Low Energy Antiproton Research Articles

LEAP 96 conference summary

The following pages represent a short summary of the many new results in low energy antiproton (p̄) physics presented and discussed at the LEAP 96 Conference. They cover a broad field of physics, from atomic physics to nuclear physics, from hadronic physics to parton physics. The impact of these results on “soft QCD”, the part of strong interactions which we do not yet understand, and on the limits that we can establish to the “fundamental symmetries” which govern Nature are original and important. Within these twelve pages, I cannot do justice to all contributions. I present my apologizes for the omissions, hoping however that the serious reader will find the missing information in these proceedings.

New measurement of the p̄p → n̄n differential cross-section from PS206

Experiment PS206 has measured with high precision the differential p̄p → n̄n cross-section at the CERN Low Energy Antiproton Ring. First results have already been published for the forward region, based on part of the collected statistics. Now the whole 601 MeV/c sample has been analysed and the new results are presented both for the forward hemisphere and for the backward region. The crucial tasks of the efficiency measurement and the studies of the systematic effects, that limit the precision of the measurement, are also discussed.

Meson spectroscopy with very low momentum antiprotons

The OBELIX spectrometer, studying at the Low Energy Antiproton Ring (LEAR) of CERN the NN annihilation, has collected annihilation data with very low momentum p(≈ 50MeV/c). The results of the spin-parity analysis on the pp annihilation into four and five pions are briefly described.

Proton-antiproton annihilation into neutral strange mesons

In a search for gluonic hadrons, the formation channels p̄p → K s K s , p̄p → ηη, p̄p → π 0 η and p̄p → π 0 π 0 were studied in the mass range from 2.1 to 2.4 GeV using the Jetset (PS202) detector and an internal molecular hydrogen cluster jet target installed in the Low Energy Antiproton Ring (LEAR) at CERN. Cross sections for p̄p → K s K s have been obtained and limits are set on the non-observation of the ξ(2230). Conversely, we find evidence for a narrow signal in a preliminary analysis of our p̄p → ηη data consistent with a narrow ξ(2230).

The Antiproton Decelerator: AD

In view of a possible future programme of physics with low-energy antiprotons, a simplified scheme for the provision of antiprotons at 100 MeV/ c has been studied. It uses the present target area and the modified Antiproton Collector (AC) in its present location. In this report the modifications and the operation are discussed.

Search for Antihelium in the Cosmic Rays

On 1987 August 22 a balloon flight was conducted using the Goddard Space Flight Center Low-Energy Antiproton configuration of the New Mexico State University balloon-borne magnet spectrometer. The launch site was Prince Albert, Saskatchewan, Canada. The balloon flew at an average atmospheric depth of 4.7 g cm-2 for more than 22 hr. During this period a sample of 4.2 × 104 helium nuclei was gathered. No antihelium candidates were found in this sample. The resultant upper limit for the ratio of antihelium to helium in cosmic rays over the rigidity interval from 1 to 25 GV/c is 9 × 10-5 at 95% confidence. This limit is below the predicted level, assuming equal matter and antimatter in the extragalactic cosmic rays.

Progress in the physics of exotic atoms at CERN

In the last few years great progress has been made at the Low Energy Antiproton Ring of CERN in three fields of exotic atoms: (i) laser spectroscopy of the anomalously long-living antiprotonic helium atoms; (ii) high-precision X-ray spectroscopy of antiprotonic hydrogen and helium isotopes and (iii) production and spectroscopy of antihydrogen. Results of the first two experiments and prospects of making the third one are summarised in the present review.

THE ANTIPROTON DECELERATOR : AD

In view of a possible future programme of physics with low-energy antiprotons, a simplified scheme for the provision of antiprotons at 100 MeV/c has been studied. It uses the present target area and the modified Antiproton Collector (AC) in its present location. In this report the modifications and the operation are discussed.

Cosmic‐Ray Antiproton Flux in the Energy Range from 200 to 600 MeV

We have studied the low-energy antiprotons in the cosmic rays by utilizing data obtained by the Balloon-borne Experiment with a Superconducting magnetic rigidity Spectrometer (BESS) flown in 1993 July from Lynn Lake, Manitoba, Canada. A detailed description of the event selection criteria and background corrections is given. Seven antiprotons are found that give an antiproton flux of 6.4 -->+ 5.5?3.5 ? 10-3(m2 sr s GeV)-1 and an antiproton/proton ratio of 5.2 -->+ 4.4?2.8 ? 10-6 in the 200-600 MeV energy range. These results are consistent with a secondary origin of low-energy cosmic-ray antiprotons within our uncertainties, but they still require the precise measurement of the spectrum shape below 500 MeV to clarify exactly the model of particle propagation and possible contributions from exotic sources.

Antihydrogen production and precision experiments. The ATHENA collaboration

The study of CPT invariance with the highest achievable precision in all particle sectors is of fundamental importance for physics. Equally important is the question of the gravitational acceleration of antimatter. In recent years, impressive progress has been achieved at the Low Energy Antiproton Ring (LEAR) at CERN in capturing antiprotons in specially designed Penning traps, in cooling them to energies of a few milli-electron volts, and in storing them for hours in a small volume of space. Positrons have been accumulated in large numbers in similar traps, and low energy positron or positronium beams have been generated. Finally, steady progress has been made in trapping and cooling neutral atoms. Thus the ingredients to form antihydrogen at rest are at hand. We propose to investigate the different methods to form antihydrogen at low energy, and to utilize the best of these methods to capture a number of antihydrogen atoms sufficient for spectroscopic studies in a magnetostatic trap. Once antihydrogen atoms have been captured at low energy, spectroscopic methods can be applied to interrogate their atomic structure with extremely high precision and compare it to its normal matter counterpart, the hydrogen atom. Especially the 1S-2S transition, with a lifetime of the excited state of 122 ms and thereby a natural linewidth of 5 parts in 1016, offers in principle the possibility to directly compare matter and antimatter properties at a level of 1 part in 1018. Additionally, comparison of the gravitational masses of hydrogen and antihydrogen, using either ballistic or spectroscopic methods, can provide direct experimental tests of the Weak Equivalence Principle for antimatter at a high precision.

Interactions of Low Energy Antiprotons with Light Atoms and Nuclei

Interactions of Low Energy Antiprotons with Light Atoms and Nuclei

Measurement of the n̄-Fe absorption cross section between 125 and 780 MeV/ c

The n̄-Fe absorption cross section σ abs nFe has been measured at LEAR (the Low Energy Antiproton Ring at CERN) using tagged n̄'s produced in the p̄p → n̄n charge-exchange reaction. With incident p̄ momenta of 875 and 545 MeV/ c σ abs nFe has been measured in the momentum interval between 125 and 780 MeV/ c, in a region where only a few measurements presently exist. The overall statistical and systematic errors range from 20 to 5%; the normalisation error is less than 3%. The measured cross sections are well reproduced by a parametrisation a + b p n , with a = 0.680 ± 0.051 barn and b = 0.228 ± 0.024 barn·GeV/ c.

AN INTENSE SOURCE OF ANTIPROTONS FOR ANTIMATTER RESEARCH AT FNAL

There is considerable interest in low energy antiprotons for atomic, nuclear, and particle physics. The LEAR facility at CERN is currently the only source of antiprotons with low enough energy to be stored and accumulated in a Penning trap. With only minimal hardware modifications, the Fermilab Linac and Booster synchrotron can be used to decelerate an initial ensemble of 1011 antiprotons with an efficiency of approximately 0.5%. Subsequent improvements will be able to increase this efficiency to near 100% transmission of 1012 antiprotons. These transfers could take place as often as two or three times a day with only a marginal decrease in Tevatron Collider luminosity.

Capture and cooling of antiprotons

Approximately one million antiprotons have been captured at a few keV in a half meter long cylindrical Penning trap from a single, fast extracted pulse of antiprotons at the Low Energy Antiproton Ring (LEAR) at CERN. By electron cooling more than 65% were collected in a harmonic Penning trap. Here they can be confined for more than three hours. This is one of the first important steps towards the synthesis of antihydrogen at rest for tests of CPT.

Metastable antiprotonic helium atoms: Past, present and future

This review briefly summarises the experimental studies carried out on metastable antiprotonic helium over the last few years, and points out a possible way ahead, assuming that very low-energy antiprotons continue to be available. Very recently, an abundance of new results has been found by the Japanese-European collaboration working in these exotic atoms at LEAR, the Low Energy Antiproton Ring at CERN. Together with tremendous progress in several of the techniques needed for antihydrogen synthesis, this has provided new hope that some way will be found to continue this fruitful line of research after the closure of this excellent machine.

Extraction of ultra-low energy antiprotons from Penning traps

Approximately one million antiprotons have been captured in a large Penning trap at the Low Energy Antiproton Ring at CERN. These antiprotons have subsequently been cooled by electron cooling. This has opened new discussions of the possible use of ultra-low energy antiprotons for nuclear, atomic, and gravitational physics. For most of these experiments, it will be necessary to extract the antiprotons from the trap in a continuous or bunched beam, allowing the timing structure to be used for post-acceleration schemes or as a time tag for the subsequent measurements. We have designed an extraction scheme to accomplish this and have tested portions of it using a smaller Penning trap loaded with protons. First results in generating a time-correlated beam of particles from a Penning trap are presented.

Antihydrogen from antiproton-positronium collisions

We survey the progress made towards the production of antihydrogen by antiproton-positronium collisions as of June 1995. The tasks remaining before such an experiment can be attempted are outlined. Some comments are made concerning the effects of the intended closure of the Low Energy Antiproton Ring (LEAR) at the end of 1996. We conclude with a personal account of the development of, in particular, the Aarhus—UCL collaboration and some memories of Bernie Deutch.

Antihydrogen beam generation using storage rings

The scheme of antihydrogen beam generation is proposed. Two storage rings should be used for effective generation of antihydrogen - an antiproton ring and a small additional one for storing and cooling of positrons. This scheme has advantages compared with those discussed earlier due to the use of a longitudinal magnetic field as the focusing system of the positron ring with the positron source immersed in this field. The electron cooling of positrons is an essential part of the project. The Low Energy Antiproton Ring (LEAR) at CERN is chosen as the basic machine for idea demonstration and all the numerical applications are made for it. The rate of antihydrogen generation is limited in the case discussed here by the positron source intensity. The simplest version of this scheme permits one to achieve a rate of about 3 × 10 4 fast (50 MeV) atoms/s and 30 slow (0.5 MeV) ones. Simultaneously such a scheme is a generator of orthopositronium with a productivity of about 2 × 10 4 and 35 “atoms”/s, correspondingly. The particle dynamics in the positron storage ring are considered.

First measurement of the spin-transfer parameter Kn00 n in the [formula omitted]p → [formula omitted]n reaction at 875 MeV/ c

The spin-transfer parameter K n00 n of the p p↑ → n ↑n charge exchange reaction has been measured for the first time at the CERN Low Energy Antiproton Ring (LEAR), at 875 MeV/ c p momentum, in the centre-of-mass scattering-angle range from 45° to 78°. To measure the transverse polarisation of the n 's, a thick scintillator counter hodoscope was used as live target, and the elastic n p scattering on the hydrogen of the scintillator was used as analysing reaction of the n transverse polarisation. Its so far unmeasured analysing power is taken as linear in momentum transfer, A np = α·q , and results are given for α· K n00 n . The values one obtains for K n00 n , estimating α from N N potential models, are less than 0.25, in agreement with the predictions.

Possibilities for antihydrogen formation by antiproton-positronium collisions

Progress made towards the production of antihydrogen by antiproton–positronium collisions is surveyed. This includes an outline of how antiprotons ejected in short bursts from the low energy antiproton ring (LEAR) at CERN are captured, cooled, and stored in a Penning trap. Details are given of how a source of low-energy positrons can be interfaced to this trap to promote antihydrogen formation. Production rates and a possible scenario are outlined. We conclude with some comments on what might happen after the closure of LEAR, currently scheduled to take place at the end of 1996.