Search

The study of hyperon resonances has entered a new era of precision with advent of high-statistics photoproduction data from the CLAS detector at Jefferson Lab. These data have multi-particle final states, allowing clean identification of exclusive reactions associated with strange mesons and baryons. Examples of physics results are: evidence for isospin interference in the decay of the Λ ( 1405 ) resonance; a strong suggestion of meson cloud effects in the structure of the Σ ( 1385 ) resonance; data from K ⁎ photoproduction that will test the existence of the purported K 0 ( 800 ) meson. Properties of other hyperon resonances will also be studied in the near future.

The spin structure functions g 1 for the proton and the deuteron have been measured over a wide kinematic range in x and Q 2 using 1.6 and 5.7 GeV longitudinally polarized electrons incident upon polarized NH3 and ND3 targets at Jefferson Lab. Scattered electrons were detected in the CEBAF Large Acceptance Spectrometer, for 0.05 Q 2 5 GeV 2 and W 3 GeV . The first moments of g 1 for the proton and deuteron are presented – both have a negative slope at low Q 2 , as predicted by the extended Gerasimov–Drell–Hearn sum rule. The first extraction of the generalized forward spin polarizability of the proton γ 0 p is also reported. This quantity shows strong Q 2 dependence at low Q 2 . Our analysis of the Q 2 evolution of the first moment of g 1 shows agreement in leading order with Heavy Baryon Chiral Perturbation Theory. However, a significant discrepancy is observed between the γ 0 p data and Chiral Perturbation calculations for γ 0 p , even at the lowest Q 2 .

The proton is made from three quarks, and mesons are made from a quark and antiquark. But what other configurations of quarks and antiquarks can form a particle? From a theoretical standpoint, this question is difficult to answer. Yet new particles with different combinations of quarks keep popping up in experiments. The latest of these, reported by the Large Hadron Collider Beauty (LHCb) Collaboration at CERN, is the pentaquark, a never-before-seen combination of five quarks that consists of a proton and a pair of heavy charm-anticharm quarks [1]. The detection of this particle comes as somewhat of a surprise: although the existence of the pentaquark doesn’t require a revision of the standard model of particle physics, earlier searches for a pentaquark composed of lighter quarks came up empty or detected candidate particles that could not be reproduced [2]. From studying this new form of quark matter, physicists stand to learn more about the complex forces between bound quarks. The concept of multiquark particles is not new [3]. But for decades after the quark model was conceived, only two categories of strongly interacting particles were known: baryons (such as protons and neutrons), consisting of three quarks, and mesons. Only within the past decade have scientists started to observe new particles that do not fit into the quark model, such as the X(3872) [4] and the Zc(3900) (see 17 June 2013 Viewpoint), which are about 4 times heavier than the proton. The Zc(3900) decays into a charm-anticharm meson (called the J/ψ), plus a light meson, suggesting the particle is made from two quarks and two antiquarks. The observation of this four-quark state—or tetraquark—implied that other multiquark states, such as a heavy pentaquark, might exist [5]. The challenge has been looking for them. Rare particles are the proverbial needle in the haystack, as scientists must hunt for them in the copious debris from high-energy collisions. In the LHCb experiment, two protons collide at very high energy, sometimes producing composite particles that have a heavy bottom quark and which stay close to the beam line. The bottom-quark particles go a short distance and then decay into other particles, such as a pentaquark plus a K meson, which are then picked up by a series of detectors. The key step that allowed LHCb’s detector to identify the candidate pentaquark was to only look for products of the bottom-quark particle Λb, which has a lifetime long enough for it to move away from the proton-proton collision point before it decays. Using the Λb decay as a filter, the researchers were able to see two new pentaquarks—or, rather, the products of their decays—sufficiently far from other confounding particle debris. Both pentaquarks that LHCb has observed are resonant-energy states of the proton and the J/ψ particle and are therefore referred to as “charmonium pentaquarks.” More formally, they are named Pc(4450) and Pc(4380), where the numbers, according to convention, indicate the particles’ masses in mega-electron-volts. A natural question to ask is what other combination of quarks—besides the resonant state of a proton and J/ψ that the LHCb researchers assume—could form a pentaquark resonance? For example, over a decade ago, theorists predicted [6] that a pentaquark could be made from a proton and a φ meson—a pair of strange-antistrange quarks. But experimental searches never uncovered this type of pentaquark. Similarly, a resonant combination of the neutron and a K+ meson, called the Θ+pentaquark, was predicted almost two decades ago. But again, the particle’s existence wasn’t confirmed, despite extensive experimental searches. (The exception was a claim by the Laser Electron Photon experiment in Japan that has been hotly contested [2].) In fact, there are many combinations of quarks that can form a pentaquark, but the only one seen so far is the charmonium pentaquark found by LHCb. What makes it so special? The short answer is we don’t know. The forces between quarks are complex, and accurately describing a manyquark system using the theory of the strong force, quantum chromodynamics (QCD), is an extremely difficult

The famous discovery of the Omega − in 1964 put the quark model on firm ground and since then a lot of effort has been spent on mapping out the baryonic and mesonic states. Over the following decades, many excited baryons with light quarks (up, down and strange) have been measured, but by most predictions, only a small percentage of those expected have been found. In this talk, I will discuss a newly developing technique using an (unflavored) photon beam to excite protons to doubly-strange "Cascade" (Xi) states. Advantages of such an experiment and associated difficulties will be presented, along with recent results from the CLAS detector at Jefferson Lab in Virginia.

The GeV photon beam at SPring-8 is produced by backward-Compton scattering of laser photons from 8 GeV electrons. The maximum energy of the photon will be above 3 GeV, and the beam intensity will be 10 7 photons/sec. Polarization of the photon beam will be 100 % at the maximum energy with fully polarized laser photons. We report the outline of the quark nuclear physics project with this high-quality high-intensity beam.

A complete set of spin observables was measured for the 2 H(p,n)2p reaction at 200 MeV at the Indiana University Cyclotron Facility (IUCF) with the high intensity polarized ion source (HIPIOS) and the Indiana Neutron Polarization Facility (INPOL). The incident proton beam was delivered successively in each of the three spin states, normal ( N ), sideways ( S ), and longitudinal ( L ). Two large-volume neutron polarimeters placed along the 0° and 24° beam lines measured the polarizations of the emitted neutrons in each of the three spin states. The measured cross sections and spin observables are compared with three-body Faddeev calculations, which generally agree with the data. The ratio of the longitudinal to transverse response is suppressed rather than enhanced, as predicted by some models.

We report the first measurement of the transverse momentum dependence of double-spin asymmetries in semi-inclusive production of pions in deep-inelastic scattering off the longitudinally polarized proton. Data have been obtained using a polarized electron beam of 5.7 GeV with the CLAS detector at the Jefferson Lab (JLab). Modulations of single spin asymmetries over the azimuthal angle between lepton scattering and hadron production planes phi have been measured over a wide kinematic range in Bjorken x and virtual photon squared four-momentum Q(2). A significant nonzero sin2 phi single spin asymmetry was observed for the first time indicating strong spin-orbit correlations for transversely polarized quarks in the longitudinally polarized proton.

We present results on the electroexcitation of the low mass resonances Delta(1232)P33, N(1440)P11, N(1520)D13, and N(1535)S11 in a wide range of Q2. The results were obtained in the comprehensive analysis of JLab-CLAS data on differential cross sections, longitudinally polarized beam asymmetries, and longitudinal target and beam-target asymmetries for pion electroproduction off the proton. The data were analysed using two conceptually different approaches, fixed-t dispersion relations and a unitary isobar model, allowing us to draw conclusions on the model sensitivity of the obtained electrocoupling amplitudes. The amplitudes for the Delta(1232)P33} show the importance of a meson-cloud contribution to quantitatively explain the magnetic dipole strength, as well as the electric and scalar quadrupole transitions. They do not show any tendency of approaching the pQCD regime for Q2 N(1520)D13 and N(1535)S11 longitudinal helicity amplitudes with good precision., 23 pages, 17 figures

The light vector mesons ($\rho$, $\omega$, and $\phi$) were produced in deuterium, carbon, titanium, and iron targets in a search for possible in-medium modifications to the properties of the $\rho$ meson at normal nuclear densities and zero temperature. The vector mesons were detected with the CEBAF Large Acceptance Spectrometer (CLAS) via their decays to $e^{+}e^{-}$. The rare leptonic decay was chosen to reduce final-state interactions. A combinatorial background was subtracted from the invariant mass spectra using a well-established event-mixing technique. The $\rho$ meson mass spectrum was extracted after the $\omega$ and $\phi$ signals were removed in a nearly model-independent way. Comparisons were made between the $\rho$ mass spectra from the heavy targets ($A > 2$) with the mass spectrum extracted from the deuterium target. With respect to the $\rho$-meson mass, we obtain a small shift compatible with zero. Also, we measure widths consistent with standard nuclear many-body effects such as collisional broadening and Fermi motion., Comment: 15 pages, 18 figures, 3 tables

1 tex file (6 pages), 4 (eps) figures; The beam spin asymmetries in the hard exclusive electroproduction of photons on the proton (ep -> epg) were measured over a wide kinematic range and with high statistical accuracy. These asymmetries result from the interference of the Bethe-Heitler process and of deeply virtual Compton scattering. Over the whole kinematic range (x_B from 0.11 to 0.58, Q^2 from 1 to 4.8 GeV^2, -t from 0.09 to 1.8 GeV^2), the azimuthal dependence of the asymmetries is compatible with expectations from leading-twist dominance, A = a*sin(phi)/[1+c*cos(phi)]. This extensive set of data can thus be used to constrain significantly the generalized parton distributions of the nucleon in the valence quark sector.

Nonrelativistic and relativistic collective deformed potential models have been used to study the excitation of seven states in 58Ni by 280 and 489 MeV unpolarized protons. Optical model potentials were determined at each energy by fitting elastic scattering data using both of the models. Deformation lengths were also obtained using both models with no dependence on incident energy observed for either model. The average value of the hadronic deformation lengths along with the charge deformation lengths were used to calculate neutron to proton multipole matrix elements ( M n M p ). Five transitions, 2+1, 1.45 MeV; 2+3, 3.04 MeV; 2+4, 3.26 MeV; 4+1, 2.46 MeV; and the 4+5, 4.75 MeV state were determined to be isoscalar, i.e. M n M p = N Z . The 4+2, 3.63 MeV state was observed to be a proton-like transition with M n M p = 0.43 ± 0.18. At small scattering angles, the relativistic model gave consistently larger cross sections than the nonrelativistic model. This effect was much larger at 280 MeV than at 489 MeV.

Measurements of the spin-flip probability S nn for inclusive inelastic proton scattering around 300 MeV from nuclei between 12 C and 90 Zr show that an enhanced spin response near 40 MeV excitation at q ∼ 100 MeV/ c is a general feature of nuclear structure. Data for 40 Ca at 800 MeV confirm that the enhancement is not a peculiarity of 300 MeV scattering. In addition, measurements in 44 Ca up to 75 MeV show that the enhancement cannot be attributed solely to a relatively narrow resonance. Continuum RPA calculations suggest that the enhancement is due to the exhaustion of most S = 0 strength at lower energy and a shift of S = 1 strength to higher energy.

The longitudinal target-spin asymmetry A_UL for the exclusive electroproduction of high energy photons was measured for the first time in p(e,e'p\gamma). The data have been accumulated at Jefferson Lab with the CLAS spectrometer using 5.7 GeV electrons and a longitudinally polarized NH_3 target. A significant azimuthal angular dependence was observed, resulting from the interference of the Deeply Virtual Compton Scattering and Bethe-Heitler processes. The amplitude of the sin(phi) moment is 0.252 +/- 0.042(stat) +/- 0.020(sys). Theoretical calculations are in good agreement with the magnitude and the kinematic dependence of the target-spin asymmetry, which is sensitive to the generalized parton distributions H and H-tilde., Comment: Modified text slightly, added references

A new baryon resonance with manifestly exotic strangeness quantum number S = +1 was first observed from the LEPS detector at the SPring‐8 facility in Japan. The photoproduction reaction γn → K−Θ+ was measured, where the neutron is bound in a 12C nucleus and the Θ+ is the new name of this resonance. The existence of the Θ+ was inferred by observing a peak in the missing mass spectrum of the K−, with the requirement of a coincident K+ in the detector from the decay of the Θ+. The mass of the resonance is 1.54 ± 0.01 GeV and a width of < 25 MeV.

Cross-sections and recoil polarizations for the reactions gamma + p --> K^+ + Lambda and gamma + p --> K^+ + Sigma^0 have been measured with high statistics and with good angular coverage for center-of-mass energies between 1.6 and 2.3 GeV. In the K^+Lambda channel we confirm a structure near W=1.9 GeV at backward kaon angles, but our data shows a more complex s- and u- channel resonance structure than previously seen. This structure is present at forward and backward angles but not central angles, and its position and width change with angle, indicating that more than one resonance is playing a role. Rising back-angle cross sections at higher energies and large positive polarization at backward angles are consistent with sizable s- or u-channel contributions. None of the model calculations we present can consistently explain these aspects of the data., 5 pages, 3 figures, submitted to Physical Review Letters

The CEBAF large acceptance spectrometer (CLAS) is used to study photo- and electro-induced nuclear and hadronic reactions by providing efficient detection of neutral and charged particles over a good fraction of the full solid angle. A collaboration of about 30 institutions has designed, assembled, and commissioned CLAS in Hall B at the Thomas Jefferson National Accelerator Facility. The CLAS detector is based on a novel six-coil toroidal magnet which provides a largely azimuthal field distribution. Trajectory reconstruction using drift chambers results in a momentum resolution of 0.5% at forward angles. Cherenkov counters, time-of-flight scintillators, and electromagnetic calorimeters provide good particle identification. Fast triggering and high data-acquisition rates allow operation at a luminosity of 10 34 nucleon cm −2 s −1 . These capabilities are being used in a broad experimental program to study the structure and interactions of mesons, nucleons, and nuclei using polarized and unpolarized electron and photon beams and targets. This paper is a comprehensive and general description of the design, construction and performance of CLAS.

Neutral pion photoproduction from a liquid deuterium target was measured in the energy region near 300 MeV at the LEGS facility of Brookhaven National Laboratory. The inclusive cross sections from deuterium are in agreement with measurments from Mainz, yet the exclusive cross sections and spin asymmetries for neutral pion production in coincidence with a detected nucleon are much smaller than expected from a quasi-free approximation. This may indicate that substantial final state interactions play a significant role, which will complicate the extraction of the desired amplitudes that would be measured if a free neutron target could be used.

The CLAS detector is a large acceptance spectrometer located at the electron accelerator at Thomas Jefferson National Accelerator Facility. Production of K+ and K* mesons along with associated hyperons were measured using the CLAS detector at electron beam energies of 2.4–4.4 GeV. In addition, polarization transfer from the electron helicity to the Λ hyperon was measured. These data are compared with theoretical models, where available, based on fits to older, less precise K+ data. The new data for CLAS indicate that improvements in the models are necessary in order to interpret the results.

The response of the nucleus to high energy probes promises to yield new information about the properties of nuclear matter. Specifically, one may learn about how a nucleon imbedded in the nucleus differs from a nucleon in free space. The nuclear response at high energies has drawn much attention in recent years, partly due to the EMC measurements. The high‐energy muon beam at Fermilab has been used by the E665 collaboration to extend studies of the nuclear response to higher energies using a 500 GeV muon beam. The present measurement emphasizes one aspect of the nuclear response to high‐energy leptons by detecting few‐MeV nucleons emitted from the nucleus. From this information, the average excitation of the residual nucleus can be inferred using standard low‐energy techniques. These data may help in developing an understanding of the hadronization process of the struck quark. The long‐standing question of the process by which the struck quark emerges dressed as a hadron jet is still open, partly because ...

The analyzing power for quasi-free nucleon-nucleon scattering has been found to differ substantially from that for the free interaction. This medium modification was first noticed in inclusive scattering experiments. In the present work, it has been demonstrated that kinematically complete exclusive measurements are capable of demonstrating this effect more clearly. This advantage arises by virtue of the possibility of selecting nucleons bound in states localized in the nuclear interior. Modification of the nucleon-nucleon interaction in the nuclear medium has been found to be an important aspect of various reactions induced by intermediate energy protons. One of the most direct ways of studying such modifications is quasi-free scattering of nucleons by polarized protons. Inclusive measurements of spin observables in this reaction have yielded some results which depart significantly from expectations based on the free interaction, even after accounting for "obvious" effects such as internal motion of the nucleons and binding energy. It is expected in the context of relativistic mean field theory and suggested by empirical fits of Dirac optical potentials to nuclear elastic scattering data that the potential experienced by a nucleon inside the nucleus includes large attractive Lorentz scaler and repulsive vector terms. It has been pointed out that the large negative scaler potential will reduce the effective mass of the nucleon upon which depend the Dirac spinors between which the nucleon-nucleon matrix elements are evaluated. In particular, the lower components of the spinors are expected to be enhanced. In one model of this effect, some spin observables are predicted to be significantly modified[l]. Most remakably, the analyzing power is substantially reduced. Although such medium effects are diluted in inclusive reactions because of emphasis of the nuclear surface where the density is not large, some indications of this medium effect have been observed[2]. In the investigation of medium effects on the basic interaction, we may expect that exclusive measurements can enjoy some significant advantages. First, the p-p and n-p interactions can be studied separately instead of being averaged together. Also, the half-off-shell kinematic conditions of each scattering event are determined experimentally instead of being averaged over, resulting in more explicit information. Most importantly, because the residual nucleus energy is determined experimentally, knockout from individual nuclear orbitals can be distinguished. To maximize medium effects, we may therefore select events corresponding to knockout from an orbital whose wave function has the bulk of its strength within the nuclear surface. The 1s orbital satisfies this criterion as well as simplifies the interpretation of the data further by precluding any initial effective polarization of the struck nucleon due to distortion effects[3]. Kinematically emphasizing the nuclear interior avoids problems associated with surface-peaked reactions Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990681 COLLOQUE DE PHYSIQUE such as inclusive quasi-free scattering for example the local density approximation is most suspect near the nuclear surface where the density gradient is large. The capability of the exclusive reaction to emphasize the nuclear interior has been studied theoretically by incorporating into a non-relativistic DWIA calculation[4] the density dependence of Horowitz and Iqbal[l]. The radial dependence was explicitly included by relaxing the usual factorization approximation. It was found that modifications to certain spin observables were indeed predicted to be large as much as a factor of two reduction in A, , for example (see TRIUMF Experimental Proposal 473, C.A. Miller et a1.(1987). In an effort to exploit the advantages of the exclusive reaction, spin observables for knockout of 1s nucleons have been measured for the first time. The choice of a l p shell nucleus, 160 , as well as a beam energy of 500 Mev were defined by the need to minimize the effects of absorption on the final state nucleons. In the first phase of the experiment, the analyzing power A, was determined while in a later run still being analyzed, the spin transfer parameters D,,,, D,,, and P were measured. The experiment was carried out with the polarized proton beam from the TRIUMF cyclotron. The Medium Resolution Spectrometer (MRS) was used to detect and identify a proton in coincidence with a lower energy proton which was detected in a crude magnetic spectrometer consisting of a dipole magnet with tracking from four vertical-drift chambers. The missing-energy resolution was adequate to easily resolve the 1s knockout from the two p states and to resolve the p states from each other via peak fitti'ng. Analyzing power measurements were done at MRS angles of 20°, 25' and 30' while the spin transfer measurement was done only at 25". In each case, the other detector arm was optimized for minimum recoil momentum at the Is separation energy of 40 MeV. Figure 1 shows analyzing power results for knockout of Is protons from 160 . They are drastically reduced in comparison with the free values shown for the same two-body kinematic conditions. The degree of "quenching" of A, is larger than that found in inclusive measurements as well as expectations based on the density-dependent DWIA calculation mentioned above. Although the optical potentials for 160 used here are only crude, they had little effect on the analyzing power in the absence of the density dependence. A more interesting comparison will be with the new finite-range Dirac-based DWIA calculation which implicitly includes modified Dirac spinors[5]. Analyzing power data for p-state knockout is shown in Figure 2. It reveals the familiar spin-orbit splitting due to the struck nucleon effective polarization induced by final state distortions. Hence it is manifested through the NN spin correlation parameter A,,. The degree of splitting therefore is sensitive to medium modification of this NN observable. However, such interpretation will require careful comparison with a calculation using reliable optical potentials. Preliminayr indications from analysis of a portion of the spin transfer data are that medium modifications are present but not large. However there is a suggestion that the polarization parameter P departs significantly from the analyzing power Ay, an effect not predicted by the nonrelativistic calculations.

The fission of nuclei with $159\ensuremath{\le}A\ensuremath{\le}232$ induced by the bombardment of 19.1 to 44.5 MeV $^{3}\mathrm{He}$ ions has been measured using solid-state detectors with time-of-flight measurements. Analysis with statistical fission theory, including precompound nucleon emission, indicates fission barriers which decrease only slightly relative to the liquid-drop model values with decreasing $Z$, approaching about 90% of the liquid-drop model barrier for $A\ensuremath{\approx}160$. These results are in contrast with measurements at higher angular momenta which indicate much lower fission barriers (60 to 70% of the liquid-drop model) for this mass region. The angular correlations indicate complete, or slightly greater than complete momentum transfer to the compound system. This is in opposition to that observed using heavier projectiles where incomplete momentum transfer is probable.NUCLEAR REACTIONS, FISSION $^{159}\mathrm{Tb}$, $^{165}\mathrm{Ho}$, $^{169}\mathrm{Tm}$, $^{\mathrm{nat}}\mathrm{Ta}$, $^{\mathrm{nat}}\mathrm{Re}$, $^{194,195}\mathrm{Pt}$, $^{197}\mathrm{Au}$, $^{202}\mathrm{Hg}$, $^{205}\mathrm{Tl}$, $^{206,208}\mathrm{Pb}$, $^{\mathrm{nat}}\mathrm{Bi}$, $^{\mathrm{nat}}\mathrm{Th}$($^{3}\mathrm{He}$,f), $E(^{3}\mathrm{He})=19.1 \mathrm{to} 44.5$ MeV; measured ${\ensuremath{\sigma}}_{F}(\ensuremath{\theta}, E)$, angular correlations, coincident fragment kinetic energies; deduced relative mass division, total kinetic energy release, statistical-model fission parameters.

The cross section at 0° for the reaction 14 C(p, n) 14 N has been measured at energies of 200, 300, 400 and 450 MeV using a nucleon charge-exchange facility recently commissioned at TRIUMF. Energy resolution of 1 MeV or better was obtained at each energy, permitting a reliable measurement of the cross section ratio for the Gamow-Teller transition to the 1 + state at 3.95 MeV and the Fermi transition to the 0 + state at 2.31 MeV. The deried values of ‖ J στ / J τ ‖ 2 are in clear disagreement with impulse approximation calculations using a free NN t -matrix.

Analyzing power and spectral data for the inclusive quasifree ( p ,p ′) reaction on 208Pb at 290 MeV are presented. Cross sections were measured over an angular range of 4°–26° and for excitation energies up to 160 MeV. The free surface response model provides a good description of the shape of the continuum. At the quasifree peak the large angle Ay(θ) data drop below the free nucleon values. This difference is a possible indication of relativistic medium effects.

The208Pb(n, p)208Tl reaction was studied at 198 and 458 MeV in a search for isovector spin giant resonances. Peaks at 5.1 MeV and 13.6 MeV excitation in208Tl are observed and discussed as candidates for the T> spin giant dipole resonance (SGDR), the spin isovector monopole resonance (SIVM), and the spin isovector quadrupole resonance (SIVQ).

A newly constructed focal-plane polarimeter for the medium resolution spectrometer at TRIUMF has been used to measure polarizations and polarization transfer coefficients for protons elastically scattered from 208Pb at 290 MeV. These results combined with existing cross section data at 300 MeV completely determine the elastic scattering amplitudes for momentum transfers q

Inclusive measurements of the linear momentum transfer distribution and exclusive studies of light-charged particles as a function of linear momentum transfer have been performed for the reaction of 150-MeV /sup 6/Li ions with /sup 238/U. Linear momentum transfer properties and an upper limit for the complete fusion cross section for this system were deduced from the folding angle between binary fission events and found to be consistent with data for heavier complex projectiles. The exclusive light-charged-particle data demonstrate the existence of fission following the transfer of all possible subdivisions of the /sup 6/Li projectile, as well as inelastic scattering. Absorptive breakup involving /sup 4/He-/sup 2/H cluster structure is found to be the dominant reaction mechanism. In addition, substantial yields of energetic protons associated with full linear momentum transfer are observed in the backward hemisphere. However, the temperature inferred from these spectra is nearly three times as large as that for a fully equilibrated compound nucleus, suggesting a nonequilibrium source for these particles.

Energy distributions of A y at backward angles are found to be identical for inclusive proton scattering from Be and C at 303 MeV. While the cross sections scale in the y -variable of West in the same way as electron and pion scattering, the A y values are on the average small and energy dependent, at variance with those expected for a single scattering mechanism.

A polarimeter which uses inclusive scattering from carbon has been mounted after the focal plane of the TRIUMF medium resolution spectrometer. Four drift chambers measure scatterings at all azimuthal angles out to 20° in polar angle over a spectrometer momentum acceptance of ± 5%. A microprocessor rejects events with small scattering angles. The efficiency for 400 MeV protons is 4%, and for 295 MeV deuterons is 1.5%. Analyzing powers for protons are similar to those previously reported for polarimeters of this type.

For the 24 Mg ( →p . →p ′) reaction at Ep=250 MeV and small momentum transfers we have measured cross sections, analysing powers and spin-transfer probabilities up to 40 MeV of excitation in 24Mg. Because of the strong suppression of the S=0 excitations in the spin-flip cross sections, the analogs of Gamow-Teller transitions could be cleanly observed. Up to Eexc=15 MeV about 80% of the L=0 strength predicted by large-scale shell model calculations has been found.

Differential cross section measurements have been made for 250 MeV proton inelastic scattering from /sup 28/Si. Hadronic deformation lengths for the states at 1.78 (2/sup +/), 4.62 (4/sup +/), 6.8 (3/sup -/ and 4/sup +/), 9.70 (5/sup -/), and 10.2 MeV (3/sup -/) have been extracted using a first-order vibration model distorted-wave Born approximation description. Deformation lengths for the 2/sup +/ and 4/sup +/ states are found to be independent of proton energy from 40 to 500 MeV. Cross sections for the excitation of the giant resonance region are found to be highly fragmented between 16 to 25 MeV. This region of excitation is found to contain an energy weighted sum rule depletion of 70% for the isovector giant dipole resonance, 26% for the isoscalar giant quadrupole resonance, and 5% for the isoscalar hexadecapole resonance. No structure was observed in the inelastic continuum region of 25 to 45 MeV of excitation energy.

Fission of /sup 238/U, /sup 209/Bi and /sup 197/Au induced by both positive and negative pion beams at incident energies of 60 to 100 MeV is examined. Energy and mass spectra and angular correlations of coincident fission fragments are compared to theoretical calculations and to other data. Excitation energies of the fissioning systems in excess of 100 MeV are deduced, implying that the fission examined in this study is caused primarily by true absorption of the pion. The masses of the coincident fission fragments are found to have a wider distribution than those from fissioning systems of similar excitation energy induced by nucleon beams. The fission cross sections are close to the reaction cross sections for /sup 238/U and scale with the fissility for the other nuclei studied.

A DWBA calculation for the 16 O ( p , n π + ) 16 O ( g.s. ) reaction is presented. The calculation is done using the Dirac equation for the proton and neutron distortions. The LLEE effect on the production vertex is considered. Observables are predicted to be measurable with the present generation of machines. Information on the NNπ vertex and reaction mechanism should be cleanly obtained.

The scattering of protons off individual nucleons in the nucleus, known as ‘quasi‐free’ scattering, gives information on the modification of the nuclear force inside the nuclear medium. In order to extract this information, we need to know the nucleon‐nucleon (N‐N) interaction in free space, and the propagation of the proton through the nucleus (from elastic scattering). Direct comparison of the analyzing powers and polarizations for quasi‐free scattering as opposed to N‐N scattering show large (40%) differences. Conventional (non‐relativistic) calculations are unable to reproduce this effect. Relativistic calculations, which use large, opposing vector and scalar potentials reproduce this effect for both isoscalar and isovector inclusive reactions and for exclusive (p, 2p) reactions.

New measurements of inelastic proton scattering to low-lying states of /sup 208/Pb at 200 and 400 MeV are reported. Deformation lengths extracted from angular distributions for the 3/sup -/ (2.614 MeV), 5/sub 1//sup -/ (3.198 MeV), 5/sub 2//sup -/ (3.209 MeV), 2/sup +/ (4.086 MeV) and 4/sup +/ (4.324 MeV) states are in good accord with values extracted at other incident proton energies. The fact that the deformation lengths are independent of incident proton energy within experimental uncertainty provides support for the validity of the collective DWBA for medium energy proton scattering to strongly excited states. Advantage is taken of this to extract statistically more precise values of the ratio of neutron to proton multipole matrix elements (M/sub n//M/sub p/). Different methods of determining the appropriate average value of M/sub n//M/sub p/ are discussed. 3 refs., 1 tab.