Search

The pentaquark is proposed to be made from four quarks and one antiquark, which is an allowed combination by the theory of quantum chromodynamics. However, there was no convincing experimental evidence before 2002, despite many searches in particle physics experiments. In 2002, an experimental group in Japan presented results showing tentative evidence for the a pentaquark called the Θ +, consisting of four light quarks and a strange antiquark. In the next few years, up to a dozen experimental groups published supporting evidence for the Θ +, followed by a number of experiments with no evidence (and where the Θ + was expected to be seen). In later years, most of the experiments supporting the Θ + were repeated, but could not reproduce the evidence with better statistics. Today, there is little belief that the Θ + is real, and it remains a mystery how so many experiments could have claimed statistically-significant evidence for the pentaquark.

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 theoretical motivation to look for S=+1 resonances is introduced and searches for such states in the literature are reviewed. The motivation for the recent pentaquark searches is discussed and the experimental details are described for several positive results of the S=+1 resonance with a mass of approximately 1.54 GeV, now called the Θ+. Finally, some new theoretical proposals to explain the recent experimental results are briefly mentioned, followed by a short discussion of future experimental prospects to learn more about pentaquark states.

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.

We describe the upgraded time-of-flight facility at the Indiana University Cyclotron which has been used to measure the polarization transfer induced in ( p → , n → ) reactions at intermediate energies, Ep=100–200 MeV. The Indiana Neutron POLarimeter (INPOL) is able to measure simultaneously two orthogonal neutron spin polarizations induced in charge exchange reactions with polarized proton beams oriented in each of the three spin states, normal ( N → ), sideways ( S → ) and longitudinal ( L → ).

The CEBAF Large Acceptance Spectrometer at 12 GeV (CLAS12) is located in Hall B, one of the experimental halls at Jefferson Lab. The forward part of CLAS12 is built around a superconducting toroidal magnet. The six coils of the toroid divide the detector azimuthally into six sectors. Each sector contains three multi-layer drift chambers for reconstructing the trajectories of charged particles originating from a fixed target. Each of the 18 planar chambers has two “superlayers” of six layers each, with the wires in the two adjacent superlayers oriented at ± 6 ° stereo angles. Each layer has 112 hexagonal cells spanning a range from about 5 ° to 40 ° in polar angle. The six-layer structure provides redundancy in track segment finding and good tracking efficiency even in the presence of some individual wire inefficiency. The design, construction, operation, and calibration methods are described, and estimates of the efficiency and resolution are presented from in-beam measurements.

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 have developed an apparatus that can polarize muonic 3He and detect the triton from the reaction μ− + 3He → ν + 3H. With this apparatus, we have measured the vector analyzing power of the reaction. This technique promises to provide a good test of QCD.

We have made simultaneous measurements of singles ($\stackrel{\ensuremath{\rightarrow}}{p}, {\stackrel{\ensuremath{\rightarrow}}{p}}^{\ensuremath{'}}$) spin-transfer observables and coincident ($\stackrel{\ensuremath{\rightarrow}}{p}, {p}^{\ensuremath{'}}\ensuremath{\gamma}$) yields and asymmetries for the 15.11 MeV, ${1}^{+}$, $T=1$ state in $^{12}\mathrm{C}$, at an incident proton bombarding energy of 200 MeV. Data were taken at four proton scattering angles ranging from ${\ensuremath{\theta}}_{p}^{\mathrm{c}.\mathrm{m}.}=5.5\ifmmode^\circ\else\textdegree\fi{} \mathrm{to} 16.5\ifmmode^\circ\else\textdegree\fi{}$. Both vertical (normal to the reaction plane) and horizontal (in-plane) incident beam polarizations were used, which allowed us to extract 16 different observables for this transition. In particular, using the ($\stackrel{\ensuremath{\rightarrow}}{p}, {p}^{\ensuremath{'}}\ensuremath{\gamma}$) reaction, we measured the sideways and longitudinal analyzing powers, ${D}_{0S}$ and ${D}_{0L}$, which vanish identically in the ($\stackrel{\ensuremath{\rightarrow}}{p}, {p}^{\ensuremath{'}}$) reaction. Detailed comparisons of all observables to calculations done in both relativistic and nonrelativistic formalisms are presented. Surprisingly, a relativistic description in which knock-on exchange processes are not explicitly included provides the best overall agreement with the data.

Differential cross sections were measured and compared to distorted-wave impulse-approximation calculations for the ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${4}^{\mathrm{\ensuremath{-}}}$(\ensuremath{\nu}${\mathit{d}}_{5/2}$,\ensuremath{\pi}${\mathit{p}}_{3/2}^{\mathrm{\ensuremath{-}}1}$) state excited in the $^{12}\mathrm{C}$(n,p${)}^{12}$B (4.52 MeV) reaction, the ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${6}^{\mathrm{\ensuremath{-}}}$(\ensuremath{\nu},${\mathit{f}}_{7/2}$\ensuremath{\pi}${\mathit{d}}_{5/2}^{\mathrm{\ensuremath{-}}1}$) state excited in the $^{28}\mathrm{Si}$(n,p${)}^{28}$Al (5.175 MeV) reaction, and the ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${10}^{\mathrm{\ensuremath{-}}}$(\ensuremath{\nu}${\mathit{h}}_{11/2}$,\ensuremath{\pi}${\mathit{g}}_{9/2}^{\mathrm{\ensuremath{-}}1}$) state excited in the $^{120}\mathrm{Sn}$(n,p${)}^{120}$In (\ensuremath{\simeq}0.0 MeV) reaction at 300 MeV. We obtained a normalization factor of 0.68 for the ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${10}^{\mathrm{\ensuremath{-}}}$(\ensuremath{\nu}${\mathit{h}}_{11/2}$,\ensuremath{\pi}${\mathit{g}}_{9/2}^{\mathrm{\ensuremath{-}}1}$) state excited in $^{120}\mathrm{Sn}$(n,p${)}^{120}$In (\ensuremath{\simeq}0.0 MeV) reaction; this is the first ``1\ensuremath{\Elzxh}\ensuremath{\omega}'' stretched state known in this mass region.

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.

The {sup 70,72}Ge({ital n},{ital p}){sup 70,72}Ga reactions have been measured at a beam energy of 200 MeV in order to study the behavior of Gamow-Teller (GT) strength near the {ital N}=40 subshell closure. The GT quenching factor extracted for {sup 70}Ge is consistent with results for {sup 54}Fe and with calculations done on the quasiparticle random-phase-approximation model if they are renormalized by a factor of about 0.6. The quasiparticle random-phase approximation predicts a drastic reduction in concentrated GT strength from {sup 70}Ge to {sup 72}Ge which is also seen in the data. These results are relevant to models of supernovae.

A gamma-gamma coincidence measurement was performed in order to search for a neutral scalar particle associated with beta decay with a mass near 17 keV. The measurement used a 35 S source and two LEPS-type Ge detectors. A coincidence signal above the background generated by random bremsstrahlung coincidences would be a signature of the two-photon decay of a light neutral scalar particle. No evidence was found for such a particle with a mass near 17 keV with a lifetime shorter than 1 nanosecond.

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.

Cross sections for the 48 Ti(n, p) reaction have been measured at angles of 0°, 6°, and 12° at an energy of 200 MeV. The measurements are compared with results of DWIA calculations to obtain estimates of transition strengths for L = 0, 1, and ⩾2 up to an excitation energy of 25 MeV. Gamow-Teller strength ( L = 0) is peaked between 3 and 4 MeV excitation energy, with a significant distribution extending to about 12 MeV. The L = 1 strength is found mainly between 6 and 20 MeV while the cross section for transitions with L ⩾2 increases from 10 MeV to the upper limit of the measurements. The distribution of Gamow-Teller strength is in poor agreement with theoretical distributions used to calculate the lifetime for double beta decay of 48 Ca.

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.

The complete set of Wolfenstein parameters, the polarization, the asymmetry of scattering and the unpolarized double-differential cross section are presented for inclusive quasielastic proton scattering from 12C at a central momentum transfer of q = 1.9 fm−1 and incident energies of 290 and 420 MeV. The spin observables D0, Dx, Dy and Dz as well as the longitudinal-to-transverse ratio of spin-flip probabilities are extracted from the data. Across the quasielastic continuum, the experimental data is compared to the variations expected from a single-scattering Fermi-gas approximation using the free NN amplitudes. Medium effects are evident in the pronounced quenching of the polarization parameter relative to the free value.

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 ratios of inclusive electron scattering cross sections of {sup 4}He, {sup 12}C, and {sup 56}Fe to {sup 3}He have been measured at 1 1.4 GeV{sup 2}, the ratios exhibit two separate plateaus, at 1.5 2.25. This pattern is predicted by models that include 2- and 3-nucleon short-range correlations (SRC). Relative to A=3, the per-nucleon probabilities of 3-nucleon SRC are 2.3, 3.1, and 4.4 times larger for A=4, 12, and 56. This is the first measurement of 3-nucleon SRC probabilities in nuclei.

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 summary of experimental results from the Pentaquark 2004 Workshop held at the SPring-8 facility in Japan is given. New results from the LEPS collaboration are highlighted, and older results are reviewed. Non-observations are also discussed in light of theoretical estimates of possible $\Theta^+$ production mechanisms. The problem of the narrow width and the parity of the $\Theta^+$ are explored and point to future experimental work that is needed., Comment: 8 pages, no figures, proceedings of the Pentaquark 2004 Workshop held at SPring-8 (Japan), July 2004

The experimental evidence for pentaquarks, both old an new, is discussed. Constraints due to $K^+N$ scattering data from previous decades is first reviewed, followed by experiments with positive evidence and those with null results. Finally, the problem of the narrow width of the Theta+ pentaquark is discussed, along with theoretical implications., Comment: Review article for Progress in Particle and Nuclear Physics (34 pages)

A critical look is taken at both positive and null evidence for the $\Theta^+$ pentaquark. Potential problems with experiments will be discussed and the question of what conclusion can be drawn from both the positive and the null results is examined. First the question of existence of the $\Theta^+$ pentaquark is considered, followed by a discussion of new experiments that are either planned or in progress to answer questions about its mass, width and isospin. Finally, indirect evidence for the parity of the $\Theta^+$ is examined, and suggestions for experiments to measure its parity directly are given., Comment: MESON2004 conference proceedings, 10 pages, 1 figure

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.

The NuMI neutrino beam line and the MINOS experiment represent a major investment of US High Energy Physics in the area of neutrino physics. The forthcoming results could decisively establish neutrino oscillations as the underlying physics mechanism for the atmospheric $\numu$ deficit and provide a precise measurement of the corresponding oscillation parameters, $\dmsq23$ and $\sinsq2t23$.neutrino sector may well be within our reach. The full potential of the NuMI neutrino beam can be exploited by complementing the MINOS detector, under construction, with a new detector(s) placed at some off-axis position and collecting data in parallel with MINOS. The first phase of the proposed program includes a new detector, optimized for $\nue$ detection, with a fiducial mass of the order of 20 kton and exposed to neutrino and antineutrino beams. In a five year run its sensitivity to the $\numutonue$ oscillations will be at least a factor of ten beyond the current limit. The future direction of the program will depend on the results of this first phase, but it is very likely that it will be a combination of a significant increase of the neutrino beam intensity via an upgraded proton source and an increase of the detector mass by a factor of five or so. Depending on the circumstances, the goals of Phase II may be a further increase of the sensitivity of a search for $\numutonue$ oscillations, or, perhaps, a measurement of the CP violating phase $\delta$ in the lepton sector.

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.

We report the first results of the beam spin asymmetry measured in the reaction e + p -> e + p + gamma at a beam energy of 4.25 GeV. A large asymmetry with a sin(phi) modulation is observed, as predicted for the interference term of Deeply Virtual Compton Scattering and the Bethe-Heitler process. The amplitude of this modulation is alpha = 0.202 +/- 0.028. In leading-order and leading-twist pQCD, the alpha is directly proportional to the imaginary part of the DVCS amplitude.

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.

Differential cross sections and analyzing powers for the /sup 7/Li(p-arrow-right,..pi../sup +- /) reactions leading to discrete states of /sup 8/B and /sup 8/Li, as well as to the continuum, are presented at T/sub p/ = 280, 354, and 489 MeV. These results are compared with previously published results at 200 and 250 MeV. At fixed four-momentum transfers squared in the region 0.6>t>0.4 GeV/sup 2//c/sup 2/, the energy dependence of the (p,..pi../sup +/) and (p,..pi../sup -/) differential cross sections is quite similar, in sharp contrast to earlier results from the /sup 13/C(p,..pi../sup +- /) reactions. The observed variations in the (p,..pi../sup -/) energy dependence are presumed to reflect differences in the relative importance of resonant and nonresonant amplitudes in the (p,..pi../sup -/) reaction mechanism. The analyzing powers of the (p-arrow-right,..pi../sup -/) and (p-arrow-right,..pi../sup +/) reactions are qualitatively very similar except for a difference in sign.

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).

Rotational sidebands in 166 Er were observed using the 24 MeV 164 Dy(α, 2nγ) reactions. The ground-state band was observed up to spin 16 + and does not backbend. A strong backbend is, however, observed in a K π = ( O + ) sideband, indicating that the 12 + state of the previously unknown S-band is at 2656 keV. The γ-band shows significant rotational alignment above I = 10 + . Levels of at least two negative-parity bands, one of which is primarily the K π = 2 − octupole vibration, are also observed.

Differences of the polarization ( P ) and analyzing power ( A y ) have been measured between 5° and 14° for the lowest isoscalar 1 + and isovector 1 + and 2 + states in 12 C excited by 400 MeV protons. This is the first measurement of P - A y in an energy regime where the impulse approximation is most valid. Values of P - A y for all states are compared to both nonrelativistic and relativistic model calculations, which only qualitatively describe the data.

Inelastic scattering to the low-lying collective states in 40Ca excited by 500 MeV protons is examined using both relativistic and nonrelativistic microscopic optical model potentials in the Tassie model. The non-standard shapes produced by these microscopic models does not produce different deformation lengths than those obtained using standard Woods-Saxon shapes. For comparison, calculations using the distorted wave impulse approximation are presented.

The half-life of 197g Pb was determined to be 6.2 ± 1.2 min by measuring the γ-decay ingrowth of 197g Pb from sources of 197m Pb (42 min). The sources were produced through the ( 6, 7 Li, x n) reaction on 197 Au using 69–95 MeV beams. Several ambiguities in the decây schemes were resolved based on these γ-decay data.

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

Rotational side-bands in 162Dy have been studied using the 160Gd(α, 2nγ)162Dy reaction. Seven side-bands are observed, with Kπ = 2+, 2−, (0)−, 0+, 5−, 4+ and (6−). Four of these bands have collective structure at low spin: the Kπ = 2+ γ-vibrational band, the Kπ = 0+ β-vibrational band, and the Kπ = 2− and (0)− octupole vibrational bands. Of the remaining bands, the 4+ band is deformation coupled while the 5− and (6−) bands are rotation-aligned. Several bandcrossings are observed in this nucleus. The β and γ-bands are crossed at I = 6 h and 12 h , respectively, by a highly aligned ( i 13 2 )2 S-band; extrapolation of this S-band to higher spin suggests that it crosses the g.s.b. between I = 18 h and 20 h . The 2− octupole band is crossed by the 5− band at I = 9 h and again by the (6−) band at I = 12 h . The latter bandcrossings are discussed in terms of two-quasiparticle plus rotor calculations.