Quarks, Hadrons, and Nuclei

2001 LECTURES AND SEMINAR SPEAKERS

2001 LIST OF PARTICIPANTS

2002 LECTURES AND SEMINAR SPEAKERS

2002 LIST OF PARTICIPANTS

PREFACE

CONTENTS

No abstract received.

The study of nuclear systems containing one or more strange-quarks provides a means of expanding our understanding of the baryon-baryon interaction. The connection between the baryon-baryon force and the hypernuclear structure is discussed along with calculational techniques used to identify hypernuclear states. A brief discussion of the S=-2 systems, including the H-Dibaryon and ΛΛ hypernuclei is presented.

Pion exchange plays a significant role in conventional models of the nuclear force. In these models, the pion contributes significantly at long and intermediate distances – this gives rise to an enhancement of the virtual pion cloud of nuclei relative to that of the free nucleon. However, several experiments that should be sensitive to nuclear pion currents, including deep inelastic scattering, and scattering, and Drell–Yan experiments, yield inconclusive results. In this talk, I will discuss pion–electroproduction as a probe of the nuclear pion field and show recent results from both Jefferson Lab and Mainz. Finally, I will discuss potential future measurements that may shed more light on this issue.

Polarization observables provide information that is inaccessible through cross section measurements. In some cases, experimental limitations do not allow information about smaller amplitudes to be extracted in practice, whereas in other cases, the information is not present in cross section measurements which average over spin states. I will introduce and review several recent topics investigated with polarization observables in Jefferson Lab experiments.

Quark-hadron duality and its potential applications are discussed. We focus on theoretical efforts to model duality. These lectures were given at HUGS 2002 by Sabine Jeschonnek, and were written up by Martin DeWitt.

Studies in nuclear and atomic physics have played an important role in developing our understanding of the Standard Model of electroweak interactions. We review the basic ingredients of the Standard Model, and discuss some key nuclear and atomic physics experiments used in testing these ideas. We also summarize the conceptual issues of the Standard Model that motivate the search for new physics.

The recent measurements at B-factories confirming a non-zero value of sin² β show that CP violation is not restricted to the kaon system. This discovery is the latest of many that support the current Standard Model of particle physics. However, particle physicists believe that the Standard Model is not a complete explanation of nature. I discuss the questions that particle physicists are seeking to answer and why precision measurements in flavor physics are one way to search for the physics beyond the Standard Model. I begin with a brief overview of the CKM matrix and its parameters. I will explain why improving the precision of measurements of the CKM matrix elements is important, and why better measurements and more measurements of CP violation are needed to help probe beyond the Standard Model. Some possible extensions of the Standard Model are described. I will discuss some of the experimental issues and then make the case for the need for a broad program of research in heavy flavor physics, comparing and contrasting different experimental techniques. The measurement of sin 2β is discussed in some detail to give the reader a feel for some of the experimental complications that need to be considered.

Some aspects of QCD, the underlying theory of nuclear interactions are discussed. Emphasis is put on simple and intuitive explanation of various phenomena.

This experiment is an update to E89-003, which was the first physics measurement performed in Hall A at Jefferson Lab in Newport News, VA. We shall examine the momentum distribution of the protons in the ¹⁶O nucleus. By doing this, we hope to determine the momentum region for which Single Particle Models such as the Relativistic Distorted Wave Impulse Approximation (RDWIA) are valid, and that where other more sophisticated models give a better description. As in E89-003, the ¹⁶O(e,e’p) reaction will be studied in the quasielastic region, but this time at Q² = 0.9 (GeV/c)² and ω = 499 MeV. The longitudinal-transverse interference response function (R_LT) and the longitudinal-transverse asymmetry function (A_LT) will be extracted from cross sections measured at a single electron angle for E_miss < 60 MeV and p_miss < 515 MeV/c. The proven self-normalizing waterfall target built by INFN and the Hall A High Resolution Spectrometers (HRS) in standard configuration will be used. The experiment is at present scheduled to run for six weeks during the fall of 2001.

The production of strange particles is one of the topics being vigorously investigated with Jefferson Lab’s CLAS spectrometer. Preliminary calculations of Λ and Σ electroproduction differential cross sections demonstrate significant deviations from predictions of several isobar model calculations. In addition, a preliminary signal for the photoproduction of the Ξ⁻ baryon has been extracted from the G6B data set.

Using a 6 GeV unpolarized electron beam and the CLAS detector at TJNAF, E94–102 experiment measured inelastically scattered electrons in coincidence with protons emitted backwards relative to the virtual photon direction in the reaction d(e, e/p)X. Using the advantages of CLAS detector (large acceptance and out-of-plane detection of the backward proton) we will study the mentioned reaction for a large range of proton momenta (0.25 – 0.6 GeV) and electron kinematics (Q² = 1 − 6GeV², x = 0.2 − 1).

In a simple spectator model, the electron scatters off a forward-moving neutron inside the deuteron, and the detected backward-moving proton is an undisturbed spectator. Its measured momentum is equal and opposite to the momentum of the neutron before was struck. By measuring the semi-inclusive cross section as the function of the spectator momentum and the direction, we can study the dependence on kinematics and off-shell behaviour of the electron-nucleon cross section in the elastic, resonance, and deep inelastic regions. At the same time we gain information on the high-momentum structure of the deuteron wave function. If the virtual photon couples to a quark in a 6-quark object, the spectator picture breaks down and there will be a different dependence of the cross section on the kinematics variables (x, Q²), and momentum of the spectator nucleon, . At the high relative nucleon momenta, in the kinematic region which favors short internucleon distances, the structure of the deuteron wave function will give a hint about the existence of the 6-quark objects.

I will present the features of this experiment, the physics involved and the directions of the analysis. I will review the Spectator Model and 6 Quark Bag Model, models tested in this experiment for describing the correlation nucleon-nucleon, as well as the applications of these models in DIS and Resonance regions. I will summarize the progress of the experiment and projected steps of the analysis.

No abstract received.

At the HUGS 2002 student seminars, an introduction to RPEX (Pr. 00-02, spokespersons: H. Gao, J. Calarco) will be given. The proton rms charge radius is a fundamental quantity, and its precise determination will have significant impacts on tests of QCD and QED. It will be measured with sub 1% precision and will run at the Bates Linear Accelerator Center located in Middleton, Massachusetts. This experiment will utilize the symmetric sectors of the BLAST detector, and the super-ratio technique to extract both the proton elastic form factor ratio at low Q² and the proton charge radius. A laser driven polarized H/D target is being developed for use in this experiment.

The Standard Model predicts the existence of so-called “exotic” states such as glueballs, hybrids, and four-quark states. The presence of low lying exotics with J^PC = 0⁺⁺ complicates the analysis of the light scalar mesons since, given the fact that they have the same quantum numbers, they will tend to mix. Here, we present a calculation of the f₀ → γγ transition form factor for the isoscalar states f₀(1370), f₀(1500), and f₀(1710) using a light-front constituent quark model. The flavor-glue content of these states is determined using a mass-squared mixing scheme. The form factor calculation involves writing down a light-cone spin-space wavefunction for the scalar meson. The radial wavefunction is taken to be a simple harmonic oscillator wavefunction with one model parameter, which is then used as a trial wavefunction of the variational method. The spin-orbit wavefunction is determined exactly by the Melosh transformation. The full wavefunction is then used to calculate the two-photon transition form factor. In the limit as q² goes to zero, the form factor can be used to calculate the f₀ → γγ decay width. Ratios of these decay widths for the different scalars can be compared with other model calculations.

The pion photoproduction has been extensively studied since the last four decades. This particular reaction is very useful in understanding the structure of nucleon and its resonances. We investigate this reaction up to the second resonance region. Though the differential cross section is relatively rather small (in mb units) but it implies the information about the role of nucleon resonances such as P₃₃(1232), P₁₁(1440), D₁₃(1520), S₁₁(1535), P₃₁(1620). We use the πN scattering model developed by F.Gross-Y.Surya. They treated the P₃₃ and D₁₃ as pure spin-3/2 particles. This model describe the πN interaction very well. By extending this model to the γN and adding more higher baryon resonances, we hope there’ll be a good agreement with the experimental data and some analysis (SAID and MAID). The other features of pion photoproduction; the role of P₃₃ or Delta, the ratio of E₂/M₁, are also interesting to be investigated. While the experimental data for higher nucleon resonance from the γN are not produced yet, this investigation gives prominently foundation for further research.

Recent double polarisation experiments at MAMI have provided accurate, model independent measurements of the Neutron Electric Form Factor, . Quasi-elastic, experiments, currently in progress at MAMI, will extract the polarisation components of the recoil neutron using a spin precession technique. The polarisation can then be used to calculate a value for . This will complete the measurement of as a function of Q² for . This will provide a stringent test for nucleon models, as is particularly model sensitive in this Q² range.

The DVCS experiment is scheduled to run in September, 2003. I will give a brief description of the DVCS reaction and the equipment we are building to test it. The main subject will be the recently completed DVCS test run and its relation to the experiment. I will present results from the elastic calibration runs for scintillator calibration and discuss how these are related to proton counting rates in the scintillator array.

An overview of SLAC experiment E158 is presented. E158 makes a precision measurement of the parity-nonconserving left-right asymmetry in the scattering of longitudinally polarized electrons from the atomic electrons in a hydrogen target (Møller scattering). The asymmetry measures the effective pseudo-scalar weak neutral-current contribution to Møller scattering at an average Q² of 0.03 (GeV/c)², and it is proportional to (¼ - sin² θ_W), where sin² θ_W is the electroweak mixing angle. The asymmetry will be measured to a precision of 7 × 10⁻⁹, which corresponds to δ(sin² θ_W) ~ 0.0007. A comparison of this measurement with precision asymmetry measurements of the Z⁰ resonance provides the first statistically significant measurement of , testing the electroweak theory at the quantum loop level away from the Z⁰ resonance. The experimental setup suited to the challenges of this precision measurement is described.

The existence of isoscalar f₀(980) and isovector a₀(980) hadronic resonances is well established but their quark structure is not yet known. Their masses and widths are to low to fit in the scalar nonet picture, and it has been suggested that they might be meson molecules or 4-quark states. The Radphi experiment at Jlab is designed to study rare radiative decays of ϕ(1020) meson, particularly ϕ → a₀γ and ϕ → f₀γ branching ratios. By measuring the ratio of these branching ratios, which is sensitive to the a₀ and f₀ quark content, it is expected to reveal the substructure of these mesons. The ϕ meson is produced in flight by diffractive photoproduction in the γp → ϕp reaction, using tagged photon beam with energy between 4-5.6 GeV. The lead glass calorimeter together with the three-level trigger system is used to select all-neutral final states. The known radiative decays of vector mesons ω → πγ and ϕ → ηγ are used to calibrate the lead glass detector (LGD). The LGD calibration is prerequisite for answering the question posed by the rare radiative ϕ decays.

No abstract received.

A measurement of the (e,e’pn) reaction has been carried out in the 3 spectrometer facility of the A1-collaboration at the Mainz 855 MeV electron microtron MAMI. The targets studied were ³He and ¹⁶O and some data were also taken with a ²H target for calibration purposes. A magnetic spectrometer was used to detect the scattered electrons, a large plastic scintillator hodoscope was used to detect the protons and a large time-of-flight scintillator array was used for detection of the neutrons. The ³He data were taken at an energy transfer of 220 MeV and momentum transfer of 375 MeV/c. The ¹⁶O data were taken at super-parallel kinematics with an energy transfer of 215 MeV and a momentum transfer of 316 MeV/c. For the kinematics studied the reaction cross section is sensitive to nucleon-nucleon correlations and two-body mechanisms like Δ-excitation and meson exchange currents.

Non-Linear σ-model plays an important role in many areas of theoretical physics. Been initially uintended as a simple model for chiral symmetry breaking, this model exhibits such nontrivial effects as spontaneous symmetry breaking, asymptotic freedom and sometimes is considered as an effective field theory for QCD. Besides, non-linear σ-model can be related to the strong-coupling limit of O(N) ϕ⁴-theory, continuous limit of N-dim. system of quantum spins, fermion gas and many others and takes important place in undertanding of how symmetries are realized in quantum field theories. Because of this variety of connections, theoretical study of the critical properties of σ-model is interesting and important.

Oscillator representation method is a theoretical tool for studying the phase structure of simple QFT models. It is formulated in the framework of the canonical quantization and is based on the view of the unitary non-equivalent representations as possible phases of a QFT model. Successfull application of the ORM to ϕ⁴ and ϕ⁶ theories in 1+1 and 2+1 dimensions motivates its study in more complicated models such as non-linear σ-model.

In our talk we introduce ORM, establish its connections with variational approach in QFT. We then present results of ORM in non-linear σ-model and try to interprete them from the variational point of view. Finally, we point out possible directions for further research in this area.

The process under study here is the exclusive electroproduction of mesons on the nucleon, in the Bjorken regime (Q², ν large and x_B finite). The concept of Generalized Parton Distributions (GPDs) which appeared recently to interpretate such exclusive reactions in this regime is reviewed. I emphasize on the richness of GPDs and especially the fact that they provide new information on the nucleon spin structure, which may shed the light on the “spin puzzle”. Furthermore, the upcoming experimental program at JLab aiming at measuring exclusive electroproduction of the vector mesons ρ,ω and ϕ in the Bjorken regime with the Cebaf Large Acceptance Spectrometer (CLAS) is presented. I discuss the relevant considerations for exclusive physics and what first step in the broad program of determining GPDs we hope to be able to make.

Exclusive measurement of quasi-free nucleon scattering provides a direct mechanism to study modifications of the free nucleon-nucleon (NN) interaction in nuclear medium. It is known that due to the density dependence of the NN interaction, quasi-free proton scattering from light targets (A≤40) at medium energies (≥400 MeV) yields analyzing powers that are substantially reduced with regards to Distorted Wave Impulse Approximation (DWIA) calculations that utilize the free NN interaction. However, at lower energies (≤200 MeV) the free NN interaction appears to be adequate for the description of proton knockout from light targets. In order to extend the current data-set to knockout from a heavy target, the 200 MeV polarized proton beam of the National Accelerator Centre (Faure, South Africa) was utilized to study the ²⁰⁸Pb(p, 2p)²⁰⁷Tl knockout reaction. The measured analyzing powers display the so-called ‘quenching’ effect with regards to standard DWIA calculations, as found for quasi-free scattering at higher energies. Agreement between theory and experiment is shown to improve only marginally when density dependent NN interactions are incorporated within the DWIA.

The Standardmodel predicts, beyond pure -states, exotic particles like glueballs and hybrids, which carry gluonic degrees of freedom. In the mass region of about 1250 MeV-1500 MeV, three J^PC = 0⁻⁺ states are known, the η(1295), η(1400) and η(1480). All theses states cannot be pure -states, thus the η(1400) is discussed to be a glueball candidate.

Data was taken with the Crystal Barrel detector at LEAR at CERN. 6026 events of the reaction were fitted and the branching ratio of the reaction determined to (1.64±0.46)·10⁻³. The partial wave analysis shows clear evidence for a η(1400) and also hints for the η(1480). The η(1295) is not needed to describe the data. The results will be presented and a new interpretation of the eta spectrum will be given.

In this paper we provide an overview on the theory and technique used to perform a partial wave analysis of the reaction . First, we will show why this channel is so interesting, and what the main questions in this meson sector are. Afterwards, we will present the main techniques of the partial wave analysis and at the end we will review the results and give a short interpretation.

An experiment has been approved to measure the exclusive, unpolarized cross section for Real Compton Scattering in the energy range 3-6 GeV and over a wide angular range (65-130° scattering angle in the CM frame), as well as to measure the longitudinal and transverse components of the polarization transfer to the recoil proton at a single kinematic point. Together, these measurements will test models of the reaction mechanism and possibly determine new structure functions of the proton that are related to the same nonforward parton densities that determine the elastic electron scattering scattering form factors and the parton densities. The experiment utilizes an untagged bremsstrahlung photon beam and the standard Hall A cryogenic liquid hydrogen targets. The scattered photon is detected in a photon spectrometer, currently undergoing construction and testing. The coincident recoil proton is detected in one of the Hall A magnetic High Resolution Spectrometers (HRS), and its polarization components are measured in the existing Focal Plane Polarimeter. At present this experiment is scheduled to run in January 2002.

One of the best tool to investigate parity violating phenomena is to consider the asymmetry where σ_R(L) indicates the cross section of a right handed (left handed) polarized electron beam scattering on an unpolarized nuclear target. Such asymmetry assumes a relatively easy form in the parton model, and if sea quark and mass corrections are neglected, then it does not depend on the nuclear structure ¹. Our goal is to study the importance of such corrections in different kinematics regions. In the next years such asymmetry might be used to extract a precise value of the Weinberg angle so it is necessary to know and take into account all the possible corrections to the naive parton model.

By using a code provided by the CTEQ group we were able to study the behaviour of the asymmetry as a function of Q² and the Bjorken variable x. The result was that our prediction differ utmost one percent with respect the one in ¹. Besides sea quark and mass corrections, another type of correction could be important in the analysis of the asymmetry: those are the so called higher twist corrections which are usually suppressed by , where Q² = − q² with q equal to the momentum transferred. Such corrections have to be taken into account, eventually, when the momentum transferred is of the order of a Gev. Unfortunately higher twists are given by matrix elements of quark currents between hadronic states, and due to the lack of knowledge of the hadronic wave functions, we have to rely on some model. In the future we plan to calculate some of these higher twist effects by using the bag model, to get an estimate of the order of magnitude in which they might enter the asymmetry.

G0 is a parity violation experiment with a dedicated large acceptance spectrometer. This experiment will measure parity violating asymmetries in electron-proton scattering to probe the sea of the nucleon between 0.1 ≤ Q² ≤ 1.0 GeV² at a beam energy of 3.0 GeV. The strange quark contributions to the magnetic and charge densities of the proton will be determined by measuring the strange form factors and . I will discuss what makes G0 different from other strange experiments, the experimental set-up with an emphasis on the detectors, and what remains to be done before the experiment begins in 2002.

In order to investigate the Q² dependence of the generalized Gerasimov-Drell-Hearn (GDH) sum for the neutron, we have measured the spin dependent longitudinal and transverse cross sections for momentum transfers ranging from 0.1 to 1 GeV² and excitation energies across the quasielastic, resonance and deep inelastic region. The longitudinally polarized (P_beam ≃ 70%) electron beam of energy ranging from 0.86 to 5.1 GeV of the Thomas Jefferson National Accelerator Facility was scattered at a fixed angle, off a high pressure polarized (P_targ ≃ 35%) ³He target in Hall A. The direction of the target polarization was maintained either parallel or perpendicular to the incident electron beam. This measurement allows the extraction of the polarized neutron spin structure functions g₁ and g₂ and the evaluation of the extended GDH sum on both ³He and the neutron. This sum, when compared to theoretical models, will help us to understand the transition from the perturbative to the non-perturbative regime of QCD.

The preferential spin orientation of a particle in the final state of a nuclear reaction is an observable which carries information about the underlying reaction mechanism. In this introductory lecture I will discuss the basic principles of polarimetry of spin ½ particles, focal-plane proton polarimeter and the extraction of the proton recoil polarization.

In the Hugs 2002 student seminar sessions, I treat a number of topical issues in neutrino physics: the phenomenology of the MSW mechanism; a brief discussion of global analysis of atmospheric and solar neutrinos; Dirac and Majorana neutrino masses and the Seesaw mechanism as an explanation for the smallness of the neutrino mass. MSW model assumes that neutrinos are created in flavor eigenstates without any definite mass but they evolve in mass eigenstates with definite mass. Latest experiments confirms the neutrino oscillations. Considering MSW oscillations in calculations provides an acceptable solution for measured neutrino flux deficiency in solar and atmospheric neutrinos. Neutrinos do not have mass in the standard model. Experimentally, it is known that neutrino mass is exceedingly small, if not zero. The seesaw mechanism predicts left handed light neutrinos with right handed heavy partners.

The constituent counting rule was showed from different exclusive experiments, including the photodisintegration and photoproduction experiments in Jlab. Some theories, including simple dimensional counting, Perturbative Chromodynamics(pQCD), Quark-Gluon Strings model(QGS) and Skewed Parton Distribution(SPD), were presented towards this behavior. But the early onset and oscillation around the scaling, as well as the breaking of Hadron Helicity Conservation(HHC), need further developments.