Low energy antiproton facility

Data on antiproton-nucleus annihilation cross section are very scarce below 5 MeV (for a review see [1]). ASACUSA has performed measurements of the antiproton-nucleus annihilation cross section at 130 keV on carbon, palladium, and platinum target foils of sub-100 nm thicknesses [2], by using the RFQD beam (see "Antiproton-nucleus cross sections"). Data at low energy and also at rest are badly needed for microscopic antiproton-nucleus models and for reliable simulations of the background contributions to experiments. Theoretical predictions are available from the Intra-Nuclear Cascade model [3] (with the influence of the Coulomb attraction stressed in [4]).

Antiproton-nucleus annihilation could be studied on a slow extraction beam line. Currently, ASACUSA traps 106 antiprotons per Antiproton Decelerator (AD) cycle (100 s), which could be slowly released between two cycles with an intensity of ~104 s1, comparable to that obtained by the Crystal Barrel experiment [5] at the former LEAR facility. However, the beam would need to be accelerated to pass through a thin target window (e.g. up to 100 keV, as described below). One could also use a jet target, as suggested earlier in our 2002 proposal [6].

With a slow extraction beam one could carry out a variety of experiments. Depending on the interest from other groups at the AD, ASACUSA’s slow extraction beam line could become a new experimental facility with a broad physics program, such as the study of rare annihilation channels, antiproton-nucleus annihilation, antiprotonic atoms, etc. As an alternative, slow extraction of a 100 keV beam directly from ELENA should also be feasible.

We are studying the feasibility of the several projects [7] such as:

1) Annihilation multiplicities: The antiprotons will be extracted either from a new trap or directly form the MUSASHI trap (see "Antihydrogen hyperfine structure"), using cylindrical mirrors and einzel lenses. Measurements of the charged annihilation prong multiplicity will be performed at rest on a large number of stable nuclei, using a detector covering a very large solid angle, based on Timepix3/Timepix4 (see "Fragmentation studies").

2) Pontecorvo reactions are antiproton annihilations on nuclei in which the final states are of a type not attainable from annihilations on free nucleons. The mechanism underlying antiproton annihilation on two or more nucleons is not understood. Attempts to describe these reactions by rescattering annihilation pions or kaons on the neighbouring nucleon(s) are prone to orders of magnitude uncertainties. A promising approach is that of the fireball model [3], in which the highly excited multiquark bag decays into the observed final state through quark annihilation or rearrangement. The predicted fireball formation probability is around 3% [1]. The fireball model can be checked by studying annihilation on three nucleons such as antiproton annihilation on 3Henp, for which no data exist. The rates are expected to be of the order of 10-6 [3], much in contrast to the 108 to 107 predicted for re-scattering.

A possible setup to measure antiproton + 3He np is sketched in the figure below. The antiprotons would be accelerated to 100 keV towards a small 3He gas target. The cryogenic target cell (20 mm long) would work at a temperature below 10 K, with an entrance window made of Kapton (0.5 –1.0 μm thick). A lead- scintillator sandwich calorimeter would cover a solid angle of 2π sr with 60% efficiency for 1 GeV neutrons. A rough estimate assuming an antiproton rate of 103 s1 and a solid angle of 2π sr leads to about 10 events/day.

Possible setup for the antiproton 3Henp experiment. The two outgoing nucleons have energies of about 1 GeV and are emitted back-to-back. The yellow arrows represent antiprotons extracted from the trap (from [7]).


[1] C. Amsler, arXiv:1908.08455, Nucleon-antinucleon annihilation at LEAR
[2] K. Todoroki et al., Nucl. Instr. Meth. in Phys. Res.
A835 (2016) 110
[3] J. Cugnon and J. Vandermeulen, Phys. Rev. C 39 (1989) 181
[4] J. Carbonell and K. Protasov, Hyperfine Interactions 76 (1993) 327
[5] see the Crystal Barrel Server Crystal Barrel Collaboration and
     C. Amsler, Rev. Mod. Phys. 70 (1998) 1293
[6] ASACUSA Status Report, 2002 CERN-SPSC-2002-002; SPSC-M-674
ASACUSA Status Report, 2020 CERN-SPSC-2020-001; SPSC-SR-264

See also:

D. Gotta, Progr. Part. and Nucl. Phys. 52 (2004) 133
Precision spectroscopy of light exotic atoms

D. Gotta, K. Rashid, B. Fricke, P. Indelicato and L. M. Simons, Eur. Phys. J.
D 47 (2008) 11 X-ray transitions from antiprotonic noble gases

CA, 11.3.20