Atomic Physics with antiprotons: experimental situation
and perspectives
Evandro Lodi Rizzini
Department of Chemistry and Physics, University of Brescia, Via Valotti 9, 25133 Brescia Italy
Abstract. Experimental results and perspectives with low-energy antiprotons at CERN are reviewed
double ionization process cannot be correctly treated
unless the electron-electron correlation is properly
accounted for.
STOPPING POWERS
Atomic collisions have been studied exstensively
during the last century. Nevertheless, it is still a
challenge to understand in detail even the simplest
collision process. This is due to the complex dynamics
of systems which consist of more than two particles
that interact via the Coulomb force. Moreover, the
understanding of the slowing down of fast particles in
matter has played a significant role in the discoveries
of the constituents of matter ever since the beginning
of the century with Thomson, Rutherford and Bohr. It
was necessary to have a good theoretical understanding of the stopping process in order to extract
information about the atomic structure. Nevertheless,
even today there is good agreement between
calculations and experiments only for high energies
(the Bethe formula).
At low impact velocity, the target electronic cloud
responds to the passage of the projectile so that it
becomes polarized during the first part of the collision.
This leads to a larger cross section for proton than for
equivelocity antiproton impact. This difference grows
when the velocity becomes smaller, but as the velocity
approaches the magnitude of the orbital velocity, the
polarization effect is counteracted by the so called
binding / antibinding effect. Here close encounters
become more important, and as the projectile passes
through the target electron cloud, the binding of the
active electron is increased or decreased, depending on
the sign of the projectile charge. This leads to a
corresponding decreaes or increase of the cross
section.
At very high impact velocities the first Born
approximation provides a convenient framework for
the treatment of single ionization. The cross section
As the energy necessary for all processes comes
from the projectile kinetic energy, a measurement of
the stopping cross section provides a consistency
check for the individual cross sections.
2
scales as q , the square of the projectile charge, due to
the perturbative nature of the interaction. For the
double ionization process, the first Born approximation is not adeguate, even at very high projectile
A velocity-linear stopping power, as predicted by
free-electron-gas models, is observed for positive
particles. Significative deviations have been found
only in the case of He and Ne targets, and a striking
difference is observed in the behaviour of the specific
mean energy loss for proton in He and H just below
of the maximum [ 1 ].
2
velocities. The cross section does not scale as q .
For energies in the range between 0.1 and 10
MeV, the antiproton double ionization cross section of
helium is considerably larger than the corresponding
cross section for equivelocity proton impact. The
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
203
Measurements of antiprotons stopping powers of
the four elements C, Al, Ni, Au were made in the
energy range 5-70 keV, and in addition in the case of
carbon down to 1.5 keV,at AD, CERN [ 2 ]. The
measurements show a reduced stopping power, the
“Barkas effect”, for antiprotons as compared to
protons. The reduction in the investigated energy
region below the stopping power maximum is almost
a factor of 2 for all the targets but Al. The main
experimental result is the linear velocity dependence
of the stopping power in the investigated low-energy
region.
ANTIPROTONIC HELIUM
Atomic capture of heavy negative particles has
been studied in a specific way since the discovery, or
even before, of such a particles ( µ , π , K, p ).
Antiprotons which stop in matter are initially
captured into high-lying states of the matter’s
constituent atoms. Thence they decay to low-lying
atomic states whose wavefunctions have a nonnegligible overlap with the atomic nucleus. The strong
nuclear interaction then causes annihilation of the
antiprotons with the emission of nuclear decay
products, including many pions. The detection of these
pions is a sign of the antiproton capture, and since
their production rate is determined by the atomic
cascade through which the antiprotons pass,
information on the physics of antiprotonic atoms can
be obtained by measuring the time spectrum of the
capture pions.
This last sentence is not true for p in H 2 and He.
The Obelix Collaboration has measured for the
first time at low energies the p energy loss in gaseous
H 2 , D 2 and He [ 3,4 ]. Differently from other
experiments based on the direct differential method,
PS201 experiment derive the stopping power by an
integral method which combines the projectile-range
distributions with the distributions of slowing-down
times. This method features high sensitivity to the
energy losses of very slow projectiles.
All the experimental results obtained by PS205
Collaboration and, more recently, by the ASACUSA
Collaboration at the new CERN-AD facility concern
with the “delayed p annihilations” in He ( ≈ 3% of
the captured antiprotons present “delayed annihilation”
with mean lifetime of 3 µ s). On the basis of these
results several detailed evaluations of the relevant
metastable energy levels in antiprotonic helium have
For H 2 and D 2 the behaviour of the stopping
power was determined for p kinetic energies ranging
from about 1.1 MeV down to the capture energy. The
evidence of differences in the nuclear stopping power
was inferred in agreement with the Wightman
prediction [ 4 ]. An unexpected result has been obtained for Xe [ 5 ].
been performed. Indeed, the p α e system is well
described as an exotic diatomic one-electron molecule
(the “atomcule”) with the antiproton as a negative
nucleus, the vibrational motion of the two nuclei being
characterized by the quantum number ν = n- l- 1.
Moreover, the different observed decays occur along
the ∆ ν = 0 decays, i.e. following a “propensity rule”.
−
In the electronic domain a negative difference between the p and p behaviours near the maximum was
clearly observed and evaluated, the “Barkas effect”.
Moreover, at energies higher than 200 ÷ 300 kev, the
p stopping power exceed that of the proton [ 6 ].
PS201 result is consistent with a positive difference in
antiproton-proton stopping powers above 250 keV and
with a maximum difference between the stopping
powers of 21% ± 3% at around 600keV.
Very recently, the initial distributions of metastable
4
3
antiprotonic He and He atoms over principal ( n )
and angular momentum ( l ) quantum numbers have
been deduced using laser spectroscopic methods [ 7 ].
4
+
In p He atoms, the region n=37-40 accounts for
nearly all of the observed (3.0 ± 0.1)% fraction of
antiprotons captured into metastable states. In
The p stopping power in helium above the
maximum is evaluated too. Obelix data indicate a p
stopping power higher than the proton’s one, the
difference being of the order of 15% ± 5% at 700keV.
p 3 He + atoms the antiprotons were monstly distributed over the region n=35-38, which accountes for
the observed (2.4 ± 0.1) % metastability.
The agreement with experiment was best for the
case of n max =40, which supports the findings that the
metastable populations in the n ≥ 41 regions are very
small.
204
As far as the totality of antiprotonic helium atoms
is concerned, there is not a common agreement on the
values and behaviour of capture cross sections as on
the initial population of (n,l) states. Diabatic, adiabatic
or quasi-adiabatic processes are invoked and
contraddictory predictions can be found in literature.
Moreover, the evolution picture of the antiprotonic
atoms can be obtained only by taking into account the
effect of interactions with surrounding atoms, helium
or foreign atoms.
The PS201 − OBELIX apparatus and the experimental strategies specifically implemented allowed for
the first time the study of the annihilation events in
the whole time range from the stopping-capture of the
antiprotons both in pure helium at different densities
and in helium with small fractions of contaminants [ 811 ]. Therefore, PS201 data make possible a correlated
and coherent study of the formation and decay
processes for the antiprotonic helium exploiting the
many different experimental conditions offered by the
apparatus [ 12 ].
ANTIHYDROGEN PRODUCTION
Antihydrogen atoms production starts last August
in the ATHENA apparatus at CERN-Antiproton
Decelerator facility: “…we demonstrate the production
of antihydrogen atoms at very low energy by mixing
trapped antiprotons and positrons in a cryogenic
environment. The neutral anti-atoms have been
detected directly when they escape the trap and
annihilate, producing a characteristic signature in an
imaging particle detector.
To compare different possible hypotheses on the
initial populations and the consequent decay
mechanism in order to reproduce the full set of
experimental data, we have simulated, within a Monte
Carlo, several possible initial state populations and
different cascade patterns by varying Stark, Auger and
radiative frequencies within the values suggested by
the literature.
...The detector is designed to provide unambiguous
evidence for antihydrogen production by detecting the
temporally and spatially coincident annihilations of the
antiproton and positron when a neutral antihydrogen
atom escapes the electromagnetic trap and strikes the
trap electrodes. An antiproton typically annihilates
into a few charged or neutral pions.
Moreover, the full set of our experimental data
−3
(density ≈ 10 ÷ 10 cm ) does not support the
hypothesis of a 97% of primary populations in levels
with fast Auger decays ( l << n-1 ). The experimental
p annihilation time distributions are well reproduced
if we assume that the annihilations occur after Stark
( ∆ n=0 and ∆ l= -1) and radiative transitions
( ∆ n= -1, ∆ l= -1) to Auger dominated final states. In
Fig.1, the Monte Carlo annihilation time distributions
20
15
...A positron annihilating with an eletron yields two or
three photons. The positron detector, comprising 16
rows, each now containing 12 scintillating, pure CsI
crystals, is designed to detect the two-photon events,
consisting of two 511-keV photons that are always
emitted back-to-back.
−3
(density ≈ 10 cm ) at the end of v = 0, 1, 2, 3
cascades are reported for a comparison with Fig. 3 in
[ 7 ].
20
It is premature to discuss absolute production rates
in our experiment, but it is noteworthy that the time
distribution of the cos( θ γγ ) ≤ -0.95 events parallels
The relevant possible conjecture refers to common
patterns for all the antiprotonic helium atom decays
from primary populations mainly in the circular or
near circular atomcule states with n= 41, 40, 39 and
l ≅ n-1, n-2.
the total annihilation rate during mixing. “ [ 13 ].
In the future it will be very nice to produce an
antihydrogen atomic beam. It will be possible to send
it in a low pressure helium target to observe helium
exotic “atomcule” production in the reaction:
205
H + 4 He 
→ ( p α e − e + e −

→ ( p α e − ) + γγ
)
6. E. Lodi Rizzini, Phys. Rev. Lett. 89, 183201 (2002).
7. M. Hori et al., Phys. Rev. Lett. 89, 093401 (2002)
This would be a matter-antimatter “chemical reaction”
instead of the normal electronic one.
8.
V. G. Ableev et al., Nuovo Cimento A 107, 1325
(1994).
9.
A. Bertin et al., Nuovo Cimento A 109, 1505 (1996).
10. A. Bertin et al., Nuovo Cimento A 110, 419(1997).
REFERENCES
11. OBELIX Coll., Nucl. Phys. A 655, 283c (1999)
1. A. Schiefermuller et al., Phys. Rev. A 48, 4467 (1993).
12. E. Lodi Rizzini et al., Phys. Lett. B507, 19 (2001).
2. S. P. Moller et al., Phys. Rev. Lett. 88, 193201 (2002).
13. Amoretti, M. et
al., Production of cold antihydrogen atoms Nature advance online publication.
3. M. Agnello et al., Phys. Rev. Lett. 74, 371 (1995).
4. A. Bertin et al., Phys. Rev. A 54, 5441 (1996).
5. E. Lodi Rizzini., Phys. Lett. B 513, 265 (2001).
206