Publications (87)

Article: π+–p elastic scattering at 47.9 MeV
[Show abstract] [Hide abstract] ABSTRACT: The differential cross section for π+–p elastic scattering at 47.9 MeV has been measured in the angular range from 30 to 170° in the laboratory system. The three phase shifts extracted from these data, δS3 = −4.7 ± 0.5°, δP31 = −0.53 ± 0.14°, and δP33 = 5.1 ± 0.3°, are consistent with phase shifts obtained at other energies.  [Show abstract] [Hide abstract] ABSTRACT: The direct annihilation rate of positrons in He, A, and Kr has been studied. Deviation from an exponential for the initial portion of the time distribution of these events is observed and is interpreted as a velocity dependence of the direct annihilation rate. The product of the gas pressure and the width of the nonexponential portion of the time distribution has a value of 270 nsec atm and is approximately independent of the pressure. An estimate of the fraction of positrons forming positronium in argon yields a value of 0.37 ± 0.03. Using this result, a check of the statistical ratio of ortho to parapositronium formation indicates that 69 ± 7% of the positronium is formed in the ortho state.
 [Show abstract] [Hide abstract] ABSTRACT: We present a measurement of the spindependent cross sections for the (3)(He) over right arrow((e) over right arrow ,e')X reaction in the quasielastic and resonance regions at a fourmomentum transfer 0.1 <= Q(2)<= 0.9 GeV2. The spinstructure functions have been extracted and used to evaluate the nuclear BurkhardtCottingham and extended GerasimovDrellHearn sum rules for the first time. The data are also compared to an impulse approximation calculation and an exact threebody Faddeev calculation in the quasielastic region.
 [Show abstract] [Hide abstract] ABSTRACT: The generalized forward spin polarizabilities gamma(0) and delta(LT) of the neutron have been extracted for the first time in a Q2 range from 0.1 to 0.9 GeV2. Since gamma(0) is sensitive to nucleon resonances and delta(LT) is insensitive to the Delta resonance, it is expected that the pair of forward spin polarizabilities should provide benchmark tests of the current understanding of the chiral dynamics of QCD. The new results on delta(LT) show significant disagreement with chiral perturbation theory calculations, while the data for gamma(0) at low Q2 are in good agreement with a nexttoleadingorder relativistic baryon chiral perturbation theory calculation. The data show good agreement with the phenomenological MAID model.
 [Show abstract] [Hide abstract] ABSTRACT: We have measured the spin structure functions g(1) and g(2) of 3He in a doublespin experiment by inclusively scattering polarized electrons at energies ranging from 0.862 to 5.058 GeV off a polarized 3He target at a 15.5 degrees scattering angle. Excitation energies covered the resonance and the onset of the deep inelastic regions. We have determined for the first time the Q2 evolution of Gamma(1)(Q2)= integral (1)(0)g(1)(x,Q2)dx, Gamma(2)(Q2)= integral (1)(0)g(2)(x,Q2)dx, and d(2)(Q2)= integral (1)(0)x(2)[2g(1)(x,Q2)+3g(2)(x,Q2)]dx for the neutron in the range 0.1< or =Q2< or =0.9 GeV2 with good precision. Gamma(1)(Q2) displays a smooth variation from high to low Q2. The BurkhardtCottingham sum rule holds within uncertainties and d(2) is nonzero over the measured range.
 [Show abstract] [Hide abstract] ABSTRACT: We have measured the spin structure functions g_1 and g_2 of ^3He in a doublespin experiment by inclusively scattering polarized electrons at energies ranging from 0.862 to 5.058 GeV off a polarized ^3He target at a 15.5° scattering angle. Excitation energies covered the resonance and the onset of the deep inelastic regions. We have determined for the first time the Q^2 evolution of Γ_1(Q^2)=∫_0^1g_1(x,Q^2)dx, Γ_2(Q^2)=∫_0^1g_2(x,Q^2)dx, and d_2(Q^2)=∫_0^1x^2[2g_1(x,Q^2)+3g_2(x,Q^2)]dx for the neutron in the range 0.1 ≤ Q^2 ≤0.9 GeV^2 with good precision. Γ_1(Q^2) displays a smooth variation from high to low Q^2. The BurkhardtCottingham sum rule holds within uncertainties and d_2 is nonzero over the measured range.

Article: The ( π +,pd) and ( π +,dd) reactions on light nuclei at 100 and 165 MeV incident pion energies
[Show abstract] [Hide abstract] ABSTRACT: The (pi(+), pd), and (pi(+), dd) reactions on Li6 and C12 targets were investigated with pions of 100 and 165 MeV kinetic energy. The experimental apparatus employed for this work is characterized by good energy resolution (< 3%, DeltaE/E) and angle definition (Deltatheta = 1.6degrees), and the measurements span a large fraction of the finalstate particle phase space. Results are compared to extensive Monte Carlo simulations in order to investigate the role of singlestep and multistep absorption processes in these nuclei. In comparison with previously published (pi+, pp) data on the same targets and at the same beam energies, specific kinematic regions were identified in which the neutron hard pickup process n + p > d dominated the observed deuteron yield. The contribution of direct quasitriton absorption is significant only at 100 MeV. 
Article: Precision PionProton Elastic Differential Cross Sections at Energies Spanning the Delta Resonance
[Show abstract] [Hide abstract] ABSTRACT: A precision measurement of absolute pi+p and pip elastic differential cross sections at incident pion laboratory kinetic energies from T_pi= 141.15 to 267.3 MeV is described. Data were obtained detecting the scattered pion and recoil proton in coincidence at 12 laboratory pion angles from 55 to 155 degrees for pi+p, and six angles from 60 to 155 degrees for pip. Single arm measurements were also obtained for pi+p energies up to 218.1 MeV, with the scattered pi+ detected at six angles from 20 to 70 degrees. A flatwalled, supercooled liquid hydrogen target as well as solid CH2 targets were used. The data are characterized by small uncertainties, ~12% statistical and ~11.5% normalization. The reliability of the cross section results was ensured by carrying out the measurements under a variety of experimental conditions to identify and quantify the sources of instrumental uncertainty. Our lowest and highest energy data are consistent with overlapping results from TRIUMF and LAMPF. In general, the Virginia Polytechnic Institute SM95 partial wave analysis solution describes our data well, but the older KarlsruheHelsinki PWA solution KH80 does not. Comment: 39 pages, 22 figures (some with quality reduced to satisfy ArXiv requirements. Contact M.M. Pavan for originals). Submitted to Physical Review C 
 [Show abstract] [Hide abstract] ABSTRACT: The (pi+,pd), and (pi+,dd) reactions were investigated with pions of 100 and 165 MeV kinetic energy on 6Li and 12C targets. In comparison with previously published (pi+,pp) data on the same targets and at the same beam energies, kinematic regions were identified in which the neutron pickup process n+p>d dominated the observed deuteron yield. The importance of this mechanism increases with energy, contributing half of the observed cross section at 165 MeV. The contribution of direct quasitriton absorption is significant only at 100 MeV. Comment: 23 pages, 12 figures
 [Show abstract] [Hide abstract] ABSTRACT: The total cross section for the πp>ππ+n reaction has been measured at incident pion kinetic energies of 200, 190, 184, 180, and 176 MeV, and cross sections for the π+p>π+π+n reaction were obtained at 200 and 184 MeV. A fit of these cross sections by heavy baryon chiral perturbation theory yields values of 8.5+/0.6(m3π) and 2.5+/0.1(m3π) for the reaction matrix elements A10 and A32, and, for the swave isospin0 and isospin2 ππ scattering lengths, the values a0=0.23+/0.08(m1π) and a2=0.031+/0.008(m1π), respectively.
 [Show abstract] [Hide abstract] ABSTRACT: Analyzing powers for (p) over right arrow n > pp(S1(0))pi() were measured at beam energies 353, 404, and 440 MeV by extracting the quasifree process from (p) over right arrow d > ppp pi(). Partial wave amplitude analysis yields a significant contribution from the isospin 1, swave channel. This contribution is relatively much larger than that expected from theoretical models which have been successful in describing the isospin 1, swave channel behavior of pp > pp pi(0) cross sections at threshold.
 [Show abstract] [Hide abstract] ABSTRACT: A new Q–QClamshell magnetic spectrometer called SASP has been commissioned in the proton hall at TRIUMF. The principal optical characteristics are: a solid angle acceptance of 13.5 msr, a momentum bite of 25%, a momentum resolution of Δp/p=2×10−4, and a maximum central momentum of 720 MeV/c. We describe the optical design, the magnetostatic design, the magnetic field mapping procedure, and the performance of this instrument.
 [Show abstract] [Hide abstract] ABSTRACT: The double differential cross section for pn→pp(1S0)π at three beam energies has been extracted from the quasifree process pd→pppπ. A comparison is carried out with single differential cross section measurements for 3He(π,pn)n, where the pion is thought to be absorbed onto a pp(1S0) “diproton” state. A significant difference is observed in the shape of the angular distribution between the production and absorption data. This difference is ascribed to the effects of the 3He nuclear environment characterizing the absorption process; however, an adequate theoretical explanation is not available.
 [Show abstract] [Hide abstract] ABSTRACT: The total cross section for the pi() p > pi()pi(+) n reaction has been measured at incident pion kinetic energies of 200, 190, 184, and 180 MeV. In addition, the pi(+) p > pi(+) pi(+) n reaction was measured at 200 and 184 MeV. A fit of the cross sections by heavy baryon chiral perturbation theory yields values of 8.5 +/ 0.6(m(pi)(3)) and 2.5 +/ 0.1(m(pi)(3)) for the reaction matrix elements A(10) and A(32), which correspond to values for the swave isospin0 and isospin2 pipi scattering lengths of a(0) = 0.23 +/ 0.08(m(pi)(1)) and a(2) = 0.031 +/ 0.008(m(pi)(1)), respectively.

Article: Multinucleon contributions to the 12C(π+, pp) reaction at 100 and 165 MeV incident pion energies
[Show abstract] [Hide abstract] ABSTRACT: Differential and total cross sections for π+ absorption on 12C are presented at incident pion energies of 100 and 165 MeV. This is the first time that this reaction has been measured with a combination of good energy and angle information over an extensive region of phase space. Results are compared to extensive Monte Carlo simulations in order to investigate the role of two and multinucleon absorption processes in this nucleus. Signatures of ISI and FSI in the pion absorption process are also investigated, in order to explain the complex reaction mechanism implied by the data. 
Article: Search for the Δ  Δ Dibaryon
[Show abstract] [Hide abstract] ABSTRACT: Models of hadron structure based on quark confinement at large distances and colormagnetic spinspin interactions within the confinement region, predict the existence of a dibaryon, d*, with isospin 0, J^π=3^+. The Quark Delocalization Color Screening Model predicts a mass in the range 2073  2143 MeV. This dibaryon may be regarded asymptotically as a deeply bound system of two Δ's, and is expected to be a stronger dibaryon candidate than the famous H particle. Our collaboration will search for the d* by studying the reaction p darrowp d*, d*arrowppπ^, in the DoubleArm Spectrometer System at TRIUMF. The d* would manifest itself as a narrow spike in the histogram of scattered proton momenta. The theoretical motivation and details of the experiment will be discussed in the oral presentation. 
Article: Search for deeply bound pionic states in^{208} Pb via radiative atomic capture of negative pions
[Show abstract] [Hide abstract] ABSTRACT: A search for narrow, deeply bound pionic atom states via atomic radiative capture of negative pions in a target of 208Pb was carried out for pion kinetic energies of 20 and 25 MeV. Although no clear signature of any such gamma ray emission could be observed in the data, fits of the gamma ray spectra between the energies of 12 and 42 MeV involving a quadratic background together with a pair of peaks (1s, 2p) whose relative intensity was taken from theory yielded an overall strength for the peaks which are consistent (to a 67% confidence level) with radiative capture whose integrated cross section is 20.0 ± 10.0 μb/sr at 90° for 20 MeV incident pions. A lower probability (40% confidence level) result was obtained when the fit was carried out without the peaks included, just the continuum background.  [Show abstract] [Hide abstract] ABSTRACT: The first spintransfer experiment performed for the πd→→p→p reaction is described. Three spintransfer parameters for this πabsorption process were determined, KLSa, KSSa, and KNNa, which correspond to the πproduction parameters, KSLp, KSSp, and KNNp, of the timereversed p→p→d→π process. Each observable was measured at a single angle for a number of energies spanning the Δ resonance of this system. The results are compared with the predictions of published partial wave amplitude fits which are primarily based on existing data for the timereversed pp→dπ reaction, and also with the predictions of two current theories. The failure of these theories to describe the fundamental features of the data clearly demonstrates the need for further theoretical work in this area.

Article: Multinucleon Contributions to the ^natC(pi^+,pp) Reaction at 100 and 165 MeV Incident Pion Energies
[Show abstract] [Hide abstract] ABSTRACT: Differential and total cross sections for pi^+ absorption on ^natC are presented at incident pion energies of 100 and 165 MeV. This is the first time that this reaction has been measured both with good energy and angle information. Results are compared to extensive Monte Carlo simulations in order to investigate the role of two and multinucleon absorption processes in this nucleus. Signatures of ISI and FSI in the pion absorption process are also investigated in order to explain the complex reaction mechanism implied by the data.
Publication Stats
930  Citations  
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Institutions

19682002

University of British Columbia  Vancouver
 Department of Physics and Astronomy
Vancouver, British Columbia, Canada


19792001

TRIUMF
Vancouver, British Columbia, Canada


1991

Università degli Studi di Trieste
 Department of Physics
Trst, Friuli Venezia Giulia, Italy
