Measurement of thep→p→dπ+Cross Section and Analyzing

Abstract

Differential cross sections and analyzing powers for the pp ! dp 1 reaction have been measured to within T cm p 85 keV of threshold. The S-wave strength is extracted and found to be ϳ13% larger than that determined from recent measurements of np ! dp 0 . Both the analyzing power and the anisotropy in the angular distribution show the expected energy dependence and these data are used to obtain values for the two contributing P-wave amplitudes. [S0031-9007(96)00564-9] PACS numbers: 13.75. Cs, 24.70. + s, 25.40.Ve Threshold pion production has received renewed attention in recent years both because technological developments have made high resolution experiments possible and because of the realization of the pivotal role played by this fundamental process. These threshold reactions provide direct information on the nuclear axial charge and indirect information regarding the pNN form factor and certain weak nuclear transitions [1-3]. Additionally, precision measurements of the pp ! dp 1 cross section and spin observables could potentially distinguish between competing production mechanisms such as mesonexchange currents (MEC) and off-shell rescattering.Considering that the NN ! ͑NN͒p reaction has been extensively studied for 50 years, it may seem surprising that there could be anything new to learn about it. However, the vast majority of the work is concentrated at energies where contributions from resonant production mechanisms dominate. Only a handful of experiments examine the threshold region where it is expected that nonresonant mechanisms are important [4][5][6][7][8].We present in this Letter a measurement of the cross section and analyzing power for the pp ! dp 1 reaction at nine energies between 287.7 and 294.1 MeV (E thresh 287.511). The lowest energy corresponds to T cm p ϳ 85 keV; the lowest energy at which analyzing powers were previously measured is more than 1 MeV above threshold [6].The measurements were carried out at the electroncooled proton storage ring (the Cooler Ring) at the Indiana University Cyclotron Facility (IUCF) using an internal H 2 gas jet target. The proton beam was 70% polarized and had an energy spread of approximately 60 keV FWHM.Sufficiently close to threshold, the pions, which are directly detected, are confined to a narrow cone about the beam direction with an opening angle given by sinu max h͞b cm g cm , where b cm and g cm are the boost parameters of the center-of-mass system and h p cm p ͞m p c. This energy dependence of the pion is used to determine the absolute beam energy to within 620 keV. To our knowledge, no other investigation of threshold pion production has obtained this level of energy precision which becomes essential at the lower end of the covered energy range. The detector (Fig. 1) contained left-right symmetric plastic scintillators which measured DE and E of the forward-going particles. Particles not stopping in the DE and E counters (E1 and E2, respectively) were vetoed by a double layer of plastic scintillators. Pions with energies less than 24 MeV and p...

Study of the reaction pp $ \to$ pp $\pi^0$ within 10 MeV above the threshold

“…We systematically found larger cross sections than given in [1], [3]. Since the results we deduced from the data for the concurring charged π-producing reactions agreed nicely with the published values [7], [10], [11], whereas the cross sections for π 0 -production remained high, we decided to perform a second experiment to clarify that matter.…”

“…For comparison, angular distributions as well as analyzing powers extracted from a study of the π + -producing reactions show clear evidence for the presence of Sp, and hence l = 1 contributions at energies as low as 2 MeV above threshold [10][11].…”

“…With the advent of medium-energy accelerators in Bloomington/USA (IUCF), Uppsala/Sweden (CELSIUS), and Jülich/Germany (COSY) a wealth of high precision data on total and differential cross sections as well as polarization observables was measured for the reactions pp → ppπ 0 [1][2][3][4][5][6], pp → pnπ + [7][8][9] and pp → dπ + [10][11], which in turn induced a flurry of theoretical activity [12][13][14][15][16][17][18]. So far, good agreement is found in the theoretical description of the reactions pp → dπ + and pp → pnπ + , larger discrepancies are observed for the pp → ppπ 0 reaction.…”

Pion reactions with few-nucleon systems

“…Pion reactions with few-nucleon systems [17][18][19][20][21]. Note that the green diamond and the blue triangle correspond to the most resent measurements from pionic deuterium atom [20,21] at η = 0.…”

Spin correlation coefficients in from 325 to 400 MeV

“…In recent studies it was found that within 10-20 MeV of threshold, i.e. below 300 MeV, only Ss for pp → ppπ 0 , [1][2][3] or at most two partial waves (Ss and Sp) for pp → dπ + and pp → pnπ + [4][5][6][7][8][9][10] were important, and direct comparisons with simple theoretical approaches can be made. The expected dominance of Ss contributions (l N N = 0, L π = 0) for the final states was seen, but at the same time it was found that for pp → ppπ 0 traditional models under-predicted the cross sections by factors of 3-5.…”