K. Shigaki

Formation of long-lived gas-phase antiprotonic helium atoms and quenching by H2

IN 1964 Condo1 suggested that the decay characteristics of negative (π− and K−) mesons in helium bubble chambers could be explained by the capture of these particles in large-angular-momentum meta-stable orbitals of exotic helium atoms. Russell2 predicted that similar atoms might be formed by antiprotons in liquid helium. Nearly two decades later the postulated metastability of K− and π− mesons in liquid helium was observed experimentally3–5. We recently observed6 that about 3% of the antiprotons stopped in liquid helium survive for several microseconds before annihilating in the helium nuclei. This is more than a million times longer than the typical (picosecond) lifetimes of antiprotons that come to rest in matter, and it represents the signature of the formation of metastable antiprotonic atoms. Here we show that the same phenomenon is observed in gas-phase helium, but that surprisingly the lifetime of the atoms is the same as in the liquid phase, despite the reduction in collisional de-excitation. In addition, we show that the presence of trace amounts of hydrogen gas greatly reduces the lifetime, suggesting that a single collision with H2 is sufficient to destroy the metastability.

Global transverse energy distributions in Si+Al, Au at 14.6A GeV/c and Au+Au at 11.6A GeV/c

Abstract Measurements of the global transverse energy distributions dσ / dE T and dE T / dη using the new AGS beam of 197 Au at 11.6 A GeV/ c on a Au target, as well as a beam of 28 Si at 14.6 A GeV/ c on Al and Au targets, are presented for a leadglass detector with acceptance 1.3 ≤ η ≤ 2.4 and 0 ≤ φ π . The dσ / dE T spectra are observed to have different shapes for the different systems and simple energy rescaling does not account for the projectile dependence. The Au+Au dσ / dE T spectrum is satisfactorily constructed from the upper edge of Si+Au by the geometric Wounded Projectile Nucleon Model after applying a correction for the beam energy.

A forward magnetic spectrometer system for high-energy heavy-ion experiments

Abstract A small aperture magnetic spectrometer has been built to study hadron production in 197 Au + 197 Au collisions at the AGS energy of 11.6Axa0GeV/c. It operates in the forward angular range from 6 to 30° with respect to the incident beam axis and covers the mid-rapidity region for heavy particles such as protons. The detector components of the spectrometer system include two time projection chambers, four drift chamber modules and a time-of-flight scintillation counter wall. A few new technologies are implemented in the design of the system to achieve the performance goals. The spectrometer has proved to function properly under the high particle-density environment encountered in experiments with the heavy-ion colliding system. The achieved momentum resolution is 1.3% in r.m.s. for pions at 1xa0GeV/c and 1.6% for protons at the same momentum. With the time-of-flight resolution of 76xa0ps in r.m.s., the particle identification momentum limit extends to 4xa0GeV/c for pions, 3xa0GeV/c for kaons, 5xa0GeV/c for protons, and 4.5xa0GeV/c for anti-protons. The tracking efficiency stays above 86% for tracks up to 5xa0GeV/c with as many as 10 tracks in the spectrometer aperture.

Backward emission of protons in Au+Au collisions at 11.7 A.GeV/c

We present preliminary results for the emission of target rapidity protons in minimum bias and central 11.7 A.GeV/c Au+Au collisions. The data span the pseudo-rapidity range {vert_bar}{eta}{vert_bar} {le} 0.76 and proton kinetic energy range of 50 {le} E {le} 200 MeV. The slopes of the kinetic energy spectra and dN/d{eta} values for central and minimum bias collisions are strikingly similar. Comparison of the results to results for Si+Au and p+A shows that the shape of the dN/d{eta} distribution is independent of the reaction system or centrality suggesting that the spectator matter does not play a decisive role in determining the shape of the proton distributions at back angles for these systems at AGS energies.

Single particle spectra from NA44

Abstract We have measured π ± , K ± , P and P spectra from proton and sulfur collisions with nuclear targets at the at beam energies of 450 Gev and 200 Gev per nucleon respectively. For central SA collisions we see that kaons and (anti)protons have large transverse energies. Protons are moved into central rapidity by rescattering where they annhilate antiprotons. The RQMD event generator qualitatively reproduces these trends while Fritjof does not.

A naturally occurring trap for antiprotons

The phenomenon of delayed annihilation of antiprotons in helium is the first instance of a naturally occurring trap for antimatter in ordinary matter. Recent studies of this effect at CERN are summarised, and plans are described for laser excitation experiments to test its interpretation in terms of metastable exotic helium atom formation.

The centrality dependence of the source size for AuAu collisions at the AGS

Abstract Recent π − π − correlation measurements and deuteron to proton yield ratio measurements for Auue5f8Au collisions from Brookhaven E866 are presented. These measurements allow us, with some interpretation, to deduce the size of the participant region in a heavy ion collision. An unpredicted rise of the apparent source size is seen in the π − π − data at midrapidity. No corresponding drop in the deuteron to proton yield ratio is seen. Further experimental study is needed to improve the statistical significance of the correlation result and to determine whether it can be explained by conventional hadronic means.