proton and neutron , but within that programme each of the individual experiments address specific unresolved issues relating to nucleon resonances. For example, we shall complete the proposal on the measurement of the G double-polarization observable for π0 to investigate the basic parameters of the P11(1440) [Roper] resonance (spokesperson Livingston), where we aim to measure the M1 partial wave contribution. In helicity dependence of single and double pion photoproduct ion processes and the GDH integral on the neutron(spokesperson Dr Annand ) we will investigate D13(1520), D15(1675) and P33(1600) resonances, and in photoproduction of h mesons off the neutron - polarization observables with circularly polarized photons (E,T,F) we will continue to investigate the structure around W=1675MeV where there is currently intense debate about the possible existence of a narrow nucleon resonance. The Glasgow group will continue to have a leading role in the whole Mainz programme: The Glasgow Photon Tagger will be maintained by Mr Lumsden; Dr Livingston will be responsible for the linearly polarized photon beam and DrAnnand will be responsible for the development and setup of the trigger, DAQ, and online / offline analysis software. Drs Annand and Livingston will oversee the analysis of the data from this programme and will continue to supervise current and future PhD students working on these experiments. Dr Protopopescu will develop detector simulation and analysis techniques for extraction of polarization observables. Dr MacGregor will oversee the development of analysis tools that are common to the programmes at MAMI and CLAS.
Meson spectroscopy: Understanding quark confinement in QCD is one of the outstanding issues in physics. For several years we have been involved with GlueXat JLab, an experiment dedicated to the search for hybrid mesons in the light quark sector up to 4 GeV. The Glasgow group developed a design for the photon tagging spectrometer and provided expertise in the design of the coherent bremsstrahlung facility for the linearly polarized photon beam . We have recently identified an outstanding opportunity for continuing this work within the CLAS collaboration, where a new meson spectroscopy programme is being developed around a forward tagger. This technique, involving electron scattering at very small angles, provides a high photon flux and a high degree of linear polarization. This will be complimentary to GlueX, but with the advantage of high linear polarization and good kaon identification, and will run at the same time as the CLAS12 electron scattering programme. Since we already have a strong involvement in the CLAS collaboration, and will carry out a major DVCS programme at CLAS12, we will focus our meson spectroscopy effort with CLAS12 to make optimum use of resources and experience within the group. We have therefore recently taken the tough decision to withdraw from the GlueX collaboration.
We will lead part of a comprehensive study of the meson spectrum, including the precise determination of resonance masses and properties. In particular, the mass range 1.5-2.5 GeV will be studied to look for mesons with exotic quantum numbers . We already have considerable experience in tagged photons and linear polarization, and will exploit this by participating in the development of the forward tagger; we have contributed to the detector design and the PAC proposal. We will collaborate closely with our colleagues from the Edinburgh group on this project. Dr Protopopescu will develop a detector simulation and software based on GPU programming, and Dr Livingston will develop FPGA based trigger techniques.
Short-Range Nuclear Structure
Overview: In this theme, we seek to address the questions How do nuclear forces arise from QCD? What is the equation of state of nuclear matter? as outlined in the 2009 Nuclear Physics Advisory Panel report and in the recent long range plans of both NuPECC and NSAC . We carry out a range of investigations of nuclear structure which fall into three broad categories: Short-Range Correlations (SRC) of nucleons within nuclei, (JLab Halls-A and CLAS, MAMI A1 and A2 collaborations); coherent A(g,po) measurements of matter form factors of nuclei to determine possible differences in neutron and proton spatial distributions (MAMI A2) and photo-disintegration observables of few-body nuclei to test ab-initiocalculations of the few-body nuclear wavefunction (MAMI A2, MAX-lab).
We shall exploit the experimental platform, built up at JLab, MAMI and MAX-lab, to advance our studies of nuclear structure in parallel with our nucleon-structure and hadron spectroscopy investigations. This will involve pushing to higher momentum transfer in the virtual-photon measurements, and extending the range of polarization observables measured in the real-photon experiments. At JLab, Hall-A we will exploit the BigBite spectrometer (trigger system developed by Dr Annand) and the two high resolution spectrometers (HRS). At MAMI we will harvest data from the Glasgow tagged photon spectrometer (upgraded by Drs Annand andMcGeorge ), the 4p Crystal Ball detector (particle identification systems developed by Glasgow and Edinburgh) and new polarized deuteron and 3He targets. At MAX-lab we will extend the high-pressure, active gas target (developed by Dr Annand) to 3He and polarized-photon experiments.
A. Work carried out in recent years
Short Range Correlations: Two nucleon knockout can be a highly selective probe of the fundamental, short-range 2 (and 3) - body interactions between nucleons. However, to study the evolution of the interaction mechanism with range, it is necessary to study reactions over a range of momentum or energy transfer, ideally using both real and virtual photon probes. Measuring the different final state charge channels provides valuable information on the reaction mechanism, and polarized observables are vital to untangle amplitudes in more complex reaction processes. Each of these provides complementary information on different aspects of the reaction and all are needed to obtain a fuller picture of short-range correlations (SRC).
The interplay between attractive and repulsive, short-range components of the NN force leads to effective pairs of nucleons which have low centre-of-mass momentum, but high relative momentum (2N-SRC). At short distances, equivalent to high momentum, the tensor and short-range repulsive components dominate the NN interaction and may be probed very cleanly in Hall-A using the (e,e'pN)reaction measured at high momentum transfer and high values of xB. In such kinematics isobar excitation and meson exchange are small effects and most of the measured cross section is due to SRC. The E01-015 experiment 12C(e,e'pN) was sensitive to internal proton momenta of 300-600 MeV/c, and measured the ratio ofpp to pn pairs to be 6%, demonstrating the dominance of the tensor component in this region and leading to a publication in Science . This was the first experiment to use the BigBite spectrometer, equipped with a trigger system designed and built byDr Annand, which has subsequently become a mainstay of the Hall-A apparatus.
With the CLAS detector, the 3He(e,e’pp)n reaction has been measured over a wide kinematic range . The ratio of pp to pn pairs was measured as a function of total pair momentum. The data show the importance of tensor correlations at low relative momenta. At MAMI, the A1 collaboration measured the two nucleon knockout reactions 3He(e,e’pn) and 16O(e,e’pn) . These are primarily sensitive to tensor components in the 2N interaction. The extensive set of 3He measurements were a factor of 5 lower in cross section than continuum Faddeev calculations using Argonne V18 and Bonn-B nucleon-nucleon potentials. This large difference has prompted considerable theoretical interest, and highlights deficiencies in the present theoretical calculations (e.g. strictly sub-pion threshold). The initial 16O measurements similarly illuminated deficiencies in the theoretical predictions,prompting improvements to the description of final state interactions (FSI) and two-body currents. The improved theory now agrees with the experimental measurements, and the next, obvious step is to extend the scope of the tests.
The MAMI A2 collaboration made a high resolution measurement of the 16O(g,pn) reaction using an array of hyper-pure Germanium detectors and the Glasgow-Tübingen TOF time of flight neutron detector array. The resolution was sufficient to separate a group of low lying states in the residual 14N. The data were consistent with theoretical models. An extensive 12C(g,pp) measurement [103] (MacGregor co-spokesperson), made with Eg=200-450 MeV linearly polarized photons using the Glasgow tagged photon spectrometer and the Crystal Ball, shows a strong photon asymmetry signal at low missing energies. The asymmetry is enhanced for back-to-back nucleon emissions, showing a strong sensitivity to photon polarization. This work was the subject of a PhD study and is currently being written up for publication.
Matter Form Factors: The matter form factor of nuclei may be obtained from an analysis of the coherent A(g,po) photoproduction differential cross-section. By comparison of the matter and proton (from electron scattering) spatial distributions the neutron distribution may be determined. The extent of the “neutron skin” in the heavy nucleus 208Pb has been measured at MAMI using the ~4p Crystal Ball + TAPS calorimeter which has the excellent energy and angle resolution necessary to determine the po-production diffraction pattern precisely. The measurements were made, in collaboration with colleagues from the University of Edinburgh, at energies well below the D resonance, where pion FSI is small. They spanned sufficient range to test the systematics of FSI corrections. This work will shortly be published. Measurements were also made on 12C and in this case the incoherent pocross section was measured for the first time with the final state of 12C tagged by measurement of the 4.4 MeV de-excitation gamma ray. Such data will provide a window on the transition form factor for excitation of this state, which is strongly collective in character.
Few-body Nuclei: Measurements of polarized-target asymmetries in the 3He(e,e'd) reaction were carried out in JLab Hall-A with the HRS and BigBite spectrometers to test Faddeev few body calculations . The data do not agree well with either theBochum or Hannover calculations. This work illustrates that polarization observables provide a very sensitive test of few body theories.
The photodisintegration of 4He , 6Li and 7Li has been studied from threshold, through the giant resonance region, using the tagged-photon beam at MAX-lab. The 4He investigations include both inclusive g+4He and exclusive 4He(g,n) cross section measurements and have been made with a gas-scintillator active target designed and built by Dr Annand. They are the subject of an STFC funded PhD, which will be submitted in 2011. These first results and a technical description of the active target will be published in 2011, along with inclusive g+6,7Li total cross sections which were measured in collaboration with the University of Tübingen, using the photon-attenuation technique. All of these data are guiding ab-initiocalculations of the structure of few-body nuclei. Recent developments mean that these can now incorporate not only “fully realistic”, phenomenological NN and 3N potentials, but also potentials derived from chiral perturbation theory, thus providing a tangible link between nuclear structure and QCD.
Current activities and future plans
Short Range Correlations: In JLab Hall-A, following on from the earlierexperiment on 12C, we will study SRC in nuclei at the repulsive core limit, via the triple coincidence (e,e'pN) reaction . It will run in 2011 and extend the range of proton momenta probed so that the repulsive-core of the NN interaction can be accessed more cleanly. This will use BigBite as the second proton detector. Experiment PR-09-010 will investigate the isospin dependence of 2N SRC by inclusive electron scattering on the mirror nuclei 3H and 3He. Measurements will extend to the xB > 2 region, constituting the first tests of the isospin dependence of 3N clusters. Experiments with the new Super Bigbite Spectrometer (SBS) are currently being developed. These will have the large acceptance and the ability to withstand >1038s-1cm-2 luminosity, necessary to measure the tiny cross sections associated with SRC at high momentum transfer. In the CLAS collaboration, DrsMacGregor and Ireland will lead the Glasgow contribution to a “Data Mining” initiative, which is the subject of a proposal to the DOE , to re-analyse CLAS detector data from a wide range of previous experiments on a variety of nuclear targets in order to extract information about SRC in nuclei.
At low photon energies, where several reaction mechanisms contribute to two-nucleon knockout reactions, it is especially important to obtain accurate data on a variety of observables, against which theoretical calculations can be tested. Further experiments at MAMI with the Crystal Ball detector will focus on improving the accuracy of measurements of the (g,pp) channel and will also use detection of decay gamma rays to separate ground and excited states in the residual nucleus.
Matter Form Factors: The coherent (g,π0) reaction can be used to obtain information on the matter form factor of the nucleus and previous measurements at MAMI on 208Pb (Tarbert et al., to be published) have shown that the technique has the sensitivity necessary to characterise the neutron skin of stable nuclei. A resolution of 0.02 fm is expected compared with an expected variation in skin thickness of 0.1 to 1.5 fm across the isotopic chain. An accepted MAMI PAC proposal (Dr MacGregor co-spokesperson) will use this technique to map out the variation in neutron skin thickness across up to four Sn isotopes.
Few-body Nuclei: In JLab Hall-A we will measure 2H(e,e'p) at Q2 = 3.5 (GeV/c)2for missing momenta pm = 0.5 – 1.0 GeV/c . This will test the applicability of theeikonal-approximation and the hypothesis that FSI are small and possibly independent of pm. The data will guide the study of SRC in nuclei and high density fluctuations in nuclei. After the 12 GeV CEBAF upgrade, Hall-A will have a tritium gas target which we will use to measure inclusive, deep inelastic scattering on the mirror nuclei 3H and 3He . This will yield a determination of the EMC effect for these nuclei, obtain the ratio of inelastic structure functions F2p/F2n and provide information on the ratio of down to up quark distributions in the nucleon embedded within a nucleus. The experiment will use one HRS and Super BigBite.
Ab-initio calculations of deuteron photo and electro disintegration by Arenhövelhave proved in general to be outstandingly successful, but so far the predictions of the polarizations of the final-state nucleons have often proved wide of the mark. The induced and transferred (from a polarized beam) polarization of the recoiling neutron will be measured with tagged photons at MAMI using two different techniques. Dr Annand is the co-spokesperson for both measurements, which will use: 1) the Crystal Ball, TAPS and a graphite polarization analyser ; 2) the Glasgow TOF array and a plastic-scintillator polarization analyser .
At MAX-lab, we plan to extend photodisintegration measurements to 3He (DrAnnand spokesperson), using the active target employed previously for 4He and an existing stock of 3He held in Glasgow. Both differential cross sections and photon asymmetries will be measured for 3,4He(g,n). The linearly polarized photon beam will be set up using the expertise of Dr Livingston. The Helium active target is also being considered for a new 3He Compton scattering experiment which will test recently developed chiral effective field theory calculations for A=3
