Hard Quark-quark Scattering with Exclusive Reactions

We present data from ir~p •*• elastic and p~p final states for scattering at 90° center of mass, -t = 9 GeV/c. A large p~p signal is seen and the p" are strongly polarized. This polarization tests a QCD prediction that quarks cannot flip helicity. The test fails dramatically. Exclusive two-body to two-body scattering at large momentum transfer represents a new laboratory for the study of hard scattering processes. In general, several types of quark diagrams may contribute, as shown in Fig. 1 for meson-baryon scattering. Elastic scattering may proceed via any or all of the graphs, as can it"p + p~p. A reaction such as ir~p + K°A cannot occur via pure gluon exchange or quark interchange. There are a large number of two-body exclusive reactions experimentally accessible with n± and K± meson beams, and each is sensitive to different mixtures of the graphs shown in Fig. 1. If the quark graphs are flavor-independent, as expected for hard scattering where the asymptotic quark masses are small on the scale of the momentum transferred in the interaction, the amplitudes for each of the two-body exclusive reactions can be written in terms of the same quark scattering amplitudes, with corresponding relationships expected between the reaction cross sections. In addition, for many possible two-body exclusive reactions polarization may be measured for a final state particle through its decay, and this can further constrain the quark amplitudes. For example, we report here on the reaction n~p~ * p~p where the angular distribution of the ir" from the p~ + irir° decay analyzes the helicity state of the p. If the pure gluon exchange graph (Fig.la) were to dominate this reaction, helicity conservation at the quark level, a prediction of quantum chromodynamics, would require that the p" helicity be the same as that of the incident ir", or zero. Helicity-flip amplitudes are expected to be suppressed by a factor m_/^ s ~ 10 for our case where / s ~ 2 GeV and we assume the asympotitically free quark mass of about 5 MeV. The other graphs, quark annihilation and interchange, can "give a p~ with helicity ±1. The momentum transfer above which one can successfully apply perturbative QCD is debatable. However, many experimental phenomena indicate that an asymptotic region sets in for pT > 1.5 GeV/c or t > 5 GeV /c. Examples are the Q dependence of the proton form factor (constant for Q > 5 ) , 3 that fixed angle elastic scattering follows dimensional counting predictions for -t > 5 GeV/c,* and that elastic cross *Work performed under the auspices of the U.S. Department of Energy and the National Science Foundation. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985211 C2-116 JOURNAL DE PHYSIQUE sections develop a flat central region at this value of momentum tran~fer.~ For this experiment, -t = 9 &v2/c2. Exclusive cross sections may be calculable with perturbative QCD, but the calculation requires knowledge of the wave functions and each quark must be accounted for. Farrar has developed a computer code to calculate cross sections for such rea~tions.~ In addition, there are other theoretical models for exclusive scattering. (a) Pure gluon exchange: n-p + n-p, p-p. (b) Quark interchange. (c) Annihilation: n-p + AK. (d) Annihilation and interchange: n-p + &-, n+~-. Fig. 1. Quark diagrams for meson-baryon exclusive scattering. Example reactions for the diagram are shown. The reactions listed in (a) can proceed via diagrams (b), (C), (d). Similarly, n ~ + KA can proceed via (d). We report on an experiment performed at the Brookhaven AGS with an intense 10 GeV/c nbeam incident on a hydrogen target. The first results, on elastic scattering and on the p p final state, will be presented. The apparatus (Fig. 2) consisted of a single arm magnetic spectrometer which selected events with a positive particle with momentum greater than 5 GeV/c at 22' in the laboratory or near 90' in the R-p elastic center of mass system. A large-aperture array of three proportional wire chambers recorded track information on the opposite side. With an event trigger for n-p + positive + X, and PT> 1.9 GeV/c, events were collected simultaneously for n-p, p-p, I&-, n-A-, and other exclusive final states. For elastic scattering at 90°, PT= 2.1 GeV/c; The spectrometer arm was located in a building which could pivot about the center of the target to select the scattering angle 8. The analyzing magnet was placed on its side so that its gap of 1 8 defined a small range of laboratory angles, A8 = f 2.5'. The magnet deflected positives down with a transverse kick of 0.8 GeVIc. The vertical deflection decoupled the momentum measurement from the large horizontal projection of the 1 meter long target at 8 = 2'. Assuming a point target, a momentum could be determined using a matrix trigger between drift cells in DWCl and DWC2 after the magnet. We also required a matrix trigger between scintillator hodoscope elements in HODO 2 and HODO 3, which reduced accidental triggers. All detectors downstream of the magnet were mounted on a table which was tilted 8.1' to match the central momentum for elastic scattering. Two threshold Cerenkov counters on the tilt table, one with Ythreshold = 21.5, the other with ythreshold = 9.6, were used to distinguish between pions, kaons, and protons in the spectrometer arm. The momentum resolution of the arm, with proportional wire chambers upstream and narrow-cell drift chambers downstream was Ap/p = 0.5% at 5 GeV/c. p%% r H2 TARGET