Annual Report 2011 / 2012 - E21 - Technische Universität München

Chapter 3. Positron Physics 27
A Precise Measurement of the Decay Rate of the
Negative Positronium Ion Ps −
Hubert Ceeh 1 , Klaus Schreckenbach 1 , Christoph Hugenschmidt 1, 2 , Stefan Gärtner 3 ,
Peter Thirolf 3 , Dirk Schwalm 4
1 Physik Department E21, Technische Universität München, D-85747 Garching, Germany
2 Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II), Technische Universität München, D-85747 Garching, Germany
3 Ludwig-Maximillians-Universität München, Am Coulombwall 1, D-85748 Garching, Germany
4 Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany
The negative positronium ion Ps − is a bound state consisting
only of three leptons, two electrons and a positron (see
fig. 1). Therefore, Ps − is an ideal object to study the quantum
mechanics of a three-body system. The ground state of
Ps − is stable against dissociation but unstable against annihilation
into photons. A precise measurement of the Ps −
ground state decay rate Γ was carried out at the high intensity
positron source NEPOMUC at the research reactor
FRM II in Garching. A value of Γ = 2.0875(50)ns −1 was
obtained, which is three times more precise compared to
previous experiments and in agreement with most recent
theoretical predictions [1]. The experimental precision that
was achieved is at the level of the leading corrections in the
theoretical predictions.
of the leading order QED corrections (see fig. 2). As these
terms also factor into the ortho- and parapositronium decay
rates, respectively, we may use them to determine the
distinct three body quantity 〈δ +− 〉 from the measured Ps −
decay rate [1]. The cusp 〈δ +− 〉 describes the probability of
finding one of the electrons and the positron at the same
position. This results in
〈δ +− 〉 = 0.020729(50), (1)
and has is to be compared with the theoretical value of
〈δ +− 〉 = 0.020733... assumed to be known up to an accuracy
of 10 −11 [5].
Figure 1: Artistic view of the Ps − ion. The two electrons (green)
are in a singlet state with their spins being aligned anti-parallel,
while the spin orientation of the positron (red) is random. Averaged
distances between the constituents are taken from [2].
Experiment
The method applied already in the previous experiment [3]
was adapted an refined. Details can be found in [4]. Ps − ions
are produced transmitting positrons through a thin diamondlike
carbon foil. The Ps − ions are are accelerated to an
energy of several keV. The number of Ps − ions surviving
the passage through a gap of adjustable width is determined
by stripping the electrons off the ions and detecting the
remaining positrons. The decrease of the number of surviving
Ps − with increasing gap width is directly reflecting the
decay rate Γ Ps −.
Results
The present result is in very good agreement with the most
recent theoretical value of Γ = 2.087963(12)ns −1 [5], which
contains now all correction terms up to order O(α 2 ). Despite
the recent progress in experimental accuracy it is obvious
that due to the recent work of Puchalski et al. theory
is again far ahead of experiment. However, with the experimental
precision we achieved we are now able to probe
theoretical calculations of the decay rate to the precision
Figure 2: Calculated Ps − decay rate with and without QED
corrections according to [5] in comparison to the measured value
from the present work and previous experiments [1, 3, 6].
The precision of the present result allows to experimentally probe
QED correction to the decay rate for the first time.
Outlook
Complementary to the decay rate measurement an experiment
for the Ps − photo detachment and the production of
a mono energetic orthopositronium beam is in preparation
and will soon be operational. It will allow for the measurement
of the photo detachment cross section for different
photon energies in the off-resonant region, as well as the
production of a mono energetic and energy tunable orthopositronium
beam with an intensity of up to a few 10 per
second.
References
[1] Ceeh et al., Phys. Rev: A 84 2011 062508 (2011)
[2] A. M. Frolov, Phys. Rev. A 60 2834 (1999)
[3] Fleischer et al., Phys. Rev. Lett. 96 063401 (2006)
[4] Ceeh et al., J. Phys.: Conf. Ser. 262 012011 (2011)
[5] Puchalski et al., Phys. Rev. Lett. 99 203401 (2007)
[6] A. P. Mills, Phys. Rev. Lett. 50 671 (1983)