A search for narrow lines in the gamma spectra from $\text{P}$D annihilation at rest

Abstract

The γ ray spectrum from $\text{p}$d annhilations at rest was studied by stopping antiprotons in liquid deuterium and analyzing the gamma rays using a magnetic pair spectrometer. No clear evidence is found for monochromatic γ rays and the upper limit for the branching ratio $\text{p}\text{p}\text{→ γ}\text{X with}\text{1100}\text{MeV/c}{}^2\text{<}\text{m}{}_{\mathrm{<mtext>x</mtext>}}\text{< 1770}\text{MeV/c}{}^2$ is less than 5 × 10³ at the 95% confidence level.

The search for narrow lines in the gamma ray spectrum produced in antiproton-proton annihilations at rest has been intense. Despite the predictions of various models¹ and earlier experimental evidence for their existence,² exotic four quark states (qq$\text{q}$$\text{q}$), known as baryonium, have not been confirmed by recent experiments.³ Evidence for bound $\text{p}$N states has also been claimed in the pionic and gamma ray spectra resulting from the annihilation of antiprotons in deuterium.⁴ Prompted by such claims, we conducted an experiment to measure the inclusive gamma ray spectrum from the annihilation of antiprotons at rest in liquid deuterium. In this report we present the results of a high-resolution, high-statistics measurement of the gamma spectrum from the annihilation of antiprotons at rest in liquid deuterium, using the well-understood PS183 magnetic pair spectrometer.

The 350 MeV/c antiproton beam entered a 7 cm diameter, 70 cm long liquid deuterium target and the antiprotons annihilated at rest after ranging through approximately 17 cm of deuterium. A cylindrical multiwire proportional chamber surrounding the target was used to measure the charged multiplicity of each event and planar drift chambers were used to determine the stopping vertex. The gamma rays from the annihilation were converted into electron-positron pairs in a thin lead foil (10% of a radiation length) which was situated at the front of the magnet aperture and subtended 1.5% of 4π solid angle. Depending upon the momentum of the particle, the magnetic field of 7.5 kG either reflected the particle back out of the magnet (R track), trapped it within the magnet aperture (T track), or allowed it to penetrate (P track). The gamma ray events analysed in this report fall into one of three different types; RR, RT, TT. The acceptance for penetrating tracks was too low at this magnetic field setting to accumulate sufficient statistics. Details of the detector can be found elsewhere.⁶

The momentum of a particle was reconstructed using the radius of a circle fit to the multiwire proportional chamber hits times a correction factor to correct for the inhomogeneities in the field. Hit patterns in the chambers and time-of-flight between various scintillators ensured a high-quality sample of events from annihilations at rest. Of 2.8 million gamma-triggered events written to tape, 750000 survived all the cuts and made their way into the spectra presented in Figure 1. The acceptance and resolution functions of the spectrometer were obtained from a Monte Carlo simulation of the detector using the GEANT3 code developed at CERN.

The spectra were fit with low-order polynomials and the search for narrow states was carried out by stepping through the spectra bin-by-bin and fitting with the global polynomial plus a gaussian constrained to have a width consistent with the spectrometer resolution at the energy of the bin. No clear evidence for narrow structures is seen in the spectra. Figure 2 shows the 95% confidence level upper limits on the yield of narrow structure.