Physics with diproton final states at ANKE-COSY

Abstract

Abstract. The study of processes with production of proton pairs with small excitation energy (diprotons) provides a new approach to hadron interactions at intermediate energies. It is especially instructive due to comparison with processes where a deuteron is produced in the same kinematics. Provided that the momentum transfer is high enough the diprotons are found presumably in the 1 S 0 state, and this simplifies the theoretical analysis. The ANKE spectrometer provides the possibility to select the diprotons with sufficiently high excitation energy resolution. A number of processes including pd → {pp} s n, production of one and two pions in pN collisions, and the pp → {pp} s γ process, have been investigated. Experiments with diproton final states at ANKEFew nucleon systems have long been a subject of intensive theoretical and experimental study. A particularly important role in this study is played by processes with production of the deuteron, a bound state of proton and neutron. In this respect, a particular interest arises for the investigation of the diproton, an unbound system of two protons with small excitation energy. The diproton, being the isospin partner of the deuteron, possesses different quantum numbers. Selection of the excitation energy of the proton pair E pp < 3 MeV ensures the dominance of the 1 S 0 state of the diproton, which simplifies significantly the theoretical interpretation. The reactions with formation of a diproton involve transitions in the NN system, different from the case of the deuteron, and, in particular, the role of the ∆ isobar is expected to be much suppressed because the S-wave ∆N intermediate state is forbidden.Detection of the proton pair with E pp < 3 MeV in an experiment requires an excellent excitation energy resolution and the ability to resolve the two spatially close trajectories of the protons. The ANKE spectometer [1] provides a resolution of σ(E pp ) = 0.2 − 0.6 MeV in the E pp = 0 − 3 MeV range and is particularly well suited for this purpose.The diproton program at ANKE involves several experiments: It should be mentioned that investigation of single pion production and the charge-exchange reaction are the key double polarization experiments in the future physics program at ANKE. Deuteron break-up reactions High momentum transferThe process to be studied is pd → {pp} s n with a forward emitted diproton. The kinematics of the process is the same as in pd-backward scattering and the same mechanisms can be considered for its description. In contrast to pd → d p, the amplitude of excitation of ∆ in the intermediate state is expected to be suppressed by a factor of 3 due to selection of the diproton in an S -wave. Thus, the one nucleon exchange mechanism, which is sensitive to the short range part of the NN-interaction potential and to the high momentum part of the deuteron wave function, is expected to play a more important role.The differential cross-section of pd → {pp} s n with a forward emitted diproton has been measured in the range of T p = 0.5 − 2.0 GeV...

Near-threshold pion production in diproton reactions with polarised beams and target at ANKE-COSY

“…It is shown in [5] that the magnitude of one of the pwave amplitudes is then fixed completely by the measurement of (1 − A x,x ) · dσ/dΩ for np → {pp} s π − and that the magnitude of the other p-wave amplitude and its relative phase can be deduced from a combined analysis of this with our cross section and analysing power data for pp → {pp} s π 0 and np → {pp} s π − . These data will provide two determinations of the LEC d. Measurements of the mixed spin-correlation parameters A z,x are not required for the extraction of the p-wave amplitudes, though such information is vital in order to identify the d-wave terms.…”

“…[5], the experiment is essentially self-analysing. The values of A x,x and A y,y depend on the product of the beam and target polarisations.…”

“…For the np → {pp} s π − process, (1−A x,x )·dσ/dΩ ∼ |δ| 2 sin 2 ϑ π , where |δ| 2 is the square of one of the p-wave production amplitudes that is linked to the 4Nπ contact interaction [5]. The differential cross 01020-p.5 section and the preliminary spin-correlation data give (1 − A x,x ) · dσ/dΩ(90 • ) = (77 ± 25) nb/sr.…”

Measurement of the spin-correlation coefficientsAx,xandAy,yin the quasi-freenp→ {pp}sπ−reaction near threshold at ANKE-COSY

“…The synchrotron produced a vector polarized deuteron beam with kinetic energy 706 MeV which interacted with the internal polarized hydrogen cluster target. This target consisted of an Atomic Beam Source (ABS), a storage cell, and a Lamb-shift polarimeter [4]. The density of the polarized hydrogen gas in the cell was 1.34 × 10 13 cm −2 .…”

Near-threshold pion production in diproton reactions at ANKE

“…It is shown in [13] that the magnitude of one of the p-wave amplitudes is then fixed completely by the measurement of (1 − A x,x )dσ/dΩ for np → {pp} s π − and that the magnitude of the other p-wave amplitude and its relative phase can be deduced from a combined analysis of this with our already taken cross section and analysing power data for pp → {pp} s π 0 and np → {pp} s π − . These data will provide two determinations of the LEC d. Measurements of the mixed spin-correlation parameters A x,z are not required for the extraction of the p-wave amplitudes, though such information is vital in order to identify the d-wave terms.…”