RIKEN Accel. Prog. Rep. 50 (2017)

Ⅰ. HIGHLIGHTS OF THE YEAR

S tudies of the muon transfer process in a mixture ofhydrogen and higher Z gas
A. Vacchi,1,2,3 K. Ishida, *1 E. Mocchiutti,3 N. Rossi,2 on behalf of the FAMU collaboration
The final aim of this experiment is a precision
spectroscopic measurement of hyperfine splitting (hfs) in
the 1S state of muonic hydrogen ΔEhfs(μ-p)1S - providing
crucial information on proton structure and muon-nucleon
interaction in order to provide the proton Zemach radius rZwith higher precision than previously possible.1) This willallow discordant theoretical values to be disentangled andevidence any level of discrepancy that may exist between
values of rZ extracted from normal and muonic hydrogenatoms. It will also set a needed cornerstone result about not
yet explained anomalies within the proton charge meansquare radius rch.2,3) By measuring the ΔEhfs(μ-p)1S transition
wavelength with an unprecedented precision of δλ/λ< 10-5,
the final experiment will establish new limits on the proton
structure parameters and shed light on the low momentum
limit of the magnetic-to-charge form factor ratio.4) The
physical process behind this experiment is the following:muonic hydrogen atoms are formed in a gas target
containing a mixture of hydrogen and a higher-Z gas. A
muonic hydrogen atom in the ground state, after absorbing aphoton of energy equal to the hyperfine-splitting
resonance-energy ΔEhfs ≈ 0.182 eV, is very quickly
de-excited in subsequent collision with the surrounding H2molecules. At the exit of the collision, the muonic atom isaccelerated by ~2/3 of the excitation energy ΔEhfs, which istaken away as kinetic energy. The experiment will observe
the products of a reaction whose rate depends on the kinetic
energy of the muonic atoms. The observable is the time
distribution of the characteristic X-ray emitted from themuonic atoms formed by muon transfer from hydrogen to
the atom of the admixture gas (μp) + Z → (μZ)* + p and its
response to variations of the laser radiation wavelength. The(μ-p)1S hfs resonance is recognized by the maximal responseto the tuned laser wavelength of the time distribution of
X-ray K-lines cascade from the (μZ)*. By means of Monte
Carlo simulations based on the existing data it has beenshown that the described method will provide the expectedresults.5)
During the first tests in 2014 at the beam delivery Port 4of the RIKEN-RAL facility, the detection system and thebeam condition allowed a satisfactory backgroundsituation.6,7) Because the efficiency of the method is boundto the collisional energy dependence of the muon transferrate, the main focus in 2015-2016 has been a detailed
experimental analysis of the muon transfer mechanism bymeasuring the muon transfer rate at various temperatures.
While for many gases the transfer rate at low energies isnearly constant, there is experimental evidence of a relevantenergy dependence for oxygen.7) Our team has performed,
at PORT4, the needed dedicated study of muon transferfrom hydrogen to the atom of an admixture gas, and, inparticular, to oxygen, at temperatures between 300 and 100
K to confirm the energy dependent muon transfer rate foroxygen and the profitability of this method (see Fig. 1).
While the final data are still subject to careful verification,
the presently extracted behavior of the transfer rate
confirms the expectations and the proposed experimentalmethod.
To achieve this result, great care has been put into thesimulation,8) final design, and construction of theexperimental layout with particular regard to the cryogenic
high-purity gas-target able to work at temperatures below
50K and pressures up to 40 atmospheres. The tasks were thefollowing: to minimize the material at the beam entrance
window of the thermally isolated structure so as to keep aminimal induced spread of the low momentum beam, tominimize the thickness of the lateral walls so as to allow
high transparency for the X-rays of the muonic cascade ofinterest, and finally to coat the internal vessel with thinlayers of high Z material in order to promote fast nuclear
muon capture for non-hydrogen muonic atoms andminimize the non-prompt noise induced by decay electrons.

Fig. 1. Transfer rate to oxygen as function of temperature.
References
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5) D. Bakalov et al., Can. J. Phys.
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7) A. Vacchi et al., RIKEN Accel.
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8) P. Danev et al., J. of Inst.,

1

RIKEN Nishina CenterDepartment Math, Info, Physics, University of Udine
3
INFN Trieste
2- 24 -