Study of nuclear properties with muonic atoms
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Study of nuclear properties with muonic atoms A. Knecht1,a
, A. Skawran1,2,b , S. M. Vogiatzi1,2,c
1 Paul Scherrer Institut, Villigen, Switzerland 2 Institut für Teilchen- und Astrophysik, ETH Zürich, Zürich, Switzerland
Received: 7 April 2020 / Accepted: 14 September 2020 © The Author(s) 2020
Abstract Muons are a fascinating probe to study nuclear properties. Muonic atoms can easily be formed by stopping negative muons inside a material. The muon is subsequently captured by the nucleus and, due to its much higher mass compared to the electron, orbits the nucleus at very small distances. During this atomic capture process, the muon emits characteristic X-rays during its cascade down to the ground state. The energies of these Xrays reveal the muonic energy level scheme, from which properties like the nuclear charge radius or its quadrupole moment can be extracted. While almost all stable elements have been examined using muons, probing highly radioactive atoms has so far not been possible. The muX experiment has developed a technique based on transfer reaction inside a high-pressure hydrogen/deuterium gas cell to examine targets available only in microgram quantities.
1 Preface This article is based on a lecture given at the 2019 summer school of the University of Pisa titled “Rewriting Nuclear Physics Textbooks: one more step forward” [1]. The intent of this write-up is to give a broad and general introduction to the topic (with plenty of references for the interested student to dig even deeper) and to give an example of the current progress of the field. Therefore, in terms of experiments, we focus almost completely on our own muX experiment, which marks the current edge of what is possible with muonic atom spectroscopy.
2 Introduction Muons are fascinating particles with experiments being conducted in the context of particle, nuclear and atomic physics [2]. Additionally, also applied research is possible by measuring the spin precession and dynamics of muons inside materials through the μSR technique [3] thus probing the internal magnetic fields of the sample under study. Muons are so-called leptons—such as electrons—and classified into the second family within the Standard Model of particle physics [4]. The properties of the muon have been extensively studied over the past 84 years since its discovery in 1936 [5,6]. Its lifetime [7],
a e-mail: [email protected] (corresponding author) b e-mail: [email protected] c e-mail: [email protected]
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for example, is the best measured lifetime of any unstable particle and searches for rare decays of the muon violating lepton family number have reached unprecedented sensitivity [8]. With a mass of 105.66 MeV/c2 , they are about 207 times heavier than an electron. Their lifetime of 2.197 μs is short, but still long enough in order to allow the large range of experiments alluded to above. Muons are also at the center of several current discrepancies between the Standard Model o
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